Abstract:
A method is provided for rate matching a number of input bits in a time interval to a fixed number of output bits in the time interval, whereby the input bits consist of a set of at least two different bit classes, each of the classes having a certain number of bits in the time interval, whereby the rate matching is performed in two rate matching stages, whereby the first rate matching stage operates only on a selection out of the set of different bit classes, thus establishing a proportion between the number of bits of the different classes, and the second rate matching stage operates on all bit classes such that the proportion is exactly or approximately maintained after the second rate matching stage and the fixed number of output bits consisting of bits of the different bit classes is achieved.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to a rate matching process which is performed in more than one rate matching stage.  
         [0002]     Transmission channels in communications systems provide certain data rates for data transfer. This is due to their embedding in a communications system which entails restrictions on the data rate. However, the data rate of an application might differ from the data rate provided by the transmission channel. Therefore, the individual data rates have to be matched at an interface. This will be further detailed using an example in UMTS (Universal Mobile Telecommunications System).  
         [0003]     A major aspect of UMTS is to offer enhanced data rates in comparison to previous mobile communications systems. Therefore, a special downlink transmission is currently being established, the so called High Speed Data Access (HSDPA), which uses the HS-DSCH (High Speed Dedicated Downlink Shared Channel) as data channel and the HS-SCCH (HSDSCH related Shared Control Channel) as related control channel.  
         [0004]     The HSDPA data channel is basically an enhancement of the existing UMTS downlink shared channel. HSDPA allows coding of multiplex different users or mobile stations on up to 15 codes with a spreading factor of 16. The primary multiple access, however, is in the time domain, where different users can be scheduled every transmission time interval (TTI), which corresponds to 3 UMTS slots (i.e., 2 ms) so the TTI is a special time window. Also, the number of codes allocated to one user can change from TTI to TTI. Depending on the system load and channel conditions, the base station or Node B adapts modulation and code rate for each user. A certain combination of code rate and modulation is referred to as MCS (Modulation and Coding Scheme) level, which may change every TTI. A certain MCS is selected on the basis of channel condition measurements.  
         [0005]     To achieve the high data rates, modulation and coding schemes are used which allow a high information bit rate per code. For channel coding a rate R=1/3 code, a turbo code is typically used.  
         [0006]     For channel coding, bits are added to the actual information carrying bits in order to be able to reconstruct the actual information carrying bits in the receiver; also, in the case that during the data transfer alterations occurred. The actual information carrying bits are referred to as systematic bits or load bits, the additional bits used for the protection as parity bits.  
         [0007]     The rate of a turbo code denotes the relation between the systematic bits and the total number of bits to be transferred.  
         [0008]     In  FIG. 1 , a fragment of the transport channel coding structure for the HS-DSCH is shown. After channel coding follows the so called Physical Layer Hybrid ARQ (Automatic Repeat Request) functionality, which includes the rate matching functionality which matches the number of bits at the output of the channel (turbo) coder to the total number of bits of the HS-DSCH physical channels.  
         [0009]     The Hybrid-ARQ functionality is controlled by the parameter RV (Redundancy Version); i.e., the exact set of bits at the output of the physical-layer Hybrid-ARQ functionality depends on the number of input bits, the number of output bits, and the RV parameter, which itself includes the parameters s and r. The RV parameter s can take the value 0 or 1 to distinguish self-decodable (1) or non self-decodable transmissions (0). For self-decodable transmissions, preference is given to the systematic bits which are identical to the bits input to the turbo decoder. Apparently, if all systematic bits (plus some parity bits) are transmitted, then the packet can be decoded, at least in good enough channel conditions. If not all systematic bits are transmitted, the packet may not be decodable, depending on the exact selection of parity bits. For non-self-decodable transmissions preference is given to the parity bits. These transmissions are, in particular, effective for retransmissions because they contain different bits than the initial transmission and therefore offer good performance when combined with an initial self-decodable transmission. The RV parameter r does not change the relative number of systematic and parity bits, but establishes which set of bits are used within these classes. In this way different retransmissions can be selected, even if s is selected equal.  
         [0010]     The physical-layer Hybrid ARQ functionality consists of two rate-matching stages as shown in  FIG. 2 .  
         [0011]     There is a first rate-matching stage where only the parity bits are punctured, if the number of bits after channel coding exceeds the storage capability of the UE. For rate-matching two possibilities exist, either cutting out bits, which is referred to as puncturing, or repeating bits.  
         [0012]     The number of output bits does not yet match the number of physical-channel bits available in the HS-DSCH TTI. Instead, the number of output bits matches the available user equipment (UE) soft-buffering capability, information about which is provided by higher layers. Note that, if the number of input bits does not exceed the UE soft-buffering capability, the first rate-matching stage is transparent. The UE soft-buffering capability is also referred to as the virtual incremental redundancy (IR) buffer.  
         [0013]     The second rate-matching stage matches the number of bits at the output of the first rate-matching stage to the number of physical-channel bits available in the HS-DSCH TTI. The second rate-matching stage uses an algorithm for puncturing or repeating bits. The algorithm may use different values for the rate-matching parameters; e.g., the e ini  parameter that basically shifts the puncturing pattern. The e ini  parameter depends again on the so-called redundancy version or RV parameter; in particular, on r. Different redundancy versions all select the same number of bits of the relevant type, but different bits. This will be explained later in more detail. Depending on the value of the RV parameter, the rate matching algorithm may also be applied differently to a different set of input bits (systematic and parity bits).  
         [0014]     The above-described rate-matching can be generally described as adapting the rates of the various input bit streams to the externally fixed rate of the transmission channel. Instead of speaking of a “rate” or “bit stream,” the situation can be described in a time window. In that time window a number of bits depending on the respective bit rate will appear. Then, instead of comparing the rates, one can compare the number of bits in the time window.  
         [0015]     The above-described approach with the two rate matching stages is currently in discussion. Even if it offers a range of advantages, like simple buffer management in the user terminal or user equipment (UE), simple parameter calculation and reuse of the already existing rate matching algorithm, which is used in the Release 99 version of the UMTS specification in certain cases this type of rate matching becomes disadvantageous. This is especially the case when the UE (user equipment) buffer rate matching punctures due to limited UE soft-buffering capabilities and the channel rate matching stage performs repetition. Repetition occurs in the second stage, when the number of available channel bits within the TTI is higher than the UE virtual IR (incremental redundancy) buffer capability.  
         [0016]     However, from the point of view of improving the possibility of a good detection on the receiver side, this is disadvantageous, as investigations have shown.  
       SUMMARY OF THE INVENTION  
       [0017]     Based on the forgoing description it is an object of the present invention to optimize a rate matching process performed in more than one stage such that an improved information content is achieved at the output of the last rate matching stage.  
         [0018]     The present invention is directed toward roughly maintaining the proportion between parity bits and systematic bits achieved after the first rate matching stage and also after the last rate matching stage in cases where repetition is performed in the second rate matching stage. The proportion can only be roughly maintained, because only integer number of bits can be repeated in the second rate matching stage. This correspondingly can lead to a slight change of the proportion. However, this change is small as it does correspond only to at most one or two bits. This is based on investigations on the decodability of data transferred via a physical channel. It has been found out that an imbalance of the proportions leads to deterioration of the decodability of the transferred bits. In other words, the repetition rate of the second rate matching step is roughly the same for all bit classes, regardless of the procedures at the first rate matching stage.  
         [0019]     Additional features and advantages of the present invention are described in, and will be apparent from, the following Detailed Description of the Invention and the Figures. 
     
