Time Domain Synchronous OFDM Research Articles

Carrier Phase Ranging with DTMB Signals for Urban Pedestrian Localization and GNSS Aiding

China developed its Digital Television (DTV) standard in 2006, known as Digital Television Terrestrial Multimedia Broadcasting (DTMB), which employs time-domain synchronous orthogonal frequency division multiplexing (TDS-OFDM) as the modulation method. In contrast to Global Navigation Satellite Systems (GNSSs), DTV signals have higher transmitting power, wider coverage, larger bandwidth, and fixed transmitter location. This paper explores the positioning performance of DTMB signals, and the potential to improve GNSS positioning accuracy in urban environments. Specifically, a solution is proposed, and a software-defined radio receiver is developed for wireless localization. Without changing the current signal structure, the pseudorandom noise (PN) sequences in the signal are used for signal acquisition and carrier phase ranging. The carrier phase of the first arrived path is extracted by the least squares matching pursuit method. Both static and dynamic field tests were conducted to verify the proposed ranging and positioning method. Centimeter-level ranging accuracy was achieved in the static scenario, while meter-level ranging accuracy was achieved in the dynamic scenario. As one possible application, the proposed ranging method was combined with GPS pseudorange measurements to achieve higher accuracy position results in an urban pedestrian scenario, especially when there is only a limited number of visible satellites.

X-Transform Time-Domain Synchronous IM-OFDM-SS for Underwater Acoustic Communication

Time-domain synchronous orthogonal frequency division multiplexing (TDS-OFDM) is presented to enhance the spectral and energy efficiency over the conventional cyclic prefix OFDM and zero padding OFDM, but the interbloc interference (IBI) between the data block and adjacent pilot symbols deteriorates its performance. The IBI is considered as the main TDS-OFDM drawback in the harsh long-delay underwater acoustic (UWA) multipath channel. This article proposes a new scheme called X-transform time-domain synchronous index modulation (IM) OFDM to eliminate the TDS-OFDM IBI, explore the multipath diversity, and reduce the peak-to-average power ratio (PAPR). Particularly, X-transform, which merges the discrete Hartley transform and discrete Fourier transform in a very low-complexity unitary matrix, and IM techniques have been employed to remove the effects of IBI existing in underwater acoustic (UWA) TDS-OFDM. Moreover, compressive sensing has been adopted to guarantee a reliable UWA channel estimation based on a guard interval IBI-free region. The proposed scheme is evaluated over simulated and experimental UWA channels. The computer simulation and the real experiment show the outperformance of the proposed scheme in terms of bit error rate, computational complexity, PAPR, energy efficiency, and spectral efficiency over its conventional benchmarks.

TDS-OFDM based Digital Television Terrestrial Multimedia Broadcasting Standards

As the most popularly utilized broadcasting network, digital terrestrial television broadcasting (DTTB) can provide multimedia information coverage for the broad audience in a very efficient way because of its characteristic of wide-range coverage and mobile reception ability. After promulgating the first generation DTTB standard, digital terrestrial/television multimedia broadcasting (DTMB), in 2006, China began to research and develop the next generation DTTB standard, namely DTMB-advanced (DTMB-A), aiming to support higher spectrum efficiency and further improve transmission reliability. In 2019, DTMB-A was accepted by ITU as the second generation international DTTB standard (as System C). Similar to DTMB, time-domain synchronous - orthogonal frequency division multiplexing (TDS-OFDM) based multi-carrier modulation scheme is adopted by DTMB-A. Thanks to the more flexible frame structure, advanced error correction coding and improved constellation mapping, DTMB-A offers 30% higher transmission capacity than DTMB under the same transmission conditions. Thus, DTMB-A can support both fixed and mobile reception more efficiently, and provide users with higher quality services such as ultra-high definition television (UHDTV). This paper first gives details of key technologies at the transmitter of DTMB/DTMB-A and introduce core algorithms at the receiver. Both laboratory test and field trial results will then be provided and analyzed, especially for the application of 4K UHDTV and single frequency network (SFN).

