Design and Analysis of Digitized RF-Over-Fiber Links

In this paper, we present a performance analysis of microwave signal transmission by deploying digitized RF transport over a standard optical fiber link. The bandpass sampling technique is used to digitize the RF signal. An analytical model, which is transparent to any modulation format, was developed to assess the transmission performance of a digitized RF signal in M-level quadrature amplitude modulation (M-QAM) format. Using this analytical model, key design parameters such as bit resolution, link bit rate, receiver sensitivity, and link signal-to-noise ratio (SNR) were analyzed for a digitized 2.475 GHz RF signal with 16-QAM data modulation format and 6 MSymbols/s baud rate over a 20-km optical link. The analytical results show that the link SNR is predominantly determined by the performance of the analog-to-digital converter (ADC). We experimentally demonstrated the digitized RF transmission over a 20-km optical link. The analytical model and experimental results show that the performance of the link SNR is governed by jitter noise beyond 7 bits of resolution of the ADC. Using an ADC with 8-bit resolution, a 2.475 GHz RF signal with 16-QAM 6 MSymbols/s data was successfully transmitted and recovered with very good error vector magnitude of 1.77.