Pre-equalization And Cooperative Precoding For Interference Suppression In Cellular Wireless Communication Systems

With the rapid development of wireless communications, the demand for cellular network keeps increasing with respect to higher data rate, larger cell coverage and higher supported user density. Higher user density leads to more severe interference, which prevents the data rate and system throughput from further increasing and becomes the major bottleneck of cellular communications. The diverse interference in wireless network could be very intricate, due to the various communication scenarios, user mobility, network topologies and user behaviors. Hence, how to eliminate the interference efficiently is not only a key factor to improve the system performance but also a challenging problem which requires much attention. This thesis aims at interference suppression issues from the aspects of channel equalization and precoding. To be more specific, serveral interference suppression schemes are proposed based on the pre-equalization and cooperative precoding to address mainly the co-channel interference (CCI) and inter-symbol interference problems.The first part of the thesis discusses the elimination scheme of inter-symbol interference (ISI) which are caused by the channel delay spread. A pre-equalized transmission scheme based on a finite field transform is proposed to overcome the well known problems of orthogonal frequency-division multiplexing (OFDM) transmission, including large peak-to-average power ratio (PAPR) and sensitivity to carrier frequency offset (CFO). In the proposed scheme, the signals are processed with the finite field transform called basefield Hartley transform (BHT), which has the similar convolution property as discrete Fourier transform (DFT) for the ISI mitigation by applying a finite field pre-equalizer and quantization pre-equalizer at the transmitter. Padding zeros into the source data enables the error correction capability of proposed scheme with various coding rate. At the receiver, only a decoding scheme is required to recover the source data without any FFT operations, which leads to a simple receiver with low processing complexity. Simulation results show the proposed scheme outperforms coded OFDM in terms of bit-error rate and PAPR performance.Then, a downlink multi-user transmission scheme is proposed for the amplify-and-forward (AF)-based multi-relay cellular network to support the multi-relay multiplexing and suppress the intra-cell co-channel interference. During the two phases of transmission, TH (Tomlinson-Harashima) precoding is firstly performed at basestation(BS) to support the data streams transmitted to both mobile-stations (MS) and relay-stations (RS), and then interference alignment (IA) is performed at both BS and RSs to achieve the interference-free communication in the second phase. Theoretical analysis is provided with respect to the throughput of different types of users, resulting the upper-bounds of ergodic channel capacity. The analysis and simulation results show that the joint applications of TH precoding and IA outperform other schemes in the presented multi-relay cellular network.Thirdly, the thesis focuses on the design of downlink transmission protocols in multi-cell multi-user mobile networks, where co-channel interference has been recognized as a challenging issue particularly for the users close to the boundary of cells. The key idea of designed scheme is to jointly apply interference alignment and pre-cancelation to the addressed scenario, where the former technique can effectively increase the overall system throughput and the latter can significantly boost the diversity gains and reception reliability. To ensure that the proposed interference alignment protocols implemented efficiently in practice, a precoder optimization scheme is developed based on the well-known iterative interference leakage minimization scheme. Both analytic and simulation results have been developed to show the capacity and diversity gains obtained by using the proposed scheme.Next, this thesis proposes a multi-cell cooperative transmission scheme based on unitary space-time modulation for the high mobility scenario, in which the channel state information (CSI) are not required at both transmitters and receivers. In the proposed scheme, each cooperative BS transmits an individual unitary signal from the common constellation set to the mobile unit which is located at the cell edge and suffers from severe inter-cell interference. Compared with traditional unitary space-time modulation, cooperation among nalti-cells not only eliminates the CCI but also increases the transmission rate by expanding the constellation size. Performance of error probability is analyzed for the proposed scenario with maximum-likelihood decoding, in which the exact pairwise error probability (PEP) is derived. Additionally, constellation optimization for cooperative transmission is also discussed to achieve a balance between transmission efficiency and reliability. Simulation results are provided to confirm the effectiveness of the proposed scheme in both block-fading channels and fast-fading channels.The last part of the thesis still studies the multi-cell cooperation under the high mobility scenario, in which the channels are assumed to be both time and frequency selective. A novel precoding scheme is proposed to eliminate both ISI and CCI based on the two-cell transmission. In the proposed scheme, short size blocks are applied to alleviate the time variation before both ISI and CCI are aligned to same dimension for suppression. The avoidance of cyclic prefix (CP) saves spectrum and power for the system. Additionally, since the feedback CSI could be outdated due to the time-variation of channels and estimation delay, an enhanced maximum-likelihood(ML) receiver is derived based on original ML receiver, in which the channel time correlation and feedback delay are taken into consideration. Furthermore, to make the proposed scheme applicable in practical scenario, a structure of embedded pilot is proposed and the mean square error (MSE) of channel estimation is analyzed by using the least-square (LS) method. Simulation results confirm the validity of the proposed scheme in doubly selective channels with both perfect CSI and estimated CSI.