Time domain synchronous OFDM system for optical fiber communications

A spectrum efficient OFDM scheme named Time Domain Synchronous-OFDM (TDS-OFDM) is introduced into coherent optical transmission system, in which the pseudo noise (PN) sequence is exploited as guard interval to realize frame synchronization, compensate the carrier frequency offset (CFO), and estimate and equalize channel simultaneously. Since there is no pilot signals or training symbols in TDS-OFDM, the proposed scheme can achieve higher spectral efficiency (SE) above 10% improvement comparing with CP-OFDM. The proposed method is implemented and verified in a 28GBaud QPSK OFDM system and a 28GBaud 16QAM OFDM system. It is demonstrated that the proposed scheme shows high CFO estimation accuracy and synchronous accuracy. Under CFO and linewidth of laser source set as 100MHz and 100kHz respectively, BER of QPSK OFDM system is below 3.8e-3 at the optical signal-to-noise ratio (OSNR) of 13dB, and BER of 16QAM OFDM system is below 3.8e-3 at the OSNR of 20dB.

Channel Estimation for TDS-OFDM Systems in Rapidly Time-Varying Mobile Channels

This paper explores the performance of time-domain synchronous orthogonal frequency-division multiplexing (TDS-OFDM) systems operated under rapidly time-varying mobile channels. Since a rapidly time-varying channel contains more unknown channel coefficients than the number of observations, the mobile channel can conveniently be modeled with the discrete Legendre polynomial basis expansion model to reduce the number of unknowns. The linear minimum mean square error (LMMSE) estimate can be exploited for channel estimation on inter-block-interference-free received signal samples owing to transmitting pseudo-noise (PN) sequences. In conventional TDS-OFDM systems, the channel estimation performance is limited due to estimating channel responses only from the beginning part of the channel. Therefore, a new system model named “ partitioned TDS-OFDM system ” is proposed to improve the system performance by inserting multiple PN sequences to the middle and end parts of the channel as well. In addition to providing the reconstruction error performance, Bayesian Cramer–Rao lower bound is derived analytically. Also, the LMMSE-based symbol detection is employed. To alleviate the negative effects of inter-carrier-interference (ICI) occuring in mobile channels, ICI cancellation is applied to enhance the detection performance. The simulation results demonstrate that the proposed TDS-OFDM system is superior to the conventional system and its corresponding performance is able to approach the achievable lower performance bound.

Priori-Information Hold Subspace Pursuit: A Compressive Sensing-Based Channel Estimation for Layer Modulated TDS-OFDM

Time-domain synchronous orthogonal frequency division multiplexing (TDS-OFDM) has been widely recognized as a fundamental OFDM block transmission scheme because of its advantages in spectral efficiency. However, it suffers from severe performance loss under strong frequency selective channels and thus has difficulty supporting high-order modulations such as 256QAM. In this paper, we proposed a compressive sensing (CS)-based channel estimation (CE) algorithm for layer modulated TDS-OFDM (LM-TDS-OFDM). In the proposed CE algorithm, the low-order modulated symbols will be recovered and then sent to a feedback loop for precise CE. The priori-information hold subspace pursuit algorithm is investigated to achieve accurate estimation of channel. Analyses and simulations show that the proposed algorithm can successfully obtain high-accuracy CE and significantly reduce the computational complexity. Based on the proposed CE scheme, LM-TDS-OFDM can well support 256QAM transmission under severe channel conditions.

