Development of remote optical local oscillator for radio over fibre system

Abstract

Radio over Fiber (RoF) is a promising technology capable of serving huge demands in the ever expanding wireless communication system. A number of studies have employed four RoF system architectures namely optical heterodyning, external modulation, optical transceiver, and up- and down-conversion for signal generation to solve the problem of system congestion. In this study, the millimeter-wave (mmwave) frequency band has been identified in resolving the congestion problem. In particular an up-conversion RoF system architecture which uses a Remote Optical Local Oscillator (ROLO) is proposed. The optical signal, generated using a 10 GHz radio frequency (RF) signal utilises the Stimulated Brillouin Scattering (SBS) technique at the Central Station (CS). At the Base Station (BS), this signal is used to up-convert a modulated intermediate frequency (IF) signal by using a microwave mixer. The mixer is developed using a Heterojunction Bipolar Transistor (HBT) as its main active component due to its high internal gain. In addition, the mixer also functions as the frequency conversion stage. This study discovered that the proposed RoF-ROLO system is effective in reducing the dispersion effect which normally restricts the performance of mm-wave RoF system, in which the frequency conversion is done at the BS. The system was designed and simulated using the OptiSystem software for up to 40 GHz mm-wave carrier. Besides, the HBT mixer configuration has been successfully modelled and simulated using Microwave Office (MWO) software. Verification was carried through real-time measurement. The simulated conversion gain of the mixer achieved ranges between 2.11 dB to 7.97 dB for modulated IF input power ranging from -30 dBm to -10 dBm, respectively. These values were obtained by fixing the Local Oscillator (LO) power to 0 dBm. Moreover, the system has practically up-converted RF signal at 12.92 GHz. The new configuration between the SBS mm-wave signal generation with the up-conversion technique has been found to be practical by omitting the necessity of mm-wave LO at the BS; yet the frequency conversion still can be done at the BS.