US20110199975A1

US20110199975A1 - Wireless base station device using collaborative harq communication system, wireless terminal device, wireless communication system, and wireless communication method

Info

Publication number: US20110199975A1
Application number: US13/093,394
Authority: US
Inventors: Jianming Wu
Original assignee: Fujitsu Ltd
Current assignee: Fujitsu Ltd
Priority date: 2008-10-28
Filing date: 2011-04-25
Publication date: 2011-08-18
Also published as: AU2008363468A1; CN102197617B; EP2763342B1; AU2008363468B2; ES2715967T3; EP2346272A4; EP2763342B8; RU2012141645A; JPWO2010049970A1; KR20110071143A; EP2763342A3; US20140079006A1; CA2741498C; CN102197617A; US10034272B2; US20140098765A1; US9974072B2; RU2544746C2; US9992771B2; RU2526285C2

Abstract

In a transmission device on a serving eNB side, a first packet transmission unit performs an operation of transmitting a retransmission data packet. On the other hand, in a transmission device on a collaborative eNB side, a second packet transmission unit performs an operation of transmitting a new data packet corresponding to information transferred from the serving eNB by the packet transfer unit. The control information about a communication to a UE by the serving eNB and the collaborative eNB is communicated by using only a PUCCH from the UE to the serving eNB and a PDCCH from the serving eNB to the UE. The serving eNB and the collaborative eNB perform communications of a new data packet and communication control information etc. through an X2 interface.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of International PCT Application No. PCT/JP2008/003080 which was field on Oct. 28, 2008, now pending the contents of which are herein wholly incorporated by reference.

FIELD

The present invention relates to collaborative transmission system technology using a distributed antenna. Packet communication technology includes, for example, E-UTRA (Evolved Universal Terrestrial Radio Access) communication technology which has been studies as a next generation mobile telephone communication standard.

BACKGROUND

Relating to the spread-spectrum code division multiple access, widely studied is the soft handoff technology for preventing the communications from being interrupted by being transmitted and received the same signals simultaneously between two base stations when a mobile terminal moves from one cell to an adjacent cell. As the prior art relating to a collaborative transmission, for example, a system described in the patent document 1, the following non-patent document 1, etc. is disclosed. In the prior art, a collaborative transmission system for successfully increasing the link capacity is disclosed.
Based on a similar concept, a collaborative transmission system using a distributed antenna arranged in a different base station is proposed in relation to the multi-input and multi-output (MIMO) technology corresponding to macroscopic fading. As the prior art obtained by combining the MIMO technology and the collaborative transmission technology, for example, the systems described in the following non-patent documents 2 through 6 are proposed. These systems aim at attaining both a macroscopic diversity effect and a MIMO effect.
The discussions of the macroscopic diversity with a collaborative transmission have been made in a planning project of a new mobile telephone communication standard such the LTE (Long Term Evolution) etc. for which a standardizing operation is performed by a standardizing organization 3GPP (3rd Generation Partnership Project), for example. These discussions are disclosed by, for example, the following non-patent document 7. However, since it has been hard to distribute data of a high layer to different base stations, the collaborative transmission has not been realized, but a system of distributing data only to one base station has been used for simple implementation.
Recently, the LTE advanced standard as a next generation standard of the LTE has been developed as the fourth generation system (4G). In the standard, especially at a system performance request relating to the frequency efficiency for downlink (DL) and uplink (UL), a rather positive target is set. A practical discussion of the problem above has been disclosed in, for example, the following non-patent document 8.
To attain the above-mentioned target, some corporations have presented useful propositions about a beam forming transmission, intra-cell interference control, and relay control. In the propositions, the point of the discussion relating to the collaborative transmission has been taken up again to reconsider the possibility of the implementation. To be concrete, it is disclosed in, for example, the following non-patent document 9 or 10. In the LTE advanced, the target of the throughput of a user at the edge of a cell is set as approximately 1.4 times as high as that in the release 8 of the LTE communication standard. By taking this into account, the collaborative transmission system is expected as an important candidate in the LTE advanced technology.
Before adopting the collaborative transmission technology in the next generation communication standard such as the LTE advanced etc., there are a number of points to be discussed. It is, for example, a search of data and control channel, transmission timing, user packet scheduling, hybrid automatic repeat request (HARQ) process, etc. between eNodes-B through the X2 interface. The most important search among them is that relating to the HARQ.
In the LTE communication standard etc., the packet communication technology is required to enable the high-speed communications at a mobile terminal. In the packet communication, a reception device receives communication information while detecting an error based on the error correction code added to a communication packet by transmission device. Then, the reception device returns to the transmission device an ACK (acknowledgement) or a NAK (negative acknowledgement) about the reception status of the communication packet. The transmission device retransmits transmission information when the reception device returns a NAK or when no transmission status confirmation can be received before a certain period has passed after a packet is transmitted.
In the HARQ technology adopted in the LTE etc., for example, the retransmission pattern is determined on the transmission device side after considering that the data whose decoding has failed by the reception device is not discarded but decoded by a combination with retransmission data in the process of a layer 1 protocol hierarchical level of the LTE etc. On the reception device side, the data whose reception has failed is not discarded, but decoded by a combination with retransmission data. Thus, retransmission control is realized with high efficiency and high accuracy.
Therefore, in the next generation packet communication system, it is an important problem to determine how the HARQ is to be realized in the collaborative transmission system to realize a collaborative transmission system with a high diversity effect.
However, in the prior art disclosed as Patent Document 1 or non-patent documents 1 through 10, no practical technology for realizing the HARQ in the collaborative transmission has not been disclosed.
In addition, the system described in the following patent document 2 is disclosed as prior art obtained by combining the HARQ and the MIMO technology. Patent Document 2 refers to a practical system for realizing the HARQ in the packet transmission using a MIMO multiple transmission antenna.
However, the MIMO is based on that a plurality of antennas are accommodated in one base station while the collaborative transmission is based on that the antennas of a plurality of base stations arranged in a distributed manner perform a collaborative transmission in the downlink direction toward a mobile terminal. To realize a collaborative transmission including a HARQ between the base stations arranged in the distributed manner, it is necessary to solve the problems, which is not necessary in the MIMO, of the communication system for user data and channel data, timing, etc. among the base stations. Especially, the combination of a new data packet and a retransmission data packet in the HARQ with the collaborative transmission is not disclosed by the above-mentioned prior art, which remains as an unsolved problem.

Patent Document 1: National Publication of International Patent Application No. 2008-503974
Patent Document 2: National Publication of International Patent Application No. 2008-517484
Non-patent Document 1: A. J. Viterbi, A. M. Viterbi, K. S. Gilhousen, and E. Zehavi, “Soft handoff extends CDMA cell coverage and increases reverse link capacity”, IEEE J. Sel. Areas Commun., vol. 12, pp. 1281-1288, October, 1994.
Non-patent Document 2: W. Roh and A. Paulraj, “MIMO channel capacity for the distributed antenna systems”, in IEEE VTC′ 02, vol. 3, pp. 1520-1524, September 2002.
Non-patent Document 3: Z. Ni and D. Li, “Impact of fading correlation and power allocation on capacity of distributed MIMO”, IEEE Emerging technologies: Frontiers of Mobile and Wireless Communication, 2004, Volume 2, May 31-Jun. 2, 2004 Page (s): 697-700 vol. 2.
Non-patent Document 4: Syed A. Jafar, and S. Shamai, “Degrees of freedom region for the MIMO X Channel”, IEEE Transactions on Information Theory, Vol. 54, No. 1, pp. 151-170, January 2008.
Non-patent Document 5: D. Wang, X. You, J. Wang, Y. Wang, and X. Hou, “Spectral Efficiency of Distributed MIMO Cellular Systems in a composite Fading Channel”, IEEE International conference on, Communications, 2008. ICC '08, pp. 1259-1264, May 19-23, 2008.
Non-patent Document 6: O. Simeone, O. Somekh, H. V. Poor, and S. Shamai, “Distributed MIMO in multi-cell wireless systems via finite-capacity links”, Communications, Control and Signal Processing, 2008. ISCCSP 2008. 3rd International Symposiumon, pp. 203-206, Mar. 12-14, 2008.
Non-patent Document 7: 3GPP TR 25.814 v7.0.0. Physical layer aspects for evolved UTRA, release-7, June 2006.
Non-patent Document 8: 3GPP TR 36.913 V7.0.0., Requirements for Further Advancements for E-UTRA, release-8, V8.0.0, June 2008.
Non-patent Document 9: 3GPP TSG RAN WG1 Meeting #53bis Warsaw, Poland, “Collaborative MIMO for LTE-A downlink”, Jun. 30-Jul. 4, 2008, R1-082501.
Non-patent Document 10: 3GPP TSG RAN WG1 Meeting #53bis Warsaw, Poland, “Network MIMO Precoding”, Jun. 30-Jul. 4, 2008, R1-082497

