Full-duplex communications for noise-limited systems

Full-duplex communications has become a hot area of research. A full-duplex can potentially double the amount of available spectrum, has a positive impact on the design of the protocol stack, and simplifies the design of acquisition and tracking. Most, if not all, of the recent research in this area has been focused on implementing FD for commercial terrestrial networks such as WiFi and cellular. The links in these networks are typically limited by multi-user interference and fading. To the best of the author's knowledge, very little work has been done in applying FD to noise-limited systems. A noise limited system, for the purpose of this paper, is one where additive white noise (AWGN) is the primary channel distortion. Typical examples include satellite communications (both ground to satellite and satellite to satellite) and point-to-point microwave relay links [13]. Data throughput in these systems is limited by the classical Shannon capacity - fixed available spectrum and fixed available transmit power. Both of these limitations can be due to either technical or regulatory considerations. These systems are usually `simpler' than typical multi-user networks such as WiFi and cellular. The point-to-point communications and lack of mobility motivate the use of highly directional, high gain antennas. These antennas can be either fixed (e.g. dish) or electronically steered beams (e.g. phased array). In either case, MIMO techniques are usually not applicable since the channel is trivially static (i.e. line of sight). Powerful error correction codes allow operation close to the Shannon limit. The receiver noise figure is the single most important factor in determining the link throughput. This paper presents a detailed implementation of a full-duplex receiver front end for a noise-limited system. The design trade-offs are driven by the desire to minimize the overall receiver noise figure.