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0020]      FIG. 1  shows a fragment of the transport channel coding structure for HS-DSCH.  
         [0021]      FIG. 2  shows a physical layer hybrid ARQ functionality in the case of HSDPA in UMTS.  
         [0022]      FIG. 3  shows schematically an embodiment of the data transfer via HSDPA in a communications network between a base station and a mobile station, where the rate matching takes place in the base station and the respective undoing takes place in the mobile station or user equipment.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0023]     In the work preparatory to the present invention it has been found out that the conjunction of puncturing in the first stage and repetition in the second stage should, in fact, occur only with more or less low repetition rates in the second stage. Otherwise, a different coding scheme will yield better results and will probably be used. Nevertheless, a combination of puncturing and repetition might also occur due to other reasons related to network status, granularity in available transport block size, etc. Regardless, the second rate matching can be refined, as it is detailed below, beginning with an analysis of the calculation of rate matching parameters for the second stage with reference to  FIG. 2 . The first rate matching is performed on parity bits, in the shown case on parity 1 and parity 2 bits to account for the limited capacity of the virtual IR buffer (BUF). The following distinction has to be made here:  
         [0024]     a) the number of incoming bits is equal to or smaller than the capacity of the virtual IR buffer BUF, with the first rate matching stage (FRS1) being transparent for all bit classes (i.e., systematic and parity bits);  
         [0025]     b) the number of incoming bits is greater than the capacity of the virtual IR buffer BUF, with puncturing being performed on the parity bits, wherein after the first rate matching unit, a certain ratio or proportion between systematic bits, parity 1 bits and parity 2 bits is established, and the total number of bits of all classes after the first rate matching unit RMU1 is N sys +N p1 +N p2 .  
         [0026]     In the second rate matching stage (RMU2) this number N sys +N p1 +N p2  has to be matched to the given number of bits N data  of the transmission channel.  
         [0027]     In the second rate matching stage RMU2, all bits can experience puncturing or repetition, if this is required for the matching. The present invention proposes methods for a puncturing or repeating that conserves the encoded information satisfyingly.  
         [0028]     Theoretically, from a performance point of view, it is not reasonable to do first puncturing and after that repetition. But, as explained above, this can occur due to the limitation of UE buffer.  
         [0029]     The present invention will be explained now using embodiments with relation to the above-mentioned HSDPA.  
         [0030]     Since for HSDPA puncturing in the first stage is done conformingly to the standard set in R99 (release 99) concerning the puncturing of turbo codes, only the parity bits are punctured and the systematic bits are transparent; i.e., they do not experience puncturing. The proportion of systematic bits to parity 1 and parity 2 bits after the first stage differs from 1:1:1 if puncturing is applied. This is due to the fact that in the first rate matching stage the parity bits experience puncturing, if this is required due to the limited soft buffer capability, whereas the systematic bits do not. However, it was shown in simulations for R99 that not puncturing the systematic bits gives an improved performance.  FIG. 1  shows the two step rate matching approach described in [1], which already has been detailed above.  
         [0031]     In  FIG. 2 , a two step rate matching (physical layer HARQ functionality) is depicted, wherein the following is denoted.  
         [0032]     RM Pi_j: The rate matching at the stage j for the class i of parity bits.  
         [0033]     RM S: The rate matching for the class of systematic bits.  
         [0034]     N k : Number of respective bits k (systematic or parity bits).  
         [0035]     N t,k : Number of respective bits k to be transmitted.  
         [0036]     Currently, the second rate matching rule, as described in the introduction, is not well-balanced and does not maintain the proportion between systematic and parity bits after first rate matching. The available number of repetition bits are simply subdivided equally to systematic, parity 1 and parity 2 bits. The outcome of this is that the repetition rate is unequal for parity and systematic bits. To avoid this, a method according to the present invention proposes to have approximately equal repetition on all three bit streams of the channel rate matching.  
         [0037]     In detail for N data &gt;N sys +N p1 +N p2 , repetition is performed in the second rate matching stage. N data  denotes the number of available physical channel bits per TTI. To calculate the amount of repetition for the systematic, parity 1 and parity 2 bits in an embodiment the following formula to calculate the number of systematic bits for transmission is used:  
         N     t   ,   sys       =     ⌈       N   sys     ·       N   data         N   sys     +     N   p1     +     N   p2           ⌉         
 
