Abstract:
The invention relates to a method for converting a signal data rate and to a transmitter in a digital data transmission system, the transmitter comprising a number of data sources that generate a signal having a first data rate, and a coder for forming the signal into frames of a given length, which length corresponds to a second data rate, and a coder for grouping the signal into a first block the length of which corresponds to the first data rate. In order to convert the data rate flexibly, the transmitter comprises a coder, which calculates the difference between the frame and the number of symbols in the first block by subtracting the frame length from the number of symbols in the first block, which equalizes the difference by removing or repeating every ith symbol in the first block, i being determined as the nearest bigger or an equal integer when the number of symbols in the first block is divided by the absolute value of the difference that was calculated, which updates the number of the symbols that do not fit in the frame by subtracting therefrom the number of the symbols that have been repeated, and which forms the first block to consist of the symbols that have not been repeated or removed.

Description:
FIELD OF THE INVENTION 
     The invention relates to a method for converting a signal data rate from a lower data rate to a higher data rate in a digital data transmission system, wherein a signal is transmitted by using frames of a given length, which length corresponds to the higher data rate, in which method the signal is grouped to a first block the length of which corresponds to the lower data rate, and wherein symbols to be transmitted are subjected to repetition coding. 
     The invention also relates to a method for converting a signal data rate from a higher data rate to a lower data rate in a digital data transmission system, wherein a signal is transmitted by using frames of a given length, which length corresponds to the lower data rate, in which method the signal is grouped to a first block the length of which corresponds to the higher data rate, and wherein symbols to be transmitted are subjected to removal coding. 
     BACKGROUND OF THE INVENTION 
     The requirements set for data transmission systems continue to increase. This concerns especially wireless data transmission systems, such as cellular radio systems, which are required to provide more and more varied services, including different data and video services. 
     Traditionally, wireless data transmission systems have been used to transmit only speech. The increasing number of different services to be transmitted means especially in wireless systems that a system must be able to transmit signals with different capacities over the radio path, for example to transmit speech at the data rate of 8 kbps and data at the rate of 64 kbps. It is also necessary to transmit signals with different quality levels, which typically require different data rates simultaneously. A typical example is a video connection where the image requires a high data rate and a high quality level, but the voice can be transmitted at a lower data rate and with a lower quality level. Therefore, a data transmission system should be able to operate effectively in an environment where transmissions of several different data rates, quality levels and service types are forwarded. 
     It is typical of digital data transmission systems that signals are transmitted in frames and that the size of a frame is predetermined. When different data rates are used, it is not always possible to select for all data rates a frame which is exactly of the correct size and which has space for the symbols to be transmitted, but it is necessary to change the data rate so that the information to be transmitted can be forwarded in the available frames. 
     A data rate can be either increased or decreased, as required. The best known and the simplest method of increasing the data rate is repetition coding, wherein user data symbols are repeated as many times as there are free symbol positions in the frame. Correspondingly, the data rate is decreased in the simplest manner by removing user data symbols until the remaining symbols fit in the available frame. The problem in the prior art arrangement is how to select the symbols to be repeated or removed so that they are distributed evenly among the symbols to be transmitted. 
     BRIEF DESCRIPTION OF THE INVENTION 
     The object of the present invention is to implement the method such that the aforementioned problems can be solved. This is achieved with a method of the type described in the preamble, characterized in that the method comprises the steps of D1) calculating the number of symbols missing from the frame by subtracting from the frame length the number of symbols in the first block, and if the number of the missing symbols is greater than one, E1) repeating every ith symbol in the first block, i being determined as the nearest bigger or an equal integer when the number of symbols in the first block is divided by the number of symbols missing from the frame, F1) updating the first block by removing therefrom the symbols that have been repeated, G1) updating the number of symbols missing from the frame by subtracting therefrom the number of the symbols that have been repeated, and if the number of the missing symbols is greater than one, H1) proceeding to step E1). 
     The method according to the invention is also characterized in that the method comprises the steps of D2) calculating the number of symbols that do not fit in the frame by subtracting the frame length from the number of symbols in the first block, and if the calculated number of symbols is greater than one, E2) removing every ith symbol from the first block, i being determined as the nearest bigger or an equal integer when the number of symbols in the first block is divided by the number of symbols that do not fit in the frame, F2) forming the first block to consist of the symbols that have not been removed, G2) updating the number of symbols that do not fit in the frame by subtracting therefrom the number of the repeated symbols, and if the number of the missing symbols is greater than one, H2) proceeding to step E2). 
