Patent Publication Number: US-2015063239-A1

Title: Multiple-input-multiple-output (mimo) communication

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method for supporting multiple-input-multiple-output communications with a base station of a wireless telecommunications network, user equipment and a computer program product. 
     BACKGROUND 
     Wireless telecommunications networks are known. Base stations in such networks provide wireless connectivity to user equipment within a geographical area or cell, associated with the base station. The wireless communication links between the base station and each of the user equipment typically include one or more downlink (or forward) channels for transmitting information from the base station to the user equipment and one or more uplink (or reverse) channels for transmitting information from the user equipment to the base station. 
     Multiple-input-multiple-output (MIMO) techniques may be employed when the base station and, optionally, the user equipment include multiple antennas. For example, user equipment that includes multiple antennas can transmit multiple independent and distinct signals to base stations on the same a frequency, using the same code and within the same transmission time interval (TTI) which can be resolved by the base stations. For example, individual user equipment is able to transmit two streams of Enhanced Data Channel (E-DCH) in the uplink using the same uplink resource (frequency, time and code) which means that the user equipment is capable of performing a rank two transmission (as opposed to a rank one transmission which occurs with when transmitting a single E-DCH stream). 
     The E-DCH consists of one or more E-DCH Dedicated Physical Data Channels (E-DPDCH) and is accompanied by one E-DCH Dedicated Physical Control Channel (E-DPCCH]. The E-DPDCH carries the data traffic and the E-DPCCH carries control information to enable the E-DPDCH to be decoded. 
     In uplink MIMO, a secondary E-DCH stream is sent together with the primary E-DCH stream. Similar to the primary E-DCH stream, the secondary E-DCH stream consists of one or more secondary E-DPDCH (S-E-DPDCH) and one secondary E-DPCCH (S-E-DPCCH). 
     For E-DCH operation, resources known as grants are scheduled by the network (such as from a base station) to the user equipment. The grant indicates the ratio of the power of the E-DPDCHs (the data) to the power of the E-DPCCH (the pilot). By carefully scheduling the grants, the network can manage the Rise over Thermal (RoT), i.e. the interference, in the uplink. 
     A relationship, function or mapping is defined between the grant and the transport block size that can be transmitted on the E-DCH. The higher the grant, the larger the transport block size the user equipment can send in a TTI. 
     Although providing uplink MIMO facilitates transmissions from the user equipment to the network, unexpected consequences can occur. 
     Accordingly, it is desired to provide an improved technique for supporting MIMO communications. 
     SUMMARY 
     According to a first aspect, there is provided a method of determining a transport block size for a secondary stream transmitted in addition to a primary stream by user equipment when supporting Multiple Input Multiple Output communications with a base station of a wireless telecommunications network, the method comprising the steps of: establishing a grant associated with the secondary stream; and determining a transport block size for the secondary stream in accordance with a relationship between grant and transport block size, the relationship differing from a primary relationship between grant and transport block size used to determine a primary transport block size for the primary stream. 
     The first aspect recognizes that a problem with existing techniques is that they lead to the inappropriate utilization of the primary or secondary stream which may lead to sub-optimal communication using those streams. In particular, existing techniques may lead to an inappropriately selected transport block size for a stream, as will now be explained. 
     The first aspect recognises that existing techniques assume that the transmit power of the E-DPDCH and S-E-DPDCH are equal as illustrated in  FIG. 1 . This is in order to provide for simplified power allocation between the two E-DCH streams. The S-DPCCH is usually transmitted at a lower offset power with respect to the transmit power of the DPCCH. If the secondary grant is selected to be the power ratio of S-E-DPDCH to DPCCH, then both the primary stream and the secondary stream will have exactly the same grant since, as mentioned above, the transmit power of E-DPDCH and S-E-DPDCH are equal. However, the first aspect recognizes that the secondary stream usually has a lower gain (due to a lower received signal to interference plus noise ratio (SINR)) than that of the primary stream. Hence, with an equal grant on both streams, the secondary stream may not be received with a sufficient signal quality to transmit the same transport block size as that of the primary stream. Alternatively, if the grants are on both the streams were to be reduced so that the transport block size could be set to a value that fits the signal quality of the secondary stream, then the transport block size carried by the primary stream (which usually has a higher gain, and a better signal quality), will be unnecessarily low. 
