I. Introduction
With the increasing demand for high-data-rate transmission for broadband wireless access, the provision of wireless services over wavelength division multiplexing (WDM) passive optical networks (PONs) has been a topic of research interest recently. In a radio over WDM-PON, to ease the system installation and maintenance, colorless optical network units (ONUs) are essential. Numerous methods for colorless operation have been proposed in recent years, including spectrum slicing of a broadband light source that covers a spectral range of all the transmission wavelengths [1], the use of tunable laser sources (TLSs) with a tunable wavelength over a spectral range of transmission [2], and the reuse of the downstream wavelengths [3]– [14]. For the schemes using a broadband light source that covers a spectral range of transmission [1], the modulation bandwidth is usually very narrow due to the poor quality of the spectrum-sliced broadband light source. In addition, such schemes suffer from slicing losses. The use of TLSs at an ONU [2] can solve the two problems. However, it is not a good solution for low-cost implementation because of the very high cost of TLSs. In addition, prior information is required for wavelength tuning at the ONU, which makes the end-to-end control complicated. To reduce the cost and relax the requirements for wavelength management at the ONU side, several schemes have been proposed based on wavelength reuse of a downstream optical signal for upstream transmission, including injection locking of a Fabry–Pérot laser diode (LD) [3]– [6], and the utilization of gain-saturation of a semiconductor optical amplifier (SOA) [7] or a reflective SOA (RSOA) [8]– [10]. However, these schemes require a low modulation depth of the downstream optical signal to reduce the crosstalk to the upstream signal, which limits the transmission performance of the downlink. In addition, the analog bandwidth of an SOA or RSOA is very limited (typically around 1 GHz). To overcome this limitation, two radio over fiber (RoF) systems have been proposed based on the reuse of a phase-modulated downstream optical signal for upstream transmission using a polarization modulator [11]– [13]. However, since two orthogonally polarized light waves, which are phase modulated by a radio signal, are utilized for the purpose of wavelength reuse, polarization multiplexing cannot be employed, which may decrease the spectral efficiency. To solve this problem, recently we have proposed a RoF system with wavelength reuse based on polarization multiplexing and coherent detection [14] . In [14], the polarization multiplexing can be employed, but the system is not symmetrical, since the polarization multiplexing can be only applied for the downlink but not for the uplink, which results in that the data rate for the downlink is twice of that for the uplink.