WO2006049537A1

WO2006049537A1 - A method and element for wireless uplink packet data communications

Info

Publication number: WO2006049537A1
Application number: PCT/SE2004/001592
Authority: WO
Inventors: Andreas Olsson; Krister Sundberg
Original assignee: Telefonaktiebolaget Lm Ericsson (Publ)
Priority date: 2004-11-02
Filing date: 2004-11-02
Publication date: 2006-05-11
Also published as: CN101053265B; EP1808034B1; EP1808034A1; EP2822329A1; US20080095105A1; CN101053265A

Abstract

The present invention relates to wireless communications. More especially it relates to wireless packet data communications. Particularly it relates to latency reduction when scheduling a plurality of temporarily inactive users or TBFs for a communications resource. Users or TBFs are pooled at controlled collision rate.

Description

A method and element for wireless uplink packet data communications

TECHNICAL FIELD OF THE INVENTION

The present invention relates to wireless communications. More especially it relates to wireless packet data communi- cations. Particularly it relates to latency reduction when scheduling a plurality of users for a communications re¬ source.

BACKGROUND AND DESCRIPTION OF RELATED ART

Multiplexing of a plurality of users on a common resource is well known in prior art. FDM (Frequency Division Multi¬ plex) , TDM (Time Division Multiplex) and CDM (Code Division Multiplex) are well known examples of multiplexing princi¬ ples.

Also a number of queuing disciplines are known for schedul- ing traffic on the multiplexed resource.

Antonio Capone, Mario Gerla and Rohi Kapoor, 'Efficient Polling Schemes for Bluetooth picocells, ' IEEE Interna¬ tional Conference on Communications, Helsinki, Finland, June 2001, describes single server polling systems. They compare an exhaustive benchmark polling scheme and three practical schemes: Pure Round Robin, Exhaustive Round Robin and Exhaustive Pseudo-cyclic. An extension of Exhaustive Round Robin scheme called Limited and Weighted Round Robin limits the number of transmissions per cycle reducing the capture effect and weights the queues according to observed queue status is argued to provide performance close to Ex¬ haustive Round Robin.

Yeng Zhong Lee, Rohit Kapoor and Mario Gerla, 'An Efficient and Fair Polling Scheme for Bluetooth, ' MILCOM 2002 - IEEE Military Communications Conference, vol. 21, no. 1, October 2002, presents a Pseudo-Random Cyclic Limited slot-Weighted Round Robin scheme using a pseudo-random ordering of slaves in a cycle. It is claimed in the publication that a poll¬ ing scheme that does not employ a pseudo-random ordering can easily lead to unfairness among TCP connections.

Hossam Fattah, Cyril Leung, ¹An Overview of Scheduling Al¬ gorithms in Wireless Multimedia Networks, ' IEEE Wireless Communications, pp. 76-83, June 2002 describes a plurality of scheduling algorithms and among other things scheduling in CDMA, networks. One algorithm, Scheduled CDMA, reveals data exchange between BS and MS in fixed-size unit called capsule, comprising one or more packets. For uplink sched¬ uling a capsule transmission request is sent to base sta¬ tion by mobile station whenever the MS has new packets to transmit. For each time slot the scheduler selects capsule transmission requests from a common queue ordered according to priority or delay sensitivity. The base station sends transmission permission capsules to selected mobile sta¬ tions to inform them of their capsule transmission times and power levels.

