WO2009069047A1

WO2009069047A1 - Link-based transmission queue structure for wireless networks

Info

Publication number: WO2009069047A1
Application number: PCT/IB2008/054882
Authority: WO
Inventors: Hans J. Reumerman; Yunpeng Zang; Lothar Stibor; Bernard Walke; Hui Chen
Original assignee: Koninklijke Philips Electronics N.V.; Philips Intellectual Property & Standards Gmbh
Priority date: 2007-11-26
Filing date: 2008-11-20
Publication date: 2009-06-04
Also published as: TW200943833A

Abstract

The invention relates to an apparatus, a method and a computer program product for controlling transmission of data to at least one receiving unit, wherein at least two queues are provided per channel for storing data packets of a predetermined access category of respective transmission links to the at least one receiving unit. The at least two queues are allocated to different ones of the respective transmission links, and data packets to be transmitted via the respective transmission links are stored in the at least two queues based on the allocation.

Description

LINK-BASED TRANSMISSION QUEUE STRUCTURE FOR WIRELESS NETWORKS

FIELD OF THE INVENTION

The present invention generally relates to an apparatus, a control method, a transmitter device and a computer program product for transmitting data to a wireless mobile unit.

BACKGROUND OF THE INVENTION

In high-mobility wireless networks such as for example wireless local area network (WLAN) based vehicular communication systems, e.g. the Wireless Access in Vehicular Environments (WAVE) system, quality of service (QoS) may not be supported properly for unicast communications among vehicles and/or between vehicles and roadside units (RSUs) provided along roads, highways, or the like. A reason is that the structure of a transmission queue used in the transmission protocol, e.g. current WAVE Medium Access Control (MAC) protocol, cannot efficiently deal with link changes caused by the high mobility of vehicles. QoS support is a crucial design aspect for vehicular communication systems, e.g. vehicular ad-hoc Networks (VANET) such as the WAVE system, in which two-fold QoS supports may be defined:

First, safety applications should always have a higher priority than non-safety applications, due to their requirements on high reliability and low latency communications for disseminating safety messages. In the WAVE system, for example, this is realized through a synchronized channel coordination scheme on the basis of multiple frequency channels. Second, two classes of frequency channels may be defined. At least one control channel (CCH) which may be exclusively used by high priority safety applications and system control messages, and at least one service channel (SCH) which may be allocated to non-safety applications. All WAVE devices may be synchronized to a global time basis, e.g. the Universally Coordinated Time (UTC), and may have to monitor the CCH during common CCH time intervals. Whenever WAVE devices have non-safety applications to perform, they can switch to an SCH during a common SCH interval which may alternatively be provided with CCH intervals. Fig. 1 shows an exemplary schematic diagram indicating a synchronization pattern 4 with a synchronization interval 3 (e.g. Sync Interval) which is formed by an alternation of a CCH interval 1 and an SCH interval 2. In this way, high priority safety applications can be physically separated from non-safety applications. When multiple applications share the same channel, different channel access priorities may be allocated to different applications. As an example of such a differentiation, the current WAVE MAC protocol follows an enhanced distributed channel access (EDCA) mechanism, such as the EDCA mechanism specified in the IEEE 802.1 Ie specification for providing multiple channel access priorities. A total of eight user priority levels can be mapped to four access categories (ACs), where each access category corresponds to one transmission queue.

Fig. 2 shows a schematic block diagram of a transmit control structure or arrangement, where transmission data is processed in a link layer control unit 41 and then supplied to a channel router 42 which is adapted to separate the transmission data in accordance with the desired transmission channel, e.g., CCH for WAVE service advertisement (WSA) data or SCH for WSA or Internet protocol (IP) data, and to supply the data to a CCH processing unit 43 and to an SCH processing unit 44, respectively. The CCH and SCH processing units 43, 44 supply their channel data to respective buffer units 45 which comprise one transmission queue per access category (AC). According to the EDCA rules, each transmission queue provided in the respective buffer units 45 has its own backoff entity 46 that counts the probability of getting access to a channel. All backoff entities 46 work independently from each other. Different parameter settings of different backoff entities statistically differentiate the probabilities of queues trying to get access to the channel. These parameter settings may comprise at least one of an arbitration inter- frame space (AIFS) setting, a contention window (CW) setting, and a transmission opportunity (TXOP) setting, which may all depend on the access category allocated to the respective transmission queue, As shown in Fig. 2, internal contention resolution units 47 employ an internal contention resolution scheme for resolving possible collisions, when multiple transmission queues residing in one physical device try to get access to the channel at the same time. The queue with higher priority (i.e. access category) can win the opportunity for transmission against the one with lower priority. The multi- transmission queue and backoff entity structure and the internal contention resolution mechanism allow traffics with higher priority to have higher probability of getting the channel resource than the ones with lower priorities. A channel selector 48 selects the desired channel based on the outputs of the internal contention resolution unit 47 and initiates a transmission attempt 49. As an additional option, management data may be supplied to the channel selector 48 from a management queue 40.

