Multi-antenna Orthogonal Frequency Division Multiplexing Research Articles

Channel Shortening by Large Multiantenna Precoding in OFDM

A channel delay spread larger than the cyclic prefix (CP) creates inter-carrier/symbol interference (ISI/ICI) in orthogonal frequency-division multiplexing (OFDM). Recent interests in low-latency applications have motivated the usage of shorter OFDM symbols where one can either downscale the CP at the cost of interference, or maintain it but with larger overhead. Alternatively, this paper studies channel shortening methods exploiting the properties of large multi-antenna precoding in order to steer the transmitted signal energy toward channel paths inside an insufficient CP. It is shown that ISI/ICI can asymptotically be canceled by conventional subcarrier-based precoding with an infinite number of antennas. This is achieved by introducing time-delay selectivity inside frequency-selective precoders in order to remove undesired delayed signals, providing a trade-off between interference mitigation and multi-path combining gains, and leading to subsequent gains in high SNR. This frequency-domain precoding method, coined time-frequency (TF) precoding, is compared to time-reversal (TR) filtering whose asymptotic rate is optimal but introduces post-modulation processing with channel-dependent signal distortion. In addition to maintain the legacy precoded multi-antenna OFDM structure, finite-size analysis shows that TF-precoding converges faster to its asymptotic rate than TR-filtering, so that TF-precoding can outperform TR-filtering in the high-SNR regime with not-so-many antennas.

Code-Frequency Block Group Coding for Anti-Spoofing Pilot Authentication in Multi-Antenna OFDM Systems

A pilot spoofer can paralyze the channel estimation in multi-user orthogonal frequency-division multiplexing (OFDM) systems by using the same publicly known pilot tones as legitimate nodes. This causes the problem of pilot authentication (PA). To solve this, we propose, for a two-user multi-antenna OFDM system, a code-frequency block group (CFBG) coding-based PA mechanism. Here multi-user pilot information, after being randomized independently to avoid being spoofed, is converted into activation patterns of subcarrier-block groups on code-frequency domain. Those patterns, though overlapped and interfered mutually in the wireless transmission environment, are qualified to be separated and identified as the original pilots with high accuracy, by exploiting CFBG coding theory and channel characteristic. Particularly, we develop the CFBG code through two steps, i.e., 1) devising an ordered signal detection technique to recognize the number of signals coexisting on each subcarrier block, and encoding each subcarrier block with the detected number and 2) constructing a zero-false-drop code and block detection-based code via $k$ -dimensional Latin hypercubes and integrating those two codes into the CFBG code. This code can bring a desirable pilot separation error probability, inversely proportional to the number of occupied subcarriers and antennas with a power of $k$ . To apply the code to PA, a scheme of pilot conveying, separation, and identification is proposed. Based on this novel PA, a joint channel estimation and identification mechanism is proposed to achieve high-precision channel recovery and simultaneously enhance PA without occupying extra resources. Simulation results verify the effectiveness of our proposed mechanism.

Position-Based Interference Elimination for High-Mobility OFDM Channel Estimation in Multicell Systems

Orthogonal frequency-division multiplexing (OFDM) and multicell architecture are widely adopted in current high-speed train (HST) systems for providing high-data-rate wireless communications. In this paper, a typical multiantenna OFDM HST communication system with multicell architecture is considered, where the intercarrier interference (ICI) caused by high mobility and the multicell interference (MCI) are both taken into consideration. By exploiting the train position information, a new position-based interference elimination method is proposed to eliminate both the MCI and the ICI for a general basis expansion model. We show that the MCI and the ICI can be completely eliminated by the proposed method to get the ICI-free pilots at each receive antenna. In addition, for the considered multicell HST system, we develop a low-complexity compressed channel estimation method and consider the optimal pilot pattern design. Both the proposed interference elimination method and the optimal pilot pattern are robust to the train speed and position, as well as the multicell multiantenna system. Simulation results demonstrate the benefits and robustness of the proposed method in the multicell HST system.

Compressive Sensing Based Beamforming forMillimeter-Wave OFDM Systems

In this paper, we address codebook-based beamforming for multi-antenna orthogonal frequency division multiplexing (OFDM) systems operating in 60-GHz millimeter-wave band. Given a pre-specified codebook, the task of the beamforming process is to identify the best transmit and receive beam vectors to maximize the spectral efficiency. By properly modeling the multipath structure in 60-GHz channels, we transform the above task into the estimation of several sparse signal vectors. Furthermore, we tailor compressive sensing (CS) technology to efficiently estimate such sparse signals, wherein the CS measurement matrix is specifically designed by minimizing the inner products between different columns of the CS effective dictionary. As a result, CS-based iterative and non-iterative beamforming schemes are proposed. Due to the ability to judiciously exploit multipath sparsity inherent in 60-GHz channels, the proposed schemes significantly outperform the existing counterparts in terms of training overhead and success probability, which is confirmed by theoretical analysis and extensive simulations.

