Qinfei Huang

Pilot Design for MIMO OFDM Systems With Virtual Carriers

We consider the problem of pilot design for channel estimation in multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) systems in the presence of virtual carriers. The design criterion is the average mean-square error (MSE) of the least square (LS) estimates of the channel frequency response over the data carriers. To maximize bandwidth efficiency, the number of pilots is set to its minimum which is equal to the number of unknown channel coefficients. Both the phase shift orthogonal and disjoint pilot schemes are investigated. For a given pilot placement, a closed-form expression for the pilot power distribution for the disjoint scheme is provided. It is found that the optimal power distribution allocates less power to the pilots that are close to the virtual carriers. Pilot placement is obtained using an exhaustive search. The latter is too complex in systems with large number of subcarriers. A suboptimum pilot placement design for the disjoint pilot scheme is presented. It is shown that, unlike fully loaded systems where the two pilot design schemes have similar performance, in the presence of virtual carriers the optimal disjoint scheme not only has lower complexity but also yields more accurate channel estimation and bit error rate performance.

Maximizing the Sum-Rate of Amplify-and-Forward Two-Way Relaying Networks

This letter addresses the problem of beamforming design for an amplify-and-forward (AF) based two-way relaying network (TWRN) which consists of two terminal nodes and several relay nodes. Considering a two-time-slot relaying scheme, we design the optimal beamforming coefficients to maximize the sum-rate of AF-based TWRN under total relay power constraint (TRPC). Although the optimization problem is neither convex nor concave, we show that the global optimal solution can be obtained by the branch-and-bound algorithm. To address the computational complexity concern, we also propose a low-complexity suboptimal solution which is obtained by optimizing a cost function over one real variable only. Simulation results show that the proposed optimal solution outperforms existing schemes significantly. Moreover, we show that the suboptimal solution only suffers small sum-rate losses compared to the optimal solution.

Timing and frequency synchronization for OFDM based cooperative systems

In this paper, we investigate the timing and carrier frequency offset (CFO) synchronization problem in decode and forward cooperative systems operating over frequency selective channels. A training sequence which consists of one OFDM block having a tile structure in the frequency domain is proposed to perform synchronization. Timing offsets are estimated using correlation-type algorithms. And since some subcarriers are nulled in the proposed tile structure, CFOs are readily estimated using subspace-based methods. By judiciously designing the size of the tile, these algorithms are shown to have better performance, in terms of synchronization errors and bit error rate, than the computationally demanding SAGE algorithm.

Data Detection in Cooperative STBC-OFDM Systems With Multiple Frequency Offsets

This paper addresses the problem of data detection in cooperative space-time block coded (STBC) orthogonal frequency division multiplexing (OFDM) systems in the presence of multiple carrier frequency offsets (CFO). An enhanced iterative maximum-likelihood detector (EIMLD) is proposed. This method consists of first removing inter-carrier interference (ICI), and then performing iterative symbol detection and inter-symbol interference reduction. Simulation results show that EIMLD significantly outperforms existing iterative methods. Comparisons with the zero-forcing and minimum-mean square error detectors, which require complex matrix inversion, are also carried out.

Multiple carrier frequency offsets tracking in co-operative space-frequency block-coded orthogonal frequency division multiplexing systems

This study addresses the problem of carrier frequency offset (CFO) tracking in co-operative space-frequency block-coded orthogonal frequency division multiplexing (OFDM) systems with multiple CFOs. Considering that the inserted pilot tones are decayed by data subcarriers in the presence of multiple CFOs, a novel recursive residual CFO tracking (R-RCFOTr) algorithm is proposed. This method first removes CFO-induced inter-carrier interference from data subcarriers, and then updates the residual CFO (RCFO) estimation of each OFDM block recursively. When used in conjunction with a multiple CFOs estimator, the proposed R-RCFOTr can effectively mitigate the impacts from the multiple RCFOs with affordable complexity. Finally, simulation results are provided to validate the effectiveness of our proposed R-RCFOTr algorithm, which has performance close to that of perfect CFO estimation at moderate and high signal-to-noise ratio, and significantly outperforms conventional CFO tracking algorithm for large CFOs.

