 reuse pattern. Figure 27.8 Distance-to-radius (D/R) ratio. 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 Conventional Reuse Principles N = 9 = 7 N = 4 Received Power dBm Figure 27.9 Signal and interference for different values of N. 27.5 Sectors Using the fourth-power-of-distance rule from Sec. 1.7, we get an S/I ratio of 9N2. For N = 4, this works out to 21.5 dB. 11 While this sounds adequate for GSM, we would point out that this is just one interferer, not all six pictured in Fig. 27.7 and that ter- rain, buildings, and vegetation add more variability to the simple distance calculation. When we take all these factors into account, we get a tenth percentile of about 7 dB for N = 4, not at all adequate for GSM. While we could get an extra 5 dB of S/I by going from N = 4 to N = 7, we could get the same 5 dB by dividing the cell coverage area into three sectors and using antennas with a 120-degree radiation pattern. Figure 27.10 shows how each sector only covers two of the six first-ring interferers. The factor of 3 works both ways. In the reverse di- rection, the receive antenna only picks up interfering user terminals in two of the six interfering cells. In the forward direction, the transmitting antenna faces point away from the user terminal community for four of the six interferers. The 15 carriers and 120 channels are divided into three sectors of 5 carriers and 40 channels each served on a separate antenna face. We can perform the same three-way division on an N = 7 AMPS channel set to break the 57 channels into three sectors of 19 channels each. Figure 27.11 shows how the three-sector antenna plan has the same benefit for N = 7 as it does for N = 4. Breaking cells into sectors is not a new cellular concept. At Bell Telephone Labora- tories, the original High Capacity Mobile Telephone Service (HCMTS) pap