WO2007108751A2

WO2007108751A2 - Method of reducing feedback signalling using sic

Info

Publication number: WO2007108751A2
Application number: PCT/SE2007/000272
Authority: WO
Inventors: Per Johan Torsner; Shyam Chakraborty; Bo Göransson; Stefan Parkvall
Original assignee: Telefonaktiebolaget L M Ericsson (Publ)
Priority date: 2006-03-20
Filing date: 2007-03-20
Publication date: 2007-09-27
Also published as: WO2007108751A3

Abstract

The present invention relates to a method and arrangements for reducing signalling in a communications system by reducing feedback and/or forward transmission signalling. The system comprises receivers and transmitters for transmission of signal streams. Each stream is encoded. The receiver indicates an identity of the latest correctly decoded uniquely encoded stream.

Description

Method and Arrangement in a Telecommunication System

FIELD OF THE INVENTION

The present invention relates to methods and arrangements in cellular mobile communication systems, in particular to achieve a limited Multiple-Input Multiple-Output (MIMO) feedback.

BACKGROUND OF THE INVENTION

In Long Term Evolution (LTE) a hybrid Automatic Repeat-Request (HARQ) scheme is used between radio base station and user equipment (UE) to perform fast retransmissions in case of transmission errors. When MIMO is applied the transmission is divided into a number of transmission streams (e.g. 4 streams for 4x4 MIMO). The question arises how to integrate HARQ with MIMO.

When it comes to HARQ retransmissions, the obvious choices are to treat all MIMO streams as one transmission (one transport block) and in case of transmission failure of this transmission retransmit all streams. This mechanism requires only a single bit ACK/NACK HARQ feedback per transmission time interval (TTI) to indicate if the transmission was successful or not.

As an alternative the transmission can be divided into one transport block per MIMO stream. If a CRC is added for each stream, transmission failure can be detected for each stream individually. To be able to make retransmissions of individual streams one bit ACK/NACK HARQ feedback per stream is needed, i.e. 4 bits for 4x4 MIMO.

With the short frame lengths that are used for LTE any physical layer signaling is rather costly. The difference in feedback needed for these two approaches is therefore a relevant criteria when choosing between the two approaches. The drawback with handling all MIMO schemes as one transmission and always perform retransmissions on all streams is that this is inefficient since some of the streams may be received correctly and others have transmission errors.

Performing retransmissions of individual MIMO streams can however cause additional complexity in the transmitter, especially if the streams that are not retransmitted should be used to make new transmissions of other data. Some discussions therefore suggest to perform retransmissions of individual MIMO streams but not to insert new data on the successful streams. Instead these streams are left empty and the available power can be used only on the retransmitted streams. This scheme however requires an indication of which streams that are retransmitted and which are "empty" unless a blind detection can be used based on power estimation.

When successive interference cancellation is used, the streams are decoded one by one. When the first stream has been decoded, the interference caused by this stream can be accounted for in the decoding of the following stream. That implies that with equal coding/modulation etc for each stream the error probability decreases for each decoded stream. An alternative and more feasible choice is however to transmit an increasing amount of information in each stream e.g. so that the error probability for each stream is constant. In figure 1 four MIMO streams are shown with different information data rate for each stream. The height of each bar illustrates the information data rate. The streams may be explicitly ordered (e.g. the stream ID is indicated on a control channel) or the order can be implicitly derived from the data rate such that the streams are ordered in order of increasing data rate.

SUMMARY OF THE INVENTION

Solutions based on multiple antennas in both transmitter and receiver are considered for several wireless systems, e.g. WLAN, WiMAX, evolved WCDMA, and in the work currently ongoing in 3GPP on UTRAN long term evolution (LTE). The solutions denoted as Multiple Input Multiple Output (MIMO) is expected to significantly increase the peak rates achievable over the air interface. One potential drawback with some MIMO schemes is however the rather extensive signalling that is needed to feedback information about channel quality from the receiver to the transmitter and to indicate the transmission parameters for each transmission stream from the transmitter to the receiver to make decoding of the information possible. This invention aims at reducing the feedback needed for a MIMO scheme using separate encoding per stream and possibly a successive interference cancellation based receiver, and targets the interaction between MIMO and the HARQ scheme that is deployed between the transmitter and the receiver. The idea is focused on LTE but could in principle be applied in any wireless system

The solution according to the present invention aims at exploiting the properties caused by the successive interference cancellation to minimize the HARQ ACK/NACK feedback and minimizing the signalling needed in the forward direction by not indicating explicitly which streams that are retransmitted and instead using the fact that the receiver has information about which streams that where correctly received in the previous transmission attempt.

