EP2057773A1

EP2057773A1 - Indicating retransmission processes in multi-beam systems

Info

Publication number: EP2057773A1
Application number: EP07826237A
Authority: EP
Inventors: Matthew P. J. Baker; Timothy J. Moulsley
Original assignee: Koninklijke Philips Electronics NV
Current assignee: Koninklijke Philips NV
Priority date: 2006-08-21
Filing date: 2007-09-03
Publication date: 2009-05-13
Also published as: WO2008125923A1; RU2009110149A

Abstract

The invention provides an efficient signaling mechanism with a predetermined mapping relationship between retransmission processes and transmission beams for indicating the identities of retransmission processes being transmitted on each beam. Another aspect of the invention provides a mode of transmission with improved performance for frames in which only a single retransmission process has data available for transmission in a multi-beam transmission system, by making a suitable selection of the available beams, or using a decorrelated diversity mode (such as a space-time or space-frequency block code).

Description

Indicating retransmission processes in multi-beam systems

FIELD OF THE INVENTION

The present invention relates to a method, computer program product, system, transmitter device and receiver device for indicating a retransmission process in a multi-beam transmission system.

BACKGROUND OF THE INVENTION

Wireless services have become more and more important. Likewise the demand for higher network capacity and performance has been increased. Several options like higher bandwidth, optimized modulation or even code-multiplex systems offer practically limited potential to increase the spectral efficiency. MIMO (Multiple Input Multiple Output) Systems utilize space-multiplex by using antenna arrays to enhance the efficiency in the used bandwidth.

MIMO systems use multiple inputs and multiple outputs from a single channel. These systems are defined by spatial diversity and spatial multiplexing. Spatial diversity is known as receiver (Rx) and transmitter (Tx) diversity. Signal copies are transferred from another antenna or received at more than one antenna. With spatial multiplexing the system is able to carry more than one spatial data stream over one frequency simultaneously.

MIMO was established in IEEE 802.1 In, 802.16-2004 and 802.16e as well as in 3^rd Generation Partnership Project (3GPP). Further standards which include MIMO are IEEE 802.20 and 802.22.

MIMO channels have multiple links and operate on the same frequency. The challenge of this technology is separation and equalization of all the signal paths. Spatial multiplexing can provide higher capacity but no better signal quality. Instead of improving signal quality, spatial multiplexing may decrease it. Spatial diversity improves the signal quality and achieves a higher signal-to-noise ratio at the receiver-side. Especially in extensive network areas, spatial multiplexing may be pushed to its limits. The larger the network environment is, the higher the signal strength has to be. In wireless broadband systems, the available time, frequency and spatial diversity can be exploited using space-time codes, space-frequency codes or a combination thereof. As an example, space-time block codes are known as a way of gaining diversity in systems with multiple antennas. A block of symbols is transformed and transmitted from one antenna and the same data with a different transformation is transmitted from another antenna. The concept can be generalized into the frequency domain as space- frequency block codes or can be extended to cover both time and frequency. For the known two transmission antenna Alamouti scheme as described for example in S. M. Alamouti, "A simple transmitter diversity scheme for wireless communications", IEEE J. Select. Areas Commun., vol. 16, no. 8, pp. 1451-1458, Oct. 1998, and a single receiver antenna, the received SNR becomes ((hi)² + (Ii2)²)/n². This means that (at least in principle) all the received power can be recovered.

Improved performance and coverage of a wireless communication system can be reached by spatial multiplexing in the near field and spatial diversity in the far field. Space-time codes additionally improve the performance and make spatial diversity useable. The signal copy is not only transmitted from another antenna but also at another time. This delayed transmission is called delayed diversity. Space-time codes combine spatial and temporal signal copies which are multiplexed in two data chains.

Fig. 3 shows a schematic block diagram of a conventional transmitter in a multi-beam transmission system. In this example, four HARQ processes are supplied to a data packet selection unit 20 and selectively switched to two beamformers 32, 34. Thus, data packets from any HARQ retransmission process can be selected for transmission on any beam and at any time.

