Adbul Karim S. Al-Bayati

Block phase-precoding for blind multiuser detection of BPSK/DS-CDMA signals

A new approach for blind multiuser detection of BPSK/DS-CDMA signals is proposed. This approach is based on precoding the phase of the transmitted data, uniquely for each user, over a block of data, to enable separation and detection of a desired users signal in an unknown multipath channel. Detection does not involve channel estimation, and does not even need knowledge of the desired users code. The method does not involve any sacrifice in data rate or system bandwidth. With some additional complexity, the method can be made to deal with the case of carrier frequency offsets. Computer simulations show good performance compared to some existing methods.

Joint carrier frequency offset and channel estimation for OFDM systems with two receive antennas

Summary Carrier frequency offset (CFO) in OFDM systems results in loss of channel orthogonality and hence degrades the systems performance. In this paper, we propose a new method for joint CFO/channel estimation for OFDM systems with two receive antennas. Our method avoids the complexity of full search methods in one or two dimensions. Using one training OFDM symbol and utilizing the knowledge of the structure of the inter-channel interference that results from CFO, we develop a two-stage estimation procedure. The first stage derives an initial CFO/channel estimate based on a one shot minimization step. The second stage refines this joint estimate by conducting a small CFO search in the vicinity of the initial estimate. This procedure provides CFO estimates over a full range of −N/2–N/2, (N: number of subcarriers), as well as the channel estimates. Computer simulations show an excellent performance that is very close to the Cramer–Rao lower bound and superior to some existing methods. The effect of antenna correlation on performance is also investigated through computer simulations, showing small performance degradation even at medium correlation coefficients (0.3–0.6). Copyright

Blind Detection of Co-channel QPSK Signals: A Phase Precoding-based Approach

Quadrature Phase Shift Keying (QPSK) is one of the important and commonly used modulation formats. Phase precoding was recently proposed as a new approach to achieve blind recovery of co-channel QPSK signals in the CDMA context. In this paper, we propose a new blind receiver for phase precoded QPSK signals, whose performance is significantly better, and close to that of an MMSE receiver. The receiver is derived by exploiting the phase-precoding knowledge, as well as the special structure of the QPSK constellation. Obtained in a closed-form manner, the proposed receiver avoids convergence and initialization problems associated with some of the earlier blind co-channel receivers that were based on the constant modulus (CM) property or finite alphabet (FA) property or Higher Order Statistics (HOS) based receivers. Considering the good performance achieved and the closed-form nature of the detector, phase precoding may be seen as an attractive candidate for co-channel systems employing QPSK signaling to enable efficient blind detection while saving significant data throughput.

Constellation-switching precoding for blind detection of co-channel signals: application to 8-ary signaling

Blind detection of co-channel signals in wireless scenarios may be achieved with the help of data precoding techniques. 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 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.

Performance Analysis of Optimum Diversity Combining for Partially Coherent Frequency-Selective Fading Channel With Intersymbol Interference

The performance of coherent BPSK and QPSK systems in the presence of frequency-selective fading and carrier phase errors is analyzed. Frequency-selective fading channels are modeled as multipath Rayleigh, Nakagami- m, or Rician two-ray fading models wherein the presence of a delayed ray causes intersymbol interference (ISI). A diversity system with L-branches is used for signal detection using an optimum equal-gain-combining receiver. The effect of ISI on system performance is assessed using a truncated pulse train of the ISI signal. For Rayleigh-faded channels, the simulation result that demonstrates the influence of Doppler spread on system performance was also included. The following two performance criteria are considered: the average bit error probability and the signal-to-noise-ratio (SNR) gain penalty due to ISI.

Blind space-time constrained minimum variance detection of DS/CDMA signals

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 users 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.

Subspace-based blind channel estimation in nearly saturated down link multicarrier code division multiple access systems

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.

Block phase precoding for blind multiuser detection in QPSK/DS-CDMA systems

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.

Block spectral precoding for blind multiuser detection of DS/CDMA signals

A new approach for blind multiuser (MU) detection of DS/CDMA signals is proposed. This approach is based on modifying the data spectrum, uniquely for each user, through a two stage coloring-whitening precoding scheme. This enables separation and detection of a desired users signal in an unknown multipath channel, regardless of the power of the interferers. This technique does not involve any sacrifice in data rate or system bandwidth. The method can be considered as an extension of the approach of J. Xavier et al. (see IEEE Trans. on Sig. Processing, vol.49, p.1073-86, 2001) to the multipath DS/CDMA channel. Detection does not involve channel estimation, and does not use code knowledge. Besides, the proposed method can tolerate severe carrier frequency offsets, with reasonable performance.