Time-domain Spreading Research Articles

DFT-s-OFDM With Enhanced Time-Domain Spreading for B-IFDMA

Enhanced time-domain spreading is proposed for block-interleaved frequency division multiple access (B-IFDMA) with discrete Fourier transform spread orthogonal frequency division multiplexing (DFT-s-OFDM). By repeating each consecutive modulation symbol prior to the spreading, we show that the DFT of the spreading sequence becomes a window function, which compared to the existing method of repeating the whole block of modulation symbols, offers a signal with both lower cubic metric (CM) and block error rate (BLER). Modulation symbol-specific spreading sequences are further shown to improve the BLER. The new transmission scheme is supported by both time- and frequency domain despreaders, as well as by two channel estimation methods for B-IFDMA.

Tensor-Based Joint Channel Estimation and Symbol Detection for AF MIMO Relay Networks

A study on the joint channel and symbol estimation issue is provided for two hop relay networks which employ the amplify-and-forward (AF) relaying approach. The encoding scheme at the source node introduces the time-domain spreading with a time-varying linear constellation precoding. Then, a set of amplifying factors matrices is utilized by the relays to amplify and forward the received data to the destination. The received signal at the destination can be constructed as a fourth-order tensor model, which is referred to as the nested parallel factor (PARAFAC) model. And then, we present a novel Levenberg-Marquardt (LM) algorithm based on this tensor model. The proposed method does not require complex signal processing at the relay, which effectively reduces the burden of relay. As a semi-blind method, which does not require the pilot signal, the proposed receiver can jointly recover the channels and information symbols. Moreover, the proposed semi-blind receiver is robust as it can work in different wireless channel scenarios. Simulations are conducted to demonstrate the efficiency of the proposed semi-blind approach.

A Novel Tensor-Based Receiver for Joint Symbol and Channel Estimation in Two-Hop Cooperative MIMO Relay Systems

In this letter, we develop a novel tensor-based receiver for two-hop cooperative multiple-input multiple-output (MIMO) relay systems. At the source node, the proposed transmit signal design combines the linear constellation precoding with time-domain spreading. We show that the received signal at the destination node is formulated as a fourth-order tensor satisfying a nested parallel factor (PARAFAC) model, and then a new tensor-based receiver is derived by resorting to the proposed receiver algorithm. Under the assumption that channel state information (CSI) is not available at each node, the proposed receiver jointly estimates the channel parameters and transmitted symbols. Numerical results show that the proposed receiver outperforms some competing tensor-based receivers and operates close to the zero forcing (ZF) receiver with perfect CSI.

Semi‐blind parallel factor based receiver for joint symbol and channel estimation in amplify‐and‐forward multiple‐input multiple‐output relay systems

In this study, the authors consider a new space-time coding scheme for two-hop amplify-and-forward (AF) relay multiple-input multiple-output (MIMO) relay communication systems. This scheme, called multiple Khatri–Rao space-time coding, which combines symbol stream blocking, spatial precoding and time-domain spreading. The authors show that the received signals at the destination node can be formulated as two parallel factor (PARAFAC) models, and then a novel semi-blind receiver is derived using a two-stage iterative fitting algorithm for joint symbol and channel estimation. Under the assumption that channel state information (CSI) is not available neither at the relay nodes nor at the destination node, the proposed semi-blind receiver operates close to the zero forcing receiver with perfect CSI. Moreover, without any channel training sequence, the proposed semi-blind receiver provides the destination node with full knowledge of all channel matrices involved in the transmission. Simulation results illustrate the performance of the proposed semi-blind receiver in comparison with alternative approaches.

Double Khatri–Rao Space-Time-Frequency Coding Using Semi-Blind PARAFAC Based Receiver

We first introduce a new class of tensor models for fourth-order tensors, referred to as “nested PARAFAC models.” Then, we present a space-time-frequency (STF) coding scheme for multiple antenna orthogonal frequency division multiplexing systems. This scheme, called double Khatri-Rao STF (D-KRSTF) coding, combines time-domain spreading with space-frequency precoding and provides an extension of Khatri-Rao space-time (KRST) coding . We show that the received signals define a fourth-order tensor satisfying two nested PARAFAC models, and a semi-blind receiver is then derived using a two-step alternating least squares algorithm for joint channel and symbol estimation. Simulation results show that our receiver offers superior performance compared with previously proposed tensor-based solutions and operates close to the zero forcing receiver with perfect channel state information.