SUMMARY

The problem of the present invention is to realize an appropriate and efficient HARQ process in the collaborative transmission system.
The aspect described below is based on the wireless communication system in which the first wireless base station device and the second wireless base station device perform a collaborative transmission process to allow the wireless terminal device not to discard a packet on which decoding has failed but to combine the packet with a retransmitted packet and decode the resultant packet while controlling the retransmission of a packet according to the transmission status information returned from the wireless terminal device, the wireless base station device or the wireless terminal device which belong to the wireless communication system, or the wireless communication method for realizing the process.
A first packet transmission unit transmits as a first packet a new data packet or a retransmission data packet corresponding to a retransmit request from the first wireless base station device to the wireless terminal device when the retransmit request is issued to the collaborative transmission process by the wireless terminal device.
A packet transfer unit transfers the information about a second packet different from the first packet between the new data packet and the retransmission data packet from the first wireless base station device to the second wireless base station device. The packet transfer unit performs a transfer process using, for example, an X2 interface regulated between the first wireless base station device and the second wireless base station device.
The second packet transmission unit transmits the second packet according to the information transferred from the packet transfer unit in synchronization with the transmission process of the first packet by the first packet transmission unit from the second wireless base station device to the wireless terminal device when the retransmit request is issued.
With the above-mentioned configuration, each of the first wireless base station device and the second wireless base station device has a retransmission buffer unit, and the first wireless base station device can be configured to hold the information about the packet on which a collaborative transmission process is performed for the wireless terminal device in the retransmission buffer unit in the first wireless base station device, and the second wireless base station device can be configured not to hold the information about the packet on which the collaborative transmission process is performed for the wireless terminal device in the retransmission buffer unit in the second wireless base station device.
With the above-mentioned configuration, the first packet can be configured as a retransmission data packet, and the second packet can be configured as a new data packet. In this instance, the packet transfer unit reads the information about the retransmission data packet from the retransmission buffer unit in the first wireless base station device, and transfers the information to the second wireless base station device. The packet transfer unit transfers, for example, the communication control information relating to the second wireless base station device for communication between the first wireless base station device and the wireless terminal device and the information relating to the transmission timing of the second packet by the second wireless base station device.
With the configurations up to the aspects above, a control information communication unit for communicating the control information about the communication by the first wireless base station device to the wireless terminal device and the control information about the communication by the second wireless base station device to the wireless terminal device between the first wireless base station device and the wireless terminal device can be further included. For example, the control information communication unit can perform the transmission of control information from the first wireless base station device to the wireless terminal device through a physical downlink control channel and perform the transmission of the control information from the wireless terminal device to the first wireless base station device through a physical uplink control channel. The physical uplink control channel in this case includes at least, for example, the individual channel quality indication information for each of the first wireless base station device and the second wireless base station device, and the precoding matrix indication information and the rank indication information common to the first wireless base station device and the second wireless base station device. In addition, the physical downlink control channel includes at least, for example, the individual modulation and coding scheme information and the individual precoding information for each of the first wireless base station device and the second wireless base station device.
With the configuration described above, the control information from the wireless terminal device to the first wireless base station device can be configured to include the transmission status information (HARQ-ACK/NAK) indicating a reception result of the packet from the first wireless base station device and a reception result of the packet from the second wireless base station device, respectively.
With the configuration above, the first wireless base station device can be configured to centrally control at least the assignment of a wireless terminal device, the assignment of communication resources, and the control of transmission timing associated with the collaborative transmission process.
The wireless terminal device for performing the communication by the wireless communication system having the above-mentioned configuration has the following aspects.
A retransmission data packet reception unit performs a receiving process on a retransmission data packet when a retransmit request is issued.
When the retransmission data packet reception unit successfully performs the receiving process on the retransmission data packet, a new data packet reception unit performs a successive interference cancellation process on the received signal received by the wireless terminal device through the retransmission data packet on which the receiving process has been successfully performed, and the receiving process of a new data packet according to a resultant received signal is performed.
With the configuration of the aspect of the wireless terminal device, a collaborative transmission process determining unit for determining whether or not the collaborative transmission process is to be performed and determining the first wireless base station device and the second wireless base station device for performing the process when the execution of the collaborative transmission process is determined can be further included. For example, the collaborative transmission process determining unit makes a determination according to the information about the reception power for the reference signal to be received from each wireless base station device currently in communication.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an explanatory view of a network model based on which the present embodiment is designed;

FIG. 2 is a configuration of an embodiment of the transmission device;

FIG. 3 is a configuration of an embodiment of the reception device;

FIG. 4 is an explanatory view of grouping cases in which two eNodes-B collaboratively operate;

FIG. 5 is an explanatory view of the collaborative downlink HARQ transmission system for a scenario 2;

FIG. 6 is an explanatory view of the collaborative downlink HARQ transmission system for a scenario 3;

FIG. 7 is an example of an operation sequence of a determining process of a serving eNB and a collaborative eNB;

FIG. 8 is an explanatory view of a data channel and a control channel;

FIG. 9 is an example of a data format of a UCI and a DCI;

FIG. 10 is an example of the transmission timing between a control channel and a data channel;

FIG. 11 is a graph indicating the a BLER to geometry for each UE on the initial transmission, retransmission # 1, #2, and #3 in the simulation result;

FIG. 12 is a graph indicating the CDF of the SINR to a S-eNB and a C-eNB with and without SIC in the simulation result;

FIG. 13 is a graph indicating the probability of a link gap between a serving eNB and a collaborative eNB;

FIG. 14 is a graph indicating the SINR to link gap between a serving eNB and a collaborative eNB with and without SIC at the CDF point of 0.5; and

FIG. 15 is a graph indicating the gain to link gap by the cancellation between the serving eNB and the collaborative eNB at the CDF point of 0.5.

