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As an alternative to the traditional cyclic prefixed OFDM (CP-OFDM) technology, time-domain synchronous OFDM (TDS-OFDM) provides higher spectrum efficiency due to no pilot in the transmitted signal. However, the pilot-Aided channel estimation techniques in CP-OFDM cannot be applied to TDS-OFDM. In this paper, channel impulse responses are estimated...
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... 无人机(Unmanned Aerial Vehicle，UAV)由于其 成本较低、作战效率高以及规避有生力量伤亡等优 收稿日期：2022-11-10 基金项目：国家重点研发计划项目(2019YFB1803200) 点成为了信息化作战的关键组成部分，其在监视侦 查、中继通信、精确打击以及电子战等军事领域发 挥了至关重要的作用 [1] 。 无人机可能飞行在山丘、 海 洋、平原、森林、沙漠或者城市，由于地形、地貌环 境多种多样，无线电波容易被反射甚至阻挡，呈现 出较强的多径效应，直接影响无人机数据链路的通 信质量，因此常采用 OFDM 技术作为通信体制 [2] 。 由于信号传输受多径衰落的影响，获取准确的信道 信息对于 OFDM 信号的恢复起到了至关重要的作 用 [3] 。传统 OFDM 系统通过在信号的时域和频域插 入导频符号，辅助接收端估计信道信息 [4] 。然而，在 多径时延扩展严重的场景下，例如处于低仰角通信 状态，系统设计时需要减小导频符号在频域的插入 间隔，致使系统吞吐量降低 [5] 。另外，强多径衰落还 会使系统误码性能显著恶化，进而影响通信系统的 可靠性。因此研究低导频符号插入且低解调损失的 信道估计与均衡技术具有重要的意义。 现有信道估计与均衡技术主要分为 3 类：非盲 信道估计 [6][7] 、盲信道估计 [8] 以及判决反馈信道估计 [9] 。非盲信道估计方法的发送信号对于接收端是部 分或全部已知的，接收机可以利用已知的导频信息 对当前信道状态进行估计，经典的非盲信道估计包 括最小二乘(Least Squares, LS)算法，线性最小均方 ...
Under the environment of severely channel frequency selective fading, the traditional orthogonal frequency division multiplexing (OFDM) non-blind channel estimation and equalization method has poor system bit error rate (BER) performance, and needs to increase the insertion of pilot symbols in the signal, which reduces the system throughput. Focusing on the above problems, a Low-Density Parity-Check (LDPC) code aided decision feedback channel estimation and equalization method is proposed. This method introduces the LDPC coding gain into the decision feedback loop, which improves the convergence. And it achieves asymptotic performance with respect to non-blind channel estimation and equalization methods under the condition of few pilot symbols inserted. Meanwhile, an improved channel estimation algorithm based on conditional maximum likelihood (CML) principle is proposed. This method detects effective tap signals based on the principle of constant false alarm to improve the channel estimation accuracy, and can be combined with the decision feedback method proposed. The simulation results show that, under the channel environment selected in the simulation, the LDPC aided decision feedback channel estimation and equalization method has 11~13dB improvement in bit error performance compared with the traditional decision feedback method, and has excellent system error performance and robustness.
... Thus, the complex algorithm with the deteriorated performance over fast time-varying channels is still required for channel estimation and OFDM equalization [14]. Furthermore, increasing the inserted pilot in the OFDM symbols will result into a much higher average power than the normal OFDM system data [15], thus the equivalent signal-to-noise ratio (SNR) at the receiver will be reduced. The OFDM system eliminates (ISI) and (ICI) through the use of a cyclic prefix. ...
... In order to eliminate ICI and ISI in an OFDM system with high mobility scenarios, an increase in a prefix value is necessary and thus the energy consumption will increase. An OFDM system can have adaptive frequency equalizers such as decision-feedback equalizers [15] which are exploited to reduce the ICI and ISI, however, equalizers such as Decision-feedback equalizers are computationally complex and can introduce errors when wrong decisions are made. As shown in the left side of Figure 2, and also illustrated more in details by [16], the channel is at and thus the signal is distorted. ...
