Youngtaek Bae

Low complexity antenna selection for V-BLAST systems with OSIC detection

Multiple-input multiple-output (MIMO) systems have an advantage of spectral efficiency compared to single-input single-output systems, which means that the MIMO systems have significantly higher data throughput. The V-BLAST (Vertical Bell Laboratories Layered Space Time) scheme is a popular transceiver structure which has relatively good performance. In the V-BLAST scheme, ordered successive interference cancellation (OSIC) technique was proposed as a possible efficient detection method in terms of performance and complexity. However, MIMO systems suffer from high complexity and implementation cost. As a practical solution, a technique called antenna selection has been introduced. Since the existing literature considered only the capacity-based selection, we develop an optimal selection method for V-BLAST scheme using OSIC detection with respect to error rate performance in this article. Its complexity is shown to be proportional to the fourth power of the number of transmit antennas. To reduce the complexity without significant performance degradation compared to the optimal selection method, a near-optimal selection method is also proposed. Simulation results show that the proposed selection method is very close to the performance of optimal selection.

Antenna Selection for MIMO Systems with Sequential Nulling and Cancellation

Multiple-input multiple-output (MIMO) wireless systems have been drawing a lot of attention recently due to significant gain in capacity or diversity improvement. However it may be desirable to use only subset of the available transmit and/or receive antennas to reduce the implementation cost and complexity. In this paper we propose a selection criterion (metric) for antenna subset in spatial multiplexing mode when sequential nulling and successive cancellation techniques are used in receiver such as VBLAST. We assume to use fixed modulation/demodulation and uniform power allocation per independent substream for each transmit antenna. Therefore the symbol error rate (SER) is a more important performance measurement than the capacity. Based on minimum signal to noise ratio (SNR), we derive optimal selection criteria for several different transmit/receive structures. Further, we show how the channel state information (CSI) known to transmitter is used for the antenna selection. Performance analysis and simulations that validate our proposal are also provided.

Capacity Comparison of Orthogonal and Non-Orthogonal Cooperative Relay Systems

For a single relay channel, we study the average throughput of two different amplify-and-forward (AF) protocols, which are orthogonal-AF (OAF) and nonorthogonal-AF (NAF). The NAF protocol has been proposed to overcome a significant loss of performance of OAF in the high spectral efficiency region, and it was also shown that NAF performs better than OAF in terms of the diversity-multiplexing tradeoff. However, the results have been evaluated at the asymptotically high signal to noise ratio (SNR), and the power allocation problem between the source and the relay was neglected. We examine which protocol has a better performance in a practical system operating at a finite SNR. We also study where a relay should be located with consideration of the power allocation. In this paper, we show that the performance depends on both SNR and power allocation ratio, which indicates OAF may perform better in a certain environment.

Power Allocation for MIMO Systems with Multiple Non-Regenerative Single-Antenna Relays

We consider a multiple-input multiple-output (MIMO) system assisted by multiple non-regenerative single-antenna relays. This system where the antennas of all the relays are distributed geographically is fundamentally different from a single relay case with multiple antennas. The optimal power allocation for a relay with multiple antennas has already been known. In that solution, the power is distributed among the eigenmodes which are computed by the singular value decompositions of the first and the second hop channels. However, no closed form solution is known for the proposed system. In this paper, we propose a near optimal inter-relay power allocation method that combines the relay selection with the water-filling type closed form solution based on an asymptotic approach. In fact, the asymptotic approach itself is effective only for a specific antenna configuration, but we demonstrate through numerical simulations that if we combine this approach with the relay selection technique, the performance is comparable to the optimal case by exhaustive search even for a general antenna configuration.

On power allocation for relay networks with nonorthogonal amplify-and-forward protocols

We consider a power allocation problem for three-node cooperative diversity systems using a relay node. Specifically, we focus on an amplify-and-forward (AF) protocol due to the simple operation at the relay node which makes the mathematical analysis tractable. This protocol consists of two phases. The source transmits the signal to both relay and destination in the first phase. In the second phase, the relay amplifies the received signal, and transmits to the destination. Depending on the source participation in the second phase, the protocol can be categorized into nonorthogonal AF (NAF) or orthogonal AF (OAF). In this paper, we consider the power allocation problem of the NAF protocol in a more general scenario than in previous literatures. We then show that there are only two modes of transmission which can be used if the optimal power allocation process is carried out. They correspond to the direct transmission and OAF with optimal power allocation. That is, the solution for optimal power allocation in the systems with NAF as a relay protocol excludes the nonorthogonal transmission. Instead, the selection between the direct transmission and OAF is optimal, i.e., using NAF protocol all the time is not optimal.

