Spread spectrum and coding for multiple access communications

In this paper, we discuss the use of spread-spectrum modulation and error correction coding for multiple access communications such as mobile communications. Our emphasis is on the connection between, and interdependence of, these two techniques. This connection is illustrated through three examples arising from our recent studies in this area. First, we consider frequency-hopped (FH) spread-spectrum multiple access communications employing M-ary modulation and error correction coding. We show that a mismatch between, for example, Q-ary Reed-Solomon (RS) codes and M-ary modulation, which is most likely to occur in practice, can drastically reduce network capacity. As a remedy, a matched M-ary BCH code is shown to outperform an RS code with a comparable code rate and decoding complexity when the number of users in the system is large. Frequency hopping is a nonlinear multiple access system. For a linear multiple access system such as a cellular, direct-sequence, code division multiple access (CDMA) system, it is pointed out that the capacity is linearly inversely proportional to the signal-to-noise ratio (E
b
/N
0
) required at the receiver to maintain a given error performance. Therefore, the key to increasing the CDMA capacity is to increase the coding gain to reduce the required E
b
/N
0
. In fact, differences in the prediction of CDMA capacities are largely due to differences in the prediction of the required E
b
/N
0
value in a mobile fading channel. Finally, we demonstrate a connection between spread spectrum and coding by considering the analysis of the multiple access interference in an FH system. The system employs RS hopping sequences derived from an RS code. This system is shown to have better access performance than a random hopping system, due to the coding structure of RS codes.