Abstract:
A system, method, and devices for communicating from a primary station to a plurality of secondary station are disclosed. The method includes the step of allocating a resource to the secondary stations over time on the basis of a hash function such that for a particular subframe the control channel elements to be searched in the search space are assigned at an equal probability for different secondary stations, and such that for any pair of subframes, the number of secondary station identifiers which can generate output values i and j is approximately the same for any values of i and j.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims priority under 35 USC §120 to and is a continuation of U.S. application Ser. No. 13/766,850, filed Feb. 14, 2013, and entitled “Method for Communicating in a Mobile System” which is a continuation of U.S. application Ser. No. 12/933,911, now U.S. Pat. No. 8,385,960, filed Sep. 22, 2010, entitled “Method for Communicating in a Mobile System,” which claims the benefit of and is the National Stage of International Application No. PCT/IB2009/051263, filed Mar. 26, 2009, which claims the benefit of and right of priority to under 35 USC §119 (b) of European Applications No. EP08305070.8 filed Mar. 26, 2008, the contents of each of the aforementioned applications being incorporated by reference herein in their entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to a method for communicating between a primary station and a plurality of secondary stations. 
         [0003]    This invention is, for example, relevant for telecommunication systems like a mobile telecommunication system. More specifically, this invention is relevant for the UMTS. 
       BACKGROUND OF THE INVENTION 
       [0004]    In a conventional UMTS system, a PDCCH (Physical Downlink Control Channel) 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. 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 in such a system 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. 
         [0005]    In an example, in order to determine the starting point of the search space, mobile stations (or secondary stations, also termed as UEs, for User Equipments in 3GPP parlance) compute a hash function f(UE_ID,s), where UE_ID is the identifier of the UE (different for distinct UEs) and s a time-varying subframe number. It is desirable that different UEs collide (have equal hash value) as infrequently as possible. 
         [0006]    The hash function presently proposed within 3GPP is of the form 
         [0000]        f (UE_ID, s )= K (UE_ID*16+ s )+ L  modulo  M,    
         [0007]    where K, L and M are constants, UE ID is the identifier of the UE, and s is the subframe number. It is clear that with this particular hash function f, two UEs that collide for some subframe number collide persistently, i.e., for all subframe numbers. 
       SUMMARY OF THE INVENTION 
       [0008]    It is an object of the invention to propose a method for communicating which permits the probability of collisions to be reduced. 
         [0009]    Another object of the invention is to provide a method for communicating preventing two UEs from repeatedly colliding. 
         [0010]    To this end, according to a first aspect of the invention, a method is proposed for communicating from a primary station to a plurality of secondary stations, comprising the step of at the primary station allocating a resource to the secondary stations over time on the basis of a hash function, wherein the hash function is such that the probability that two secondary stations are allocated common resources in two subframes substantially equals the product of the probability that the two secondary stations are allocated a common resource in the first subframe and the probability that the two secondary stations are allocated a common resource in the second subframe. 
         [0011]    As a consequence, the hash functions proposed here aim to reduce the likelihood of persistent collisions. In fact, the hash functions are such that the probability that different UEs collide in two subframes is approximately equal to the probability that two UEs collide in the first of these subframes times the probability that two UEs collide in the second of these subframes. Thus, it is unlikely that two UEs collide repeatedly. 
         [0012]    In a specific embodiment of the method, the hash function has the form: f(x, s)=(h(x)mod g(s)mod M, where x is a parameter of each secondary station, s is the subframe number, h is a function dependent on x, g is a function dependent on s, M is a constant, and mod is the modulo function. In another specific embodiment of the method, h is a constant multiplier. 
         [0013]    The present invention also relates to a secondary station comprising means for communicating with a primary, the secondary station further comprising 
         [0014]    control means configured to search at least one of a plurality of search spaces, each search space comprising at least one resource set, where at least one resource set might be used to transmit a message to the considered secondary station, wherein the search space of the secondary station is determined on the basis of a hash function, wherein the hash function is such that the probability that two secondary stations are configured to have common resources in the search spaces in both of any two subframes substantially equals the product of the probability that the two secondary stations are configured to have a common resource in the first subframe and the probability that the two secondary stations are configured to have a common resource in the second subframe; 
         [0015]    wherein the control means are configured for searching in the configured at least one search space for a control message from the primary station addressed to the considered secondary station, and receiving the control message. 
         [0016]    In accordance with still another aspect of the invention, it is proposed a primary station comprising means for communicating with a plurality of secondary stations, the primary station further comprising 
         [0017]    allocating means to allocate at least one resource set to a given secondary station into at least one of a plurality of search spaces, each search space comprising at least one resource set, wherein the search space of the given secondary station is determined on the basis of a hash function, wherein the hash function is such that the probability that two secondary stations are configured to have common resources in the search spaces in both of any two subframes substantially equals the product of the probability that the two secondary stations are configured to have a common resource in the first subframe and the probability that the two secondary stations are configured to have a common resource in the second subframe. 
         [0018]    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 
         [0019]    The present invention will now be described in more detail, by way of example, with reference to the accompanying drawings, wherein: 
           [0020]      FIG. 1  is a block diagram of a system in accordance with the invention comprising a primary station and at least a secondary station. 
           [0021]      FIG. 2  is a time chart representing the allocated search spaces in accordance an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0022]    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 . 
         [0023]    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. 
         [0024]    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. 
         [0025]    In accordance with a first embodiment of the invention, results of various simulations carried out by the inventors are described. With these simulations, 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. 
         [0026]    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).
   