 Further Embodiments of the Present Invention: 
 
 a) Parameter of Second Rate Matching Stage (Channel Rate Matching) 
 
         [0038]     The parameters of the second rate matching stage depend on the value of the RV parameter s, which can take the value 0 or 1 to distinguish self-decodable (1) and non-self-decodable (0) transmissions, and the RV parameter r (range 0 to r max ) which changes the initial error variable e ini .  
         [0039]     The number of bits are denoted before second rate matching as N sys  for the systematic bits, N p1  for the parity 1 bits, and N p2  for the parity 2 bits, respectively. The number of available physical channel bits per TTI is N data . Bit separation is used and the rate matching parameters are determined as follows.  
         [0040]     For N data ≦N sys +N p1 +N p2 , puncturing is performed in the second rate matching stage. The number of transmitted systematic bits in a retransmission is 
 
N t,sys =min{N sys , N data }
 
 for a transmission of self-decodable type (s=1) and 
 
 N   t,sys =max{ N   data −( N   p1   +N   p2 ),0}
 
 in the non-self-decodable case, i.e., s=0. 
 
         [0041]     For N data &gt;N sys +N p1 +N p2  repetition is performed in the second rate matching stage. A similar repetition rate in all bit streams is achieved by setting the number of transmitted systematic bits to  
         N     t   ,   sys       =       ⌈       N   sys     ·       N   data         N   sys     +     N   p1     +     N   p2           ⌉     .         
 