Joint Detection and Decoding in LDPC-Based Space-Time Coded MIMO-OFDM Systems via Linear Programming

This work proposes a novel linear programming approach for the joint detection and decoding of LDPC-based space-time (ST) coded signals in multi-antenna orthogonal frequency division multiplexing (OFDM) systems. While traditional receivers typically decouple the detection and decoding processes as two disjunctive blocks or require iterative turbo exchange of extrinsic information between the soft detector and decoder, we formulate a joint linear program (LP) by exploiting the constraints imposed on the data symbols, training symbols, noise subspace as well as channel code. In consideration of the vast amount of LDPC parity check inequalities, we further present an adaptive procedure to significantly reduce the complexity of the joint LP receiver. Our LP-based receivers outperform existing receivers with substantial performance gains. Moreover, the proposed joint LP receiver demonstrates strong robustness when pilot symbols are sparsely arranged on subcarriers.

Enhancement in Spectral Efficiency using Transmit-side Channel Shortener for MISO-OFDM Systems

Time-domain channel shortening filters (CSFs) are used in orthogonal frequency division multiplexing (OFDM) systems to increase the spectral efficiency. We consider transmit-side CSF to reduce the cyclic prefix (CP) length required for the OFDM symbol. We first construct a simple CSF scheme which, however, has a poorer spectral efficiency (SE) when compared to a system with adequate CP length (which we refer to as a full-CP system). We then propose a modified CSF (MCSF) structure that exploits the null-space of an under-determined system of equations, and provides independent equivalent channels to the receiver. The SE of the MCSF is shown to be generally higher than the full-CP system when the chosen CP length is significantly smaller than the channel delay spread. Simulated BLER performance for turbo-coded multi-antenna OFDM systems reveals that the MCSF can provide up to 2 dB gain in <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX">${{\bf E}_{\bf b}}/{{\bf N}_{\bf 0}}$</tex></formula> when compared with the full-CP system for the same transmit information rate.

Performance Evaluation of MIMO OFDM Systems in On-Ship Below-Deck Environments

Below-deck compartments on naval vessels provide a challenging environment for wireless networks. The metallic walls of the compartments produce multiple reflections that can degrade signal integrity. Between compartments, the metal bulkheads impede the propagation of electromagnetic waves, limiting network connectivity. Orthogonal frequency-division multiplexing (OFDM) is proposed to mitigate the effects of intersymbol interference (ISI) caused by multiple reflections. Additionally, the use of multiple antennas for channel diversity has shown to improve communications reliability and capacity. Single and multiantenna OFDM physical layers were tested within several below-deck spaces aboard Thomas S. Gates (CG 51), a decommissioned Ticonderoga-class US Navy cruiser. Measurements were taken with four OFDM-based schemes typical of current-generation Wireless Local Area Network (WLAN) technologies. The performance of multiantenna signaling techniques, including 2 × 2 Alamouti space-time coding and 2 × 2 multiple-input-multiple-output spatial multiplexing (MIMO-SM), were compared to the performances of 1 × 2 maximal ratio combining (MRC) and a conventional single-input-single-output (SISO) system. Results indicate that the tested MIMO techniques can approximately double the channel capacity. Throughput as high as 36 Mb/s was achieved in conventional situations where SISO links only admitted rates of 18 Mb/s.

Scalable Video Multicast Over Multi-Antenna OFDM Systems

We propose a framework for efficient scalable video multicast over downlink orthogonal frequency-division multiplexing (OFDM) systems with multiple transmit antennas. In conventional video multicast systems, the achievable transmission rate is determined by the user with the worst channel condition, and the system saturates the capacity when the number of users increases. To accommodate the heterogeneous channel conditions and device capabilities of various users, scalable video coding (SVC) encodes video streams into base and enhancement layers. We exploit the advances in multi-antenna OFDM and the layered nature of SVC, and propose a framework for scalable video multicast which guarantees the base layer quality for all users while making best use of limited resource for the enhancement layer of users with good channel conditions. We show that the resource allocation that includes the transmit precoding, subcarrier allocation, and bit and power allocation is a very difficult optimization problem. A low-complexity suboptimal algorithm is proposed which is suitable for practical implementations. Simulation results are provided to show the effectiveness of the proposed solution.

Performance of peak-to-average power ratio reduction in single- and multi-antenna OFDM via directed selected mapping

Selected mapping (SLM) is a popular scheme for peak power reduction in orthogonal frequency-division multiplexing (OFDM) systems. In this letter, the performance of various versions of SLM, among them ordinary and directed SLM, in single- and multi-antenna point-to-point OFDM systems is assessed. Analytic expressions for the distribution of the PAR are derived. Numerical results cover that significant gains over conventional SLM can be achieved by directed SLM.

