Subspace method is well-known in CDMA channel estimation. But a basic problem, channel identifiability by subspace method, is still not well-solved. In this paper, two subspace based blind methods for estimating the channel responses of a OFDM-CDMA system in downlink and uplink are discussed respectively. Under some reasonable assumptions, it is mathematically proved that subspace method for downlink can estimate the channel subject to a scalar ambiguity, and the method for uplink can give the channel responses subject to a diagonal matrix ambiguity. The methods do not need precise channel order information (only an upper bound for the orders is required). Simulations show that the methods are effective and robust.
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