Pilot-aided Orthogonal Frequency Division Multiplexing Research Articles

Eliminating Impulsive Noise in Pilot-Aided OFDM Channels via Dual of Penalized Atomic Norm

In this paper, we propose a novel estimator for pilot-aided orthogonal frequency division multiplexing (OFDM) channels in an additive Gaussian and impulsive perturbation environment. Due to sensor failure which might happen because of man-made noise, a number of measurements in high rate communication systems is often corrupted by impulsive noise. High power impulsive noise is generally an obstacle for OFDM systems as valuable information will be completely lost. To overcome this concern, an objective function based on a penalized atomic norm minimization (PANM) is provided in order to promote the sparsity of time dispersive channels and impulsive noise. The corresponding dual problem of the PANM is then converted to tractable semidefinite programming. It has shown that one can simultaneously estimate the time dispersive channels in a continuous dictionary and the location of impulsive noise using the dual problem. Several numerical experiments are carried out to evaluate the performance of the proposed estimator.

Pseudo pilot-aided OFDM system with high spectral efficiency for high performance coaxial transmission

High performance network over coax (HINOC) has become next-generation broadcasting network (NGB) industry standard of China, as well as International Telecommunication Union (ITU) telecommunication standard, which adopts orthogonal frequency division multiplexing (OFDM) technique and high order modulation formats. As is known, in OFDM system, phase noise destroys the orthogonality of the subcarriers and degrades the performance of the system especially when the data are transmitted in high order modulation formats. In this paper, we propose a high spectral efficiency OFDM system utilizing a novel and almost blind phase noise compensation method mainly used for high performance coaxial transmission. In our system, some subcarriers, which remain to transmit unknown useful data in lower order modulation formats, are selected as pseudo pilots instead of pilots to suppress phase noise effectively and high spectral efficiency can be achieved with the aid of puncturing mechanisms. Simulation results show that the proposed pseudo pilot-aided OFDM system can achieve the performance of the pilot-aided counterpart yet acquiring higher spectral efficiency.

Robust CFO Acquisition in PN-Padded OFDM Systems

As an alternative to the traditional pilot-aided orthogonal frequency division multiplexing (OFDM), the time-domain pseudonoise (PN)-padded OFDM provides a higher spectral efficiency. However, the carrier frequency offset (CFO) attenuates peaks of the conventional PN correlation output, which limits the CFO estimation range of the OFDM synchronizer. An improved correlation is proposed in this letter to remove the CFO-induced amplitude attenuation of correlation peaks. For a synchronizer adopting the designed correlator, a larger range of CFO acquisition is obtained through using wider correlation windows with a smaller interval between them. The proposed method of CFO acquisition is verified in a digital terrestrial multimedia broadcast receiver, in which the synchronizer is able to acquire CFOs up to ±320 kHz in the DVB-T F1 channel. Furthermore, the acquisition range can be expanded in more favorable channels.

On Communication Over Unknown Sparse Frequency-Selective Block-Fading Channels

This paper considers the problem of reliable communication over discrete-time channels whose impulse responses have length L and exactly S ≤ L non-zero coefficients, and whose support and coefficients remain fixed over blocks of N >; L channel uses but change independently from block to block. Here, it is assumed that the channel's support and coefficient realizations are both unknown, although their statistics are known. Assuming Gaussian non-zero-coefficients and noise, and focusing on the high-SNR regime, it is first shown that the ergodic noncoherent channel capacity has pre-log factor 1-(S)/(N) for any L. It is then shown that, to communicate with arbitrarily small error probability at rates in accordance with the capacity pre-log factor, it suffices to use pilot-aided orthogonal frequency-division multiplexing (OFDM) with S pilots per fading block, in conjunction with an appropriate noncoherent decoder. Since the achievability result is proven using a noncoherent decoder whose complexity grows exponentially in the number of fading blocks K, a simpler decoder, based on S+1 pilots, is also proposed. Its e-achievable rate is shown to have pre-log factor equal to 1-(S+1)/(N) with the previously considered channel, while its achievable rate is shown to have pre-log factor 1-(S+1)/(N) when the support of the block-fading channel remains fixed over time.

