【正文】 he effects of cochannel interference. We will analyze a simple multipleuser cellular system, including the antenna and propagation effects of a typical system. Despite the simplicity of the example system considered in this chapter, the analysis presented can easily be extended to include other features of a cellular system. Cellular radio system SystemLevel Description： Cellular systems provide wireless coverage over a geographic service area by dividing the geographic area into segments called cells. The available frequency spectrum is also divided into a number of channels with a group of channels assigned to each cell. Base stations located in each cell are equipped with wireless modems that can municate with mobile users. Radio frequency channels used in the transmission direction from the base station to the mobile are referred to as forward channels, while channels used in the direction from the mobile to the base station are referred to as reverse channels. The forward and reverse channels together identify a duplex cellular channel. When frequency division duplex (FDD) is used, the forward and reverse channels are split in frequency. Alternatively, when time division duplex (TDD) is used, the forward and reverse channels are on the same frequency, but use different time slots for transmission. Highcapacity cellular systems employ frequency reuse among cells. This requires that cochannel cells (cells sharing the same frequency) are sufficiently far apart from each other to mitigate cochannel interference. The RF spectrum available for the geographic service area is assigned to each cluster, such that cells within a cluster do not share any channel. If M channels make up the entire spectrum available for the service area, and if the distribution of users is uniform over the service area, then each cell is assigned M/N channels. As the clusters are replicated over the service area, the reuse of channels leads to tiers of cochannel cells, and cochannel interference will result from the propagation of RF energy between cochannel base stations and mobile users. Cochannel interference in a cellular system occurs when, for example, a mobile simultaneously receives signals from the base station in its own cell, as well as from cochannel base stations in nearby cells from adjacent tiers. In this instance, one cochannel forward link (base station to mobile transmission) is the desired signal, and the other cochannel signals received by the mobile form the total cochannel interference at the receiver. The power level of the cochannel interference is closely related to the separation distances among cochannel cells. If we model the cells with a hexagonal shape, as in Figure32, the minimum distance between the center of two cochannel cells, called the reuse distance ND , is RDN N3? （ 31） The level of cochannel interference received within a given cell is also dependent on the number of active cochannel cells at any instant of time. As mentioned before, cochannel cells are grouped into tiers with respect to a particular cell of interest. The number of cochannel cells in a given tier depends on the tier order and the geometry adopted to represent the shape of a cell (., the coverage area of an individual base station). For the classic hexagonal shape, the closest cochannel cells are located in the first tier and there are six cochannel cells. The second tier consists of 12 cochannel cells, the third, 18, and so on. The total cochannel interference is, therefore, the sum of the cochannel interference signals transmitted from all cochannel cells of all tiers. However, cochannel cells belonging to the first tier have a stronger influence on the total interference, since they are closer to the cell where the interference 。 4) Higher frequency band。 2) Networking wide band。s guidance, the mobile service rapid development, it satisfied the people in any time, any place to carry on the correspondence with any individual the desire. The mobile munication realizes in the future the ideal persontoperson munication service way that must be taken. In the information support technology, the market petition and under the demand bined action, the mobile munication technology39。 The six, puters have developed from central processing to the distributional server and intellectualized processing。 The four, information processing have developed from the voice to the data and the image。 The three, software39。s life39。s foundation, has had the land honeyb mobile munication, the satellite munication as well as the wireless Inter munication, these mailing address caused the correspondence appearance to have the huge change, used the digital technique the modern wireless munication already to permeate the national economy each domain and people39。s QoS involves the radio resource management and the dispatch, the admission control and the mobility management and so on, the mobility management mainly includes the terminal mobility, individual mobility and service mobility. Wired side39。s Rake receive and the track technology, the OFDMA technology which declines from the time domain and the frequency range resistance time and the frequency selectivity, the link autoadapted technology, the union coding technique. 2) Frequency spectrum use factor lift technique The fundamental research pointed out: In the independent Rayleigh scattering channel, the data rate and the antenna several tenth linear relationships, the capacity may reach Shannon 90%. Is launching and the receiving end may obtain the capacity and the frequency spectrum efficiency gain by the multiantenna development channel space. The MIMO technology mainly includes the spatial multiplying and the spac