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

Chapter 6Subcarrier-Multiplexed ROF Uplink

In Chapter 5 the downlink of a dual-carrier Fi-Wi, transmitting both cellular radio and Wi-Fi signal, was analyzed. In this chapter the uplink performance of the same system will be analyzed.

Note that in the uplink direction, these subcarrier-multiplexed radio signals travel first over the wireless channel and then over the nonlinear ROF channel. Therefore, there are a few noteworthy points as listed below.

• Since the RF signal experiences random fluctuation due to fading and shadowing effects in air first, the envelope of the RF signal that modulates the laser is essentially a random signal.
• Since the WCDMA uplink transmission is asynchronous, there will be multiuser interference among WCDMA users. This multiuser interference happens before the signal goes through the nonlinear optical link.
• Nonlinearity in the ROF link creates both co-channel interference within the same RF subcarrier and cross-talk between the RF subcarriers. These are functions of wireless channel fading as well.
• Noise is added twice at the optical and wireless channels. The RIN is a function of total RF power over fiber.
• The SNR and nonlinear distortion of any subcarrier is a function of total RF power in the system. Hence, the noise and nonlinearity may adversely affect the low-power RF subcarrier when the other subcarrier has significantly higher power.

The functional block diagram of the subcarrier-multiplexed uplink portion is shown in Fig. 6.1. This figure ...