Radio Frequency (RF) communications are generally reliable and well understood but cannot support emerging data rate needs unless they use a large portion of the precious radio spectrum. Free Space Optical (FSO) communications offer enormous data rates but operate much more at the mercy of the environment. The perennial limitations of FSO communications are manifested in the channel attributes of scintillation (optical turbulence) and path obscurations. Both phenomena reduce the availability of the optical channel to support reliable communications. Since RF paths are relatively immune to the same phenomenology, combining the attributes of a high data rate but bursty link (FSO) with the attributes of a low data rate (by comparison) but reliable link (RF) could yield attributes better than either one alone: high availability with high data rates.
This paper is based on a recently devised system that uses ultra-short pulses of laser light that provide greater bandwidth and improved reliability over conventional optical wireless links. The approach uses a technique called "Fractal Modulation", which is a form of Wavelet Packet Modulation (WPM), to produce wavelets that can co-exist in a signal channel without interference, and provide frequency and time diversity, concurrently. By sending the same message at several different rates (multi-rate), one can get through adverse weather conditions.
Also, in this paper we investigate channel equalization and channel coding for such a multi-rate communications scenario. As for the equalization, we considered a MMSE linear equalization after photo-detection. For the channel coding, we have used Fountain Codes, a new class of erasure correction codes, in concatenation with an inner convolutional code. We argue that for a parallel multi-rate system Fountain coding is a flexible method for receiving data from multiple streams.