In UTRA (Universal Mobile Telecommunications System Terrestrial Radio Access) Release 7 was established, which is based on WCDMA (Wideband Code Division Multiple Access) and uses Alamouti space-time in antenna constellations of 2x1 or 4x2. The release 7 includes a TDD (Time Division Duplex) mode and a FDD (Frequency Division Duplex) code. The latter mode uses the D-TxAA (Double Transmit Antenna Array) proposal which is based on the STTD (Space-Time Transmit Diversity) principle defined in Release 99. It can be seen as a twofold transmit diversity chain. Each chain can be rate-controlled depending on the channel feedback. D-TxAA is derived from an existing closed loop transmit diversity scheme (TxAA mode 1) where the mobile terminal signals to the network complex weights which should be applied to the signals from each of two transmit antennas. In D- TxAA, two different data streams are transmitted using orthogonal weight vectors, one weight vector being based on those transmitted from the mobile terminal, and the other vector being derived deterministically from the first.

For the operation of D-TxAA, the following features are assumed:

Orthogonal pilot channels are transmitted from each antenna of the respective base station device (i.e., Node B in UMTS terminology).

No dedicated (i.e. beamformed) pilots are available (assuming that the fractional dedicated physical channel (F-DPCH) is used, which does not carry pilot bits).

Feedback information (FBI) for the first beam is derived by the terminal device (i.e. user equipment (UE) in UMTS terminology) and transmitted to the base station device, indicating the desired beamforming vector.

The first beam is transmitted using a restricted code book of weight vectors (for example the codebook currently used for TxAA mode 1).

The identity of the antenna weight vector for the first beam is signaled to the UE on the High-Speed Shared Control Channel (HS-SCCH). - The second beam is transmitted using a deterministic phase vector which is orthonormal to the vector for the first beam.

Channel quality information (CQI) is signaled by the terminal device to the base station device, enabling the base station device to derive a different rate for each of the two beams. The transmissions on the two beams are comprised of separate codewords, each being protected by a dedicated cyclic redundancy code (CRC). This further implies that independent HARQ retransmission schemes (such as the hybrid automatic repeat request (HARQ) scheme) operate on the two beams.

The decoding time available for the terminal device to decode a packet is unchanged from previous releases of UMTS HSDPA (High-Speed Downlink Packet Access). This implies that the constraint on the minimum number of retransmission processes in existence applies independently per beam for D-TxAA. In HSDPA, five parallel retransmission processes are needed (eight are actually provided) in order to ensure full channel utilization during the decoding time of the terminal device. For D-TxAA, this implies that at least five processes are needed per beam, as the terminal device cannot be receiving data from one process on one beam simultaneously with decoding data from the same process from the other beam (assuming that the retransmission process identification (ID) can be considered to be an identifier of the soft buffer position to be used in the terminal device for combining the retransmissions). The existence of a large number of retransmission processes transmitted on two independent beams poses a problem of how the identity of the transmitted processes can be signaled efficiently in each sub-frame.

Full flexibility requires enough signaling bits to indicate the transmission of any of the retransmission processes or none on either of the beams. For example, for a total of N retransmission processes, where N is equal to the number of beams multiplied by the number of processes required on a single beam, full flexibility requires the ability to signal N+l values (one of N processes or none) for the first beam, and N values (one of N-I processes or none) for the second beam, i.e., a total of N(N+ 1) values. For five processes, this results in 110 values, requiring 7 signaling bits.

Further, the independence of the retransmissions on each beam results in the start and end of a retransmission sequence being unsynchronized between the two beams. A mechanism is therefore needed to determine which beam should be used for transmission when retransmissions are continuing on one beam only.

SUMMARY OF THE INVENTION

An object of the present invention is to provide transmission or signaling mechanism by means of which retransmission performance can be improved and signaling overhead for indicating the identity of retransmission processes transmitted on different beams can be reduced.

This object is achieved by a method as claimed in claim 1, a computer program product as claimed in claim 7, a multi-beam transmission system as claimed in claim 8, a transmitter device as claimed in claim 9, and a receiver device as claimed in claim 15. Accordingly, a predetermined mapping relationship is established between each retransmission process and a transmission beam (e.g., in sub-frames where the number of the retransmission processes simultaneously transmitted is equal to the number of simultaneously transmitted beams). This reduces the number of possible combinations which need to be signaled. When a data packet on only one of the beams is continuing to be retransmitted while the data stream on the other beam has ceased, it is desirable to maximize the SINR for the ongoing retransmissions. This may be achieved by either: mapping the ongoing retransmissions to the strongest beam regardless of the retransmission process identity; mapping the ongoing retransmissions to the first beam regardless of the retransmission process identity; or transmitting the ongoing retransmissions simultaneously on both beams, applying a decorrelation transformation to the transmissions from one beam to decorrelate the data on that beam from the data transmitted on the other beam.

The above mapping and processing steps can be implemented by providing respective mapping and/or processing units at the transmitter device.

