WO2007069959A1 - Method and arrangement for improved re-transmission in a wireless communication system - Google Patents

Method and arrangement for improved re-transmission in a wireless communication system Download PDF

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Publication number
WO2007069959A1
WO2007069959A1 PCT/SE2005/001951 SE2005001951W WO2007069959A1 WO 2007069959 A1 WO2007069959 A1 WO 2007069959A1 SE 2005001951 W SE2005001951 W SE 2005001951W WO 2007069959 A1 WO2007069959 A1 WO 2007069959A1
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Prior art keywords
data blocks
user
ranking
retransmissions
fraction
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PCT/SE2005/001951
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French (fr)
Inventor
Henrik Nyberg
Ylva Timner
Peter De Brun
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Telefonaktiebolaget Lm Ericsson (Publ)
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Priority to PCT/SE2005/001951 priority Critical patent/WO2007069959A1/en
Publication of WO2007069959A1 publication Critical patent/WO2007069959A1/en

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Classifications

    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04LTRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L1/00Arrangements for detecting or preventing errors in the information received
    • H04L1/12Arrangements for detecting or preventing errors in the information received by using return channel
    • H04L1/16Arrangements for detecting or preventing errors in the information received by using return channel in which the return channel carries supervisory signals, e.g. repetition request signals
    • H04L1/18Automatic repetition systems, e.g. Van Duuren systems
    • H04L1/1867Arrangements specially adapted for the transmitter end
    • H04L1/1887Scheduling and prioritising arrangements
    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04WWIRELESS COMMUNICATION NETWORKS
    • H04W72/00Local resource management
    • H04W72/12Wireless traffic scheduling
    • H04W72/121Wireless traffic scheduling for groups of terminals or users
    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04LTRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L1/00Arrangements for detecting or preventing errors in the information received
    • H04L2001/0092Error control systems characterised by the topology of the transmission link
    • H04L2001/0093Point-to-multipoint
    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04WWIRELESS COMMUNICATION NETWORKS
    • H04W72/00Local resource management
    • H04W72/30Resource management for broadcast services

Definitions

  • the present invention relates to multimedia broadcast/ multicast services
  • MBMS massive machine type communications
  • MBMS Multimedia Broadcast/ Multicast
  • the so called Multimedia Broadcast/ Multicast (MBMS) service is an example of a solution on this subject, which provides the possibility to utilize an uplink channel for interactions between the service and its users.
  • MBMS is a unidirectional, point-to-multipoint service in which data is transmitted from a single source to a group of users in a specific area.
  • the service in practice has two modes: broadcast mode and multicast mode.
  • a broadcast service is a unidirectional point-to-multipoint service in which data is transmitted from a single source to multiple terminals, or users in the associated broadcast service area.
  • broadcast services can be called push-type services.
  • a multicast service is a unidirectional point-to-multipoint service in which data is transmitted from a single source to a multicast group in the associated multicast service area. Only users that subscribe to the specific multicast service and have joined the multicast group associated with that service can receive the multicast services. As a difference, a broadcast service can be received without separate indication from the customers.
  • multicast users as well as broadcast users in MBMS need a return channel for the interaction procedures in order to be able to subscribe to the desired services.
  • MBMS is in the process of standardization in the 3GPP (Third Generation Partnership Project).
  • MBMS enables offering a complete set of Multicast and Broadcast services over GSM/ EDGE as well as WCDMA.
  • MBMA is an example of the previously described Broadcast/ Multicast operation (or service) where the same payload data is transmitted to multiple end users in a cellular network.
  • this type of Broadcast/ Multicast operation (service) in cellular networks is referred to as Multicast operation.
  • Multicast operation this type of Broadcast/ Multicast operation (service) in cellular networks.
  • Multicast operation In order to provide nearly error-free data transmissions for Multicast operation, multiple transmissions (or retransmissions) is an efficient means, compared to providing a radio environment with high enough quality to completely avoid retransmissions.
  • retransmissions in MBMS One problem regarding retransmissions in MBMS is that a single data block can generate as many retransmission requests as there are users in the systems. As the number of users increase, the likelihood of retransmission requests increases drastically. Since no separate channel exists for retransmissions, the occurrence of retransmissions limits the bandwidth available for the actual primary transmissions. At one extreme, for a system with a large number of users, this could in a worst case scenario prevent any primary transmissions from being sent.
  • BBR Blind Block Repetition
  • Packet Downlink ACK/ NACK In this mode of operation, re- transmissions are based on requests (ACK/NACK reporting) from up to 16 users in one cell. To meet requirements on guaranteed bit rate, the number of retransmissions must be limited (or controlled), such that sufficient transmission time is spent on sending new data (primary transmissions). A typical requirement is that each RLC block is sent at least once. This requirement, together with the arrival bit rate of new data, specifies the bit rate that must be reserved for "primary transmission" (i.e., first transmission attempts).
  • the primary purpose of re-transmitting data units is to provide an error-free transmission over the radio interface.
  • the amount of retransmission (absolute or relative) must be restricted since retransmissions share the radio link with primary transmissions.
  • some residual errors may be preferred, rather than no errors, since the correctly received data may be obsolete by arrival.
  • An object of the present invention is to provide improved MBMS in a cellular radio network and to keep the quality at an acceptable level for as many users as possible.
  • a further object is to provide retransmission strategies for MBMS.
  • Another object is to provide improved retransmission strategies for MBMS to limit the bandwidth used for retransmissions.
  • Yet another object is to provide selective retransmissions for MBMS.
  • a further object is to provide selective retransmissions based on ACK/ NACK reports from multiple users.
  • the present invention comprises evaluating the reception success of a data block for multiple users, ranking the transmitted data blocks based at least on the evaluation and selectively retransmitting requested data blocks based on the ranking.
  • the evaluation is performed based on received ACK/ NACK reports, i.e., potentially retransmission requests, from the respective users.
  • the ranking can be performed based on the user priority for the set of users, the age of the requested data blocks, or some other quality requirement.
  • the available bandwidth for transmissions (and retransmissions) is divided into one minimum section reserved for primary transmission, and one remaining section for retransmission, thus, ensuring a minimum guaranteed bitrate for the users.
