## 1 Introduction  
Radio-over-fiber (RoF) supports broadband interconnections between base stations (BS's) and the central office (CO) to release the heavy burden of wireless throughput [1; 2; 3; 4]. For an adequate RoF transmission, multiplexing techniques are applied to both optical and electrical domains. Optical time-division multiple access (TDMA) and wavelength-division multiple access (WDMA) are main schemes to multiplex signals from different users. For TDMA, users are allocated in different time slots. When the user number increases, the queue time between two successive transmissions becomes longer. Additionally, there is a stringent requirement of synchronization between transmitter and receiver. WDMA has a simple structure and fairly increases the transmission rate, but it is not efficient in bandwidth utilization, since only one wavelength can be used by a certain user. On the other hand, optical code-division multiple-access (OCDMA), providing multiple users with asynchronous access without scheduling, is a promising solution to implementing RoF [2; 3; 4]. Spectral amplitude coding (SAC) has been studied in many RoF schemes [3; 4] since it is free from multiple access interference (MAI), and the data can be encoded on an optical carrier without pre-sampling.  
In this paper, we propose a new system including two novel characteristics: supporting multi-service transmission and adapting wavelength reuse structure. These two issues have not been studied in previous research on SAC-based RoF. Due to the variant data traffics and diverse service types, scheduling algorithms on a wireless [5] and processing control scheme on an optical signal [6] is proposed to provide the requested quality of service (QoS). The optical control scheme is preferable because of its short processing time and low cost.  
We propose a power control scheme that classifies the total users into several classes. Different power levels are assigned to each class according to the specific class-of-service. Figure 1 shows the user classification of a 2-class scheme. \(K_{i}\) and \(P_{i}\) is the number of users and power in class \(i\), respectively,where \(i=1,2\). The power level \(P_{2}\) is higher than \(P_{1}\), so a higher signal-to-noise ratio (SNR) is detected for Class 2 and better service is achieved. Furthermore, the desired user can be identified through MAI elimination even if all coded signals are multiplexed in the same wavelength band.  
Our second contribution is the designation of a cost-effective BS. Wavelength reuse is commonly used for reducing the cost of a RoF network [7; 8], where both downlink and uplink share the same light source. This has advantages such as the centralization of the light source at the CO and a simplified BS structure. However, the broadband light source (BLS) used for SAC encoding is not suitable for generating the optical millimeter, which is required by most wavelength reuse schemes. A coherent light source, generally a laser, is used to generate optical millimeters for up- and downlink transmissions by modulation schemes such as a double side band (DSB) [9] or a single side band (SSB) [10]. Unlike a laser with a near impulse spectrum, the broadband light source (BLS) for SAC encoding occupies a range of wavelength band of 1 to 10 THz. Employing DSB or SSB on BLS would result in an overlapping between the down- and uplink carriers. Thus, in this paper, we employ the concept of wavelength-division multiplexing (WDM). The spectrum of BLS from the CO is divided by a de-multiplexer (De-Mux) into two carriers. Only one of the carriers is modulated by the downlink data, while the other is for the uplink transmission.  
To investigate the proposed system performance, the bit-error rate (BER) is used to quantify QoS by considering the effects of thermal noise and phase-induced intensity noise (PIIN). Along with simulations, the numerical results show that we achieve significantly differential service between individual users when employing power control in the proposed RoF architecture.