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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Use of digital radio relay stations for the last mile. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Civil radio communications The digital radio relay stations (CRRS) of the 37 GHz band that have appeared on the international telecommunications equipment market over the past three or four years have proven to be a very effective means of passing, including the "last mile" for delivering E1 digital streams to the end user. A practically free frequency range and minimum weight and size parameters that allow mounting radio modules on simple supports ensure a consistently high demand for this type of equipment in the near future. The table shows the main characteristics of the CRRS in the range of 37 ... 39 GHz of domestic and foreign manufacturers. Manufacturer*Model name*Bandwidth*Configuration (+0, +1 - no or hot standby)*Service channels kbit/s*Fact. syst. 4XЕ1 BER=10-3,dB*Type of modulation*Eg. supply, V*Temperature range, °С
Abroad, radio relay lines (RRL) of this range are used, in particular, for the organization of urban paths and connections of cellular switching centers in mobile communication systems. In the domestic communications complex, mainly single-span RRLs are used. The main limiting factor for the introduction of CRRS in local networks (a tree-like structure characteristic of the Russian Federation) for domestic signalmen is the low weather resistance of multi-span RRLs. The standard calculation method, created on the basis of summarizing numerous experimental data, shows that for a single-span RRL with a length of 6 km, the total duration of fading (fading is an increase in the attenuation of the radio signal due to changes in weather conditions on the RRL route; deep fading can lead to a deterioration in the quality of the digital signal at the output radio relay equipment - an increase in the error rate, loss of synchronization, etc.) can be 0,03-0,04% of the worst month for regions of the European part of Russia. When using a linear topology, the unavailability times of individual hops are summed up, which leads to a significant decrease in the stability of the network as a whole. One of the ways to increase stability is hot standby, which means that the equipment contains not only the main set, but also a permanently switched on backup set that is not loaded with a digital stream. When the quality of the signal at the output of the main set of equipment deteriorates, the digital signal is switched to the backup set using an automatic switching system. But even hot standby can only improve the hardware availability of the network, but does not reduce communication interruptions due to weather conditions. At the same time, the cost of RRL is 2...3 times lower than the cost of cable communication lines (CLS), which, with a limited number and low territorial density of subscribers, can have a decisive impact on the profitability of the reconstruction of rural communication networks. Below, we will describe a way to eliminate the contradiction between the cost of a network using RRL and its survivability using the example of a linear circuit with a total length of 24 km, connecting five nodes using a four-span RRL. The attenuation of electromagnetic waves in hydrometeors (raindrops or fog, snowflakes, etc.) has a decisive influence on the fading statistics on the RRL path of bands above 20 GHz. This is caused by the scattering of waves on the particles of hydrometeors and their absorption of electromagnetic energy. The amount of attenuation depends on the ratio of the size of the drop formations and the wavelength of the radio signal. In the frequency range above 20 GHz, these sizes become commensurate, especially during heavy rainfall, when the average droplet diameter increases as the intensity of the rain increases. For reasonably designed lines, rains with an intensity above 30 mm/h, localized inside the centers with a radius of 3...5 km, with an average distance between the centers of about 30 km, are critical (see figure).
With such a spatial distribution of rains, more than one rain cell cannot be located on the territory of the network and, therefore, there are no simultaneous signal fading on non-neighboring intervals. If you organize the connection of the access network to the higher PBX via FOCL (or CRRL) from two terminal nodes of the network, the loss of communication with it immediately by a group of nodes can be completely eliminated. Quantitative calculation in accordance with the standard methodology shows that the connection of any of the nodes of the linear network will be absent for less than 8 minutes per year. Thus, the technique associated with the construction of bypass routes and territorial reservation of lines can be a strong argument in favor of the use of relatively cheap RRL in local communication networks. Based on the actual material, averaged initial data on the distribution of the required numbering capacities in rural areas on the territory of the European part of the Russian Federation were established. Based on the number of settlements and the desired degree of telephonization, it was concluded that practically on the territory of each district it is possible to organize one or two rural networks with the following parameters: occupied territory - 300 ... 600 km2, number of nodes - 8 - 16, the number of potential subscribers is -1000 - 3000, the average distance between nodes is 5...7 km. For such networks, it seems promising to use RRL connected in a ring scheme as a transmission medium. Connecting the ring access network to the higher-level exchange of two nodes geographically separated by more than 15 km will minimize network unavailability due to weather conditions. The optimal capacity of each direction within the ring is determined by the numbering capacity of the switching nodes and ranges from 8 to 34 Mbps, which will allow expanding both the numbering capacity and the network area in the future. Taking into account that in 90% of cases the height of the antennas in medium-rough terrain on the paths of the above length does not exceed 30 m, and the weight and size parameters of the CRRS of the 37 GHz band make it possible to place radio modules on the roofs of buildings, cheap lighting towers and simple supports, the specific capital costs for creating a ring access networks based on CRRS alone can amount to $ 150-170 per subscriber without taking into account the cost of automatic telephone exchange capacities. The cost of building linear structures for such a network based on CL or FOCL exceeds $350 per subscriber. Thus, the use of radio relay equipment in the 37 GHz band in suburban and rural networks, combined with the reorganization of the structure of such networks according to the ring principle and territorial redundancy of connection paths, makes it possible to reduce costs when switching to digital subscriber capacities in rural regions while maintaining acceptable communication stability. Authors: S. Burdin, T. Gogobegidze, A. Abramov, A. Didenko, Kazan, Tatarstan
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