In the specific example of the current WAVE MAC protocol, as shown in Fig. 2, the transmission queues are associated with four different access categories (ACs), e.g., AC=O...3, to which different channel access priorities can be allocated. In other words, packets with the same AC will be put into the same queue regardless of the receiver of each packet, as they all have the same priority.

Because the channel accesses to CCHs and SCHs are alternatively coordinated by the CCH interval 1 and SCH interval 2, which are temporally orthogonal, the WAVE devices have two buffer units 45 with two sets of transmission queues (AC=O...3) for CCHs and SCHs, respectively, as shown in Fig. 2. Hence, one transmission queue per channel type (e.g. CCH or SCH) is allocated to every access category.

However, the above structure of transmission queues has drawbacks in high mobility environments. The reason is that packets having the same priority will be put into their single allocated queue in a first-in- first-out (FIFO) manner, even if they are addressing different receivers. A link failure between transmitter and receiver, which may have been caused by a movement of the vehicle of the receiver, may lead to a failure of the respective unicast packet transmission, which in turn leads to the result that no acknowledgement can be received by the transmitter. According to current access protocols, such as the WAVE MAC protocol, a packet failed in getting an acknowledgement has to be retransmitted multiple times before reaching a retry limit. Once the retry limit has been reached the packet can be discarded from the respective queue of its access category. However, the retransmission does not help, when the link failure is caused by the mobility, i.e., the receiver is leaving the transmitter and is no longer within the transmission range of the transmitter. The channel resource is wasted and all following packets in the same queue are blocked, even if they are addressing other receivers which are still within the communication range.

The following examples of Fig. 3 illustrate the problem in more detail. On board units (OBUs) on vehicles 50, 51 try to access the Internet through a road ride unit (RSU) 52. A unicast link between each OBU and the RSU 52 is established for supporting this service.

As shown in Fig. 3, the RSU 52 is the providing an Internet access service where all packets from the RSU 52 to users of this service at the vehicles 50, 51 have the same AC, e.g. AC=2, and are thus stored in the same transmission queue of the buffer unit 45 provided at the RSU 52.

On CCH, the RSU 52 has to broadcast a WSA packet periodically in each CCH interval. When a first user at the first vehicle 51 reaches the transmission range of the RSU 52 and successfully receives the WSA packet, a specific service request will be sent by the OBU of the first user to the RSU 52 for initiating a unicast data transmission from the RSU 52 to the OBU on the first vehicle 51 in the following SCH intervals. According to the EDCA QoS scheme, packets from the RSU 52 to the OBU on the first vehicle 51 of this service are queued into the transmission queue of AC=2, as shown in the upper diagram of Fig. 3.

After a short period, due to the mobility of the second vehicle 50, the OBU on the second vehicle 50 reaches the transmission range of the RSU 52 and carries out the same procedures to initiate the same service from the RSU 52. The packets from the RSU 52 to the OBU on the second vehicle 50 have the same AC as the ones to the OBU on the first vehicle 51 , as they are of the same service.

Problems arise when the OBU on the first vehicle 51 leaves the transmission range of the RSU 52. A packet transmitted from the RSU 52 to the OBU on the first vehicle 51 cannot be acknowledged anymore, but will not be discarded until the retry limit is reached, according to the current WAVE MAC protocol. As all packets are waiting in the same FIFO queue of AC=2 at the RSU 52, the packets to the OBU on the second vehicle 50 have no chance to access the channel, before all obsolete packets to the OBU on the first vehicle 51 are discarded, as shown in the lower diagram of Fig. 3.