Antenna Selection for Noncoherent Space–Time–Frequency-Coded OFDM Systems

This paper studies receive antenna selection (AS) for multiantenna systems that operate over frequency-selective channels, where the channel state information (CSI) is known neither at the transmitter nor at the receiver. We consider AS for noncoherent space-time-frequency (STF)-coded orthogonal frequency-division multiplexing (OFDM) systems that employ unitary signals. An OFDM receiver with a single radio-frequency chain is employed, and the selection is based on the received signal power, where the receive antenna with the largest received signal power averaged over all subcarriers is chosen. Theoretical analysis and simulations show that by using AS, the maximum spatial and multipath diversity can still be obtained without CSI at the receiver, which is the same as the full-complexity multiantenna OFDM systems. We simplify code design and decoding using subcarrier grouping, which enables us to apply existing full-diversity codes for noncoherent and differential STF-OFDM systems to each group, to obtain maximum spatial and multipath diversity when AS is used. We also present a low-complexity suboptimal decoder that can easily be implemented in practice.

Analysis and Compensation of Transmitter and Receiver I/Q Imbalances in Space-Time Coded Multiantenna OFDM Systems

The combination of orthogonal frequency division multiplexing (OFDM) and multiple-input multiple-output (MIMO) techniques has been widely considered as the most promising approach for building future wireless transmission systems. The use of multiple antennas poses then big restrictions on the size and cost of individual radio transmitters and receivers, to keep the overall transceiver implementation feasible. This results in various imperfections in the analog radio front ends. One good example is the so-called I/Q imbalance problem related to the amplitude and phase matching of the transceiver I and Q chains. This paper studies the performance of space-time coded (STC) multiantenna OFDM systems under I/Q imbalance, covering both the transmitter and the receiver sides of the link. The challenging case of frequency-selective I/Q imbalances is assumed, being an essential ingredient in future wideband wireless systems. As a practical example, the Alamouti space-time coded OFDM system with two transmit and M receive antennas is examined in detail and a closed-form solution for the resulting signal-to-interference ratio (SIR) at the detector input due to I/Q imbalance is derived. This offers a valuable analytical tool for assessing the I/Q imbalance effects in any STC-OFDM system, without lengthy data or system simulations. In addition, the impact of I/Q imbalances on the channel estimation in the STC-OFDM context is also analyzed analytically. Furthermore, based on the derived signal models, a practical pilot-based I/Q imbalance compensation scheme is also proposed, being able to jointly mitigate the effects of frequency-selective I/Q imbalances as well as channel estimation errors. The performance of the compensator is analyzed using extensive computer simulations, and it is shown to virtually reach the perfectly matched reference system performance with low pilot overhead.

Partial Transmit Sequences for Peak-to-Average Power Ratio Reduction in Multiantenna OFDM

The major drawback of orthogonal frequency-division multiplexing (OFDM) is its high peak-to-average power ratio (PAR), which gets even more substantial if a transmitter with multiple antennas is considered. To overcome this problem, in this paper, the partial transmit sequences (PTS) method--well known for PAR reduction in single antenna systems--is studied for multiantenna OFDM. A directed approach, recently introduced for the competing selected mapping (SLM) method, proves to be very powerful and able to utilize the potential of multiantenna systems. To apply directed PTS, various variants for providing a sufficiently large number of alternative signal superpositions (the candidate transmit signals) are discussed. Moreover, affording the same complexity, it is shown that directed PTS offers better performance than SLM. Via numerical simulations, it is pointed out that due to its moderate complexity but very good performance, directed or iterated PTS using combined weighting and temporal shifting is a very attractive candidate for PAR reduction in future multiantenna OFDM schemes.

Theory and design of near-optimal MIMO OFDM transmission system for correlated multipath Rayleigh fading channels

We consider channel-coded multi-input multi-output (MIMO) orthogonal frequency-division multiplexing (OFDM) transmission and obtain a condition on its signal for it to attain the maximum diversity and coding gain. As this condition may not be realizable, we propose a suboptimal design that employs an orthogonal transform and a space-frequency interleaver between the channel coder and the multi-antenna OFDM transmitter. We propose a corresponding receiving method based on block turbo equalization. Attention is paid to some detailed design of the transmitter and the receiver to curtail the computational complexity and yet deliver good performance. Simulation results demonstrate that the proposed transmission technique can outperform the conventional coded MIMO OFDM and the MIMO block single-carrier trans- mission with cyclic prefixing.

