Wireless Ad Hoc and Sensor Networks

Background on Networking 19in urban areas, owing to the greater RF noise floor and the large numberof base stations and mobiles. Interference has been recognized as a majorbottleneck in increasing capacity and is often responsible for droppedcalls. The two major types of system-generated cellular interference arecochannel interference and adjacent channel interference. Interferingsignals often generated within the cellular network are difficult to controlin practice. Even more difficult to control is interference due to out-ofbandusers, which arises without warning owing to front end overloadof subscriber equipment or intermittent intermodulation products.1.3.1.4 Cochannel Interference and Network CapacityThe concept of frequency reuse assigns the same set of frequencies toseveral cells in a given coverage area to increase the capacity of the cellularnetwork. These cells, which share a common set of frequencies, are calledcochannel cells, and the interference between signals from these cells isknown as cochannel interference. Interference cannot simply be kept incheck by raising the power of the transmitter, as is the case with thermalnoise. The increase in power increases the interference to neighboringcells, decreasing the system capacity. Interference can only be reduced byphysical separation of cochannel cells by a minimum distance.When the size of each cell is considered to be equal and the base stationsare considered to transmit with the same power, the cochannel interferenceratio is independent of the transmitted power and becomes a function of theradius of the cell (R) and the distance between the centers of the nearestcochannel cells (D). By increasing the ratio of D/R, the spatial separationbetween cochannel cells relative to the coverage distance of a cell is increased.Thus, interference is reduced from improved isolation of RF energy from thecochannel cell. The parameter Q, called the cochannel reuse ratio, is relatedto the cluster size. For a hexagonal geometry (Rappaport 1999),Q = D = 3NR(1.3)A small value of Q provides larger capacity because the cluster size Nis small, whereas a large value of Q improves the transmission qualitydue to a smaller level of cochannel interference. A trade-off must be madebetween these two objectives in actual cellular design. Let i 0 be the numberof cochannel interfering cells. Then, the signal-to-interference ratio (SIR)or referred as signal-to-noise ratio (SNR) for a mobile receiver, whichmonitors a forward channel, can be expressed asSI=Si0I ii=∑1,(1.4)