Design and implementation of channel estimation for low-voltage power line communication systems based on OFDM

An optimized channel estimation algorithm based on a time-spread structure in OFDM low-voltage power line communication (PLC) systems is proposed to achieve a lower bit error rate (BER). This paper optimizes the best maximum multi-path delay of the linear minimum mean square error (LMMSE) algorithm in time-domain spread OFDM systems. Simulation results indicate that the BER of the improved method is lower than that of conventional LMMSE algorithm, especially when the signal-to-noise ratio (SNR) is lower than 0 dB. Both the LMMSE algorithm and the proposed algorithm are implemented and fabricated in CSMC 0.18 μm technology. This paper analyzes and compares the hardware complexity and performance of the two algorithms. Measurements indicate that the proposed channel estimator has better performance than the conventional estimator.

Blind frequency-offset tracking scheme for multiband orthogonal frequency division multiplexing using time-domain spreading

A blind scheme for estimating the residual carrier-frequency offset of multiband orthogonal frequency division multiplexing (MB-OFDM)-based ultra-wideband (UWB) systems is proposed. In the MB-OFDM UWB system, time-domain spreading (TDS) is used by transmitting the same information across two adjacent OFDM symbols. By using the TDS structure, the proposed frequency estimation scheme does not require the use of pilot symbols. To demonstrate the usefulness of the proposed estimator, analytical expression of the mean square error is derived and the performance is compared with a conventional pilot-assisted estimator.

Layered Steered Space–Time-Spreading-Aided Generalized MC DS-CDMA

We present a novel trifunctional multiple-input-multiple-output (MIMO) scheme that intrinsically amalgamates space-time spreading (STS) to achieve a diversity gain and a Vertical Bell Labs layered space-time (V-BLAST) scheme to attain a multiplexing gain in the context of generalized multicarrier direct-sequence code-division multiple access (MC DS-CDMA), as well as beamforming. Furthermore, the proposed system employs both time- and frequency-domain spreading to increase the number of users, which is also combined with a user-grouping technique to reduce the effects of multiuser interference. Further system performance improvements can be attained by serially concatenating our proposed scheme with an outer code that is amalgamated with a unity-rate code for the sake of improving the achievable decoding convergence behavior of the proposed system, which is evaluated with the aid of extrinsic information transfer charts. We also propose a novel logarithmic likelihood ratio (LLR) postprocessing technique to improve the iteratively detected system's performance. Explicitly, the proposed system can attain a second-order spatial diversity gain and a frequency diversity gain of order V, where V is the number of subcarriers. Additionally, the proposed system attains a beamforming gain and a multiplexing gain that is twice that of a single-input-single-output system. Furthermore, after I = 10 decoding iterations and employing an interleaver depth of Dint = 160 000 bits, a time-domain spreading factor of Ne = 4, and V = 4 subcarriers, the overloaded system supporting K = 8 users requires an Eb/N0 that is only about 0.45 dB higher than the single-user system.

Time domain spreading and frequency domain maximal ratio combining reception for frequency diversity enhancement in single carrier UWB communication systems

Ultra-wideband technology is a promising candidate for short-range broadband communication systems. Ultra-wideband channels are characterized by dense multiple paths that provide multipath diversity, also referred to as frequency diversity. Single carrier systems with frequency domain equalization have attracted much interest for high data rate communications. In this paper, frequency diversity in single carrier ultra-wideband systems is enhanced by employing simple frequency domain maximal ratio combining (FDMRC) reception with time domain complex quadratic spreading (CQS) sequences. Since ultra-wideband channels exhibit dense multiple paths and strong frequency selectivity, the proposed system can attain significant performance improvement over traditional single carrier frequency domain equalization (SC-FDE) systems. Through the comparison of the impact of the cyclic prefix length, the proposed system shows stronger robustness to inter-block interference than the conventional SC-FDE system. Compared with multicarrier systems, like orthogonal frequency-division multiplexing and multi-carrier code division multiple access, the proposed receiver complexity is reduced greatly. To accommodate multiuser communication, block spreading is further introduced to combine with the frequency diversity enhancement technique for single carrier block transmission UWB systems, realizing multiuser interference-free reception.

Pilot-Assisted Channel Estimation for Orthogonal Multi-Carrier DS-CDMA with Frequency-Domain Equalization

Orthogonal multi-carrier direct sequence code division multiple access (orthogonal MC DS-CDMA) is a combination of time-domain spreading and orthogonal frequency division multiplexing (OFDM). In orthogonal MC DS-CDMA, the frequency diversity gain can be obtained by applying frequency-domain equalization (FDE) based on minimum mean square error (MMSE) criterion to a block of OFDM symbols and can improve the bit error rate (BER) performance in a severe frequency-selective fading channel. FDE requires an accurate estimate of the channel gain. The channel gain can be estimated by removing the pilot modulation in the frequency domain. In this paper, we propose a pilot-assisted channel estimation suitable for orthogonal MC DS-CDMA with FDE and evaluate, by computer simulation, the BER performance in a frequency-selective Rayleigh fading channel.

