Orthogonal Frequency Division Multiplexing Symbol Period Research Articles

OFDM tolerance to additive white Gaussian and laser phase noises in optical heterodyning systems accompanied by the feasible pilot-assisted equalization

It is well known that the performance of orthogonal frequency division multiplexing (OFDM) is limited by phase noise, which depends on the laser linewidths for coherent-detection radio over fiber (RoF) systems. In this manuscript, we present numerical and theoretical results to analyze the combined effects of phase and additive white Gaussian noises in the performance of RoF-OFDM schemes with the feasible pilot-based channel corrector. This phase-noise compensator is adopted due to its effectiveness as well as simplicity. It is shown that as the subcarrier modulation format increases, the required radio frequency linewidth for overcoming the FEC limit decreases by a factor of 10 times. Also, there is a signal–noise ratio penalty of 7 dB. Furthermore, it is demonstrated that the bit error rate enhances as the product of the laser linewidths and OFDM symbol period decreases because the pilot-assisted equalization method acts as a high-pass filter for phase noise. We finally discovered that the intermediate-frequency (IF) carrier suppression is not mandatory; the system performance is not affected by selecting properly the carrier–signal ratio and IF. To confirm the previous observation, a novel way to measure phase noise in RoF-OFDM signals accompanied by the IF carrier is proposed. The mean integral phase noise comes from the adaptation of the root mean square phase deviation of the single-carrier systems. This work serves as a guideline for the study and design of OFDM-based RoF schemes susceptible to direct and coherent detections.

Modulation in the Air: Backscatter Communication Over Ambient OFDM Carrier

Ambient backscatter communication (AmBC) enables radio-frequency (RF) powered backscatter devices (BDs) (e.g., sensors and tags) to modulate their information bits over ambient RF carriers in an over-the-air manner. This technology, also called “modulation in the air,” has emerged as a promising solution to achieve green communication for future Internet of Things. This paper studies an AmBC system by leveraging the ambient orthogonal frequency division multiplexing (OFDM) modulated signals in the air. We first model such AmBC system from a spread-spectrum communication perspective, upon which a novel joint design for BD waveform and receiver detector is proposed. The BD symbol period is designed as an integer multiplication of the OFDM symbol period, and the waveform for BD bit “0” maintains the same state within the BD symbol period, while the waveform for BD bit “1” has a state transition in the middle of each OFDM symbol period within the BD symbol period. In the receiver detector design, we construct the test statistic that cancels out the direct-link interference by exploiting the repeating structure of the ambient OFDM signals due to the use of cyclic prefix. For the system with a single-antenna receiver, the maximum-likelihood detector is proposed to recover the BD bits, for which the optimal threshold is obtained in closed-form expression. For the system with a multi-antenna receiver, we propose a new test statistic which is a linear combination of the per-antenna test statistics and derive the corresponding optimal detector. The proposed optimal detectors require only knowing the strength of the backscatter channel, thus simplifying their implementation. Moreover, practical timing synchronization algorithms are proposed for the designed AmBC system, and we also analyze the effect of various system parameters on the transmission rate and detection performance. Finally, extensive numerical results are provided to verify that the proposed transceiver design can improve the system bit-error-rate performance and the operating range significantly and achieve much higher data rate, as compared with the conventional design.

On Detection in MIMO-OFDM Systems Over Highly Mobile Wireless Channels

High mobility support of multiple-input multiple-output (MIMO)-orthogonal frequency division multiplexing (OFDM) systems plays a major role in wireless communication systems. Conventional detection approaches for MIMO-OFDM systems assume the channel is constant for at least one OFDM symbol duration. However, the time variation of the channel on account of mobility over an OFDM symbol period causes a loss of subchannel orthogonality which results in inter-carrier interference. In this paper, we propose the space alternating generalized expectation maximization (SAGE) based signal detection for MIMO-OFDM systems and compared by the zero forcing (ZF), minimum mean square error (MMSE), vertical Bell laboratories layered space-time (VBLAST) detection methods by considering complexity and performance tradeoffs. Simulation results show that the SAGE based detection has comparable performance with the VBLAST while needs lower computational complexity. It is also shown that both SAGE and VBLAST have a clear BER performance advantage over the MMSE and ZF detections. We also investigate the sensitivity of detection MIMO-OFDM signals to channel estimation errors by the application of a simple frequency domain based pilot based channel estimation. It is shown that the superiority of the SAGE scheme is also valid in the case of channel imperfections.