     The first aspect also recognizes that if the secondary grant is instead selected to be the power ratio of S-E-DPDCH to S-DPCCH, then the secondary grant will always be equal to or larger than that of the primary grant since the power of S-DPCCH is equal to or smaller than that of DPCCH. If the same relationship, function or mapping between grant and transport block size is used on the secondary stream as that used on the primary stream, then this would cause the secondary stream (which has a poor signal quality) to support a larger transport block size than that of the primary stream. This will result in a high block error rate (BER) on the secondary stream. 
     The first aspect also recognizes that if, alternatively, two Enhanced Relative Grant Channels (E-RGCH) are provided, each one of which schedules the grant for an associated stream, then the use of the same power for E-DPDCH and S-E-DPDCH presents the following problems. Although the use of two E-RGCH allows for different grants for the different streams, by forcing the user equipment to transmit the same power for E-DPDCH and S-E-DPDCH, the final grant used would result in either both streams having the same transport block size or the secondary stream having a larger transport block size, as described above. In addition, this approach requires the user equipment to need to monitor two E-RGCH instead of just one. Also, the rank of the channel is not constant and by having independent grants, the user equipment transmit power may increase suddenly when it transmits two streams and using the secondary grant. Since only a very high format is considered for uplink MIMO, the transmission power would almost double and it would be difficult for network to manage its RoT). 
     Accordingly, a method of determining a transport block size for a secondary stream is provided. The secondary stream may be transmitted in addition to a primary stream in uplink between user equipment and a base station when supporting MIMO communications. The method may comprise the step of establishing a grant associated with the secondary stream. The method may also comprise the step of determining a transport block size for the secondary stream. The transport block size may be determined using a relationship, mapping, function or lookup table which defines the transport block size for each grant. That relationship, mapping or function which defines the transport block size for each grant may be a different relationship, mapping or function compared to that used to define the relationship between each grant and transport block size for the primary stream. 
     By using this approach, where a different relationship, mapping, function or lookup table is used to derive the transport block size for the secondary stream to that relationship, mapping or function used to derive the transport block size for the primary stream from the grant of the primary stream enables different transport block sizes to be used, each of which may be appropriate to its associated stream. Typically, at least the grant for the primary stream may be signalled to the user equipment from the network, and the grant for the secondary stream may be either signalled or derived deterministically from the power and resources allocated to the secondary stream. 
     According to one embodiment, for an identical grant for the primary stream and the secondary stream, the relationship and the primary relationship determine differing transport block sizes for the secondary stream and the primary stream. Accordingly, even when the primary stream and secondary stream have an identical grant, the transport block sizes for the secondary stream and the primary stream may be determined not to be identical, each of which may be appropriate to its associated stream. 
     In one embodiment, for an identical grant for the primary stream and the secondary stream, the relationship determines the transport block size which is smaller than the primary transport block size determined by the primary relationship. Accordingly, even when the grant for the primary stream and the secondary stream is the same, the primary transport block size for the primary stream would be determined to be larger than that for the secondary stream. Using a separate relationship, mapping, function or lookup table to derive the transport block size of the secondary stream allows the grants of the secondary stream (which can be larger than the grant of the primary stream) to map to a smaller transport block size compared to that of the primary stream, so as to be better matched to the reception quality of the secondary stream. 
     In one embodiment, the relationship determines the transport block size by utilising the primary relationship to determine an intermediate transport block size from the grant which is then factored by a scalar to determine the transport block size. Accordingly, the relationship, mapping, function or lookup table used to define the relationship between grant and transport block size for the primary stream may be reused to derive the transport block size for the secondary stream. 
