Tradeoffs between frequency-hop and direct-sequence signaling for frequency-selective fading channels

Spread-spectrum modulation can provide protection from the selective fading that is typically encountered in mobile radio networks. Because the methods of combating frequency-selective fading are quite different for frequency-hop (FH) and direct-sequence (DS) spread spectrum systems, these two types of modulation behave very differently as the communication channel varies. The purpose of this paper is to compare the performance of FH and DS systems under identical conditions for a general wideband fading channel model. Each system has the same bandwidth, transmits over the same frequency-selective Gaussian wide-sense-stationary uncorrelated-scattering channel, and uses error-correction coding to combat thermal noise and fading. The probability of bit error at the output of the decoder is determined for each system by a combination of analysis and simulation. Results are presented for systems with a single transmitter-receiver pair and for systems with multiple simultaneous transmissions (i.e., multiple-access communications). These results illustrate the tradeoffs in performance between the two types of spread-spectrum systems as a function of the parameters of the spread-spectrum signals and the frequency-selective fading channel.