| In asynchronous slow frequency-hop spread spectrum (FHSS) systems, different parts of a packet are transmitted at different frequencies according to a pseudorandom hopping pattern. If the hopping patterns from different transmitters are not orthogonal, then packets may collide during periods in which the signals occupy the same frequency band. In comparison with interference in direct-sequence spread spectrum systems, interference mitigation in FHSS is not well studied. In this paper, we show that in FHSS systems using binary frequency-shift key (BFSK) modulation, the structure of asynchronous multiuser interference that comes from the symbols not being time aligned with the desired signal can be exploited for interference cancellation. We apply an EM-based approach that results in an iterative algorithm to estimate the desired packet in the presence of one or more interfering signals. The receiver consists of a soft demodulator and a soft decoder, such as as BCJR decoder for a convolutional or turbo code. The soft demodulator runs a trellis algorithms that calculates the probability that each symbol in a dwell interval experiences multiple access interference and the probability of the desired signal in presence of the interference (if any). The output of this soft demodulator is used as the input to a soft-input soft output decoder for the channel code. The decoder calculates the probabilities of the information bits and the code symbols and passes this information back to the soft demodulator. Analysis and simulation results show that this approach is particularly effective in the presence of strong interfering signals. |
| John M. Shea (S'92-M'99) received the B.S. (with highest honors) in Computer Engineering from Clemson University in 1993 and the M.S. and Ph. D. degrees in electrical engineering from Clemson University in 1995 and 1998, respectively.
Dr. Shea is currently an Associate Professor of electrical and computer engineering at the University of Florida. Prior to that, he was an Assistant Professor at the University of Florida from July 1999 to August 2005 and a post-doctoral research fellow at Clemson University from January 1999 to August 1999. He is currently engaged in research on wireless communications with emphasis on error-control coding, cross-layer protocol design, cooperative diversity techniques, and hybrid ARQ.
Dr. Shea was selected as a Finalist for the 2004 Eta Kappa Nu Outstanding Young Electrical Engineer Award. He received the Ellersick Award from the IEEE Communications Society in 1996. Dr. Shea was a National Science Foundation Fellow from 1994 to 1998. He is an Associate Editor for the IEEE Transactions on Vehicular Technology. |