Low Complexity ICI Mitigation for MIMO-OFDM in Time-Varying Channels

Time-varying channels destroy the orthogonality among subcarriers in orthogonal frequency division multiplexing (OFDM) systems, and introduce intercarrier interference (ICI). Lots of efforts have been devoted to mitigate ICI in OFDM systems, with different frame structures and channel models, but the computational complexity of the methods is usually very high. In multiple-input multiple-output (MIMO) systems, the complexity is even higher. In this paper, a low-complexity ICI mitigation method is proposed for MIMO-OFDM systems under the assumption of linear time-varying channels. It reduces the complexity of ICI compensation from $ {O(K^{3}(N^{3}+MN^{2}+MN)+NK\log (K))}$ to $ {O(K(N^{3}+2MN^{2}+2MN+2M^{2}+N\log (K)))}$ , where ${K}$ is the number of subcarriers, ${M}$ the number of transmitters, and ${N}$ the number of receivers. It requires channel estimation based on the linear time-varying channel model, and no transmission overhead is needed. The proposed algorithm applies to all OFDM systems as long as linear time-varying channel estimation is applicable. Time-domain synchronous-OFDM naturally suits for the proposed ICI mitigation algorithm because its receiver is able to easily estimate linear time-varying channels. Simulation with QPSK and 16QAM modulation demonstrates the performance of the proposed method, in comparison with no ICI mitigation case. It shows that 2 dB signal to noise gain is achieved when the uncoded bit error rate is $ {10^{-3}}$ and the normalized Doppler frequency is 0.1.

Iterative Identification of Sparse Mobile Channels for TDS-OFDM Systems

Spectral efficiency improvement is essential for mobile systems development in the light of growing demand for high data transmission rate. The Time-domain synchronous OFDM transmission system increases bandwidth utilization by time-domain channel estimation without any support from frequency-domain pilot signals. Channel identification methods use a training sequence, added at the beginning of every OFDM symbol. Sparse nature of the channel impulse response, which manifests itself during wireless broadband transmission, enables one to implement compressed sensing techniques for channel identification. Channel sparsity reduces the number of measurement samples required in channel reconstruction procedure, but they should be taken from the inter-block interference (IBI) free part of the received training sequence. If maximum delay spread of the channel is comparable with the length of the training sequence, the number of measurements within the IBI-free region may be insufficient for accurate channel identification. This paper discusses a new iterative approach to sparse channels estimation, which uses the measurements from outside the IBI-free region. The proposed solution is examined for simulated transmission over randomly generated doubly-selective channels when compared with the conventional CS technique and perfect channel state information condition. The experiments reveal an enhancement of the transmission quality in terms of SER for slow-time selective channels when both delays and gains of the channel impulse response are estimated with the orthogonal matching pursuit algorithm. Further improvement, also for transmission over moderate varying channels, is observed when the path delays are found in advance. A suitable frame structure is proposed to do so.

Approach to suppress out‐of‐band emission for dual pseudo noise padded time‐domain synchronous‐orthogonal frequency division multiplexing systems

The dual pseudo noise padded (DPNP) time-domain synchronous-orthogonal frequency division multiplexing (TDS-OFDM) which utilises the second pseudo noise (PN) sequence for channel estimation is able to reduce the complexity and improve the accuracy of channel estimation compared to the classical TDS-OFDM. However, because of the duplicate PN sequence, DPNP TDS-OFDM will suffer from severer out-of-band emission, which is a headache issue for the conventional TDS-OFDM. In this study, a novel out-of-band suppression approach based on a new PN design criterion as well as a windowing operation is proposed for the DPNP TDS-OFDM systems. The frame structure and system model are also modified to accommodate to the approach, while the overlap-and-add approach is adopted to guarantee the spectral efficiency. Both simulation and experimental results show that this approach could achieve satisfactory performance with much less complexity and no spectral efficiency loss compared to the classical DPNP TDS-OFDM system. In addition, an iterative channel estimation method is also proposed for the modified frame structure to combat against the long channel delay spread.

