annel is 5 dB tougher than AMPS because it can withstand 12 dB of S/I at the same audio qual- ity as AMPS at 17 dB of S/I. We can use this 5 dB to go from N = 7 to N = 4, an im- provement of 1.75 in capacity. The 6-dB rule would suggest that a 5-dB performance improvement would get us a factor of 1.78, very close to what we actually get. If we fac- tor in the 30/25 efficiency factor for GSM's smaller bandwidth, we get a combined fac- tor of 2.1 for GSM over AMPS, SO calling the GSM channel twice as efficient is a per- fectly reasonable claim. For another example, suppose that we have a 12.5-kHz channel proposal and we want to know what S/I distribution it would have to have to be equivalent in efficiency to the 25-kHz GSM channel with its 12-dB requirement. Well, the spectrum usage is two times better (20/10), SO we can allow 6 dB more S/I, or 18 dB, for equivalent cellu- lar efficiency. By the same relationship, if the channel were a narrowband 5-kHz chan- nel, five times the bandwidth reduction, then we could tolerate 14 dB more S/I, or 26 dB, for cellular efficiency equivalent to GSM. 27.11.3 Sectors Since three-sector cells remove about two-thirds of the interference, they are worth about a factor of 3 in radio performance, or 5 dB. By the previous rule of thumb, would expect an N = 7 system with omnidirectional cells to be equivalent to an N = 4 system with three-sector cells.17 Six-sector cells are worth another 3 dB because they remove one of the remaining two first-ring interferers. Some AMPS systems use six-sector cells, but GSM does not, mostly because the very small channel groups required for six-sector cells would be dif- ficult in the eight-channel GSM carrier environment. 18 The earliest AMPS deployment plans started off with N = 12 omnidirectional systems (for cov- erage) that would migrate into N = 7 systems with three-sector cells. This did not last long be- cause the migration is an administrative nightmare and because the early growth rates were SO rapid that it did not make sense to have a se