  • Fig. 1 is a schematic flow diagram of an embodiment of a method according to the invention
  • Fig. 2 is a schematic illustration of the known sliding window principle
  • Fig. 3 is a diagram illustrating the effect of various embodiments according to the invention
  • Fig. 4 is a diagram illustrating the effect of various embodiments according to the invention
  • Fig. 5 is a diagram illustrating the effect of various embodiments according to the invention.
  • Fig. 6 is a diagram illustrating the effect of various embodiments according to the invention.
  • Fig. 7 is a schematic illustration of a system according to the invention.
  • the present invention will be described in the context of MBMS in a GSM/ EDGE communication system. However, it is equally applicable to any other packet based communication system utilizing a common channel for transmissions and retransmissions, where retransmission requests are typically sent from each user on dedicated channels in the uplink.
  • MBMS bit stream has the (average) bit rate n, the bit rate available for retransmissions is riacs - n.
  • the proportion (1 - n/rMcs) of the data blocks can be used for retransmissions without starving the primary bit stream.
  • the current MBMS standard does not specify strategies for how to accomplish efficient retransmission strategies.
  • the error probability for each of 10 users is 10%, their combined error probability is 65%, meaning that several retransmissions might be necessary before all 10 users have received the data block correctly.
  • the capacity required for retransmissions may become large in comparison with the capacity required for primary transmissions.
  • the basic approach is therefore to prioritize all or a limited number of primary transmissions, and for the remaining capacity or bandwidth select data blocks for retransmission based on a retransmission priority scheme taking the selection of primary transmissions into account. The selection is generally based on the probability that a retransmission will keep the quality at a high level for as many users as possible.
  • the idea is further to use an approach to regulate retransmission attempts. This can be accomplished by a "leaky bucket" flow control that ensures an average bit rate for primary transmissions but allows for short periods with many retransmissions.
  • retransmission strategies could be based on, but not limited to, quality requirements and current radio conditions such as the ones mentioned below:
  • Radio quality metric example (residual) block error rate (BLER) per user, i.e. expected fraction of RLC blocks not correctly received.
  • BLER block error rate
  • the invention comprises transmitting SO a plurality of data blocks to a plurality of users on a common channel (it is understood, due to the inherent nature of MBMS, that the same data blocks are transmitted to all users), the users respond to the transmission by reporting S 1 successful and unsuccessful reception of each data block to the transmitting node e.g. ACK/NACK reports. Subsequently, the received reports from all users are evaluated S2 together or jointly and the transmitted data blocks are sorted or ranked S3 accordingly. Finally, retransmissions are selectively effected S4 based on the previous ranking.
  • the above mentioned ranking or prioritizing based on the reception success /failure reports can be performed based on some predetermined function or relation.
  • the age of the data blocks can be taken into account, e.g. by discarding old data blocks in favor of new data blocks or vice versa.
  • Another possible parameter is a user priority for each user. The priority could be based on subscriber data, or the time the user has been in the system.
  • an additional step comprises initially allocating a predetermined fraction of the total available bandwidth to primary transmissions and allocating the remaining fraction for retransmissions.
  • TTI basic transmit time intervals
  • RLC block The amount of data sent during a TTI
  • reception reporting e.g. ACK/NACK reporting is performed by each user for each such data block.
  • Each user needs one TTI exclusive uplink channel time to send such a report comprising up to a number M of data blocks.
  • Decisions on new primary transmissions and retransmissions are taken after each period of K block transmissions (primary transmissions and retransmissions). Alternatively, after K time steps.
  • K the period between decisions is long enough, i.e. K equal to or larger than M.
  • K M to keep retransmission delays fairly low.
  • An RLC ACK/ NACK sliding window is used to limit the range of RLC blocks that may be retransmitted.
  • the window spans the most recent data block and a fixed number of data blocks preceding the last block. Any data block within the range of the window and which has been reported as NACK is a candidate form retransmission.
  • the window is slid forward at a pace corresponding to the rate of transmitting new data blocks.
  • the ACK/ NACK report history contained in the current window is used for prioritization decisions.
  • each data block within the range of the current ACK/ NACK window is considered as a candidate for retransmission.
  • decisions on which data block to retransmit are based at least on the ACK/ NACK reports from all users within the current window.
  • the number of retransmissions on the common channel is limited, according to the invention, by in the first place reserving capacity or bandwidth for primary transmissions (so that a guaranteed bit rate is achieved). This means that for each period of K transmissions, a subset is reserved for new data blocks (however, not more than necessary). The capacity or bandwidth that remains is available for selected retransmissions.
  • a basic way of taking block age into account is by using an ACK/ NACK window that limits the number of previous blocks that may be considered for retransmission.
  • the ACK/NACK window is slid in accordance with the number of new data blocks transmitted (primary transmissions) . Old blocks that thereby fall outside the window are discarded with respect to further retransmissions, meaning that retransmissions of these blocks are stopped.
  • each data block in the window is assigned a ranking based on the following factors:
  • Blocks that have not yet been reported by all users may be considered for retransmission if there is spare transmission time after considering blocks with complete reports;
  • the age of the data block (or block number).
  • the ACK/NACK reports from each user indicate to what extent retransmissions of a certain data block is requested, whereas the scores or rankings indicate which users are already close to a good quality and also indicate the radio link quality for each user.
  • the scoring or ranking strategy can, according an embodiment of the present invention, be represented by a predetermined function, hereinafter referred to as a score function, and calculated for each block.
  • the decisions on which blocks to retransmit are made based on ranking blocks with respect to at least the current scores, and how optionally on how many retransmissions that are allowed for the next retransmission period (i.e. number of retransmissions during a period of K transmissions) .
  • score function determines which quality requirements will be best met.
  • three different score functions or strategies will be described below: Basically, the three different embodiments according to the invention can be referred to and summarized as follows:
  • An exemplary first score function is defined as:
  • the second exemplary score function is defined by:
  • P 1 (U)I an estimate of the block error probability (BLEP) for user u.