A New Equalizer in Doubly-Selective Channels for TDS-OFDM

Various factors contribute to the intercarrier interference (ICI) in orthogonal frequency-division multiplexing (OFDM), which violates the orthogonality among subcarriers and greatly degrades the performance. Traditional ICI compensation methods incorporate iterative estimation and cancelation for pilot subcarriers, bearing too much complexity and reducing the spectral efficiency. In this paper, a novel channel equalization method is proposed for time domain synchronous-orthogonal frequency-division multiplexing (TDS-OFDM), where fast and accurate channel estimation is achieved by utilizing the padded pseudo-noise sequence in consecutive blocks. The new equalizer refers to fast suboptimal-minimum mean square error structure, other than traditional overlap-add structure, inspired by the fact that linear convolution structure is better preserved than circular convolution structure in TDS-OFDM under time-varying channels. With the approximation of linear time-varying channel, the proposed equalizer achieves considerable performance with low complexity (linear) and no additional spectral overhead.

Sparsity-Aware Adaptive Channel Estimation Based on SNR Detection

Time domain synchronous orthogonal frequency division multiplexing OFDM (TDS-OFDM) has a higher spectral efficiency and faster synchronization than standard cyclic prefix OFDM, but suffers from the performance loss over multipath fading channels due to severe interblock-interference. Recently, compressive sensing (CS) is explored to solve those problems, but it heavily relies on the sparsity level of the channel, which is unknown in practice. In this paper, a signal-to-noise ratio-based sparsity estimation method is proposed to detect the sparsity level of the channel. Then we propose the priori aided subspace pursuit algorithm if the channel sparsity meets the theoretical CS model, otherwise we use an improved iterative channel estimation method with the help of the channel sparsity. Simulation results demonstrate that the proposed scheme could improve the robustness and performance of TDS-OFDM systems compared with the conventional iterative methods or recently proposed CS based methods.

Energy‐efficient orthogonal frequency division multiplexing scheme based on time–frequency joint channel estimation

Time-domain synchronous orthogonal frequency division multiplexing (TDS-OFDM) enjoys the higher spectrum efficiency and faster synchronisation than the classical cyclic prefix OFDM (CP-OFDM). However, TDS-OFDM suffers from performance degradation especially under severely fading channels with long delays. To solve these problems, the authors propose an energy-efficient OFDM scheme called time-frequency-training orthogonal frequency division multiplexing (TFT- OFDM) based on the time-frequency joint channel estimation under the framework of compressive sensing (CS). The power of the guard interval (GI) in the proposed scheme can be reduced to achieve higher energy efficiency, which is infeasible for CP-OFDM. This method first utilises the time-domain pseudo noise sequence to acquire partial support information of the channel, and then some frequency-domain pilots are used for the exact channel estimation. Simulation results show that TFT- OFDM with CS can achieve much higher energy efficiency than the classical CP-OFDM, and outperforms the conventional OFDM schemes in both static and mobile environments. Moreover, for the channel with long delay spread, the TFT-OFDM scheme with CS can demonstrate robustness and much better performance than the conventional OFDM schemes. In this way, the TFT-OFDM scheme can use the same GI length for larger broadcasting coverage and hence further achieve higher energy efficiency.

Compressive Sensing Based Channel Estimation for OFDM Systems Under Long Delay Channels

Time-domain synchronous orthogonal frequency division multiplexing (TDS-OFDM) has advantages in spectral efficiency and synchronization. However, its iterative interference cancellation algorithm will suffer from performance loss especially under severely fading channels with long delays and has difficulty supporting high-order modulations like 256 QAM, which may not accommodate the emerging ultra-high definition television service. To solve this problem, a channel estimation method for OFDM under the framework of compressive sensing (CS) is proposed in this paper. Firstly, by exploiting the signal structure of recently proposed time-frequency training OFDM scheme, the auxiliary channel information is obtained. Secondly, we propose the auxiliary information based subspace pursuit (A-SP) algorithm to utilize a very small amount of frequency-domain pilots embedded in the OFDM block for the exact channel estimation. Moreover, the obtained auxiliary channel information is adopted to reduce the complexity of the classical SP algorithm. Simulation results demonstrate that the CS-based OFDM outperforms the conventional dual pseudo noise padded OFDM and CS-based TDS-OFDM schemes in both static and mobile environments, especially when the channel length is close to or even larger than the guard interval length, where the conventional schemes fail to work completely.