The detecting unit of the receiver device may be arranged to detect data packets associated with at least one of said retransmission processes, wherein the retransmission control unit if the receiver device may be arranged to request a retransmission based on the signaled combination and an incorrect reception of at least one of said data packets.

The invention may be implemented using concrete hardware units, or alternative as a computer program product, e.g., embodied on a computer-readable medium or downloadable from network system, comprising code means for generating the steps of the above method when run on a computer device, e.g., provided at a respective transmitter or receiver device.

Further advantageous embodiments are defined in the dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the present invention will be described in greater detail based on embodiments with reference to the accompanying drawings in which:

Fig. 1 shows an exemplary packet transmission sequence according to a first embodiment; Fig. 2 shows an exemplary packet transmission sequence according to a second embodiment;

Fig. 3 shows a schematic block diagram of a transmitter in a conventionalmulti-beam transmission system; and

Fig. 4 shows a schematic block diagram of a transmitter according to an embodiment.

DESCRIPTION OF THE EMBODIMENT

The embodiments of the present invention will now be described in greater detail based on a HARQ retransmission process in a wireless network environment with multi-beam transmission, such as for example a UMTS network environment.

Automatic retransmission request (ARQ) is a system where a receiving station will send an acknowledgement (ACK) message to the sending station when a data packet has been successfully received. The success of the reception is usually by comparison to a checksum (CRC) sent with the data. In the event that the checksum calculated by the receiving station does not match the checksum included with the data packet, the receiving station will send a negative acknowledgment (NAK) to the sender and discard the erroneous data packet. This will cause the sending unit to retransmit the erroneous data packet. In HARQ systems, this scheme has been modified to minimize retransmission by either "chase combining" or "incremental redundancy (IR)".

In chase combining, if the terminal device (i.e., UE) detects that a data packet has been received in error, it will send a NAK to the sending base station device (i.e., Node B). Rather than discarding the erroneous packet, it will be stored. In the event the retransmitted packet is also received in error, the previous packet and the current packet will be combined in a attempt to recover from the data errors. Each time a packet is resent, the same coding scheme is used. Eventually the packet will either be received without error and the terminal device will recover from the errors, or the maximum number of resends will be reached and error recovery will be left to higher protocol levels. IR is similar to chase combining. However, retransmitted data is coded using additional redundant information to improve the chances that the packet will be received either without errors or with enough errors removed to enable combing with previous packets to allow error correction.

According to the embodiments, reduced retransmission signaling overhead can be achieved by applying a predetermined relationship between HARQ process IDs and transmission beams.

In the following specific example, two HARQ retransmission processes are provided per transmission beam. For a two-beam case like the initially mentioned D-TxAA scheme, a total of four HARQ processes are therefore provided. A fully flexible approach would require 5x4 = 20 combinations to be able to be signaled on the HS-SCCH, requiring five signaling bits.

According to the first embodiment, each beam is allocated a predetermined subset of the available HARQ retransmission processes, for example mapping processes Pl and P2 to beam Bl and processes P3 and P4 to beam B2. Thus the allowed combinations are as follows:

Note that "0" is used here to designate "no process". Note that the case "0 0" does not need to have a specific signaling designation, as the absence of signaling would be used to indicate this state to the terminal device.

As the proposed scheme leads to a total number of eight combinations, only a reduced number of three bits is required for the signaling. In general, the number of signaling bits required to indicate the HARQ process IDs to the terminal device (e.g. using the HS- SCCH) is reduced from N²+N to (N/2)²+N.

Typically, the signal-to-interference-plus-noise ratio (SINR) of the first beam will be higher than the SINR of the second beam, as the first beam is formed using the optimally-derived and requested beamforming weight vector transmitted by the terminal device. Therefore, for sub-frames when no process is transmitted on beam Bl, the mapping of the other processes to the beams would be reversed, so that transmissions take place on beam Bl. This implies a modification to the signaling combinations, as shown for example below:

Note that in this case, the arrival of a packet on process Pl or P2 during retransmission of process P3 or P4 would necessarily cause further retransmissions of processes P3 or P4 to revert to beam B2.

For some larger numbers of processes, the number of signaling possibilities and the corresponding number of signaling bits required are shown below for a dual-beam system:

It can be seen from the above table that the proposed signaling scheme enables the number of signaling bits required to be reduced and/or the number of available processes to be increased. For example, the proposed signaling can support seven processes per beam with just six signaling bits, while the conventional method can only support five processes per beam with seven signaling bits. A first exemplary sequence of packet transmissions on the two beams according to the first embodiment is shown in Fig. 1 , where the numerals denote the HARQ process ID and the letters denote the packet ID on a given process.