We present in this paper a method for enhancing equalization of a dynamic channel. A dynamic channel is characterized and modeled by a high relative velocity between transmitter and receiver and fast changes of environment conditions for wave propagation. Based on Jakes model, an auto-regressive model (AR) [1] for such a dynamic system, i.e., a time variant channel is developed. More specifically, the enhanced equalization method we are proposing is a combination of a multi-stage time and frequency domain equalizer with a feed-forward loop. The underlined wok presents a unified approach to the equalization method that employs both time and frequency domains data to enhance the equalization scheme. In an OFDM system, the channel coefficients for each tap, in time domain for consecutive blocks, are partially independent thus correlated. Such correlation can improve the channel estimation if it is taken into account. The method in this paper enhances the performance of equalization by dynamically selecting the number of previous OFDM symbols based on the Doppler frequency. In order to decrease the complexity of the system model, we utilize the autocorrelation and Doppler frequency to dynamically select the previous OFDM symbols that will be stored in the memory. In addition to deriving earlier results in a unified manner, the approach presented also leads to enhanced performance results without imposing any restrictions or limitations on the OFDM system such as increasing the number of pilots or cyclic prefix.
... In practice, pilot symbols are inserted into OFDM signal in either the time or frequency domain. The former insertion pattern is regularly applied to time-domain synchronous OFDM (TDS-OFDM) system [28], [29], in which CIR is estimated by using the pseudorandom (PN) correlation method, and transformed into CFR through the FFT operation [30]. In this paper, we concentrate on using the proposed schemes to facilitate the CE design in OFDM with the latter insertion pattern. ...
In OFDM system, channel estimator with fixed-coefficient interpolation gives rise to significant estimation errors when the estimator mismatches the experienced time-varying channel. This paper presents a pilot-aided channel estimator adopting a double one-dimensional (1-D) polyphase subfilter structure, in which a few interpolating coefficients are applicable to all subcarriers of an OFDM symbol. By analyzing the channel interpolation for OFDM receiver, we propose a two-dimensional (2-D) adaptation scheme of the interpolating coefficient, based on a block least-mean-square algorithm, which requires none of knowledge on channel statistics. The 2-D adaptation scheme is simplified as two types of 1-D schemes respectively for the time and frequency dimensions, based on the known frequency- and time-direction interpolating coefficients. By simultaneously exploiting channel correlation in the time and frequency dimensions, the adaptive interpolators are trained to match channel properties. Furthermore, the proposed schemes of adaptive interpolation demand lower implementation cost, compared with the existing methods. Simulation results show that the adaptive interpolators converge at a high rate, and that the simplified schemes outperform the conventional methods in the high-speed mobile channel and the single-frequency-networks channel with long-delay echoes, while preserving a low implementation complexity.
... In practice, pilot symbols are inserted into OFDM signal in either the time or frequency domain. The former insertion pattern is regularly applied to time-domain synchronous OFDM (TDS-OFDM) system [28], [29], in which CIR is estimated by using the pseudorandom (PN) correlation method, and transformed into CFR through the FFT operation [30]. In this paper, we concentrate on using the proposed schemes to facilitate the CE design in OFDM with the latter insertion pattern. ...
In an orthogonal frequency division multiplexing system, conventional interpolation techniques cannot correctly balance performance and overhead when estimating dynamic long-delay channels in single frequency networks (SFNs). In this study, classical filter analysis and design methods are employed to derive a complex interpolator for maximizing the resistible echo delay in a channel estimator on the basis of the correlation between frequency domain interpolating and time domain windowing. The coefficient computation of the complex interpolator requires a key parameter, i.e., channel length, which is obtained in the frequency domain with a tentative estimation scheme having low implementation complexity. The proposed complex adaptive interpolator is verified in a simulated digital video broadcasting for terrestrial/handheld receiver. The simulation results indicate that the designed channel estimator can not only handle SFN echoes with more than 200 μs delay but also achieve a bit-error rate performance close to the optimum minimum mean square error method, which significantly outperforms conventional channel estimation methods, while preserving a low implementation cost in a short-delay channel.
