Time Domain Synchronous Orthogonal Frequency 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.

Study on An Improved Accuracy Algorithm Based on Channel Estimation in 5G Application

5G has ultra-high spectrum utilization, energy efficiency and resource utilization. Orthogonal frequency division multiplexing modulation is a key technology in order to achieve the characteristics of 5G communication such as ultra-high spectrum utilization. In recent years, a pseudo-random sequence (PN) filled with cyclic prefix time-domain synchronous orthogonal frequency division multiplexing modulation technology has emerged as a modulation technology in high-performance 5G communication systems. And the technology has been maturely applied in the field of terrestrial digital multimedia wireless broadcasting because of its higher channel estimation and synchronization performance. At the same time, 5G has higher requirements in terms of transmission rate and so on, so it has stricter performance requirements for transmission delay control and timing recovery. However, the TDS-OFDM system in 5G application does not allocate pilot subcarriers, which are used for system channel estimation and various parameters. Since the autocorrelation function of the time-domain PN sequence filled in the TDS-OFDM system is not an ideal shock function, when the phase difference with the received sequence is not 0, the autocorrelation value is not 0 as well. When used as a channel estimation, it will cause mutual interference of various multipath components, reduce the accuracy of the estimation, and may also cause some weak multipath Loss of components. This paper proposes an improved channel estimation scheme for TDS-OFDM based systems in 5G application, which can effectively improve the accuracy of channel estimation in the case of high signal-to-noise ratio.

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.

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.

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.

A Generalized Analytical Solution to Channel Estimation With Intersymbol Interference Cancelation and Co-Channel Interference Cancelation for Single Input Single Output/Multiple Input Single Output Digital Terrestrial Multimedia Broadcasting Systems

A novel generalized channel estimation scheme without the need of matrix inversion or iterative operations is proposed for single input single output (SISO)/multiple input single output (MISO) digital terrestrial multimedia broadcasting (DTMB) system, adopting the time-domain synchronous orthogonal frequency division multiplexing technology. Simultaneously exploiting time domain correlation channel estimation and the constant coefficients of the imperfect autocorrelation of m-sequence, a simple and generalized closed-form solution related to each channel tap can be acquired by solving the constructed linear equations. On the basis of the specified frame head structure for the SISO DTMB system, quasi-orthogonal frame head sequences over multiple antennas are designed by employing different cyclic shifts of m-sequence. In addition, in order to effectively recover the transmitted signal in the frequency domain, the tail cancelation and cyclic reconstruction process is elaborated primarily for MISO systems. It is shown that, for the SISO DTMB system, the implementation complexity of the intersymbol interference cancelation (ISIC) of the proposed approach is reduced by approximately 90% over the existing iterative scheme, and for the MISO DTMB system, the implementation complexity of the proposed scheme with the co-channel interference cancelation (CCIC) and ISIC dramatically decreases compared with the existing scheme. Simulation results validate that the proposed channel estimation scheme can achieve almost the same performance as the existing channel estimation methods, but with a significant reduction in computational complexity in both SISO and MISO DTMB systems.

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

Novel Transmit Diversity Scheme for TDS-OFDM System With Frequency-Shift m-Sequence Padding

This contribution proposes a design method for training sequence (TS) defined as frequency-shift m-sequence (FSMS) and a novel transmit diversity scheme with FSMS padding as guard interval (GI) for the further evolution of time domain synchronous orthogonal frequency domain multiplexing (TDS-OFDM) systems. The good auto-correlation characteristic of FSMS can facilitate synchronization and channel estimation at the receiver. Due to the cross-correlation characteristics of FSMSs, the accuracy of channel estimation is affected by the mutual interference among the transmit antennas. To further improve the accuracy of channel estimation, the channel impulse response (CIR) should be estimated in an iterative manner to suppress the effects of the mutual interference and additive white Gaussian noise. Compared with the transmit diversity schemes based on the space-frequency coded TS and space-time coded TS for TDS-OFDM system, the proposed transmit diversity scheme with FSMS padding can provide diversity gain and superior system performance over time-varying and/or frequency-selective propagation conditions.