Novel time-frequency differential space-time modulation for multi-antenna OFDM systems

Abstract Differential space-time (DST) modulation has been proposed recently for multiple-antenna systems over Rayleigh fading channels, where neither the transmitter nor the receiver knows the fading coefficients. Among existing schemes, differential modulation is always performed in the time domain and suffers performance degradations in frequency-selective fading channels. In order to combat the fast time and frequency-selective fading, a novel time-frequency differential space-time (TF-DST) modulation scheme, which adopts differential modulation in both time and frequency domains, is proposed for multi-antenna orthogonal frequency division multiplexing (OFDM) system. A corresponding suboptimal yet low-complexity non-coherent detection approach is also proposed. Simulation results demonstrate that the proposed system is robust for time and frequency-selective Rayleigh fading channels.

LDPC based time-frequency double differential space-time coding for multi-antenna OFDM systems

Differential space-time coding was proposed recently in the literature for multi-antenna systems, where neither the transmitter nor the receiver knows the fading coefficients. Among existing schemes, double differential space-time (DDST) coding is of special interest because it is applicable to continuous fast time-varying channels. However, it is less effective in frequency-selective fading channels. This paper’ls authors derived a novel time-frequency double differential space-time (TF-DDST) coding scheme for multi-antenna orthogonal frequency division multiplexing (OFDM) systems in a time-varying frequency-selective fading environment, where double differential space-time coding is introduced into both time domain and frequency domain. Our proposed TF-DDST-OFDM system has a low-complexity non-coherent decoding scheme and is robust for time-and frequency-selective Rayleigh fading. In this paper, we also propose the use of state-of-the-art low-density parity-check (LDPC) code in serial concatenation with our TF-DDST scheme as a channel code. Simulations revealed that the LDPC based TF-DDST OFDM system has low decoding complexity and relatively better performance.

A Near-Optimal Low-Complexity Transceiver for CP-Free Multi-Antenna OFDM Systems

Conventional orthogonal frequency division multiplexing (OFDM) system utilizes cyclic prefix (CP) to remove the channel-induced inter-symbol interference (ISI) at the cost of lower spectral efficiency. In this paper, a generalized sidelobe canceller (GSC) based equalizer for ISI suppression is proposed for uplink multi-antenna OFDM systems without CP. Based on the block representation of the CP-free OFDM system, there is a natural formulation of the ISI suppression problem under the GSC framework. By further exploiting the signal and ISI signature matrix structures, a computationally efficient partially adaptive (PA) implementation of the GSC-based equalizer is proposed for complexity reduction. The proposed scheme can be extended for the design of a pre-equalizer, which pre-suppresses the ISI and realizes CP-free downlink transmission to ease the computational burden of the mobile unit (MU). Simulation results show that the proposed GSC-based solutions yield equalization performances almost identical to that obtained by the conventional CP-based OFDM systems and are highly resistant to the increase in channel delay spread.

OFDM Channel Estimation in the Presence of Interference

We develop a frequency-domain channel estimation algorithm for single-user multiantenna orthogonal frequency division multiplexing (OFDM) wireless systems in the presence of synchronous interference. In this case, the synchronous interferer's signal on each OFDM subcarrier is correlated in space with a rank one spatial covariance matrix. In addition, the interferer's spatial covariance matrix is correlated in frequency based on the delay spread of the interferer's channel. To reduce the number of unknown parameters we develop a structured covariance model that accounts for the structure resulting from the synchronous interference. To further reduce the number of unknown parameters, we model the covariance matrix using a priori known set of frequency-dependent functions of joint (global) parameters. We estimate the interference covariance parameters using a residual method of moments (RMM) estimator and the channel parameters by maximum likelihood (ML) estimation. Since our RMM estimates are invariant to the mean, this approach yields simple noniterative estimates of the covariance parameters while having optimal statistical efficiency. Hence, our algorithm outperforms existing channel estimators that do not account for the interference, and at the same time requires smaller number of pilots than the MANOVA method and thus has smaller overhead. Numerical results illustrate the applicability of the proposed algorithm.

A Semi-Blind Channel Estimation Method for Multiuser Multiantenna OFDM Systems

A subspace-based blind method is proposed for estimating the channel responses of a multiuser and multiantenna orthogonal frequency division multiplexing (OFDM) uplink system. It gives estimations to all channel responses subject to a scalar matrix ambiguity and does not need precise channel order information (only an upper bound for the orders is required). Furthermore, the scalar ambiguity matrix can be easily resolved by using only one pilot OFDM block, given that the number of users is smaller than the number of symbols in the pilot symbol block. Equalization methods are discussed based on the estimated channels. By using partial knowledge of the channels, a multipath subspace method is proposed that reduces the computational complexity. Simulations show that the methods are effective and robust.