Number Of Pilot Subcarriers Research Articles

Efficient Channel Estimation for Wideband Millimeter Wave Massive MIMO Systems With Beam Squint

Massive multiple-input-multiple-output (MIMO) and millimeter wave have been adopted as the enabling technologies for the 5G and beyond 5G (B5G) systems. A challenging problem introduced by the use of large antenna size and wide bandwidth is beam squint, i.e., spatial-wideband effect. Beam squint can significantly degrade the channel estimation performance for conventional channel estimators. Research effort on channel estimation under beam squint conditions has been very limited. For the few available work that attempts to address this problem, they require either all subcarriers or multiple symbols used as pilot for channel estimation, so large overhead becomes inevitable. Therefore, in this paper, we propose an efficient channel estimation method that only requires a small number of subcarriers. The channel estimation problem is formulated as a nonlinear least squares optimization problem. Initial parameter estimation is critical, which will affect the efficiency and convergence of the proposed algorithm. Using a densely-spaced antenna structure and consecutive subcarriers assignment approach, we can effectively avoid the aliasing effect and reduce the ambiguity during the initialization phase. A subcarrier assignment criterion is proposed to achieve the optimal performance. Closed-form expressions of the Cramér-Rao lower bound (CRLB) and the achievable rate are derived to evaluate the performance. Both simulation results and theoretical analysis show that even with a small number of subcarriers, the estimation error closely approaches the CRLB, and its effect is negligible compared with the noise when evaluating the signal-to-noise ratio with a simple linear detector. Furthermore, the number of pilot subcarriers has little impact on the achievable rate.

A DFT-based Low Complexity LMMSE Channel Estimation Technique for OFDM Systems

The linear minimum mean square error (LMMSE) channel estimation technique is often employed in orthogonal frequency division multiplexing (OFDM) systems because of its optimal performance in the mean square error (MSE) performance. However, the LMMSE method requires cubic complexity of order O(N 3 p ), where Np is the number of pilot subcarriers. To reduce the computational complexity, a discrete Fourier transform (DFT) based LMMSE method is proposed in this paper for OFDM systems in the frequency selective channel. To validate the proposed method, the closed form mean square error (MSE) expression is also derived. Finally, a computer simulation is carried out to compare the performance of the proposed LMMSE method with the classical LS and LMMSE methods in terms of bit error rate (BER) and computational complexity. Results of the simulation show that the proposed LMMSE method achieves exactly the same performance as the conventional LMMSE method, with much lower computational complexity.

Research and Development of a Highly Reconfigurable OFDM MODEM for Shallow Water Acoustic Communication

Orthogonal frequency division multiplexing (OFDM) provides an effective approach to combat multipath with high bandwidth efficiency for underwater acoustic communication, thus design and implementation of acoustic OFDM MODEMs draw more and more attention. Extensive work has been performed to mitigate multipath, Doppler shift, and peak average power ratio from the viewpoint of theoretical analysis. Meanwhile, design and implementation of a parameter-configurable OFDM to adapt to highly diverse underwater channels and various missions become more and more important. Traditional MODEM with fixed parameters cannot achieve the best tradeoff between data rate and bit error rate. In order to obtain the best communication performance under long time delay channels, Doppler channels, and frequency-selective channels, in this paper, we provide the design and implementation of a parameter-configurable underwater acoustic OFDM MODEM with the purpose to achieve flexibility. Specifically, the reconfigurable parameters of the OFDM MODEM include the number of receivers, the number of null subcarriers, the number of pilot subcarriers, as well as the type of channel estimation algorithms. The number of pilot subcarriers and type of channel estimation methods are determined based on channel characteristics, the number of null subcarriers is determined based on the Doppler shift, and the number of the received channels is based on fading channel. Moreover, in this paper, the Doppler estimation performance under different number of null subcarriers, and the channel estimation performance under different number of pilot subcarriers and under different channel estimation algorithms, are analyzed in terms of bit error rate. Finally, the effectiveness of the proposed highly configurable OFDM MODEM is demonstrated by two sea trials corresponding to different communication scenarios.

