 CHANNELS Figure 2.25. band. Multiple Access conventional FDMA system with MFSK waveforms but the frequency band WMA is hopping over a wide frequency range once¹¹ every chip time Tc. With L radios transmitting synchronized FH/MFSK waveforms with the same frequency hopping pattern, the receiving radio terminal's front end requires only one frequency synthesizer to dehop all the transmitted signals simultaneously. For the first part of this section we shall assume this type of multipoint-to-point system. We shall, however, examine alternative ways in which the L radios can use the multiple access frequency band of WMA Hz. Assume for the multiple access frequency band that there are M fre- quency slots where tones of duration T are orthogonal. This is roughly Mo = WMAT for non-coherent chip waveforms. The simplest approach to divid- ing up the available capacity and frequency WMA Hz and T seconds is to use FDMA as discussed above and sketched in Figure 2.25. If there are more possible transmitting radios than available FDMA slots using MFSK wave- forms,12 then there must be some means of sharing these slots, as discussed earlier in Section 2.2. Demand assignment or polling schemes are possible but these require some central control capabilities. Another approach which requires no control is to allow each user to randomly hop within the avail- able non-overlapping MFSK frequency sub-bands in an SSMA mode. These systems will be discussed next. Goodman, Henry, and Prabhu [117] extended a technique first proposed by Viterbi [118] for multiple access by low rate mobile users employing a satellite transponder. They suggested that each radio be assigned a unique address of m symbols belonging to {0, 1, , M - 1} where M = M, the number of orthogonal tone positions in the frequency band of WM Hz. An M-ary symbol is transmitted by each radio by sending m MFSK chips using the unique address as follows: Let the l-th radio's m symbol address be R(6), R(R) (2.39) where , M - 1} and M=Mo. If X is an M-ary data symbol, then the tra