  • BLEP can, e.g., be estimated by the recursion
  • P 1 (U) Q- p crr (u;M) + (l - ⁇ ) - p ⁇ (u) where p ⁇ (u) is the previous estimate used (initially set to 0), and a is between 0 and 1.
  • the quantity p en .(u;M) N orr (u;M)lM is the fraction of NACKs in the last report of M blocks and N err (M) is the number of errors for the last report.
  • P w ⁇ u) a- p err (u; W) + (1 - ⁇ ) • p' ⁇ (u) where p' ⁇ ( «) is the previous estimate used, and a is between 0 and 1.
  • the quantity p err (u;W) N err ⁇ u;W) I W is the fraction of NACKs for user u in the current window of size W. Note that there may be better functions of the ACK/ NACK history to estimate the residual BLEP.
  • strategy 1 A retransmission strategy that employs e.g. score function 1 will in the following be called strategy 1.
  • Strategies 2 and 3 are defined correspondingly.
  • time is omitted in the score functions, they are (at least inherently) functions of the time since the ACK/ NACK window changes between retransmission decisions.
  • Ranking of all current data blocks in the window is done with respect to the value of the score function and optionally the age of the block.
  • the age of the block it is equally possible to use the age of the block as an integral part of the actual score function. It is purely a mathematical action to include age of a block into the score function.
  • the strategies described can be modified by the way the block age is used when ranking blocks for retransmission. This can be important for certain real-time broadcast transmissions.
  • An additional factor to consider when ranking blocks for retransmission is the time spent in the broadcast session by the user requesting a specific block.
  • the media i.e. data blocks are sent with constant bit rate.
  • Data blocks are numbered in ascending order as they are transmitted the first time. If a block is retransmitted at a later time, it keeps the original number. (Alternatively, it could re-numbered so that it is considered as a more recent block.)
  • ACK/ NACK reporting is scheduled cyclically among n users so that each user is scheduled each Mth transmitted block (alternatively: each M:th time step).
  • the number of reporting users (n) may vary in time but must be less than or equal to M.
  • M equals the current number of users and is changed whenever n changes.
  • Each user reports the M least recent receptions among the received but not yet reported blocks (ACK for a correctly received block, NACK for an incorrectly received block).
  • the transmitting node has a retransmission window of size W.
  • Time is here assumed to be measured in units of number of transmission time intervals (TTIs).
  • Block error rates for the different users are assumed to be uniformly distributed within the given interval, unless otherwise stated. Below referred to as the simplified radio link model.
  • Table 1 The calculations for a simplified system scenario with three users are shown in Table 1 below.
  • the BLER and residual BLER estimates used for strategy 2 and 3 respectively are for simplicity both set to the fraction of NACKs in the present window.
  • the 3 blocks with largest scores (and least block age) are marked for each strategy.
  • the effects of different retransmission strategies, according to the invention, in simulations (with a simplified radio link model) with corresponding calculations are shown in Figure 3 (16 users with different link quality) and Figure 4 (16 users with equal link quality).
  • the simulations comprise 30000 data blocks transmitted (broadcast or multicast) during 48000 TTIs.
  • the primary bitrate is supported if 10 out of 16 TTIs are spent on primary transmissions.
  • One reference strategy is the so called "Retransmission prio" where each data block is retransmitted until all users have received it correctly.
  • the ACK/ NACK window may stall the flow of primary transmissions since the window may not be moved until the block at the back end has been acknowledged by all users.
  • data blocks that are not sent during the simulation are counted as lost and included in the residual BLER statistics.
  • the high residual BLER should in this case be interpreted as a failure to reach the required primary bit rate. E.g., with an average residual BLER of 22%, only 78% of the required primary bit rate has been reached. See also the analysis of Figure 5 and 6 below.
  • the Figures show that the retransmission strategy affects the BLER distribution.
  • strategy 3 the distribution is tilted so that the number of users with very low BLER is increased.
  • Strategy 1 yields least quality dispersion among users.
  • Strategy 3 yields a quality distribution that is between strategy 1 and 3 with respect to dispersion when there is dispersion of link quality to different users (Figure
  • strategy 2 estimates and schedules retransmissions with respect to link quality.
  • Strategy 3 in addition takes into account which users have already received much service (they are prioritized so that they stay satisfied) .
  • the core of the invention is to use specific score functions for ranking, and in particular use a score function that aims at maximizing the fraction of satisfied users. More generally, the score functions correspond to different objectives of broadcast/ multicast retransmissions.
  • Figures 5 and 6 show the fraction of satisfied users (i.e. max 1% residual BLER) for different numbers of users and for different strategies.
  • the simulations comprise 10000 blocks offered over a time of 16000 TTIs.
  • the user quality breaks down (for all users) when the load (number of simultaneous users) becomes high enough.
  • Limiting the number of retransmissions maintains the quality for more users.
  • the choice of strategy becomes increasingly important as more users are present.
  • strategy 3 is always best for variable link quality, see
  • strategy 1 yields a higher FSU at a specific number of present users, see Figure 6.
  • strategy 3 generally yields the highest FSU when resources get very scarce.
  • bit rate required for "primary transmissions” can be guaranteed by (explicitly or implicitly) limiting the number of retransmissions. (However, it is not guaranteed that the primary transmissions are correctly received by a large enough fraction of users.)
  • Figure 7 illustrates an exemplary embodiment of a system according to the invention.
  • the system comprises a transmitting node 10 configured for broadcasting or multicasting data blocks to n users Ul... Un.
  • the users respond to the transmitted data blocks by sending ACK/ NACK reports ACK/ NACK Ul... ACK/ NACK Un indicating correctly received or incorrectly received data blocks.
  • the system comprises an evaluation unit 11 for jointly evaluating the received ACK/ NACK reports from all users, a ranking unit for ranking or prioritizing among the data blocks based on the received reports.
  • the transmitting node 10 comprises a retransmission unit 13 for retransmitting data blocks based on a resulting ranking from the ranking unit 13.
  • embodiments of the present invention are described as implemented to handle retransmissions to users within one network cell, it is equally possible, with appropriate modifications, to use for retransmissions to users in multiple cells.
  • One possibility could be to perform the method of the invention in multiple cells, one at a time.
