Generation of A 16-Star/Square quadrature amplitude modulation (QAM) signal in radio over fiber system

Radio-over-fiber (RoF) system, which is a promising integration solution of wireless and optical systems, has been attracting increasing attention for its significance in providing broadband wireless access with increased capacity and mobility of the access network and reduced cost of the system. 16-Star and 16-Square QAM signals, carrying four information bits by a single symbol, are attractive modulation format in radio over fiber system because of high spectral efficiency and low symbol rate. In this paper, using a quad-parallel Mach-Zehnder modulator followed by a single-drive Mach-Zehnder modulator, we propose a scheme to generate a 16-Star/Square QAM radio frequency (RF)-signal in radio-over-fiber system. The RF carrier is obtained by quintupling the frequency of the employed local oscillator signal, thus our proposal is cost-effective.

1. INTRODUCTION

Recently, quadrature amplitude modulation (QAM) has received increasing attention because of its high spectral efficiency and low symbol rate in radio-over-fiber (RoF) systems. Several schemes have been demonstrated to generate the 16-Square QAM RF-signals [1]–[4]. All of these methods can not obtain 16-Star QAM RF-signals. It is easy to verify that, in the wireless scenario, the 16-star QAM signal outperforms the 16-squre QAM signal in terms of signal-to-noise ratio (SNR). Moreover, in these methods, the RF-carrier is produced by frequency doubling technique. Hence, to obtain a high-frequency RF-carrier, a high-frequency local oscillator (LO) is indispensable, which in turn increases the cost and the complexity of the system.