WO2007084065A2

WO2007084065A2 - Method and arangement for multiplexed feedback information using harq

Info

Publication number: WO2007084065A2
Application number: PCT/SE2007/050022
Authority: WO
Inventors: Stefan Parkvall; Erik Dahlman; Jung-Fu Cheng
Original assignee: Telefonaktiebolaget Lm Ericsson (Publ)
Priority date: 2006-01-18
Filing date: 2007-01-16
Publication date: 2007-07-26
Also published as: WO2007084065A3

Abstract

The present invention relates to methods and arrangements in cellular mobile communication systems for transmitting of multiple feedback information elements when using multiple HARQ-processes. Multiple feedback messages, e.g. positive or negative acknowledgements, are transmitted at a single time instant without any resorting for defining multiple structures for the feedback signaling and in such a way that the linkage between a certain feedback message and a certain hybrid ARQ process (and/or MIMO stream) remains unambiguous.

Description

a device for supporting an air supply front panel

FIELD OF THE INVENTION

The present invention relates to methods and arrangements in cellular mobile communication systems, in particular to methods and arrangements for transmitting of multiple feedback information elements .

BACKGROUND OF THE INVENTION

The use of hybrid ARQ with soft combining is regarded as one of the key features for telecommunication systems applying

High-Speed Downlink Packet Access (HSDPA) or WCDMA enhanced uplink (HSUPA) , as well as for upcoming systems such as the long-term evolution (LTE) systems currently in the process of standardisation at the 3rd Generation Partnership Project

(3GPP) .

For instance, in HSDPA a base station transmits a data packet to a user equipment, e.g. a mobile terminal, which attempts to decode this packet and provides a feedback in the form of a positive acknowledgement (ACK), i.e. the packet has been correctly decoded, or a negative acknowledgement (NAK), i.e. the decoding attempt has failed. If the base station receives a negative acknowledgement, the packet is retransmitted and the terminal then tries to combine the retransmitted data packet with the data packet of the original transmission attempt to make an attempt to decode this combination. Again, a status message informing the base station about the success or failure is sent and, if needed, subsequent retransmissions may take place. The same mechanism is used for WCDMA enhanced uplink (although the direction is reversed, i.e., the terminal transmits the packet and the base station performs the decoding) .

To keep the amount of feedback signaling, i.e. the amount of positive or negative acknowldegements , as low as possible, a scheme with multiple parallel stop-and-wait processes 15 is used in HSDPA. While one process is decoding the received data, more data can be transmitted to the other processes. This is illustrated in figure 1. A transmitting station 16 transmits a packet 10 to a receiving station 17 whereupon, after a predefined time 16 for receiver processing, the receiving station 17 transmits a single bit indicating a positive or negative acknowledgement back to the transmitting station 16. In case of a transmission failure 11 when transmitting the packet to the receiving station, the receiving station 17 returns a negative feedback message 12. In cases of no transmission failures 13, the receiving station 17 returns a positive feedback message 14. As the time relation 16 between the packet transmission and the corresponding feedback transmission is known, it is implicitly also known which hybrid ARQ process the acknowledgement relates to; there is thus no need to include a process number in the feedback message.

A similar scheme, where the timing relation between data transmission and feedback message is known and used to implicitly indicate the process the feedback message relates to, is envisioned for LTE for both uplink and downlink transmissions. Further, LTE-systems will support both frequency-division duplex (FDD) and time-division duplex (TDD) as illustrated in figure 2. In FDD, uplink 21a and downlink 21b transmissions are separated in frequency. Typically, uplink transmissions 21a can therefore take place regardless of whether there is a downlink transmission 21b or not. In TDD on the other hand, uplink 23a and downlink 23b transmissions are separated in time. Hence, uplink transmissions 23a cannot take place at the same time as downlink transmissions 23b (and vice versa) . Yet another alternative may relate to a combined FDD/TDD transmission 22.

LTE-systems will also support multi-stream transmissions, also known as MIMO (multiple-input, multiple-output). In MIMO, both the transmitter and the receiver are equipped with multiple antennas. By exploiting the properties of the channel in combination with the multiple antennas, several data streams can be transmitted in parallel to a single receiver, thereby significantly increasing the data rates. This is schematically illustrated in figure 3.

