Highly linear transmission and EVM improvement of vector modulation signals for radio-over-f iber applications

Abstract

The principle of error vector magnitude (EVM) against modulator nonlinearity for vector modulation signal (VMS) transmission in radio-over-fiber (RoF) systems is theoretically and experimentally investigated. A highly linear modulation scheme is proposed and demonstrated using a single-drive dual parallel Mach-Zehnder modulator (MZM). This method improves EVM performance and enlarges the linear input dynamic range of the VMS transmission. An index of maximum allowable input power difference (MAIPD) that reflects the difference of upper input power limits between these two schemes is measured. An EVM limitation of 5% MAIPD has 5 dB. Both 16-and 64-QAM results indicate that the proposed scheme supports VMS transmission better than the MZM one.OCIS codes: 060.2360, 060.4080, 060.5625. doi: 10.3788/COL201210.100606.The combination of wireless and fiber access technology makes radio-over-fiber (RoF) application a promising tool in bringing high-speed broadband wireless services into reality [1] . Recent studies have increasingly focused on complex modulation formats, such as quadrature phase shift keying (QPSK), quadrature amplitude modulation (QAM), and even orthogonal frequency-division multiplexing (OFDM), including the transmission quality of these formats. Previous reports have demonstrated the performances of each signal in different RoF systems [2−4] . Moreover, input signal power increase is effective, to some extent, in combating noise [3] and fiber dispersion [4] . Unfortunately, the applicable range is too narrow because nonlinearity, especially modulator nonlinearity, seriously worsens signal-to-noise ratio (SNR) and error vector magnitude (EVM) when more power is added. We have previously reported a highly linear modulation by optically mitigating the third-order inter-modulation distortion (IMD) [5] . The proposed scheme demonstrates better linear transmission capability, with 16-QAM modulation signals compared with the results of a quadrature biased Mach-Zehnder modulator (QB-MZM). Nevertheless, such finding lacks theoretical explanation for EVM improvement, an area that should be studied further for guidance in real applications.In this letter, experiments are conducted between systems that use a single-drive dual-parallel MZM (SD-DPMZM) and a QB-MZM, respectively. The study is conducted with EVM serving as critical evaluations of vector modulation signal (VMS) transmission in a RoF system. The proposed technique is then extended to incorporate the capability of reducing the EVM of VMS for application in high-power input scenarios. To the best of our knowledge, it is the first time to present the evolution of EVM versus modulator nonlinearity in RoF schemes.Compared with previous studies, the work presented here focuses on lower RF carrier frequency (5 GHz) and bit-rates (up to 300 Mbps). This work, however, is acceptable in cellular and WLAN applications, where the wireless band rangs from 900 MHz to 5.2 GHz [2] . As previously demonstrated, the proposed configuration works at a higher frequency ban...