Bandwidth efficient optimal superimposed pilot design for channel estimation in OSTBC based MIMO–OFDM systems

This paper proposes optimal superimposed pilot (SIP) sequence design and pilot placement towards mean square error (MSE) minimization for channel estimation in spatially correlated/uncorrelated MIMO–OFDM channels. For uncorrelated channels, a closed form expression is derived for the optimal pilot sequence, number of pilot subcarriers and pilot locations. For correlated channels, the problem of designing the optimal SIP sequence is formulated as a semidefinite program. The proposed schemes lead to a substantial increase in the bandwidth efficiency since it employs pilot symbols on a significantly fewer number of subcarriers in comparison to existing schemes. It is shown that the proposed techniques achieve the Cramer–Rao bounds. In addition, a closed form analytical expression is derived for the bit-error rate of orthogonal space–time block codes (OSTBC) based MIMO–OFDM systems including the effect of channel estimation error arising from the proposed SIP based estimation techniques. Simulation results demonstrate the improved estimation performance and bandwidth efficiency of the proposed schemes.

Accurate OFDM Sparse Channel Estimation by Optimal Deterministic Pilot Pattern

ABSTRACTIn orthogonal frequency division multiplexing (OFDM) systems the wireless channel impulse response can be modelled as a sparse vector. Recently emerged compressed sensing (CS) theory can be used for reconstruction and estimation of unknown non-zero channel taps. In CS-based channel estimation, the number of pilot sub-carriers can be decreased, so the spectral efficiency of the system can be increased. The efficiency and performance of sparse channel estimation (SCE) highly depend on the measurement matrix, which in turn depends on the pilot pattern. In this paper, based on minimization of the correlation between all columns of measurement matrix, a new criterion is proposed. In addition, a deterministic pilot pattern optimization algorithm for OFDM SCE is introduced. Greedy CS recovery methods, such as orthogonal matching pursuit (OMP) and regularized OMP are used for estimation of the channel taps. The simulation results confirm the efficiency and accuracy of proposed criterion and optimization algorithm in terms of mean square error.

An Improved Technique of Channel Estimation for OFDM in DVB-T2

Orthogonal Frequency Division Multiplexing (OFDM) with pilot assisted channel estimation is widely applied in wireless communication systems due to its high rate transmission capability and its robustness to multi-path fading and delay. In this paper an improved channel estimation method for OFDM in second generation Digital Video Broadcasting for Terrestrial (DVB-T2) is presented based on a pilot arrangement in time-varying frequency-selective fading channels. A new method which is based on an improved Domain Transform Least Square Estimation method (DTLSE) and a Two Dimensional Interpolation method (2-DI) have been proposed. The proposed methods give a good BER performance compared to other considered methods .The estimation matrix size for obtaining Channel Impulse Response (CIR) depends only on the length of the channel not on the number of pilot subcarriers nor the size of OFDM symbol, it does not require knowledge of channel statistics and works for any pilot pattern. The Minimum Mean Squared Error (MMSE) channel estimation was considered as the reference method that gives the best BER performance but suffers from a high computational load. The proposed method is compared with the two other methods: the improved DTLSE and MMSE. The proposed method proves to give a reduction in complexity compared to the MMSE technique with a comparable performance but better than the DTLSE one. General Terms Pilot-assisted channel estimation, wireless systems, DVB-T2.

