EP1303997A1

EP1303997A1 - A method for the air interface to support variable data rate

Info

Publication number: EP1303997A1
Application number: EP00934846A
Authority: EP
Inventors: Daoben; Li; Shiu Ming Joseph Ho; Haiyang Gao; Yongsheng; Zhang; Weidong Chen; Yuanhua Deng; Frederic Leroudier
Original assignee: Linkair Communications Inc
Current assignee: Linkair Communications Inc
Priority date: 2000-06-05
Filing date: 2000-06-05
Publication date: 2003-04-23
Also published as: EP1303997A4; CN1452843A; WO2001095644A1; CN1193627C; AU5057900A

Abstract

This invention describes a method for the air interface to support variable data rate, wherein the air interface is divided into a plurality of partitions in either the time domain, the space domain, the frequency domain, or any combination of them, each partition is allocated a set of LS codes as the orthogonal spread code, and at least two partitions are allocated different sets of LS codes with different code lengths. By utilizing different LS code lengths within different partitions, and by the flexible allocation of different partitions to each mobile terminal, to meet the required variable data rate of the present invention. In the time domain, the method is achieved by utilizing different LS code lengths, within different sub frames and by the flexible allocation of sub frames to each mobile terminal.

Description

A Method for the Air Interface to Support Variable Data Rate

Field of the Invention:

The present invention relates to a method for the air interface to support variable data rate in a wireless communications system. More particularly, to a method and means for configuring and allocating physical channel resources of the LS coded wireless system to different mobile terminals with data rate requirements. Such a configuration and allocation method allows the efficient transmission of packet data traffic, such as Internet Protocol (IP) packets, with different quality of service requirements while providing multiplexing gain and increased channel utilization.

Background of the invention:

This invention concerns the support of multiple mobile terminals with different data rate requirements. In CDMA network, multiple access is achieved by the allocation of different multiple access codes to different mobile terminal. Furthermore, in classic CDMA systems, the support of both best effort packet data services (e.g. Internet browsing, file transfer) on the same radio resource as real-time circuit-like applications such as voice poses the problem of system-wide power and interference management. Preventing a high speed data burst from consuming resource reserved for real-time users is an extremely difficult problem to manage. This is mainly due to the impact of interference on classic CDMA systems. This problem is even more arduous when considering a network where multiple cells can interfere with each others (typical of a high density cellular network).

In PCT application PCT/CN00/00028, invented by Li Daoben and entitled "A Method for Spread Spectrum Multiple Access Coding with Interference Free Window," a coding scheme called LS code was disclosed. LS code has the special property that the auto correlation of an LS code is zero, except at the origin, when the two copies of the LS codes are synchronized to with a window of n chips. In addition, LS code has another special property that the cross correlation between two different LS codes are zero when the two codes are synchronized to with a window of n chips. Thus, there is a Zero Correlation Window of size n chips, which is also referred to as Interference Free Window. This means when LS code is used as a spreading code for the wireless air interface, the Inter- Symbol-Interference (ISI) is negligible as long as the time offset between two paths of the same channel stay within the Zero Correlation Window (in other words, as long as the delay spread of each transmission is bound by the IFW) . Similarly, the Multiple Access Interference (MAI) between two channels using two different LS codes is negligible as long as the time offset between the two channels and delay spread of each channel stay within the Zero Correlation Window.

Hereinafter the wireless systems, the signals, the air interface which utilize LS codes as the orthogonal spread codes will be respectively referred to as LS coded wireless systems, LS coded signals, and LS coded air interface.

Hereinafter the wireless systems, the signals, the air interface which utilize orthogonal spread codes with an IFW property will be respectively referred to as IFW wireless systems, IFW signals, and IFW air interface.

In an LS coded wireless network, a method is needed that allows the allocation of radio interface resources to different mobile terminals such that the data rate requirements of different mobile terminals can be satisfied. The radio interface resource of an LS coded wireless system can be allocated in multiples of sub frames. The problem of resource allocation is thus a problem of how to configure these sub frames and how to allocate the sub frames to the mobile terminals in an LS coded wireless system, such that the data rate requirement of the mobile terminals can be satisfied. At the same time, this method should make use of the special intermittent transmission characteristic of packet data traffic such that multiplexing gain can be realized and thus increase radio channel utilization and capacity.

Summary of the invention:

It is the object of the present invention to provide a method for allocating channel resources to its supported mobile terminals for the transmission of data packets.

It is another object of the invention to provide a method for supporting variable data rate by using different LS code length and different sub frame and LS code allocations.

It is a another object of this invention to provide a method for support intermittent packet data traffic in a LS coded system such that multiplexing gain can be achieved..

