Analog vs Digital Radio-Over-Fiber: A Spectral Efficiency Debate From the SNR Perspective

For decades, wireless communication networks have applied the digital interface like the common public radio interface (CPRI) between remote sites and base stations for data transport. Due to the fine digitizing resolution, such a digital radio-over-fiber (RoF) solution has long been blamed for its high optical bandwidth consumption. In post-3G era when the wireless traffic significantly increases, analog RoF has revived in research owing to its intrinsic high spectral efficiency (SE). A number of metrics are proposed to compare the SE across RoF interfaces, among which a popular one is the CPRI equivalent data rate (CPRI-EDR). CPRI-EDR bridges analog and digital worlds by universally converting their SE to the digital speed requirement of CPRI, but it ignores another critical metric of link fidelity, namely, an interface with a CPRI-EDR may not offer an equivalent signal-to-noise ratio (SNR). More broadly, there is a tradeoff between SE and SNR for most RoF interfaces. This is commonly realized by the flexible-rate data compression for digital RoF. In terms of analog RoF, we recently exploited angle modulations to enhance the SNR at the sacrifice of spectrum expansion. Therefore, it is desired to find an SNR-related metric to fairly compare the SE. In this paper, we propose an SE metric as a function of SNR i.e.,
R
(
SNR
), defined as the ratio between the required optical bandwidth for a target SNR and the radio signal bandwidth. We analyze the factors for both digital and analog RoF that influence
R
, based on which a comprehensive SE comparison is illustrated. The results shed light on the future design of RoF links aiming to provide a flexible quality of service cost effectively.