     In one embodiment, the relationship determines the transport block size by factoring the grant by a scalar to determine an intermediate grant and by then utilising the primary relationship to determine the transport block size from the intermediate grant. Again, this enables the reuse of the functionality used to derive the transport block size of the primary stream. 
     In one embodiment, the scalar is a value of no greater than 1. This helps to ensure that the transport block size of the secondary stream is smaller than the transport block size of the primary stream since the secondary stream is typically weaker than that of the primary stream. 
     In one embodiment, the method comprises the step of receiving the scalar from a network node of the wireless telecommunications network. It will be appreciated that the scalar may be semi-static or may be dynamic. 
     In one embodiment, a plurality of differing transport block sizes are supported for communication between the user equipment and the base station and the method comprises the step of selecting one of the plurality of differing transport block sizes for the secondary stream for whose size closest but no larger than the transport block size. 
     It will be appreciated that the determined transport block size for the secondary stream may not be a valid transport block size since these are typically a specific set of discrete transport block sizes. Therefore, the determined transport block size may need to be rounded to a valid transport block size. For example, the determined transport block size may need to be rounded down to the next lowest valid transport block size in the set. 
     In one embodiment, the relationship determines the transport block size by utilising the grant to identify an index of a lookup table containing the transport block size. Typically, the transport block size increases as the index of the lookup table increases (although the reverse may be true) and there may be a one-to-one mapping between the index and a transport block size. 
     In one embodiment, the index comprises an Enhanced Data Channel Transport Format Combination Identifier determined from the grant. 
     In one embodiment, the index comprises an Enhanced Data Channel Transport Format Combination Identifier determined from the grant and an offset value. 
     In one embodiment, the grant is established from a power of a pilot associated with one of the primary stream and the secondary stream. 
     In one embodiment, the grant is signalled separately from a grant associated with the primary stream. 
     According to a second aspect, there is provided user equipment operable to determine a transport block size for a secondary stream transmitted in addition to a primary stream by user equipment when supporting Multiple Input Multiple Output communications with a base station of a wireless telecommunications network, the user equipment comprising: establishing logic operable to establish a grant associated with the secondary stream; and determining logic operable to determine a transport block size for the secondary stream in accordance with a relationship between grant and transport block size, the relationship differing from a primary relationship between grant and transport block size used to determine a primary transport block size for the primary stream. 
     In one embodiment, for an identical grant for the primary stream and the secondary stream, the relationship and the primary relationship determine differing transport block sizes for the secondary stream and the primary stream. 
     In one embodiment, for an identical grant for the primary stream and the secondary stream, the relationship determines the transport block size which is smaller than the primary transport block size determined by the primary relationship. 
     In one embodiment, the relationship determines the transport block size by utilising the primary relationship to determine an intermediate transport block size from the grant which is then factored by a scalar to determine the transport block size. 
     In one embodiment, the relationship determines the transport block size by factoring the grant by a scalar to determine an intermediate grant and by then utilising the primary relationship to determine the transport block size from the intermediate grant. 
     In one embodiment, the scalar is a value of no greater than 1. 
     In one embodiment, the user equipment comprises receiving logic operable to receive the scalar from a network node of the wireless telecommunications network. 
     In one embodiment, a plurality of differing transport block sizes are supported for communication between the user equipment and the base station and the determining logic is operable to select one of the plurality of differing transport block sizes for the secondary stream for whose size closest but no larger than the transport block size. 
     In one embodiment, the relationship determines the transport block size by utilising the grant to identify an index of a lookup table containing the transport block size. 
     In one embodiment, the index comprises an Enhanced Data Channel Transport Format Combination Identifier determined from the grant. 
     In one embodiment, the index comprises an Enhanced Data Channel Transport Format Combination Identifier determined from the grant and an offset value. 
     In one embodiment, the grant is established from a power of a pilot associated with one of the primary stream and the secondary stream. 
     In one embodiment, the grant is signalled separately from a grant associated with the primary stream. 
