WO2009156929A2

WO2009156929A2 - Method for allocating transmission resources in a telecommunication system

Info

Publication number: WO2009156929A2
Application number: PCT/IB2009/052648
Authority: WO
Inventors: Matthew P.J. Baker; Timothy J. Moulsley
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2008-06-26
Filing date: 2009-06-19
Publication date: 2009-12-30
Also published as: WO2009156929A3

Abstract

The present invention provides a means of configuring channel quality indication CQI and acknowledgment ACK/NACK signalling corresponding to a plurality of cells without requiring simultaneous transmissions of the said signalling, while supporting continuous simultaneous data transmission from said plurality of cells.

Description

Method for allocating transmission resources in a telecommunication system

FIELD OF THE INVENTION

The present invention relates to a method for allocating transmission resources in a telecommunication system, a system of telecommunication using this method, a primary station and a secondary station using the same method.

This invention is, for example, relevant for Multiple-cell or multiple-carrier transmission systems such as dual-cell High-Speed Downlink Packet Access HSDPA for Universal Mobile Telecommunications System UMTS.

BACKGROUND OF THE INVENTION

A dual-carrier (otherwise known as dual-cell) packet communication system transmits data packets from a primary station to a secondary station using two different carrier frequencies. Data packets may be transmitted to the secondary station on both carrier frequencies simultaneously, or on each carrier frequency at different times.

Each data packet is carried on a data packet channel called High-Speed Downlink Shared Channel HS-DSCH on one or other carrier frequency.

Feedback signalling is transmitted by the secondary station, to provide acknowledgements ACK/NACK in response to the data packets (each data packet being acknowledged independently), and Channel Quality Indication (CQI), relating to the radio links on one or both of the carrier frequencies, to assist the primary station with scheduling and modulation/coding-scheme selection for the data packets.

Typically the ACK/NACK and CQI signalling are transmitted one after the other for a single carrier frequency.

For two carrier frequencies the ACK/NACKs would be transmitted at the same time. Also the CQIs for the two carrier frequencies would be transmitted at the same time.

A similar system is also known in which the two different carrier frequencies are used by two different primary stations.

SUMMARY OF THE INVENTION

The transmission of multiple simultaneous uplink signalling channels causes a high peak-to-average transmission power ratio for the secondary station (or alternatively a high cubic metric of the transmitted signal). This is undesirable for efficient power amplifier design. It also increases the peak transmission power, which limits the coverage of the communication system as the secondary stations move away from the primary station(s).

Therefore it is an object of the present invention to provide a mechanism for signalling multiple ACK/NACK and CQI indications without simultaneous transmission for multiple carriers or cells.

To this end, there is provided a method for allocating resources for transmissions from a secondary station to a primary station, comprising a step of of configuring channel quality indication (CQI) and acknowledgment (ACK/NACK) signalling corresponding to a plurality of cells without requiring simultaneous transmissions of the said signalling, while supporting continuous simultaneous data transmission from said plurality of cells.

The present invention also extends to a primary station and a secondary station for implementing such a method.

The invention is based on the recognition that CQI may not need to be transmitted for every cell for every data packet. In conventional systems, CQI is typically transmitted at every nth occasion for each cell.

According to the first aspect of the invention, a time offset is configured for the CQI reporting cycle for one cell relative to the CQI reporting cycle for another cell, so that the CQI reporting occasions of the two cells do not overlap. This is shown in Figure 2.

According to a second aspect of the invention, unused CQI reporting occasions may be reused for ACK/NACK signalling to enable ACK/NACK signalling for different cells to be transmitted sequentially rather than simultaneously. This is shown in Figure 3.

In general, the invention entails defining a relationship between the cell or carrier frequency used for the data transmission and the timing of the corresponding respective feedback (which may comprise ACK/NACK, CQI, Channel State Information CSI, channel rank, and precoding information).

In an embodiment of the invention, the use of the invention is configured and deconfigured independently for different secondary stations. In particular, the invention may be configured for secondary stations which are far from the primary station, and deconfigured for secondary stations which are close to the primary station.

In another embodiment, the signalling channels may be carried by different sub- carriers in the frequency domain.

In another embodiment, the different carrier frequencies for data transmission may instead be different scrambling codes. These and other aspects of the invention will be apparent from and will be elucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described in more detail, by way of example, with reference to the accompanying drawings, wherein:

Fig. 1 is a block diagram of a system comprising a primary station and a secondary in accordance with the invention;

Fig. 2 shows Offset CQI reporting patterns;

Fig. 3 shows delayed ACK/NACK transmission; and

Fig. 4 shows CQI reporting in conjunction with staggered ACK/NACK.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a system of communication 300 as depicted in Figure 1, comprising a primary station 100, like a base station, and at least one secondary station 200 like a mobile station.

