Seok-Hyun Yoon

Orthogonal frequency division multiple access with an aggregated sub-channel structure and statistical channel quality measurement

In this paper, an orthogonal frequency division multiple access (OFDMA) is considered for the mobile air interface. To reduce the reverse link overheads for CQI feedback, we consider an aggregated sub-channel structure, where a set of adjacent sub-carriers are tied up to a sub-channel to be used as a unit of user-multiplexing and the corresponding power/rate allocation. Modeling the SNR distribution over the bandwidth of a sub-channel as Rician, the channel quality of a sub-channel is summarized with the mean and variance of channel gain envelop divided by noise standard deviation. Then, we develop a generalized two step channel/resource allocation algorithm, which uses the two statistical measurements, and analyze the spectral efficiency of the OFDMA system in terms of average frequency utilization.

Superposition of Broadcast and Unicast in Wireless Cellular Systems

Practical application of superposition coding in multiplexing broadcast and unicast for OFDM-based mobile cellular systems is discussed. Superposition coding, whose gain is increased as the superposed signals have larger difference in signal-to interference-plus-noise ratio, is well matched for unicast and broadcast in a single-frequency network configuration. Combined with interference cancellation technique, broadcast signals from multiple base stations can be cancelled in a single step, minimizing the interference from broadcast to unicast. Issues related with scheduling and strategy for time, frequency and power resource sharing between broadcast and unicast are discussed. The application of superposed broadcast and unicast is then extended to MIMO systems. Simulation results in a practical mobile cellular environment are also provided, showing significant throughput gain of superposed broadcast and unicast.

Hybrid beam-forming and beam-switching for OFDM based wireless personal area networks

In this paper, we propose a hybrid beam-forming and beam-switching technique for OFDM based wireless personal area networks (WPAN). In order to compromise between performance and computational cost and overheads for the channel side information feedback, the proposed system employs hybrid beam-forming and beam-switching, where the block beam-switching using a predefined beam codebook is used at the transmitter while the optimum per-subcarrier beam-forming is used at the receiver. To verify the performance of the proposed scheme, the effective SNR gain over the single antenna transmission system is investigated along with the spectral efficiency bound for some of the channels developed by the IEEE 802.15.3c task group. For comparison, the performance of the optimum per-subcarrier beam-forming and the block beam-switching are also examined.

System level performance of OFDMA forward link with proportional fair scheduling

System level performance of OFDM/ OFDMA is evaluated for various system parameters, such as frequency reuse factor, scheduling algorithm and sub-channel structures. Specifically, the generalized two step sub-channel/power allocation algorithm, with either aggregated or distributed sub-channel structure, is first extended to provide proportional fairness. Then, by simulation, the sector throughput is evaluated in practical OFDMA based cellular system environment with frequency reuse of 1 and 3, respectively.

A Detection Algorithm for Multi-Input Multi-Output (MIMO) Transmission using Poly-Diagonalization and Trellis Decoding

A MIMO detection algorithm utilizing poly- diagonalization and tail-biting trellis is proposed. Linear MIMO detection, such as zero-forcing or minimum mean squared error (MMSE) equalization, is basically a channel diagonalization technique, where interferences from other data streams are decoupled for the separate decoding. It is well known, however, that such decoders suffer from noise enhancement, which causes considerable performance degradation. In this paper, we propose poly-diagonalization of the channel matrix in zero-forcing and MMSE senses. The idea behind poly-diagonalization is to allow interferences partially in order to alleviate the noise enhancement and, then, to use post trellis decoding for the joint detection utilizing the poly-diagonal structure of the effective channel. Under the proposed framework, the zero-forcing and the MMSE equalizer can be regarded as special cases of poly-diagonalization, i.e., of the first order. The proposed scheme can provide a tradeoff between the complexity and performance by choosing an appropriate order of poly-diagonalization. According to the simulation results, considerable gain can be obtained even with the second order, i.e., bi-diagonalization, for which the decoding complexity is far less than that of the maximum likelihood decoding.