In this example, the packet A on HARQ process P3 is retransmitted three times. The second retransmission, which occurs in sub-frame No. 5, takes place on beam Bl, as no packets are then being transmitted on beam Bl. The next retransmission, in sub-frame No. 7, reverts to beam B2, as a packet is transmitted from HARQ process Pl on beam Bl. The next packet on HARQ process P3, packet 3B, is also able to be transmitted on beam Bl, as there are no packets from processes Pl or P2 requiring to be transmitted in sub- frame No. 9.

In the second embodiment shown in Fig. 2, packets are transmitted on both beams in sub-frames when there is only a single codeword (HARQ process) available for transmission.

In the second embodiment, packet 3A in sub-frame No. 5 and packet 2C in sub-frame No. 8 are both transmitted on both beams simultaneously, as there is in each case only one HARQ process with data available for transmission.

In this case, with D-TxAA, it is necessary to employ a decorrelation transformation to the packet transmitted on one of the two beams (otherwise the deterministic function for deriving the second antenna weight vector in D-TxAA will cause the transmissions from the second antenna to cancel, resulting in a single omni-directional (or at least unmodified) antenna pattern from the first antenna). This decorrelation transformation may be: scrambling interleaving - a space time block code (STBC), or a space frequency block code (SFBC), such as the well-known Alamouti scheme, otherwise known as STTD (Space-Time Transmit Diversity) in UMTS.

The possibilities of using either selection of beam Bl or the decorrelated diversity mode can also be incorporated in the signaling scheme for indicating the HARQ process ID described above. For each of the N/2 cases when only one HARQ process is transmitted, two signaling values can be provided to indicate that the packet is either transmitted on beam Bl or is transmitted using a predetermined decorrelated diversity mode such as STTD applied to the two beams. The signaling values and numbers of bits required in this case are shown in the two following tables:

The number of signaling bits required then becomes (N/2) +2N, as shown in the table below.

Thus the proposed signaling scheme according to the second embodiment can support six processes per beam together with a choice of single-codeword mode with just six signaling bits, while the conventional method can only support five processes per beam with seven signaling bits and no choice of single-codeword mode. Fig. 4 shows a schematic block diagram of a transmitter according to an embodiment for simultaneously transmitting information via two beams. In this example, a mapping unit 10 is provided, which can handle four HARQ retransmission processes and which is adapted to define a predetermined mapping relationship between each retransmission process and each transmission beam. Furthermore, a signaling unit 50 is provided for indicating the retransmission process by signaling a respective combination derived from the mapping relationship. In the mapping unit 10, data from any HARQ retransmission process can be mapped to any beam generated by respective beamformers 32, 34 which control at least one of signal phases and signal amplitudes at respective antenna structures. This mapping relationship can be predetermined, while a control unit 40 can be adapted to decide whether to use a different mapping, e.g., to map a concerned retransmission process to the strongest beam if there is no transmission on other transmission beams. The control unit 40 may also control the selection of data packets to be transmitted on each beam. This can be achieved by a data packet selection unit 22 (e.g. with a multiplexing functionality) which is controlled by the control unit 40. The signaling unit 50 may then indicate which HARQ retransmission processes are associated with transmitted data packets.

As a further modification of the above embodiments, the multi-beam mode of transmission for a single packet can be used when the signal-to-noise ratio becomes too low for the system to support a satisfactory data rate on one or both beams independently. Thus this can be seen as a possible fallback mode for D-TxAA when the number of supportable layers drops. The conventional assumption for the fallback mode of D-TxAA is closed- loop TxAA mode 1 , but this does not necessarily capture the maximum signal energy from all paths in the channel, as one beam would carry no signal.

It is noted that despite the above description of the embodiments in terms of "beams", the invention is equally applicable to antennas, virtual antennas, or other forms of spatial multiplexing layers.

Furthermore, it is noted that although the above description is largely related to the case of two simultaneously-transmitted beams, it is also applicable to larger numbers of transmitted beams, and to correspondingly larger numbers of simultaneously-transmitted retransmission processes and codewords.

For example, in case of three simultaneously-transmitted beams, a system operating in accordance with the invention might transmit a codeword of one retransmission process on two beams simultaneously using a space-time block code, while a codeword of a second retransmission process could be transmitted simultaneously on the third beam.