Low-Complexity Data-Detection Algorithm in Cooperative SFBC-OFDM Systems With Multiple Frequency Offsets

This paper addresses the problem of data detection in cooperative space-frequency block-coding (SFBC) orthogonal frequency-division multiplexing (OFDM) systems in the presence of multiple carrier frequency offsets (CFOs). A new ordered-successive parallel interference cancellation (OSPIC) detector is proposed. This method consists of first carrying out coarse ordered interference cancellation detection, where the interference components are successively eliminated, and then performing fine interference cancellation detection using parallel interference reduction. Simulation results show that the proposed detector significantly outperforms existing detectors and performs close to that of CFO-free systems for practical signal-to-noise ratios (SNRs). To further reduce the computational complexity, a minimum mean-square error (MMSE)-based reduced-complexity OSPIC (RC-OSPIC) detector is proposed. Compared with the MMSE detector, which requires a full-size complex matrix inversion, the proposed RC-OSPIC detector performs better and is computationally much more effective.

Cooperative beamforming for OFDM-based amplify-and-forward relay networks☆

Abstract In this paper, we investigate the cooperative beamforming (BF) design problem for amplify-and-forward relay networks that are operating over frequency-selective channels using Orthogonal frequency division multiplexing (OFDM) signaling. We focus on the time-domain (TD) BF due to the lower implementation complexity and lower feedback requirement from the destination. Our aim for the BF design is maximizing the minimum signal-to-noise-ratio (SNR) over all subcarriers at the destination. We show that the BF designs lead to non-convex problems generally. Three approaches to approximate these problems into convex problems are proposed. In the first approach, semidefinite relaxation (SDR) techniques with randomization methods are applied. In the second approach, an existing iterative method for cooperative beamforming of multi-group multicasting (MGM) relay networks is extended to solve our problem. The third approach consists of improving the second approach by appropriately choosing the initial phase rotation. Simulation results demonstrate that the third approach always outperforms the other two approaches. Moreover, the performance of the second approach is close to that of the first approach when the filter length is relatively small. When a longer TD filter is employed, the second approach outperforms the first approach.

Transmit beamforming for MISO frequency-selective channels with per-antenna power constraint and limited-rate feedback

This paper addresses the problem of transmit beamforming (BF) design for cyclic prefixed transmissions over MISO frequency selective channels. Unlike [1] which is based on convex optimization tools, we here give two more computationally effective solutions in close-form to minimize the arithmetic mean of the effective error probabilities (ARITH-EEP) under the per-antenna power constraint (PPC). Moreover, limited-rate feedback methods needed for transmit BF is considered and a simple codebook design method for frequency selective channels is proposed. Unlike the conventional method where the codebook is designed for the BF vectors directly, we construct codebooks for the amplitude and phase of the time domain channel state information (CSI) to reduce the rate of feedback. The simulation results show that the proposed suboptimal solutions not only have lower complexities but also outperform the PPC solutions given in [1] in terms of BER. The effectiveness of the proposed limited-rate feedback method is also demonstrated in the simulation.

Transmit Beamforming for MISO Frequency-Selective Channels With Per-Antenna Power Constraint and Limited-Rate Feedback

In this paper, we consider transmit beamforming (BF) design under per-antenna power constraint (PPC) for multiple-input-single-output (MISO) frequency-selective channels. Both cyclic-prefixed (CP) single carriers and orthogonal frequency-division-multiplexing (OFDM) systems are investigated. Since there is no closed-form expression for the optimal PPC BF coefficients, and the optimization tools employed to find the PPC solution are usually complicated, here, we propose three computationally more effective suboptimal solutions to minimize the arithmetic mean of the effective error probabilities. Moreover, we address the issue of limited-rate feedback, and a simple codebook design method for frequency-selective channels is proposed. Unlike conventional methods where the codebook is directly designed for BF vectors, here, we construct codebooks for the amplitude and phase of the time-domain channel state information (CSI) to reduce the rate of feedback. Further, the nonuniform feedback strategy is investigated. Simulation results show that the proposed suboptimal solutions have much lower complexities than the convex optimization-based PPC solutions. The effectiveness of the proposed uniform and nonuniform limited-rate feedback methods is also demonstrated via simulations.

Transmit beamforming for MISO frequency-selective channels with total and per-antenna power constraints

We consider the problem of transmit beamforming (BF) design for cyclic prefixed (CP) transmissions over MISO frequency selective channels. Both CP single carriers (SC) and orthogonal frequency-division multiplexing (OFDM) systems are investigated. To reduce receiver complexity, frequency domain BF is adopted. The BF is designed by minimizing the arithmetic mean of the error probabilities at the receiver, first under the total power constraint (TPC) and then under the per-antenna power constraint (PPC). The solutions under the PPC are obtained using convex optimization tools. The simulation results show that although BF for SC only slightly outperforms BF for OFDM under TPC, the gap in performance becomes large under PPC. It is also shown that for large number of transmit antennas, the phase-rotation BF (PRB) is nearly optimal under PPC.