Blind Estimation of OFDM Parameters in Cognitive Radio Networks

This paper presents a blind parameter estimation algorithm for orthogonal frequency division multiplexing (OFDM) signal affected by a time-dispersive channel, timing offset, carrier frequency offset and additive Gaussian noise. Unlike the previous studies, this paper presents the second-order cyclostationarity of OFDM signal considering the effect of time-dispersive channel. The cyclostationarity properties of received OFDM signal in time-dispersive channel is exploited to estimate the OFDM parameters. These parameters includes OFDM symbol period, useful symbol period, cyclic prefix factor, number of subcarriers and carrier frequency offset. Simulations are performed to investigate the performance of OFDM parameter estimation algorithm in diverse channel conditions.

Differential Quasi-Orthogonal Space-Frequency Trellis Codes

Two rate-one differential quasi-orthogonal space-frequency trellis codes (DQOSFTCs) for multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) channels are proposed. The DQOSFTCs are systematically constructed within an OFDM symbol period, by combining unitary quasi-orthogonal trellis codes and differential modulation over the frequency domain (DF-QOSFTC) or time domain (DT-QOSFTCs). Besides multipath diversity, our DQOSFTCs achieve high-coding gain and simple decoding when the channel state information is not available at both the transmitter and the receiver. Simulation results show that our proposed codes significantly outperform the existing differential space-frequency trellis codes.

Blind carrier frequency offset estimator for multi-input multi-output-orthogonal frequency division multiplexing systems over frequency-selective fading channels

This study presents a new blind carrier frequency offset (CFO) estimation technique for multi-input multi-output (MIMO) orthogonal frequency division multiplexing (OFDM) systems employing space-time coding (STC). CFO estimation is crucial for OFDM systems to avoid the performance degradation because of the inter-carrier interference that results when the CFO is not estimated and compensated accurately. Based on the assumptions that the data symbols are selected from a constant modulus constellation and the channel is varying slowly over time, a new blind CFO estimator is proposed by minimising the power difference between all subcarriers in two consecutive STC blocks. Therefore the proposed system exploits all subcarriers in time and frequency domain, which provides a remarkable performance improvement over other techniques reported in the literature. The complexity of the proposed estimator is substantially reduced by approximating the cost function by a sinusoid that can be minimised using direct closed-form computations within one OFDM symbol period. Monte Carlo simulations are used to assess the performance of the proposed system by means of mean squared error (MSE) in both static and time-varying frequency-selective fading channels. The simulation results demonstrate that the proposed estimator can eliminate the MSE error floors that usually appear at moderate and high signal-to-noise ratios for the estimators that work only in frequency domain.

Low-Complexity ICI Mitigation Methods for High-Mobility SISO/MIMO-OFDM Systems

In orthogonal frequency-division multiplexing (OFDM) systems, it is generally assumed that the channel response is static in an OFDM symbol period. However, the assumption does not hold in high-mobility environments. As a result, intercarrier interference (ICI) is induced, and system performance is degraded. A simple remedy for this problem is the application of the zero-forcing (ZF) equalizer. Unfortunately, the direct ZF method requires the inversion of an N times N ICI matrix, where N is the number of subcarriers. When N is large, the computational complexity can become prohibitively high. In this paper, we first propose a low-complexity ZF method to solve the problem in single-input-single-output (SISO) OFDM systems. The main idea is to explore the special structure inherent in the ICI matrix and apply Newton's iteration for matrix inversion. With our formulation, fast Fourier transforms (FFTs) can be used in the iterative process, reducing the complexity from O (N <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> ) to O (N log <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> N). Another feature of the proposed algorithm is that it can converge very fast, typically in one or two iterations. We also analyze the convergence behavior of the proposed method and derive the theoretical output signal-to-interference-plus-noise ratio (SINR). For a multiple-input-multiple-output (MIMO) OFDM system, the complexity of the ZF method becomes more intractable. We then extend the method proposed for SISO-OFDM systems to MIMO-OFDM systems. It can be shown that the computational complexity can be reduced even more significantly. Simulations show that the proposed methods perform almost as well as the direct ZF method, while the required computational complexity is reduced dramatically.

