外文翻譯---理論研究通用分組無線業(yè)務(wù)(文件)
【正文】 interference from another cell and thus maximum capacity can be achieved. The cell is usually depicted as a hexagon, but in reality the actual shape varies according to the geographic environment and radio propagation. Channel allocation is chosen based on the density of the users. If a cell has many users to serve, usually more channels are allocated. The channels are then reused in adjacent cells or cluster of cells. The spatial separation of the cells with the same radio channels, in conjunction with the low transmission power and antenna orientation, keeps the cochannel interference at an acceptable level. Mobility is one of the key features in wireless munication systems. There is a need to track the users moving into different cells and changing radio channels. A mobile switched to another channel in a different cell is called handoff. A signaling and call processing procedure is needed to support user mobility and handoff such that a mobile phone can be pleted successfully. Paging is another key feature in cellular systems. It uses a mon shared channel to locate the users within the service area and to broadcast some signaling messages. Multiple Access Technique Multiple access is a technique to allow users to share a munication medium so that the overall capacity can be increased. There are three monly used multiple access schemes: FDMA (Frequency Division Multiple Access), TDMA (Time Division Multiple Access), CDMA (Code Division Multiple Access). In FDMA, each call is assigned its own band of frequency for the duration of the call. The entire frequency band is divided into many small individual channels for users to access. In TDMA, users share the same band of frequencies. Each call is assigned a different time slot for its transmission. In CDMA, users share the same band of frequencies and time slots. Each call is assigned a unique code, which can spread the spectrum to the entire frequency band. The spectrum spread calls are sent on top of each other simultaneously, and are separated at the receiver by an inverse operation of the unique codes. A bination of the three multiple access schemes can also be applied. Power Control Power control is one of the most important design features in wireless munication including FDMA, TDMA, and CDMA systems. It ensures each user transmits and receives at a proper energy level to convey information successfully while reducing the interference to other users. Power control is needed in FDMA and TDMA systems because of the cochannel interference management. This type of interference is caused by the frequency reuse in the limited available spectrum. Via a proper power level adjustment, the cochannel interference can be reduced. This allows a higher frequency reuse factor and thus increases the system capacity. ower control is the most essential requirement in CDMA systems. Without power control, all the mobiles transmit to the base station with the same power not taking into account path loss and fading effect. Mobiles close to the base station will cause significant interference to mobiles that are farther away from the base station. This effect is the socalled near/far effect. Therefore, a welldesigned power control algorithm is crucial for proper operation of a CDMA system. In the absence of power control, the system capacity is very low pared to other systems. Another advantage of power control is that it can prolong battery life by using a minimum required transmission power. Power control on a reverse link is more stringent than on a forward link because of the near/far effect. On a forward link, power control is still necessary to reduce the intercell interference. Power co
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