In summary, an efficient signaling mechanism has been described with a predetermined mapping relationship between retransmission processes and transmission beams, for indicating the identities of retransmission processes being transmitted on each beam. Another aspect of the invention provides a mode of transmission with improved performance for frames in which only a single retransmission process has data available for transmission in a multi-beam transmission system, by making a suitable selection of the available beams, or using a decorrelated diversity mode (such as a space-time or space-frequency block code).

It is to be noted that the present invention can be applied to any wireless communication multi-beam or multi-antenna system involving retransmission. Moreover, any kind of space-time coding, space-frequency coding or combined space-time-frequency coding could be used to explore the desired multi-site diversity effects. The above embodiments may thus vary within the scope of the attached claims.

Claims

CLAIMS:

1. A method of indicating a retransmission process in a multi-beam transmission system where retransmissions are allocated to at least two different transmission beams, said method comprising: a. defining a predetermined mapping relationship between each retransmission process and each transmission beam; and b. indicating said retransmission process by signaling a respective combination derived from said mapping relationship.

2. The method according to claim 1, further comprising ignoring said predetermined mapping relationship and mapping said retransmission process to the strongest beam of said at least two transmission beams, if there is no transmission on other transmission beams.

3. The method according to claim 1, further comprising ignoring said predetermined mapping relationship and mapping said retransmission process to a predetermined first transmission beam of said at least two transmission beams, if there is no transmission on other transmission beams.

4. The method according to claim 1, further comprising ignoring said predetermined mapping relationship and mapping said retransmission process to at least two transmission beams of said at least two transmission beams, if there is no transmission on other transmission beams.

5. The method according to claim 1, further comprising ignoring said predetermined mapping relationship and mapping said retransmission process to at least two transmission beams of said at least two transmission beams, if the transmission quality falls below a predetermined threshold.

6. The method according to claim 4 or 5, further comprising applying at least one of scrambling, interleaving, space-time coding and space-frequency coding to data packets associated with said retransmission process, if said retransmission process is mapped to at least two of said at least transmission beams.

7. A computer program product comprising code means for generating the steps of method claim 1 when run on a computer device.

8. A multi-beam transmission system, wherein information is simultaneously transmitted via a plurality of transmission beams, said system comprising: a. a mapping unit for defining a predetermined mapping relationship between each retransmission process and each transmission beam; b. a signaling unit for indicating a retransmission process by signaling a respective combination derived from said mapping relationship; and c. at least one receiver device for receiving said signaled combination and for controlling retransmission based on said signaled combination.

9. A transmitter device for simultaneously transmitting information via a plurality of beams, said transmitter comprising: a. a mapping unit for defining a predetermined mapping relationship between each retransmission process and each transmission beam; and b. a signaling unit for indicating said retransmission process by signaling a respective combination derived from said mapping relationship.

10. The transmitter device according to claim 9, wherein said mapping unit is configured to ignore said predetermined mapping relationship and to map said retransmission process to the strongest beam of said at least two transmission beams, if there is no transmission on other transmission beams.

11. The transmitter device according to claim 9, wherein said mapping unit is configured to ignore said predetermined mapping relationship and to map said retransmission process to a predetermined first transmission beam of said at least two transmission beams, if there is no transmission on other transmission beams.

12. The transmitter device according to claim 9, wherein said mapping unit is configured to ignore said predetermined mapping relationship and to map said retransmission process to at least two transmission beams of said at least two transmission beams, if there is no transmission on other transmission beams.

13. The transmitter device according to claim 9, wherein said mapping unit is configured to ignore said predetermined mapping relationship and to map said retransmission process to at least two transmission beams of said at least two transmission beams, if the transmission quality falls below a predetermined threshold.

14. The transmitter device according to claim 12 or 13, further comprising a processing unit for applying at least one of scrambling, interleaving, space-time coding and space-frequency coding to data packets transmitted associated with said retransmission process, if said retransmission process is mapped to at least two of said at least transmission beams.

15. A receiver device for receiving information via a plurality of beams, said receiver comprising: a. a detecting unit for detecting a signaled combination of a predetermined mapping relationship defined between each retransmission process and each of said plurality of beams; and b. a retransmission control unit for controlling retransmission based on said signaled combination.

16. The receiver device according to claim 15, wherein said detecting unit is arranged to detect data packets associated with at least one of said retransmission processes, and wherein said retransmission control unit is arranged to request a retransmission based on said signaled combination and an incorrect reception of at least one of said data packets.