Blind frequency-offset estimator for OFDM with general symbol constellation

An efficient feedforward blind carrier frequency offset (CFO) estimator for orthogonal frequency division multiplexing (OFDM) systems with general symbol constellation is presented. The proposed estimator requires only one OFDM symbol period to blindly and accurately estimate the CFO in Gaussian and multipath fading channels.

LMMSE 등화기법을 적용한 OFDM-DSRC 시스템의 성능분석

무선 다중경로 채널에서 데이터를 고속으로 전송할 경우, 신호는 페이딩, 심볼간 간섭 등의 영향으로 높은 에러율을 갖게 된다. 그러므로 현재 개발된 DSRC(dedicated short-range communication) 시스템의 변복조 방식은 1 Mbps이상의 데이터 서비스가 어려울 것으로 예상되므로 새로운 채널등화 기법 및 개선된 변복조 방식이 요구된다. OFDM(orthogonal frequency division multiplexing) 방식은 보호간격의 삽입을 통하여 ISI를 방지할 수 있으므로 고속 데이터 전송에 적합하다. 그러나 보호기간이 OFDM 방식에서의 각각의 심볼주기에 사용되는 채널지연 확산보다 길어지므로 채널활용의 효율성에서 상당한 손실이 야기된다. 그러므로 등화기를 고속의 데이터 전송율과 긴 채널 지연확산 조건을 가지는 ITS(intelligent transport system)에 적용하기 위해서는 ISI(inter symbol interference)를 제거할 필요가 있다. 본 논문에서는 OFDM-DSRC 시스템을 위한 채널 등화기를 설계하였으며, 다중경로 페이딩 환경에서 컴퓨터 시뮬레이션을 통해 그 성능을 분석하였다. The signal in wireless multi-path channel is affected by fading and ISI because of high data rate transmission, so the signal has the high error rate. The present modulation and demodulation method of DSRC system can not expect sufficient for providing data service over 1 Mbps, so the channel equalization and advanced modulation and demodulation methods are required. OFDM is generally known as an effective technique for high data rate transmission system, since it can prevent ISI by inserting a guard interval. However, a guard interval longer than channel delay spread has to be used in each OFDM symbol period, thus resulting a considerable loss in the efficiency of channel utilization Therefore the equalizer is necessary to cancel ISI to accommodate advanced ITS service with higher bit rate and longer channel delay spread condition In this paper, the channel equalizer for the OFDM -DSRC system was designed and its performance in a multi-path fading environment was evaluated with computer simulation.

Doubly selective fading channel estimation in MIMO OFDM systems

High data transmission rates and high mobility give rise to time- and frequency-selectivity in wireless communication channels. This paper investigated time and frequency doubly selective channel estimation using pilot tones among Multi-Input Multi-Output (MIMO) orthogonal frequency division multiplexing (OFDM). Firstly, a complex exponential basis expansion channel model (BECM) was introduced to represent doubly selective channel during an OFDM symbol period; then, based on BECM, an effective MIMO-OFDM doubly selective channel estimation method was presented; finally, the optimality in designing pilot tones parameters was done according to the minimization of channel estimation MSE, mainly including the number, the placement and the structure of pilot tones. Simulation results show that the proposed estimation method has good performance in doubly selective channel scenarios and confirms the theoretical analysis findings.