Abstract:
The present invention relates to a method for communicating between a primary station and at least one secondary station, comprising configuring each secondary station to search at least one of a plurality of search spaces having a first structure, said first structure consisting of at least a first number of resource sets having a first size, where at least one resource set might be used to transmit a message to the considered secondary station, changing the search space structure to a second structure different from the first structure in response to a signalling message.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method for communicating in a network. More specifically, the present invention relates to communication between a primary station and a secondary station, in a telecommunication network, like a cellular telecommunication network (e.g. UMTS, GSM). 
     BACKGROUND OF THE INVENTION 
     In UMTS LTE the downlink control channel PDCCH (Physical Downlink Control Channel) carries information such as resource allocation for uplink or downlink transmission. A PDCCH message can use 1, 2, 4 or 8 Channel Control Elements (CCEs or resource elements)—referred to as CCE aggregation levels 1, 2, 4 or 8. 
     The UE does not know in advance the location in CCE space of messages intended for it. In principle a UE could attempt to blindly decode all the possible PDCCHs with different starting positions in the CCE space and thus receive any messages intended for that UE. However, if the CCE space is large the processing complexity is prohibitive. Therefore a more limited search is configured which consists of a number of search spaces. 
     A search space is a set of aggregated CCEs (with a certain aggregation level) within which a mobile station (or user equipment (UE) or secondary station) performs blind decoding of all PDCCH payloads possible for that aggregation level. Search spaces are defined per aggregation level; a secondary station thus can have up to four search spaces. For example, the search space of a UE for aggregation level 1 (referred to as 1-CCE) could consist of the CCEs indexed 3, 4, 5, 6, 7, 8, while its search space for aggregation level 8 could consist of the two resource sets of aggregated CCEs consisting of the CCEs indexed by 1, 2, . . . , 8 and 9, 10, . . . , 16, respectively. In this example, the UE thus performs six blind decodings for 1-CCEs and two blind decodings for 8-CCEs. 
     The LTE specification currently requires the UE to perform the following: 
     6 decoding attempts of 1-CCE aggregation 
     6 decoding attempts of 2-CCE aggregation 
     2 decoding attempts of 4-CCE aggregation 
     2 decoding attempts of 8-CCE aggregation 
     The larger aggregations are intended to be used for large messages, and/or small messages when a lower code rate is required, for example under bad channel conditions. However, restricting the search spaces to reduce processing complexity limits the availability of suitable aggregations for different conditions as conditions vary. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to propose a method of communication which alleviates the above mentioned problem. 
     It is another object of the invention to provide a method enabling the search space to be adapted to the situation, without causing more signalling or overhead. 
     To this end, in accordance with the invention, a method is proposed for communicating between a primary station and at least one secondary station, comprising 
     (a) configuring each secondary station to search at least one of a plurality of search spaces having a first structure, said first structure consisting of at least a first number of resource sets having a first size, where at least one resource set might be used to transmit a message to the considered secondary station, 
     (b) changing the search space structure to a second structure different from the first structure. 
     As a consequence, the structure of the search space may be changed in accordance with a particular situation. For instance in case of a change in the transmission characteristics, like the channel conditions, it is possible to change the search space. The channel conditions may change because of new interference sources or because of mobility of a secondary station approaching the cell edge. 
     The present invention also relates to a primary station comprising means for communicating with at least one secondary station, comprising configuring means for configuring each secondary station to search at least one of a plurality of search spaces having a first structure, said first structure consisting of at least a first number of resource sets having a first size, where at least one resource set might be used to transmit a message to the considered secondary station, and wherein the configuring means are arranged for changing the search space structure to a second structure different from the first structure. 
     In accordance with still another aspect of the invention, a secondary station is proposed, said secondary station comprising means for communicating with a primary station, said secondary station comprising control means for searching at least one of a plurality of search spaces having a first structure, said first structure consisting of at least a first number of resource sets having a first size, where at least one resource set might be used to transmit a message to the considered secondary station, wherein the control means are arranged for changing the search space structure to a second structure different from the first structure in response to a indication from the primary station of a change in the search space structure. 
     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 drawing, wherein: 
         FIG. 1  is a block diagram of a system in accordance with the invention comprising a primary station and at least a secondary station. 