OFDM With Iterative Blind Channel Estimation

A new blind channel estimation technique is presented for uncoded orthogonal frequency-division multiplexing (OFDM) systems. Instead of using pilots to sound the channel, a decision algorithm first makes primary estimates of the data symbol for each subcarrier based on a constrained linear minimum mean square error (MMSE) criterion. Then, these estimates are applied to optimal MMSE channel estimation. The technique requires only one value from the time–frequency correlation of the channel transfer function. Performance is evaluated by simulation so that comparison can be made with known optimal coherent/differential detection. Compared with known decision-directed Kalman-based estimation and two pilot-aided OFDM schemes (block pilots and comb pilots), the presented technique performs better for regions with mid to high signal-to-noise ratios (SNRs). Its robustness to the time variation of the channel is also quantified by simulation, showing only small degradation in performance relative to the quasistatic case of wireless local area network (WLAN) systems. Finally, the impact of covariance assumptions in the channel modeling is quantified using simulation, offering a feel for the performance with mismatch between the channel model and the receiver assumptions.

Sampling Frequency Offset Estimation for Pilot-Aided OFDM Systems in Mobile Environment

In orthogonal frequency division multiplexing (OFDM) systems, most of the conventional sampling frequency offset (SFO) estimation methods work under the assumption of time-invariant or slow time-variant channels. In mobile environment, the time-variant channel significantly degrades the accuracy of SFO estimation. To solve the problem, we first analyze the properties of time-variant channels. If terminal moves within some tens of the wavelength of radio frequency (RF) signal, channel path delay almost remains unchanged. For most practical OFDM systems, our analysis indicates that channel path delay can be regarded as unchanged during the interval of some tens of OFDM symbols in time-variant channels. Based on the analysis, we propose a novel SFO estimation method for pilot-aided OFDM systems. Different from the conventional methods, the proposed method estimates SFO by detecting the variation of the symbol timing error caused by SFO. The detection is finished by implementing correlation between the channel impulse responses (CIRs) estimated by different OFDM symbols. Performance of the proposed method is simulated and compared with that of two conventional post-FFT methods. Numerical results show that, the proposed SFO estimation method performs better than the conventional methods not only in time-variant channels, but also at low SNRs and large residual carrier frequency offsets (CFOs).

Optimizing Pilot Locations Using Feedback in OFDM Systems

Pilot-aided orthogonal frequency-division multiplexing (OFDM) channel estimation has, so far, been optimized only for open-loop systems for which the uniform spacing of pilots is optimal. In this paper, we examine closed-loop OFDM systems and show how uniform pilots may no longer be optimal. In doing so, we propose a feedback technique in which the pilot allocation mechanism is adapted to the mobile channel. After deriving a general expression for the symbol error rate as a function of pilot allocation, we optimize the pilot locations and propose two practical solutions using bounds on the objective function. We proceed to formulate pilot allocation based on the maximal average OFDM channel capacity. We examine the feedback overhead that is associated with our method and make comparisons with the conventional uniform framework. Finally, we suggest the use of vector quantization in the context of the generalized Lloyd algorithm to reduce feedback and extend our method to a multiple-input-multiple-output (MIMO)-OFDM system based on the Alamouti space-time block code. Monte-Carlo simulations illustrate the error-rate/capacity performance gains via nonuniform pilots and illustrate the effects of Doppler frequency and carrier frequency offset.

Robust and fast frequency offset estimation for OFDM based satellite communication

A pilot-aided Orthogonal Frequency Division Multiplexing (OFDM) frequency offset estimator designed for satellite communication system is proposed in the paper. The estimator focuses on the acquisition of the integer part of carrier frequency offset and the acquisition range is as large as the whole signal bandwidth. Making full use of the phase difference between received pilot and local referential pilot, a fast estimation is obtained. Compared with existing method, our method can also work well even in SNR as low as 0dB. Simulations verify the good performance of our method.