  • Another possibility could be to exploit the possibility of macrodiversity by having some functionality which coordinates the ranking of data blocks for different cells, i.e. by identifying data blocks suitable for macrodiversity and coordinating retransmissions of theses blocks in suitable cells (together providing macrodiversity). As an example, if the data block is requested for retransmission from multiple users distributed over a plurality of cells.
  • the invention comprises the following advantages: Enabling more efficient retransmission strategies

Abstract

In a method of retransmission in a wireless communication system, comprising a transmitting node communicating (SO) a plurality of data blocks to multiple user nodes on a common radio channel, and in response to receiving reports (Sl) of successful/unsuccessful reception of said data blocks from each of the multiple users, jointly evaluating (S2) the reception results based on the reports, ranking (S3) the data blocks for retransmission based at least on the joint evaluation, and, selectively retransmitting (S4) data blocks to the multiple users based at least on the ranking to limit the bandwidth used for retransmissions.

Description

METHOD AND ARRANGEMENT FOR IMPROVED RETRANSMISSION IN A WIRELESS COMMUNICATION SYSTEM
TECHNICAL FIELD The present invention relates to multimedia broadcast/ multicast services
(MBMS) in general, specifically to retransmissions systems supporting such services.
BACKGROUND At present, there is a rapid increase in the demand for providing multimedia services such as video and audio in communication systems such as GSM and UMTS. The so called Multimedia Broadcast/ Multicast (MBMS) service is an example of a solution on this subject, which provides the possibility to utilize an uplink channel for interactions between the service and its users. MBMS is a unidirectional, point-to-multipoint service in which data is transmitted from a single source to a group of users in a specific area. The service in practice has two modes: broadcast mode and multicast mode.
In general, a broadcast service is a unidirectional point-to-multipoint service in which data is transmitted from a single source to multiple terminals, or users in the associated broadcast service area. In other words, broadcast services can be called push-type services. On the other hand, a multicast service is a unidirectional point-to-multipoint service in which data is transmitted from a single source to a multicast group in the associated multicast service area. Only users that subscribe to the specific multicast service and have joined the multicast group associated with that service can receive the multicast services. As a difference, a broadcast service can be received without separate indication from the customers. In practice, multicast users as well as broadcast users in MBMS need a return channel for the interaction procedures in order to be able to subscribe to the desired services. At present, MBMS is in the process of standardization in the 3GPP (Third Generation Partnership Project). MBMS enables offering a complete set of Multicast and Broadcast services over GSM/ EDGE as well as WCDMA. MBMA is an example of the previously described Broadcast/ Multicast operation (or service) where the same payload data is transmitted to multiple end users in a cellular network. Hereafter, this type of Broadcast/ Multicast operation (service) in cellular networks is referred to as Multicast operation. In order to provide nearly error-free data transmissions for Multicast operation, multiple transmissions (or retransmissions) is an efficient means, compared to providing a radio environment with high enough quality to completely avoid retransmissions.
One problem regarding retransmissions in MBMS is that a single data block can generate as many retransmission requests as there are users in the systems. As the number of users increase, the likelihood of retransmission requests increases drastically. Since no separate channel exists for retransmissions, the occurrence of retransmissions limits the bandwidth available for the actual primary transmissions. At one extreme, for a system with a large number of users, this could in a worst case scenario prevent any primary transmissions from being sent.
Consequently, for the standardized MBMS [1] two different re-transmission strategies are specified: Blind Block Repetition (BBR) - In this mode of operation, multiple transmissions (of identical data) are made a specified number of times, not considering the number of users or the current radio quality (received signal strength and/ or interference level)
Packet Downlink ACK/ NACK (PDAN) - In this mode of operation, re- transmissions are based on requests (ACK/NACK reporting) from up to 16 users in one cell. To meet requirements on guaranteed bit rate, the number of retransmissions must be limited (or controlled), such that sufficient transmission time is spent on sending new data (primary transmissions). A typical requirement is that each RLC block is sent at least once. This requirement, together with the arrival bit rate of new data, specifies the bit rate that must be reserved for "primary transmission" (i.e., first transmission attempts).
The primary purpose of re-transmitting data units (e.g., RLC blocks) is to provide an error-free transmission over the radio interface. However, the amount of retransmission (absolute or relative) must be restricted since retransmissions share the radio link with primary transmissions. For broadcast type of transmission, some residual errors may be preferred, rather than no errors, since the correctly received data may be obsolete by arrival.
SUMMARY
An object of the present invention is to provide improved MBMS in a cellular radio network and to keep the quality at an acceptable level for as many users as possible.
A further object is to provide retransmission strategies for MBMS.
Another object is to provide improved retransmission strategies for MBMS to limit the bandwidth used for retransmissions.
Yet another object is to provide selective retransmissions for MBMS.
A further object is to provide selective retransmissions based on ACK/ NACK reports from multiple users. These and other objects are achieved in accordance with the attached claims.
Briefly, the present invention comprises evaluating the reception success of a data block for multiple users, ranking the transmitted data blocks based at least on the evaluation and selectively retransmitting requested data blocks based on the ranking.
Specifically, the evaluation is performed based on received ACK/ NACK reports, i.e., potentially retransmission requests, from the respective users.
Additionally, the ranking can be performed based on the user priority for the set of users, the age of the requested data blocks, or some other quality requirement.
According to another specific embodiment, the available bandwidth for transmissions (and retransmissions) is divided into one minimum section reserved for primary transmission, and one remaining section for retransmission, thus, ensuring a minimum guaranteed bitrate for the users.
Advantages of the present invention comprise:
Enabling more efficient retransmission strategies
Enabling providing a minimum guaranteed bitrate for transmissions
Reducing total number of retransmissions
BRIEF DESCRIPTION OF THE DRAWINGS
The invention, together with further objects and advantages thereof, may best be understood by making reference to the following description taken together with the accompanying drawings, in which:
Fig. 1 is a schematic flow diagram of an embodiment of a method according to the invention; Fig. 2 is a schematic illustration of the known sliding window principle;
Fig. 3 is a diagram illustrating the effect of various embodiments according to the invention; Fig. 4 is a diagram illustrating the effect of various embodiments according to the invention;
Fig. 5 is a diagram illustrating the effect of various embodiments according to the invention;
Fig. 6 is a diagram illustrating the effect of various embodiments according to the invention;
Fig. 7 is a schematic illustration of a system according to the invention.
