Sidestepping the Rayleigh limit for LoS spatial multiplexing: A distributed architecture for long-range wireless fiber

While existing wireless systems rely on rich scattering environments to obtain spatial multiplexing gains, such gains are also available for line-of-sight (LoS) point-to-point links if the range is small enough. For fixed antenna separation (limited by node form factor) and carrier frequency (limited by available electronics), the range up to which multiple degrees of freedom are available is limited by the Rayleigh criterion. In this paper, we propose a distributed architecture for sidestepping this criterion, leading to spatial multiplexing gains at ranges much larger than those dictated by node form factor and carrier frequency constraints. Although the antenna spacings at the transmitter and the receiver are not large enough to support spatial multiplexing, the use of relays spread out over a larger area enables synthesis of a full rank MIMO channel. We focus specifically on the design of air-to-ground wireless fiber links over ranges of tens of kilometers, which have the following features in addition to relay-enabled spatial multiplexing: (a) the small carrier wavelength enables large beamforming gains even for nodes of compact form factor, (b) the large available bandwidths enable multiGigabit speeds for each spatially multiplexed data stream.