Blind channel estimation for CDMA systems with orthogonal modulation
M.K. Tsatsanis, Zhengyuan Xu · Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE · 1998
Most recent efforts on blind channel estimation and interference suppression for CDMA systems have concentrated on Direct-SEquence CDMA employing linear modulation. Several existing CDMA systems however, do not conform to the above model as they employ orthogonal modulation. For example, the reverse channel of the IS-95 cellular system employs 64-ary Walsh orthogonal modulation as a means of expanding the signal's bandwidth. Unfortunately, orthogonal modulation destroys the linear, time invariant property of the baseband equivalent overall channel model and severely limits the applicability of common linear blind equalization techniques. In this paper, the structure of the Walsh codes is exploited in order to derive chip-rate receivers and channel estimation algorithms. It turns out that a partial pilot signal may be identified in the Walsh code structure, casting the problem as a semi-blind channel estimation task. Based on this observation, batch and adaptive methods are derived to identify the chip-rate channel. The design of optimal receivers based on the channel estimates is also studied.