Time Domain Synchronous OFDM Systems Research Articles

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.

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.

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

A novel 3780-point FFT processor scheme for the time domain synchronous OFDM system

The 3780-point FFT is a main component of the time domain synchronous OFDM (TDS-OFDM) system and the key technology in the Chinese Digital Multimedia/TV Broadcasting-Terrestrial (DMB-T) national standard. Since 3780 is not a power of 2, the classical radix-2 or radix-4 FFT algorithm cannot be applied directly. Hence, the Winograd Fourier transform algorithm (WFTA) and the Good-Thomas prime factor algorithm (PFA) are used to implement the 3780-point FFT processor. However, the structure based on WFTA and PFA has a large computational complexity and requires many DSPs in hardware implementation. In this paper, a novel 3780-point FFT processor scheme is proposed, in which a 60×63 iterative WFTA architecture with different mapping methods is imported to replace the PFA architecture, and an optimized CoOrdinate Rotation DIgital Computer (CORDIC) module is used for the twiddle factor multiplications. Compared to the traditional scheme, our proposed 3780-point FFT processor scheme reduces the number of multiplications by 45% at the cost of 1% increase in the number of additions. All DSPs are replaced by the optimized CORDIC module and ROM. Simulation results show that the proposed 3780-point FFT processing scheme satisfies the requirement of the DMB-T standard, and is an efficient architecture for the TDS-OFDM system.

Pilot Design and Channel Estimation for TDS-OFDM System with Transmit Diversity

To realize transmit diversity for the time domain synchronous OFDM (TDS-OFDM) system, this letter proposes the space-time-frequency orthogonal training sequence and the corresponding flexible channel estimation methods. Simulation results indicate that an significant performance improvement could be achieved for low-density parity-check code (LDPC) coded TDS-OFDM system over multi-path fading channels.

Complexity Reduced Transmit Diversity Scheme for Time Domain Synchronous OFDM Systems

This paper proposes a novel scheme to reduce the complexity of existing transmit diversity solutions to time domain synchronous OFDM (TDS-OFDM). The space shifted constant amplitude zero autocorrelation (CAZAC) sequence based preamble is proposed for channel estimation. Two flexible frame structures are proposed for adaptive system design as well as cyclicity reconstruction of the received inverse discrete Fourier transform (IDFT) block. With regard to channel estimation and cyclicity reconstruction, the complexity of the proposed scheme is only around 7.20% of that of the conventional solutions. Simulation results demonstrate that better bit error rate (BER) performance can be achieved over doubly selective channels.

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.

LDPC Coded TDS-OFDM for PLC Systems

Powerline communications (PLC) have drawn great interest in recent years. However, most PLC standards such as HomePlug AV use the cyclic-prefix OFDM (CP-OFDM) technology. This paper presents a broadband PLC system using low density parity check (LDPC) coded time domain synchronous OFDM (TDS-OFDM), whose spectrum efficiency is about 10% higher than that of CP-OFDM. With the same bandwidth and the ability to combat the time delay spread as HomePlug AV, this system can provide a maximum throughput of 199.7 Mbps physical layer data rate. Simulations over the measured practical powerline channel in Beijing, China, show that LDPC in the TDS-OFDM system dramatically improves the bit error rate performance, and verify the feasibility and performance of the TDS-OFDM technology for PLC systems.

A Novel PN Complementary Pair for Synchronization and Channel Estimation

In this paper, a novel pseudo-random noise complementary pair (PNCP) is proposed and adopted as the guard intervals in the time-domain synchronous OFDM (TDS-OFDM) system. The proposed PNCP has nearly ideal aperiodic auto-correlation property and inherits the differential property of the PN sequence. Simulations demonstrate the proposed TDS-OFDM system padded with PNCP could achieve better performance in both synchronization and channel estimation than the conventional TDS-OFDM system.

A multi-user uplink TDS-OFDM system based on dual PN sequence padding

The adoption of pseudo-noise (PN) sequence instead of cyclic prefix (CP) as guard interval (GI) enables time domain synchronous orthogonal frequency division multiplexing (TDS-OFDM) outperform Cyclic Prefixed- OFDM (CP-OFDM) by spectrum efficiency but at the expense of higher complexity. Up to now, TDS-OFDM is only used in unidirectional data transmission such as downlink broadcasting. The difficulty to eliminate the implicitly mixed interferences between different PN sequences and inverse discrete Fourier transform (IDFT) blocks caused by multiple users hinders the application of TDS-OFDM in multi-user uplink scenarios. To deal with this problem, this paper proposes a multi-user uplink TDS-OFDM system based on dual PN padding (DPNP) where the cancellation of mixed interference is avoid by joint cyclicity reconstruction for all users. In addition, joint channel estimation and separation is also proposed to restore the transmitted signal for each user. Theoretical analysis and computer simulation not only verify the feasibility of the proposed system, but also show that, compared with the conventional single-user TDS-ODFM system, it holds lower implementation complexity yet even better system performance in mobile environments.

