A comparison of frequency-hop and direct-sequence spread-spectrum communications

Abstract

Mobile radio networks in tactical environments are typically subject to frequency-selective fading. Many techniques, including error-control coding, modulation schemes, and sophisticated receivers, are used to provide diversity and improve communication over a fading channel. Spread-spectrum modulation, in particular, has properties that make it suitable for mitigating the effects of fading. Two types of spread-spectrum modulation combat fading in different ways. Frequency-hop (FH) modulation and block coding with interleaving can provide diversity among the symbols of a codeword in a frequency-selective channel. On the other hand, direct-sequence (DS) modulation has anti-multipath capability that allows the receiver to discriminate against interference caused by multipath propagation. If a RAKE receiver is used, the copies of the signal that are received with different delays can be combined to provide diversity. Each system takes advantage of different features of the fading channel, so channel variation affects the behavior of these two methods differently. For example, an increase in the correlation bandwidth improves the performance of a DS system while degrading the performance of a FH system. Previous research on the relative performance of these two formats is limited to specific channel models and environments. The article considers a general wideband channel model ann studies each system under identical conditions. The purpose is to explore the effects of the channel's characteristics on the performance of both FH and DS spread-spectrum systems.