Pilot-aided Channel Estimation Research Articles

Analysis of Pilot-Aided Channel Estimation with Optimal Leakage Suppression for OFDM Systems

This letter analyzes the effect of the leakage on the mean square error (MSE) performance of the pilot-aided channel estimator using discrete Fourier transform (DFT)-based interpolation in orthogonal frequency division multiplexing (OFDM) systems with virtual carriers. The optimal linear estimator for leakage suppression is derived to minimize the MSE. Numerical results show that the pilot-aided channel estimator with optimal leakage suppression improves the MSE performance significantly over the conventional one.

Pilot-Aided Multicarrier Channel Estimation via MMSE Linear Phase-Shifted Polynomial Interpolation

Current wireless multicarrier systems commonly adopt pilot-aided channel estimation, for which a simplest and least restrictive technique is polynomial interpolation in the frequency domain. For channels with large delay spreads, however, the performance of low-order polynomial interpolation suffers from modeling error. The problem may be remedied by adding a linear phase shift to the interpolator, or equivalently, a "window shift" in the time domain. We derive a method to estimate the optimal window shift, in the minimum mean-square error (MMSE) sense, for polynomial-interpolative channel estimation of arbitrary order. As a practical application, we show how to apply the resultant technique to Mobile WiMAX downlink channel estimation. In addition, we propose a method to automatically select the interpolation order based on some estimated MSE.

Channel Estimation Based in Comb-Type Pilots Arrangement for OFDM System over Time Varying Channel

Orthogonal Frequency Division Multiplexing (OFDM) has been recently applied widely in wireless communication systems, due to its high data rate, transmission capability with high bandwidth, efficiency and its robustness to multipath delay .Channel estimation is an essential problem in OFDM system. Pilot-aided channel estimation has been used; a good choice of the pilot pattern should match the channel behavior both in time and frequency domains. We explored comb pilot arrangements. The advantage for comb type pilots arrangement in channel estimation is the ability to track the variation of the channel caused by doppler frequency, it is observed that the doppler effect can be reduced, and so this will increase the system mobility. Kalman and Least Square (LS) estimators have been proposed to estimate the Channel Frequency Response (CFR) at the pilots location, then CFR at data sub channels are obtained by mean of interpolation between estimates at pilot locations. Different types of interpolations have been used such as; low pass interpolation; spline cubic interpolation and linear interpolation. Kalman estimation has better performance than LS estimation. The estimators perform about the same for SNR lower than 10 dB. The performances of all schemes have been compared by finding Bit Error Rate (BER), where BPSK modulation scheme was used.

Effective Symbol Timing Recovery Based on Pilot-Aided Channel Estimation for MISO Transmission Mode of DVB-T2 System

This paper proposes an effective symbol timing recovery (STR) based on a pilot-aided channel impulse response (CIR) estimation for multi-input single-output (MISO) transmission mode of DVB-T2 system. In particular, this paper focuses on fine STR capable of resolving an ambiguity effect of the CIR which is caused by an inaccurate coarse symbol timing offset (STO). In the proposed fine STR, the CIR of the MISO channel is estimated after performing coarse STR. Then, the ambiguity of the CIR is investigated by categorizing it into four regions under the assumption of an inaccurate STO. Finally, accurate STO is estimated by changing the fast Fourier transform (FFT) window with respect to the ambiguity categorization. Performance evaluations are accomplished by comparing the proposed STR with the conventional STR in large delay channels.

Performance Analysis of a Complex Multicode Tracking Loop in Bandlimited Rayleigh Fading Channels

This paper presents the root-mean-squared tracking error performance analysis of a class of coherent digital delay-locked loops for multicode direct sequence spread spectrum signals in bandlimited correlated Rayleigh fading channels. In the transmit side, multiple independent PSK-modulated data streams in the in-phase and qudrature phase branches are spread by short mutually orthogonal codes before being further complex spread by a long complex PN code. We assume that the system employs a pilot code channel to assist the receiver's synchronization and channel estimation. The proposed code tracking loop incorporates the pilot-aided channel estimator and derives the timing error from all short-code-spread subchannels. Our analysis takes into account the effects of imperfect channel estimate, correlated frequency selective Rayleigh fading and band-limiting. Numerical results are presented to quantify the impact of the resulting multipath interference within the same code channel and amongst different code channels.