     The invention also relates to a transmitter in a digital data transmission system, comprising means for generating a signal having a first data rate, and means for forming the signal in frames of a given length that corresponds to a second data rate, and means for grouping the signal in a first block the length of which corresponds to the first data rate. The transmitter according to the invention is characterized in that the transmitter comprises means for calculating the difference between the frame and the number of symbols in the first block by subtracting the frame length from the number of symbols in the first block, means for equalizing the difference by removing or repeating every ith symbol in the first block, i being determined as the nearest bigger or an equal integer when the number of symbols in the first block is divided by the absolute value of the difference that was calculated, means for updating the number of symbols that do not fit in the frame by subtracting therefrom the number of the symbols that have been repeated, and means for forming the first block to consist of the symbols that have not been repeated or removed. 
     The preferred embodiments of the invention are disclosed in the dependent claims. 
     The method and the transmitter according to the invention provide several advantages. By means of the method according to the invention, the symbols to be repeated or removed can be selected as evenly as possible from among the symbols to be transmitted. In this manner the quality of the coding is the best possible. The method according to the invention enables flexible rate adaptation between data rates of different types. On the other hand, the method according to the invention enables unambiguous repetition or removal, so that it is possible to deduce the repeated symbols at the receiving end if the algorithm is known. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be described in greater detail in connection with the preferred embodiments and with reference to the accompanying drawings, in which 
     FIG. 1 shows an example of a wireless data transmission system where the method according to the invention can be applied, 
     FIG. 2 is a block diagram illustrating a possible structure of a transmitter implementing the arrangement according to the invention, 
     FIGS. 3 a  to  3   d  illustrate the arrangement according to the invention by means of different frame alternatives, 
     FIG. 4 is a block diagram illustrating the structure of another transmitter according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 illustrates a digital data transmission system where the arrangement according to the invention can be applied. This system is a part of a cellular radio system comprising a base station  100  that communicates via a bidirectional connection  102  to  106  with subscriber terminals  108  to  112 . The digital data transmission system used as an example is a cellular radio system, and the invention will be described in the following when it is applied in a cellular radio system but without restricting the invention thereto, as it is evident for a person skilled in the art. The invention is also applicable in other systems. 
     Assume that in the system the terminal equipments can transmit information at several different data rates depending on the data transmission capacity required. FIG. 2 is a block diagram illustrating a possible structure of a transmitter implementing the arrangement according to the invention. The transmitter comprises a number of data sources  200  to  204 , which may be provided for example for speech transmission or different data services. In the data sources, the information is subjected to channel coding. Signals  206  to  210  from the data sources are in the form of frames and their data rates differ from one another. The transmitter further comprises a switch or a multiplexer  212 , which selects or combines the signal from the active data source(s) to coding means  214 . Control means  216  control the operation of the switch or the multiplexer  212  and the other parts of the transmitter. The control and coding means  214 ,  216  are implemented most preferably with a processor by means of software. A signal  218  supplied from the coding means  214  is in the form of a frame. In the coding means  214 , the data rates of the signals from the data sources are adapted to the frame by using either repetition or removal coding by means of the arrangement according to the invention. It should be noted that the transmitter of FIG. 2 only comprises components that are essential for describing the invention. An actual transmitter also comprises other parts, such as filters and amplifiers, that are evident for a person skilled in the art, but they are not described herein for the sake of clarity. 
     FIGS. 3 a  to  3   d  illustrate the arrangement according to the invention by means of different frame alternatives. The purpose is to convert the data rate of the signal to be transmitted into a data rate that is suitable for the length of the available frame. The number of symbols that fit in the frame is denoted by N T  and the number of symbols in the signal to be transmitted after the channel coding is denoted by N C . FIG. 3 a  shows a data frame  300  of a channel, and a frame  302  of a data source. In the example of FIG. 3 a , the channel data rate N T  is higher than the information data rate N C . This requires the implementation of repetition coding so that the channel frame can be filled with symbols. FIG. 3 b  shows the data frame  300  of the channel and the frame  304  of the data source. In the example of FIG. 3 b , the channel data rate N T  is lower than the information data rate N C . This requires the implementation of removal coding or puncturing so that the information could be transmitted in the desired frame. 
     Examine the arrangement according to the invention in case of repetition coding, i.e. in the situation shown in FIG. 3 a , where N T &gt;N C . A first block S 0 ={d 1 , d 2 , . . . , d ns } is formed of the N c  symbols to be transmitted. Here n s  is the number of symbols in the block S 0 , and at the beginning of the procedure n s =N c . The purpose is to convert the lower data rate into a higher data rate by repeating N T −N C  symbols from the block S 0 , such that after the repetition the block size is N T  symbols. The repetitions are preferably distributed as evenly as possible in the frame to be transmitted. 
