Heba Y. Soliman

Reducing the peak to average power ratio in MIMO OFDM systems

Multiple input multiple output (MIMO) orthogonal frequency division multiplexing (OFDM) systems have been receiving a great attention, as one of the solutions for achieving high speed, efficient, and high quality of service for wireless communications. However MIMO OFDM suffers from high PAPR, this demands expensive linear amplifiers with wide dynamic range. In MIMO OFDM systems, a straightforward way for PAPR reduction is to apply existing techniques separately on each transmit antenna, then the overall PAPR reduction is obtained. A proposed technique based on modified partial transmit sequences (PTS) using circular shifting and phase rotation is employed to achieve more reduction in the overall PAPR in MIMO OFDM, it exploits the extra degree of freedom provided by MIMO to reduce the overall PAPR. Two sub-optimal solutions for modified PTS are suggested, to reduce the complexity and side information in MIMO OFDM, with slight loss of PAPR reduction performance.

Modified PTS with circular shifting for PAPR reduction in MIMO OFDM systems

Multiple input multiple output (MIMO) orthogonal frequency division multiplexing (OFDM) technology is one of the most attractive candidates for fourth generation (4G) mobile radio communications. It effectively combats the multipath fading channel and improves the bandwidth efficiency. At the same time, it also increases system capacity so as to provide a reliable transmission. However, the main drawback of MIMO OFDM systems is high peak to average power ratio (PAPR), which result in many restrictions for practical applications. In this paper, the low complexity partial transmit sequences (PTS) is adopted to be modified using circular shifting, it exploits the extra degree of freedom provided by MIMO, to improve PAPR reduction in MIMO OFDM systems. Merging the circular shifting with phase rotation in low complexity PTS is also proposed, to provide more PAPR reduction.

Joint 2D-DOA and Carrier Frequency Estimation Technique Using Nonlinear Kalman Filters for Cognitive Radio

The problem of jointly estimating carrier frequencies and their corresponding two-dimension direction of arrivals (DOA) of band-limited source signals is considered in this paper for cognitive radio. The main problem of estimating carrier frequencies spread over a wideband spectrum is the requirement of high sampling rates. Thus, the Kalman filters are applied in the spatial domain instead of the temporal domain in the proposed algorithm to relax hardware complexity. The proposed algorithm exploits both the azimuth and elevation angles instead of a single DOA to increase the spatial capacity. Two approaches are proposed using two different types of nonlinear Kalman filter: extended Kalman filter (EKF) and unscented Kalman filter (UKF). Using simulations, the factors that affect the performance of both the filters are discussed. Scaling the estimated parameters to the same range and the proper tuning and initialization of the filters are crucial factors to prevent the filter divergence. Although UKF is supposed to have a better performance than EKF, reducing the inter-element spacing of the employed arrays and the proper filter initialization can make EKF approach the performance of UKF. On the other hand, UKF suffers from high processing time. Overall, both filters are able to converge to the true values of the unknown parameters using a number of relaxed analog-to-digital converters equal to the number of the array elements in the employed arrays. However, the approaches can detect a number of source signals higher than one-third of the total number of the array elements.

Sensing-Throughput tradeoff with primary user traffic and cooperative sensing in cognitive radio

In cognitive radio (CR), changing the primary users status diminishes the achievable throughput for secondary user. Traffic of primary user is inversely proportional to throughput of secondary user. Increasing sensing time increases the primary user protection, but has a negative effect on the throughput of secondary user. Solving the dilemma of sensing-throughput tradeoff is to find the optimal sensing time to achieve the maximum throughput. Cooperative spectrum sensing is used as a solution. Enhancement of the achievable throughput can be done by using cooperative spectrum sensing instead of local spectrum sensing. Moreover, optimal spectrum sensing time is increased.

Power performance enhancement of underlay spectrum sharing using microstrip patch ESPAR antenna

Improving antenna arrays characteristics such as power consumption, cost and complexity has gained wide attention in the context of cognitive radio networks. One antenna array configuration that offers great potential is the electronically-steerable parasitic array radiator (ESPAR) antenna. In this paper, we propose a transmitter patch ESPAR antenna system for underlay spectrum sharing in cognitive radio networks. We transmit symbols over switchable weakly-correlated beampatterns of the antenna system so as to maximize the transmitted power to a secondary user receiver, while constraining the interference to the primary users. Our results show the superior power performance of the proposed patch ESPAR antenna system over the dipole ESPAR antennas previously used in the underlay spectrum sharing paradigm. Furthermore, we calculate the bandwidth over which the switchable patterns of the patch ESPAR antenna remains weakly correlated, and it is found to be 131.2 MHz at a center frequency of 4.902 GHz.

Low Complexity V-BLAST MIMO-OFDM Detector by Successive Iterations Reduction

V-BLAST detection method suffers large computational complexity due to its successive detection of symbols. In this paper, we propose a modified V-BLAST algorithm to decrease the computational complexity by reducing the number of detection iterations required in MIMO communication systems. We begin by showing the existence of a maximum number of iterations, beyond which, no significant improvement is obtained. We establish a criterion for the number of maximum effective iterations. We propose a modified algorithm that uses the measured SNR to dynamically set the number of iterations to achieve an acceptable bit-error rate. Then, we replace the feedback algorithm with an approximate linear function to reduce the complexity. Simulations show that significant reduction in computational complexity is achieved compared to the ordinary V-BLAST, while maintaining a good BER performance.

Performance analysis of a combined STC-SVD MIMO-OFDM system

Multiple-Input Multiple-Output (MIMO) systems using orthogonal frequency-division multiplexing (OFDM) technique provide promising features for the next generation communication systems. MIMO-OFDM systems can be enhanced using Space-Time Coding (STC). In this paper we introduce a combined Alamouti Space Time Block Coding/Repetition coding (STBC/STRC) with Singular Value Decomposition (SVD) system for future generation WLANs. The introduced system improves the Symbol Error Rate (SER) performance of MIMO-OFDM systems. Two schemes are applied on a (2×2) MIMO-OFDM antenna configuration. MATLAB® simulations show that the proposed system performed better over its standalone Alamouti-STC and SVD counterparts. Moreover, it is shown that better performance is not associated with a significant increase in the computational complexity of the algorithm, especially in the STRC-SVD case.

Hybrid constant modulus array

The hybrid implementation of the multistage constant modulus CM array is designed for separating correlated signals. The canceller weights in the cascade structure provide estimates of the direction vectors of the captured signals. These estimates are then used in a parallel implementation of the linearly constraint CM (LCCM) array leading to the hybrid structure. In this paper, the hybrid implementation is extended to a three stage case for separating three correlated signals. The three correlated signals are of equal power and randomly generated. Mean Square Error (MSE) learning curves indicate that the proposed hybrid LCCM algorithm converges faster and has lower MSE than the conventional cascade implementation.