DESCRIPTION OF EMBODIMENTS

The best embodiments are described below in detail with reference to the attached drawings.
First, the system network model is described according to the embodiments of the present invention.
FIG. 1 is an explanatory view of a network model based on which the present embodiment is designed.
To hold generalities, a network is configured as a packet communication system including two wireless base stations for collaboratively performing a service on a wireless mobile terminal (UE: User Equipment) such as a mobile telephone terminal etc. A packet communication system can be realized as, for example, an E-UTRA (Evolved Universal Terrestrial Radio Access) system in accordance with the LTE communication standard on which a standardizing operation is performed by 3GPP.
In the LTE etc., a base station is referred to as an eNode-B (evolved Node B). In the present embodiment, in the description below, a base station is referred to as an eNode-B or an eNB for short.
As illustrated in FIG. 1, one of the two wireless base stations is a serving base station (serving eNode-B, hereinafter referred to as a “serving eNB” or a “S-eNB” for short), and the other is referred to as a collaborative base station (collaborative eNode-B, hereinafter referred to as a “collaborative eNB” or a “C-eNB” as necessary). The determination as to which the eNB belongs, a serving eNB or a collaborative eNB, depends on the long-period power intensity received by each UE. Therefore, the positioning of the eNB for each UE can be different. As a reasonable definition, the long-period power intensity from the serving eNB received by each UE is higher than that of the collaborative eNB.
FIG. 2 is a configuration of a packet transmission device according to an embodiment configured in the eNode-B on the network illustrated in FIG. 1. FIG. 3 is a configuration of a packet reception device according to an embodiment configured in the UE illustrated in FIG. 1. The transmission device in FIG. 2 is provided on the downlink side of the eNode-B, and the reception device in FIG. 2 is provided on the downlink side of the UE. The configuration of the transmission/reception device on the uplink channel side of the devices has a common configuration, and the detailed description is omitted here.
The transmission device illustrated in FIG. 2 includes a new data packet transmission unit 201, a retransmission data packet transmission unit 202, a channel assignment unit 203, a modulation unit 204, a wireless processing unit 205, a transmission control unit 206, an uplink control channel reception unit 207, and an X2 control channel transmission/reception unit 208. The new data packet transmission unit 201 is further configured by a block generation unit 201-1, a new portion acquisition unit 201-2, and a new data packet coding unit 201-3. The retransmission data packet transmission unit 202 is further configured by a retransmission buffer unit 202-1, a retransmission portion acquisition unit 202-2, and a retransmission data packet coding unit 202-3.
The reception device illustrated in FIG. 3 includes a wireless processing unit 301, a retransmission data packet reception unit 302, a new data packet reception unit 303, a reception control unit 304, and an uplink control channel transmission unit 305. The retransmission data packet reception unit 302 is further configured by a retransmission data packet demodulation unit 302-1, a retransmission buffer unit 302-2, a retransmission portion combination unit 302-3, a retransmission data packet decoding unit 302-4, and a output distribution unit 302-5. The new data packet reception unit 303 is further configured by a retransmission data packet re-coding unit 303-1, a retransmission data packet re-modulation unit 303-2, a canceller unit 303-3, a new data packet demodulation unit 303-4, and a new data packet decoding unit 303-5.
Described below in detail are the operations of the embodiments of the transmission device and the reception device with the above-mentioned configurations.
A very unique and important behavior for the HARQ can be the block error rate of normally 1% or less when a retransmission data packet is decoded after the HARQ combining process performed by the retransmission portion combination unit 305-3 illustrated in FIG. 2. In the embodiment illustrated in FIG. 2, in the successive interference cancellation process (SIC) performed by the canceller unit 303-3, a decoded retransmission data packet is positively used, thereby realizing an effective SIC process. That is, in the embodiment illustrated in FIG. 2, a retransmission packet is first detected in the UE, and then other packets (new or retransmission packets) are detected.
Next, in the present embodiment, one new packet and one retransmission packet are delivered in complete synchronization toward one UE from two collaboratively operating eNodes-B which implement a transmission device of a downlink system illustrated in FIG. 1.
FIG. 4 is an explanatory view of grouping cases in which two eNodes-B collaboratively operate. In this example, a collaborative transmission is grouped into four types of scenarios. Each scenario refers to a different channel resource assignment, and a different control channel design. For simplicity, the explanation here refers to the case of one UE only, but the scenario for a plurality of UEs is described later.
In the scenario 1 illustrated in FIG. 4( a), it is assumed that only a new data packet is delivered to a UE positioned at the cell edge from the serving eNB. To realize a macroscopic transmission collaboratively, some new data packets are transferred from the serving eNB to the collaborative eNB through the X2 interface. Then, the new data packets are delivered simultaneously to a corresponding UE from both eNodes-B. On the UE side, the receiving process is performed while suppressing the interference from each other.
In the scenario 2 illustrated in FIG. 4( b) it is assumed that two types of transmission packet are delivered to the UE positioned at the cell edge. One is a retransmission data packet, and another packet is a new data packet. The retransmission data packet is delivered from a serving eNB to a UE simultaneously when the new data packet transferred from the serving eNB through an X2 interface is delivered from a collaborative eNB to a UE. In the UE, as described later, the new data packet reception unit 303 illustrated in FIG. 3 performs the receiving process while suppressing the interference from each other in the SIC process.
In the scenario 3 illustrated in FIG. 4( c), as in the scenario 2, the two types of transmission packets, that is, the retransmission data packet and the new data packet, are delivered. In the scenario 3, unlike the scenario 2, a new data packet is delivered from the serving eNB to the UE simultaneously when a retransmission data packet is delivered from the collaborative eNB to the UE. In this case, the retransmission data packet is transferred from the serving eNB to the collaborative eNB. In the UE, as described later, the new data packet reception unit 303 illustrated in FIG. 3 performs the receiving process while suppressing the interference from each other in the SIC process.
In the scenario 4 illustrated in FIG. 4( d), it is assumed that only the retransmission data packet is delivered from the serving eNB to the UE at the cell edge. To collaboratively realize a macroscopic transmission, some retransmission data packets are transferred from the serving eNB to the collaborative eNB through the X2 interface. Then, the retransmission data packets are simultaneously delivered to the corresponding UE from both eNBs. The UE performs the receiving process while suppressing the interference from each other.
It is considered that the scenario 2 illustrated in FIG. 4( b) and the scenario 3 illustrated in FIG. 4( c) are better transmission systems for providing the highest diversity gain by a macroscopic transmission analysis and a cancellation gain by the SIC process because since the BLER (block error rate) for the retransmission data packet after a HARQ combination is sufficiently low, the retransmission data packet can be first extracted, and then the new data packet can be extracted by the SIC process, thereby acquiring a better result. Therefore, it is preferable that one new data packet and one retransmission data packet can be constantly acquired as a rule of the collaborative transmission, and they can be transmitted simultaneously from both the serving eNB and the collaborative eNB. According to the system level simulation result described later, it is certain that if an UE moves at the speed of 3 km/h, the probability of a retransmission is 8-10%. However, if it moves at the speed of 30 km/h, the probability of a retransmission increases up to 70-80%. Therefore, when there are terminal groups coexisting and moving at different speeds, the probability of retransmissions can be estimated as 30-40%. It means the possibility of the collaborative HARQ transmission between the new data packet and the retransmission data packet is 23-29%. It is considered that the probability that the scenario 1 illustrated in FIG. 4( a) as a normal collaborative transmission without a retransmission is approximately 70%. However, since the scenario 4 illustrated in FIG. 4( d) indicates a low occurrence probability of a HARQ packet, it does not occur in a practical system. Therefore, the probability that the scenario 4 is adopted is nearly zero.
By the search above, the description below is concentrated on the cases of the scenario 2 illustrated in FIG. 4( b) and the scenario 3 illustrated in FIG. 4( c) as an operation of the transmission device of the eNode-B downlink system illustrated in FIG. 2. One of these scenarios is selected and designed during the implementation. A more preferable scenario between them is described later.
FIG. 5 is an explanatory view of the collaborative downlink HARQ transmission system for the scenario 2.
First, in FIG. 5( b), if a new data packet received at the UE (for example, a new data packet #0) enters an erroneous state, the data is retransmitted from the serving eNB simultaneously with the new packet (for example, a new data packet #12) delivered from the collaborative eNB (C-eNB) to the synchronous transmission timing determined by the serving eNB (S-eNB). A similar process occurs with a retransmission packet #4 (or #11) transmitted with the new data packet #17 (or #15).
FIG. 5( a) is a block diagram of the configuration of the process of the transmission device for the scenario 2. When the transmission device in FIG. 2 is implemented as a downlink system on the serving eNB side, a retransmission buffer unit 504 on the serving eNB side in FIG. 5( a) corresponds to the retransmission buffer unit 202-1 illustrated in FIG. 2. A first packet transmission unit 501 on the serving eNB side corresponds to the portion excluding the retransmission buffer unit 202-1 in the retransmission data packet transmission unit 202 illustrated in FIG. 2. Furthermore, an RF 503 on the serving eNB side corresponds to the portion configured by the channel assignment unit 203, the modulation unit 204, and the wireless processing unit 205 illustrated in FIG. 2. On the other hand, when the transmission device is implemented as a downlink system on the collaborative eNB side, the second packet transfer unit 503 on the collaborative eNB side in FIG. 5( a) corresponds to the new data packet transmission unit 201 in FIG. 2. An RF 505 on the collaborative eNB side corresponds to the portion configured by the channel assignment unit 203, the modulation unit 204, and the wireless processing unit 205 in FIG. 2. Furthermore, a packet transfer unit 502 for transferring a new data packet from the serving eNB to the collaborative eNB corresponds to an X2 control channel transmission/reception unit 108 illustrated in FIG. 2.
As understood from the process configuration described above, when the serving eNB and the collaborative eNB each having a transmission device of a downlink system illustrated in FIG. 2 operate according to the scenario 2, the first packet transmission unit 501 performs an operation of transmitting a retransmission data packet 507 in the transmission device on the serving eNB side. On the other hand, in the transmission device on the collaborative eNB side, the second packet transfer unit 503 performs the operation of transmitting a new data packet 508 corresponding to the information transferred from the serving eNB by the packet transfer unit 502.
FIG. 6 is an explanatory view of the collaborative downlink HARQ transmission system for the scenario 3.
First, in FIG. 6( b), when the new data packet (for example, a new data packet #0) received by the UE enters an erroneous state, the data is transferred through the X2 interface along a corresponding control channel to the collaborative eNB. Then, it is retransmitted from the collaborative eNB simultaneously with a new packet (for example, a new data packet #4) delivered from the serving eNB to the synchronous transmission timing determined by the serving eNB. A similar process is generated with a retransmission packet #5 (or #14) transmitted with a new data packet #9 (or #7).
FIG. 6( a) is a block diagram of the process configuration of the transmission device for the scenario 3. When the transmission device in FIG. 2 is implemented as a downlink system on the serving eNB side, a retransmission buffer unit 604 on the serving eNB side in FIG. 6( a) corresponds to the retransmission buffer unit 202-1 in FIG. 2. A first packet transfer unit 601 on the serving eNB side corresponds to the new data packet transmission unit 201 in FIG. 2. Furthermore, an RF 605 on the serving eNB side corresponds to the portion configured by the channel assignment unit 203, the modulation unit 204, and the wireless processing unit 205. On the other hand, when the transmission device in FIG. 2 is implemented as a downlink system on the collaborative eNB side, the second packet transfer unit 603 on the collaborative eNB side in FIG. 6( a) corresponds to the portion excluding the retransmission buffer unit 202-1 in the retransmission data packet transmission unit 202 in FIG. 2. In addition, an RF 605 on the collaborative eNB side corresponds to the portion configured by the channel assignment unit 203, the modulation unit 204, and the wireless processing unit 205 in FIG. 2. Furthermore, a packet transfer unit 602 for transferring a retransmission data packet from the retransmission buffer unit 604 in the serving eNB to the collaborative eNB corresponds to the X2 control channel transmission/reception unit 108 in FIG. 2.
As understood from the process configuration described above, when the serving eNB and the collaborative eNB each having a transmission device of a downlink system illustrated in FIG. 2 operate according to the scenario 3, the first packet transmission unit 601 performs an operation of transmitting a new data packet 607 in the transmission device on the serving eNB side. On the other hand, in the transmission device on the collaborative eNB side, the second packet transfer unit 603 performs the operation of transmitting a retransmission data packet 608 corresponding to the information transferred from the retransmission buffer unit 604 in the serving eNB by the packet transfer unit 502.
With respect to the entire complexity, the scenario 2 is more preferable than the scenario 3 because, according to the scenario 2, the collaborative eNB receives a new block transferred from the serving eNB through the X2 interface, and can deliver a new data packet generated based on the received block without considering whether or not the packet has been correctly received on the UE side as described later in the explanation of the control channel. As described later, the serving eNB is totally responsible including the control channel access for the receiving process and the HARQ. This simplifies the design of the collaborative eNB. However, it is obvious that the configuration of the scenario 3 can be adopted.