         FIG. 2  is a time chart representing the allocated search spaces in accordance an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention relates to a method for communicating in a network, like a cellular network. For instance, the network may be a UMTS network as depicted on  FIG. 1 . 
     Referring to  FIG. 1 , a radio communication system in accordance with the invention comprises a primary station (BS)  100  and a plurality of secondary stations (MS)  110 . The primary station  100  comprises a microcontroller (μC)  102 , transceiver means (Tx/Rx)  104  connected to antenna means  106 , power control means (PC)  107  for altering the transmitted power level, and connection means  108  for connection to the PSTN or other suitable network. Each MS  110  comprises a microcontroller (μC)  112 , transceiver means (Tx/Rx)  114  connected to antenna means  116 , and power control means (PC)  118  for altering the transmitted power level. Communication from primary station  100  to mobile station  110  takes place on a downlink channel, while communication from secondary station  110  to primary station  100  takes place on an uplink channel. 
     One of the downlink control channels received by the secondary stations is the PDDCH, where each secondary station has to blindly decode a plurality of sets of CCEs to find which set was allocated to it as set out in the preamble of the description. 
     In accordance with a first embodiment of the invention, it is assumed that 48 CCEs are available. This corresponds to the illustrative exemplary first embodiment of the invention. Various sets of 48 search spaces for the 1-CCEs have been considered; to each user to which a 1-CCE is to be sent, one of these 48 search spaces is assigned at random (the choice corresponds to the outcome of a hash function of that UE that we model as being uniform over the numbers 1, 2, . . . , 48). Each search space consists of six CCEs in this example. 
     The following sets of search spaces have been considered: 
     S — 1: all search spaces contiguous—i.e. of the form {i,i+1,i+2,i+3,i+4,i+5} with 0≦i≦47 where i is the CCE index, and all elements modulo 48. 
     S — 5: all search spaces of the form {i,i+5,i+10,i+15,i+20,i+25} with 0≦i≦47, and all elements modulo 48. 
     S — 7: all search spaces of the form {i,i+7,i+14,i+21,i+28,i+35} with 0≦i≦47, and all elements modulo 48. 
     S_d: all search spaces of the form {i,i+1,i+3,i+7,i+12,i+22} with 0≦i≦47, and all elements modulo 48. S_d is designed so that all search spaces overlap in just 1 CCE. 
     So, for example, the search space of S — 5 corresponding to i=25 consists of the CCEs indexed by 25, 30, 35, 40, 45, 2 (as 50 modulo 48 equals 2). 
       FIG. 2  illustrates the use of a pattern enabling the number of resource elements in common to be minimized, in accordance with the first embodiment, compared with the prior art. On  FIG. 2 , a set of available resources  200  are depicted. 
     In a conventional system, if only sets of 1-CCEs and 8-CCEs are considered, the search space for one secondary station or UE for 8-CCE messages (2 positions  208  are constructed from contiguous groups of CCEs) is depicted on  FIG. 2 . The positions  201  of 1-CCE messages (6 contiguous positions) are such that it is likely that all possible positions are blocked if another UE is receiving an 8-CCE message. 
     In accordance with the first embodiment of the invention, the set of available resources  300  comprises search space for one UE for 8-CCE messages  308 , as on  FIG. 2  where 2 positions are constructed from contiguous groups of CCEs. Regarding the search space for a UE for 1-CCE messages, 6 non-contiguous positions  301  are represented. These positions are non contiguous, so that they reduce overlap with higher aggregation-level search space and therefore increase likelihood that a position can be found to send a small message. 
     A fixed set of search spaces can be a good compromise for typical situations. However, in some cases, for example where a secondary station or a UE near the cell border experiences bad channel conditions over an extended period, it would be advantageous to modify the search spaces to allow more blind decodings for 4 and 8-CCEs, like in a first variant of the first embodiment of the invention, and fewer for 1-CCE. 
     For example: 
     2 decoding attempts of 1-CCE aggregation 
     2 decoding attempts of 2-CCE aggregation 
     6 decoding attempts of 4-CCE aggregation 
     6 decoding attempts of 8-CCE aggregation 
     This modification could be carried out using explicit RRC signalling. However, other solutions are of interest, for example with lower signalling overhead. 