The retransmission of obsolete packets thus consumes bandwidth and degrades network performance. The problem becomes even more serious in high density scenarios, e.g., in traffic jams, where more service links are initiated.

SUMMARY OF THE INVENTION

It is an object of the present invention to improve efficiency of data transmission when dealing with link changes caused by high mobility. This object is achieved by an apparatus according to claim 1 and a method according to claim 9.

Accordingly, a link-based transmission queue structure and control scheme are provided, which could be implemented separately or as extensions to the current transmission protocols, such as the WAVE MAC protocol. More than one unique transmission queue is allocated to a specific access category. The unique transmission queues may be allocated to different transmission links, wherein at least one transmission link is allocated to one unique transmission queue. Thereby, efficiency is improved in cases where many transmission links of the same category are handled by one transmitter apparatus, e.g., RSU. Namely, active links are allocated to available transmission queues of the buffer unit. If all available transmission queues have been allocated and new transmission links have to be established, it is started to share transmission queues by two or more links. In such sub-optimal cases, still an increase in efficiency can be obtained.

Moreover, the proposed solution enables QoS support with an accent on dealing with users with low offered load and strict delay requirements in wireless networks, such as multiple input multiple output (MIMO) networks, while keeping at the same time high throughput for other users.

In a specific example, each unique transmission queue is allocated to one transmission link. If all available transmission queues per access category have been used up, no further transmission links can be established and service requests are rejected. In this example, each unique transmission queue is thus never shared with other links. In this way queues can be controlled independently according to the status of each link.

A transmission queue can be created when a new link is established for a unicast transmission. Packets of this link are buffered in its private queue. A queue of a unicast link can be identified by the receiver and the access category of the packets.

According to the synchronized scheme described initially, all queues for the SCH at the transmitter could be suspended at the starting point of a CCH Interval and resumed again when an SCH interval begins and a request of service from the user is received again. In other words, if a service user has left a provider's transmission range during a CCH Interval, the service provider will notice it in time by missing the service request from this user and can thus avoid transmitting obsolete packets to it.

Similarly, when a transmission of a data packet fails continuously for a multiple of times, it can be assumed that the service user has left the transmission range. The queue and the corresponding backoff entity can then be suspended. They could be resumed again, when a service request is received from the service user within a predefined period of time, e.g. a predetermined number of synchronization intervals.

Furthermore, a transmission queue can be suspended, deleted, dropped or destroyed and optionally all packets can be discarded if a service request has not been received from the corresponding service user for a predefined time period, e.g. a predetermined number of consecutive synchronization intervals.

As there can be more than one queue per channel of the same access category, contention resolution could be extended to support two levels. In addition to the contention resolution among queues of different access categories, another level of contention resolution may be introduced for resolving the contention among multiple queues with the same access category. When more than one transmission queue are claiming for channel access at the same time, a mechanism of arbitration can be applied for determining the winner of the contention. For example, depending on the creation time of the queues, the queue which has been created or established earlier wins the contention.

The present invention can be implemented as a computer program product which comprises code means for performing the steps of the above method when run on a computer or computing device, module, or chip provided at the transmitter apparatus.

Further advantageous modifications are defined in the dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described on the basis of an embodiment with reference to the accompanying drawings, in which:

Fig. 1 shows a schematic diagram of a synchronization pattern which can be used in the embodiment;

Fig. 2 shows a schematic block diagram of a conventional transmission control structure for prioritized channel access;

Fig. 3 shows diagrams of a scenario which leads to an obsolete packet problem in the conventional control structure; Fig. 4 shows a schematic diagram of a link based transmission control structure according to the embodiment; and

Fig. 5 shows an example of a software-based implementation of the embodiment.

DETAILED DESCRIPTION OF AN EMBODIMENT

In the following, an exemplary embodiment will be described based on WAVE system as standardized by the IEEE specifications P1609 and 802.1 Ip, as described for example in IEEE Standard for Information technology -Part 11 : Wireless Medium Access Control (MAC) and Physical Layer (PHY) specifications: Amendment: Wireless Access in Vehicular Environments (WAVE), IEEE Draft Amendment P802.1 lp/Dl.O, Feb. 2006 and in IEEE P 1609.4, Wireless Access in Vehicular Environments (WAVE) Multi-Channel Operation, Draft Standard, D06, Nov. 2005. The proposed embodiment increases efficiency of unicast links by using a new structure of the MAC transmission queue, which is based on the state of links to overcome the problem caused by the high mobility of receiving devices (provided e.g. on vehicles) and to guarantee a required QoS of unicast communications in vehicular or other high-mobility environments.

