Chan-Ho Choi

SC-FDMA with iterative multiuser detection: improvements on power/spectral efficiency

This article outlines the basic principles of single-carrier frequency division multiple access with iterative multiuser detection, called grouped frequency division multiple access. GFDMA allows multiple users to share a common set of subcarriers and separates the signals of users by employing distinct interleavers and frequency-domain multiuser detection. Some attractive features of GFDMA are explained, including low-cost iterative multiuser detection, multiuser and frequency diversity gains, and flexibility in resource allocation. We also discuss the applications of GFDMA to spatial diversity schemes and to intercell interference mitigation in the cell edge. It is shown that GFDMA offers advantages over SC-FDMA with respect to power/spectral efficiency. Such power/spectral efficiency can provide considerable performance improvements in future wireless communication networks.

MIMO-OFDM with insufficient cyclic prefix

For orthogonal frequency division multiplexing (OFDM), cyclic prefix (CP) should be longer than the length of channel impulse response, resulting in a loss of bandwidth efficiency. In this letter, the CP reconstruction (CPR) technique is first applied to a multi-input multi-output (MIMO)-OFDM system with insufficient CP. The intercarrier interference (ICI) from multiple transmit antennas is so large for MIMO system that it can not be sufficiently suppressed with the conventional CPR procedure used in single-input single-output (SISO) system. A new minimum mean-square error (MMSE) equalization and ordering process is proposed for MIMO system to suppress the ICI during the CPR procedure. By applying the proposed CPR algorithm to MIMO-OFDM system, we can obtain both the benefits of multiplexing gain and spectral efficiency gain.

Unique-word-based single carrier system with decision feedback equalization for space-time block coded transmissions

This letter proposes a unique-word-based single-carrier (UW-SC) system with frequency-domain decision feedback equalization (FD-DFE) for space-time block coded (STBC) transmissions. The STBC scheme can not be directly applicable to the UW-SC system, because its transmit sequence is reversed in the time-domain. To cope with this problem, we first design a new transmit sequence to fulfill the theorem of circular convolution, and propose a new corresponding receiver structure to have the transmit diversity. Unlike the cyclic prefixed SC system with FD-DFE, the proposed UW-SC system with FD-DFE greatly reduces an error propagation, thus yielding an improved performance as the number of feedback taps increases

Iterative Multiuser Detection for Single-Carrier Modulation with Frequency-Domain Equalization

In this letter, we propose a new multiuser detection scheme for single-carrier modulation with frequency-domain equalization (SC-FDE). We first present mathematical formulations of multiuser detection for the SC-FDE system with user- specific interleaving. The proposed structure employs spreader and different interleaver for each user at the transmitter, and re moves multiuser interference with an iterative detection method in the frequency domain at the receiver. Simulation results show that the proposed scheme outperforms the interleave- division multiple-access (IDMA) scheme in the block error rate even for severely frequency-selective channel, while providing considerably lower computational complexity.

Single Carrier Frequency-Domain Equalization with Transmit Diversity over Mobile Multipath Channels

This paper discusses a cyclic prefixed single carrier frequency-domain equalization (SC-FDE) scheme with two types of transmit diversity. Firstly, we propose a SC-FDE system with space-frequency block coding (SFBC). The transmit sequence of the proposed system is designed to have spatial and frequency diversities, which is equivalent to the SFBC. The corresponding combining receiver is derived under a minimum mean square error (MMSE) criterion. It is shown that the proposed system significantly outperforms the SC-FDE system with space-time block coding (STBC) over fast fading channels, while providing lower computational complexity than orthogonal frequency division multiplexing (OFDM) combined with SFBC. We verify the performance of two-branch transmit diversity systems including the proposed one through bit error rate (BER) analysis. Secondly, as a scheme that combines STBC and SFBC, a space-time-frequency block code (STFBC) SC-FDE system is presented. Computer simulation results show that the proposed STFBC SC-FDE system has better immunity to the distortion caused by both fast fading and severe frequency selective fading, compared to the SC-FDE system with the STBC or the SFBC scheme. Complexity analysis is also conducted to compare their computational loads of the transceiver. It is shown that the proposed STFBC SC-FDE system has lower computational complexity than the STFBC OFDM system.

