Changyong Shin

Blind Channel Estimation for MIMO-OFDM Systems

By combining multiple-input multiple-output (MIMO) communication with the orthogonal frequency division multiplexing (OFDM) modulation scheme, MIMO-OFDM systems can achieve high data rates over broadband wireless channels. In this paper, to provide a bandwidth-efficient solution for MIMO-OFDM channel estimation, we establish conditions for channel identifiability and present a blind channel estimation technique based on a subspace approach. The proposed method unifies and generalizes the existing subspace-based methods for blind channel estimation in single-input single-output OFDM systems to blind channel estimation for two different MIMO-OFDM systems distinguished according to the number of transmit and receive antennas. In particular, the proposed method obtains accurate channel estimation and fast convergence with insensitivity to overestimates of the true channel order. If virtual carriers (VCs) are available, the proposed method can work with no or insufficient cyclic prefix (CP), thereby potentially increasing channel utilization. Furthermore, it is shown under specific system conditions that the proposed method can be applied to MIMO-OFDM systems without CPs, regardless of the presence of VCs, and obtains an accurate channel estimate with a small number of OFDM symbols. Thus, this method improves the transmission bandwidth efficiency. Simulation results illustrate the mean-square error performance of the proposed method via numerical experiments

On the Design of Interference Alignment Scheme for Two-Cell MIMO Interfering Broadcast Channels

The interference alignment (IA) is a promising technique to effectively mitigate interferences in wireless communication systems. To show the potential benefits of such an IA scheme, this letter focuses on a two-cell multiple-input multiple-output (MIMO) Gaussian interfering broadcast channels (MIMO-IFBC) with M transmit antennas and N receive antennas. It corresponds to a downlink scenario for cellular networks with two base stations (BSs) with M transmit antennas per BS, and two users with N receive antennas per user, on the cell-boundary of each BS. In this scenario, we propose a novel IA technique jointly designing transmit and receive beamforming vectors in a closed-form expression without iterative computation. It is also analytically shown that the proposed IA algorithm achieves the optimal degrees of freedom (DoF) of 2N in the case of [¾N] ≤ M <; 2N. The simulations demonstrate that not only the analytical results are valid, but the sum-rate of our proposed scheme also outperforms those of conventional techniques, especially in the high signal-to-noise ratio (SNR) regime.

An Efficient Design of Doubly Selective Channel Estimation for OFDM Systems

We find that placing each pilot tone in an equally spaced manner according to the conventional placement scheme is not suitable for doubly selective channel estimation. In this paper, we propose an efficient pilot tone placement scheme enabling accurate channel estimation in OFDM systems regardless of time variations of a channel. Since the number of channel impulse response taps to be estimated is typically much greater than the number of pilot tones, linear minimum mean square error (LMMSE) estimation schemes for time-invariant channels cannot be straightforwardly extended to doubly selective channel estimation. To overcome this problem, we propose an accurate LMMSE channel estimator that exploits a small number of pilot tones located according to the derived pilot placement. To achieve performance close to the LMMSE estimator but with lower complexity, an approximate LMMSE (ALMMSE) channel estimator is also proposed. Finally, we propose a novel iterative ALMMSE channel estimator that achieves better performance than the LMMSE and ALMMSE estimators, while having complexity in between the two.

Non-Redundant Precoding-Based Blind and Semi-Blind Channel Estimation for MIMO Block Transmission With a Cyclic Prefix

By combining multiple-input-multiple-output (MIMO) communication with block transmission using a cyclic prefix (CP), MIMO block transmission systems can not only achieve high data rates but also simplify channel estimation and equalization. In this paper, to provide a bandwidth-efficient solution for channel estimation in MIMO block transmission systems with a CP, we present a framework for blind channel estimation based on a general non-redundant precoding. Using this framework, we propose a blind estimator exploiting a simplified non-redundant precoding that is robust against overestimates of a true MIMO channel order. Furthermore, in the case with the number of transmit antennas greater than that of receive antennas, we show under specific system conditions that the proposed blind algorithms can be used for estimation of the MIMO channel without oversampling the received signals. With the simplified precoding conditions established in this paper, the proposed method does not impose the strict identifiability conditions on the MIMO channel, which are required for the existing methods. Furthermore the proposed estimator achieves accurate channel estimation with a small number of symbol blocks. In addition, we derive a simplified precoder that is optimized in the sense of minimizing the impact of unknown additive noise. We discuss the tradeoff between channel estimation accuracy and bit error rate performance associated with the simplified precoder. Finally, we develop a technique for resolving the channel ambiguity in the proposed blind estimator to create a semi-blind channel estimator relying on only a few known symbols.

