Multistage Decoding of Frequency-Hopped FSK System

In a recent paper 1 a new decoding scheme for a frequency-hopped multilevel FSK system was described. This system, where M users share a common frequency band, has been examined for applications in multiple access satellite communication 2 and mobile radio telephony.13 Each user conveys a if-bit message every T seconds by transmitting a sequence of L tones (chips) chosen from an alphabet of 2 k sinewaves of duration r ( = T / L ) . Each user is assigned an address (code) and the message is modulated onto the address. The receiver, knowing the address, decodes the received signal and extracts the message. However, transmissions by other users can combine to cause an erroneous message resulting in an ambiguous reception. Thus, even without channel impairments the number of simultaneous users the system can support at a given error probability is interference limited. Whenever an ambiguous reception occurs, i.e., more than one mes471 sage value is decoded, the conventional decoder can't identify the correct message. The new decoder 1 makes use of the algebraic structure of the addresses to eliminate erroneous messages that come from interference. Each one of the L chips comprising a message is checked to determine whether it is part of an interference pattern, i.e., a possible sequence transmitted by some other user. An interference message will always be identified as such. The correct message will usually fail to have interference patterns for some chips and thus will be identified and correctly decoded.