     The number of symbols y missing from the frame is calculated first by subtracting from the frame length N T  the number of symbols N C  in the first block. If the number of missing symbols is one, the first symbol in the block is repeated and the procedure is over. Otherwise term i is defined, such that        i   =     ⌈       n   s     y     ⌉                            
     wherein n s  is the number of symbols in the block S 0 . In other words, i is determined as the nearest bigger or an equal integer when the number of symbols in the first block is divided by the number of symbols missing from the frame. The first step of the repetition coding is carried out by repeating every ith symbol in the block S 0 . The number n R  of the repeated symbols is obtained by determining the nearest smaller or an equal integer when the number of symbols in the block is divided by i, i.e.          n   R     =       ⌊       n   s     i     ⌋     .                            
     The block S 0  is updated next by removing therefrom the symbols that have been repeated. After this, the number of symbols missing from the frame to be transmitted is updated by subtracting from it the number of the symbols that have been repeated, and the above-described procedure is repeated until all the symbol positions in the frame to be transmitted are in use. 
     Examine the arrangement according to the invention in case of removal coding or puncturing, i.e. in the situation shown in FIG. 3 b , where N T &lt;N C . A first block S 0 ={d 1 , d 2 , . . . , d ns } is formed of the N C  symbols to be transmitted. Here n s  is the number of symbols in the block S 0 , and at the beginning of the procedure n S =N C . The purpose is to convert the higher data rate into a lower data rate by removing N C −N T  symbols from the block S 0 , such that after the removals the block size is N T  symbols. The removals are preferably distributed as evenly as possible in the frame to be transmitted. 
     The number of symbols y that do not fit in the frame is calculated by subtracting the frame length N T  from the number of symbols N C  in the first block. If the number of symbols is one, the last symbol in the block is removed and the procedure is over. Otherwise term i is defined, such that        i   =     ⌈       n   s     y     ⌉                            
     wherein n S  is the number of symbols in the block S 0 . In other words, i is determined as the nearest bigger or an equal integer when the number of symbols in the first block is divided by the number of symbols that do not fit in the frame. The first step of the removal coding is carried out by removing every ith symbol in the block S 0 . The number n R  of the removed symbols is obtained by determining the nearest smaller or an equal integer when the number of symbols in the block is divided by i, i.e.          n   R     =       ⌊       n   s     i     ⌋     .                            
     The block S 0  is updated next to consist of the symbols that have not been removed. After this, the number of symbols that do not fit in the frame to be transmitted is updated by subtracting therefrom the number of the symbols that have been removed, and the above-described procedure is repeated until the symbols to be transmitted fit in the frame to be transmitted. 
     FIG. 3 c  shows a data frame  300  of the channel and a frame  306  of the data source. In the example shown in FIG. 3 c , the channel data rate N T  is higher than the information data rate N C , such that                N   T     -     N   C         N   C       &gt;   1     ,                          
     i.e. the size of the frame to be transmitted is more than double the number of symbols in the information to be transmitted. In such a case, in the preferred embodiment of the invention each symbol to be transmitted is repeated a sufficient number of times and the first block S 0  is formed of this symbol group. After this, the procedure is as described above. 
     In the example of FIG. 3 d , the channel data rate N T  is lower than the information data rate N C , such that                N   C     -     N   T         N   C       &lt;   A     ,                          
     wherein A is a predetermined positive constant from (0,1). A may be for example 0.2. FIG. 3 d  shows a frame  310  of a data source and two frames  300 ,  308  of different lengths that are used in the system and that contain N T  and N T1  symbols, such that N T &lt;N T1 . In a preferred embodiment of the invention, when the data rate of the data source exceeds the corresponding data rate of the smaller frame by a predetermined threshold, the adaptation is carried out by selecting for signal transmission the larger frame, which is subjected to repetition coding. In the example of FIG. 3 d , the symbols in the frame  310  of the data source are not transmitted through puncturing in the shorter frame  300 , but the larger frame  308  is selected for use and the repetition coding is executed with the method described above. In this manner it is possible to avoid a large number of removals that might deteriorate the signal quality. Another alternative in the situation of FIG. 3 d  is to divide the symbols in the frame  310  of the data source into two smaller frames  300  where the repetition coding according to the invention is carried out. 