It is a further object of this invention to provide a method for supporting both real time circuit-based traffic, such as voice calls, and best effort packet-based traffic in an LS coded carrier simultaneously.

The present invention provides a channel resource configuration and allocation method that allows:

1. The support of high and variable data rate traffic using LS coded wireless

system.

2. The support of both real-time circuit-based and best-effort packet-based

traffic within the same LS coded carrier.

In the present invention, the air interface is divided into a plurality of partitions in either the time domain, the space domain, the frequency domain, or any combination of them, each partition is allocated a set of LS codes as the orthogonal spread code, and at least two partitions are allocated different sets of LS codes with different code lengths. By utilizing different LS code lengths within different partitions, and by the flexible allocation of different partitions to each mobile terminal, to meet the required variable data rate of the present invention.

In the time domain, the method is achieved by utilizing different LS code lengths, within different sub frames and by the flexible allocation of sub frames to each mobile terminal.

Brief description of the attached drawings:

The accompanying drawings which are incorporated in and constitute a part of this specification, illustrate particular embodiments of the invention, and together with the description, serve to explain, but not restrict, the principles of the invention.

Figure 1 shows the LAS-2000 frame structure as a preferred embodiment of the present invention, and demonstrates how the frame is further divided into sub frames and time slots. LAS-2000 is an operation mode of the LS coded wireless systems, that is compatible with the IS-2000 standard.

Figure 2 shows a (17, 136, 2559) LA code, which contains LA 17 pulses, and 2559 chips in length. This is intended as an illustrative example only since many other codes of the same family (LA codes) can be used.

Figure 3 shows a time-slot within the LS-2000 frame structure. It demonstrates the position of the LS code in relation to the LA pulse.

Figure 4 shows the LS code generation tree and demonstrates the relationship between LS code length and the number of available LS codes at each level of the

- A - tree.

Figure 5 shows the time offset overlap LA code and demonstrates how it is generated from an original LA code.

Figure 6 shows the time slots and how multiple LS-codes are arranged within the time slot.

Figure 7 shows an example of a time-slot, which carries eight 16 chips (8 chips C code and 8 chips S code) long LS codes.

Figure 8 shows the LS code generation tree and explain the concept of sub tree and least common ancestor.

Figure 9 demonstrates an example of supporting variable data rate within the LAS-2000 frame structure using LS codes of different length.

Figure 10 shows a preferred embodiment of the LAS-2000 data frame structure. LAS-2000 is one example of how this invention can be used. Other examples exist based on the use of other wireless standards such as W-CDMA and TD-SCDMA.

Figure 11 demonstrates an example of supporting different data rates in different sub frames of the LAS-2000 frame structure.

Figure 12 shows an example of supporting both circuit-based traffic and packet- based traffic within the same LAS-2000 carrier.

Figure 13 shows the state diagram of an enhanced 16QAM modulation. Detailed Description of the Invention:

The invention relies on the notion that the transmission from a fixed or mobile transmitter of an LS coded wireless system can be separated by the receiver from the transmission from other transmitters by a number of different means. This separation can for example be realized:

- in the time domain by limiting some transmitters to transmit within certain

time slots;

- in the space domain by using adaptive antenna arrays to distinguish areas of

different delay spreads;

- in the frequency domain by allocating different RP carriers to transmitters

with different delay properties;

by allocating different code divided channels to mobiles belonging to

different delay spread categories.

The method according to the present invention is composed of the following steps:

1. Divide the air interface into a number of partitions in either the time, the space,

and the frequency domains. Each partition supports a different IFW. For

example, in the time domain, this can be achieved by allocating different IFW

to different SF. In the space domain, this can be achieved by allocating

different IFW to different directional antenna. In the frequency domain, this

can be achieved by allocating different IFW to different carrier.

2. Each partition is allocated a set of LS codes as the orthogonal spread code, and

at least two partitions are allocated different sets of LS codes with different code lengths. By utilizing different LS code lengths within different partitions,

and by the flexible allocation of different partitions to each mobile terminal, to

meet the required variable data rate of the present invention.

In PCT application PCT/CN98/00151, invented by Li Daoben and entitled "A Spread Spectrum Multiple Access Coding Method," a coding scheme called Large Area (LA) code was disclosed. The present invention utilizes the LA code as described in PCT/CN98/00151 such that each SF carries a (136, 17, 2259) LA code. This LA code consists of 17 pulses with a period of 2559 chips. Each LA pulse corresponds to a time-slot within the SF. The first time-slot may be used as the Pilot and the remaining 16 time-slots can be used for carrying user data or signaling information. Figure 2 shows a LA code with 17 pulses.

