In this PhD dissertation, an opto-electronic oscillator (OEO) implementation for a radio access network (RAN) is introduced. The OEO is an excellent solution for producing a low phase noise signal in the microwave and millimetre wave ranges. The implementation of a single-loop OEO in the RAN is proposed in order to simplify the current configuration of the base stations in the RAN. Simplification is assured due to the fact that there is no need for local oscillators in the base station thanks to the oscillator signal distributed from the centralized OEO. This is achieved by employing the single-loop OEO in the central station of the RAN and distributing the OEO’s signal to base stations via an optical distribution network called passive optical network (PON).
As the first step of this work, the preliminary idea for the single-loop OEO implementation in the RAN is clarified. The general idea and the way in which the OEO signal is distributed from a central station to multiple base stations are explained. In addition, the possible challenges of the proposed idea are investigated. A literature review is performed and possible solutions are offered to overcome the potential challenges of the implementation.
One of the main challenges is the power penalty due to chromatic dispersion for high-frequency signal distribution. The power penalty can seriously degrade the OEO’s signal distribution to some base stations via PONs because chromatic dispersion in the 1550-nm wavelength is very dominant in the microwave and millimetre wave ranges. Chromatic dispersion causes a phase shift and power loss for the distributed optical signal. As a solution in this dissertation implementation of a tunable-dispersion compensation module (TDCM) for each base station to tune for compensation of the chromatic dispersion is proposed. The OEO is combined with the TDCMs to compensate the chromatic dispersion and to avoid the power penalty. With that system configuration, the penalty-free distribution of signal in the 1550-nm wavelength for the frequencies from 10 MHz to 45 GHz is achieved.
Multi-mode operation is another challenge of single-loop OEO implementation in the RAN. The multi-mode operation of the OEO is a result of the non-ideal filtering of the electrical bandpass filter (BPF). Undesired OEO’s spurious modes are filtered but are not totally removed because of bandwidth insufficiency of the electrical BPF. Manufacturing very narrow-bandwidth electrical BPF is not an easy engineering task because of physical limitations. A cascaded connection of the two non-ideal electrical filters is introduced. One of cascaded filters has a shifted main frequency to left while other has shifted main frequency to the right of wanted central frequency. Those cascaded electrical BPFs are implemented in the single-loop OEO. With that idea, the side-mode suppression ratio (SMSR) is improved by 8.3 dB. This approach is useful for increasing the efficiency of single-loop OEO implementation in RANs.
This dissertation also presents a novel efficient approach to measure the free spectral range (FSR) and SMSR of the single-loop OEO for various optical fibre lengths. The innovative approach comprises the combination of an automated optical fibre path selector with a single-loop OEO. With this measuring approach, the optimum optical fibre length of the single-loop OEO can be selected regarding the SMSR and FSR of the electrical BPF. This measuring approach is useful for testing, characterizing, or investigating different kinds of electrical BPFs to provide the best performance in single-loop OEO configurations.
In a subsequent chapter of the dissertation, the phase-noise degradation of the OEO signal is measured where the microwave signal is distributed to the base stations via an optical distribution network called a PON. The single-loop OEO is operating at 10.5 GHz and the produced signal is distributed to base stations via PONs. The phase noise measurement is made by a signal source analyser at the output of the single-loop OEO and at the base station. The base station position is changed from 5 km to 35 km with 5-km increments using the automated optical fibre path selector. The phase noise is degraded by less than 2 dB for optical distances up to 20 km. Thus, the phase noise degradation is practically negligible and this supports the main hypothesis of the dissertation proposal for centralized OEO implementation in the next-generation RAN.