A Comparative Study of Channel Estimation Techniques for OFDM in DVB-T2

pilot symbol assisted channel estimation is a well-known multicarrier technique for high rate transmission over wireless frequency selective fading channels. In this paper a comparative study of channel estimation techniques is presented based on a pilot arrangement used in the second generation for Digital Video Broadcasting Terrestrial (DVBT2) system in time-varying frequency-selective fading channels. A modified method based on Domain Transform Least Square Estimation (DTLSE) has been proposed which reduces the computational complexity by avoiding the use of the matrix inversion .The estimation matrix size for obtaining Channel Impulse Response (CIR) depends only on the length of the channel not on the number of pilot subcarriers nor the size of OFDM symbol. Two Dimensional Linear Interpolation (2-DLI) is discussed which performs better than one Dimensional Linear Interpolation (1-DLI)but suffers from long processing delay and higher sensitivity to Doppler frequency shifts. The Minimum Mean Squared Error (MMSE) channel estimation is also considered as a reference method because it has a good performance close to the perfect estimation at the cost ofhigh computational complexity and the necessity of prior knowledge ofchannel statistics. The proposed method is compared with three other methods DTLSE, 2-DLI and MMSE.The proposed method provesto give a better performance compared to the other methods which suffer from either a much higher computational load or less robustness to Doppler shifts.

Research on Channel Estimation Algorithm in 60GHz System Based on 802.15.3c Standard

GHz communication occupies huge transmission bandwidth and presents a diffuse multipath transmission characteristic, so there is a certain complexity in channel estimation. This paper using actual 60GHz-HSI-OFDM system model and realistic 60GHz channel model, presents the estimation scheme based on training sequence and pilot signal respectively. 60GHz channel usually belongs to time-invariant channel, while in reality, each frame may last several hundred milliseconds. So channel status may change during this time. The OFDM data symbols in each frame using the channel estimated value by the training sequence may appear deviation, which led to a decline in system performance. This moment the pilot subcarriers are designed to estimate channel. But because the number of pilot subcarriers is only 16, the estimation precision may be inadequate. To solve this problem, this paper proposes pilot subcarriers jointing null subcarriers, guard subcarriers to estimate the channel information and improves the estimated precision. Meanwhile, in order to improve the poor estimation performance when using pilot subcarriers, compressed sensing (CS) technology is introduced to 60GHz channel estimation. The simulation results show that CS algorithm could get better performance than LS and DFT algorithm when using OFDM subcarriers to estimate channel information. Finally the best subcarriers estimated solution is proposed for 60GHz LOS and NLOS channels respectively by analysis.

A Study of Pilot-Assisted OFDM Channel Estimation Methods With Improvements for DVB-T2

We provide a comparative study of channel estimation methods for pilot-assisted orthogonal frequency-division multiplexing (OFDM) communication systems in time-varying frequency-selective fading channels. We also propose an improved low-complexity least-squares (LS) estimation method based on the domain-transform process, which estimates time-domain channel impulse response (CIR) using the LS estimate of pilot subchannels. Our method is particularly suited to commercial OFDM systems such as the second generation of digital terrestrial television broadcasting systems (DVB-T2) because it has the following properties: 1) It uses only one OFDM symbol; 2) it does not require knowledge of channel statistics; 3) it works for any pilot pattern; and 4) the size of the estimation matrix for obtaining CIR only depends on the number of pilot subcarriers, but not on the size of OFDM symbol. Because of the first three properties, our method is robust to high Doppler frequency shifts, timing synchronization errors, and pilot patterns in commercial systems. Moreover, through computer simulations, we verify that our method provides competitive bit error rate and channel estimation error performance, compared with some well-known methods, which suffer from either a much higher computational load or less robustness to Doppler shifts or timing errors. For irregular pilot patterns, efficient yet simple regularization methods are suggested to solve the ill-conditioned problem of the estimation matrix. An upper bound on the mean squared error of the estimated CIR is also provided.

Channel Estimation and Data Detection for OFDM Systems Over Fast-Fading and Dispersive Channels

The Doppler shift of fast-fading channels will generate intercarrier interference (ICI) and, hence, degrade the performance of orthogonal frequency-division multiplexing (OFDM) systems. In this paper, a novel channel-estimation and data-detection method for OFDM systems over the fast-fading dispersive channels has been proposed and investigated. A new pilot pattern is also proposed, and it is composed of a comb-type pilot pattern and a grouped pilot pattern. The required number of pilot subcarriers is, therefore, significantly reduced compared with that of using the grouped pilot pattern only. We present the MSE analysis of the proposed channel estimation based on the proposed pilot pattern and thoroughly investigate the MSE performance of channel estimation by a numerical method. A closed-form mathematical expression has been derived to express the MSE of the proposed channel estimation as a function of the Doppler shift, the number of resolvable paths of the channel, the polynomial order, and so on. Both computer simulation and numerical results show that the proposed channel estimation and data detection based on the proposed pilot pattern have effectively eliminated the ICI in terms of the MSE of the channel estimation and bit-error-rate (BER) performance, whereas the required pilot number per OFDM symbol is reduced by about 92.3% in comparison with that of using the grouped and equispaced pilot pattern.