A Study on Channel Estimation Methods for Time-Domain Spreading MC-CDMA Systems

As a candidate for the transmission technology of next generation mobile communication systems, time-domain spreading MC-CDMA systems have begun to attract much attention. In these systems, data and pilot symbols are spread in the time domain and code-multiplexed. To combat fading issues, we need to conduct channel estimation by using the code-multiplexed pilot symbols. Especially in next generation systems, frequency bands higher than those of current systems, which raise the maximum Doppler frequency, are expected to be used, so that a more powerful channel estimation method is expected. Considering this, we propose a channel estimation method for highly accurate channel estimation; it is a combination of a two-dimensional channel estimation method and an impulse response-based channel estimation method. We evaluate the proposed method by computer simulations.

Performance analysis of MMSE detectors for high speed VSF-OFCDM

This letter investigates the effect of Doppler frequency shift on the performance of MMSE receivers for high speed variable spreading factor (VSF) orthogonal frequency and code division multiplexing (OFCDM) systems in broadband wireless communications. In VSF-OFCDM systems, the existence of multicode interference (MCI) resulting from multiple code channels occupying the same bandwidth degrades the performance. A full MMSE receiver is analytically studied in this letter, exploiting correlation properties among transmitted data on different subcarriers to combat the MCI. Furthermore, considering the fact that the fast time-varying fading destroys the orthogonality of the time domain spreading codes and thus introduces time domain MCI, its effect on system performance is evaluated accurately. It is also shown that it is expected to use time domain spreading factor NT of 8 for a large range of Doppler frequencies, and frequency domain spreading factor as large as possible, especially in high signal to noise ratio. Moreover, in order to improve performance, the pilot channel should occupy around 20% to 25% of the total transmitted power for a large range of Doppler.

A study on channel estimation methods for time-domain spreading MC-CDMA systems

In time-domain spreading MC-CDMA systems, which are candidates for the 4G system, data and pilot symbols are spread in the time domain and code-multiplexed. To combat fading issues, we need to conduct channel estimation by using the code-multiplexed pilot symbols. In this paper, we propose a channel estimation method for highly accurate channel estimation; it is a combination of a two-dimensional channel estimation method and an impulse response-based channel estimation method. We evaluate the proposed method by computer simulations.

Flexible and reliable transmission techniques for scale-free UWB system

The speedy dissemination of digital consumer electronics devices within home and personal area causes increments of multimedia communication and advent of entertainment networking. This feature of communication requires variable data-rate and reliable transmission. In order to meet the various demands, in this paper, the multiband orthogonal frequency division multiplexing (MBOFDM) ultra-wideband (UWB) system by exploiting a time domain spreading factor and multiple-access scheme is proposed for variable data-rate transmission from 20kbps to 960Mbps in wireless communication. This article presents the basic structure and main functions of the variable data-rate (VR)-UWB system. With an emphasis on a proposed idea, we address the multiple-access in MB-OFDM system and apply adaptive bit allocation schemes to OFDMA system. It is shown in this article that scale-free UWB can accept multiple users and provide variable data-rate as well as support reliable transmission, while most conventional studies focus on the fixed rate transmission. The proposed system will be promising solution in ubiquitous environment to support multimedia traffic with various rates.

Downlink Transmission of Broadband OFCDM Systems-Part V: Code Assignment

To provide high speed mobile services, orthogonal multi-code transmission is employed in the orthogonal frequency and code division multiplexing (OFCDM) system. However, in a wireless frequency selective channel either with or without Doppler shift, code orthogonality is distorted and multi-code interference (MCI) is caused. The variance of the MCI is dependent on which codes to be used. This paper investigates the code assignment in the OFCDM system, aiming to mitigate the MCI. The code assignment can be explored in terms of the code distance defined in this paper. Generally, the codes with large code distances to each other should be assigned with high priority. Moreover, to avoid large interference from the powerful code-multiplexed pilot channel, the time spreading code adjacent to the pilot code in the code tree (Adachi et al., 1998) is suggested not be used when the time domain spreading factor is large. The performance of the proposed non-sequential code assignment scheme is analytically evaluated. It has been shown that the system performance can be improved significantly by the non-sequential scheme under various channel conditions, especially when a low to medium system load is assumed.

