In-Soo Hwang

Interference Aware-Coordinated Beamforming in a Multi-Cell System

In this paper, we propose jointly optimized linear transceiver algorithms called interference aware-coordinated beamforming (IA-CBF) for a two-cell system where each base station is equipped with multiple transmit antennas. To generalize IA-CBF to more than two-cell scenarios, a new beam-switching mechanism combined with IA-CBF is proposed. For a two-cell system, we derive a minimum-mean-square-error-type IA-CBF algorithm based on a lower bound on the achievable sum rate. We propose optimal (under an assumption of zero other-cell interference) and suboptimal transmit/receive beamforming vectors through zero-forcing IA-CBF algorithms. We also investigate the optimality of the proposed IA-CBF algorithms with respect to the number of receive antennas. Numerical results confirm that the proposed system with two transmit/receive antennas achieves the full degrees of freedom (a.k.a. multiplexing gain) of the two-cell multiple-input multiple-output channel while showing a better sum rate performance than competitive solutions such as non-cooperative eigen-beamforming and interference nulling. A three-dimensional ray tracing tool is also used to evaluate the proposed multi-cell IA-CBF algorithm.

Multicell cooperative systems with multiple receive antennas

Multicell cooperation may play an important role in achieving high performance in cellular systems. Multicell cooperation with a single receive antenna at the mobile station has been widely investigated. Applying cooperation with multiple receive antennas, allowed in several emerging wireless standards, has met with some challenges. This is primarily because the transmit precoding/beamforming vector and receive processing have to be jointly optimized in multiple cells to combat out-of-cell interference. In this article, we first discuss the role of the receive antennas in a multicell environment, and then review recently proposed multicell cooperative algorithms and receive antenna techniques for different interference statistics. Finally, we highlight recent work on the fundamental limits of cooperation and the possibility of overcoming such limits by using multiple receive antennas in multicell cooperative networks.

A practical transmit beamforming strategy for closed-loop MIMO communication

A new beamforming strategy is proposed for multiuser systems with N transmit antennas at the transmitter and M ⩽ N single antenna receivers. The proposed scheme remarkably improves on the classical spatial division multiple access, and achieves the same data rates as spatial multiplexing for all users but with significantly superior performance/diversity gain. When compared with the Bell labs layered space–time system, the symbol rate is the same and the performance is much superior because of the presence of diversity gain. In addition, unlike the Bell labs layered space–time system, the receivers do not need to know each others vector channels. Finally, the proposed algorithm is based on dirty-paper coding, but does not require much complexity and is implementable. Copyright

Jointly optimized two-cell MIMO systems

Jointly optimized interference aware coordinated beamforming (IA-CBF) algorithms for a two-cell system are investigated, where each base/mobile station is equipped with multiple transmit antennas. We first derive a minimum-mean-square-error IA-CBF algorithm based on a lower bound on the achievable sum rate. We next propose optimal (under an assumption of zero other-cell interference) and suboptimal transmit/receive beamforming vectors through zero-forcing IA-CBF algorithms. Numerical results confirm that the proposed system shows better sum rate performance than competitive solutions such as non-cooperative eigen-beamforming and interference nulling.

Precoded dirty-paper coding with signalto-interference plus noise ratio-based ordering for multiuser multiple-input multiple-output communication

The authors propose a joint dirty-paper coding (DPC) and beamforming scheme for use in multiuser multiple-input multiple-output (MIMO) systems. Unlike conventional maximum likelihood (ML)-based DPC schemes, this joint DPC and beamforming precoding scheme is intended as a suboptimal strategy in that it cancels only causal interference, thus offering a practical implementation option. In addition, a signal-to-interference plus noise ratio (SINR)-based ordering strategy that can be readily applied to conventional DPC schemes is also proposed as a way of improving the performance of the joint precoding scheme. The proposed schemes markedly improve classical spatial division multiple access (SDMA), and achieve the same data rates as spatial multiplexing (SM) for all users, but with significantly superior performance/diversity gain. When the number of active users per sector is much greater than the number of transmit antennas, the proposed schemes are able to achieve enough multiuser diversity to asymptotically approach the optimal DPC capacity. In addition, unlike BLAST, the receivers do not need to know each others vector channels. Simulation results confirm that the proposed schemes provide significant gain over conventional zero-forcing SDMA in terms of average sector throughput for downlink multi-cell multiuser systems and symbol error rate as well. Finally, the proposed interference cancellation strategies at the transmitter can be expandable to MIMO systems with any number of multiple antennas.