The Effect of Subcarrier-Based Channel Estimation on the BER Bound of Turbo Coded OFDM/SDMA System

In this paper, we study the performance of the channel estimation in a Turbo coded, orthogonal frequency division multiplexing (OFDM) with multiple antennas at the receiver. We survey the accuracy of the channel estimation for different number of users and different number of pilot subcarriers. Moreover, since there is not a closed form solution to the BER of the Turbo coded system for different number of constellation size. We develop an equation to obtain the BER bound of the Turbo coded OFDM/SDMA system with M-ary quadrature amplitude modulation (MQAM) in the Rayleigh flat fading channel as a function of the signal constellation and characteristic of the Turbo code with a curve fitting and then propose a method for selecting switching threshold in adaptive modulation case.

Optimisation of the pilot-to-data power ratio in the wireless MIMO-OFDM system with low-complexity MMSE channel estimation

Multiple-input-multiple-output orthogonal frequency division multiplexing (MIMO-OFDM) communication systems are currently being viewed as a main candidate for the next generation broadband wireless access interfaces providing high throughput. However, a considerable potential gain in spectral efficiency is challenged by the receiver’s ability of accurate data symbol identification at the output of the radio channel. Performance of the MIMO detectors strongly depends on both the received signal-to-noise power ratio (SNR) and the retrieved channel state information (CSI), which can be obtained using the channel estimation techniques relying on pilot symbol transmission on the dedicated subcarriers. Necessity to improve channel estimation and detection performance concurrently leads to the dilemma of the optimal power assignment to the data and pilot subcarriers as these are the conflicting requirements under the total transmit power constraint. We derive an analytical solution of the described problem in the form of weak and tight upper bounds on the pilot-to-data power ratio (PDR) given the system design parameters (number of subcarriers, number of pilots, number of transmit antennas, model order of the channel response, SNR). The resultant PDR equations can be applied to OFDM systems with arbitrary arrangement of the pilot subcarriers operating in an arbitrary multipath channel, not just limited to the equispaced optimal pilot pattern and the channel with the uniform sample-spaced power-delay profile as studied in the previous works. In an adaptive system, which is capable to adjust transmit signal configuration (e.g., block length, number of pilot subcarriers or antennas) according to the established channel conditions, an optimal parameter tuning is desirable for each operational mode. The closed-form PDR expressions and their analysis presented in this work are designated to alleviate the challenging task of the on-the-fly system optimisation as they represent relatively simple functional relationships.

Channel estimation approach with variable pilot density to mitigate interference over time-selective cellular OFDM systems

We propose a new channel estimation technique to mitigate interference with the aid of pilot density discrepancy in a cellular OFDM system, especially for 3GPP systems. The proposed estimator applies interference mitigation to pilot symbols, using repeated interference features according to the pilot subcarrier distance. In order to use this property, the proposed estimator employs a specific pilot structure which consists of two types of pilot symbols with different pilot density. The combination of interference alleviation and pilot rearrangement not only makes the channel estimation robust to the time-selectivity of the channel but also reduces the number of pilot subcarriers needed to estimate the channel. To clarify the advantages of the proposed method, the average mean square errors (MSE) of the frequency channel estimate are derived for the proposed estimator with unequal pilot density, and it is compared with general estimators with equal pilot density. Numerical analysis and simulation results confirm that the proposed estimator with unequal density relationship outperforms the estimators over practical time-varying environments in terms of MSE performance and pilot overhead efficiency.