 
         [0031]      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. 
         [0032]    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. 
         [0033]    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. 
         [0034]    In order to determine the start of the search space of each secondary station, each secondary station uses a hash function. The hash functions disclosed in accordance with this embodiment aim to reduce the likelihood of persistent collisions. In fact, the hash functions are such that the probability that different UEs collide in two subframes is approximately equal to the probability that two UEs collide in the first of this subframes times the probability that two UEs collide in the second of these subframes. Stated differently, collision events in different subframes are approximately independent. 
         [0035]    In fact, we describe functions f s (x) with x ∈ X, s ∈ {0,1, . . . , T-1} into {0,1, . . . , M-1}. The variable x corresponds to the UE_ID in the present situation, and s to the subframe number. The functions have the following properties. 
         [0036]    1. For each s ∈ {0,1, . . . , T-1}, the function f s  attains each element in {0,1, . . . , M-1} approximately equally often. 
         [0037]    2. For all distinct s,t in {0,1, . . . , T-1}, the number of elements x in X such that f s (x)=i and f t (x)=j is approximately the same for all values of i and j. 
         [0038]    We propose to use sets of hash functions of the form 
         [0000]        f   s ( x )=( Ax  mod  M   s )mod  M    
         [0039]    where A is a constant number and M 0 , M 1 , . . . , M T-1  are different numbers. It is advantageous if M 0 , M 1 , . . . , M T-1  are relatively prime to each other and to M. 
         [0040]    As a variant of the first embodiment, the following parameters are selected: 
         [0041]    T=10, UE ID in X={0,1, . . . , 2 24 −1}, M=47, and A=1. For the multipliers M 0 , M 1 , . . . , M 9 , we take ten prime numbers close to 2 12 , as depicted in the following table. 
         [0042]    To test the “uniformity” of each of the T=10 hash functions, i.e., Property 1 above, we counted for i=0, . . . , M-1, the number of elements x ∈ X for which f t (x)=i. The quotient of the smallest of these numbers and the largest of these numbers are computed. In case of a uniform distribution, this quotient would equal one; we thus wish that the quotient should be approximately one. For our specific choice of M 0 , M 1 , . . . , M 9 , the computed quotients range from 09885 to 09906. 
         [0000]    
       
         
               
             
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 List of multipliers 
               
             
          
           
               
                   
                 s 
                 M s   
               
               
                   
                   
               
               
                   
                 0 
                 4057 
               
               
                   
                 1 
                 4073 
               
               
                   
                 2 
                 4079 
               
               
                   
                 3 
                 4091 
               
               
                   
                 4 
                 4093 
               
               
                   
                 5 
                 4099 
               
               
                   
                 6 
                 5003 
               
               
                   
                 7 
                 5009 
               
               
                   
                 8 
                 5011 
               
               
                   
                 9 
                 5021 
               
               
                   
                   
               
             
          
         
       
     
         [0043]    To test the independence of the hash functions f s  and f t , i.e. Property 2 above, we computed for all pairs (i,j) the number elements x ∈ X for which f s (x)=i and f t (x)=j. 
         [0044]    Next, we computed the quotient of the smallest of these M 2  number and the largest of these M 2  numbers. Ideally, we would like this quotient to be equal to one. For our specific choice of M 0 , M 1 , . . . , M 9 , the computed quotients range from 0.9752 to 0.9808. 
         [0045]    We can conclude that in the embodiment, the hash functions are approximately uniform and approximately independent. 
         [0046]    In the envisioned application, the values of T and the range X is fixed while M may vary. For implementation reasons, it is advantageous that M 0 , M 1 , . . . , M T-1  do not depend on M. If we change M to 24, the computed quotients for uniformity range from 0.9941 to 0.9952; the computed quotients for testing independence range from 0.9779 to 0.9889. So also for this case, the proposed hash functions are approximately uniform and approximately independent. If we change M to 120, the computed quotients for uniformity range from 0.9706 to 0.9762; those for testing independence range from 0.9330 to 0.9474. 
         [0047]    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. 
         [0048]    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. 
         [0049]    The inclusion of reference signs in parentheses in the claims is intended to aid understanding and is not intended to be limiting. 
         [0050]    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.