... Due to the no-pilot signaling, the receiver parameters regarding channel state and synchronization are estimated only by making full use of the inserted PN. A majority of PN correlation-based estimation schemes can be found in [5]- [10] for synchronization or in [11], [12] for channel estimation. However, the carrier frequency offset (CFO) attenuates correlative gain, which leads to a poor accuracy of parametric estimation, degrading the performance of the PN-padded OFDM receiver. ...
... Regarding the configuration of the estimator, refer to [5]. In the CFO tracking stage, the CPC with L=L m and P=N, which can be shared with the channel estimator in [12], is adopted to provide accurate CFO tracking. Note, L m is the m-sequence length in the middle of the TGI. ...
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 kHz in the DVB-T F1 channel. Furthermore, the acquisition range can be expanded in more favorable channels.
... [1] uses the cross-correlation results of the transmitted and received PNs as the channel estimates ignoring the fact that the correlation of the PN is not a perfect impulse function, which introduces mutual interference between different channel paths in the estimation result. To overcome this problem, [4] suggests to remove interference components of several strongest paths and [5] proposes to iteratively detect the significant paths in the correlation results. Both of them did not thoroughly eliminate the interference lying in the PN correlation. ...
An accurate channel estimation is crucial for the novel time domain
synchronous orthogonal frequency-division multiplexing (TDS-OFDM) scheme in
which pseudo noise (PN) sequences serve as both guard intervals (GI) for OFDM
data symbols and training sequences for synchronization/channel estimation.
This paper studies the channel estimation method based on the cross-correlation
of PN sequences. A theoretical analysis of this estimator is conducted and
several improved estimators are then proposed to reduce the estimation error
floor encountered by the PN-correlation-based estimator. It is shown through
mathematical derivations and simulations that the new estimators approach or
even achieve the Cramer-Rao bound.
... For example, the scattered-pilot-aided channel estimate in the DVB-H receiver is treated as , and the entries of can be chosen from the sequence . Another example is time domain synchronous OFDM (TDS-OFDM) system, in which the PN-correlation-based CE [26] results in an estimate of , and similarly the entries of may come from the sequence . In this paper, we concentrate on the equalization scheme of CP-OFDM systems. ...
... In the frequency domain, the PIB channel estimate is approximated as the DFT of . Thus , i.e. the DFT of , indicates time variations of the channel response and the corresponding estimation is given by (25) Therefore, the symbol estimation in (17) is reduced to (26) with the coefficient (27) The ICI mitigation only needs CMs, which is an extremely low-complexity equalization scheme. Note that in the above simplified equalization scheme, the subscript is dropped temporarily, since only the first-order dynamic component is considered here. ...
... Note that in the above simplified equalization scheme, the subscript is dropped temporarily, since only the first-order dynamic component is considered here. For time-invariant channels, , then (26) returns to the conventional one-tap equalization. ...
... If orthogonality is destroyed, the receiver will have a serious bit error rate due to (ICI). A cyclic extension of the signal in time domain called cyclic prefix (CP) is inserted between each OFDM symbol to eliminate ISI almost completely for it is larger than maximum of time delay, which keeps orthogonality between each subcarriers as well [20,21]. ...