Wireless Positioning Using TDS-OFDM Signals in Single-Frequency Networks

Wireless positioning using digital television (DTV) signals is a promising complementary to global positioning system due to high transmission power and wide coverage of DTV transmitters. Without changing the current infrastructure of Chinese DTV broadcasting network, this paper proposes a positioning scheme using time-domain synchronous (TDS) orthogonal frequency division multiplexing (OFDM) signals in single-frequency networks. The transmission parameter signaling (TPS) embedded in the TDS-OFDM signals is time-division multiplexed as orthogonal frequency-domain pilots, and then the time-domain pseudorandom noise sequence and the frequency-domain TPS are jointly utilized for accurate time of arrival (TOA) estimation for each transmitter. The theoretical bound of the TOA estimation accuracy is also derived. The proposed wireless positioning scheme has no impact on the normal TV program reception. Simulation results show that the positioning accuracy of less than 1 m can be achieved in SFN scenarios. The idea of time-frequency joint positioning can also be applied to other OFDM-based wireless systems.

On Channel Estimation and Equalization in 2 $\times$ 1 MISO TDS-OFDM Based Terrestrial DTV Systems

This paper proposes a 2 × 1 Multiple Input Single Output (MISO) scheme for Chinese Terrestrial DTV transmission, which is based on Time-domain Synchronous Orthogonal Frequency Division Multiplexing (TDS-OFDM). Our proposed scheme adopts the modified Alamouti code, and thus the frame structure is backward compatible to the Single Input Single Output (SISO) DTV standard. To achieve satisfying performance, we insert shifted PN sequences as the frame head, and develop an improved channel estimation (CE) algorithm for MISO co-channel interference cancellation (CCIC). Simulation results show that our MISO scheme can benefit from the transmit diversity gain with the help of MISO-CCIC channel estimation.

A Novel Two-Layer Data Transmission Scheme in TDS-OFDM System

A scheme that modulates the training sequence is proposed to support two-layer data streams in the time domain synchronous orthogonal frequency division multiplex (TDS-OFDM) systems. A theoretical analysis and computer simulation show that the proposed scheme works well and that the two layer data streams are compatible with each other.

TDS-OFDMA: a novel multiple access system based on TDS-OFDM

As the key technology of Chinese national digital television terrestrial broadcasting (DTTB) standard, time domain synchronous OFDM (TDS-OFDM) utilizes known training sequence instead of cyclic prefix (CP) as the guard interval between adjacent inverse discrete Fourier transform (IDFT) blocks to achieve better system performance. Until now, TDS-OFDM was primarily used in downlink broadcasting. When TDS-OFDM is directly used in the uplink, it is difficult to eliminate the superposed interferences caused by different training sequences and IDFT blocks from multiple users. To solve this problem, a novel scheme called time domain synchronous orthogonal frequency division multiple access (TDS-OFDMA) is proposed in this paper. The novel space-time frame structure of the TDS-OFDMA scheme is proposed to achieve joint cyclicity reconstruction as well as joint channel estimation at the receiver. Simulation results show that the proposed scheme can achieve better bit error rate (BER) performance than conventional OFDMA system under highspeed mobile environments.

Synchronization for TDS-OFDM over multipath fading channels

Time domain synchronous orthogonal frequency division multiplex (TDS-OFDM) is an effective multi-carrier modulation scheme to improve the spectrum efficiency. However, conventional synchronization algorithm based on the correlation property of the time-domain pseudo-noise (PN) sequence may not work well over severe multipath fading channels when large carrier frequency offset (CFO) exits. In this paper, a robust synchronization algorithm is proposed where the PN sequence in the frame head is considered as a cyclic prefix (CP) OFDM training symbol. The time-domain slide auto-correlation based on the cyclic structure of the PN sequence is adopted for timing and fine CFO estimation, while in the frequency domain, a differential correlation is applied for large CFO estimation. Simulations demonstrate that the proposed method could achieve the satisfactory performance over multipath fading channels. In addition, to reduce the implementation complexity, the frame head of the TDS-OFDM system is preferred to be binary PN sequences in the frequency domain with cyclic structure in the time domain.