Zero‐pseudorandom noise training OFDM

A novel transmission scheme called zero-pseudorandom noise training sequence orthogonal frequency division multiplexing (ZPN-OFDM) is proposed, where the frame structure of dual-pseudorandom noise (PN) padding time domain synchronous OFDM is modified by replacing the first PN sequence by a zero sequence. The new ZPN-OFDM system offers better energy efficiency than previous OFDM systems. The ZPN-OFDM system is tested for significant tap delay multipath channels. Simulation results show that ZPN-OFDM can provide significant bit error rate improvement as well as energy efficiency improvement.

Spectrum- and Energy-Efficient OFDM Based on Simultaneous Multi-Channel Reconstruction

Time domain synchronous OFDM (TDS-OFDM) has a higher spectrum and energy efficiency than standard cyclic prefix OFDM (CP-OFDM) by replacing the unknown CP with a known pseudorandom noise (PN) sequence. However, due to mutual interference between the PN sequence and the OFDM data block, TDS-OFDM cannot support high-order modulation schemes such as 256QAM in realistic static channels with large delay spread or high-definition television (HDTV) delivery in fast fading channels. To solve these problems, we propose the idea of using multiple inter-block-interference (IBI)-free regions of small size to realize simultaneous multi-channel reconstruction under the framework of structured compressive sensing (SCS). This is enabled by jointly exploiting the sparsity of wireless channels as well as the characteristic that path delays vary much slower than path gains. In this way, the mutually conditional time-domain channel estimation and frequency-domain data demodulation in TDS-OFDM can be decoupled without the use of iterative interference removal. The Cramér-Rao lower bound (CRLB) of the proposed estimation scheme is also derived. Moreover, the guard interval amplitude in TDS-OFDM can be reduced to improve the energy efficiency, which is infeasible for CP-OFDM. Simulation results demonstrate that the proposed SCS-aided TDS-OFDM scheme has a higher spectrum and energy efficiency than CP-OFDM by more than 10% and 20% respectively in typical applications.

Compressive Sensing Based Time Domain Synchronous OFDM Transmission for Vehicular Communications

Time domain synchronous OFDM (TDS-OFDM) has higher spectral efficiency and faster synchronization than standard cyclic prefix OFDM (CP-OFDM), but suffers from the difficulty of supporting 256QAM in low-speed vehicular channels with long delay spread and the performance loss over fast time-varying vehicular channels. This paper addresses how to efficiently use the compressive sensing (CS) theory to solve those problems. First, we break through the conventional concept of cancelling the interferences if present, and propose the idea of using the inter-block-interference (IBI)-free region of small size to reconstruct the high-dimensional sparse multipath channel, whereby no interference cancellation is required any more. In this way, without changing the current signal structure of TDS-OFDM at the transmitter, the mutually conditional time-domain channel estimation and frequency-domain data detection in conventional TDS-OFDM receivers can be decoupled. Second, we propose the parameterized channel estimation method based on priori aided compressive sampling matching pursuit (PA-CoSaMP) algorithm to achieve reliable performance over vehicular channels, whereby partial channel priori available in TDS-OFDM is used to improve the performance and reduce the complexity of the classical CoSaMP signal recovery algorithm. Simulation results demonstrate that the proposed scheme can support the 256QAM and gain improved performance over fast fading channels.