It is thus an advantage of the present invention to use the properties of successive interference cancellation and information available in HARQ to minimize the needed signalling for MIMO.

These and other advantages described below are obtained by means of a method of reducing signalling in a communications system by reducing feedback and/or forward transmission signalling. The system comprises receivers and transmitters for transmission of signal streams. The method comprises: encoding each stream uniquely, indicating by the receiver an identity of the latest correctly decoded uniquely encoded stream used for retransmission decision. The method may further use information about previously correct received stream. In one embodiment, a successive interference cancellation scheme is used for obtaining information about previously correct received stream.

Moreover, the streams may implicitly or explicitly be ordered in order of increasing information bit rate. The streams are decoded one by one in increasing stream order. If a certain stream, having a value, is not decoded, its interference is not accounted for in the decoding of the subsequent stream and the indication comprises an identity of a highest decoded stream. Then the feedback is encoded as a number of bits, n, such that 2ⁿ is equal to a maximum number of streams in the system and a transmitted feedback with the value means that all streams with an identity equal to or less the certain stream are retransmitted. Thus, the necessity of the retransmission is indicated by absence of feedback. The coding may be modified such that the absence of a Hybrid Automatic Repeat-request (HARQ) ACK or NACK feedback indicates that all streams are to be retransmitted.

The method may further comprise using a certain number of bits for encoding and for indicating retransmissions and using bit values together with the certain number of bits. In these cases, the signalling is realized using different code words, different modulation or any other unique signalling.

According to one embodiment a length of a HARQ ACK/NACK feedback is fixed to a maximum number of streams and using a coding depending on a number of configured or active streams.

According to yet another embodiment, depending on the number of configured or active streams, a number of bits are used for: indicating an ACK/NACK for each stream individually, or indicating that same value (ACK or NACK) are combined to increase the reliability of the signaling. Thus, the reliability is achieved through soft combination of messages.

The method may further comprise retransmission of individual streams whereby not retransmitted streams are empty. Thus, information about a HARQ reception in a previous transmission attempt can be used. Information may be received by a receiver, which sends HARQ NACKs for a stream in a previous transmission attempt, will be retransmitted and the remaining streams will be empty.

The method may be employed in at least one of MIMO, multiple transmit/receive systems such as Single-lnput-Multiple-Output (SIMO), or Multiple-Input- Single-Output (MISO).

The invention further relates to a communications network infrastructure arrangement in a communications network adopted for transmission of data streams from a number of transmitters to a number of receivers. The arrangement comprises a portion for encoding each data stream uniquely, means for obtaining an identity of the latest correctly decoded uniquely encoded stream, and means for executing a retransmission based on the obtained identity. The arrangement may be arranged to communicate with a number of receive/transmit antennas. Preferably, the arrangement is used in a MIMO system. The invention also relates to a receiver for reception of a number of uniquely encoded data streams in a wireless communication network. The receiver comprises means for controlling the received streams, means for deciding the quality of controlled streams and means for generation of an indication, an identity of the latest correctly decoded uniquely encoded stream in the controlling means.

DESCRIPTION OF THE DRAWINGS

In the following, the invention will be exemplified with reference to a number of embodiments, as illustrated in the drawings, in which:

Fig. 1 shows four MIMO streams with different information data rate for each stream,

Fig. 2 is a schematic MIMO system,

Fig. 3 is a block diagram illustrating an arrangement implementing the invention, and

Fig. 4 is a block diagram illustrating a user unit implementing the invention.

DESCRIPTION OF THE INVENTION

Fig. 2 illustrates an exemplary and simplified MIMO system 100 constructed by MTx transmit antennas 103 and N Rx receive antennas 104. The antenna spacing between the Tx antennas and Rx antennas in the MIMO system in Fig. 2 is generally big enough, to guarantee the spatial un-correlation of signals. As Fig. 2 shows, in the transmitter, MIMO architecture unit 101 first transforms a channel of data stream into M channels of parallel sub data streams; then, multiple access transform unit 102 performs multiplex processing; finally, the corresponding MTx antennas 103 transmit the signal simultaneously into the wireless channels. The MIMO architecture unit 101 can adopt any one of the MIMO processing methods, such as STTC (Space Time Trellis Code), space-time block code, space-time Turbo code, BLAST code and etc.