Described below is a further detailed operation of the transmission device in FIG. 2 with the process of the scenarios 2 and 3 above.
In FIG. 2, the block generation unit 201-1 generates a block of a predetermined size from an information bit to be transmitted. The size of a block generated by the block generation unit 201-1 is equal to the amount of information bit which can be stored in one packet. That is, a normal packet to be transmitted by a transmission device includes information bits corresponding to one block.
The retransmission buffer unit 202-1 temporarily holds for a retransmission a block of the information bits generated by the block generation unit 201-1. The retransmission buffer unit 202-1 can sequentially discard the block which has been correctly decoded by the reception device and is not to be retransmitted.
The transmission control unit 206 controls the new portion acquisition unit 201-2 and the retransmission portion acquisition unit 202-2 according to the control signal received by the uplink control channel reception unit 207 from the UE side through a control channel.
Practically, when the transmission device in FIG. 2 operates as a serving eNB for a certain UE according to the scenario 1 (refer to FIG. 4( a)), and if a transmission of a retransmission data packet does not be instructed by the UE side, then the following operation is performed. That is, the transmission control unit 206 first instructs the new portion acquisition unit 201-2 to acquire a new block generated by the block generation unit 201-1 and corresponding to the UE to be processed, and output it to the new data packet coding unit 201-3 for a transmission. The transmission control unit 206 instructs the retransmission portion acquisition unit 202-2 to stop the operation. Furthermore, the transmission control unit 206 instructs the new portion acquisition unit 201-2 to output the new block also to the X2 control channel transmission/reception unit 208, and transfer it also to the collaborative eNB corresponding to the UE to be processed.
On the other hand, when the transmission device in FIG. 2 operates as a collaborative eNB for a certain UE according to the scenario 1, and if the UE side does not instruct the serving eNB corresponding to the UE to transmit a retransmission data packet, then the following operation is performed. That is, the transmission control unit 206 instructs the new portion acquisition unit 201-2 to acquire a new block received by the X2 control channel transmission/reception unit 208 and transferred from the serving eNB corresponding to the UE to be processed, and output it to the new data packet coding unit 201-3 for a transmission.
Next, when the transmission device in FIG. 2 operates as a certain serving eNB for a UE according to the scenario 2 (refer to FIG. 4( b)), and if the number of received NAKs received for the certain UE by the uplink control channel reception unit 207 has reached a predetermined number, the following process is performed. That is, the transmission control unit 206 instructs the retransmission portion acquisition unit 202-2 to acquire a transmitted block (retransmission block) corresponding to the NAK held in the retransmission buffer unit 202, and output it to the retransmission data packet coding unit 202-3 for a retransmission. In addition, the transmission control unit 206 instructs the new portion acquisition unit 201-2 to acquire a new block generated by the block generation unit 201-1 and corresponding to the UE to be processed, and output it not to the new data packet coding unit 201-3 but to the X2 control channel transmission/reception unit 208 to transfer it to the collaborative eNB corresponding to the UE to be processed.
On the other hand, when the transmission device in FIG. 2 operates as a collaborative eNB for a certain UE according to the scenario 2, and if the number of received NAKs received by the uplink control channel reception unit 207 in the serving eNB corresponding to the certain UE has reached a predetermined number, then the following process is performed. That is, the transmission control unit 206 instructs the new portion acquisition unit 201-2 to acquire a new block received by the X2 control channel transmission/reception unit 208 and transferred from the serving eNB corresponding to the UE to be processed, and output it to the new data packet coding unit 201-3 for a transmission.
When the transmission device in FIG. 2 operates as a serving eNB for a certain UE according to the scenario 3 (FIG. 4( c)), and if the number of received NAKs received by the uplink control channel reception unit 207 for the UE has reached a predetermined number, then the following process is performed. That is, the transmission control unit 206 instructs the retransmission portion acquisition unit 202-2 to acquire a transmitted block (retransmission block) corresponding to the NAK held in the retransmission buffer unit 202 to output it not to the retransmission data packet coding unit 202-3 but to the X2 control channel transmission/reception unit 208 and transfer it to the collaborative eNB corresponding to the UE to be processed. The transmission control unit 206 instructs the new portion acquisition unit 201-2 to acquire a new block generated by the block generation unit 201-1 and corresponding to the UE to be processed, and output it to the new data packet coding unit 201-3 for a retransmission.
On the other hand, when the transmission device in FIG. 2 operates as a collaborative eNB for a certain UE according to the scenario 3, and if the number of received NAKs received by the uplink control channel reception unit 207 in the serving eNB corresponding to the certain UE has reached a predetermined number, then the following process is performed. That is, the transmission control unit 206 instructs the retransmission portion acquisition unit 202-2 to acquire a retransmission block received by the X2 control channel transmission/reception unit 208 and transferred from the serving eNB corresponding to the UE to be processed, and output it to the retransmission data packet coding unit 202-3 for a transmission.
An ACK and a NAK are control signals stored with user data, transferred from a certain UE to be processed, and received by the uplink control channel reception unit 207 in the transmission device operating as a serving eNB for the certain UE as uplink control information (UCI) described later. These ACK and NAK indicate whether or not a reception error of a packet has occurred in the UE, and is returned from the UE to the corresponding serving eNB for each received packet.
In the transmission device in FIG. 2, when a new block is input from the new portion acquisition unit 201-2, the new data packet coding unit 303-1 in the new data packet transmission unit 201 generates a new packet in which the new block is included in an information bit section and a corresponding parity bit is included in a parity bit section.
When a retransmission block is input from the retransmission portion acquisition unit 202-2, the retransmission data packet coding unit 202-3 in the retransmission data packet transmission unit 202 generates a retransmission packet in which the retransmission block is included in an information bit section and a corresponding parity bit is included in a parity bit section.
The channel assignment unit 203 assigns the new packet generated by the new data packet coding unit 201-3 or the retransmission packet generated by the retransmission data packet coding unit 202-3 to a communication channel corresponding to the UE to be processed, and outputs the resultant frame data to the modulation unit 204.
The modulation unit 204 modulates the frame data output from the channel assignment unit 203, and outputs the data to the wireless processing unit 205.
The wireless processing unit 205 performs a predetermined wireless transmitting process on the frame data after the modulation, and transmits the resultant data through an antenna not illustrated in the attached drawings.
Described next is the detailed operation of the reception device illustrated in FIG. 3 and implemented in the downlink system in the UE.
As illustrated in FIG. 3, the reception device is provided with the retransmission data packet reception unit 302 and the new data packet reception unit 303.
In FIG. 3, the reception control unit 304 can recognize whether a received packet is a new data packet or a retransmission data packet according to the new data indication information (refer to FIG. 9( b)) included in the downlink control information (DCI) transmitted from the serving eNB with the received packet through a physical downlink control channel as described later. The recognition is similar to the identification between the scenario 1 and the scenario 2, or between the scenario 1 and the scenario 3. The reception control unit 304 performs the identifying process based on the output of the retransmission data packet demodulation unit 302-1 which constantly performs the demodulating process.
By the identification, when the reception device operates according to the scenario 1 (FIG. 4( a)) described above, the retransmission data packet reception unit 302, the retransmission data packet re-coding unit 303-1, the retransmission data packet re-modulation unit 303-2, and the canceller unit 303-3 in the new data packet reception unit 303 do not operate, and the received signal received by the wireless processing unit 301 through an antenna passes through the canceller unit 303-3 in the new data packet reception unit 303 and enters the new data packet demodulation unit 303-4.
The new data packet demodulation unit 303-4 demodulates the received packet from each communication channel configuring the received signal input from the wireless processing unit 301, and outputs the received packet to the new data packet decoding unit 303-5.
The new data packet decoding unit 303-5 decodes the input new data packet, and outputs resultant new information bits to the processing unit at the subsequent stage but not illustrated in the attached drawings.
On the other hand, in the identifying process by the reception control unit 304, when the reception device illustrated in FIG. 3 operates as the scenario 2 (FIG. 4( b)) or the scenario 3 (FIG. 4( c)), both retransmission data packet reception unit 302 and new data packet reception unit 303 operate under the control of the reception control unit 304.
Described first is the operation of the retransmission data packet reception unit 302.
The retransmission data packet demodulation unit 302-1 demodulates the received packet from each communication channel configuring the received signal input from the wireless processing unit 301, and outputs the received packet to the retransmission portion combination unit 302-3. The retransmission data packet demodulation unit 302-1 performs a demodulating process regardless of whether the received packet is a retransmission data packet or a new data packet to enable the identifying process by the reception control unit 304.
With the timing of processing on a retransmission packet indicated by the reception control unit 304, the retransmission portion combination unit 302-3 combines the retransmission data packet input from the retransmission data packet demodulation unit 302-1 with the past data packet held in the retransmission buffer unit 302-2 after a first reception failure. Then, the retransmission portion combination unit 302-3 outputs the combination result to the retransmission data packet decoding unit 302-4. The reception control unit 304 receives retransmission sequence information and other control information as a part of downlink control information (DCI) transmitted with a received packet from the serving eNB through the physical downlink control channel, and notifies the retransmission portion combination unit 302-3 of these pieces of control information. The retransmission portion combination unit 302-3 performs the process of combining retransmission packets in the HARQ system according to the control information.
The retransmission data packet decoding unit 302-4 decodes the input retransmission data packet, and outputs the resultant reconstructed information bits to the output distribution unit 302-5.
When the information bits are successfully reconstructed, the output distribution unit 302-5 outputs them to the processing unit at the subsequent stage but not illustrated in the attached drawings. Simultaneously, the output distribution unit 302-5 outputs the reconstructed information bits to the retransmission data packet re-coding unit 303-1 in the new data packet reception unit 303.
Described next is the operation of the new data packet reception unit 303.
When the reconstructed information bits are input from the output distribution unit 302-5, the retransmission data packet re-coding unit 303-1 and the retransmission data packet re-modulation unit 303-2 are operated, and a replica of a successfully received retransmission data packet is generated.
The canceller unit 303-3 performs a cancelling process on the interference signal components in the retransmission data packet received from the serving eNB (in the case of the scenario 2) or the collaborative eNB (in the case of the scenario 3) for the received signal input from the wireless processing unit 301 as a successive interference cancellation process. Thus, the canceller unit 303-3 appropriately extracts only the received signal components of the new data packet received from the collaborative eNB (in the case of the scenario 2) or the serving eNB (in the case of the scenario 3), and outputs the result to the new data packet demodulation unit 303-4.
the new data packet demodulation unit 303-4 demodulates the received packet from each communication channel configuring the received signal from which the interference components input from the canceller unit 303-3 are removed, and outputs the received packet to the new data packet decoding unit 303-5.
The new data packet decoding unit 303-5 decodes the input new data packet, and outputs the resultant new information bits to the processing unit at the subsequent stage but not illustrated in the attached drawings.
If the reconstructing process on the retransmission data packet fails in the retransmission data packet reception unit 302, and no input is performed from the output distribution unit 302-5 to the retransmission data packet re-coding unit 303-1, then the input from the retransmission data packet re-modulation unit 303-2 to the canceller unit 303-3 is set to zero. Thus, the operation of the canceller unit 303-3 becomes invalid equivalently. As a result, the new data packet demodulation unit 303-4 and the new data packet decoding unit 303-5 extract a new data packet without the cancelling process.
In FIG. 3, the reception control unit 304 correctly recognizes the physical downlink control channel from the serving eNode-B described later according to, for example, the reference signal (RS) in the received signal. As an RS group between the serving eNB and the collaborative eNB, a signal group in which signals have the same patterns but different phase shifts, for example, those orthogonal to each other, can be used to easily identify the channel between the serving eNB and the collaborative eNB.
As an example of a variation of a system of processing the above-mentioned reception device, the following interactive system capable of improving the system performance can also be applied.