ABBREVIATIONS
MBMS Multimedia Broadcast/ Multicast Service
BBR Blind Block Repetition
PDAN Packet Downlink ACK/ NACK RLC Radio Link Control
MCS Modulation and Coding Scheme BLEP/ BLER Block Error Probability/ Rate
TTI Transmit Time Interval
DETAILED DESCRIPTION
The present invention will be described in the context of MBMS in a GSM/ EDGE communication system. However, it is equally applicable to any other packet based communication system utilizing a common channel for transmissions and retransmissions, where retransmission requests are typically sent from each user on dedicated channels in the uplink.
As stated in the background, one major disadvantage of MBMS in GSM/ EDGE is the sheer number of retransmission requests and correspondingly number of retransmissions necessary in a system with many users. Regarding for example one cell in a GSM/ EDGE system utilizing MBMS, the capacity available for retransmissions can be analytically calculated.
Assuming a fixed MCS (Modulation and Coding Scheme) for MBMS in a cell, the total available bit rate mcs for MBMS is well defined. Assuming that the
MBMS bit stream has the (average) bit rate n, the bit rate available for retransmissions is riacs - n. In other words, the proportion (1 - n/rMcs) of the data blocks can be used for retransmissions without starving the primary bit stream.
Since the capacity available for primary transmissions and re-transmissions is limited, there might be situations when not all RLC (Radio Link Control) blocks reported as erroneous can be re-transmitted, at least not without jeopardizing the required primary bit rate. Thus, it may be necessary to limit the total load of primary transmissions and retransmissions. In particular, a re-transmission strategy will then be necessary for how to determine which data blocks to retransmit.
Moreover, if re-transmissions are handled efficiently, resources for primary transmissions are freed and it may be possible to offer higher data rates or, alternatively, more (parallel) users or Multicast sessions.
As stated previously, the current MBMS standard does not specify strategies for how to accomplish efficient retransmission strategies. In fact, there is a risk that a large number of retransmissions would lower the available capacity for primary transmissions below the required. This will occur when retransmissions are prioritized over primary transmissions, the number of multicast users in a cell is large and their combined error probability is larger than what is possible to support by traditional unlimited retransmissions.
For example, if the error probability for each of 10 users is 10%, their combined error probability is 65%, meaning that several retransmissions might be necessary before all 10 users have received the data block correctly. Thus, even with relatively low error probabilities, the capacity required for retransmissions may become large in comparison with the capacity required for primary transmissions.
Consequently, basic requirements for MBMS should:
-Ensure capacity for primary transmissions; -Aim at providing "good quality for as many users as possible" in situations when not all users can be fully supported. The limitation to implement and to consider is that it will not always be possible to retransmit a radio block every time a user reports an error. Therefore, a careful selection of which requested retransmissions to serve will be necessary. The basic approach, according to the present invention, is therefore to prioritize all or a limited number of primary transmissions, and for the remaining capacity or bandwidth select data blocks for retransmission based on a retransmission priority scheme taking the selection of primary transmissions into account. The selection is generally based on the probability that a retransmission will keep the quality at a high level for as many users as possible.
In order to ensure capacity for primary transmissions (first transmission attempts), the idea is further to use an approach to regulate retransmission attempts. This can be accomplished by a "leaky bucket" flow control that ensures an average bit rate for primary transmissions but allows for short periods with many retransmissions.
To be efficient, retransmission strategies could be based on, but not limited to, quality requirements and current radio conditions such as the ones mentioned below:
Radio quality metric example: (residual) block error rate (BLER) per user, i.e. expected fraction of RLC blocks not correctly received.
User quality requirement example: residual BLER not exceeding a specified limit, e.g. 1%.
Basically, with reference to Figure 1, the invention comprises transmitting SO a plurality of data blocks to a plurality of users on a common channel (it is understood, due to the inherent nature of MBMS, that the same data blocks are transmitted to all users), the users respond to the transmission by reporting S 1 successful and unsuccessful reception of each data block to the transmitting node e.g. ACK/NACK reports. Subsequently, the received reports from all users are evaluated S2 together or jointly and the transmitted data blocks are sorted or ranked S3 accordingly. Finally, retransmissions are selectively effected S4 based on the previous ranking.
Some key elements for the principles of the invention thus comprise the following:
ACK/ NACK reporting (or retransmission requests) of users (which is already specified in [I]);
Ranking or prioritizing data blocks for retransmission using a score function based on ACK/ NACK reports; Retransmission of data blocks with predetermined e.g. highest rank.
The above mentioned ranking or prioritizing based on the reception success /failure reports can be performed based on some predetermined function or relation. Optionally, also the age of the data blocks can be taken into account, e.g. by discarding old data blocks in favor of new data blocks or vice versa. Another possible parameter is a user priority for each user. The priority could be based on subscriber data, or the time the user has been in the system.
In order to provide a specified or guaranteed bit rate, an additional step comprises initially allocating a predetermined fraction of the total available bandwidth to primary transmissions and allocating the remaining fraction for retransmissions.
A few specific embodiments will be described below:
Consider a system wherein time is measured as the number of basic transmit time intervals (TTI :s). The amount of data sent during a TTI is referred to as a data block (e.g. RLC block). It is also assumed that reception reporting e.g. ACK/NACK reporting is performed by each user for each such data block. Each user needs one TTI exclusive uplink channel time to send such a report comprising up to a number M of data blocks. Decisions on new primary transmissions and retransmissions are taken after each period of K block transmissions (primary transmissions and retransmissions). Alternatively, after K time steps. To guarantee reporting from up to M users before any new decisions are taken, assume that the period between decisions is long enough, i.e. K equal to or larger than M.
Typically; K=M to keep retransmission delays fairly low.