As a preferred embodiment, The said partitions are sub-frames in the time domain, the number of sub-frame in each frame can be determined by the periodicity of selected LA codes. The said sub-frame is divided into a plurality of time slots, in which the number of the said time slots can be determined by the number of pulses of the said LA codes, and the said time slot length varies with the variation of the pulse interval of the said LA codes. The said LS codes fill the said time slot by modulation. Herein after we call such kind of division described above as LAS-CDMA, so that this allocating scheme could be referred to more easily.

LAS-CDMA is a wireless air interface allocating scheme that combines the benefits of CDMA and TDMA, two of the most popular wireless multiple access technologies in use today. LAS-CDMA employs a new coding technique that reduces the interference among users, and thus resulting in higher user capacity and better quality. At the same time, LAS-CDMA employs a transmission frame structure similar to those used in TDMA based systems. This allows efficient allocation of bandwidth resources and QoS management that is desirable for supporting high-speed and variable rate data services.

While according to LAS-CDMA, each frame consists of N (e.g. 24576 in the case of LAS-2000) chips and is divided into M (e.g. 10 for LAS-2000) Sub-Frames (SFs). The first Sub-Frame, SFO, consists of NO (e.g. 1545 chips for LAS-2000) and it can be used for carrying control channels.

Figure 1 shows the LAS-2000 frame structure as a preferred embodiment of the present invention, and demonstrates how the frame is further divided into sub frames and time slots. LAS-2000 is an operation mode of LAS-CDMA and the LS coded wireless systems, that is compatible with the IS-2000 standard. Referred to Figure 1, the downlink channel of a LAS-2000 system consists of consecutive 20 ms frames.

As an example, in LAS-2000, SFO can be used as the Broadcast and Synchronization Channel (BSCH) in the downlink and as the Access Channel (ACH) in the uplink. The remaining 9 SFs, SF1 to SF9, carry the traffic and signaling channels. Each of these SFs consists of 2559 chips and is further divided into 17 time- slots.

Referring back to the LS codes, it is demonstrated in Figure 1 that the LS code is made up of two components, the S and C components, of equal length. For the preferred embodiment as shown in Figure 1, both the S and C components are 64 chips long. The S and C components and are separated by a 4 chips gap. There is a 4 chips gap before the C component and there is a variable length gap after the S component. Each LS code carries one modulation symbol. When 16QAM modulation scheme is used, each LS code carries 4 information bits. Assuming 128 chips LS codes are used and TSO is used for other purposes, the number of information bits carried by a SF per LS code is 16x4=64 bits. Thus, the number of information bits carried by the 20 ms frame per 128-chip LS code is 64x9 = 576 bits. If each user is assigned an LS code in all the SFs of the 20 ms frame, then the per user data rate is 28.8 kbps. The combined LA and LS code is called the LAS code. Figure 3 shows the relation between the LA pulse and the LS code within a given time-slot.

Packet data traffic is differentiated from circuit based traffic, such as voice, in a number of aspects. In general, voice traffic generates a constant rate data stream, while the data rate and arrival pattern of packet data traffic is highly variably. For example, a packet data session that supports real-time traffic, such as voice conversation, requires a constant bit rate connection, with low latency and delay variation. Whereas, packet data session that supports file download requires relatively high data rate with low bit error rate, while latency is not a major concern. In addition, packet data arrivals follow an intermittent on and off pattern. During a packet data session, burst of data arrival is followed by a silent period. This alternating on and off cycle continues until the data session terminates. This data arrival model has a significant impart on the design of channel allocation protocol. The channel allocation must be flexible and dynamic enough to support a wide range of data rates and, at the same time, allows the dynamic reallocation of resource such that multiplexing gain can be achieved.

This invention provides a method for supporting packet data traffic in a LS coded wireless system, such that variable data rate, up to the LS coded system limit, can be allocated to each users. At the same time, the method allows the sharing of codes and partitions (sub frames in the time domain) among a number of users. This results in multiplexing gain and provides in higher throughput and capacity.

In order to support different data rates for different users, one approach is to support LS codes of different code length within the same SF. Another approach is to support the flexible allocation of the SF and LS codes on a per user basis. This invention provides a method that supports both approaches.

Figure 4 shows the LS code generation tree. At the root of the tree, there is a pair of 2-chip LS codes, called the originator. This originator is used for generating longer LS codes base on the LS code generation method as described in PCT application PCT/CNOO/00028. In general, for each level down the tree, the length of the LS code as well as the number of LS codes double. Figure 4 clearly demonstrates the number of LS codes and their lengths at each level of the tree. Note that in the current implementation, the lowest level of the tree contains 128 LS code of length 128 chips. Since each LS code is used to spread a modulation symbol, the shorter the LS code, the larger the number of LS codes that can be sent within a fixed size time- slot, and thus the higher the data rate.