Improved Channel Estimation for TDS-OFDM Based on Flexible Frequency-Binary Padding

By adopting the pseudo-random noise (PN) sequences as the guard interval (GI) as well as the training sequence, time domain synchronous orthogonal frequency division multiplexing (TDS-OFDM) outperforms the conventional OFDM using the cyclic prefix (CP) in the spectral efficiency at the cost of the iterative padding subtraction (IPS) in channel estimation (CE). To avoid the high computational complexity of the IPS, an improved frequency domain CE scheme based on a newly designed frame structure is proposed. To further improve the CE accuracy, the time domain training sequences, which are frequency binary (FB) after the fast Fourier transform (FFT), are flexibly padded as the frame header instead of the PN sequences. Theoretical analyses and computer simulations show that the proposed scheme can improve the system performance under the time frequency doubly selective channels, even in the presence of large channel delays in the single frequency networks.

Secure communication in TDS-OFDM system using constellation rotation and noise insertion

Time-domain synchronous orthogonal frequency division multiplexing (TDS-OFDM) is the key technology of Chinese digital television terrestrial broadcasting (DTTB) standard. However, the issue of security was not considered, and this limits its application in commercial or military scenarios where secure communication is very important. This paper proposes an encryption scheme based on pseudo random constellation rotation and weak artificial noise insertion, which jointly utilizes the pseudo random key and the irreversible property of wireless channel to guarantee the physical layer security of TDS-OFDM system. Simulation results show that the proposed security scheme can protect the system from eavesdroppers, while the legal receivers only suffer a negligible bit error rate (BER) performance loss compared with the conventional TDS-OFDM system without security.

Low cost VLSI architecture of resisting long echo channel estimation for DTMB system

Digital television/terrestrial multimedia broadcasting (DTMB) was announced as the Chinese digital television terrestrial broadcasting (DTTB) standard in August, 2006. It adopts TDS-OFDM (time-domain synchronous-orthogonal frequency division multiplexing) as the baseline modulation technology. There is not pilot information in order to avoid reduction of channel throughput. It uses pseudo-noise (PN) sequence as guard interval (GI) and the training sequence for both multi- and single-carrier block transmissions which makes the synchronization faster than CP-OFDM (cyclic prefix-orthogonal frequency division multiplexing) systems. It can resist the inter-symbol interference (ISI) and can be used for channel estimation and synchronization. In this paper, a full-mode channel estimation scheme for DTMB demodulator is developed. This method utilizes the PN sequence to obtain channel estimation and equalization both in three kinds of frame head (FH) mode as well as in single and multi-carrier modulation transmission mode. To implement on hardware realization, interference cancellation is predigested, then equalization and filter for channel impulse response are optimized. A low cost VLSI architecture for channel estimation and equalization is proposed which works effectually and achieves low complexity as well. Computational simulation results show that the proposed scheme has little BER loss in performance.

A novel time-domain synchronous block transmission frame structure for broadcasting return channel

In time-domain synchronous (TDS) block transmission systems, known symbols are padded in the guard interval between data blocks using some specific patterns. The complexity of channel estimation is relatively high for the time domain synchronous orthogonal frequency division multiplexing (TDS-OFDM) system. As a result, original TDSOFDM is only used for downlink broadcasting. This paper proposes a new frame structure based on TDS block transmission. The proposed frame structure can simplify the channel estimation and support the multiuser scenarios, due to the proposed padding sequence and its specific time and frequency allocation pattern. It supports the variable number of frames in one superframe, which provides the flexibility to combat various channel conditions and maintains the system efficiency. Because of these characters, it can be used in wireless uplink channel as an alternative orthogonal frequency division multiple access (OFDMA) structure. Simulation results show that the performance of the proposed frame structure is almost the same as the conventional OFDMA scheme, but has better spectrum efficiency. Therefore, it is preferred to be used in the return channel of terrestrial broadcasting systems.