3^rd Generation Partnership Project (3GPP) : Technical Speci¬ fication Group GSM/EDGE Radio Access Network, General Packet Radio Service (GPRS) , Mobile Station (MS) - Base Station System (BSS) interface, Radio Link Control/Medium Access Control (RLC/MAC) protocol (Release 5) , 3GPP TS 44.060 v5.13.0, France, September 2004, specifies proce¬ dures for Radio Link Control, RLC, layer and Medium Access Control, MAC, layer, including physical link control func¬ tions of the radio interface between GSM/EDGE Radio Access Network, GERAN, and Mobile Station, MS. An Uplink State Flag, USF, is used on Packet Data Channel(s), PDCH(es) to allow multiplexing of uplink radio blocks from different mobile stations. An RR (Radio Resource) connection is a physical connection established between a mobile station and the network to support exchange of information flows. A TBF (Temporary Block Flow) is, in A/Gb mode, a physical connection used by the two RR peer entities to support the unidirectional transfer of LLC (Logical Link Control) PDUs on packet data physical channels. (A/Gb mode is a mode of operation of the MS when connected to the Core Network, CN, via GERAN and the A and/or Gb interfaces; the A interface being the interface between a BSS (Base Station Subsystem) and a 2G MSC (Mobile Switching Center) and the Gb interface being the interface between a BSS and a 2G SGSN (Serving GPRS Support Node) .) In Iu mode, a TBF is a logical con¬ nection offered by two MAC entities to support the unidi¬ rectional transfer of RLC PDUs on basic physical sub- channels. (Iu mode is a mode of operation of the MS when connected to the CN via GERAN or UTRAN and the Iu inter¬ face; the Iu interface being the interface between a BSS or an RNC (Radio Network Controller) and a 3G MSC or a 3G SGSN.) In extended uplink TBF mode, the uplink TBF may be maintained during temporary inactive periods, where the mo¬ bile station has no RLC information to send.

The mobile station generally operates with a sliding trans¬ mission window of RLC data PDUs. In the extended uplink TBF mode of Technical Specification 3GPP TS 44.060, if there is no RLC data block available within the window, the mobile station shall stop sending RLC data blocks. The mo¬ bile station shall continue sending RLC data blocks when an RLC data block becomes available in the window.

3^rd Generation Partnership Project (3GPP) : Technical Speci- fication Group GSM/EDGE Radio Access Network, Multiplexing and multiple access on the radio path (Release 5) , 3GPP TS 45.002 v5.12.0, France, April 2004, defines the physical channels of the radio sub system required to support the logical channels. It includes a description of the logical channels and the definition of frequency hopping, TDMA

(Time Division Multiple Access) frames, timeslots and bursts. In the uplink part for channels other than PACCH (Packet Associated Control Channel) transmitted as access bursts on PRACH (Packet Random Access Channel) or CPRACH

(Compact Packet Random Access Channel) , the logical channel type shall be indicated by the message type contained in the block header part. For PACCH transmitted as access bursts, the logical channel type is indicated by the corre¬ sponding polling message on the downlink. For the PRACH or CPRACH case the logical channel type is indicated by the USF, set on the downlink on a block-by-block basis.

The MAC layer is responsible for sharing of communications resource (the air interface) common to data and voice us¬ ers, according to an allocation strategy.

In e.g. GSM/GPRS, MAC of BSS (Base Station Subsystem) is responsible for management of uplink and downlink schedul¬ ing of RLC blocks belonging to different TBFs over avail- able time slots, resolving conflicts due to e.g. request collisions, assigning uplink TBFs to requesting MTs (Mobile Terminals) if there are time-slots available, notifying of uplink TBF deallocation if MT has been inactive during a predefined period, associating respective voice calls to a pair of time-slots and signaling as need be for deallocat¬ ing of a TBF to render the time-slot pair available for speech communications. In uplink direction, MAC of MT is responsible for initiating transmission of requests of up¬ link TBFs to BSS for transfer of data for which no TBF is yet established. Once the TBF setup is acknowledged, MAC of MT forwards RLC PDUs, carrying one or more segmented LLC PDUs, over a time-slot allocated by BSS. MT continues sending until there is no more data to send, or it has transmitted a maximum number of RLC blocks allowed. The TBF is then released. Each TBF is assigned by the network a temporary flow identity, TFI, which is unique in both di¬ rections.

Figure 1 illustrates schematically segmentation/reassembly of LLC PDUs and RLC PDUs. The LLC PDU comprises a frame header «FH», LLC data or control information «Information field», and a frame check sequence «FCS». A radio block consists of a 1-byte MAC header «BH» followed by RLC data «Info field», or an RLC/MAC control block «Info field», fi¬ nalized by a 16-bit block check sequence, «BCS». The radio block is carried on the physical channel by four normal bursts.