Performance Analysis of OFDM Systems with Selected Mapping in the Presence of Nonlinearity

In the presence of nonlinearity, we analyze the impact of the selected mapping (SLM) technique on bit-error-rate (BER) performance of orthogonal frequency division multiplexing (OFDM) systems in an additive white Gaussian noise channel. The peak-to-average-ratio (PAR) reduction gain of SLM can be increased by improving the PAR statistics at the cost of complexity in the OFDM transmitter, thereby helping to decrease the required power amplifier (PA) output backoff (OBO). However, since the PAR statistics focus only on the statistical distribution of the highest peak in an OFDM symbol, the statistics cannot be used to quantify BER performance degradation in the presence of nonlinearity such as that caused by a PA or digital-to-analog converter (DAC). We first derive a closed-form expression for the envelope power distribution in an OFDM system with SLM. Then, using this derived envelope power distribution, we investigate the BER performance and the total degradation (TD) of OFDM systems with SLM under the existence of nonlinearity. We discuss peak backoff (PBO) and the clipping ratio, which determine the operating point of the PA and dynamic range of the DAC, respectively. Lastly, we consider the total degradation (TD), which indicates the tradeoff between the OBO and the E_b/N_o penalty due to nonlinearity, and numerically compute the PBO and clipping ratio that minimize the TD. The TD-minimizing PBO and clipping ratio are given as functions of the number of candidate signals in SLM.

Interfering channel alignment and degrees of freedom for downlink multiceli MIMO networks

To solve the multi-cell interference problem in downlink cellular networks, we study G-cell multiple input multiple output Gaussian interfering broadcast channels (MIMO-IFBC) with K users on the cell-boundary of each base station (BS). The main idea is to align interfering channels from each BS to its non-intended users onto a small dimensional subspace. This is called interfering channel alignment (ICA). We analytically derive both the feasibility conditions for the ICA to achieve a total number of degrees of freedom (DoF) of GK and the maximum number of DoF, which is achieved by the ICA for a given antenna confguration. The numerical results show that our analysis is valid and the achievable DoF of the proposed ICA outperforms that of conventional schemes.

Distributed uplink intercell interference control in heterogeneous networks

Heterogeneous cellular networks which consist of macrocells and small cells can offer significant capacity gain by utilizing the resources of the small cells. However, to achieve this, the interference between the macrocells and the small cells must be carefully managed. In this work, we propose an uplink intercell interference control (ICIC) scheme which is a unified ICIC approach of handover based interference control and rate-split based interference control. The handover based interference control scheme is a win-win strategy which enhances both interfering users rate and interfered users rate. On the other hand, the rate-split based interference control scheme is a yield-win strategy where an interfering user sacrifices his rate to save the interfered users rate. In this paper, we assume that users have their target QoS such as minimum rate when they send their data. The proposed uplink ICIC scheme reduces the interference as best as possible while it guarantees the minimum QoS. Simulation results shows that the proposed ICIC scheme offers enhanced rate or fairness than legacy ICIC schemes which are not considering user QoS. The proposed ICIC scheme works in distributed manner with low-complexity so that it can be applied to self-organizing network and mobile ad-hoc networks as well as heterogeneous cellular networks.

User cooperation-assisted multi-cell MIMO networks

We explore the interplay between interference, cooperation, and feedback in multi-cell networks. Specifically, this paper focuses on G-cell multiple-input multiple-output Gaussian interfering broadcast channels (MIMO-IFBC) with K cooperating users on the cell-boundary of each base station. It corresponds to a downlink scenario for cellular networks with G base stations, and K users equipped with Wi-Fi interfaces enabling to cooperate among users on a peer-to-peer basis. In this scenario, we propose a novel interference alignment technique exploiting user cooperation. Our proposed algorithm obtains the total degrees of freedom (DoF) of GK to be achieved by transmitting one stream to each user from its corresponding BS, when each BS and cell-edge user has M = K + (G − 1)dalign transmit antennas and N = ⌊(M(G − 1)(K − dalign) + 1)/K⌋ receive antennas as a minimum antenna configuration, respectively, where ∀dalign ∈ {1,2,…K}. Furthermore, the algorithm requires only a small amount of channel feedback information with the aid of the user cooperation channels. The simulations demonstrate that not only are the analytical results valid, but the achievable DoF of our proposed algorithm also outperforms those of conventional techniques.

Energy-efficient data synchronization for cooperative context-aware computing

In cooperative context-aware computing, the synchronized context information among devices is critical to the system performance. In this work, we propose an energy efficient context synchronization scheme using the reference-credit concept. Simulation results show that it improves energy efficiency by approximately 50% compared to the legacy random synchronization scheme which is popular in use. The proposed scheme can be employed as a key context synchronization function of the context-aware devices and computing platforms.

Doubly Selective Channel Estimation for OFDM Systems

In order to estimate doubly selective channels in orthogonal frequency division multiplexing (OFDM) systems, we derive a linear minimum mean square error (LMMSE) channel estimator by considering fast time-variant channel estimation by Wiener filtering. To reduce the computational complexity of the LMMSE estimator, we also present an approximate LMMSE (ALMMSE) estimator. This estimator obtains partial channel information by exploiting LMMSE estimation and finds time-variant channel impulse responses by interpolating the channel information with the discrete orthonormal Legendre polynomials. Furthermore, we propose an iterative LMMSE estimation scheme to significantly improve channel estimation performance. By exploiting both detected information symbols and given pilot symbols as new pilot symbols, this scheme iteratively uses the proposed approximate LMMSE estimator to update channel estimates. Simulation results demonstrate the effectiveness and good performance of the proposed channel estimation schemes