Radio over Fiber for Wireless Communications: From Fundamentals to Advanced Topics

Chapter 3Power Link Budget and Cumulating SNR

3.1 Introduction

In the last chapter, we reviewed the basic characteristics of the elements of a Fi-Wi system. In this chapter, we will review two, closely interrelated issues of a Fi-Wi system; namely the link power budget and the SNR issues.

Although the optical fiber has huge bandwidth, its power-handling capability is naturally poor. A single-mode optical fiber gets the ability to provide a large bandwidth by essentially having only a few micrometers of core radius that will allow only a single ‘ray’ (or mode) of light to propagate. This inherently limits the fiber's power-handling capability. In addition, the RF-optical modulation depth of ROF links should be kept small to limit nonlinear distortions at optical modulators. This means only a fraction of the total power will be allocated to RF sidebands. Also note that power losses in the optical domain will appear twice in the electrical domain due to the square-law detection nature of the photodiode. As a result, the received RF power at the optical receiver is typically very low (30 dBm or less) depending on the fiber length [1999]. This has to be amplified significantly (by 40–50 dB) at the RAP before driving an antenna to overcome wireless channel path losses.

The second issue is that in a Fi-Wi channel, the analog RF signal undergoes two lossy channels (optical and wireless) ...