Low-Complexity MIMO Detection Based on Belief Propagation Over Pairwise Graphs

This paper considers a belief propagation algorithm over pairwise graphical models to develop low-complexity iterative multiple-input multiple-output (MIMO) detectors. The pairwise graphical model is a bipartite graph where a pair of variable nodes are related by an observation node represented by the bivariate Gaussian function obtained by marginalizing the posterior joint probability density under the Gaussian input assumption. Specifically, we consider two types of pairwise models: the fully connected and ring-type. The pairwise graphs are sparse, compared with the conventional graphical model introduced by Bickson et al., insofar as the number of edges connected to an observation node (edge degree) is only two. Consequently, the computations are much easier than those of maximum likelihood (ML) detection, which are similar to the belief propagation (BP) that is run over the fully connected bipartite graph. The link level performance for non-Gaussian input is evaluated via simulations, and the results show the validity of the proposed algorithms. We also customize the algorithm with Gaussian input assumption to obtain the Gaussian BP run over the two pairwise graphical models, and for the ring-type, we prove its convergence to the linear minimum mean square error (MMSE) estimates. Since the maximum a posterior (MAP) estimator for Gaussian input is equivalent to the linear MMSE estimator, it shows the optimality of the scheme for Gaussian input.

Dynamic subchannel and bit allocation in multiuser OFDM with a priority user

We consider multiuser orthogonal frequency division multiplexing (OFDM) with adaptive subcarrier allocation and adaptive modulation, especially when there is one priority user who must be provided with a fixed data rate. We develop the optimum subcarrier/bit allocation method that minimizes total transmission power employing integer programming (IP) which is an NP-hard problem. To reduce the complexity, a suboptimum two-step algorithm is proposed: firstly, subcarriers are allocated to the priority user and then the remaining subcarriers are distributed to other users considering the best channel gain for each subcarrier; in the second step, using the Levin-Campello algorithm, the bits are loaded into the priority user and the other users separately. Numerical results show that total transmission power of the proposed optimum/suboptimum algorithms is significantly smaller than that of fixed modulation. In addition, the difference of total transmission power between the optimum and suboptimum algorithms is within about 0.5 dB when the number of subcarriers is 64 and the required data rate of the priority user is identical to the average required data rate of each user.

A New MIMO System for Gbps Transmission

In this paper, a receiver algorithm for an 8times8 MIMO system is considered for Giga-bps data transmission, in which the decoding complexity and latency are crucial factors for implementation of MIMO system with large number of transmit and receive antennae. To reduce computational complexity and the decoding latency, a 3-stage receiver algorithm is proposed. The MIMO receiver consists of multi-dimensional detection, partial interference cancellation and the weighted zero-forcing equalization, where the transmit antennae are divided into two group. The first group is encoded by a lower rate code and multi-dimensional search algorithm is used for demodulation and decoding, while the second group is encoded by higher rate code and weighted zero-forcing equalization is used. Once the first group is decoded successfully, they are cancelled out from the received vector for the decoding of the second group. The complexity and the error rate performance are investigated and compared with a well-known sphere decoding algorithm.

A Comparison of Opportunistic Transmission Schemes with Reduced Channel Information Feedback in OFDMA Downlink

In this paper, we consider downlink throughput performances of multiuser orthogonal frequency division multiplexing (multiuser OFDM) with reduced channel information feedback schemes. Specifically, two types of reduced feedback schemes, namely, 1-bit per sub-carrier and selective feedback scheme are considered and compared with each other in terms of average network throughput. For the latter, since the exact analysis is complicated, we resort to an approximate analysis. Simulations results will also be provided to verify the approximate analysis. Since the strict throughput comparison for given number of feedback bits per user is quite difficult, rather we compare their general behaviors in various system configurations with different system parameters, which can give us an insight into practical system design with those reduced feedback schemes.

Interference mitigation in heterogeneous cellular networks of macro and femto cells

We investigate the impact of co-channel deployment of femtocells on the existing macro-cellular systems considering the techniques to resolve the laud neighbor problem. There are several approaches to this aim, e.g., femtocell power control, interference coordination, opening access to femtocells and so on. Among them, the coordinated scheduling, including power control, and their impact will be the main focus of this work. In the context of 3GPP-LTE, we examine under various operational scenarios the performance in terms of the average and 5% worst user throughput, a useful measure of fairness among users, both for femto and macro cells. Although recent studies have shown that co-channel femtocell has minor impact on the macrocell performance in average sense, non-negligible percentage of users may lose their opportunity to get satisfactory data service and, hence, our focus will be put more on 5% worst users.