Without losing generality, in the following a set of queues for a single channel, e.g. the SCH, is described, while the invention applies to both sets of queues for CCH and SCH, or to any number of channels of any type.

Fig. 4 shows a schematic block diagram of a link based transmission control structure according to the embodiment. A new queue structure is shown for SCH as an exemplary channel, where each transmission queue has two attributes: AC and link ID (LID). AC may be the same as initially described in connection with the current WAVE MAC protocol, while LID identifies to which transmission link this queue is associated or allocated. The same queue structure can be used as well for the CCH access structure.

According to Fig. 4, channel data is processed in a link layer control unit 26 and then supplied to a MAC based transmission control unit 22 which is adapted to separate the transmission data of the concerned channel, e.g. SCH. The channel data is then supplied by the transmission control unit 22 to the respective buffer unit 21 which comprises several transmission queues per access category (AC). An LID which identifies an associated transmission link is allocated to each transmission queue.

Similar to Fig. 2, each transmission queue provided in the buffer unit 21 has its own backoff entity that counts the probability of getting access to a channel. All backoff entities work independently from each other. Different parameter settings of different backoff entities statistically differentiate the probabilities of queues trying to get access to the channel. These parameter settings may comprise at least one of an arbitration inter- frame space (AIFS) setting, a contention window (CW) setting, and a transmission opportunity (TXOP) setting, which may all depend on the access category allocated to the respective transmission queue,

Again similar to Fig. 2, a first internal contention resolution unit 24 employs an internal contention resolution scheme for resolving possible collisions, when multiple transmission queues residing in one physical device try to get access to the channel at the same time. The queue with higher priority (i.e. access category) can win the opportunity for transmission against the one with lower priority. The multi-transmission queue and backoff entity structure and the internal contention resolution mechanism allow traffics with higher priority to have higher probability of getting the channel resource than the ones with lower priorities. In addition to the above first internal contention resolution unit 24 among queues of different access categories, a second internal contention resolution unit 23 may be introduced for resolving the contention among multiple queues with the same access category. When more than one transmission queue are claiming for channel access at the same time, a mechanism of arbitration can be applied for determining the winner of the contention. For example, depending on the creation time of the queues, the queue which has been created or established earlier wins the contention. Of course, any other arbitration mechanism could be used in the second internal contention resolution unit 23.

A channel selector functionality (not shown) of the transmission control unit 22 may select the desired channel based on the output of the second internal contention resolution unit 23 and initiates a transmission attempt towards a physical layer control unit 25.

A transmission queue of the buffer unit 21 can be created by the transmission control unit 22 when a new link is established for a unicast transmission. Packets of this link are buffered in its private queue. A queue of a unicast link can be identified by the receiver or LID and the access category (AC) of the packets. According to the synchronized scheme described initially, all queues for the SCH at the transmitter could be suspended at the starting point of a CCH interval and resumed again when an SCH interval begins and a request of service from the user is received again.

Similarly, when a transmission of a data packet fails continuously for a multiple of times, the transmission control unit 22 assumes that the respective service user has left the transmission range. The queue and the corresponding backoff entity can then be suspended. They can be resumed again by the transmission control unit 22, when a service request is received from the service user within a predefined period of time, e.g. a predetermined number of synchronization intervals. Furthermore, the transmission control unit 22 can be adapted to suspend, delete, drop or destroy a transmission queue and optionally to discard all packets, if a service request has not been received from the corresponding service user for a predefined time period, e.g. a predetermined number of consecutive synchronization intervals. Fig. 5 shows a schematic block diagram of a software-based implementation of the proposed link-based transmission control structure and scheme. Here, the proposed transmission device 200 comprises a processing unit 210, which may be provided on a single chip or a chip module and which may be any processor or computer device with a control unit which performs control based on software routines of a control program stored in a memory 212. Program code instructions are fetched from the memory 212 and are loaded to the control unit of the processing unit 210 in order to perform the processing steps of the above functionalities described in connection with Fig. 4. The processing steps of blocks 21, 23 and 24 may be performed on the basis of input data DI and may generate output data DO, wherein the input data DI may correspond to data blocks to be queued for transmission and the output data DO may correspond to data blocks scheduled for transmission by the physical layer control unit 25.