     FIG. 4 is a block diagram illustrating a possible structure of a transmitter implementing the arrangement according to the invention. The transmitter used as an example in the figure utilizes the CDMA multiple access method, but the idea of the invention can naturally also be applied in other types of transmitters. The transmitter according to the invention therefore comprises a number of data sources  456  to  460 , which have output signals  400  to  404  with different quality level requirements and different data rates, and which may produce signals that are to be transmitted simultaneously. Possible data transmitters typically include a speech coder, a telefax terminal, a video coder or some other source producing digital information. 
     The transmitter according to the invention further comprises first coders  406 ,  408  for subjecting at least some of the signals to be transmitted to external coding, such that due to the coding the quality level requirement of the signals coded in this manner equals the lowest quality level requirement of the active data sources. Therefore, all the signals to be transmitted are provided with the same quality level requirement. The coding to be used can be carried out in known manners and it is preferable to use the Reed-Solomon coding method. The transmitter may also comprise first interleavers  410 ,  412  for interleaving the coded signals, which means that the positions of the symbols in the signal are changed according to a known algorithm. 
     The signals  400 ,  416 ,  418  that are obtained from all the data sources and that have thus a common quality level requirement are supplied to a multiplexer  414 , wherein the signals are combined on a time domain into frames of equal length that contain bits in numbers that may vary from frame to frame, depending on the signals at the input of the multiplexer. The output of the multiplexer  414  is functionally connected to a second coder  420 , where the signal is subjected to internal coding. This coder  420  may be any desired coder. The preferred embodiment utilizes a convolutional coder with which it is possible to lower the common quality level requirement. 
     The signal  450  coded in this manner is supplied to means  422 , where the frames to be transmitted are subjected to equalization of the number of symbols, such that the length of the symbols to be transmitted in each frame equals a multiple of a known time span that is substantially shorter than the symbol length. In connection with the CDMA method, this may be a multiple of the bit length or chip length of the spreading code. 
     The number of the symbols can be equalized by either repeating or removing symbols in the above-described manner according to the invention. The means  422  can be preferably implemented by means of software through signal processing with a signal processor. In the means  422 , the symbols of the signal are grouped into a first block the length of which corresponds to a desired equalized data rate. In the means  422 , the difference between the symbols in the frame and the desired number of symbols is calculated by subtracting the desired number from the number of symbols in the first block. The difference is equalized in the means  422  by removing or repeating every ith symbol in the first block, i being determined as the nearest bigger or an equal integer when the number of symbols in the first block is divided by the difference calculated. The means  422  form the first block so that it consists of the symbols that have not been repeated or removed, and they repeat the above-described procedure until the desired number of symbols is achieved. 
     In the transmitter according to the invention, it is also possible to form frames of different lengths, and when the higher data rate exceeds the lower data rate by a certain threshold, a larger frame is selected for signal transmission, and repetition coding is carried out. 
     The transmitter according to the invention further comprises a multiplexer  434 , which combines data about the frame structure of each frame, e.g. about the bit rate, to the signal frames that are to be transmitted and that contain an equal number of symbols. The data  426  concerning the frame structure is supplied first to a coder  432 , where the desired coding is executed to protect the information against transmission errors, and the coded signal is supplied to the multiplexer  434 . This data may comprise, for example, a description of the frame structure of the frame in question or the next frame to be transmitted. The signal  452  combined in this manner is supplied further to an interleaver  436 . 
     The transmitter may comprise a multiplexer  438 , where the interleaved signal is supplemented with additional information, such as power control data  428  or reference symbols  430  for coherent reception. When the transmitter is a CDMA transmitter, the combined signal  440  is supplied to a multiplier  442 , where the signal is multiplied by a spreading code characteristic of the connection, so that the signal spreads to the entire frequency band used. The signal multiplied in this manner is supplied further to radio-frequency parts, where the signal is converted into a radio frequency and amplified for transmission by an antenna  446 . 
     The transmitter according to the invention comprises control means  454 , which control the operation of the above-described parts and which may be implemented for example by means of a microprocessor, separate logic circuits or the like. The control means  454  also provide control to the radio-frequency parts  444  concerning the transmit power of each frame to be transmitted, and this power may depend on the number of symbols in the frame after the multiplexer  414 . 
     The transmitter according to the invention naturally also comprises other components, such as filters and converters, as it is evident for a person skilled in the art, but for the sake of clarity they are not mentioned in FIG.  4  and in the description related thereto. 
     Even though the invention is described above with reference to an example according to the accompanying drawings, it is clear that the invention is not restricted thereto but it can be modified in several ways within the scope of the inventive idea disclosed in the appended claims.