Information Rate loss from radiation decoherence

The information rates achievable by using electromagnetic radiation affected by thermal noise and signal decoherence are studied. The classical coherent-state Gaussian model is compared with an alternative photon gas model which represents lack of a shared phase reference between transmitter and receiver. At any frequency, the information rates in the presence of decoherence essentially coincide with those over a Gaussian model when the signal-to-noise ratio is below a threshold. Only above the threshold does decoherence cause a significant loss in information rates; the loss can amount to half of the capacity. The threshold exceeds 40 dB for radio frequencies and vanishes at optical frequencies.