Pilot-Aided OFDM Channel Estimation in the Presence of the Guard Band

In this letter, pilot design and channel estimation are discussed for orthogonal frequency-division multiplexing (OFDM) systems with guard subcarriers. First, we investigate the effects of guard band on channel estimation errors. From this, we propose pilot placement having a maximum distance between adjacent pilots except for the guard band, and show that it achieves minimum channel estimation errors among partially equispaced pilots using equivalence of the Toeplitz and circulant matrices. Also, an efficient channel estimator is developed by introducing an extended channel and its finite impulse response (FIR) approximation to overcome high numerical complexity caused by the presence of guard subcarriers and the use of a large number of subcarriers. Simulation results are presented for OFDM and orthogonal frequency division multiple access (OFDMA) systems consistent with IEEE 802.16a standards.

Performance analysis of channel estimation for OFDM systems with residual timing offset

The authors present an analysis of the effect of timing offset on channel estimation for comb-type pilot-aided orthogonal frequency division multiplexing (OFDM) systems. Residual timing offset does not negatively affect the channel estimation of the pilot subcarrier, but does corrupt the channel information obtained via interpolation. This paper provides the mean square error (MSE) channel estimation performance when a linear interpolation technique is used in a comb-type pilot-aided OFDM system. Analysis shows that the performance degradation of the channel estimator due to imperfect frame synchronization is dependent on the frequency correlation of the channels and the amount of timing offset

Clipping noise mitigation for channel estimation in OFDM systems

This letter provides a new technique to mitigate the clipping noise on pilot symbols in clipped and pilot-aided orthogonal frequency division multiplexing (OFDM) systems. The basic principle is to directly filter the clipping noise on the positions of pilots in the frequency domain before the insertion of pilots. Simulation results show that the new technique can effectively improve channel estimation and system performance

Pilot-aided OFDM∕SDMA channel estimation with unknown timing offset

The paper presents a novel approach to estimating the multi-path channel of orthogonal frequency-division multiplexing (OFDM)/spatial-diversity multiple-access (SDMA) systems. Having OFDM/SDMA symbols transmitted not aligned with base station (BS) timing, demodulated signals may severely suffer from an inter-OFDM-symbol interference (ISI), namely the misalignment interference. Consequently, the interference degrades the channel estimation extensively. By appropriately designing pilots, the misalignment effects on the channel estimation can fully be cancelled and, the channel is optimally estimated in the minimum mean square error (MMSE) sense by a new estimation scheme. Owing to the interference cancellation, the new scheme is more robust against timing offsets (TO) and gives a better performance in estimating the SDMA channel than the existing approaches.

ICI mitigation for pilot-aided OFDM mobile systems

Orthogonal frequency-division multiplexing (OFDM) is robust against frequency selective fading due to the increase of the symbol duration. However, for mobile applications channel time-variations in one OFDM symbol introduce intercarrier-interference (ICI) which degrades the performance. This becomes more severe as mobile speed, carrier frequency or OFDM symbol duration increases. As delay spread increases, symbol duration should also increase in order to maintain a near-constant channel in every frequency subband. Also, due to the high demand for bandwidth, there is a trend toward higher carrier frequencies. Therefore, to have an acceptable reception quality for the applications that experience high delay and Doppler spread, there is a need for ICI mitigation within one OFDM symbol. We introduce two new methods to mitigate ICI in an OFDM system with coherent channel estimation. Both methods use a piece-wise linear model to approximate channel time-variations. The first method extracts channel time-variations information from the cyclic prefix. The second method estimates these variations using the next symbol. We find a closed-form expression for the improvement in average signal-to-interference ratio (SIR) when our mitigation methods are applied for a narrowband time-variant channel. Finally, our simulation results show how these methods would improve the performance in a highly time-variant environment with high delay spread.