The radio system 300 may comprise a plurality of the primary stations 100 and/or a plurality of secondary stations 200 (also noted UE for User Equipment). The primary station 100 comprises a transmitter means 110 and a receiving means 120. An output of the transmitter means 110 and an input of the receiving means 120 are coupled to an antenna 130 by a coupling means 140, which may be for example a circulator or a changeover switch. Coupled to the transmitter means 110 and receiving means 120 is a control means 150, which may be for example a processor. The secondary station 200 comprises a transmitter means 210 and a receiving means 220. An output of the transmitter means 210 and an input of the receiving means 220 are coupled to an antenna 230 by a coupling means 240, which may be for example a circulator or a changeover switch. Coupled to the transmitter means 210 and receiving means 220 is a control means 250, which may be for example a processor. Transmission from the primary radio station 100 to the secondary station 200 takes place on a first channel 160 and transmission from the secondary radio station 200 to the first radio station 100 takes place on a second channel 260.

The present invention provides a means of configuring CQI and ACK/NACK signalling corresponding to a plurality of cells without requiring simultaneous transmissions of the said signalling, while supporting continuous simultaneous data transmission from said plurality of cells.

A Work Item for dual-cell HSDPA operation was already approved. As the HS- DSCH from the two cells will be independent, the key challenge for this Work Item is the design of the control signalling. In this document we discuss the design of the HS-SCCH and High-Speed Dedicated Physical Control Channel HS-DPCCH to support dual-cell operation.

We assume fully independent scheduling between the two cells - unlike for the two beams in Rel-7 MIMO (for Multiple-Input and Multiple-Output) where if only transport block is transmitted it is always transmitted on the primary beam. For dual-cell operation, a single transport block may be transmitted on either carrier.

Moreover, it is assumed that in case of simultaneous scheduling of both cells to a User Equipment UE, the number of channelisation codes and modulation scheme in the two cells would also be independent in order to maximise scheduling flexibility.

Consequently, there is less opportunity to save HS-SCCH signalling bits with dual- cell operation compared to with MIMO. We show below the comparison between MIMO and dual-cell operation, indicating number of bits required for 2 HS-SCCHs and for a single combined HS-SCCH.

It can be seen that for dual-cell operation although the saving from a single combined HS-SCCH is reduced compared to the MIMO case, a 25% saving is still possible, arising from the common cyclic redundancy check CRC / UE identity ID, and the linkage of the Hybrid Automatic Repeat-Request HARQ process IDs. More importantly, as the downlink DL is generally code-limited not power- limited, the number of SF 128 codes used for HS- SCCH could be halved. This was a major motivation in selecting the single combined HS- SCCH for Rel-7 MIMO.

For dual-cell operation, the DL Orthogonal Variable Spreading Factor OVSF code space is more than doubled by the provision of a second carrier which does not have to carry all the channels which have to be carried in a normal cell. Therefore the need to avoid doubling the number of HS-SCCH codes is not so critical as it was in the MIMO case. If a single HS-SCCH approach were to be chosen, the HS-SCCH set for each UE could be semi- statically configured to be on in either one cell or the other. By carrying the HS-SCCHs for some UEs in one cell and for other UEs in the other cell, the total number of SF 16 codes available for data transmission could be maximised: if one cell would lose another SF 16 code for HS-DSCH transmission by allocating another HS-SCCH, the HS- SCCH could instead be carried by the other cell, thus optimising the usage of the code trees in the two cells. In particular, the anchor cell which has to provide other channels is likely to risk blocking SF 16 codes from HS-DSCH usage if too many HS-SCCH codes have to be provided.

This principle could be adopted even if independent HS-SCCHs are adopted: each HS-SCCH in the monitored set of a UE would be configured by Radio Resource Control RRC signalling to relate to one cell or the other. Alternatively, the monitored set size could be reduced by means of one bit in the HS-SCCH part 1 to indicate which cell it related to.

The solution selected for the HS-DPCCH depends on the solution selected for the HS- SCCH. If a single combined HS-SCCH is selected, the Rel-7 MIMO design of HS-DPCCH can be almost completely re-used for dual-cell operation, with the exception that the PCI bits are not needed. This would avoid concerns regarding cubic metric arising from the transmission of multiple codes.

If on the other hand independent HS-SCCHs are selected, the joint ACK/NACK (Acknowledgment / Non-acknowledgment) signalling of the MIMO HS-DPCCH does not allow the possibility that only one of the HS-SCCHs was detected. This suggests that two HS-DPCCHs should be used to allow fully independent ACK/NACK signalling.

However, a number of contributions have shown that the transmission of a second HS-DPCCH code channel has an adverse impact on cubic metric, which is undesirable for the cell-edge coverage which is expected for the increased bit-rate provided by dual-cell operation.

It is possible, however, to avoid simultaneous transmission of the two HS-DPCCHs as much as possible by using time-multiplexing.

In many cases Channel Quality Indication CQI does not need to be transmitted in every subframe. If a reporting periodicity greater than one subframe is configured, a sub frame offset could be applied to the CQI reporting pattern for one of the cells so that the CQI reports for the two cells never occur in the same sub frames. A trade-off exists here between maximising discontinuous transmission DTX for battery saving purposes and maximising coverage. Making the subframe offset configurable by RRC signalling on a UE- specific basis would allow a cell-edge UE to benefit from the reduced peak power, while UEs closer to the centre of the cell could transmit their CQIs simultaneously and benefit from longer DTX periods. This is illustrated in Figure 2.