In summary, an apparatus, a method and a computer program product for controlling transmission of data to at least one receiving unit have been described, wherein at least two queues are provided per channel for storing data packets of a predetermined access category of respective transmission links to the at least one receiving unit. The at least two queues are allocated to different ones of the respective transmission links, and data packets to be transmitted via the respective transmission links are stored in the at least two queues based on the allocation. While the invention has been illustrated and described in detail in the drawings and the foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive. The invention is not limited to the disclosed embodiment of the WAVE environment and also applies to any wireless network with high- mobility stations and contention based access scheme with QoS support. More specifically, the invention is not restricted to high-mobility vehicular environments, in an effort to solve the problem brought by the high mobility of the vehicles. It applies for all wireless networks, such as wireless ad-hoc networks or wireless mesh networks with changing network topology, which utilize contention based channel access with QoS support, e.g. WAVE and IEEE 802.i l WLAN. The invention is also applicable to all wireless networks implementing MIMO technologies. It is especially interesting for small office or home office (SO-HO) scenarios, with the network traffic composed of applications with different delay and throughput requirements, where QoS support is necessary for a successful operation. The invention might also be applied to wired networks, provided that they implement MIMO technologies in some way. It might even be contemplated to apply the invention to other kinds of networks.

Variations to the disclosed embodiments can be understood and effected by those skilled in the art, from a study of the drawings, the disclosure and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality of elements or steps. A single processor or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. A computer program may be stored/distributed on a suitable medium, such as an optical storage medium or a solid-state medium supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems. Any reference signs in the claims should not be construed as limiting the scope thereof.

Claims

CLAIMS:

1. An apparatus for transmitting data to at least one receiving unit, said apparatus comprising: a) a buffer unit (21) adapted to provide per channel at least two queues for storing data packets of a predetermined access category of respective transmission links to said at least one receiving unit; and b) a control unit (22) adapted to store said data packets to be transmitted via said respective transmission links in said at least two queues, wherein said at least two queues are allocated to different ones of said respective transmission links.

2. An apparatus according to claim 1, wherein said control unit (22) is further adapted to control said at least two queues independently from one another according to a status of an allocated one of said respective transmission links.

3. An apparatus according to any one of the preceding claims, wherein said control unit (22) is further adapted to suspend at least one of said at least two queues, if a transmission of a data packed from said at least one queue fails for a predetermined number of times or if a predetermined time period has elapsed since the receipt of the last service request of a transmission link to which said at least one queue is allocated.

4. An apparatus according to claim 3, wherein said control unit (22) is further adapted to suspend said at least one of said at least two queues at the starting point of a control channel interval.

5. An apparatus according to claim 3 or 4, wherein said control unit (22) is further adapted to resume said suspended at least one queue when a service channel interval begins and when a service request is received within a predefined period of time.

6. An apparatus according to claim 5, wherein said predefined period of time corresponds to a multiple of a number of synchronization intervals.

7. An apparatus according to any one of claims 3 to 6, wherein said buffer unit

(21) is further adapted to discard data packets of a suspended queue.

8. An apparatus according to any one of the preceding claims, further comprising a decision unit (23) adapted to allocate a transmission priority to said at least two queues in dependence on a timing of creation of said at least two queues.

9. A method of controlling transmission of data to at least one receiving unit, said method comprising: providing per channel at least two queues for storing data packets of a predetermined access category of respective transmission links to said at least one receiving unit; allocating said at least two queues to different ones of said respective transmission links; and storing data packets to be transmitted via said respective transmission links in said at least two queues based on said allocation.

10. A transmitter device comprising an apparatus according to one of claims 1 to

11. A transmitter device according to claim 10, wherein said transmitter device comprises a roadside unit for wireless transmission of data to on board units of vehicles.

12. A computer program product comprising code means for performing the steps of claim 9 when run on a computer device.