The feedback scheme described above assumes a fixed delay between the reception of the data transmission and the transmission of the acknowledgements . Such a scheme works well in FDD, where the feedback message (the ACK or NAK) can be transmitted at any time. In TDD, however, there may not be an uplink transmission slot available when the acknowledgement should be transmitted. The scheme can still be applied if the timing relation is redefined in such a way that the feedback message is transmitted at the first available uplink transmission slot after a certain time. As the division between uplink and downlink transmission slots is known and typically does not change at a fast rate, the data transmitter can then still relate the feedback messages to a given hybrid ARQ process .

Another alternative would be to include a hybrid ARQ process number in the feedback message. However, this increases the overhead of the feedback message. SUMMARY OF THE INVENTION

Regarding the prior art solution as presented above it has been observed that a problem may arise if there have been multiple transmissions in one direction before there is a possibility to send a feedback message in the other direction. Figure 4 illustrates an exampel where no uplink slot (UL) is available for transmitting the ACK- or NAK- feedback related to a first downlink transmission (DL) . Although the feedback transmission related to said first downlink transmission can be postponed to the first available uplink slot, this would however cause an ambiguity with regard to the downlink transmission that the ACK/NAK- feedback in the uplink slot relates to.

The present invention aims to solve the problem of transmitting multiple feedback messages, e.g. positive or negative acknowledgements, at a single time instant without any resorting for defining multiple structures for the feedback signaling. It is thus the basic idea of the present invention to transmit multiple feedback messages, e.g. positive or negative acknowledgements, at the same time in such a way that the linkage between a certain feedback message and a certain hybrid ARQ process (and/or MIMO stream) remains unambiguous.

The present invention implies the advantage that, if the number of ACK/NAKs to be transmitted at a certain time instant is smaller then the maximum number the system is designed to support, the reliability of each ACK/NAK is increased (which can be translated into a lower transmission power) . BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 illustrates a multiple hybrid ARQ-process.

Figure 2 illustrates schematically FDD and TDD transmission schemes .

Figure 3 illustrates schematically a MIMO-transmission scheme.

Figure 4 illustrates problems that can occur in case of multiple transmissions in one direction without possibility to send a feedback message in the reverse direction.

Figure 5 illustrates a time multiplexing with multiple ACK/NAK in a single time slot.

Figure 6 illustrates a code multiplexing of multiple ACK/NAK in a single time slot.

Figures 7a and 7b show two examples of schematic illustrations of a minimum distance, whereof figure 7a shows the case of a single ACK/NAK and figure 7b shows the case of two ACK/NAK.

Figure 8 illustrates a part of a telecommunication network within which the present invention can be applied.

DESCRIPTION OF THE INVENTION

Although the background and the invention is described in terms of ACK/NAK and multiple hybrid ARQ processes, this should be seen as an example application only. The invention is applicable to any problem requiring a varying amount of feedback information to be transmitted in a fixed amount of time.

Further, a similar problem as described in the scenario above can arise in MIMO schemes if each stream is acknowledged independently. The number of transmitted streams may depend on the rapidly varying channel conditions. Hence, either the feedback scheme can be designed for the worst case, e.g. always transmit n bits of

ACK/NAK even if only one stream is transmitted, or another MIMO scheme has to be chosen.

Yet another scenario where multiple ACK/NAKs are of interest is a packet-centric retransmission protocol. In this case, multiple packets transmitted during the same time interval need one hybrid ARQ process each, i.e., multiple stop-and- wait hybrid ARQ processes for a single time slot (as opposed to only one process per time slot for HSDPA) . Hence, multiple ACK/NAKs may need to be fed back at the same time instant.

According to a first embodiment of the present invention a series of multiple independent feedback messages, e.g. positive or negative acknowledgements, are transmitted in each feedback message. This is illustrated in figure 5. If the feedback message is received in a time slot n the first feedback relates to the data transmission (or hybrid ARQ process) in slot n-k, the second feedback relates to the data transmission (hybrid ARQ process) in slot (n-k-1) and so on. In this way, a feedback message that cannot be transmitted at the desired time instant, e.g. due to the lack of an uplink slot in a TDD system, can be transmitted at a later time instant. This implies that in cases where there is only a single ACK/NAK to be transmitted some of the bit fileds in the feedback message remain unused. In the example illustrated in figure 5, ACK/NAK3 53 and ACK/NAK4 54 both are unused but still occupy bandwidth in the feedback channel. Furthermore, the transmission power has to be set taking the short duration of each ACK/NAK into account, while the power amplifier is not fully used during the latter part of the feedback message, i.e. related to ACK/NAK3 and ACK/NAK4, in figure 5.