Physical‐layer transceiving techniques on data‐aided orthogonal frequency‐division multiplexing towards seamless service on vehicular communications

Frequency error, non-ideal channel estimation (CE) and inefficient seamless road side unit (RSU) service are critical issues occurring in conventional systems that are completely specified by current IEEE 802.11p standards. This study investigates novel techniques for achieving accurate frequency offset compensation and effective CE and RSU selection for handover in signal-overlapping areas. Recently, communications using data-aided orthogonal frequency-division multiplexing (DA-OFDM), such as pseudo-random-postfix OFDM and time-domain-synchronous OFDM (TDS-OFDM), have been actively studied because of their higher effectiveness, efficiency and better transmission quality. This study consists of three parts: (i) derivation of the maximum likelihood estimation for DA-OFDM, (ii) design of an accurate RSU selection scheme and (iii) implementation of a superior TD CE technique. Performance comparisons between the conventional and the proposed techniques are conducted through comprehensive computer simulations.

Joint Time-Frequency Channel Estimation for Time Domain Synchronous OFDM Systems

As the key technology of Chinese national digital television terrestrial broadcasting (DTTB) standard, time domain synchronous OFDM (TDS-OFDM) has high spectral efficiency at the cost of performance loss over fast fading channels. In this paper, a novel time-frequency signal structure is proposed to improve the system performance of TDS-OFDM over fast time-varying channels. Each TDS-OFDM symbol adopts the time-domain one-sample shifted training sequence (TS) and the frequency-domain grouped pilots as the time-frequency training information. The modulable orthogonal sequence (MOS) with perfect autocorrelation property is adopted as the TS, while each pilot group has only one non-zero central pilot surrounded by several zero pilots. The corresponding joint time-frequency channel estimation utilizes the time-domain TS for path delay estimation and the frequency-domain grouped pilots for path gain estimation, thus accurate tracking of the fast time-varying wireless channel could be achieved. Only about 1% of the total subcarriers will be occupied by the redundant grouped pilots, thus the loss in spectral efficiency is negligible. Simulation results demonstrate that the proposed scheme outperforms the conventional solutions over fast fading channels.

TDS-OFDM based HDTV transmission over fast fading channels

Consumer electronics are expecting high-definition television (HDTV) services not only at home, but also in high-speed vehicles like cars, train, subways, etc. However, time domain synchronous OFDM (TDS-OFDM) could only provide HDTV services in static or low-speed channels, but not in fast fading channels due to the severe mutual interferences between the time-domain training sequence (TS) and the OFDM data block. Without changing the original TDS-OFDM signal structure, this paper firstly proposes the idea of regarding transmission parameter signaling (TPS) symbols embedded in the OFDM data block as frequency-domain pilots. This new perspective breaks the conventional concept that the training information of TDS-OFDM exists only in the time domain. Then, the two-dimensional channel estimation method jointly exploiting the time-domain TS as well as the frequency-domain pilots is proposed to improve the system reliability over fast fading channels. Simulation results demonstrate that reliable HDTV transmission could be achieved when the moving speed of consumer electronics is up to 120 km/h <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sup> .

Unified Time-Frequency OFDM Transmission with Self Interference Cancellation

Time domain synchronous orthogonal frequency division multiplexing (TDS-OFDM) has higher spectral efficiency than the standard cyclic prefix OFDM (CP-OFDM) OFDM by replacing the random CP with the known training sequence (TS), which could be also used for synchronization and channel estimation. However, TDS-OFDM requires suffers from performance loss over fading channels due to the iterative interference cancellation has to be used to remove the mutual interferences between the TS and the useful data. To solve this problem, the novel TS based OFDM transmission scheme, referred to as the unified timefrequency OFDM (UTF-OFDM), is proposed in which the time-domain TS and the frequency-domain pilots are carefully designed to naturally avoid the interference from the TS to the data without any reconstruction. The proposed UTF-OFDM based flexible frame structure supports effective channel estimation and reliable channel equalization, while imposing a significantly lower complexity than the TDS-OFDM system at the cost of a slightly reduced spectral efficiency. Simulation results demonstrate that the proposed UTF-OFDM substantially outperforms the existing TDS-OFDM, in terms of the system’s achievable bit error rate. key words: orthogonal frequency division multiplexing, training sequence, interference cancellation, low complexity, spectral efficiency