Downlink transmission rate-control strategies for closed-loop multiple-input multiple-output systems

A novel downlink transmission rate-control and feedback reduction strategy for closed-loop multiple-input multiple-output (MIMO) multiple-input multiple-output wireless systems is presented. Unlike conventional systems that use signal to interference plus noise ratio at the receiver as an indicator of channel quality, we propose using instantaneous MIMO capacity as an indicator for the downlink transmission rate-control. A set of instantaneous capacity thresholds is first chosen such that the expected weighted capacity loss because of thresholding effects are minimised. While computing the thresholds, we also consider the quality of service and weight function to meet different traffics and user needs. Then a set of codebooks can be constructed minimising the overall capacity loss with given quality of service constraint. Simulation results show that, with only four data rate-control bits, our algorithm gives only 12% capacity loss in 4×4 MIMO systems and almost twice better than the current IS-856 standard in single-input single-output systems. In case of 5-bit feedback scenario, the proposed algorithm outperforms conventional systems by minimising instantaneous capacity loss.

Analysis of a MIMO Relay System with HARQ

We present a multiple-input multiple-output (MIMO) relay system with hybrid automatic repeat request (HARQ) protocols which can achieve better outage behaviors than simple MIMO cooperative diversity schemes. Using repetition based and Alamouti based HARQ scheme, we can achieve better diversity gains without increasing additional transmit power. This system can be a practical solution for a new system which requires lower deployment costs and limited power in each nodes

Antenna Selection Strategies for MIMO Systems

Antenna selection strategies for multiple-input multiple-output (MIMO) wireless systems is presented. In our scheme, both transmit and receive antenna selection is done at the receiver, significantly reducing feedback information to the transmitter. Row/column probability density function (pdf) is used for antenna selection to reduce computational complexity without performance degradation. Unlike other schemes, we used two different antenna selection algorithms both for high SNR and low SNR regime, achieving additional performance gain in comparison with the single antenna selection algorithm. Simulation results show that our scheme nearly approaches to the optimal closed-loop capacity (known as water-filling capacity).

A New Practical Dirty-Paper Coding Strategy in MIMO System

In this paper, we propose a new broadcast strategy for a MIMO system with N transmit antennas at the transmitter and M les N single antenna receivers. The proposed method spatially separates the M users but does not suffer from the power loss of classical SDMA. For the special case of M = N = 2 and when the two single antenna receivers are assumed to be co-located, the proposed scheme produces a 2 transmit, 2 receiver antenna MIMO scheme. This gives a MIMO transmission scheme that doubles the symbol rate of MIMO STBC systems (Alamouti scheme) from one to two symbol per transmission time. It is proved that it provides the same performance level as that of the Alamouti STBC for the first symbol, and the same performance as the BLAST system for the second symbol. When compared to the BLAST system, our scheme has the same symbol rate, but enjoys significant performance improvements, since it provides 2 level diversity per symbol on the first symbol while the BLAST system does not provide any diversity.

A Broadcast Scheme for MIMO Systems with Channel State Information at the Transmitter

We propose a new broadcast strategy for a multiple-input multiple-output (MIMO) system with N transmit antennas at the transmitter and M≤N single antenna receivers. The proposed method, based on dirty-paper coding (DPC), spatially separates the M users but does not suffer from the power loss of classical spatial division multiple access (SDMA). For the special case of M=N=2 and when the two single antenna receivers are assumed to be co-located, the proposed scheme produces a 2 transmit, 2 receiver antenna MIMO transmission system that doubles the symbol rate of MIMO space-time block code (STBC) systems from one to two symbol per transmission time. It is proved theoretically and experimentally that the proposed scheme provides the same performance level as that of MIMO STBC systems (i. e., the Alamouti scheme) for the first symbol, and the same performance as the Bell labs layered space-time (BLAST) system for the second symbol. When compared to the BLAST system, the proposed scheme has the same symbol rate, but achieves significantly better performance, since it provides 2 level diversity per symbol on the first symbol while the BLAST system does not provide any diversity.