Enhanced multiple access schemes for future broadband satellite communications

A new structure of next generation integrated communication system was proposed, which is composed of space segment based on satellites and terrestrial segment. Moreover, the characteristics of enhanced multiple access schemes based on orthogonal frequency division multiplexing (OFDM) technique were analyzed for satellite links. However, OFDM is a doubtful candidate as its higher peak-to-average power ratio (PAPR) that causes the distortion of high power amplifier (HPA). Furthermore, different schemes were evaluated and compared in terms of the HPA nonlinearity and the link level performance in detail. And the pilot-aided channel estimation and equalization techniques were also considered for analyzing the problem. Simulation results show that the bit error rate (BER) and block error rate (BLER) performance of orthogonal frequency division multiple access (OFDMA) outperforms that of single carrier-frequency division multiple access (SC-FDMA) for the satellite links in the proposed structure, though discrete Fourier transform-spread OFDM DFT-S OFDM has low PAPR, especially the BER performance of OFDMA is 3.6 dB larger than that of SC-FDMA at the target BER.

Performance Analysis of Analytic Interpolator for Comb-Type Pilot-Aided Channel Estimation in OFDM Systems Without ICI

Pilot-aided channel estimation in orthogonal frequency division multiplexing (OFDM) systems often needs an interpolation technique to construct the channel frequency response on nonpilot subcarriers. Many studies on interpolations have been published; however, the performance of interpolations such as cubic spline interpolation and linear interpolation is limited by either the number or the position of the pilot subcarriers. This paper proposes an analytic interpolator developed using a linear least square criterion to replace conventional interpolation techniques for comb-type pilot-added channel estimation in OFDM systems without inter-carrier interference. The performance evaluation of the analytic interpolator is performed both on the basis of theory and through simulation, and the results from both the analyses are found to be in good agreement. The simulation results show that the proposed method can effectively mitigate the ill-influences of limitations on the number and position of the pilot subcarriers and that it also outperforms the cubic spline interpolation and linear interpolation, especially for short or nonequidistant pilot patterns.

Analysis and Comparison of Time Replica and Time Linear Interpolation for Pilot Aided Channel Estimation in OFDM Systems

This paper analyzes and compares two time interpolators, i.e., time replica and time linear interpolator, for pilot aided channel estimation in orthogonal frequency division multiplexing (OFDM) systems. The mean square error (MSE) of two interpolators is theoretically derived for the general case. The equally spaced pilot arrangement is proposed as a special platform for these two time interpolators. Based on this proposed platform, the MSE of two time interpolators at the virtual pilot tones is derived analytically; moreover, the MSE of per channel estimator at the entire OFDM symbol based on per time interpolator is also derived. The effectiveness of the theoretical analysis is demonstrated by numerical simulation in both the time-invariant frequency-selective channel and the time varying frequency-selective channel.

Pilot-Aided Channel Estimation for WiMAX 802.16e Downlink Partial Usage of Subchannel System Using Least Squares Line Fitting

This paper presents a pilot-aided channel estimation method which is particularly suitable for mobile WiMAX 802.16e Downlink Partial Usage of Subchannel mode. Based on this mode, several commonly used channel estimation methods are studied and the method of least squares line fitting is proposed. As data of users are distributed onto permuted clusters of subcarriers in the transmitted OFDMA symbol, the proposed channel estimation method utilizes these advantages to provide better performance than conventional approaches while offering remarkably low complexity in practical implementation. Simulation results with different ITU-channels for mobile environments show that depending on situations, enhancement of 5dB or more in term of SNR can be achieved.

Performance of Coded CS-CDMA/CP with M-ZCZ Code over a Fast Fading Channel

This paper studies the performance of a coded convolutional spreading CDMA system with cyclic prefix (CS-CDMA/CP) combined with the zero correlation zone code generated from the M-sequence (M-ZCZ code) for downlink transmission over a multipath fast fading channel. In particular, we propose a new pilot-aided channel estimation scheme based on the shift property of the M-ZCZ code and show the robustness of the scheme against fast fading through comparison with the W-CDMA system empolying time-multiplexed pilot signals.

