We consider the problem of designing transmit beamformer and power for downlink cooperative base-station (BS) system with a large antenna arrays. Since the design of the beamforming vector at the transmitter requires high computational complexity, in a large antenna arrays, we utilize the zero-forcing transmit beamformer, which is the simplest form and the optimal performance in a large antenna arrays. Therefore, this paper focuses on the design of power allocation with fixed transmit beamformer for minimizing the transmit power while meeting target signal-to-interference-and-noise-ratio (SINR) of each user and power constraints. We consider two scenarios according to the power constraints of cooperative BSs. One scenario is the sum power constraint on the cooperative base-stations. In this case, the cooperative BSs share the total available transmit power. However, each BS exists a maximum available transmit power in practical implementations. Thus, we consider a more realistic per BS power constraints.We proposed the solution strategies for both scenarios: For the sum power constraint case, a simple intuitive solution, where the power is allocated without regard to the power constraint until the SINR constraints is satisfied, is presented. For the per BS power constraints case, we use the properties of a large antenna arrays to find the solution of closed form. We also demonstrate, via numerical simulation, the performance of proposed strategy is convergent to the optimal performance which is achieved by using the iterative algorithm.

A common model for transmission over wireless links is that of a multiantenna system affected by an additive interfering signal. In some scenarios of interest, such as when the interferer is located close to the transmitter and performs retransmission, interference may be learned by the transmitter, but remain unknown at the receiver. In this case, it is well known that, if transmitter and receiver have perfect channel state information (CSI), then a technique called Dirty Paper Coding (DPC) is able to fully mitigate the interference. This paper studies the impact of imperfect CSI on a multiple-input multiple-output (MIMO) system with interference and compares the performance of DPC with that of a scheme where interference is decoded at the receiver, which we refer to as beamforming with joint decoding (BF-JD). Unlike DPC, which models the interference as an "unstructured" random process, BF-JD exploits the fact that the interfering signal is a codeword of the interferer's codebook. It is demonstrated by analysis and numerical results that BF-JD provides advantages over DPC when CSI is imperfect at the transmitter but perfect at the receiver, whereas this is not true for the case of imperfect CSI at both transmitter and receiver.

In this paper, we study an energy efficient two-way amplify-and-forward (AF) relay network equipped with multiple antennas at both sources and relay. To minimize the transmit power while satisfying quality of service (QoS) of both sources, transmit beamformers and receive combiners are designed with ZF-based relay processing matrix. Due to the non-convexity of the optimization problem, the original problem is decomposed into two simpler optimization problems which can be solved through iteration algorithm. As an alternative, eigen-beamforming based technique is also provided to reduce the complexity of the iterative solution at the expense of the power usage. Numerical results show that the proposed solution efficiently reduces the additional power usage of the system compared to the eigenbeamforming based technique while meeting the QoS constraint of each source. Moreover, we can see the proposed algorithm converges to the steady-state value in a reasonable number of iterations.

In this paper, we propose a new limited channel feedback algorithm for Orthogonal Frequency Division Multiple Access (OFDMA) systems. The proposed method reports channel information of relative maximum and minimum points of the channel gain from users to base station (BS). Based on the feedback information, BS reconstructs the channel for all the subcarriers using linear interpolation. Compared with the full CSI reporting, the proposed scheme requires only 1/60 feedback information at the expense of 1% throughput loss. Simulation results show that the proposed scheme provides an attractive tradeoff between uplink feedback amount and downlink system performance compared to the conventional limited feedback techniques.

This paper presents the achievable capacity expressions with analytical closed forms of spatial multiplexing transmission (SMT) and single antenna transmission (SAT) for multicell downlink distributed antenna systems (DAS). Considering that SMT and SAT yield high transmission gains of the center and the edge of the cell, respectively, we also propose switching algorithms depending on a user location. The transmission scheme decides a favorable transmission technique between SMT and SAT through instantaneous or ergodic capacity comparison. Simulation results show that the analytical closed forms well match to the empirical expressions. Furthermore, the proposed transmission schemes are superior to SMT and SAT.

Cooperative multiple input multiple output (MIMO) system is capable in mitigating co-channel interference (CCI) that limits the downlink capacity of cellular network. Among the joint transmission designs for base station (BS) cooperation, the linear joint transmissions with per-base power constraint is preferred due to its computational simplicity and reality. However, since the transmissions with per-base power constraint sometimes make BSs cannot fully utilize their usable power, it leads considerable capacity loss. In this paper, we propose a BS selection technique for MMSE joint transmitter in downlink cooperative MIMO system to relive the inefficient usage of power under per-base power constraint. To find the BS group which yields the highest capacity, we compare the system capacity of all the possible BS groups. Simulation results show that the BS selection significantly improves the system capacity compared to the conventional linear joint transmissions by overcoming the aforementioned power allocation problem.

In this paper, a computationally efficient iterative detection algorithm (IDA) for D-STTD systems is presented. To find an optimal sequence by iterative searching, the proposed scheme consists of two stages. Firstly, the IDA establishes the candidate sequences. Then, it iteratively finds an optimal sequence by examining the candidate sequences until the early termination condition is satisfied. The adaptation of the number of candidate sequences makes the proposed IDA more efficient in computational complexity. The proposed detector is shown to achieve the near-ML performance with significantly reduced complexity.

Space-time block coded orthogonal frequency division multiplexing (STBC-OFDM) from coordinate interleaved orthogonal design (CIOD) is considered to be an effective space- time code (STC) to achieve full-rate and full-diversity with a simple maximum-likelihood (SML) decoder. However, the SML decoder cannot prevent the performance degradation completely due to time-varying effects such as co-channel interference (CCI). In this paper, we propose a decoding scheme to reduce the CCI after comparing the performance of the various STCs in time-varying environment. To fully exploit the special characteristic of the CIOD, the proposed decoding scheme consists of two stages: two symbols are decoded in advance and then used for decoding of the rest symbols in the form of interference cancellation. Simulation results demonstrate that the proposed receiver achieves significantly enhanced performance over time-varying channel compared to that of the SML. The complexity analysis of the proposed scheme and the conventional decoding schemes is also delivered.