Orthogonal Frequency Division Multiplexing (OFDM) is the most promising modulation technique. It has been adopted by most wireless and wired communication standards. The idea is to utilize a number of carriers, spread regularly over a frequency band, in such a way so that the available bandwidth is utilized to maximal efficiency. In this paper the design and implementation of OFDM on a Field Programmable Gate Array (FPGA) device has been presented. This attractive architecture uses Radix-2 serialized 512-point FFT (Fast Fourier Transform) algorithm that enhances its speed. The system design is optimized in terms of both area and speed. The implementation was made on FPGA since it allows flexibility in design and also it can achieve higher computing speed than digital signal processors. ASIC-like performance with lower development time and risks can also be achieved. The results show that the design has been simulated up to 227.355 MHz and it achieves higher speed and lower area.
... Due to its no-pilot frame format, TDS-OFDM differs considerably from conventional cyclic-prefixed OFDM (CP-OFDM) in the receiver design. Generally speaking, the parameter estimation, including channel estimation [14]- [16] and synchronization [17]- [20], is implemented through calculating the received signal's correlation with local PN sequence. It is hence crucial to analyze the PN correlation in the presence of the CCI, which can roughly indicate the effects on estimating the time-domain parameters. ...
... Here, we also design a comb filter for DTMB as a reference. In the designed DTMB receiver, the sampling rate [20], and then we can replace the notch filtering in (13) with the comb filtering (16) where the delay determines the frequency interval between notches, and the frequency offset adjusts the notches aiming at the corresponding carrier of the co-channel PAL interference. Fig. 7 shows sequences of the simulated DTMB signal as well as the co-channel PAL interference when the SIR is 5 dB. ...
... The dispersion is caused by the filtering process. Fig. 9 demonstrates the frequency-domain channel estimate (CE) using the method in [16], and the CCI spectrum (the dotted curve) is also plotted as a reference. In the presence of the CCI, the probability of the CIR false detection is very high, consequently the frequency-domain channel response is fluctuant, leading the receiver is not able to produce replica of the transmitted data normally. ...
During digital television (TV) transition, analog signal disturbs digital terrestrial television (DTT) transmission in an area where a simulcast service is available. In the TDS-OFDM- based DTTB system, the receiver is extremely sensitive to the co-channel interference (CCI) introduced by analog TV service. The CCI not only generates strong narrow-band interference in the frequency domain, but also greatly degrades the estimation of time-domain demodulating parameters in TDS-OFDM receiver. In this paper, a method of the time-domain autocorrelation of the received DTT signal is developed to detect the CCI, and it utilizes the line synchronization mechanism of analog TV signals. Also an IIR filter, setting one notch at each carrier of analog TV signal, is designed to reject the CCI in the time domain. From the filtered signal, synchronization errors and channel responses can be estimated by means of conventional algorithms for TDS-OFDM receiver. A DTMB receiver is designed to verify the proposed solutions and its robustness against the CCI is enforced about 20 dB in terms of signal interference ratio (SIR).
... In the past few years, there have been many literatures [133,134,135,136,137,138,5,76,139,140,141,142,143,144] dedicated to the channel estimation problem in the TDS-OFDM context, more specifically in the DTMB system case. ...
... A notable degradation can be observed with a moderate Doppler frequency (10 Hz comparing to the subcarrier spacing of 2 kHz) as shown in [143]. Another method proposed in [139,145] involved a decision feedback loop in the PN-correlation-based estimation. The hard-decided data symbols of the previous OFDM symbol are used to eliminate the ISI from the PN sequence. ...
This thesis mainly focuses on the challenging channel estimation problem in the new Chinese digital television (DTV) standard-DTMB. First, we compare DTMB with DVB-T in terms of system specifications, power efficiency and BER. Secondly, we study channel estimation methods exploring the pseudo noise (PN) sequence in frequency and time domains. The frequency domain method is easy to implement. The time domain technique based on correlation of the PN provides a better performance, but suffers from an estimation error floor. Consequently we propose several methods to mitigate the error floor. Finally, we propose new data-aided channel estimation methods which exclude decoding and interleaving from the feedback loop to reduce the computational complexity. Moving average and Wiener filtering in frequency and time domains have been studied to refine the estimates. The new algorithms provide superior performance over the typical methods in the literatures in different applications and channels.