Relay selection for amplify-and-forward systems with differential and double-differential modulation

Most of existing research on cooperative diversity systems have assumed that the destination node and the relay node have perfect channel state information. However, it is difficult to obtain all the channel state information in the systems, especially in a multi-relay node environment. To greatly reduce the cost required for those channel estimations, a differential modulation technique can be applied to multi-node cooperative communications. However, the performance of this system can be severely degraded if the transmission among the relays is not synchronized. Therefore we first combine relay selection schemes with differential modulation. Even if some existing literatures considered how to select a relay, the selection methods in those papers still require the channel state information. In this paper, we propose a simple relay selection scheme which does not require any channel state information. Our method is based only on the received signal power at the destination. This selection method can also be applied to double-differential modulation and show that the proposed selection scheme can work well for both differential modulation schemes. Our relay selection scheme combined with these differential modulation can significantly reduce the receiver complexity and also improve data throughput by eliminating pilot symbol period needed for channel estimation.

Performance Comparison of Orthogonal and Non-orthogonal AF Protocols in Cooperative Relay Systems

For a single relay channel, we compare the capacity of two different amplify-and-forward (AF) protocols, which are orthogonal AF (OAF) and non-orthogonal AF (NAF). The NAF protocol has been proposed to overcome a significant loss of performance of OAF in the high spectral efficiency region, and it was also theoretically proved that NAF performs better than OAF in terms of the diversity-multiplexing tradeoff. However, existing results have been evaluated at the asymptotically high signal to noise ratio (SNR), thus the power allocation problem between the source and the relay was neglected. We examine which protocol has better performance in a practical system operating at a finite SNR. We also study where a relay should be located if we consider the power allocation problem. A notable conclusion is that the capacity performance depends on both SNR and power allocation ratio, which indicates OAF may perform better than NAF in a certain environment.

Selection based inter-relay power allocation for multiple AF single-antenna relays

This article deals with inter-relay power allocation for multiple-input multiple-output systems with multiple single-antenna relays. It is difficult to derive an optimal solution in a closed form for the case of multiple multi-antenna relays as well as the case of multiple single-antenna relays. In this article, we propose an approximate solution which is effective only for the case of multiple single-antenna relays. A key contribution of this article is a low complexity inter-relay power allocation method which is based on relay selection. This approach also reduces the feedback information of the gain factors of all the relays. An incremental greedy search algorithm is also proposed to further reduce the complexity of the relay selection process with negligible performance degradation. Simulations indicate that the performance is comparable to the optimal exhaustive search algorithm.

Relay gain optimization for multiple-relay wireless networks

Multiple-input multiple-output (MIMO) systems assisted by multiple relays with single antenna are considered. The signal transmission consists of two phases. In the first phase, the source node broadcasts the vector symbols to all relays, then all relays forward the symbols multiplied by gain parameters to the destination simultaneously. Unlike the case of full cooperation among relays such as a single relay with multiple antennas, in this case there may be no closed form optimal solution for the relay gains with respect to utility and/or cost functions due to the distributed relays. Therefore we propose an alternative approach in which we use the relay selection scheme along with conventional methods to find the relay gains. As a utility or cost function, we choose the determinant and trace of mean square error (MSE) matrix which are related to capacity and MMSE respectively. Further we combine this approach with a greedy search algorithm in order to reduce computational complexity. We provide simulation results to validate that the proposed methods incur a negligible performance loss compared to the optimal solution.

Low Complexity Antenna Selection for Spatial Multiplexing Systems with VBLAST Detection

Multiple-input multiple-output (MIMO) systems have the advantage of spectral efficiency in comparison with single-input single-output (SISO) systems. Through this advantage, data throughput can be significantly increased. The VBLAST (vertical Bell Laboratories layered space-time) scheme was proposed as one possible transceiver structure in terms of performance and complexity. Recently a technique called antenna selection has been introduced as a practical solution to the cost and the complexity problem. In this paper we develop an optimal selection criterion for MIMO systems with VBLAST detection. A suboptimal selection criterion will then be proposed to reduce the complexity without significant performance degradation compared to the optimal criterion. As a performance metric, we used the vector symbol error rate (VSER). Simulations that validate our proposed algorithm are also provided.