In an alternative solution, which is illustrated in figure 6, each feedback signal 61, e.g. positive or negative acknowledgements, is used to modulate an (orthogonal) code sequence 62, e.g. a Walsh code, such as to distinguish code sequences 63 related to a data transmission in slot (n-k) and code sequences 64 related to a data transmission in slot (n-k-1) . Each hybrid ARQ process that is to be acknowledged at the same time is assigned a specific code sequence. This implies the advantage of a more efficient power usage in the typical case that feedback information needs to be transmitted only for some previous slots. Also, given a constant transmission power for the feedback message, the probability of a misdetection of the different ACK/NAKs is smaller the smaller the number of ACK/NAKs that are to be multiplexed. This means, in terms of mutual distance between the possible signal points, that the distance is larger in the case of a single ACK/NAK (there are only two signal points, ACK or NAK) than in the case of multiple ACK/NAKs.

In a generalization of the scheme above, channel coding is used to encode all the ACK/NAKs that are to be transmitted at the same time. A family of flexible channel codes can be used to encode the messages . The exact code to use depends on the number of ACK/NAK messages to be transmitted. Note that there is no need to additionally signal which code is adopted in the feedback channel since the sender knows how many ACK/NAKs shall be fed back. Consequently, the sender knows which of the code words the channel coder may generate. For example, as illustrated in figures 7a and 7b, only two different code words are possible if a single ACK/NAK is to be fed back. In case of two ACK/NAKs to be fed back, four different code words are possible (they may or may not be a superset of the two-codeword case) . Preferably, the channel code is designed such that the minimum distance, d_mi_n, between the signal points is larger the smaller the number of simultaneous ACK/NAKs that are transmitted.

One non-limiting example of a good choice of code family is the expurgated 2^nd order Reed-Muller codes of length 64. For a single ACK/NAK bit, we have d_min=64, for 2-7 ACK/NAKs, d_min>32, and for 8-10 ACK/NAKs, d_min>16. With a soft decision decoding, the metric can serve a soft check on the reliability of the decoded results.

Claims

1. A method in a network node (81) of a telecommunication system comprising a plurality of nodes (82) equipped for multiplexed transmitting and receiving of data packets, said network node (81) receiving data packets from another node (82) in a multiplexed transmission with multiple subsequent transmissions of data streams in one direction,

c h a r a c t e r i s e d b y

multiplexing two or more feedback information elements for a number of corresponding hybrid ARQ-processes within one HARQ feedback message to be transmitted during one time slot.

2. The method according to claim 1, applying a time-based multiplexing by

assigning specific bit field positions within the HARQ feedback message to feedback information elements to be transmitted for a corresponding hybrid ARQ-process such that there is a fixed relation of said position and the slot number of the slot that the feedback information element refers to;

transmitting the multiplexed feedback information elements for the hybrid ARQ-processes within the same time slot.

3. The method according to claim 2, whereby subsequent bit fields of the multiplexed HARQ feedback message contain feedback information for subsequent time slots .

4. The method according to claim 1, applying a code-based multiplexing by assigning specific orthogonal code sequences to feedback information elements to be transmitted for a corresponding hybrid ARQ-process ;

modulating said assigned code sequences with the respective feedback information elements;

5. The method according to claim 4, whereby the code sequence is a Walsh-code.

6. The method according to claim 2 or 4 , further comprising the step of channel coding the feedback signals.

7. The method according to claim 6, whereby the channel code is designed to maximise the minimum distance between signal points representing the feedback signals.

8. The method according to claim 1, whereby the feedback information elements consist of positive and negative acknowledgements .

9. A network node (81) in a telecommunication system (80) comprising a plurality of nodes (82) equipped for multiplexed transmitting and receiving of data packets, said network node (81)

c h a r a c t e r i s e d i n

a multiplexer (82) for multiplexing two or more feedback information elements for a number of corresponding hybrid ARQ-processes within one HARQ feedback message to be transmitted during one time slot.

10. The network node (81) according to claim 9, wherein said multiplexer (82) includes means for assigning specific bit field positions within a time slot to feedback information elements to be transmitted for a corresponding hybrid ARQ-process such that there is a fixed relation of said position and the slot number of the slot that the feedback information element refers to;

means for transmitting the multiplexed feedback information elements for the hybrid ARQ-processes within the same time slot.

11. The network node (81) according to claim 9, wherein said multiplexer (82) includes

means for assigning specific orthogonal code sequences to feedback information elements to be transmitted for a corresponding hybrid ARQ-process;

means for modulating said assigned code sequences with the respective feedback information elements;