Downlink Resource Management in the Frequency Domain for Multicell OFCDM Wireless Networks

Recently, various versions of combining code division multiple access (CDMA) with orthogonal frequency-division multiplexing (OFDM) have drawn much attention for IMT-advanced (4G) and new broadcasting systems. In particular, variable-spreading-factor orthogonal frequency and code multiplexing (OFCDM) and multicarrier CDMA adopt 2-D spreading, i.e., time-domain and frequency-domain spreading. Maeda , Yang and Hanzo, Zheng , and Zhou have mentioned a few strong points of 2-D spreading. However, what is missing in their analysis/simulations is the system-level aspect of 2-D spreading. In particular, we are interested in the following questions: ldquoWhat are the advantages of frequency-domain spreading in multicell environments?rdquo and ldquoWhat is the optimal frequency reuse factor in 2-D spreading systems?rdquo Answers to these questions are investigated in this correspondence.

Performance of Multiband OFDM Systems with Extra Diversity

This letter presents the performance of ultra-wideband multi-band orthogonal frequency division multiplexing (UWB MB-OFDM) systems with an extra diversity. To fully obtain diversity gain in the current MB-OFDM system when a time-domain spreading (TDS) is adopted, two consecutive OFDM symbols are designed to be cyclic shifted against each other. Simulation results indicate that the MB-OFDM system using additional frequency diversity outperforms conventional MB-OFDM system.

Adaptive Subcarrier Allocation in Synchronous Reverse Links of a Multicarrier CDMA System with Time and Frequency Spreading

Multicarrier code-division multiple access (CDMA) with time and frequency spreading has been recently considered as a candidate for fourth-generation (4G) wireless systems. This signaling scheme simultaneously utilizes code spreading in the time and frequency domains to simultaneously improve frequency diversity and minimize multiuser access interference. As a result, it is capable of outperforming multicarrier CDMA systems that employ 1-D spreading. In this paper, a novel adaptive subcarrier allocation algorithm is developed for multicarrier CDMA with time and frequency spreading to improve the overall bit error rate (BER) performance for all spreading configurations. This algorithm assigns users to subcarrier groups that provide favorable fading characteristics while simultaneously reducing the amount of interference caused to other users. The proposed algorithm is shown to provide a performance improvement, ranging from 1.5 dB with 2times16 (time times frequency) and spreading to 7 dB with 16times2 (time times frequency) spreading. The algorithm is also shown to maintain or improve the BER floor for each spreading configuration. It is concluded that at higher and lower levels of Eb/No, a higher frequency- and time-domain spreading should be, respectively, employed to improve BER performance. Furthermore, the Eb/No threshold level to switch between time and frequency spreading for the analyzed system is found to be 2.5 dB.

BLIND SOURCE SEPARATION FOR TWO-DIMENSION SPREAD SPECTRUM SYSTEM BASED ON TRILINEAR DECOMPOSITION

This paper links the two-dimension-spread-spectrum-system source separation problem to the trilinear model, which is an analysis tool rooted in psychometrics and chemometrics. Exploiting this link, it derives a blind source separation algorithm. The proposed algorithm capitalizes on time-domain spread, frequency-domain spread and temporal diversity-combining. The simulation results reveal that the performance of the blind source separation algorithm for two-dimension spread spectrum system is very close to nonblind minimum mean-squared error method, and this algorithm works well for small sample size. The blind source separation algorithm does not require channel fading information and spread codes, so it has blind and robust characteristics.

Orthogonal Multi-Carrier DS-CDMA with Frequency-Domain Equalization

Orthogonal multi-carrier direct sequence code division multiple access (orthogonal MC DS-CDMA) is a combination of orthogonal frequency division multiplexing (OFDM) and time-domain spreading, while multi-carrier code division multiple access (MC-CDMA) is a combination of OFDM and frequency-domain spreading. In MC-CDMA, a good bit error rate (BER) performance can be achieved by using frequency-domain equalization (FDE), since the frequency diversity gain is obtained. On the other hand, the conventional orthogonal MC DS-CDMA fails to achieve any frequency diversity gain. In this paper, we propose a new orthogonal MC DS-CDMA that can obtain the frequency diversity gain by applying FDE. The conditional BER analysis is presented. The theoretical average BER performance in a frequency-selective Rayleigh fading channel is evaluated by the Monte-Carlo numerical computation method using the derived conditional BER and is confirmed by computer simulation of the orthogonal MC DS-CDMA signal transmission.