None of the cited documents above discloses uplink group scheduling of data transmissions or sending out of a par¬ ticular RLC/MAC message or a particular bit in a poll mes¬ sage to each mobile station, MS, in order to trigger or stop uplink group scheduled TBFs, the trigger giving all of. them access to the physical resource of a possibly multi- plexed resource.

SUMMARY OF THE INVENTION

A general problem of multiple access systems is to fulfill various requirements of a session as regards, e.g. QoS. Another problem is how to incorporate such requirements when allocating traffic to communications resources and scheduling of transmission instances.

In multi-user access, delay sensitive applications are ob¬ served to often also be bursty in nature. Individual scheduling of TBFs then risks being inefficient in terms of underutilized resources and increased latency as perceived by an end user. Generally, this is less of a problem in downlink direction, where a base station transmits data to a plurality of users and resources efficiently can be allocated and scheduled in relation to information available at sender side.

In uplink direction a base station receiving information from a plurality of user devices, the queue status of the mobile entity is not always available, at least not if lim¬ ited time restrictions also need to be met. Further it may not be efficient to spend communication resources on commu- nicating such information to a scheduling entity, such as a base station or base station controller.

Consequently, there is a need of providing efficient uplink scheduling of packet data transmissions for temporarily in¬ active users multiplexed on the same communication re- sources.

Another object of the invention to keep the risk of colli¬ sions of data packets of different users below a selected level.

Finally, it is an object to provide control messages that fulfills the above-mentioned objects.

These objects are met by a method and system of uplink scheduling and associated signaling.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 illustrates schematically segmentation/reassembly of LLC PDUs and RLC PDUs according to prior art.

Figure 2 illustrates an example of equal share splitting of a resource. Figure 3 illustrates schematically example delay and waste of capacity with individual scheduling according to prior art.

Figure 4 illustrates uplink scheduling according to the invention grouping or pooling of a plurality of inactive TBFs.

Figure 5 illustrates an information field element of a trigger message according to the first variation of a first basic embodiment of the invention and of a stop message ac- cording to a second basic embodiment of the invention.

Figure 6 illustrates information field elements of a trig¬ ger message according to a first mode of a first embodiment of the invention.

Figure 7 illustrates information elements of a , trigger^' message according to a second mode of the first embodiment of the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

For delay sensitive applications it is important with low latency.

The invention identifies that scheduling of a number of temporarily inactive TBFs (temporarily carrying no data) allowing occurrence of collisions when transmitting on a communications resource, such as a PDCH, would reduce la¬ tency as compared to splitting the resource in time dedicating the fractions to particular TBFs.

An example of equal share splitting of a resource is sche¬ matically illustrated in figure 2. In the example there are three TBFs «TBF1», «TBF2», «TBF3» of uplink scheduling for a communications resource R₃- (t) at various time in- stances t «R_j(l)», «Rj(2)», «Rj(3)», «R_d(4)», «Rj(5)». The communications resource can be, e.g., one or more recurring time slots of a time multiplexed system, which is antici¬ pated in the figure. Each TBF is preceded by a correspond- ing USF «USF1», «USF2», «USF3».

If the respective data sources of the TBFs «TBF1», «TBF2», «TBF3» are bursty in nature, the dynamic individual sched¬ uling imposes a great risk that one or more of the time- shares allotted to one or more TBFs are temporarily unused.