An RLC ACK/ NACK sliding window, illustrated in Figure 2, is used to limit the range of RLC blocks that may be retransmitted. In the Figure 2 a window of size W=5 is illustrated, where the window is slid after each transmitted new data block. Typically, the window spans the most recent data block and a fixed number of data blocks preceding the last block. Any data block within the range of the window and which has been reported as NACK is a candidate form retransmission. The window is slid forward at a pace corresponding to the rate of transmitting new data blocks. In embodiments according to the invention, the ACK/ NACK report history contained in the current window is used for prioritization decisions.
When considering which data blocks are to be retransmitted, each data block within the range of the current ACK/ NACK window is considered as a candidate for retransmission. As mentioned before, decisions on which data block to retransmit are based at least on the ACK/ NACK reports from all users within the current window.
The number of retransmissions on the common channel is limited, according to the invention, by in the first place reserving capacity or bandwidth for primary transmissions (so that a guaranteed bit rate is achieved). This means that for each period of K transmissions, a subset is reserved for new data blocks (however, not more than necessary). The capacity or bandwidth that remains is available for selected retransmissions.
A basic way of taking block age into account is by using an ACK/ NACK window that limits the number of previous blocks that may be considered for retransmission. The ACK/NACK window is slid in accordance with the number of new data blocks transmitted (primary transmissions) . Old blocks that thereby fall outside the window are discarded with respect to further retransmissions, meaning that retransmissions of these blocks are stopped.
Subsequently, each data block in the window is assigned a ranking based on the following factors:
ACK/NACK reports from all users for that particular data block. Blocks that have not yet been reported by all users may be considered for retransmission if there is spare transmission time after considering blocks with complete reports;
Current user scores or rankings (based on e.g. ACK/NACK history for each user);
Optionally, the age of the data block (or block number).
The ACK/NACK reports from each user indicate to what extent retransmissions of a certain data block is requested, whereas the scores or rankings indicate which users are already close to a good quality and also indicate the radio link quality for each user.
The scoring or ranking strategy can, according an embodiment of the present invention, be represented by a predetermined function, hereinafter referred to as a score function, and calculated for each block. The decisions on which blocks to retransmit are made based on ranking blocks with respect to at least the current scores, and how optionally on how many retransmissions that are allowed for the next retransmission period (i.e. number of retransmissions during a period of K transmissions) .
The choice of score function determines which quality requirements will be best met. As exemplary embodiments of the present invention, three different score functions or strategies will be described below: Basically, the three different embodiments according to the invention can be referred to and summarized as follows:
Fair. Retransmit most requested blocks; Throughput: Aims at maximizing the expected number of users to successfully receive a requested block (factors: number of users requesting a block, link quality of user) and thereby gives priority to users experiencing better radio conditions;
User quality: Aims at increasing the fraction of satisfied users (FSU) by retransmitting primarily to selected users. Also, this strategy gives priority to users experiencing better radio conditions.
An exemplary first score function is defined as:
where b is the data block number, u is the user and /„ (Jb) equals 1 if block b is reported as NACK (Negative ACKnowledgement) by user u. (Consequently, a reported ACK (ACKnowledgment) results in a score function value = 0).
This so called Fair-strategy in some aspect assumes that a frequently requested block is caused by some error at the transmitting node, thereby enabling frequently requested blocks to be retransmitted
The second exemplary score function is defined by:
where P1(U)Is an estimate of the block error probability (BLEP) for user u. BLEP can, e.g., be estimated by the recursion
P1(U) = Q- pcrr(u;M) + (l -ά) - p\ (u) where p\ (u) is the previous estimate used (initially set to 0), and a is between 0 and 1. The quantity pen.(u;M) = Norr(u;M)lM is the fraction of NACKs in the last report of M blocks and Nerr (M) is the number of errors for the last report.
Finally, an exemplary third score function is defined by:
where p («) is an estimate of the residual block error probability (BLEP) for user u (0/0 should be interpreted as 0). It is here estimated by the recursion
Pw {u) = a- perr (u; W) + (1 - α) • p' (u) where p' («) is the previous estimate used, and a is between 0 and 1. The quantity perr(u;W) = N err{u;W) I W is the fraction of NACKs for user u in the current window of size W. Note that there may be better functions of the ACK/ NACK history to estimate the residual BLEP.
A retransmission strategy that employs e.g. score function 1 will in the following be called strategy 1. Strategies 2 and 3 are defined correspondingly.
Although time is omitted in the score functions, they are (at least inherently) functions of the time since the ACK/ NACK window changes between retransmission decisions.
Ranking of all current data blocks in the window is done with respect to the value of the score function and optionally the age of the block. For the described strategies, the age of a block (age=number of last transmitted block minus block number of considered block) is only used to rank among blocks with equal scores so that newer blocks are prioritized ("last in, first served"). However, it is equally possible to use the age of the block as an integral part of the actual score function. It is purely a mathematical action to include age of a block into the score function.
Strategies 2 and 3 both give a higher score to users with low BLEP values, residual or not.
The strategies described can be modified by the way the block age is used when ranking blocks for retransmission. This can be important for certain real-time broadcast transmissions.
An additional factor to consider when ranking blocks for retransmission is the time spent in the broadcast session by the user requesting a specific block.
Further examples visualizing the effect of applying the previously described different score functions to a system will be discussed in relation to Fig. 3 to Fig. 6.
For the exemplary embodiments, the following assumptions are made:
The media i.e. data blocks are sent with constant bit rate. Data blocks are numbered in ascending order as they are transmitted the first time. If a block is retransmitted at a later time, it keeps the original number. (Alternatively, it could re-numbered so that it is considered as a more recent block.)
ACK/ NACK reporting is scheduled cyclically among n users so that each user is scheduled each Mth transmitted block (alternatively: each M:th time step). The number of reporting users (n) may vary in time but must be less than or equal to M. A special case is that M equals the current number of users and is changed whenever n changes. Each user reports the M least recent receptions among the received but not yet reported blocks (ACK for a correctly received block, NACK for an incorrectly received block).
The transmitting node has a retransmission window of size W. The window is sliding as time is stepped, meaning that at time t the window covers ACK/ ANCK reports for the W most recent blocks, i.e. those from time t-x, where x>= W+l, up to and including time t. Time is here assumed to be measured in units of number of transmission time intervals (TTIs).
