## 2 Wireless Signals Transport Strategies for Fiber-Wireless Link
Since the wireless signals are transported in a digital format, the dynamic range of the optical link is independent of the fiber transmission distance until the received signal goes beyond the link sensitivity. In this scheme, the complex signal processing functionality is carried out in the CO while only a minimal set of frontend components consisting of the analog-to-digital (ADC) and digital-to-analog (DAC) converters are located in the antenna BS. The link is able to maintain its transparency as the BS does not require full processing of the wireless signals. The key enabler for this scheme is the ADC/DAC technology. To digitize the wireless signals, the analog bandwidth of the ADC must be larger than the wireless carrier frequency and with sufficient sampling rate and resolution. This requirement becomes more stringent for wireless signals beyond the microwave bands. To overcome this limitation, digitized IF-over-fiber is introduced whereby the wireless signal is first downconverted to an IF frequency to cater for the limitations of current ADC technology. This scheme is depicted in Figure 2e. Open base station architecture initiative (OBSAI) [20] and Common Public Radio Interface (CPRI) [21] are two initiatives to create an open market for cellular networks by providing specifications for interface of wireless base stations. OBSAI and CPRI are based on digitized IF-over-fiber scheme and the base station architecture is shown in Figure 3a. In the downlink direction, the optical digital datastream is recovered using a photodetector (PD) and digitally processed before the analog signal at an IF frequency is reconstructed using a DAC. The wireless signal is then generated after two stages of frequency upconversion process. Likewise in the uplink direction, the wireless signal is downconverted using two frequency translation stages before the IF signal is digitized, processed and modulated onto an optical carrier to be transported back to the CO.  
We can further relax the requirements on the ADC technology by using bandpass sampling technique [22] rather than direct sampling, especially given that for most wireless applications the required information bandwidth is only a small fraction of the wireless carrier frequency. Bandpasssampling undersamples the wireless signal using a much lower sampling frequency that is comparable to the wireless information bandwidth instead of the wireless carrier frequency [22]. Therefore upon sampling, it will generate an infinite number of image replicas of the wireless signal. The sampling frequency has to be carefully chosen to minimize noise aliasing from adjacent images. Since an infinite number of image replicas are generated, the wireless signal can be recovered using appropriate bandpass filter. Hence we are able to perform frequency translation without using LO and mixers. Shown in Figure 3b is the BS architecture of a fiber-wireless link incorporating digitized IF-over-fiber transport based on bandpass sampling. The benefit of using bandpass sampling reflects in the uplink path where no frequency translation stage is required which can further simplify the base station design.