If e.g. a first TBF «TBF1» has a substantial amount of data queuing up in sending entity, e.g. mobile station, there is a waste of capacity and resources to schedule transmissions for a second «TBF2» and third TBF «TBF3» having the first «TBF1» to wait from time instance two, which would have been the next, until, e.g., time instance four for its next transmission if there is no data to transmit for TBFs two and three «TBF2», «TBF3». Further, if there is no data to transmit for TBFs two and three «TBF2», «TBF3», TBF one «TBF1» should be scheduled for a greater share of the transmission resource, but with individual scheduling of prior art this is not detected at BSS and communicated to the mobile station such that it can transmit data more in¬ tensively until a propagation round-trip time has lapsed. This is illustrated schematically in figure 3. The first TBF «TBF1» does not start transmitting intensively until time instance 7, when it starts transmitting on scheduled resources «Rj(7)», «R-j(8)», «Rj(9)», «Rj(10)» at time in¬ stances 7-11 or more time instances (depending on traffic situation) . An example study for GSM/GPRS reveals that for four simultaneous TBFs (users) in a cell, this kind of problems contributes to an estimated excessive delay corre¬ sponding to approximately 80 ms on average. Also greater delays could arise depending on e.g. data packet sizes and number of users.

Figure 4 illustrates uplink scheduling according to the in¬ vention remedying both aforementioned deficiencies by grouping or pooling of a plurality of inactive TBFs. The pooling is triggered by e.g. sending of a particular trig¬ ger message «Trigger» concerning the plurality of selected TBFs rendering the communications resource accessible to the selected TBFs, letting the various TBFs «TBFl/2/3» com- pete for the resource «Rj(l)», «Rj(2)», «Rj(3)», «R-j(4)», «Rj(5)». By proper selection of the TBFs allowed to compete within a pool, the risk of collision is kept at a minimum due to the bursty nature of packet data traffic. Traffic characteristics affect the collision rate and consequently the number of TBFs to be selected or pooled.

Collisions are preferably handled as follows. When colli¬ sions of grouped TBFs occur and the received PDU is not considered valid according to a checksum, the very schedul¬ ing is considered empty or otherwise treated in correspon- dence to data transmissions corrupted by noise or interfer¬ ence. If there have been no correctly (according to check¬ sums) received transmissions during a predefined period from any user or TBF of the pooled group of TBFs, the TBFs are scheduled individually and are individually checked for data.

There are two basic embodiments, primarily discriminating by how a particular pooling period is ended, of the inven¬ tion presented in further detail below:

1. by setting of a stopper (e.g. a time-out or maximum number of consecutive PDUs or data packets) , and

2. by sending of an explicit stop message. Preferably, inactive users or TBFs are those considered for grouped or pooled scheduling. Users or TBFs active to a greater extent than set by the stopper are preferably not grouped or pooled but scheduled individually. Thereby the risk of collisions is reduced further. For the first basic embodiment, when an inactive user or TBF becomes active to a greater extent than set by the stopper, or is identified as such, it is preferably excluded from grouped scheduling.

As mentioned above, the group scheduling of TBF pooling is triggered by an appropriate message of control signaling. In GSM/GPRS the message triggering the group scheduling is preferably a particular trigger message of RLC/MAC proto¬ col, or a new bit in a poll message.

According to the first basic embodiment of the invention, a pooling period is set on beforehand by e.g. a BSS. The pe¬ riod is preferably included in the message triggering pool¬ ing (first variation of first basic embodiment), or is sent as System Information on a broadcast channel of a cell or base station (second variation of first basic embodiment) . Figure 5 illustrates an information field element of a trigger message according to the first variation of the first basic embodiment. The information field comprises affected TBF or TBFs «Affected TBFs». In a first mode of the first variation of the first basic embodiment, the pe- riod as seen by an individual TBF is determined by a maxi¬ mum number of RLC data blocks allowed to send. Figure 6 illustrates information field elements of a trigger message according to the first mode of the first variation of the first basic embodiment. A field of affected TBF or TBFs «Affected TBFs» indicate to which TBFs the message is rele¬ vant and a field of maximum number of PDUs to send «Maximum # PDUs Sent» indicates to a TBF when pooling is ended. In a second mode of the first variation of the first embodi- ment, for which information fields of a trigger message is illustrated in figure 7, the period as seen by all TBFs in the pool is limited by a maximum time of the pooling period «Maximum Pool Time». Also in the trigger message according to the second mode of the first embodiment there is an in¬ formation field of affected TBF or TBFs «Affected TBFs» identical to the corresponding information field of figure 6.