Block error rates for the different users are assumed to be uniformly distributed within the given interval, unless otherwise stated. Below referred to as the simplified radio link model.
There are no gains from combination of different (re-)transmission attempt, i.e., no so called incremental redundancy has been considered.
The calculations for a simplified system scenario with three users are shown in Table 1 below. Table 1 shows results for an RLC ACK/ NACK window of size W = 9 (more typically W= 128-256) at some arbitrary time t. In this case, the BLER and residual BLER estimates used for strategy 2 and 3 respectively are for simplicity both set to the fraction of NACKs in the present window. The 3 blocks with largest scores (and least block age) are marked for each strategy. Although the calculations are performed for a simplified system scenario with only three users, the same strategy can be implemented for any number of users.
Figure imgf000017_0001
Table 1: Calculations for a simplified system scenario with three users and three alternative strategies
The effects of different retransmission strategies, according to the invention, in simulations (with a simplified radio link model) with corresponding calculations are shown in Figure 3 (16 users with different link quality) and Figure 4 (16 users with equal link quality). The simulations comprise 30000 data blocks transmitted (broadcast or multicast) during 48000 TTIs. The primary bitrate is supported if 10 out of 16 TTIs are spent on primary transmissions. The capacity available for retransmissions in these examples is 6 out of 16 TTIs (K=M=lβ and W=256).
As a comparison, a few reference strategies are added to the diagrams, in addition to the above described three exemplary strategies. One reference strategy is the so called "Retransmission prio" where each data block is retransmitted until all users have received it correctly. In this case, the ACK/ NACK window may stall the flow of primary transmissions since the window may not be moved until the block at the back end has been acknowledged by all users. In this case, data blocks that are not sent during the simulation are counted as lost and included in the residual BLER statistics. The high residual BLER should in this case be interpreted as a failure to reach the required primary bit rate. E.g., with an average residual BLER of 22%, only 78% of the required primary bit rate has been reached. See also the analysis of Figure 5 and 6 below.
Another reference strategy is so called "Random", where the data blocks to retransmit are drawn randomly among requested retransmissions within the current window. The window is advanced by 10 data blocks per period of 16 TTIs for this and the other strategies.
The Figures show that the retransmission strategy affects the BLER distribution. For strategy 3, the distribution is tilted so that the number of users with very low BLER is increased.
Strategy 1 yields least quality dispersion among users. Strategy 3 yields a quality distribution that is between strategy 1 and 3 with respect to dispersion when there is dispersion of link quality to different users (Figure
2). However, when all links are equal, strategy 2 yields results close to strategy 1 (Figure 3).
The important difference between strategy 2 and 3 is that strategy 2 estimates and schedules retransmissions with respect to link quality.
Strategy 3 in addition takes into account which users have already received much service (they are prioritized so that they stay satisfied) .
The core of the invention is to use specific score functions for ranking, and in particular use a score function that aims at maximizing the fraction of satisfied users. More generally, the score functions correspond to different objectives of broadcast/ multicast retransmissions.
Figures 5 and 6 show the fraction of satisfied users (i.e. max 1% residual BLER) for different numbers of users and for different strategies. The simulations comprise 10000 blocks offered over a time of 16000 TTIs. Without any limitation of retransmissions, the user quality breaks down (for all users) when the load (number of simultaneous users) becomes high enough. Limiting the number of retransmissions maintains the quality for more users. The choice of strategy becomes increasingly important as more users are present.
In these examples, strategy 3 is always best for variable link quality, see
Figure 5. For fixed link quality, strategy 1 yields a higher FSU at a specific number of present users, see Figure 6. However, strategy 3 generally yields the highest FSU when resources get very scarce.
The analysis and simulation results indicate that:
-It is necessary to limit retransmissions in order to guarantee primary transmissions;
-The choice of retransmission strategy affects user quality when the number of users is so large that retransmissions must be limited; -A score based strategy, utilizing the ACK/ NACK history for each user, is clearly advantageous.
Some advantages of the invention include:
The proposed solution to regulate the number of retransmissions when resources become scarce and to employ a "score function" (together with information on the age of a given data block) will solve the basic and initial problems of wireless multicast transmission:
-How to keep the number of retransmissions at a reasonable level? -How to decide which data units (e.g., RLC blocks) should be retransmitted (if retransmissions need to be limited)?
-How to prioritize between re-transmission requests from different users (to maintain user quality) (if retransmissions need to be limited)?
In other words, with re-transmission strategies as described according to the invention, it is possible to provide required bit rates with better "guarantees"/ higher probability. The bit rate required for "primary transmissions" can be guaranteed by (explicitly or implicitly) limiting the number of retransmissions. (However, it is not guaranteed that the primary transmissions are correctly received by a large enough fraction of users.)
Figure 7 illustrates an exemplary embodiment of a system according to the invention. Accordingly, the system comprises a transmitting node 10 configured for broadcasting or multicasting data blocks to n users Ul... Un. The users respond to the transmitted data blocks by sending ACK/ NACK reports ACK/ NACK Ul... ACK/ NACK Un indicating correctly received or incorrectly received data blocks. Further, the system comprises an evaluation unit 11 for jointly evaluating the received ACK/ NACK reports from all users, a ranking unit for ranking or prioritizing among the data blocks based on the received reports. Finally, the transmitting node 10 comprises a retransmission unit 13 for retransmitting data blocks based on a resulting ranking from the ranking unit 13.
Even though the various units 11, 12, 13 are depicted as included in the transmitting node 10, it is implied that the actual units can be located elsewhere in the system and only supply the necessary information for actuating the1 selective retransmissions in the transmitting node 10.
Although embodiments of the present invention are described as implemented to handle retransmissions to users within one network cell, it is equally possible, with appropriate modifications, to use for retransmissions to users in multiple cells. One possibility could be to perform the method of the invention in multiple cells, one at a time. Another possibility could be to exploit the possibility of macrodiversity by having some functionality which coordinates the ranking of data blocks for different cells, i.e. by identifying data blocks suitable for macrodiversity and coordinating retransmissions of theses blocks in suitable cells (together providing macrodiversity). As an example, if the data block is requested for retransmission from multiple users distributed over a plurality of cells.
