At the beginning of 2008, the reorganization of the domestic telecommunications industry became the focus of people's discussions. As far as the restructuring plan that people have predicted, future operators will have their own fixed and mobile networks, and they will operate two parts of the business. In order to minimize costs, For the optimization of the network, the operating company will definitely choose the integration of the network. In addition, from the market point of view, a survey by a research institute shows that 60.6% of the respondents believe that the main scenario in the next 5 years will be the integration of wireless and wired (FMC), and most users will have more than one To make a call through the most suitable network (either a fixed network or a wireless network).
At present, FTTH based on PON technology is in full swing in some pilot cities. At the same time, WiMAX has also sprung up and has successfully become a member of the 3G standard. When the backbone optical network has become saturated, the huge market share in the access network field will undoubtedly become the driving force for major operators to compete for investment. Optical fiber access and wireless access have their own advantages. Optical fiber has the characteristics of low loss, high bandwidth, anti-electromagnetic interference, etc., and wireless access can provide users with convenient and ubiquitous services everywhere, and eliminates the need for laying The expensive cost of optical fiber, so people wonder if it is possible to combine wired and wireless access with a technology. Radio Over Fiber (ROF) technology has emerged in response to this demand, and has become a hot topic for more and more people.
Therefore, no matter from the perspective of technology, policy or market drive, convergence will definitely become the main theme and inevitable trend of the telecommunications industry in the future. Technology will tend to converge, networks will converge, services will also converge, and ROF technology will also It will play a huge role in future network convergence.
1. ROF Technology Introduction
ROF technology is a newly developed wireless access technology that combines optical fiber communication and wireless communication to meet the needs of high-speed and large-capacity wireless communication. In the ROF system, optical fiber is used as the transmission link between the base station (BS) and the central station (CS), and the optical carrier is used to directly transmit the radio frequency signal. Optical fiber only plays the role of transmission, switching, control and signal regeneration are concentrated in the central station, and the base station only realizes photoelectric conversion. In this way, complex and expensive equipment can be concentrated in the central site, and multiple remote base stations can share these equipment. Reduce the power consumption and cost of the base station.
The radio frequency (or millimeter wave) signal transmitted by the optical fiber increases the wireless bandwidth, but the loss in the atmosphere after the antenna is transmitted will increase, so the cellular structure is required to change to a pico cell, and the simplification of the base station structure is beneficial to increase the number of base stations to reduce the number of cells The coverage area makes the network more flexible, and the multi-path fading of wireless signals in the atmosphere will be reduced. In addition, the use of optical fiber as a transmission link has the characteristics of low loss, high bandwidth and prevention of electromagnetic interference. It is these advantages that make ROF technology have broad application prospects in future wireless broadband communications, satellite communications, and intelligent transportation systems.
2. Current status of ROF technology research
In the ROF system, because the optical carrier carries the analog microwave signal, compared with the traditional digital optical fiber transmission link, the system has more performance on the optical device, as well as the link's own dispersion and nonlinear effects. For demanding requirements. At present, research on ROF technology is still concentrated on the physical layer, such as the generation of millimeter wave signal sources based on microwave photonics, analysis and improvement of the characteristics of optical modulators and filters, dispersion control of fiber links, and optical carriers in base stations The design and optimization of the reuse system.
Taking the generation technology of the millimeter wave signal source as an example, the traditional high-frequency signal generation source requires an expensive local oscillator source, and the heterodyne mixing technology of light waves can be used to obtain a high-frequency carrier. In radar or optical fiber radio (ROF) communication systems, up-converting intermediate frequency microwave signals in the optical domain can obtain millimeter wave signals carrying high data rates. Currently, more mature technologies are based on intensity modulators and EAM. XAM effect, all-optical frequency up-conversion technology based on XGM effect in SOA, XPM or FWM effect in highly nonlinear optical fiber.
At present, IEEE electronic journals and other optical authoritative journals OpTIcs Letters, OpTIcs Express, etc. have published a large number of articles about ROF, but these studies have remained at the level of signal processing technology and link system research, and the network Has few research results. The maturity of a technology must rely on market forces to generate profits. To apply ROF technology in actual communication systems, there are still many practical problems to be studied. For example: the problem of interface in network convergence, the problem of MAC protocol, the problem of higher gain of the antenna, the problem of frequent handover and Doppler effect in high-speed mobile in picocell, etc.
In the field of research, Professor Zhang's research group at Georgia Tech has done a lot of research on the 40G / 60G RF ROF system, and has built an optical wireless transmission system to modulate the high-definition TV data source stored in DVD to 40G. On the microwave, it is then modulated onto an optical carrier for transmission, detected and received and transmitted by an antenna, and the signal is sent to a high-definition TV for playback at the receiving end, and a good experimental effect is obtained. However, the distance between the transmitting antenna and the receiving antenna is very close and it is easily interfered by water vapor.
Not long ago, the OFC 2008 conference was successfully held in San Diego, California, and network convergence has become a hot topic. Many articles about ROF technology have also been included. Unlike the past, the included ROF papers tend to be applied research. Among the more typical articles are: Orthogonal Frequency Division Multiplexing (OFDM) is applied to ROF system to increase spectrum utilization and reduce inter-symbol interference; study optical wavelength reuse technology in uplink transmission to remove the light source of the base station; Research based on the combination of WiMAX or WiFi and ROF technology; research on the ring network of ROF system based on optical bifurcated multiplexers (OADMs); ROF system based on multimode optical fiber and plastic optical fiber.
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