 The available room for parity bits in a transmission is  
         N     t   ,   p1       =         ⌊         N   data     -     N     t   ,   sys         2     ⌋     ⁢           ⁢   and   ⁢           ⁢     N     t   ,   p2         =     ⌈         N   data     -     N     t   ,   sys         2     ⌉           
 
 for the parity 1 and parity 2 bits, respectively. 
 
         [0042]     Table 1 summarizes the resulting parameter choice for the second rate matching stage. The parameter a in Table 1 is chosen as in [2]; i.e., a=2 for parity 1 and a=1 for parity 2.  
                                                           TABLE 1                           Parameters for Second Rate Matching Stage in case of puncturing                    X i     e plus     e minus                              Systematic   N sys     N sys     |N sys  − N t,sys |           RM S           Parity 1   N p1     a · N p1     a · |N p1  − N t,p1 |           RM P1_2           Parity 2   N p2     a · N p2     a · |N p2  − N t,p2 |           RM P2_2                      
 
 The rate matching parameter e ini (cf. subclause 4.2.7.5 of [2]) is calculated for each bit stream according to the e ini  variation parameter r using 
 
 e   ini ( r )={[ X   i   −r·e   minus −1]mod  e   plus }+1 
 
 in the case of puncturing (i.e., N data ≦N sys +N p1 +N p2 ), and 
 
 e   ini ( r )={[ X   i −(2 ·s+r )· e   minus −1]mod  e   plus }+1 
 
 for repetition (i.e., N data &gt;N sys +N p1 +N p2 ). 
 
         [0043]     Several further embodiments of the present invention can be applied by those versed in the art. For example, rounding signs can be selected to round upwards or downwards or even to the next even integer, again possibly upwards or downwards. In this way, some fine-tuning is possible. As an example, some preferred embodiments are given hereunder.  
         [0044]     It may be the case that the first rate matching generates cases where there are some (or exactly one) first parity bits (i.e., N p1 =1) but no second parity bits (i.e. N p2 =0). In that case, the formulas presented above would cause repetition of second parity bits, which is impossible as there is no single bit to repeat. While this is an extreme case of parameterization which will probably not be used in a sensibly engineered system, it is still possible to account for this case by a small modification of the formula.  
         N     t   ,   sys       =       ⌊       N   sys     ·       N   data         N   sys     +     2   ⁢     N   p2             ⌋     .         
 
         [0045]     In this way, all the transmitted bits will be allocated for systematic bits, if no single parity 2 bit is present after the second rate matching. Then no parity bits, in particular no parity 2 bits, will need to be repeated, which avoids the problem having to repeat non-existent parity 2 bits. Depending on the particularities of the first rate matching step, the case may happen that no parity 1 bits but one parity 2 bit is present after the first rate matching. Also, the other situation is possible; i.e., that the first rate matching step creates more parity 2 bits than parity 1 bits (if the sum of both parity 1 and parity 2 bits is odd). This is the case for the first rate matching step proposed for UMTS. Then, also, the case that no parity 2 bits but one parity 1 bit is present after the first rate matching will not happen. In this case, the formulas presented above will not be directly applicable, but the principles pointed out also can be applied for the case that there are more parity 2 bits after the first rate matching than parity 1 bits. Basically, in all the formulas presented here, “parity 1” and “parity 2” and the corresponding indices have to be swapped. Therefore the formulas (and the description) to be used for an extension of the UMTS systems are not exactly the formulas shown here, but the ones with “parity 1” and “parity 2” swapped; e.g., the last formula would then be modified to be  
         N     t   ,   sys       =       ⌊       N   sys     ·       N   data         N   sys     +     2   ⁢     N   p1             ⌋     .         
 