- First, a retransmission data packet is extracted, and if it is correctly received, a new data packet is extracted in the SIC process by a canceller unit.
- If the retransmission data packet is not successfully received, a new data packet is extracted. If the new data packet is correctly received, the retransmission data packet is extracted again in the SIC process by the canceller unit.

Thus, in the present embodiment, a retransmission data packet and a new data packet are assigned to the serving eNB and the collaborative eNB (in the case of the scenario 2) or inversely (in the case of the scenario 3) to perform a collaborative transmission, thereby successfully and simultaneously transmitting a retransmission data packet and a new data packet corresponding to the same UE using the same channel resources. Thus, in the collaborative transmission system according to the present embodiment, channels can also be effectively used.
The assignment of channel resources and the user scheduling for a collaborative transmission are centrally controlled by the transmission control unit 206 (FIG. 2) in the serving eNB. As an important parameter for determining whether or not a collaborative transmission is to be performed, a link gap Δue or, in place of it, a reference signal receiving power (RSRP) difference used as a term in the LTE is used. The parameter is defined as a difference of logarithm received signal powers between the serving eNB and the collaborative eNB in the UE. If the link gap Δue is smaller than the link gap target Δ as another parameter, the collaborative transmission is performed. Otherwise, a normal transmission is preferable. Using these parameters, a band width for a collaborative transmission can be easily controlled.
The reception control unit 304 in the reception device (FIG. 3) of the UE sequentially detects the RSRP deference of each received RS during communications, and notifies the serving eNB side of the result through the uplink control channel transmission unit 305. As a result, the uplink control channel reception unit 207 in the current serving eNB (FIG. 2) receives it, and the transmission control unit 206 (FIG. 2) determines whether or not the collaborative transmission is to be continued, determines a new serving eNB, etc.
Described above is the collaborative HARQ transmitting process relating to one UE, but each UE can identify the execution status of the collaborative transmission according to an RS signal group and identify the serving eNB and the collaborative eNB as described above. Thus, each eNode-B can control whether it functions as a serving eNB or a collaborative eNB for each UE, and can perform the same process as the process mentioned above.
FIG. 7 is an example of an operation sequence of a determining process of a serving eNB and a collaborative eNB. A UE determines, for example, the eNode-B1 as a serving eNB and the eNode-B0 as a collaborative eNB according to an RS signal group in the state in which communications with the eNode-B0 and the eNode-B1 are performed using, for example, control signals 0 and 1 (S1 in FIG. 7). Thus, the UE performs communications with the eNode-B1 using, for example, a random access channel RACH. Upon receipt of a notification of a data channel and a control channel from the eNode-B1 (S2 in FIG. 7), the UE notifies the eNode-B1 as a serving eNB of the information relating to the eNode-B0 as a collaborative eNB using the control channel (S3 in FIG. 7). As a result, a notification is issued from the eNode-B1 to the eNode-B0 using the X2 interface, and the eNode-B0 notifies the UE of the data channel and the control channel (S4 in FIG. 7). Thus, the UE can receive a collaborative transmission from the eNode-B1 and the eNode-B0. In this case, it receives a packet of collaborative transmission data and control information from the eNode-B1 as a serving eNB, and receives only the packet of collaborative transmission data from the eNode-B0 as a collaborative eNB.
Described next is the control channel communicated between a control channel designing eNode-B and the UE.
In the configuration of the present embodiment, an important control signal is communicated through a link between the serving eNB and the UE. That is, the link between the serving eNB and the UE is configured so that it has a more important function that the link between the collaborative eNB and the UE.
In designing a control channel, three channels are regarded. They are a physical uplink control channel (PUCCH), a physical downlink control channel (PDCCH), and an X2 control channel (X2CCH).
In addition, a control channel is designed according to the above-mentioned scenario 2 (FIG. 4( b)) because the scenario can provide better system performance and lower complexity for both the control channel and the data channel. The selection is confirmed in evaluating the system level simulation described later.
FIG. 8 is an explanatory view of a data channel and a control channel and their communication directions. The restrictions on the two types of channels are described below.

