Frequency-Hopped Multilevel FSK for Mobile Radio

In exploring the possibility of providing digital mobile communications service to a large number of users, we have investigated a spreadspectrum modulation technique using frequency-hopped frequency shift keying ( F H - F S K ) and a majority logic receiver. This modulation is an extension of a technique proposed by Viterbi1 for multiple access by low-rate mobile users employing a satellite transponder. 1257 By assigning a distinct tone sequence called an address to each user, allows many users to share the same frequency band. This approach differs markedly from time-division or frequency-division channel assignment in that each address is a carrier spread over the entire frequency band of the system and the entire time interval of each code word. Code words are transmitted by means of uniform frequency shifts of the address. Because the address is spread in frequency, FH-FSK is resistant to the frequency-selective fading that degrades channelized systems in which each user operates within a narrow frequency band. On the other hand, performance of FH-FSK is limited by mutual interference among system users. Even if all transmitted tones could be perfectly detected, transmission errors would still occur when several users communicate simultaneously. Assuming that adequate speech quality is provided by 32 kb/s transmission with a binary error rate less than 10-3, we find that, with perfect transmission, up to 209 users can share the 20-MHz (per direction) bandwidth proposed for mobile telephony.