Multi-antenna Orthogonal Frequency Division Multiplexing Systems Research Articles

Hierarchical 2-D Feature Coding for Secure Pilot Authentication in Multi-User Multi-Antenna OFDM Systems: A Reliability Bound Contraction Perspective

Due to the publicly known and deterministic characteristic of pilot tones, pilot authentication (PA) in multi-user multi-antenna orthogonal frequency-division multiplexing systems is very susceptible to the jamming/nulling/spoofing behaviors. To solve this, in this paper, we develop a hierarchical 2-D feature (H2DF) coding theory that exploits the hidden pilot signal features, i.e., the energy feature and independence feature, to secure pilot information coding which is applied between legitimate parties through a well-designed five-layer hierarchical coding model to achieve secure multiuser PA (SMPA). The reliability of SMPA is characterized using the identification error probability (IEP) of pilot encoding and decoding with the exact closed-form upper and lower bounds. However, this phenomenon of non-tight bounds brings about the risk of long-term instability in SMPA. Therefore, a reliability bound contraction theory is developed to shrink the bound interval, and practically, this is done by an easy-to-implement technique, namely, codebook partition within the H2DF code. In this process, a tradeoff between the upper and lower bounds of IEP is identified and a problem of optimal upper and lower bound tradeoff is formulated, with the objective of optimizing the cardinality of sub-codebooks such that the upper and lower bounds coincide. Solving this, we finally derive an exact closed-form expression for IEP, which realizes a stable and highly reliable SMPA. Numerical results validate the stability and resilience of H2DF coding in SMPA.

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.

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.

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.

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.