     According to a third aspect, there is provided a computer program product operable, when executed on a computer, to perform the method steps of the first aspect. 
     Further particular and preferred aspects are set out in the accompanying independent and dependent claims. Features of the dependent claims may be combined with features of the independent claims as appropriate, and in combinations other than those explicitly set out in the claims. 
     Where an apparatus feature is described as being operable to provide a function, it will be appreciated that this includes an apparatus feature which provides that function or which is adapted or configured to provide that function. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the present invention will now be described further, with reference to the accompanying drawings, in which: 
         FIG. 1  illustrates a relationship between the transmit powers; and 
         FIG. 2  illustrates the main processing steps according to one embodiment. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Overview 
     Referring now to  FIG. 2 , the general approach of embodiments is to use a different function to derive the transport block size TBS 2  for the secondary stream from the grant for the secondary stream, than the function used to derive the transport block size TBS 1  for the primary stream from the grant for the primary stream. Typically, at least the grant for the primary stream is signalled to the user equipment by the base station, and the grant for the secondary stream is either signalled or derived deterministically at step S 1  by the user equipment according to a predefined rule. From this grant, a transport block size for at least the secondary stream is derived at step S 2 . 
     As an example, consider a case where the primary stream&#39;s transport block size TBS 1  is a function f 1  of the grant G 1  (E-DPDCH:DPCCH power ratio) used for transmission in the primary stream, i.e.: 
         TBS   1    =f   1  ( G   1 )   Equation 1
 
     Currently, the function f 1  is implemented as a look-up table defined by 3GPP TS25.321, although it will be appreciated that the function f 1  may be implemented as a relationship, mapping or function. 
     The grant G 1  is the power ratio of the E-DPDCHs to the DPCCH (pilot power). 
     In general approach of embodiments, a separate function is used to derive the transport block size TBS 2  for the secondary stream from the grant for the secondary stream, i.e.: 
         TBS   2   =f   2  ( G   2 )   Equation 2
 
     where G 2  is the grant provided for transmission in the secondary stream. 
     As described above, G 2  can be the power ratio of S-E-DPDCHs to DPCCH or the power ratio of S-E-DPDCHs to S-DPCCH, and since the power of E-DPDCHs and S-E-DPDCHs are typically assumed to be the same, G 2  is always equal or greater than G 1 . 
     If the existing function fi is used on the secondary stream (with G 2  as the input), this will result in TBS 2 ≧TBS 1 , i.e., the secondary stream (which has a weaker radio condition) has to support a larger transport block size than that in the primary stream. However, by using a separate function f 2  to derive the secondary stream transport block size, it allows G 2  (which can be≧G 1 ) to map to a smaller transport block size compared to that in the primary stream, so as to be better matched to the reception quality of the secondary stream. 
     Embodiment 1 
     Function Scaling Factor 
     In one embodiment the function f 2  is equal to function f 1  multiplied by a scaling factor α, i.e.: 
         TBS   2   =f   2 ( G   2 )=α f   1 ( G   2 )   Equation 3
 
     This embodiment allows the same lookup table used in f 1  to be reused to derive TBS 2 . Advantageously, the value of α is signalled by the network and can be semi-static or dynamic. 
     It should be noted that TBS 2  derived using Equation 3 may not be a valid transport block size since the E-TFCIs indexes to a specific set of discrete transport block sizes. Therefore the calculated TBS 2  may need to be “rounded” to a valid transport block size, for example rounded down to the next lower valid transport block size. 
     Embodiment 2 
     Grant Scaling Factor 
     In one embodiment, the function f 2  is equal to f 1  but with the grant G 2  scaled by a scaling factor α, i.e.: 
         TBS   2   =f   2 ( G   2 )= f   1 (α G   2 )   Equation 4
 
     This embodiment reuses function fi and since function fi maps to a valid transport block size, the calculated TBS 2  will be a valid transport block size. 
