A new technique based on the auto-correlation function is described for the estimation of the Doppler spread in mobile communication systems. We first propose to divide a uncertainty region of Doppler spread into multiple frequency bins. Based on the given multiple bins the correlator compares the estimated value at a certain time index to the theoretical exact value and then decides which bin the Doppler spread is estimated in. The certain time index can be optimized to give the largest decision region among multiple bins. We derive the optimum time index algorithm to give the largest decision region for each bin based on Rayleigh fading channel. We also apply the same Doppler spread estimator to the Rician case with the slight transformation of the received signal. We show that the proposed technique is not affected significantly by the Rician factor and the SNR degradation with the reasonable number of samples for estimation which is not the case of other estimators given in the literature.

We investigate selection transmit multi-input multi-output systems where only a single transmit antenna is selected for the transmission and multiple receive antennas are employed for maximal ratio combining. Antenna selection is performed by a generalized selection criterion based on the ordinal number of the strength of the received signal-to-noise ratio.

In this paper we propose the RF domain beamforming (BF) scheme with a single analog-to-digital/digital-to-analog converters (ADC/DAC) to reduce the power consumption of the chipset for the application to mm-wave WPAN systems and THz communication systems. We also propose the codebook search algorithm for the estimation of the channel state information (CSI) which is a bottleneck to implement the RF BF. Our simulation results show that the deterioration of bit error rate (BER) performance of our proposed design compared to the optimal baseband BF techniques [1], [2] is not significant, while the power consumption and the process time is much reduced.

In this paper, we propose a hybrid M-ary Quadrature Amplitude Modulation (M-QAM) transmission scheme that jointly uses discrete-rate adaptation and selection combining for singular value decomposition (SVD)-based multiple-input multiple-output (MIMO) systems, and derive exact closed-form expressions of the performance of the proposed scheme in terms of the average spectral efficiency and the outage probability.

In multi-cell wireless systems with insufficient frequency reuse, the downlink transmission suffers from other cell interference (OCI). The cooperative transmission among multiple base stations is an effective way to mitigate OCI and increase the system sum rate. An adaptive scheme for serving one user in each cell was proposed in [1]. In this paper, we generalize the scheme in [1] by serving more than one user in each cell with adaptive OCI cancelation. Based on our derived statistics of a user for different transmission strategies, we propose a low complexity transmission scheme that achieves near-maximal ergodic sum rate. Through numerical examples, we show that the system sum rate can be improved by selecting the appropriate transmission strategy combination adaptively. As a result, our proposed system can explore spatial multiplexing gain without additional power and thus improves the system sum rate significantly.

In this paper, we propose four different strategies of node pair selection in multiple input multiple output (MIMO) interference channel where interference alignment (IA) is considered as a transceiver design method. In the first scheme, we consider the maximization of the sum rate by selecting node pairs in a brute force way. We also propose a sub-optimal sum rate maximization scheme with lower complexity than the first scheme. In the third scheme, we aim to minimize the number of links among pairs which incurs the outage in MIMO interference channel. In the fourth scheme, we suggest a max-min node pair selection scheme to enhance both the sum rate and the outage probability. Simulation results demonstrate that all our proposed node pair selection schemes can increase the sum rate but also while also reducing the outage probability compared to the scheme with random node pair selection.

Transmit diversity systems based on orthogonal space-time block coding (OSTBC) usually suffer from rate loss and power spreading. Proper antenna selection scheme can help to more effectively utilize the transmit antennas and transmission power in such systems. In this paper, we propose a new antenna selection scheme for such systems based on the idea of antenna switching. In particular, targeting at reducing the number of pilot channels and RF chains, the transmitter now replaces the antennas with the lowest received SNR with unused ones if the output SNR of space time decoder at the receiver is below a certain threshold. With this new scheme, not only the number of pilot channels and RF chains to be implemented is decreased, the average amount of feedback information is also reduced. To analyze the performance of this scheme, we derive the exact integral closed form for the probability density function (PDF) of the received SNR. We show through numerical examples that the proposed scheme offers better performance than traditional OSTBC systems using all available transmitting antennas, with a small amount of feedback information. We also examine the effect of different antenna configuration and feedback delay.

Timing and frequency offsets are caused by imperfect synchronization at the receiver. These errors degrade the performance of OFDM systems by introducing inter-carrier-interference (ICI) and inter-symbol-interference (ISI). In this paper, we derive signal-to-interference ratio (SIR) analytically with timing and frequency offsets for the case that the sampling rate of analog-to-digital converter (ADC) in OFDM receiver is an integer fraction of the signal bandwidth. We find the exact form of interference power as a function of the fractional sampling rate. Our derived analysis is confirmed by simulations and can be applied to see the exact performance of OFDM systems with fractional sampling rate.

In this paper, we propose an optimal power control algorithm with fast convergence rate for CDMA cellular systems. The new power control algorithm is based on linear quadratic control theory (LQR). Using the state feedback control designed to minimize an objective function, each mobile performs a successful transmission with optimal power. Simulation results show a fast convergence rate to target SIR with less power consumption, and an augmented channel capacity through decreased outage probability.