ed 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 Conventional Reuse Principles Figure 27.6 N = 7 reuse pattern. shows the geometric layout of seven channel sets in an N = 7 system typical for AMPS. Notice that the trip from one cell to a cochannel cell is two steps out and one step to the right. This corresponds to i = 2 and j = 1 in the N = i2 + j2 + ij formula. We have shaded a cluster of seven cells that represents all seven channel sets. Figure 27.7 shows the layout of four channel sets in an N = 4 system typical for GSM, where i = 2 and j = 0. We have similarly shaded a cluster of four cells representing the four channel sets. In this geometric format we can describe the channel reuse by comparing the dis- tance between cochannel cells and the cell radius. The distance-to-radius (D/R) ratio, shown in Fig. 27.8, works out to V3N, about 4.6 for N = 7 and about 3.5 for N = 4. The radio performance effect of changing the reuse ratio N is to change the interfer- ence radio path and, therefore, to change the average S/I ratio, as shown in Fig. 27.9. Changing N has little effect on the shape of the interference distribution. Of course, as increasing N improves S/I performance, it decreases capacity by allowing fewer chan- nels per cell. The improvement from AMPS N = 7 to GSM N = 4 is a factor of 1.75 (7 to 4) in channel capacity. 10 10 There may be more benefit when we take into account that the larger number of channels in the N = 4 configuration allows a higher occupancy at the same blocking rate, as discussed in Sec. 23.1. 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 Figure 27.7 N = 4