Hereinafter, it is assumed that the MIMO streams are implicitly or explicitly ordered in order of increasing information bit rate. When successive interference cancellation (SIC) is used, the streams are decoded one by one in increasing stream order. If a certain stream k can not be decoded, its interference can not be accounted for in the decoding of the subsequent stream k+1. The probability that a stream can be decoded when the decoding of any stream with lower order has failed is therefore low (assuming that the information bitrates are increasing to exploit the gains of successive interference cancellation). It is therefore feasible to design a HARQ ACK/NACK feedback that indicates the ID of the highest decoded stream instead of indicating an ACK/NACK feedback for each stream individually. It should be noted that the invention is not limited SIC receivers and the method of the invention can be applied as soon as a stream is decoded separately.

According to a first embodiment of the present invention the HARQ feedback is encoded as, e.g., a number of bits n such that 2ⁿ is equal to the maximum number of MIMO streams in the system. A transmitted HARQ feedback with the value k means that all MIMO streams with an identity equal to or less then k shall be retransmitted. For 4x4 MIMO this means that 2 bits can be used instead of 4 bits for the HARQ feedback and still exploit most of the gains retransmissions of individual streams.

The absence of HARQ ACK/NACK feedback, i.e. no feedback is transmitted, indicates that no retransmission is necessary.

Alternatively the coding may be modified so that the absence of HARQ ACK/NACK feedback indicates that all streams shall be retransmitted.

Here, by means of example, it is disclosed to use a certain number of bits (and their values) for indicating retransmissions. However, this signalling can be realized in many ways, e.g., as different code words, different modulation or any other unique signalling.

According to a second embodiment of the present invention, the length of the HARQ ACK/NACK feedback is fixed to the maximum number of MIMO streams but the coding used is depending on the number of currently configured or active MIMO streams. For instance, if a maximum of 4 MIMO streams are supported, the ACK/NACK feedback consists of 2 bits. When 4 MIMO streams are configured or active the 2 bits are coded as described in the first embodiment. When two MIMO streams are configured or active the two (2) bits are used to indicate the ACK/NACK for each MIMO streams individually. And when single stream transmission is used the 2 bits indicate the same value (ACK or NACK) and are combined to increase the reliability of the signalling, e.g. by means of soft combining of the two messages.

Generally, the maximum number of transport blocks in LTE may be 2. If 4x4 MIMO is used, then one transport block may be mapped to 2 layers (e.g. antennas). According to a third embodiment of the present invention it is assumed that retransmission of individual streams are performed and that the streams that are not retransmitted are empty, i.e. no power is transmitted on those streams.

Instead of indicating explicitly on a control channel which streams that are retransmitted the information about the HARQ reception in the previous transmission attempt is used. I.e. the receiver knows that the streams it sent HARQ NACKs for in the previous transmission attempt will be retransmitted and the other streams will be empty. This requires an interaction between HARQ and MIMO.

The invention may be implemented in a network node based on a MIMO system. The network node may comprise a computer unit 300 for processing signals, as illustrated very simply and schematically in Fig. 3. The network comprises receivers and transmitters (Fig.

2) for transmission of data streams. The computer unit 300 may comprise a unit 310 for detecting the transmitted data stream, encoding/decoding each stream uniquely, a processing unit 320 for processing the data stream and indicating the identity of correctly decoded encoded streams and memory means 330 for storing data application programs.

Fig. 4 illustrates in a schematic block diagram a user equipment (UE) 400 implementing teachings of the present invention, wherein a processing unit 420 handles communication data and communication control information. The UE 400 further comprises a volatile (e.g. RAM) 430 and/or non volatile memory (e.g. a hard disk or flash disk) 440, an interface unit 450. The UE 400 may further comprise a mobile communication unit 460 with a respective connecting interface. All units in the UE can communicate with each other directly or indirectly through the processing unit 470. Software for implementing the method according to the present invention may be executed within the UE 400. The UE 400 may also comprise an interface 480 for communicating with an identification unit, such as a SIM card, for uniquely identifying the UE in a network and for use in the identification of the 'SIGN' (i.e. traffic counting and digital signature of the UE). Other features often present in UE are not shown in Fig. 4 but should be understood by the person skilled in the art, e.g. for a mobile phone: MIMO antennas 410, camera, replaceable memory, screen and buttons. The computer unit according Fig. 3 may be implemented as an additional part or part of the processing unit. The invention is not limited to MIMO systems and may be implemented in any multiple transmit/receive systems such as Single-lnput-Multiple-Output (SIMO)₁ Multiple-lnput- Single-Output (MISO), etc.

