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12.1 INTRODUCTION TO CELLULAR MOBILE NETWORKS 333of multiple reflections. Suppose that mobile MS is in cell A 1 of Fig. 12.1-5 and isusing voice channel N of the base station of that cell. During a fade, the strengthof channel N received by MS from cell A 1 may fall below the strength of channelN received from cell B 1 . The user of the mobile would then hear a short burst ofspeech from the call in cell B 1 .AMPS uses color codes and supervisory audio tones (SATs) to provide protectionagainst problems of this nature. A color code is assigned to each cell. Thecodes can have three values—00, 01, or 10—and each value is associated with a particularSAT frequency. Voice channels carry both speech and a SAT frequency.Now consider the cells A 1 ,B 1 , ...,G 1 of Fig. 12.1-5, all of which use the samechannel group. The color codes and SAT frequencies could then be assigned asfollows:Color Code SAT Frequency Cells00 5970 Hz A 101 6000 Hz B 1 ,D 1 ,F 110 6030 Hz C 1 ,E 1 ,G 1In this way, the color code of cell A 1 is different from the color codes of the cellsin the adjacent clusters that use the same channel group and could cause cochannelinterference. Also, cells B 1 and D 1 , who are nearby cochannel interferers for cell C 1 ,have color codes that are different from cell C 1 , and so on.All base stations transmit the SAT frequency of their respective cells on theirforward voice channels. A mobile operating in A 1 expects to receive—andusually receives—the 5970-Hz SAT frequency of cell A 1 .However, during a fade it may receive a forward channel from an interfering BSin cell B 1 , ...,orG 1 . When receiving an unexpected SAT frequency, the mobilemutes the received speech. The mobile user then experiences a silent interval,instead of a more disturbing burst of extraneous speech.All mobiles transmit the received SAT frequency on their reverse voice channels.A base station therefore expects to receive the SAT frequency of its cell on itsreverse voice channels. When a BS receives a different SAT frequency, it mutesthe received speech, and the subscriber connected to a mobile is therefore also protectedagainst extraneous speech.The SAT frequencies are well above the highest transmitted speech frequency.The MS and BS receivers separate the speech and the SAT frequency withlow-pass and high-pass filters, and no SAT is heard by the listener at the MS orby the party at the other end of the connection.12.1.5 Cell SizeWith seven-cell clusters, each cell can have a group of up to 56 voice channels. Theload (the average number of simultaneous calls) on a group of 56 channels shouldnot exceed 42 erlangs. Otherwise, the probability that all channels are busy whena new call has to be set up becomes unacceptably high. When the traffic density