         [0046]     Similarly, all the other formulas can be modified by swapping parity 1 and parity 2 bits. As this is quite obvious, it will not be highlighted throughout this document but is still considered to be within the scope of the present invention. As a matter of fact, it can be considered to be purely a question of the nomenclature which stream to call parity 1 or parity 2 bits, at least for the purpose of the second rate matching stage.  
         [0047]     As a further enhancement, it can be desirable to have more parity 1 bits after second rate matching than parity 2 bits, or the same number. As such, if there is an odd number of bits available for parity bits, then parity 1 bits will be preferred in the sense that N t,p1 =1+N t,p2 . One reason for such a preference is that the same preference is already given for the bits after first rate matching. If the same preference is also given for the second rate matching, then the puncturing or repetition rate for the second rate matching step will be more similar between parity 1 and parity 2 bits than if the preference was inversed.  
         [0048]     An other advantage is the ease of implementation of subsequent processing steps after the second rate matching. For example, in another application of the same applicant dated Feb. 19, 2002 and named “An optimization of the bit distribution function for HSDPA,” a method is given as to how these bits are further distributed on so-called high and low reliable positions. For this purpose, in one particular processing step, it is proposed to write parity 1 and parity 2 bits alternating. If there is at most 1 more parity 1 bits that parity 2 bits (or an equal number), as proposed by this invention, then it is always possible to write parity 1 and parity 2 bits alternating starting with a parity 1 bit. Otherwise at the end two parity 2 bits may have to be written in succession, which is clearly less preferred and would also complicate the implementation due to having to cope with this exception in some particular cases.  
         [0049]     The formula to achieve this preference towards parity 1 bits would simply be to inverse the rounding for calculation, i.e. to calculate the number of parity bits in a transmission as:  
         N     t   ,   p1       =         ⌈         N   data     -     N     t   ,   sys         2     ⌉     ⁢           ⁢   and   ⁢           ⁢     N     t   ,   p2         =     ⌊         N   data     -     N     t   ,   sys         2     ⌋           
 
 for the parity 1 and parity 2 bits, respectively. 
 
         [0050]     Another embodiment achieving the same objective (i.e., to allow an efficient handling of the bits after the second rate matching step) would be to make sure that the number of bits available for parity bits is always even. Assuming that the total number of bits available for transmission is even as well (this is indeed the case for HSDPA, but the argument could be adapted also for the case the total number is odd), the number of systematic bits to be transmitted must also be selected to be even. This can be done by rounding not to the next integer for the calculation of N t,sys , the number of transmitted systematic bits, but by rounding to an even integer. Again, rounding can be performed upwards or downwards or to the nearest even integer, in the latter case again rounding can be done upwards or downwards if the result before rounding would be an odd integer or in that special case rounding can be done to the nearest integer divisible by 4.  
         [0051]     The formulas then, for example, could be written as follows.  
         [0052]     For N data ≦N sys +N p1 +N p2 , puncturing is performed in the second rate matching stage. The number of transmitted systematic bits in a retransmission is N t,sys =min{2*└N sys /2┘, N data } for a transmission of self-decodable type and N t,sys =max{2*└(N data −(N p1 +N p2 ))/2┘,0} in the non-self-decodable case.  
         [0053]     For N data &gt;N sys +N p1 +N p2 , repetition is performed in the second rate matching stage. A similar repetition rate in all bit streams is achieved by setting the number of transmitted systematic bits to  
         N     t   ,   sys       =     2   *       ⌊       N   sys     ·       N   data         2   ⁢     N   sys       +     4   ⁢     N   p2             ⌋     .           
 
         [0054]     The number of parity bits in a transmission is  
         N     t   ,   p1       =             N   data     -     N     t   ,   sys         2     ⁢           ⁢   and   ⁢           ⁢     N     t   ,   p2         =         N   data     -     N     t   ,   sys         2           
 
 for the parity 1 and parity 2 bits, respectively. Note that in this case no rounding is necessary for the calculation of N t,p1  and N t,p1  because both values will be integers. 
 
         [0055]     As a further aspect of the present invention, the exact way how to generate different redundancy versions can be enhanced. Different redundancy versions all select the same number of bits of the relevant type, but different bits. Ideally, regarding two redundancy versions:  
         [0056]     1) differ in as many as possible bits; i.e., there should be as few bits that are punctured in one redundancy version and also in another one as possible.  
         [0057]     2) Further, a reasonable number of redundancy versions should be selected. Due to constraints in the signalling, this number may typically be selected to be 2 or 4.  
         [0058]     3) As a further optimization, the bit patterns that are punctured in two redundancy versions should be well seperated from another. As such, if one redundancy version punctures bits number  1 , 9 , 17 , . . . , another redundancy version should puncture bits  5 , 13 , 21 , . . . and not bits  2 , 10 , 18 . The latter pattern is only shifted by one against the first one, while the second is shifted by 4. In this way, after a combination of two transmissions, the punctured bits of both transmissions are more evenly distributed using the second pattern than when using the third pattern (together with the first one). Similarly, if only bits number  1 , 9 , 17 , . . . are not punctured in one redundancy version, then in the next redundancy version ideally bits number  5 , 13 , 21 , . . . should not be punctured.  
         [0059]     The generation of different puncturing patterns is achieved by setting the parameter e ini  depending on the parameter r. The same principles also apply if repetition is done, in this case e ini  is calculated depending on (2s+r). Both cases are similar, wherein sometimes only puncturing will be dealt with in the following text to simplify the notation, but it should be highlighted that the present invention is equally applicable for repetition as well.  
         [0060]     The formulas from above for the calculation of e ini  are printed here again 
 
