On Key Transmission Techniques For Wireless Communication Systems

The explosive growth of wireless communications is creating the demand for high speed, reliable, and spectrally efficient communication over wireless medium. Orthog-onal Frequency Division Multiplexing (OFDM) is an attractive multicarrier approach for high throughput communication systems due to its robustness to frequency selec-tive fading, when combined with Multiple Input Multiple Output (MIMO) technology, by transmitting independent information over different antennas simultaneously, system capacity can be further enhanced, whereas multi-node cooperation proves to be an ef-ficient way to achieve spatial diversity in a distributed manner. This dissertation deals with the problem of channel estimation, interference mitigation and precoding tech-niques for future wireless communication systems, the main contributions are listed as follows:1. We propose an improved channel estimation algorithm for OFDM systems with virtual carriers. As conventional estimator can not estimate the Channel Trans-fer Function (CTF) at virtual carriers, resulting in channel energy leakage after inverse discrete Fourier transform, time domain filtering method is not directly applicable. To circumvent this problem, we derive Least Squares (LS) method to estimate the CTF at virtual carriers by using the assumption of limitation of Channel Impulse Response (CIR). Further, by exploiting the noise correlation between signal subspace and noise subspace, we use maximum a posterior (MAP) criterion to estimate the noise in signal subspace and then suppress the estimation error brought by it without the knowledge of channel statistical pa-rameters. In addition to the training mode, the proposed method can also be extended and used in the tracking mode with decision-aided feedback. Simula-tion results show that the improved scheme is free of Symbol Error Rate (SER) floor and its SER attains 2dB signal-to-noise ratio gains in comparison with that of regular LS estimator.2. A hybrid pilots assisted channel estimation algorithm for MIMO-OFDM systems under low Signal-to-Noise Ratio (SNR) and arbitrary Doppler spread scenarios is proposed. Motivated by the dissatisfactory performance of the Optimal Pilots (OP) designed under static channels over multiple OFDM symbols imposed by fast fading channels, the proposed scheme first assumes that the virtual pilot tones superimposed at data locations over specific subcarriers are transmitted from all antennas, then the virtual received pilot signals at the corresponding locations can be obtained by making full use of the time and frequency domain correlations of the frequency responses of the time varying dispersive fading channels and the received signals at pilot subcarriers, finally the channel pa-rameters are derived from the combination of the real and virtual received pilot signals over one OFDM symbol based on LS criterion. Simulation results illus-trate that the proposed method is insensitive to Doppler spread and can effec-tively ameliorate the Mean Square Error (MSE) floor inherent to the previous method, meanwhile its performance outmatches that of OP at low SNR region under static channels.3. Three InterCarrier Interference (ICI) mitigation schemes are derived. (1) We propose a frequency-domain iterative ICI cancellation algorithm. Through sepa-rating the channel transfer function matrix into data and ICI part, data is initially obtained by zero-forcing algorithm, the interference between adjacent subcarri-ers, created by ICI matrix, is then subtracted from received symbols and finally the parallel interference cancellation detection using data matrix is performed to suppress the multistream interference from different antennas. By restricting the interference to eighteen neighboring subcarriers, under the condition that the performance loss is neglectable compared with the original method, compu-tational complexity can be drastically reduced. Simulation results show the pro-posed method can effectively mitigate the effect of ICI at different doppler shifts and approach the ICI-free performance at low signal-to-noise ratio. (2) We em-ploy optimal frequency domain Partial Response Coding (PRC) in MIMO-OFDM systems over frequency selective, fast fading channels to mitigate ICI. We focus on deriving, via an analytical approach, a tractable, closed-form expression of Carrier-to-Interference Ratio (CIR) to quantify the impact of time selective fad-ing and demonstrate the effectiveness of PRC in mitigating ICI in MIMO-OFDM systems. From the numerical results, PRC can effectively increase CIR and the improvement is proportional to the number of subcarriers and the coding length. (3) Beamforming is helpful in cancelling ICI introduced by the excessive mul-tipath delay. We propose an ICI eliminating beamforming scheme employing a per-tone processing approach,thus with moderate computational complex- ity. The InterSymbol Interference (ISI) is removed by using a simple decision feedback equalizer, while the optimal steering and combining vectors are then derived to maximize the Signal to Interference plus Noise Ratio (SINR). This method not only achieves the beamforming benefit, but also significantly allevi-ates the ICI. Simulation results show that the proposed algorithm can effectively reduce the system SER, permitting good performance for multipath delay pro-files that would break conventional links.4. We investigate the ICI mitigation for the uplink of an Orthogonal Frequency Divi-sion Multiple Access (OFDMA) system in a time and frequency selective fading channel. We first develop an appropriate signal model for the OFDMA uplink in a doubly selective fading channel. Using this model, the proposed algorithm squeezes the interference of subcarrier k into 2?+1 neighboring subcarriers by preprocessing the received signal and yields a banded structure interference matrix. Specifically, the value of?mainly depends on the Carrier Frequency Off-sets (CFOs) and determines the squeezing depth which in turn influences the receiver implementation complexity. The proposed scheme exploits this banded structure and realizes a low complexity iterative soft interference cancellation Minimum Mean Square Error (MMSE) equalizer. Simulation results show that the Bit Error Rate (BER) performance of our proposed algorithm approaches the existing full MMSE equalizer at all SNR regime with a significant reduction on computational complexity.5. We investigate the problem of precoding optimization in an Amplify-and-Forward (AF) MIMO relay system. Most reported works on this problem focus chiefly on the design of relay precoder without simultaneously optimizing the direct link. In this paper, we propose a method for joint source/relay precoder design, taking both direct and relay links into account. Our design is based on maximizing the mutual information (MI) under limited transmission power constraints at the source and relay, respectively. We first formulate a constrained optimization problem before relaxing the original cost function for tractability and derive a MI lower bound which asymptotically approaches the exact expression of MI in an iterative fashion. In contrast to previous strategies, we then prove that the optimal structure of the source and relay precoders jointly convert the MIMO relay channel into a bank of Single Input Single Output (SISO) relay channels without having to assume a beamforming structure to simplify the derivation. Specifically, the linear precoding design problem degenerates into power load-ing among multiple SISO relay channels. Applying standard Lagrange technique results in a scalar convex optimization which can be readily solved by iterative water filling. Numerical examples demonstrate that the proposed scheme, ei-ther exploiting partial or full Channel State Information (CSI), significantly out-performs the existing methods.