A new subspace-based blind method for channel estimation in nearly saturated downlink multicarrier code division multiple access systems is proposed. In this method, the authors exploit the subspace structure of the received signal to estimate the channel directly in the frequency domain irrespective of the length of the finite impulse response (FIR) channel, thus allowing operation in nearly saturated systems (number of users can be one less than process gain). The method is shown to outperform some earlier blind channel estimation methods, whereas at the same time avoiding the limitations in the number of users that these methods suffer from.

Blind detection of co-channel signals in wireless scenarios may be achieved with the help of data precoding techniques [6]-[9]. In this letter we propose a new data precoding / blind detection approach that is based on a unique modification to the signal constellation of each of the co-channel signals. Detection is based on exploiting the embedded constellation property. The new approach can be seen as a generalization to some earlier precoding techniques [8][9] that were based on phase modifications. Demonstration of the proposed method involves application to 8-ary amplitude-phase modulations, with performance shown to be reasonably close to an MMSE receiver. Performance results for a semi-blind receiver for 4-ary signaling are also given to demonstrate further possible applications of the new scheme.

We address the problem of blind multiuser detection in DS/CDMA systems that employ QPSK data modulation. We show that by suitable precoding (modification) of the phase of the QPSK data at the transmitter (before spectrum spreading), it is possible to detect the data of any user blindly using only the knowledge of the preceding sequence, regardless of the power of the interferes. The proposed method can be seen as an extension to our earlier preceding approach for BPSK signals in AKS Al Bayati et al. (2004). However, the phase preceding/decoding operation of QPSK signals is a more challenging problem, especially in its decoding procedure. The proposed detection scheme has a deterministic nature and is therefore shown to provide performance advantage in the medium-to-high SNR region.

A new approach for blind multiuser detection of binary phase-shift keying/direct-sequence code-division multiple-access signals is proposed, based on precoding the phase of the transmitted data uniquely for each user, over a block of data. This would enable separation and detection of a desired user's signal in an unknown multipath channel without channel estimation. The method does not involve any sacrifice in data rate or system bandwidth. The proposed method exhibits good performance, compared with some existing methods.

We propose a blind multiuser detector based on a new data precoding technique for direct-sequence code-division multiple-access signals. The modulus of all users' data is block encoded, using a sequence that is unique for each user. This precoding method, together with the analytical constant modulus algorithm for detection, enables a closed-form, one-shot detection of the desired user's signal in a multipath channel using one or more antennas. The detection process does not involve or require a channel estimation step. The proposed detector is shown to be extremely near-far resistant, and can operate properly in the presence of severe carrier frequency offset.

Constrained minimum variance (CMV) detection has been proposed for blind multiuser detection of DS/CDMA signals in multipath channels. We suggest a new approach for implementing the CMV detection when an antenna array is used at the receiver. A simple technique is used for restricting the domain of the CMV detector to a smaller subspace based on signals directions of arrivals (DOAs), in a beamformer-like formulation. The proposed technique requires prior knowledge of only the timing of the desired user's signal. It is seen that the proposed subspace restriction is useful for accurate path DOA estimation followed by efficient data detection in heavily loaded systems and over small block sizes of the input signal. This makes this method suitable for slowly time varying channels.