     It is expected that the secondary stream is weaker than that of the primary stream and therefore a suitable value of a in Equations 3 or 4 would be &lt;1. 
     Embodiment 3 
     Index Offset 
     The E-TFCI is the index to the transport block size lookup table. The transport block size lookup table (defined in 3GPP TS25.321) increases as the E-TFCI increases and there is a one to one mapping between E-TFCI and transport block size. Hence the function fi also gives the E-TFCI number. To avoid confusion, a function hi is defined that takes the grant G 1  and gives ETFCI 1  (E-TFCI for primary stream) as follows: 
         ETFCI   1   =h   1  (G 1 )   Equation 5
 
     In one embodiment, the E-TFCI for the secondary stream ETFCI 2 , is an offset γ (an integer) from the E-TFCI calculated using function h 1 , i.e.: 
       ETFCI 2 =max( h   1 ( G   2 )−γ, ETFCI min )   Equation 6
 
     where ETFCI min  is the minimum valid E-TFCI in the transport block size lookup table. Since the E-TFCI points to a valid transport block size, the ETFCI 2  derived from Equation 6 would point to a valid transport block size. The existing transport block size lookup table used by the primary stream can therefore be reused. Similar to α, γ can be signalled by the network and can be semi-static or dynamic. 
     Accordingly, it can be seen that embodiments prevent the secondary stream from using a transport block size that it cannot support or lowers the UE primary stream transport block size to match that in the secondary stream. This alleviates the problem where uplink MIMO increases the uplink throughput but the secondary stream holds down the total throughput which would otherwise occur is both streams transmit with the same transport block size. 
     A person of skill in the art would readily recognize that steps of various above-described methods can be performed by programmed computers. Herein, some embodiments are also intended to cover program storage devices, e.g., digital data storage media, which are machine or computer readable and encode machine-executable or computer-executable programs of instructions, wherein said instructions perform some or all of the steps of said above-described methods. The program storage devices may be, e.g., digital memories, magnetic storage media such as a magnetic disks and magnetic tapes, hard drives, or optically readable digital data storage media. The embodiments are also intended to cover computers programmed to perform said steps of the above-described methods. 
     The functions of the various elements shown in the Figures, including any functional blocks labelled as “processors” or “logic”, may be provided through the use of dedicated hardware as well as hardware capable of executing software in association with appropriate software. When provided by a processor, the functions may be provided by a single dedicated processor, by a single shared processor, or by a plurality of individual processors, some of which may be shared. Moreover, explicit use of the term “processor” or “controller” or “logic” should not be construed to refer exclusively to hardware capable of executing software, and may implicitly include, without limitation, digital signal processor (DSP) hardware, network processor, application specific integrated circuit (ASIC), field programmable gate array (FPGA), read only memory (ROM) for storing software, random access memory (RAM), and non volatile storage. Other hardware, conventional and/or custom, may also be included. Similarly, any switches shown in the Figures are conceptual only. Their function may be carried out through the operation of program logic, through dedicated logic, through the interaction of program control and dedicated logic, or even manually, the particular technique being selectable by the implementer as more specifically understood from the context. 
     It should be appreciated by those skilled in the art that any block diagrams herein represent conceptual views of illustrative circuitry embodying the principles of the invention. Similarly, it will be appreciated that any flow charts, flow diagrams, state transition diagrams, pseudo code, and the like represent various processes which may be substantially represented in computer readable medium and so executed by a computer or processor, whether or not such computer or processor is explicitly shown. 
     The description and drawings merely illustrate the principles of the invention. It will thus be appreciated that those skilled in the art will be able to devise various arrangements that, although not explicitly described or shown herein, embody the principles of the invention and are included within its spirit and scope. Furthermore, all examples recited herein are principally intended expressly to be only for pedagogical purposes to aid the reader in understanding the principles of the invention and the concepts contributed by the inventor(s) to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions. Moreover, all statements herein reciting principles, aspects, and embodiments of the invention, as well as specific examples thereof, are intended to encompass equivalents thereof.