Soft decision decoding in fast frequency hopping systems with convolutional codes and turbo codes

The purpose of this paper is to show how reliability information on each received bit can be derived to enable soft decision decoding for coded fast frequency hopping/multiple frequency shift keying (FFH/MFSK) multiple-access systems. Convolutional codes as well as turbo codes are considered and their performance is compared to that of previously proposed Reed-Solomon (RS) codes. A mobile radio environment yielding a frequency selective fading channel is assumed. It is shown that the application of turbo codes and convolutional codes with soft decision decoding can allow for a considerably higher number of simultaneously transmitting users than RS codes or uncoded systems. To achieve these throughput gains, however, the calculation of soft decisions after de-spreading is mandatory; the pertinent technique is described in the paper.