 e   ini ( r )={[ X   i   −r·e   minus −1]mod  e   plus }+1 
 
 in the case of puncturing (i.e., N data ≦N sys +N p1 +N p2 ), and 
 
e ini ( r )={[ X   i −(2 ·s+r )· e   minus −1]mod  e   plus }+1 
 
 for repetition (i.e., N data &gt;N sys +N p1 +N p2 ). 
 
         [0061]     The variable e ini  is used as a starting value for the error variable e of the rate matching algorithm that is used to actually determine which bits to puncture. This algorithm is described in section 4.2.7.5 “Rate matching pattern determination” of [3]. This section is cited here.  
         [0000]     b) Rate Matching Pattern Determination  
         [0062]     The bits before rate matching are densted by: 
    x i1 ,x i2 , x i3 , . . . , x iX     i   , where i is the TrCH number and the sequence is defined in 4.2.7.3 for uplink or in 4.2.7.4 for downlink. Parameters X i , e ini , e plus , and e minus  are given in 4.2.7.1 for uplink or in 4.2.7.2 for downlink.    
 
         [0064]     The rate matching rule is as follows:  
                                                                                                                                                                                                                                                                             if puncturing is to be performed                e = e ini     -- initial error between current and               desired puncturing            ratio                m = 1   -- index of current bit                do while m &lt;= X i                  e = e − e minus     -- update error           if e &lt;= 0 then   -- check if bit number m should be               punctured                set bit x i,m  to δ where δ∉ {0, 1}                e = e + e plus     -- update error                end if                m = m + 1   -- next bit           end do            else                e = e ini     -- initial error between current and               desired            puncturing ratio                m = 1   -- index of current bit                do while m &lt;= X i                  e = e − e minus     -- update error           do while e &lt;= 0   -- check if bit number m should be               repeated                repeat bit x i,m                  e = e + e plus     -- update error                end do                m = m + 1   -- next bit                end do            end if       A repeated bit is placed directly after the original one.       End of citation                  
 
         [0065]     Note that a bit that is set to δ is actually punctured because the δ are removed.  
         [0066]     As can be seen by inspecting the formulas, if e ini  is increased by a value of e minus , then the puncturing pattern is shifted by one bit, if e ini  is increased by a sh*e minus , where sh is an integer, than the puncturing pattern is shifted by sh bits. If e ini  is incremented by a value smaller than e minus , then some puncturing positions (positions where a bit is punctured) may be shifted by one and others may be not affected, this depends on the exact values of the other parameters.  
         [0067]     Apparently the formula for the setting of e ini  as presented above is only good with respect to the criteria 1 and 2 above, but not with respect to criterion 3. Therefore, another scheme for setting e ini  is also known, which is better with respect to criterion 3: 
 
 e   ini ( r )={[ X   i −( r·e   plus   /r   max )−1]mod  e   plus }+1 in the case of puncturing, and 
 
 e   ini ( r )={[ X   i −(( s+ 2 ·r )· e   plus /(2 ·r   max ))−1]mod  e   plus }+1 for repetition. 
 
 Where rε{0,1, . . . , r max −1} and r max  is the total number of redundancy versions allowed by varying r, which was already said to be typically 2 or 4. 
 
