Rate-adaptive modulation techniques for infrared wireless communications

While infrared links can support transmission at high bit rates, it is difficult to maintain a signal-to-noise ratio (SNR) sufficient to support these high bit rates under all conditions using reasonable transmitter power levels. Under adverse conditions, instead of incurring abrupt link failure, it is preferable to achieve graceful degradation by lowering the bit rate until a suitably low error probability can be attained. We describe several coding and decoding schemes for rate-adaptive transmission, and we discuss their performance and implementation complexity. The codes considered are rate-compatible punctured convolutional (RCPC) codes and simple repetition codes. Each code is combined with L-ary pulse-position modulation. We present results of a search for families of good RCPC codes derived from rate-1/5 and rate-1/7 mother codes