Kyeongjun Ko

Multiuser MIMO User Selection Based on Chordal Distance

Multiuser MIMO (MU-MIMO) systems have advantages over single-user MIMO systems in terms of system performance. In MU-MIMO systems, inter-user interference needs to be dealt with especially when linear processing is used. The block diagonalization (BD) method is one of techniques that are widely used to eliminate the inter-user interference. In a cellular system where there are many users, the subset of users which maximizes the system performance should be selected since the base station cannot support all the users in the cell. In this paper, we propose a low complexity MU-MIMO scheduling scheme using BD with chordal distance. For a large number of users, the optimal scheduling technique needs an exhaustive search, which is impractical. One of the key ideas of this paper is to use chordal distance as a measure of orthogonality between different users. Simulation results show the proposed algorithm has throughput close to the optimal scheduling scheme with lower complexity than existing low complexity scheduling algorithms.

Low complexity multiuser MIMO scheduling with chordal distance

In multiuser MIMO systems, block diagonalization (BD) eliminates inter-user interference with perfect CSI since the transmitter decomposes a multiuser MIMO channel into multiple parallel independent single-user MIMO channels. For a large number of users, the transmitter selects a subset of users who have good channels to maximize the system throughput. In this paper, a new low complexity scheduling algorithm with BD is proposed since an exhaustive search for the optimal user set is impractical. Simulation results show that the proposed algorithm has much lower complexity while it achieves the total throughput close to the optimal algorithm. It also has lower complexity than previous low complexity scheduling algorithms.

Hybrid MU-MISO Scheduling with Limited Feedback Using Hierarchical Codebooks

The multiuser MIMO (MU-MIMO) systems tend to have inter-user interference in practical systems with limited feedback. Practical MU-MIMO systems with limited feedback can be divided into two classes in general. One is zero-forcing beamforming (ZFBF), and the other is per user unitary rate control (PU2RC) which is an extended version of random beamforming (RBF). To improve performance, ZFBF needs more feedback bits, and PU2RC needs a larger number of users. We propose a new hybrid MU-MISO system which has the advantages of the two MU-MIMO schemes simultaneously, and analyze the sum-rate performance and the quantization error of the hybrid scheme. The analysis shows how the number of feedback bits, the number of users, and the signal power affect the sum-rate. The throughput scaling laws are also derived in the high and the medium SNR regimes. Simulation results show that the proposed hybrid MU-MISO system performs better than either of the two MU-MIMO systems with the same number of feedback bits. Another advantage is that it has lower complexity for codeword search since it has a hierarchical codebook structure.

Determinant-based MIMO scheduling with reduced pilot overhead

In a multiuser multiple-input multiple-output (MU-MIMO) cellular system where there are many candidate users, it is critical to select a user group which maximizes the overall throughput of the system. However, the optimal scheduling strategy (exhaustive user selection) is computationally prohibitive when the total number of users is large. In this article, we propose a determinant-based user selection algorithm which reduces the search complexity without much performance degradation. A key contribution of this article is to use a matrix determinant as a measure of orthogonality as well as channel quality in user selection. Linear precoding schemes (zero-forcing beamforming or block diagonalization) widely used in MU-MIMO systems require two sets of pilots to estimate both raw and effective channels, which results in increased pilot overhead. In order to reduce the overhead, we also propose a new pilot scheme with only one set of pilot, which is another key contribution of this article. The new pilot scheme is combined with the proposed scheduling algorithm. Simulation results show that the proposed scheduling algorithm with a new pilot scheme reduces computational complexity and pilot overhead with negligible performance degradation compared to the exhaustive scheduling with a conventional pilot scheme.

Hybrid Codebook Design for Multiuser MISO Systems with Limited Feedback

Multiuser MIMO (MU-MIMO) system with limited feedback can be divided into two classes generally. One is zero-forcing beamforming (ZFBF), and the other is per user unitary rate control (PU2RC) which is an extended technique of random beamforming (RBF). However, in order to perform well, ZFBF requires many feedback bits, and PU2RC requires many users. These drawbacks of the existing MU-MIMO systems limit system performance in real environment. In this paper, we propose a new MU-MISO system which has merits of two MU-MIMO schemes simultaneously. Simulation results show the new MU-MISO system has good performance with simple feedback and low complexity user selection.

Hierarchical codebook design for fast search with Grassmannian codebook

It has been well known that channel aware systems (closed-loop) have better performance than channel unknown systems (open-loop). In order to know channel in the transmitter, receivers should send own channel information the transmitter through reverse channel. However, the channel information should be fed back in Frequency Division Duplexing (FDD) additionally since forward link channel and reverse link channel are different each other. In FDD, the channel information is usually quantized as a codebook and each receiver feedback the best codeword index within the codebook by any metric generally. The codebook has the characteristic that as codebook size is larger system performance increases but computational complexity to find the best codeword also increases exponentially in conventional codebook structure. In this paper, we propose some codebook design schemes which have hierarchical structure to overcome the complexity problem as well as achieves near-optimal performance.

On the SINR Distribution for an Orthogonal Random Beamforming System and Its Performance

This paper presents closed-form expressions of the distribution of signal-to-interference-plus-noise ratios (SINRs) and the average bit error rate (BER) for an orthogonal random beamforming system in multiple-antenna Gaussian broadcast channels. First, we derive the statistical distribution of the SINR of the scheduled transmit signals and confirm that the approximate form of the cumulative distribution function (cdf) of the SINR is equivalent to the formula in \cite{sharif} and \cite{letaief}. We then obtain a closed-form expression for the total average BER of the system by using the approximate cdf. Our theoretical results are compared to the Monte Carlo simulation results.

BER Based Multiuser MIMO User Selection with Block Diagonalization

It has been shown that multiuser MIMO systems have better capacity performance than single-user MIMO systems. For a large number of users, the transmitter selects a subset of users with good channels to maximize the system performance. Shannon capacity has been used most of the time as the performance measure for multiuser MIMO systems. In this paper, we consider BER as an alternative performance metric, and propose two new user selection algorithms with block diagonalization (BD), which eliminates inter-user interference in a multiuser MIMO system. The first approach is BER minimization with fixed rate, and the second approach is rate maximization with fixed target BER. Simulation results show that the proposed algorithms produce lower BER and/or higher rate when they are compared to the conventional capacity based multiuser MIMO user selection algorithm with comparable computational complexity.

Eigenmode BER based MU-MIMO scheduling for rate maximization with linear precoding and power allocation

In conventional multi-user MIMO systems, Shannon capacity has been used mostly as the performance measure for user selection. In this paper, we propose two new MIMO scheduling techniques based on BER instead of capacity as the performance measure for rate maximization with target BER. One of the key contributions of this paper is to use BER instead of capacity as the user selection metric, and another is the novel power allocation techniques considering user scheduling for the rate maximization strategy. Simulation results show that the proposed BER based algorithms produce higher rate compared to the conventional capacity based user selection algorithm with much lower computational complexity as well as the schemes meet target BER.

Regenerative hierarchical codebooks for limited channel feedback in MIMO systems

It has been well known that channel-adaptive systems (closed-loop) have better performance than the systems without channel knowledge (open-loop). For closed-loop systems, the receiver need to feed back its own channel information to the transmitter through the reverse channel. As the codebook size gets larger, the system performance improves, but the computational (search) complexity grows exponentially with respect to the size in conventional codebook structure. In this letter, we propose 2 codebook design schemes with hierarchical structure to overcome the complexity problem, which maintains near-optimal performance.