Hybrid Optical/Radio Frequency (RF) Communications

Abstract

The concept of free-space optical communications (FSOC) has been around since the late 1960s. Lasers offered the potential for small transmitters and receivers with very high antenna gain (i.e., small transmitter spot sizes). Specifically, FSOC systems could be much more efficient and could provide orders of magnitude gains in data rate compared to a radio frequency (RF) system of the same size. Unfortunately in the 1970s and 1980s, much of the potential gain in efficiency was lost because of poor electrical-to-optical efficiency, poor optical detector efficiency, the increased transmitter spot sizes necessitated by transmitter pointing error limitations, and most importantly, link degradation from optical channel effects. The result was that the advantages of optical communications over RF communications were never realized for the past 40 years, except with one exception, fiber optical communications (FOC). This is because FOC technologies overcame the detector and efficiency problems cited above; laser light could be easily launched into fiber optic cable and FOC did not suffer the channel effects that plagued FSOC.1 The FOC technology break-through can help move FSOC systems into a reality if the FSOC channel effects can be overcome. The realization that the latter aspect is possible came from the realization that neither FSOC nor RF communications can provide totally reliable, multi-gigabit/tetrabit per second (Gbps/Tbps) communications like a FOC system by themselves. However, they have the potential to move towards that goal by working together in a network infrastructure [1]. That is, through a hybrid optical-RF networking construct. This can be clearly seen in Table 9.1. 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. On the other hand, FSO communications offer enormous data rates, but operate much more at the mercy of the environment. The perennial limitations of FSOC systems are directly attributable to signal fading/scintillation (optical turbulence) and path blocking (cloud obscuration) [1]. Both phenomena reduce the 1 FOC did have its own channel effects to deal with, but their resolution is a major success story that will not be covered here.