The above mentioned and described embodiments are only given as examples and should not be limiting to the present invention. Other solutions, uses, objectives, and functions within the scope of the invention as claimed in the below described patent claims should be apparent for the person skilled in the art.

Claims

1. A method of reducing signalling in a communications system by reducing feedback and/or forward transmission signalling, said system comprising receivers and transmitters for transmission of signal streams, the method comprising: encoding each stream uniquely, and indicating by said receiver an identity of the latest correctly decoded uniquely encoded stream used for retransmission decision.

2 . The method of claim 1 , further using information about previously correct received stream.

3 . The method of claim 1, further comprising using a successive interference cancellation scheme for obtaining information about previously correct received stream.

4 . The method of claim 1 , wherein said streams are implicitly or explicitly ordered in order of increasing information bit rate.

5. The method of claim 4, wherein said streams are decoded one by one in increasing stream order.

6. The method of claim 5, wherein if a certain stream, having a value (k), is not be decoded, its interference is not accounted for in the decoding of the subsequent stream (k+1 ) and said indication comprises an identity of a highest decoded stream.

7 . The method of claim 5, wherein said feedback is encoded as a number of bits, n, such that 2ⁿ is equal to a maximum number of streams in the system and a transmitted feedback with said value (k) means that all streams with an identity equal to or less said certain stream (k) are retransmitted.

8 . The method of claim 7, wherein necessity of the retransmission is indicated by absence of feedback.

9. The method of claim 7, wherein said coding is modified such that the absence of a Hybrid Automatic Repeat-request (HARQ) ACK or NACK feedback indicates that all streams are to be retransmitted.

10 . The method of claim 1 , further comprising using a certain number of bits for encoding and for indicating retransmissions.

11. The method of claim 10, further comprising using bit values together with said certain number of bits.

12 . The method of claim 10 or 11, wherein signalling is realized using different code words, different modulation or any other unique signalling.

13. The method of claim 1, further comprising fixing a length of a HARQ ACK/NACK feedback to a maximum number of streams and using a coding depending on a number of configured or active streams.

14 . The method of claim 1 , wherein depending on the number of configured or active streams a number of bits are used for: • indicating an ACK/NACK for each stream individually, or

• indicating that same value (ACK or NACK) are combined to increase the reliability of the signaling.

15 . The method of claim 14, wherein said reliability is achieved through soft combination of messages.

16. The method of claim 1, further comprises retransmission of individual streams whereby not retransmitted streams are empty.

17 . The method of claim 16, further comprising using information about a HARQ reception in a previous transmission attempt.

18 . The method of claim 17, comprising obtaining information by a receiver, which sends HARQ NACKs for a stream in a previous transmission attempt, will be retransmitted and the remaining streams will be empty.

19. The method according to any of previous claims, wherein said communication network employees at least one of Multiple-Output Multiple-Input (MIMO), multiple transmit/receive systems such as Single-lnput-Multiple-Output (SIMO), or Multiple- Input- Single-Output (MISO).

2 O . A communications network infrastructure arrangement (300) in a communications network adopted for transmission of data streams from a number of transmitters to a number of receivers, characterized in that said arrangement comprises an portion (320) for encoding each data stream uniquely, means (320) for obtaining an identity of the latest correctly decoded uniquely encoded stream, and means (320) for executing a retransmission based on said obtained identity.

21. The arrangement of claim 19, being arranged to communicate with a number of receive/transmit antennas.

22 . The arrangement of claim 20, for use in one of a Multiple-Output Multiple-Input (MIMO), multiple transmit/receive systems such as Single-lnput-Multiple-Output (SIMO), or Multiple-Input- Single-Output (MISO) systems.

23 . A receiver (400) for reception of a number of uniquely encoded data streams in a wireless communication network, characterized in that said receiver comprising means (420) for controlling said received streams, means (420) for deciding the quality of controlled streams and means for generation of an indication, an identity of the latest correctly decoded uniquely encoded stream in said controlling means.