     One aspect of the invention is to re-configure the search spaces implicitly, based on other changes in the UE mode or configuration. These changes could be indicated by RRC signalling or otherwise. As an example, a change of the transmission rate or the selection of a particular modulation scheme induces the selection of another search space in accordance with this variant of the invention. Indeed, in case the transmission rate is lowered, it implies that the transmission conditions have been deteriorated. Thus, this variant avoids to transmit an explicit message for changing the search space, the secondary station carries out the change from the modification of the transmission characteristic. For each applicable UE mode there is a default set of search spaces. 
     We assume, as an example, that the default search spaces are configured as follows: 
     6 decoding attempts of 1-CCE aggregation 
     6 decoding attempts of 2-CCE aggregation 
     2 decoding attempts of 4-CCE aggregation 
     2 decoding attempts of 8-CCE aggregation 
     In one embodiment the invention is applied in LTE to modify the PDCCH search spaces. If the UE reports channel quality less than a certain threshold, then the search spaces could be modified to: 
     2 decoding attempts of 1-CCE aggregation 
     2 decoding attempts of 2-CCE aggregation 
     6 decoding attempts of 4-CCE aggregation 
     6 decoding attempts of 8-CCE aggregation 
     In a further embodiment the invention is applied in LTE to modify the PDCCH search spaces when the UE is configured to use UE-specific reference symbols. The configuration of UE-specific reference symbols is likely to imply that UE-specific beamforming is being used, and UE-specific beamforming is most likely to be applied at the cell border with poor SINR. Also, if beamforming is applied to data, this would make it desirable that the PDCCH is made more robust, in order to match the extended coverage of the data transmission. As an example the following could apply when beamforming is used: 
     2 decoding attempts of 1-CCE aggregation 
     2 decoding attempts of 2-CCE aggregation 
     6 decoding attempts of 4-CCE aggregation 
     6 decoding attempts of 8-CCE aggregation 
     A refinement would be to limit the set of different message formats which are blindly decoded for each aggregation size, and apply different limits depending on the UE configuration. For example if MIMO is not supported in beamforming mode (or when UE-specific reference symbols are configured), then it would not be necessary to blindly decode message formats designed for MIMO operation. 
     A further embodiment could adapt the search space depending on UE capabilities—for example if a UE does not support MIMO (e.g. due to a limitation in the number of physical antennas), message formats designed for MIMO operation would be implicitly excluded from the search. The same would be true if a UE supporting MIMO was configured not to operate in MIMO mode (e.g. by network signalling). 
     As an example the following could apply for a UE which did not support MIMO: 
     8 decoding attempts of 1-CCE aggregation 
     6 decoding attempts of 2-CCE aggregation 
     1 decoding attempts of 4-CCE aggregation 
     1 decoding attempts of 8-CCE aggregation 
     In a further embodiment the UE may be configured to use semi-persistent scheduling. In this case the same resource allocation is understood to apply periodically. RRC signalling indicates the period. Some PDCCH messages may configure the resource. Such messages may be identified by one or more of:
         a different ID (CRNTI) from that used for normal messages.   Different scrambling from that used for normal messages   Different HARQ processes from those indicated by normal messages   Different Incremental Redundancy Versions from those indicated by normal messages   Different Modulation and Coding Scheme from those indicated by normal messages   An extra bit       

     In that case the following search spaces could be used for each of the two CRNTIs: 
     3 decoding attempts of 1-CCE aggregation 
     3 decoding attempts of 2-CCE aggregation 
     1 decoding attempts of 4-CCE aggregation 
     1 decoding attempts of 8-CCE aggregation 
     As a refinement the modified search space may only be applicable in certain subframes, for example, the subframes where a persistent resource allocation is applicable. 
     As a further refinement the search space could revert to the default one following a cell change (handover), or a further change of the transmission characteristic. 
     The invention may be applicable to mobile telecommunication systems like UMTS LTE and UMTS LTE-Advanced, but also in some variants to any communication system having allocation of resources to be done dynamically or at least semi persistently. 
     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. 
     The inclusion of reference signs in parentheses in the claims is intended to aid understanding and is not intended to be limiting. 
     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.