According to the second basic embodiment of the invention a pooling period is ended by a particular stop message to af¬ fected TBFs. For GSM/GPRS, the stop message is preferably a particular stop message of RLC/MAC protocol. The stop message comprises an information field with information on affected TBF or TBFs «Affected TBF» as schematically illus- trated in figure 5.

The embodiments can be combined. Thereby, e.g., a pooling period initially set can be stopped prior to the period initially set having lapsed.

The invention applies to particularly radio communication systems but also to other wireless communications, e.g. in¬ frared communications, whenever they involve uplink sched¬ uling of a plurality of packet data connections.

In this patent application acronyms such as BSS, MT, MS, GSM or GPRS are applied. However, the invention is not limited to systems with entities with these acronyms, but holds for all communications systems operating analogously.

The invention is not intended to be limited only to the em¬ bodiments described in detail above. Changes and modifica¬ tions may be made without departing from the invention. It covers all modifications within the scope of the following claims.

Claims

1. A method of wireless uplink packet data communications c h a r a c t e r i z e d i n that packet data communica¬ tions of different connections or TBFs are scheduled in one or more groups of connections or TBFs, the connections or TBFs of a group being scheduled for data transmissions al¬ lowing the connections or TBFs to transmit simultaneously on a same wireless channel resource.

2. The method of wireless uplink packet data communica¬ tions according to claim 1 c h a r a c t e r i z e d i n that connections or TBFs are grouped during a time period of finite duration.

3. The method of wireless uplink packet data communica¬ tions according to claim 2 c h a r a c t e r i z e d i n that the time period of finite duration is initiated by a trigger message.

4. The method of wireless uplink packet data communica¬ tions according to claim 3 c h a r a c t e r i z e d i n that the trigger message is a particular RLC/MAC message or a particular bit or flag in a poll message

5. The method of wireless uplink packet data communica¬ tions according to claim 2 c h a r a c t e r i z e d i n that the time period of finite duration is sent on a broad¬ cast channel in terms of a maximum number of consecutive data blocks a connection or TBF of the group is allowed to send.

6. The method of wireless uplink packet data communica¬ tions according to claim 2 c h a r a c t e r i z e d i n that the time period of finite duration is sent on a broad¬ cast channel in terms of an explicit time period or stop time.

7. The method of wireless uplink packet data communica¬ tions according to claim 2 c h a r a c t e r i z e d i n that the time period of finite duration is sent as system information on a broadcast channel.

8. The method of wireless uplink packet data communica¬ tions according to claim 3 c h a r a c t e r i z e d i n that the time period of finite duration is included in the trigger message in terms of a maximum number of consecutive data blocks a connection or TBF of the group is allowed to send.

9. The method of wireless uplink packet data communica¬ tions according to claim 3 c h a r a c t e r i z e d i n that the time period of finite duration is included in the trigger message in terms of an explicit time period or stop time.

10. The method of wireless uplink packet data communica¬ tions according to claim 2 c h a r a c t e r i z e d i n that the time period of finite duration is ended by a stop message.

11. The method of wireless uplink packet data communica¬ tions according to any of claims 3-9 c h a r a c t e r ¬ i z e d i n that the trigger message includes an informa¬ tion element comprising identity of affected one or more connections or TBFs.

12. The method of. wireless uplink packet data communica¬ tions according to claim 10 c h a r a c t e r i z e d i n that the stop message includes an information element com¬ prising identity of affected one or more connections or TBFs.

13. A communications element for use in a wireless communications system c h a r a c t e r i z e d b y processing means for carrying out the method in any of claims 1-12.

14. The communications element according to claim 13 c h a r a c t e r i z e d i n that the communications element is included in or is a mobile station of a mobile communications system.

15. The communications element according to claim 13 c h a r a c t e r i z e d i n that the communications element is included in or is a base station subsystem of a mobile communications system.

16. A communications system c h a r a c t e r i z e d b y the communications system comprising processing means for carrying out the method in any of claims 1-12.