- A new data packet can be transmitted on the two links, that is, from the serving eNB to the UE and from the collaborative eNB to the UE.
- A retransmission packet can be transmitted only on the link from the serving eNB to the UE.
- The PUCCH indicated as a C1 is transmitted on the link from the UE to the serving eNB.
- The PDCCH indicated as a C2 is transmitted on the link from the serving eNB to the UE.
- Only a new data packet and a control signal relating to the packet are delivered from the serving eNB to the collaborative eNB using the X2 interface. The control channel in the X2 interface is indicated as C3.

By the above-mentioned design of the control channel for the collaborative transmission, the amount of control channel can be exceedingly reduced, and the system latency can be considerably shortened by the HARQ process in a single direction. Described below in more detail is the design of each of the three channels.
First described is the design of the PUCCH.
In the design described below, the PUCCH corresponds to the uplink control information (UCI) including the following two periodic signals. One includes a channel quality indication (CQI), a precoding matrix indication (PMI), and a rank indication (RI), and expressed by CQI/PMI/RI. The other includes a HARQ-ACK/NAK. A PUCCH is transmitted only on the link from the UE to the serving eNB. In FIG. 8, it is indicated by C1. The PUCCH is terminated by the uplink control channel transmission unit 305 (FIG. 3) in the UE and the uplink control channel reception unit 207 (FIG. 2) in the eNode-B operating as a serving eNB. Each active UE separates the serving eNB and the collaborative eNB by, for example, a high layer control signal.
Each UE observes a channel response according to the reference signal (RS) from the serving eNB as well as the collaborative eNB. As described above, the phases of the RS of both NBs are set so that they can be orthogonal to each other. The uplink control channel transmission unit 305 (FIG. 3) in the UE notifies the uplink control channel reception unit 207 (FIG. 2) in the serving eNB corresponding to the UE of a periodical UCI. The CQI/PMI/RI included in the UCI corresponds to the quality of both links, that is, the link from the serving eNB to the UE and the link from the collaborative eNB to the UE. Then, the UCI is only transmitted to the corresponding serving eNB for the following two reasons.

- Generally, the quality of the link from the serving eNB to the UE is better than the that from the collaborative eNB to the UE, which ensures the performance for the UL control channel.
- It exceedingly reduces the amount of control channel, and simplifies the control channel design.

FIG. 9( a) illustrates a data format of an example of a UCI for both links. The format includes individual CQI for the respective links. It also includes corresponding PMI and RI. The field information corresponding to the PMI and RI is the same for both links.
The ACK or NAK (HARQ-ACK/NAK) included in the UCI for the HARQ process is the information about whether or not a reception error of a packet has occurred in the UE. The retransmission data packet decoding unit 302-4 and the new data packet decoding unit 303-5 in the reception device illustrated in FIG. 3 notifies the uplink control channel transmission unit 305 that it is necessary to retransmit a packet being processed when an error rate is equal to or higher than a predetermined threshold and the number of repetitions of a decoding process reaches a predetermined number in each decoding process. Thus, the uplink control channel transmission unit 305 transmits, to the serving eNB corresponding to the UE to which the unit belongs, a NAK for each received packet for which a retransmission is specified. In the case other than the above-mentioned condition, when the retransmission data packet decoding unit 302-4 and the new data packet decoding unit 303-5 successfully receive each received packet, the uplink control channel transmission unit 305 transmits an ACK for each received packet which has successfully received to the serving eNB corresponding to the UE including the unit.
The HARQ-ACK/NAK included in the UCI is received by the uplink control channel reception unit 207 (FIG. 2) in the serving eNB, and the information is passed to the transmission control unit 206. The transmission control unit 206 performs the retransmitting process on the HARQ as described above. In this case, it is preferable that the retransmitting process is performed only to the UE from the serving eNB as described in the scenario 2 for the following reasons.

- The transmission latency in the HARQ process for a transmission packet can be reduced.
- The control channels including the PDCCH and the X2CCH can be simplified.
- The complexity for the collaborative eNB can be reduced because a transmitted new packet is not left in the retransmission buffer unit 302-2 (FIG. 2) arranged in the collaborative eNB. The collaborative eNB is only to transmit a new packet after the control channel (X2CCH) from the X2 interface.

The field of the HARQ-ACK/NAK on the PUCCH is designed to include the ACK/NAK signal (2 bits) corresponding to both of the serving eNB and collaborative eNB for the transmission data packet corresponding to both of the serving eNB and collaborative eNB.
Described next is the design of the PDCCH.
In the design, the PDCCH is transmitted only from the serving eNB to de destination UE so that it can be indicated as a C2 in FIG. 8. In this case, the PDCCH is terminated by the transmission control unit 206 (FIG. 2) in the eNode-B operating as a serving eNB and the reception control unit 304 (FIG. 3) in the UE.
That is, each UE decodes only the PDCCH from the serving eNB corresponding to the UE for the following two reasons.

- The quality of the link from the serving eNB to the UE is better than that from the collaborative eNB to the UE. This ensures the performance for the control channel.
- Transmitting the PDCCH from only one link considerably moderates the load of the control channel.

The downlink control information (DCI) transmitted through the PDCCH can indicate whether or not a collaborative transmission is currently being performed. For the purpose, a new bit is introduced to the DCI. As another expression, a PCI includes a bit identifying whether a transmission packet is a new data packet or a retransmission data packet, that is, whether it is the scenario 1 or the scenario 2, or whether it is the scenario 1 or the scenario 3. It is used to indicate the reception device to perform or not to perform the HARQ processing. The information can be attained by using the new data indication information (FIG. 9( b) described later) already prescribed and existing in the LTE standard.
Furthermore, the DCI includes the following information

- In addition to the modulation and coding scheme (MCS) for the serving eNB in the format 1, format 1A, and format 1C, 5 bits of additional MCS for the collaborative eNB is required.
- Additional MCS (5 bits) and precoding information in the format 2

The DCI for both links including the above-mentioned information is collectively encoded using the CRC specifying the UE. FIG. 9( b) is an example of the DCI using the format 2. In FIG. 9( b), the “RB assigning header” and the “RB assignment” are control information relating to the assignment of a resource block. The “new data indication information” is the information specifying whether a transmission packet is a new data packet or a retransmission data packet. A “redundant version” is the control information about a HARQ. The “MCS-1” and the “MCS-2” are the MCSs respectively for a serving eNB and a collaborative eNB. The precoding information 1 and the precoding information 2 are the precoding information respectively for the serving eNB and the collaborative eNB.
The PDCCH including the DCI is stored together with a user data packet in a subframe regulated in the data format in, for example, the E-UTRA communication system, and then transmitted.
Described next is the design of an X2 control channel.
Am X2 control channel (X2CCH) is delivered with a data packet corresponding to the control channel through the X2 interface indicated by C3 in FIG. 8. Practically, the X2CCH is terminated by the X2 control channel transmission/reception unit 208 in the transmission device illustrated in FIG. 2 of the serving eNB and the collaborative eNB. The X2CCH is realized on the cable link using, for example, optical fiber.
The X2CCH includes the following information.

- Resource assignment header: 1 bit
- Resource block assignment
- Modulation and coding scheme: 5 bits
- Precoding information
- Transmission timing for subframe

Described next is the timing control between the X2CCH and the PDCCH.
The transmission timing control is one of the most important problem for a collaborative transmission. It is determined by the serving eNB, and is instructed by the collaborative eNB through the X2 interface. The transmission timing is determined by considering the latency of the X2 interface.
FIG. 10 is an example of the transmission timing between a control channel and a data channel. In FIG. 10, the data and the corresponding X2CCH are transferred to the collaborative eNB prior to the relating transmission (“PDCCH” and “Data from S-eNB”) from the serving eNB to the UE with the timing t2. The transmission timing t1 of the data from the collaborative eNB (“Data from C-eNB”) is determined by the serving eNB based on the maximum latency T of the X2 interface. By the synchronous network between the serving eNB and the collaborative eNB, the data from the serving eNB and the data from the collaborative eNB are delivered with predetermined timing t1 and t2. It guarantees the reception of both data with the simultaneous timing t3.
Including the above-mentioned timing control, the collaborative transmission for each UE is centrally controlled by the serving eNB. The control includes the scheduling of the UE and data, and the transmission timing control.
A system level simulation has been performed to evaluate the performance of the above-mentioned collaborative HARQ transmission system according to the present embodiment.
In the system level simulation, a system loaded with the transmission device (FIG. 2) and the reception device (FIG. 3) according to the present embodiment is implemented in the cell network formed by 7 clusters. Each cluster is configured by 19 hexagonal cells, and each cell includes 3 sectors. The bore-sight point of the antenna of the sector is directed at the vertex of the hexagon. A surrounding inclusive network structure is adopted to generate an accurate model of the generation of interference from an external cell, the cluster to be observed is arranged at the center, and six copies are symmetrically arranged at the sides of the central cluster. Tables 1 and 2 respectively illustrate the simulation case grouping and condition assumption.

