The advent of the high-speed information age has led to the diversification of user service demand, which the currently available spectrum cannot meet. Hence, exploiting higher-frequency millimeter wave signals has become necessary. However, the transmission performance of high-frequency millimeter wave signals is unstable, and their transmission distance is significantly shorter. Radio-over-fiber (ROF) technology using optical fibers to transmit wireless signals has low attenuation loss, large capacity, and long transmission distance. A combination of wavelength division multiplexing (WDM) technology with ROF, WDM-ROF, can handle the diversification of services and load different data in multiple channels. In recent years, it has become an area of intense research interest.
The current WDM-ROF systems have some issues, such as the high cost of the laser array, poor scalability and reconfigurability, generation of single and non-adjustable millimeter wave signals, same service transmission for each base station, and being applicable only to a small number of scenarios. To overcome these shortcomings, we propose a multi-service layered WDM-ROF system with an optional frequency millimeter wave based on an optical frequency comb.
The structural diagram of the proposed multi-service layered WDM-ROF system with an optional frequency millimeter wave based on an optical frequency comb is shown in Fig. 1. The system consists of CS (central station), RN (remote node), SCS (subcentral station), and BSG (base station group). In the CS, the generation of a flat optical frequency comb, loading of data, and uplinking of a reserved optical carrier are accomplished, which reduces the cost of the laser array and improves its scalability and reconfigurability. In the RN, the required optical frequency is filtered out by a WDM filter. In the SCS, the optical switch matrix controlling the combined output of different optical frequencies can realize the selection of millimeter wave frequencies. In the BS, four millimeter wave signals with different frequencies and two service information are generated by the PD and transmitted through the electric amplifier (EA) antenna, reducing the cost due to identical transmission in each base station and making the system applicable to more situations. Simultaneously, the uplink signal is loaded to realize uplink transmission. The proposed multi-service layered WDM?ROF system with optional frequency millimeter waves based on an optical frequency comb was simulated and verified using OptiSystem simulation software. In addition, its transmission sensitivity and power cost were studied.
The proposed optical frequency comb generation scheme meets the requirements of a 21-line optical comb with a frequency interval of 20 GHz, and flatness of 0.96 dB is obtained by simulation (Fig. 2). In the downlink, two 10 Gbit/s downlink data are upconverted to four-millimeter wave signals of 35, 45, 65, and 95 GHz using the MEMS optical switch matrix in one of the control states and PD, and the bit error rate (BER) curves of four millimeter wave signals after B-T-B, 20 and 40 km SMF transmissions are shown (Fig. 6). The figure shows that the BER increases gradually with a reduction in the received optical power, and the relationship between the receiving sensitivity and BER is approximately linear. The receiving sensitivity and power cost of four millimeter wave signals in the downlink after 20 and 40 km SMF transmissions are listed in Table 1. The table shows that the receiving sensitivity gradually decreases, and the power cost gradually increases with an increase in the transmission distance. The receiving sensitivity slightly degrades with an increase in the millimeter wave signal frequency, the power cost slightly increases (within 1 dB) after 20 and 40 km SMF transmissions. The BER curves of uplink data updata1 and updata2 after B-T-B, 20 and 40 km SMF transmissions are shown in Fig. 8. The figure shows that the BER curves of uplink data updata1 and updata2 are similar to those of the downlink, the BER gradually increases with a decrease in the received optical power, and the receiving sensitivity is linear with the BER. The receiving sensitivity and power cost of updata1 and updata2 after 20 and 40 km SMF transmissions are given in Table 2. The table shows that the receiving sensitivity gradually decreases, and the power cost gradually increases with an increase in the transmission distance. The receiving sensitivities of updata1 and updata2 are similar, and the power costs after 40 km SMF transmission are 0.846 and 0.837 dB, respectively. The transmission performance of the uplink is good and better than that of the downlink.
This study proposes a multi-service layered wavelength-division multiplexing radio-over-fiber (WDM-ROF) system in which an optional frequency millimeter wave is generated based on an optical frequency comb (OFC). The transmission sensitivity and power cost characteristics of the WDM-ROF system were studied. The simulation results show that the average power loss of the downlink and uplink is less than 1 dB after 40 km of SMF transmission, while the eye diagrams of the downlink and uplink remain open after 40 km of SMF transmission; the transmission performance of the uplink is better than that of the downlink. By increasing the number of optical frequency comb lines, adjusting the frequency interval of the optical frequency comb, increasing the optical switch matrix structure, and configuring multiple groups of base stations, the system can realize BSG expansion, frequency selection, reconstruction of millimeter wave signals, and multiple service transmissions, providing an effective way for next-generation broadband optical wireless access networks.