Analysis and performance evaluation of a pilot-aided interpolated channel estimator for OFDM systems

In this paper, we propose a novel pilot-aided channel estimator through interpolation for Orthogonal Frequency Division Multiplexing (OFDM) systems that replaces part of the virtual subcarriers with pilot subcarriers (pilots) reducing the interpolation error while keeping the code rate stable. A novel thorough analysis of the Mean Square Error (MSE) of the proposed estimator is given for the general case where data subcarriers are positioned before the first and after the last pilot. Simulations show the improvement of the proposed scheme in MSE and Bit Error Rate (BER) when applied to a practical OFDM wireless local area network type of system with realistic channel conditions.

Intercarrier Interference Suppression for OFDMA Uplink in Time- and Frequency-Selective Fading Channels

This paper investigates intercarrier interference (ICI) suppression and channel estimation for the uplink of an orthogonal frequency-division multiple-access (OFDMA) system in a time- and frequency-selective fading channel. In such a doubly selective channel, channel variations within each OFDMA block disrupt the orthogonality among subcarriers and leads to ICI. We develop an appropriate signal model for the OFDMA uplink in a doubly selective fading channel and propose a minimum mean square error (MMSE) scheme and an MMSE successive detection (MMSE-SD) scheme to suppress ICI. It is shown that the MMSE scheme is the optimal linear scheme in terms of maximizing achievable data rate and that the MMSE-SD scheme is able to further remove ICI and exploit the Doppler diversity embedded in time-varying channels. As an essential component in ICI suppression, channel estimation is also considered. A basis expansion model (BEM) is formulated for the OFDMA uplink channel, and a pilot-aided channel-estimation algorithm is developed to track users' channels in the time domain. Simulation results are presented to illustrate the overall performance improvements that can be obtained from using the proposed ICI suppression and channel-estimation schemes.

Maximum a posteriori channel estimation for cooperative diversity orthogonal frequency-division multiplexing systems in amplify-and-forward mode

Transmit diversity-orthogonal frequency-division multiplexing (OFDM) systems have been proposed to mitigate the detrimental effects of channel fading. However, owing to the space and power limitations, the use of multiple transmit antennas is not practical in certain wireless devices, such as portable terminals and wireless sensors. Therefore cooperation among users at the physical layer has been proposed recently. Here, space-time block coded in amplify-and-forward (AF) relaying mode has been proposed as cooperative diversity for OFDM systems (CO-OFDM) in the presence of perfect channel-state information. Then, the channel estimation techniques for CO-OFDM systems in AF mode based on pilot symbols are investigated over frequency-selective channels. In particular, expectation-maximisation (EM) based maximum a posteriori (MAP) channel estimation is developed and compared with comp-type pilot-aided channel estimation (PACE) based the maximum likelihood (ML) estimator and the least minimum mean-square error (LMMSE) channel estimation techniques for CO-OFDM systems. To overcome the drawback owing to the receiver complexity, the Karhunen-Loeve expansion with the optimal truncation property is also considered. Simulation results that demonstrate the overall performance advantage of the EM-MAP based receiver over the PACE-ML and PACE-LMMSE based receivers are presented.

A novel CDD-OFDM scheme with pilot-aided channel estimation

Cyclic delay diversity (CDD) is a low-complexity standard-conformable transmit diversity scheme for coded orthogonal frequency division multiplexing (OFDM) systems. However, it makes channel estimation more challenging due to the increased frequency-selectivity of the equivalent single-input single-output channel. In this paper, we propose a novel CDD-OFDM scheme with pilot-aided channel estimation for any number of transmit antennas. By alternating and optimizing the cyclic delay parameter over adjacent OFDM symbols, we design a simple yet efficient channel estimation scheme and illustrate its excellent performance for the DVB-T application.

Pilot sharing method for uplink OFDMA channel estimation

This paper proposes an orthogonal pilot-aided channel estimation scheme for uplink (UL) orthogonal frequency division multiple access (OFDMA) systems. In the proposed method, the users utilizing adjacent UL subchannels in the frequency domain share some common pilot subcarriers by using orthogonal pilot patterns. In this way, each user can effectively use more pilot subcarriers and can improve the quality of channel estimation, especially for the data subcarriers located near the edges of a multiple access (MA) user subchannel in the frequency domain, and can offer a better bit error rate than user dedicated pilot allocations.