TABLE 1

MINIMAL SET OF UTRA AND EUTRA SIMULATIONS

						CHAN-
SIMULATION	CF	ISD	BW	PLoss	SPEED	NEL
CASE	(GHz)	(m)	(MHz)	(dB)	(km/h)	MODEL

1	2.0	500	10	20	3	TU
2	2.0	500	10	10	30	TU
3	2.0	1732	10	20	3	TU

TABLE 2

CONDITION ASSUMPTION FOR SYSTEM LEVEL SIMULATION

	PARAMETER	VALUE

	NUMBER OF CELLS	19
	NUMBER OF SECTORS PER	3
	CELL
	NUMBER OF UEs PER	20
	SECTOR
	CENTRAL FREQUENCY
	2 GHz
	TRANSMISSION POWER	40 watt (46 dBm)
	LOGARITHMIC SHADOWING	8 dB
	NOISE INDEX
	9 dB
	eNB TRANSMISSION
	0 dBi
	ANTENNA GAIN
	UE RECEPTION ANTENNA	14 dBi
	GAIN
	MAXIMUM CIR	30 dB
	PATH LOSS	128.1 + 37.6log10 (R),
		R in km
	eNB-TO-UE CORRELATION	0.5
	eNB-TO-UE MINIMUM	35 METERS
	DISTANCE
	THERMAL NOISE DENSITY	−174 dBm/Hz
	eNB ANTENNA PATTERN	70° BEAM WIDTH
	UE ANTENNA PATTERN	Omni-Directional
	UE RECEPTION DEVICE	MMSE
	TYPE
	CHANNEL MODEL	TU
	CHANNEL EVALUATION	IDEAL VALUE FROM RS
	MCS OPERATION POINT	10% BLER

First, by evaluating the BLER (block error rate) of the HARQ system according to the present embodiment, a full system level simulation without a collaborative transmission is performed.
FIG. 11, in (a), (b), and (c), illustrates the BLER for each UE as the function of the geometry about the initial transmission and the retransmission # 1, #2, and #3 respectively in the cases 1, 2, and 3.
Table 3 is a summary of the average BLER of the entire UE for the initial transmission and the retransmission # 1, #2, and #3 in the cases 1, 2, and 3. The BLER for the initial transmission for the cases 1 and 3 is about 9%, and that for the case 2 is 78%. However, after the first retransmission, the BLER for the cases 1 and 3 is 0.1% or less, and that for the case 2 is 25%. Thus, when the reception device for performing an appropriate SIC process according to the present embodiment is introduced, it can be expected that the system performance for the collaborative transmission can be improved.

TABLE 3

AVERAGE BLER FOR INITIAL TRANSMISSION,
RETRANSMISSION #1, #2, AND #3 IN CASES 1, 2, AND 3

	TRANSMISSION
	INDEX	CASE
1	CASE 2	CASE 3

	INITIAL	9.11E−02	7.83E−01	8.89E−02
	TRANSMISSION
	RETRANSMISSION	1.21E−03	2.56E−01	1.20E−03
	#1
	RETRANSMISSION	6.54E−05	4.79E−02	6.27E−05
	#2
	RETRANSMISSION	7.69E−06	7.59E−03	0
	#3

Described next is the SINR gain from a reception device for performing a SIC process according to the present embodiment.
As described above, the link gap target Δ is an important parameter having an influence on the collaborative transmission. In the system level simulation, the parameter is used to control the band width between the collaborative eNBs. The motive of performing the system level simulation is to clarify the gain attained by the scenario 2 with respect to the scenario 3. First, the CDF (cumulative density function) of the reception SINR (signal-to-interference and noise power ratio) in the collaborative transmission user for various set values of the link gap target Δ, or 1 dB, 10 dB, and 19 dB is plotted. Thus, the SINR at the CDF point of 0.5 can be illustrated. This enables the merit of the SINR from the scenario 2 to be correctly indicated.
The explanatory legends of the plot graphics are defined as follows.

- Serving link, No-SIC: SNR (signal-to-noise ratio) or SNR gain received by a UE from the serving eNB (or a serving link) when there is no SIC cancelling process of the interference from the collaborative eNB (or the collaborative link). It corresponds to the scenario 3.
- Collab link, No-SIC: SNR or SNR gain received by a UE from the collaborative eNB (or a collaborative link) when there is no SIC cancelling process of the interference from the serving eNB (or the serving link). It corresponds to the scenario 2.
- Serving link, SIC: SNR or SNR gain received by a UE from the serving eNB (or a serving link) when there is a SIC cancelling process of the interference from the collaborative eNB (or the collaborative link). It corresponds to the scenario 3.
- Collab link, SIC: SNR or SNR gain received by a UE from the collaborative eNB (or a collaborative link) when there is a SIC cancelling process of the interference from the serving eNB (or the serving link). It corresponds to the scenario 2.

FIG. 12, in (a), (b), and (c), illustrates the CDF of the SINR received by the UE in each case of the reception from the serving eNB and the collaborative eNB, in each case of with and without the SIC, and in each case of with each set value of Δ, or 1 db, 10 dB, and 19 dB. As the link gap target increases, the link quality between the serving eNB and the UE becomes better. In addition, the SIC process by the canceller unit 303-3 (FIG. 3) operates in a better condition with respect to the link between the collaborative eNB and the UE.
FIG. 13 is a graph indicating the probability of a UE falling into a link gap target Δ and determined as a cell edge user. For the UE, a collaborative transmission is performed. When the link gap target Δ indicates a reasonable value about, for example, 8 dB, the rate of the cell edge user is about 60%, which is sufficiently large value, and requires a collaborative transmission.
FIG. 14 is a graph indicating the SINR of the UE as a function of the value of Δ as a function of the link gap target Δ when the CDF value is 50%. FIG. 15 is a result of calculating the SINR gain of the UE for the two links with and without SIC in addition to the conditions of FIG. 14.
By comparing the link (link 1) from the collaborative eNB to the UE with the link (link 2) from the serving eNB to the UE, some observation results are obtained as follows.

- When a retransmission data packet is delivered from the serving eNB, the SINR gain for the link 1 in the SIC process is about 2 through 2.5 dB.
- When the retransmission data packet is delivered from the collaborative eNB, the SINR gain for the link 2 in the SIC process is about 1.5 through 1.75 dB.
- When the value of Δ increases, the SINR gain of the link 1 becomes larger, and the SINR gain of the link 2 becomes smaller. Thus, it is preferable that the value of Δ is not too small or large. In addition, a small value of Δ causes a too small possibility of a collaborative transmission, and a large value of Δ causes a too large possibility of a collaborative transmission. An appropriate value of Δ is between 8 dB and 10 dB. As a conclusion based on the study of the SINR gain by the SIC, the retransmission data packet is to be delivered constantly from the serving eNB.

