1 characteristics of distributed radar
With the development of radar technology, radar has put forward many new requirements for signal transmission. Many modern radar systems are operated by multiple radar stations. The radar stations are far apart, reaching tens of kilometers or even hundreds of kilometers. The long-distance transmission of radar signals is the basis for multi-station cooperation. Such multi-station cooperative radar systems include multi-base radars, fence radars, radar networks, and distributed radars. Multi-base radar, fence radar, radar network, etc. only need to transmit point and track information, and its data communication rate is low, and communication capacity is small; distributed radar needs to transmit real-time echo data after receiving signal A/D conversion. The biggest difficulty of its communication needs is that the biggest difficulty lies in the long-distance real-time transmission of ultra-wideband large-capacity data.
The distributed radar system consists of a joint processing control center and multiple transmitting stations and multiple receiving stations. The joint processing control center generates reference signals to be sent to the radars, so that each radar has the same time reference, which facilitates time synchronization between the radars; the receiving signals of different receiving stations and different frequencies are transmitted to the joint processing control center, and the signals are integrated. Level of accumulation processing (coherent and non-coherent). The essential characteristics of distributed radar are: using multiple signals of the same frequency or different frequency, using multiple receiving stations to receive signals, transmitting multiple frequency band signals received by multiple stations to the processing center for accumulation (coherent or non-coherent) )deal with.
The broadband communication network is the backbone of distributed radar. The transmission of radar signals and control information is completely dependent on the support of the communication network. Its performance directly affects the performance of the entire system.
2 optical communication technology
The optical fiber communication system is a communication system in which light waves are used as a carrier for transmitting information and an optical fiber is used as a transmission medium, and has the advantages of high speed, low loss, strong anti-interference ability, good confidentiality, light weight, small volume and the like.
In current radar communication systems, custom high speed transmission protocols are often employed. The high-speed multiplexing technology is used to multiplex the data to be transmitted into a dedicated high-speed data frame structure, which is converted into an optical signal by optical and optical conversion and transmitted in the optical fiber, and the required data is recovered after photoelectric conversion and tapping at the opposite end. The advantage of this method is the use of a custom high-speed transmission protocol with low protocol overhead and high transmission efficiency. The disadvantage is that the custom high-speed transmission protocol is not compatible with the communication standard protocol, and cannot utilize mature optical communication technologies and devices, and cannot access the public communication network. As the demand for communication capacity increases, not only the optical communication equipment needs to be added, but also the fiber laying needs to be increased, so that the cost is doubled.
SDH (Synchronous Digital Hierarchy) and WDM (Wavelength Division Multiplexing) optical communication technologies are used in distributed radar communication systems, which can make good use of mature optical communication technologies and devices, greatly improve the communication capacity of the system, and if necessary, Access to the public communication network greatly expands the coverage of the radar system.
2.1 SDH technology
SDH is a comprehensive information transmission network that integrates multiplex, line transmission and switching functions and is operated by a unified network management system.
SDH has a standard information rate structure level called STM (Synchronous Transfer Module). The most basic and important module is the STM-1 with an interface speed of 155.52 Mbit/s. The higher-level STM-N signal is formed by synchronously multiplexing N STM-1s and interleaving bytes. The SDH standard interface rate is shown in Table 1.
ITU specifies the frame structure of STM-N signals in ITU-T G.707. The STM-N frame is a rectangular block-like frame structure in bytes, consisting of 9 lines in the vertical direction and 270 & TImes; N columns in the horizontal direction. The transmission is sequentially transmitted from left to right and from top to bottom, and the frame period is 125 μs.
As shown in FIG. 1, the frame structure of the STM-N is composed of three parts: SOH (segment overhead), Au-PTR (management unit pointer), and information payload (payload). The information payload is where the various information blocks are stored in the STM-N frame structure, which also contains POH (Channel Overhead Byte).
When SDH transmits service signals, the frame structure of various service signals to enter SDH must be mapped, located and multiplexed in three steps. Mapping is a process in which signals of various rates are first loaded into a corresponding standard container (C) by code rate adjustment, and then POH is added to form a VC (virtual container). The deviation of the frame phase is called the frame offset. The positioning is the process of collecting the frame offset information into the TU (tributary unit) or AU (management unit). It passes the functions of TUPTR (TU pointer) or AUPTR (management pointer). to fulfill. Multiplexing is the process of passing multiple low-cost channel layer signals into the high-priced channel through code rate adjustment, or passing multiple high-priced channel layer signals through the code rate adjustment to enter the multiplexing layer.
With the continuous development of communication technologies, more and more different types of applications need to carry network bearers through SDH. Due to the limited number of standard interfaces that SDH can provide externally, in order to more efficiently carry services of certain rate types, cascading technology and virtual concatenation technology of VC-4 bandwidth bundling have emerged. Using VC cascading technology, multiple VCs are combined to form a container with a larger combined capacity, enabling SDH devices to meet efficient access at any rate.
2.2 WDM technology
WDM technology utilizes the broadband and low-loss characteristics of single-mode fiber, and uses multiple wavelengths of light as carriers, allowing each carrier channel to be simultaneously transmitted in the same fiber. WDM technology uses an optical multiplexer (optical multiplexer) to combine multiple optical signals of different wavelengths and transmit them in the same optical fiber; the optical path demultiplexer (optical demultiplexer) at the opposite end will be from the same root. The multiplexed signal in the optical fiber is decomposed into multiple optical signals of different wavelengths and processed separately. Compared with the universal single-channel system, WDM not only greatly improves the communication capacity of the network system, but also makes full use of the bandwidth of the optical fiber, and it has the advantages of simple expansion and reliable performance, especially it can directly access multiple services. Make its application prospects very bright.
WDM is usually called CWDM (Sparse Wavelength Division Multiplexing) with a large optical channel spacing (even on different windows of the fiber), and WDM with a small channel spacing in the same window is called DWDM (Dense Wavelength Division Multiplexing). ). With the advancement of technology, modern technology has been able to achieve nano-level multiplexing of wavelength intervals, and even achieve multiplexing with wavelengths of a few nanometers.
If CWDM is used, generally more than 10 waves can be used, and the communication distance is generally about 40 km and less than 80 km. To achieve long-distance communication, DWDM technology must be used. DWDM technology is currently available for more than 100 waves, and the relay-free transmission distance can already reach 400 km.
3 distributed radar communication system works
3.1 overall plan
The overall scheme of the distributed radar communication system is shown in Figure 2.
The communication system is composed of a central station and a plurality of peripheral stations, and uses optical fiber communication for data transmission. The central station and each peripheral station are connected by the same cable bidirectional optical fiber to form a star network structure.
The central station to the peripheral station acts as a downlink, and the peripheral station to the central station acts as an uplink.
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