Digital Coherent Receiver Based Multiband Phase Modulated Radio-over-Fiber Systems

High-performance,low-cost multi-band access network is one of the bottlenecks limiting the development of next generation mobile communication systems. Therefore, it is a challenging goal for the network infrastructure to support multi-band and having the ability to re-configuration which restricted by the structure complexity, system flexibility and high-cost.In this paper, we proposed a phase-modulated and coherent-detected radio-over-fiber (RoF) system to achieve this goal. The digital signal processing (DSP) technology and the direct bandpass sampling technique is utilized in the proposed system in order to improve the system flexibility and reduce costs.We first investigate the link performance and its transmission characteristics of the proposed phase-modulated and coherent-detected RoF system. The direct bandpass sampling technology is applied into the design in order to overcome the high sampling rate requirement and front-end hardware dependency of conventional digtal coherent receivers. In particular, the selection rule of bandpass sampling rate was defined by taking into account the free-running optical local oscillator induced frequency offset. Simulation results show that the proposed link structure can provide acceptable system performance under the conditions of 8-bit analog-to-digital converter (ADC)sampling resolution and 100 kHz laser linewidth. A 40Mbps quadrature phase shift keying (QPSK) modulated data signal at 2.4GHz radio frequency (RF)carrier is experimentally demonstrated over the proposed 50.6-km RoF link.The link budget exceeds 15 dB in the conditions of -10 dBm received optical power and 1 GSa/s sampling rate. This is enough to ensure the stability and reliability of information transmission.In order to meet the requirements of future integrated heterogeneous wireless access. An effective method for determining the valid ADC sampling frequency ranges is derived to ensure no aliasing occurs in the scenario of multi-band signals transmission in the presence of free-running optical local oscillator caused time varying frequency offset. The hybrid-transmission and direct frequency down-conversion of two-band QPSK subcarrier multiplexed signals was experimentally demonstrated over the proposed RoF link. Which makes the system independent to the high frequency analog filters and radio frequency local oscillators. Moreover, it is transparent to the sub-carriers and its data format, therefore multi-band communications re-configuration can be achieved by changing the ADC sampling rate and DSP processing algorithms.Concerning on the high cost of existing commercial high analog bandwidth photodetectors and ADCs. An optical intensity modulator is utilized as the bandpass sampler which performs encoded on-off keyed pulse sequence on the optical local oscillator. QPSK modulated data signal with 20 MHz bandwidth at 15.2 GHz RF carrier frequency is experimentally demonstrated to be successfully detected by using balanced photodiodes(BPDs) with only 800 MHz analog bandwidth. The conversion gain and bit error rate curves are obtained.