In summary, the invention comprises the following advantages: Enabling more efficient retransmission strategies
Enabling providing a minimum guaranteed bitrate for transmissions
Reducing total number of retransmissions
It will be understood by those skilled in the art that various modifications and changes may be made to the present invention without departure from the scope thereof, which is defined by the appended claims.
REFERENCES
[1] 3GPP TS 43.246, Multimedia Broadcast Multicast Service (MBMS) in the GERAN, V6.5.0 (2005-09).

Claims

1. A method of retransmission in a wireless communication system, said system comprising a transmitting node communicating a plurality of data blocks to multiple user nodes on a common radio channel, and said transmitting node receiving reports of successful/ unsuccessful reception of said data blocks from each of said multiple users, characterized by: jointly evaluating the reception results from said multiple users based on said reports; ranking said data blocks for retransmission based at least on said joint evaluation, and said transmitting node selectively retransmitting data blocks to said multiple users based at least on said ranking to limit the bandwidth used for retransmissions.
2. The method according to claim 1, characterized by ranking said data blocks based on a predetermined function.
3. The method according to claim 1, characterized by ranking said data blocks based on at least one of block error probability for a user, residual block error probability for a user, fraction of negative acknowledgements in for a user.
4. The method according to claim 2, characterized in that said predetermined function is represented by: Φ) = ∑Iu(b)
where b is the data block number, u is the user and Iu (b) equals 1 if block b is reported as NACK by user u.
5. The method according to claim 2, characterized in that said predetermined function is represented by:
■Ϊ20) = ∑/β0) - (l - A(«)) where px (u) is an estimate of the block error probability for user u.
6. The method according to claim 5, characterized in that the block error probability is estimated by the recursion px(u) = a - perr(u;M) + (l-a) - p\ (u) where p\ (u) is the previous estimate used (initially set to 0), and a is a value between 0 and 1. The quantity perr(u;M) = Nerr(u;M)/M is the fraction of NACKs in the last report of M blocks and N^rr (M) is the number of errors for the last report.
7. The method according to claim 2, characterized in that said predetermined function is represented by: s3(b) = ∑Iu(b)lpΛu)
where p (u) is an estimate of the residual block error probability for user u.
8. The method according to claim 7, characterized in that the residual block error probability for user u p(u) is estimated by the recursion P (u) = a • Pen (U>'W) + (1 ~ a) ' P' (M) where p' (u) is the previous estimate used, and a is between 0 and 1. The quantity perr(u;W) = Nerr(u;W)IW is the fraction of NACKs for user u in the current window of size W.
9. The method according to any of claims 2-8, characterized by additionally ranking said data blocks based on the age of the data blocks.
10. The method according to any of claims 2-9, characterized by additionally ranking said data blocks based on user priority.
11. The method according to claim 1, characterized by the further step of allocating at least a predetermined minimum fraction of the available bandwidth on said common radio channel to transmissions and any remaining fraction to the selective retransmissions.
12. A wireless communication system, said system comprising a transmitting node (10) configured for communicating a plurality of data blocks to multiple user nodes (Ul, ..., Un) on a common radio channel, and said transmitting node (10) is further configured for receiving reports of successful/unsuccessful reception of said data blocks from each of said multiple users (U 1,...,Un), characterized by: means (11) for jointly evaluating the reception results from said multiple users based on said reports; means (12) for ranking said data blocks for retransmission based at least on said joint evaluation, and means (13) for selectively retransmitting data blocks to said multiple users based at least on said ranking to limit the bandwidth used for retransmissions .
13. The system according to claim 12, characterized in that said ranking means (12) are adapted for ranking said data blocks based on a predetermined function.
14. The method according to claim 12, characterized in that said ranking means (12) are adapted for ranking said data blocks based on at least one of block error probability for a user, residual block error probability for a user, fraction of negative acknowledgements in for a user.
15. The system according to claim 13, characterized in that said predetermined function is represented by:
u where b is the data block number, u is the user and I11 (b) equals 1 if block b is reported as NACK by user u.
16. The system according to claim 13, characterized in. that said predetermined function is represented by:
Figure imgf000026_0001
where px(u)is an estimate of the block error probability for user u.
17. The system according to claim 16, characterized in. that the block error probability for user u is estimated by the recursion pl(u) = a-perr(u;M) + (l-a) - p\ (u) where p\ (u) is the previous estimate used (initially set to 0), and a is a value between 0 and 1. The quantity perr{u;M) = Nen(u;M)lM is the fraction of NACKs in the last report of M blocks and Nerr (M) is the number of errors for the last report.
18. The system according to claim 13, characterized in that said predetermined function is represented by:
where p (u) is an estimate of the residual block error probability for user u.
19. The system according to claim 18, characterized in that the residual block error probability p(u) *s estimated by the recursion A0 («) = α • P err (u > W) + (I - a) • p1 a (u) where p' («) is the previous estimate used, and a is between 0 and 1. The quantity perr(u;W) = Nen(u;W)IW is the fraction of NACKs for user u in the current window of size W.
20. The system according to any of claims 13-19, characterized in that said ranking means are further adapted for additionally ranking said data blocks based on the age of the data blocks.
21. The method according to any of claims 13-20, characterized in that said ranking means are further adapted for additionally ranking said data blocks based on user priority.
22. The system according to claim 12, characterized by further means for allocating at least a predetermined minimum fraction of the available bandwidth on said common radio channel to transmissions and any remaining fraction to the selective retransmissions.
23. A node in a wireless communication system, said node is adapted for communicating a plurality of data blocks to multiple user nodes on a common channel, and said node is further adapted to receive reports of successful/unsuccessful reception of said data blocks from each of said multiple users, characterized by: means (11) for jointly evaluating the reception results from said multiple users based on said reports; means (12) for ranking said data blocks for retransmission based at least on said joint evaluation, and means (13) for selectively retransmitting data blocks to said multiple users based at least on said ranking to limit the bandwidth used for retransmissions. .
24. The node according to claim 15, characterized by further means for allocating at least a predetermined minimum fraction of the available bandwidth on said common radio channel to transmissions and any remaining fraction to the selective retransmissions.
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