For the LAS code as described in Figure 3, each LS code is positioned next to an LA pulse. This LA pulse and the LS code constitute a time-slot within the frame structure. With shorter length LS codes, multiple LS codes can be placed next to the LA pulse. Figure 5 shows the time-offset overlapped version of the LA code where M-1 shifted version of an original LA codes are generated by shifting the original LA code by a distance equal to i times the length of the S or C codes of the LS code to be supported. Where i = 1 ... M-1 is the index of the shifted LA code. Figure 5 shows how this time-offset overlap version of the LA code is generated.

Given the time-offset overlap version of the LA code, each time-slot now contains a pulse from the original LA code and M-1 shifted versions of the pulse. Thus M copies of the same LS coded can be placed in the time-slot as demonstrated in Figure 6. Thus the capacity of the LS code is increased by M time compared to the case when the time-slot carries only one LS code. The number of LS codes per time- slot, M, depends on the length of the LS code. In the preferred embodiment, given that the maximum length LS code that can be carried by a time-slot is 128 chips, M is equal to 128 divided by the length of the LS code. For LS code of length 128, one LS code (one C and one S component) can be carried by the time-slot. For LS code of length 32, four LS codes (four C and four S components) can be carried by the time- slot, and so on. Figure 7 shows an example time-slot that carries eight 8-chip long LS codes.

As described earlier, the number of LS codes is reduced as the length of the LS code decreases. According to the design of LS code, when one or both LS codes at the vertex of the free are used, the sub-free under the vertex is not used. For example, in Figure 8, since vertex X is the least common ancestor of sub-tree A. When one or both LS codes at vertex X are used, the remaining LS codes below vertex X within the same sub-free (sub-free A) cannot be used. However, the other sub-frees, such as subtree B, are not affected and can still be used. In general, any LS code in the tree can be used as long as none of its ancestors has been used. This means that any LS code can be selected from the free and be used for spreading modulated data traffic as long as none of the ancestors of the selected LS code has been used.

The above-described property of the LS code can be used to support high and variable data rate under an LS coded system. Each SF can support multiple LS codes with different lengths. Different SFs with the frame structure can be configured differently and support a different set of LS codes. This enables high flexibility in radio resource and data rate allocation among multiple mobile terminals. This will be demonstrated in the following examples.

Figure 9 shows one example configuration of the LAS-2000 channels. In this configuration, the carrier is set up to support 64 128-chip LS code channel, 16 64-chip LS code channels, and 8 32-chip LS code channels. In this example, all SFs are configured in the same way such that each LS code forms a continuous channel. When 16 QAM modulation scheme is used, each LS code of length 128, 64 and 32 chips form channels with throughput of 259.2Kbps, 518.4 Kbps, and 1036.8 Kbps, respectively. To achieve lower data rate, each user can be assigned smaller number of SFs within the 20 ms frame. The throughput per SF for 128, 64, and 32 chip LS codes are 28.8, 57.6, and 115.2 Kbps, respectively. On the other hand, higher data rates can be achieved by allocating multiple LS codes to the same mobile. In general, the allocation of channel resource in a wireless system can be highly flexible. For example, a mobile terminal can be allocated LS codes 1 and 2 in SF 4, 5, and 6, only. In addition, the allocation can be for one particular 20 ms frame or for an extended duration. Also, multiple users may share the same allocation. For example, a data pipe can be carried by LS codes 1 and 2 in SFs 1 to 9. A number of mobile terminals may be assigned to this same data pipe. Multiple accesses with this data pipe can be achieved by explicit or piggybacked signaling.

Different symbol rates can be supported by using LS codes of different lengths. One remote unit can be allocated one or more LS code and for each allocated LS code, one or more time slots or sub-frames. Table 1 shows the modulation symbol rates and data rates when 16 QAM modulation is used per time slot per LS code and per sub- frame per LS code.

Table 1 Symbol Rates and Data Rates when 16QAM Modulation is used

For modulation, the present invention introduces an enhanced 16QAM, whose state diagram is illustrated in FIG. 13. Other modulations such as QPSK can also be used.

