Throughput of unslotted direct-sequence spread-spectrum multiple-access channels with block FEC coding

An analysis of unslotted random-access direct-sequence spread-spectrum multiple-access (DS/SSMA) channels with block forward error correction (FEC) coding is presented. Extending a methodology that was introduced in an earlier paper on unslotted packet code-division multiple access (CDMA) without coding, a procedure for calculating the error probability of an L-bit packet in the variable message length, FEC-coded, DS/SSMA environment is described. This procedure is then used in conjunction with appropriate flow equilibrium traffic models to compute channel throughput. Using BCH block coding as an example, the analytical model is exercised to obtain throughput versus channel traffic curves over a range of code rates, leading to an assessment of maximum achievable throughput and the associated optimum FEC code rate. The results show that the use of block FEC coding provides a significant improvement in the bandwidth-normalized channel throughput (utilization), approaching values competitive with those for comparable narrowband ALOHA channels