Symbol combining for frequency-hop hybrid ARQ communication systems subject to fading and partial-band interference

In channels affected by fading or strong interference, the throughput of frequency-hop spread-spectrum communication systems employing hybrid automatic repeat request protocols can be unacceptably low. We show that significant improvements in throughput can be achieved by combining symbols between the different transmissions. Several symbol combining techniques based on Viterbi´s (1982) ratio threshold test and on Bayesian decision theory are proposed and are shown to provide substantial improvements in performance over systems that do not employ symbol combining