In another example, the SFs can be grouped into larger units call Data Frames (DF). Each DF can be used to carry one higher layer data block, such as a radio link layer frame. Figure 11 shows an example where three SFs are combined together to form a DF. Figure 11 shows only one of the possible configurations. The number of SFs per DF can range from 1 to the maximum number of SFs per frame. The configuration for each DF can be different. Figure 12 shows an example wherein DF1, which consists of SF1, SF2, and SF3, supports 32 128-chip LS code channels, 16 64- chip LS code channels, and 16 32-chip LS code channels. DF2, which consists of SF4, SF5, and SF6, supports 32 128-chip LS code channels, and 48 64-chip LS code channels. DF3, which consists of SF7, SF8, and SF9, supports 128 128-chip LS code channels. A mobile terminal can be allocated a given number of LS codes within a selected set of DFs. In addition, a number of mobile terminals may share a given allocation.

Figure 12 shows another example where circuit-based traffic, such as voice calls, and packet data traffic are supported simultaneous within the LAS-2000 frame structure. It is demonstrated in Figure 12 that the first 64 128-chip LS codes are assigned for circuit-based service. Each of these 64 LS codes is divided in the time domain into two channels. The first channel is made up of SF1 to SF4 and the second channel is made up of SF6 to SF9. SF5 is used to carry the dedicated control channels. According to this configuration, the LAS-2000 carrier supports 128 simultaneous voice calls. In addition, the LAS-2000 carrier as shown in Figure 12 also supports 4 16-chip LS codes and 8 32-chip LS codes. These 12 LS codes are divided in the time domain into three data frame carriers that occupy SF1-SF3, SF4-SF6, and SF7-SF9, respectively. This LS codes can be used to carry link layer frames for a number of packet data sessions. A mobile terminal can be dynamically assigned a number of data frame carriers within selected LS codes when link layer frames are available for transmission. This allows the support of variable data rates. As mobile terminals are assigned data frame carriers only when needed, this allows the network to realize multiplexing gain due to the intermittence behavior of packet data traffic. This example demonstrates how circuit-based service, such as voice, and packet data service can be supported within the same carrier. The example given in Figure 12 represents one of the preferred embodiments. Other configurations that support various mixtures of voice and data fraffic and that utilize different LS code combinations are possible and are not shown in this example.

The interference reduction properties of LS coded wireless system resolve the power management challenge caused by the co-existence of time sensitive fraffic such as voice with best effort bursty fraffic of packet data services such as wireless Internet browsing or file or e-mail transfer.

It will be apparent to those skilled in the art that various modifications can be made to the present invention without departing from the scope and spirit of the present invention. It is intended that the present invention covers modifications and variations of the systems and methods provided they fall within the scope of the claims and their equivalents. Further, it is intended that the present invention cover present and new applications of the system and methods of the present invention.

Claims

1. A method for the air interface to support variable data rate, wherein:

The air interface is divided into a plurality of partitions in either the time domain, the space domain, the frequency domain, or any combination of them, each partition is allocated a set of LS codes as the orthogonal spread code, and at least two partitions are allocated different sets of LS codes with different code lengths.

2. A method of claim 1, wherein:

The said partitions are sub-frames in the time domain, the number of sub- frame in each frame can be determined by the periodicity of selected LA codes;

The said sub-frame is divided into a plurality of time slots, in which the number of the said time slots can be determined by the number of pulses of the said LA codes, and the said time slot length varies with the variation of the pulse interval of the said LA codes;

The said LS codes fill the said time slot by modulation.

3. A method of claim 2, wherein the said LS codes fill the said time slot in form of an LS frame, which has a certain length and further includes C component for C code and S component for S code, while the C code and the S code of the LS code are filled in the said C component and S component separately.

4. A method of claim 3, wherein when the length of the said allocated LS codes is shorter than length of the said C component plus the said S component, multiple LS codes can be used to fill the said C component and the said S component of the said LS frame.

5. A method of claim 1, wherein the said air interface is divided into a plurality of sub-frames in the time domain.

6. A method of claim 1, 2, 3, 4, or 5, wherein the said plurality of partitions is partitioned into a plurality of groups, and the LS codes utilized in each group has a certain code length.

7. A method of claim 1, 2, 3, 4, or 5, wherein the partitions utilizing LS codes of short length are used for low-speed data rate fraffic, and the partitions utilizing LS codes of long length are used for high-speed data rate fraffic.

8. A method of claim 1, 2, 3, 4, or 5, wherein the partitions utilizing LS codes of short length are used for low-speed data rate fraffic, the partitions utilizing LS codes of long length are used for high-speed data rate fraffic, and shortest length of the LS coed utilized is 2 chips while the longest length of the LS code is 128 chips.

9. A method of claim 1, 2, 3, 4, or 5, wherein the said modulation with the selected orthogonal spread spectrum codes is an enhanced 16QAM modulation.