In a mobile radio system which operates according to the TDMA principle, i.e., the different radio channels are time divided in different time slots and different carrier frequencies, where one radio channel occupies a given time slot and a given frequency, there is a need to be able to establish the identity of a given radio channel. The radio channel may, for instance, be an access channel, a control channel or (as is most usual) a traffic channel. It is known to introduce into the time slot a special bit field which states this identity.
In a mobile radio system, radio signals are transmitted in the different channels over media which can give rise to errors in the information transmitted and thus also in errors in the bit field denoting channel identity. The errors are caused by the properties of the radio media, such as errors caused by fading and multipath propagation.
In order to correct these transmission errors, so-called channel coding is introduced in channel transmission, which involves the addition of further bits to the information bits. This block of bits being coded in accordance with a given code prior to transmission over the radio medium. An example of one such code is the so-called convolution code for code-word lengths of 2-10 bits, where the bits in a code-word are dependent on the bits in preceding code-words. A given code, for instance a convolution code, is generated with the aid of a generator polynomial or a code polynomial of the kind EQU g(x)=1+a.sub.1 x+a.sub.2 x.sup.3 +.. a.sub.n x.sup.n
where some of the coefficients a.sub.1, ...,a.sub.n are=0.
The introduction of redundant bits and/or code information bits with the aid of generator polynomials apparently causes the block length to grow and requires greater processor capacity both when channel coding in the transmitter-and when channel decoding in the receiver. This problem is most noticeable when receiving in a mobile station in which the requisite processor capacity for channel decoding must not be too high.
Another known method of distinguishing between the identities of different radio channels involves the use of different coding schedules when channel coding. For instance, the aforesaid convolution coding can be used, although with different rates for the speech channels and the control channels respectively. Generally, the code rate is defined as EQU R=k/n,
where k=the number of information bits in each code-word and n=the number of bits in each code-word. According to the above, a given value R1 can be used for coding the speech channels and another value R2 can be used for coding the control channels.
This known method involves the necessity of completely decoding all candidates and the selection of the channel which passes the error discovery test. Although this method is reliable, the problem of high complexity upon reception in a mobile station remains.