Communication by Energy Modulation: The Additive Exponential Noise Channel

Communication across an additive exponential noise (AEN) channel is studied. The AEN channel is derived from a continuous-time Gaussian channel by assuming decoherence between the signal and noise components. Decoherence renders impossible the use of the complex amplitude and information is transmitted by modulating the signal energy. The AEN channel is shown to perform closely—in terms of channel, information rate, and error probability of binary codes, including the use of bit-interleaved coded modulation—to an equivalent discrete-time Gaussian channel of identical signal-to-noise ratio. Constellations are designed for low signal-to-noise ratios, where the minimum energy per bit is not universally attained by all modulations, and for high signal-to-noise ratios, where an equiprobable nonuniform constellation 0.76 dB away from capacity is described. The multiple-access AEN channel is also considered: its capacity region coincides with that of the Gaussian equivalent; feedback does not, however, enlarge the capacity region.