The present application has proposed the collaborative transmission system for the HARQ process to again a high SINR gain using the reception device for performing the SIC process.
The present application realizes the SIC process more easily by using the unique behavior of the HARQ constantly indicating a low BLER after the combination of HARQs.
To attain high SINR gain by the SIC process, it is preferable that a retransmission data packet is eventually delivered on the link constantly from the serving eNB to the UE and a new data packet is delivered on the link from the collaborative eNB to the UE during the delivery. However, it is obvious that an inverse process can be used.
Relating to a control channel, three channels, that is, a physical uplink control channel (PUCCH), a physical downlink control channel (PDCCH), and a X2 control channel (X2CCH), are regarded by considering the feasibility and the facility. The design of the control channels can exceedingly reduce the amount of control channel, and considerably shorten the system latency.
The above-mentioned collaborative transmission system can also be applied to an intra-eNode-B in which a collaborative transmission occurs between two transmission points in the same eNode-B.
Described below is an example of a configuration of the hardware of a wireless base station. A wireless base station includes a wireless IF (interface), a processor, memory, a logical circuit, a cable IF, etc. The wireless IF is an interface device for performing wireless communications with a wireless terminal. The processor is a device for processing data, and includes, for example, a CPU (central processing unit), a DSP (digital signal processor), etc. The memory is a device for storing data, and includes, for example, ROM (read only memory), RAM (random access memory), etc. The logical circuit is an electronic circuit for performing a logical operation, and includes, for example, an LSI (large scale integration), an FPGA (field-programming gate array), etc. The cable IF is an interface device for performing cable communications with other wireless base stations connected to a network (what is called a backhaul network) on the network side of a mobile telephone system.
The correspondence between the wireless base station illustrated in FIG. 2 and the hardware is described below for example. The wireless IF corresponds to, for example, the wireless processing unit 205. The processor, the memory, and the logical circuit correspond to, for example, the new data packet transmission unit 201, the retransmission data packet transmission unit 202, the channel assignment unit 203, the modulation unit 204, the transmission control unit 206, and the uplink control channel reception unit 207. The cable IF corresponds to, for example, the X2 control channel transmission/reception unit 208.
Described below is an example of a configuration of the hardware of a wireless terminal. A wireless terminal includes a wireless IF (interface), a processor, memory, a logical circuit, an input IF, an output IF, etc. The wireless IF is an interface device for performing wireless communications with a wireless base station. A processor is a device for processing data, and includes, for example, a CPU (central processing unit), a DSP (digital signal processor), etc. The memory is a device for storing data, and includes, for example, ROM (read only memory), RAM (random access memory), etc. The logical circuit is an electronic circuit for performing a logical operation, and includes, for example, an LSI (large scale integration), an FPGA (field-programming gate array), etc. The input IF is a device for inputting data, and includes, for example, an operation button, a mike, etc. The output IF is a device for outputting data, and includes, for example, a display, a speaker, etc.
The correspondence between the wireless terminal illustrated in FIG. 3 and the hardware is described below for example. The wireless IF corresponds to, for example, the wireless processing unit 301. The processor, the memory, and the logical circuit correspond to, for example, the retransmission data packet reception unit 302, the new data packet reception unit 303, the reception control unit 304, and the uplink control channel transmission unit 305.

Claims

1. A wireless communication system in which a first wireless base station device and a second wireless base station device perform a collaborative transmission process for realizing the process to allow the wireless terminal device not to discard a packet on which decoding has failed but to combine the packet with a retransmitted packet and decode a resultant packet while controlling the retransmission of the packet according to transmission status information returned from the wireless terminal device, comprising:

the first wireless base station including a first processor; and

the second wireless base station including a second processor;

wherein

the first processor is configured to

transmit as a first packet a new data packet or a retransmission data packet corresponding to a retransmit request from the first wireless base station device to the wireless terminal device when the retransmit request is issued to the collaborative transmission process by the wireless terminal device; and to

transfer information about a second packet different from the first packet between the new data packet and the retransmission data packet from the first wireless base station device to the second wireless base station device; and

the second processor is configured to

transmit the second packet according to information transferred from the first base station in synchronization with a transmission process of the first packet from the second wireless base station device to the wireless terminal device when the retransmit request is issued.

2. The wireless communication system according to claim 1, wherein:

each of the first wireless base station device and the second wireless base station device has a memory; and

the first wireless base station device holds information about a packet on which a collaborative transmission process is performed for the wireless terminal device in the memory of the first wireless base station device; and

the second wireless base station device does not hold the information about a packet on which a collaborative transmission process is performed for the wireless terminal device in the memory of the second wireless base station device.

3. The wireless communication system according to claim 1, wherein

the first packet is the retransmission data packet, and the second packet is the new data packet.

4. The wireless communication system according to claim 3, wherein

the first processor is configured to read information about the retransmission data packet from the memory of the first wireless base station device, and to transfer the information to the second wireless base station device.

5. The wireless communication system according to claim 1, wherein

the first processor is configured to communicate between the first wireless base station device and the wireless terminal device control information about a communication to the wireless terminal device by the first wireless base station device and control information about a communication to the wireless terminal device by the second wireless base station device.

6. The wireless communication system according to claim 5, wherein

the first processor is configured to perform:

a transmission of the control information from the first wireless base station device to the wireless terminal device through a physical downlink control channel; and

a reception of the control information from the wireless terminal device to the first wireless base station device through a physical uplink control channel.

7. The wireless communication system according to claim 6, wherein

the physical uplink control channel includes at least individual channel quality indication information for each of the first wireless base station device and the second wireless base station device, and precoding matrix indication information and rank indication information common to the first wireless base station device and the second wireless base station device.

8. The wireless communication system according to claim 6, wherein

the physical downlink control channel includes at least individual modulation and coding scheme information and individual precoding information for each of the first wireless base station device and the second wireless base station device.

9. The wireless communication system according to claim 1, wherein

the control information from the wireless terminal device to the first wireless base station device includes transmission status information indicating a reception result of a packet from the first wireless base station device and a reception result of a packet from the second wireless base station device, respectively.

10. The wireless communication system according to claim 1, wherein

the first processor is configured to transfer the communication control information relating to the second wireless base station device for communication from the first wireless base station device to the wireless terminal device and information relating to a transmission timing of the second packet by the second wireless base station device.

11. The wireless communication system according to claim 1, wherein

the first processor is configured to control centrally at least assignment of the wireless terminal device, assignment of communication resources, and control of transmission timing relating to the collaborative transmission process.

12. A wireless terminal device which performs a communication in a wireless communication system according to claim 1, comprising: a third processor;

wherein

the third processor is configured to

perform a receiving process on the retransmission data packet when the retransmit request is issued; and to

perform, when the third processor successfully performs a receiving process on the retransmission data packet, a successive interference cancellation process on a received signal received by the wireless terminal device through a retransmission data packet on which the receiving process has been successfully performed, and perform the receiving process on the new data packet according to a resultant received signal.

13. The wireless terminal device according to claim 12, wherein

the third processor is configured to determine whether or not the collaborative transmission process is to be performed and to determine the first wireless base station device and the second wireless base station device performing the process when the execution of the collaborative transmission process is determined.

14. The wireless terminal device according to claim 12, wherein

the third processor is configured to make the determination according to information about reception power for a reference signal to be received from each wireless base station device currently in communication.

15. The base station device which performs a communication in the wireless communication system according to claim 1, comprising a processor;

wherein

the processor is configured to

transmit to the wireless terminal device as a first packet the new data packet or a retransmission data packet corresponding to a retransmit request when the base station device operates as the first wireless base station device and when the retransmit request to the collaborative transmission process is issued in the wireless terminal device;

transfer information relating to a second packet different from the first packet between the new data packet and the retransmission data packet to the second wireless base station device when the base station device operates as the first wireless base station device, and when the retransmit request is issued; and to

transmit the second packet according to information transferred from the first wireless base station device to the wireless terminal device in synchronization with the first packet by the first wireless base station device when the base station device operates as the second wireless base station device, and when the retransmit request is issued.

16. A wireless communicating method in which a first wireless base station device and a second wireless base station device perform a collaborative transmission process for realizing the process to allow the wireless terminal device not to discard a packet on which decoding has failed but to combine the packet with a retransmitted packet and decode a resultant packet while controlling the retransmission of the packet according to transmission status information returned from the wireless terminal device, comprising:

transmitting by a first processor in the first wireless base station device as a first packet a new data packet or a retransmission data packet corresponding to a retransmit request from the first wireless base station device to the wireless terminal device when the retransmit request is issued to the collaborative transmission process by the wireless terminal device;

transferring by the first processor information about a second packet different from the first packet between the new data packet and the retransmission data packet from the first wireless base station device to the second wireless base station device; and

transmitting by a second processor in the second wireless base station device the second packet according to information transferred in the packet transferring step in synchronization with a transmission process of the first packet in the first packet transmitting step from the second wireless base station device to the wireless terminal device when the retransmit request is issued.

17. A wireless communication system in which a plurality of wireless base station devices perform, a collaborative transmission process on a wireless terminal device, comprising:

the wireless terminal device including a processor;

wherein

the processor is configured to

receive a control channel only from a first wireless base station device; and to

receive data collaboratively transmitted by at least the first wireless base station device and the second wireless base station device based on the received control channel.

18. The wireless communication system according to claim 17, wherein

the first wireless base station device is a serving base station device for the wireless terminal device.

19. A wireless communication terminal device which receives data from a plurality of wireless base station devices in a collaborative transmission, comprising a processor;

wherein

the processor is configured to

receive data collaboratively transmitted by at least a first wireless base station device and a second wireless base station device based on the received control channel.

20. The wireless terminal device according to claim 10, wherein

21. A wireless communication method in which a plurality of wireless base station devices perform, a collaborative transmission process on a wireless terminal device, comprising:

receiving a control channel only from a first wireless base station device by a processor in the wireless terminal device; and

receiving by the processor data collaboratively transmitted by at least the first wireless base station device and the second wireless base station device based on the received control channel.

22. The wireless communication method according to claim 21, wherein