         [0068]     In this way, e ini  is varied not in steps of e minus  but in steps of e plus /r max . Note that due to the modulo calculation included in the formula which is necessary because e ini  can reasonably only take values between 1 and e plus , a variation that exceeds e plus  is equivalent to a smaller variation. Therefore, this formula tries to maximize the variation of e ini  within the reasonable range and consequently better fulfils criterion 3. Therefore, this formula is used for HSDPA as specified in [3].  
         [0069]     However, it is still possible to enhance this formula for e ini . One crucial fact is that the formula above also can cause non integer values for e ini , if e plus *r is not divisible by r max  (or (s+2*r) is not divisible by 2*r max  in the case of repetition). This is an unnecessary complication of the implementation for the rate matching formula in section 4.2.7.5. Therefore, an objective of the present invention is to simplify the implementation by providing an alternative that does not involve this disadvantage.  
         [0070]     There is also another reason to modify the formula for some special cases; i.e., if the system uses some specific parameters e.g.  
         [0071]     A single bit needs to be punctured (N p1 =1; N t,p1 =0). The parameters that would be calculated without rounding are Xi=1, e plus =1, e minus =1, e ini =1.5. Then the rate matching algorithm would not puncture this single bit, which is contradictory to the requirement that this bit should be punctured. e ini  has to be at most 1 so that this single bit is punctured. Generally, to puncture (or repeat) all bits the following condition must apply: e plus =e minus  AND e ini &lt;=e plus . This is not achieved with the present formula. The same applies to repetition of a single bit (N p1 =1; N t,p1 =2). Here the formula also fails.  
         [0072]     In general, the condition e ini &lt;=e plus  is necessary to make sure that the correct number of bits are punctured. Due to the fact that e ini  may not be an integer, this prerequisite cannot be assured (while the prerequisite is assured for integer values due to the mod function).  
         [0073]     All these problems are solved by rounding the fraction which is calculated: 
 
 e   ini ( r )={└ X   i −( r·e   plus   /r   max )−1┘mod  e   plus }+1 in the case of puncturing, and 
 
 e   ini ( r )={└ X   i −(( s+ 2 ·r )· e   plus /(2 ·r   max ))−1┘mod  e   plus }+1 for repetition. 
 
 Note that the symbol └ ┘ is used to denote rounding towards minus infinity, also sometimes called floor( ). 
 
         [0074]     These formulas are equivalent to performing the rounding outside of the modulo operation (or actually anywhere where it covers the division that can cause fractional numbers to appear), e.g.: 
 
 e   ini ( r )=└( X   i −( r·e   plus   /r   max )−1)mod  e   plus ┘+1 in the case of puncturing, and 
 
 e   ini ( r )=└( X   i −(( s+ 2 ·r )· e   plus /(2 ·r   max )┘−1)mod  e   plus ┘+1 for repetition. 
 
         [0075]     Implementing the rounding function directly after the division minimizes the area where fractional figures have to be dealt with: 
 
 e   ini ( r )=[( X   i   −└r·e   plus   /r   max ┘−1)mod  e   plus ]1 in the case of puncturing, and 
 
 e   ini ( r )=( X   i −└( s+ 2 ·r )· e   plus /(2 ·r   max )┘−1)mod  e   plus +1 for repetition. 
 
         [0076]     This minimizing of the area where fractional numbers are used can also be applied for the other preferred embodiments.  
         [0077]     In a further variant, the division followed by rounding can also be implemented as a so-called integer division, often denoted by the symbol “div.” This is equivalent to a division yielding a fractional result which is then rounded downwards, at least for positive figures, which is the case here. The div function can be efficiently implemented, for example, in digital signal processors, where it is often available as a machine instruction. The formulas would then read: 
 
 e   ini ( r )=[( X   i −( r·e   plus )div  r   max −1)mod  e   plus ]+1 in the case of puncturing, and 
 
 e   ini ( r )=( X   i −(( s+ 2· r )· e   plus )div(2 ·r   max )−1)mod  e   plus +1 for repetition. 
 
         [0078]     Note that instead of rounding downwards, rounding upwards also can be implemented. This may cause a shift of one position for some punctured bits, but this difference typically will not be crucial. Rounding upwards is slightly more complex from an implementation point of view, as the div function cannot be used. Typically one would have to add the dividend minus one before performing the div operation. However, rounding upwards denoted by the symbol ┌ ┐ can only be performed inside the modulo function, otherwise again the same problem as identified above would occur i.e. the single bit would not be punctured, because e ini  would be calculated to be 2 (after rounding from 1.5) and therefore would be larger than e plus . Rounding inside the modulo function would give the following formula: 
 
 e   ini ( r )={( X   i   −┌r·e   plus   /r   max ┐−1)mod  e   plus }+1 in the case of puncturing, and 
 
 e   ini ( r )={( X   i −┌( s+ 2 ·r )· e   plus /(2 ·r   max )┐−1) mod  e   plus }+1 for repetition. 
 