A similar approach is possible for ACK/NACK transmission, if one of the HS- DPCCHs is offset in time by one slot. This would mean that the ACK/NACK for one cell would be transmitted with the usual timing approx 7.5 slots after the end of the HS-DSCH packet, while the ACK/NACK for the other cell would be transmitted one slot later, in the first slot of the CQI field of the first HS-DPCCH. As the HS-DSCH HARQ scheme is asynchronous in the downlink, this one-slot delay at the UE would not impact the maximum number of HARQ processes required. This is illustrated in Figure 3.

If simultaneous transmission of 2 HS-DPCCHs is to be completely avoided, the proposal in Figure 3 would mean that CQI could not be transmitted in either of the slots used for ACK/NACK in each subframe. However, one slot remains, and this could be used for CQI transmission alternating between the two cells. Thus again a trade-off is possible to enable cell-edge UEs to benefit from not having to transmit simultaneous HS-DPCCHs at the expense of a reduced update rate for CQI. This is shown in Figure 4.

We suggest that the remaining CQI slot shown in Figure 4 could be used for transmission of a 4-bit CQI word. This is similar to the dual-stream MIMO CQI reporting, which uses a reduced granularity to enable the CQI to be coded with an average of 4 bits. This generally gives very acceptable accuracy for scheduling purposes, as at l-2dB the granularity of the CQI reporting is not the limiting factor. This therefore would allow a 10,4 code to be used, which is a lower code-rate than is used for CQI/PCI reporting in Rel-7 MIMO.

Therefore, it can be noted that, in some embodiments of the invention for HS-SCCH:

• The need to avoid doubling the number of HS-SCCH codes is not so critical as it was in the MIMO case, but nevertheless a 25% saving in transmission power and a 50% saving in SF 128 code usage can be achieved by adopting a single HS-SCCH for the two cells.

• By carrying the HS-SCCHs for some UEs in one cell and for other UEs in the other cell, the total number of SF 16 codes available for data transmission in the two cells can be maximised. • Regardless of whether a single HS-SCCH or independent HS-SCCHs are adopted, each HS-SCCH in the monitored set of a UE should be able to relate to an HS-DSCH in the other cell, either semi-statically by RRC configuration of the HS-SCCH, or dymamically by means of one bit in the HS-SCCH part 1 to indicate which cell it relates to. for HS-DPCCH:

• It should be possible to offset the CQI reporting patterns for the two cells, so that simultaneous reporting of two CQI values is avoided (except in the case of CQI being required in every sub frame for both cells).

• The ACK/NACK field for one cell should be able to be delayed by one slot to avoid simultaneous ACK/NACK transmission for the two cells. In this case, CQI is coded using a 10,4 code and transmitted for each cell alternately. o This configuration could be UE-specific, for example for cell-edge UEs.

In the present specification and claims the word "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. Further, the word "comprising" does not exclude the presence of other elements or steps than those listed.

From reading the present disclosure, other modifications will be apparent to persons skilled in the art. Such modifications may involve other features which are already known in the art of radio communication and the art of transmitter power control and which may be used instead of or in addition to features already described herein.

Claims

1. A method for allocating resources for transmissions from a secondary station to a primary station, comprising a step of configuring channel quality indication (CQI) and acknowledgment (ACK/NACK) signalling corresponding to a plurality of cells without requiring simultaneous transmissions of the said signalling, while supporting continuous simultaneous data transmission from said plurality of cells.

2. A method as claimed in claim 1, wherein a time offset is configured for the channel quality indication (CQI) reporting cycle for one cell relative to the CQI reporting cycle for another cell, so that the CQI reporting occasions of the two cells do not overlap.

3. A method as claimed in claim 1, wherein unused channel quality indication (CQI) reporting occasions may be reused for acknowledgment (ACK/NACK) signalling to enable acknowledgment (ACK/NACK) signalling for different cells to be transmitted sequentially rather than simultaneously.

4. A method as claimed in claim 1, further comprising a step of defining a relationship between the cell or carrier frequency used for the data transmission and the timing of the corresponding respective feedback.

5. A method as claimed in claim 1, wherein signalling channels may be carried by different sub-carriers in the frequency domain.

6. A primary station comprising means for allocating resources for transmissions from a secondary station, and means for configuring channel quality indication (CQI) and acknowledgment (ACK/NACK) signalling corresponding to a plurality of cells without requiring simultaneous transmissions of the said signalling, while supporting continuous simultaneous data transmission from said plurality of cells.

7. A secondary station comprising means for transmitting resources in allocated resources for transmissions to a primary station, and means for configuring channel quality indication (CQI) and acknowledgment (ACK/NACK) signalling corresponding to a plurality of cells without requiring simultaneous transmissions of the said signalling, while supporting continuous simultaneous data transmission from said plurality of cells.