Multitone Frequency-Hopped MFSK System for Mobile Radio

A multiple access modulation technique that uses multilevel frequency shift keying (MFSK) to modulate frequency-hopped spread spectrum carriers has been examined for possible applications in satellite communication 1 and in digital mobile radiotelephony. 2 In every time frame, each user communicates a /£-bit message by frequency-shifting a tone sequence that is unique to the user. System efficiency depends strongly on the length of the sequence (L tones per message) and on the size of the tone alphabet. The total available bandwidth and the transmission rate of each user determine an optimum K, L pair; unless the system design parameters are very near this optimum, efficiency is prohibitively low. As Refs. 1 and 2 report, K, L, and the user bit rate determine the 3007 duration of each tone. An important transmission impairment in mobile radio is the multipath delay spread (the difference in delays between the various simultaneous propagation paths). This delay spread makes it difficult to communicate with short tones and, in fact, the 13-/is tone duration prescribed by the optimum K and L derived in Ref. 2 is uncomfortably close to delay spreads observed in practice. The purpose of this paper is to propose a modification of the modulation scheme that increases design flexibility. By increasing the size of the tone alphabet and allowing each user to transmit more than one tone simultaneously, a designer can increase the duration of each tone, thereby making the system less vulnerable to multipath delay spread without degrading overall efficiency.