         [0079]     With any of these preferred embodiments it is possible to generate redundancy versions consistently that always puncture or repeat exactly the required number of bits.  
         [0080]     Generally, the present invention in its various embodiments focuses on 
    1. Rate matching scheme, 
        using at least two rate matching steps operating serially on multiple classes of elements,     whereby in a first rate matching step the puncturing or repetition rate is different for different classes,     whereby the repetition or puncturing rate of a second rate matching step is identical or almost identical for the different classes.    
        2. A rate matching scheme as described in 1, where a preference is given to a first class compared to a second class by rounding upwards for the calculation of the number of elements to be transmitted after the second rate matching step for the first class and downwards for the second class.     3. A rate matching scheme, allowing creation of different redundancy versions by pre-setting or predetermining an initial error variable e ini  which is used in the rate matching algorithm with different values.     4. A rate matching scheme as described in 1 to 3, where e ini  is calculated as the sum of an initial error variable plus the integer fraction of e plus  divided by the number of the redundancy versions, where e plus  is the increment of the error variable which is used after each puncturing operation. 
 
 The rate matching takes place in a base station BS in a rate matching unit RU. A base station is a central unit in a communications network CN that serves, for example, a cell in a cellular network. The complementary undoing of the rate matching takes place in the terminal or mobile communications device in a unit RU −1 . This is schematically depicted in  FIG. 3 , where the HSDPA Data transfer is suggested by the arrow. 
   
 
         [0088]     Although the present invention has been described with reference to specific embodiments, those of skill in the art will recognize that changes may be made thereto without departing from the spirit and scope of the present invention as set forth in the hereafter appended claims.  
       4. REFERENCES  
       [0000]    
       
          [1] R1-02-0199, TR 25.858 “High Speed Downlink Packet access”, Espoo, Finland, January, 2002  
          [2] R1-01-0029, Siemens, “Physical Layer Hybrid ARQ Functionality for HSDPA”, Espoo, Finland, January, 2002  
          [3] 3GPP TS 25.212 V5.0.0 (2002-03), “Multiplexing and channel coding (FDD) (Release 5)”, 3GPP  
       
     
       LIST OF ABBREVIATIONS  
       [0000]    
       
          ARQ Automatic Repeat Request  
          BCH Broadcast Channel  
          BER Bit Error Rate  
          BLER Block Error Rate  
          BS Base Station  
          CCPCH Common Control Physical Channel  
          CCTrCH Coded Composite Transport Channel  
          CFN Connection Frame Number  
          CRC Cyclic Redundancy Check  
          DCH Dedicated Channel  
          DL Downlink (Forward link)  
          DPCCH Dedicated Physical Control Channel  
          DPCH Dedicated Physical Channel  
          DPDCH Dedicated Physical Data Channel  
          DS-CDMA Direct-Sequence Code Division Multiple Access  
          DSCH Downlink Shared Channel  
          DTX Discontinuous Transmission  
          FACH Forward Access Channel  
          FDD Frequency Division Duplex  
          FER Frame Error Rate  
          GF Galois Field  
          HARQ Hybrid Automatic Repeat reQuest  
          HS-DPCCH A Dedicated Physical Control Channel associated with high speed  
          downlink transmission.  
          HS-DSCH High Speed Downlink Shared Channel  
          HS-PDSCH High Speed Physical Downlink Shared Channel  
          HS-SCCH Shared Control Channel for High Speed Downlink Shared Chanel  
          MAC Medium Access Control  
          Mcps Mega Chip Per Second  
          MS Mobile Station  
          OVSF Orthogonal Variable Spreading Factor (codes)  
          PCCC Parallel Concatenated Convolutional Code  
          PCH Paging Channel  
          PhCH Physical Channel  
          PRACH Physical Random Access Channel  
          RACH Random Access Channel  
          RSC Recursive Systematic Convolutional Coder  
          RV Redundancy Version  
          RX Receive  
          SCH Synchronisation Channel  
          SF Spreading Factor  
          SFN System Frame Number  
          SIR Signal-to-Interference Ratio  
          SNR Signal to Noise Ratio  
          TF Transport Format  
          TFC Transport Format Combination  
          TFCI Transport Format Combination Indicator  
          TPC Transmit Power Control  
          TrCH Transport Channel  
          TTI Transmission Time Interval  
          TX Transmit  
          UL Uplink (Reverse link)  
          RAN WG1 Radio access network Working Group 1 (Physical layer)  
          CR Change Request  
          HI HS-DSCH Indicator  
          HSDPA High Speed Downlink Packet Access  
          MCS (Modulation and Coding Scheme) 
 
 Note that the relevant documents regarding HSDPA are maintained by 3GPP, the third generation partnership project, Address: ETSI, Mobile Competence Centre, 650, route des Lucioles, 06921 Sophia-Antipolis Cedex, web-site www.3gpp.org.