equency as another cell is using its reverse link, then there will be an interfering radio path from one base station to another. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Conventional Reuse Principles Capacity and Quality Principles 27.4 Regular Channel Reuse Our goal in conventional reuse cellular system design is to have a high S/I ratio at the tenth percentile. Having less variation in the signal distribution is an important part of this, so we want our cells to be as round as possible so that the maximum distance from the center is not too much more than the average distance from the center. Three regular shapes can cover (or tessellate) a geographic area, triangles, squares, and hexa- gons, and the shape that gives the least distance variation is the hexagon (Fig. 27.5). The hexagonal shapes are similar to the cells of a honeycomb. We divide the entire set of available channels into N channel sets. An AMPS N : 7 system has seven channel sets of 56 channels each for a total of 395 voice channels.8 A GSM N = 4 system has four channel sets of 112 channels each distributed over 14 car- riers of 8 time slots each. The geometry of hexagonal tessellation allows N channel sets in regular reuse as long as N = i2 + j2 + ij for some integers i and j. This means that we can have N = 1, N = 3, N = 4, N = 7, and N = 9, but not N = 2, N = 5, N = 6, or N = 8.9 Figure 27.6 7Those of us who have worked in cellular for a while are really good at drawing hexagonal lat- tice grids. If the wireless telephone business goes away, then we will have a generation of engi- neers who can tile public washroom floors. Since 7 X 56 = 392, we can afford to have four channel sets of 56 channels and three channel sets of 57 channels. 9N = 1 means using the same radio channels at every base station, as CDMA does. Station Figure 27.5 Cellular system layout. Download