A Simplified Equalization Method for Dual PN-Sequence Padding TDS-OFDM Systems

Time domain synchronous orthogonal frequency division multiplexing (TDS-OFDM) modulation scheme can improve the spectrum efficiency over the traditional cyclic prefix (CP) OFDM technology at the cost of computational efforts due to the iterative padding subtraction (IPS) at the receiver to mitigate the inter symbol interference caused by the multipath propagation. In this paper, a simplified equalization scheme is presented for the dual PN-sequence padding (DPNP) TDS-OFDM systems. Although the length of the padded PN sequence needed is double compared to what needed by the IPS, leading to slight decrease of the spectrum efficiency, this method can save about 90% computational effort in practice compared with that of the IPS algorithm. It also outperforms IPS method under the time-varying fading channel in terms of symbol error rate and maximum Doppler frequency, verified by the computer simulation.

Phase Noise Correction Based on TPS Symbols for the Chinese DTTB System

In orthogonal frequency division multiplexing (OFDM) system, phase noise (PHN) from oscillator can severely reduce the performance by causing: common phase error (CPE) and inter-carrier interference (ICI). The impact of PHN on received signal can be characterized by the circular convolution of the transmitted signal and the discrete Fourier transform (DFT) of PHN signal in frequency domain. Digital television/terrestrial multimedia broadcasting (DTMB), announced as the Chinese digital television terrestrial broadcasting (DTTB) standard, adopted the time-domain synchronous OFDM system (TDS-OFDM) system which has 36 continuous transmission parameter signaling (TPS) symbols in frequency domain which be used for PHN suppression. Based on a linearized parametric model for PHN, least squared (LS) estimation and fast Fourier transform (FFT) approaching method for PHN correction are proposed in this paper. The effects of the PHN on channel estimation in both TDS-OFDM system and conventional cyclic prefix OFDM (CP-OFDM) are also investigated. Simulations show that the proposed algorithm can effectively reduce the PHN and improve the symbol error rata (SER) of TDS-OFDM systems over both additive white Gaussian noise (AWGN) and broadcasting multipath channel. In practice, the FFT-approaching method can be applied to the DTMB receiver-chip design directly with low implementation cost.

A General SFN Structure With Transmit Diversity for TDS-OFDM System

In the digital television terrestrial broadcasting, the Single Frequency Network (SFN) has much advantage. SFN can serve an arbitrary large area with the same information broadcasted at the same frequency, resulting in the potential diversity gain. To improve the transmission performance of the Time Domain Synchronous-Orthogonal Frequency Division Multiplexing (TDS-OFDM) system in SFN, a general SFN structure with transmit diversity is introduced, theoretically analyzed and computer simulated. The proposed SFN structure is flexible to set up. Simulations show that this method can greatly improve the reliability of the signal transmission over the frequency selective fading channels and is suitable for the TDS-OFDM system in SFN.

Low-Complexity Equalization for TDS-OFDM Systems Over Doubly Selective Channels

Time variation of a multipath channel leads to interchannel interference (ICI) in orthogonal frequency-division multiplexing (OFDM) systems. It results in the performance degradation, therefore, limits the achievable throughput. Some methods have been proposed to suppress ICI, unfortunately, they are either computationally complex or at the price of spectral efficiency. In this paper, a low-complexity equalization method for time-domain synchronous OFDM (TDS-OFDM) systems is proposed under the assumption that the channel impulse response (CIR) varies in a linear fashion within a block period. The rationale behind our method is to use a finite power series expansion for the inverse of the equalization matrix. This method provides a desired tradeoff between the performance and the processing complexity. Theoretical analysis and simulation results demonstrate that the proposed method can effectively mitigate ICI caused by the channel variations with low computational complexity.

Design of space-time-frequency transmitter diversity scheme for TDS-OFDM system

To improve the performance of the time domain synchronous -orthogonal frequency division multiplexing (TDS-OFDM) system, an efficient space-time-frequency coding scheme is proposed. The block coding is employed in both time and frequency dimensions simultaneously and proved to be a quasi-orthogonal design. The performance improvement of TDS-OFDM system with this method is theoretically analyzed and computer simulated, showing its superior performance in the presence of both time and frequency selective environments. Comparing to the pure space-time or space-frequency coding scheme with straightforward implementations, the proposed implementation scheme reduces the computational load by about 70% in TDS-OFDM system.

On channel estimation and equalization in TDS-OFDM based terrestrial HDTV broadcasting system

In TDS-OFDM (time-domain synchronous orthogonal frequency division multiplexing) systems, pseudonoise (PN) sequences rather than cyclic prefixes are inserted as guard interval, between consecutive inverse discrete Fourier transformed (IDFT) symbol blocks. Since the PN sequences can also be used as training symbols, such system can provide higher spectrum efficiency. However, due to non-cyclic property of the signal, the simple channel estimation and equalization techniques for conventional cyclic prefixed OFDM (CP-OFDM) can not be applied to TDS-OFDM. In this paper, we propose a channel estimation and equalization method for TDS-OFDM. Channel estimation depends on time domain correlation and iterative interference cancellation techniques, while equalization is based on tail cancellation and cyclic restoration algorithm (TCCR). It is shown that our proposed method can provide satisfactory performance in TDS-OFDM based terrestrial high-definition television (HDTV) broadcasting system.