Spectral Efficient Cooperative Diversity Technique with Multi-Layered Modulation

This paper proposes a spectral efficient cooperative diversity technique with multi-layered modulation for high data rate transmissions. The proposed system transmits with different cyclic delay and phase rotation patterns, in order to achieve cooperative diversity gain for all transmit signals from the source. Since these patterns increase the interference between the layers, we derive optimal cyclic delay and phase rotation patterns to minimize mean square error. With the optimal patterns, the proposed system still suffers from inter-layer interference, especially in high-order modulations. To overcome this problem, multiple layers are separately interleaved by layer-specific interleavers and are mapped by hierarchical symbol mapping for the high data rate transmission with cooperative diversity. A destination structure with iterative equalization is also presented to separate each layer and extract diversity gain for high data rate transmissions. The proposed system can use reliable feedback from the layers and effectively remove the inter-layer interference. To estimate the performance of the proposed system, we develop a density evolution technique. It is shown that the predicted performances agree with simulated performances. Simulation results indicate that the proposed system achieves cooperative diversity and has better performance than conventional cooperative systems.

Full-rate cooperative communications with spatial diversity for half-duplex uplink relay channels

This paper proposes a full-rate cooperative communication technique with spatial diversity for single-carrier transmissions. In order to achieve both the spatial diversity and the full-rate, data streams are simultaneously transmitted through all direct and relay channels with different phase rotation and cyclic delay patterns. The phase rotation and cyclic delay patterns are derived in the sense of minimizing interlayer interference and a corresponding destination structure is presented for decoupling each layer at the destination. Simulation results show that the proposed technique achieves spatial diversity without sacrificing spectral efficiency and outperforms conventional full-rate cooperative communication techniques.

Spectral Efficient Multiuser Technique with Channel-Dependent Resource Allocation Schemes

This paper proposes a spectral efficient multiuser scheme for single-carrier transmission. In the proposed scheme, a combination (hybrid) of localized and distributed subcarrier mapping types is employed, where the signals of users with different subcarrier mapping types are superimposed on a common set of subcarriers. We first present mathematical formulations of multiuser detection and derive the equalization coefficients for the hybrid type. In fading environments, the proposed scheme allows users to employ any subcarrier mapping types (localized, distributed, hybrid types), which can support low and high speed users. To estimate the performance of the proposed scheme, we develop a density evolution technique and show that the predicted performances are consistent with simulated performances. Further, based on the performance estimation of the proposed scheme, we discuss channel-dependent resource allocation algorithms: opportunistic scheduling and adaptive modulation and coding (AMC) schemes. These resource allocation algorithms increase the spectral efficiency of the proposed scheme with guaranteed quality-of-service (QoS). Simulation results show that the proposed multiuser scheme achieves both multiuser and frequency diversity gains in fading environments.

Space-Time Block Coded Transmission with Phase Feedback for Two-User MIMO-MAC

This paper proposes a space-time block coded (STBC) transmission with phase feedback for two-user multi-input multi-output multiple access channels (MIMO-MAC). In the proposed system, the phase feedback is transmitted to only one user, which reduces the overhead of the reverse link. We derive a closed-form solution for the proposed phase feedback, in order to maximize the overall received signal-to-interference-plus-noise power ratio (SINR). This allows the receiver to separate the two users with a linear detection. We also propose an efficient search method for the phase feedback, which reduces the complexity of computation. For practical applications, the quantized feedback of the proposed system is designed. Simulation results show that the proposed system with only 1-bit feedback considerably improves the performance of the MIMO-MAC system.

User Scheduling and Adaptive Multilevel Modulation for SC-FDMA with Iterative Multiuser Detection

This letter investigates the problem of user selection with adaptive multilevel modulation for a single-carrier frequency division multiple access system with iterative multiuser detection. Multiple users sharing a common set of subcarriers, called a chunk, cause multiple access interference (MAI) which degrades quality-of-service (QoS). To guarantee QoSs of the users and achieve multiuser diversity, the proposed algorithm allocates each chunk to users with favorable channel conditions. However, despite this user scheduling, the improvement of spectral efficiency is limited at high SNR due to the MAI. In order to effectively mitigate MAI, we employ adaptive multilevel modulation which adjusts the number of layers at each user selection. Simulation results show that the proposed algorithm improves spectral efficiency compared to conventional algorithms.