Analysis of Coded FHSS Systems with Multiple Access Interference over Generalized Fading Channels

We study the effect of interference on the performance of coded FHSS systems. This is achieved by modeling the physical channel in these systems as a block fading channel. In the derivation of the bit error probability over Nakagami fading channels, we use the exact statistics of the multiple access interference (MAI) in FHSS systems. Due to the mathematically intractable expression of the Rician distribution, we use the Gaussian approximation to derive the error probability of coded FHSS over Rician fading channel. The effect of pilot-aided channel estimation is studied for Rician fading channels using the Gaussian approximation. From this, the optimal hopping rate in coded FHSS is approximated. Results show that the performance loss due to interference increases as the hopping rate decreases.

An OFDM Symbol Design for Reduced Complexity MMSE Channel Estimation

In this paper we revisit the minimum mean square error (MMSE) pilot-aided channel estimation for broadband orthogonal frequency division multiplexing (OFDM) systems. The careful design of OFDM symbol leads to the proposal of a simplified time-domain (TD) MMSE estimator. By exploring the Fourier properties of the symbol, the investigated method eliminates the need to use direct or inverse discrete Fourier transforms (DFT/IDFT) before the channel estimation, by using the TD received symbol samples as the input to the MMSE filter. Moreover, performing the estimation in TD (where the channel impulse response (CIR) energy is mainly limit to a small set of samples), makes way to a simple, nonetheless efficient estimation of the filter parameters. The performance of the channel estimation scheme, using the estimated parameters, presents a tolerable degradation when compared with the ideal situation (a-priori knowledge of the channel correlation and noise variance), while exhibiting a reduced computational load. The feasibility of the investigated method is substantiated by system simulation using indoor and outdoor wireless channel models.

Iterative Turbo Channel Estimation for OFDM System over Rapid Dispersive Fading Channel

Coherent OFDM detection requires accurate channel state information (CSI). Mobile radio channels are both time and frequency dispersive, especially at high vehicular speeds, which makes channel estimation a challenging problem in system design. Conventional preamble-based and pilot-aided channel estimation require numerous reference signals, which significantly compromises the system throughput. This paper proposes a novel low complexity iterative turbo channel estimation technique which makes use of preamble, pilots and soft decoded data information in an iterative fashion to improve the system performance over the time and frequency selective fading channel while maintaining the system throughput. The numerical and analytical results show that the proposed technique can approach the performance of systems with perfect CSI with much fewer preamble and pilots symbols compared to existing channel estimation methods.

A method for analyzing the performance of comb-type pilot-aided channel estimation in power line communications

In this paper, we propose an original method for analyzing the performance of comb-type pilot-aided channel estimation for power line communications. Our method is based on a unified mathematical decomposition of the channel estimation error, which takes both the signal-to-noise ratio of the pilot sub-carrier locations and the interpolation kernel used into account. We demonstrate that the channel estimation error is composed of two terms that evolve differently according to the pilot spacing and the interpolation technique used. The error due to additive noise over channel transmission is formulated by a simple easily used analytic expression. Our results for some practical situations show that good performances can be achieved with a simple linear interpolation.

Multiuser channel estimation and prediction in two dimensions for MIMO–OFDM uplinks

This paper addresses the pilot-aided multiuser channel estimation for the uplink of multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) systems. The channel estimate is performed in two dimensions, i.e., time and frequency (2D), by exploiting their correlation. With the properly designed separable pilot symbols, a multiuser signal separation can be achieved and a multiuser channel estimator can be designed as well. To reduce the high computational complexity for a truly 2D estimator, two cascaded one dimensional (1D) estimators are used instead, which corresponds to two estimation stages with two different filters. At the 1st stage, the Wiener filter is implemented in the frequency direction, which separates the received multiuser signals and outputs the tentative estimate corresponding to an individual user. Whereas, at the 2nd stage, the Kalman filter is adopted in the time direction to estimate and predict the desired channel state information.