Linear Precoded Orthogonal Frequency Division Multiplexing Research Articles

Use of Chaotic Oscillations for Precoding and Synchronization in OFDM

This paper proposes a novel linear precoding method for Orthogonal Frequency Division Multiplexing (OFDM) based on the employment of the chaotic waveforms generated by the fourth-order chaotic oscillator and orthonormalized by the Gram-Schmidt process. The proposed linear precoding method is aimed to increase resilience to the multipath propagation issues and reduce the Peak-to-Average Power Ratio (PAPR) of the transmitted signal. Moreover, the chaotic waveform enables novel timing synchronization methods to be implemented in the receiver. The modelling of baseband Linear Precoded OFDM (LP-OFDM) data transmission system with Rayleigh channel has been performed in Simulink environment to validate the proposed method and to compare the performance to the classic precoding methods, such as Walsh-Hadamard Transform (WHT). Experiments have shown that in a high Signal-to-Noise Ratio (SNR) scenario, the employment of the novel precoding scheme allows reducing Bit Error Ratio (BER) by several dB compared to non-precoded OFDM. The proposed precoding method leads to the reduction of PAPR; however, it is not as efficient as classical precoding schemes, such as WHT. Experimental evidence of synchronization of the chaotic oscillators within 50 samples long time interval is presented.

Quasi-Cyclic Symbol Extensions for Shaping the OFDM Spectrum

$N$ -continuous orthogonal frequency division multiplexing (OFDM) is a promising solution to the out-of-band (OOB) emission issue in OFDM systems where the $N$ th-order continuity between OFDM symbols leads to lower OOB emission than the plain OFDM; however, high self-interference in such systems is an obstacle. In this paper, we propose a new class of quasi-cyclic symbol extensions, which makes the OFDM system $N$ -continuous. The new extension leads to low OOB emission while the additive interference is less than other $N$ -continuous OFDM systems. Moreover, we propose a general analysis of the spectrum and interference that occurs in a linear precoded OFDM system. The framework is applicable to many $N$ -continuous OFDM systems including our proposed method.

Double Allocation of Spreading Code Minimizing SI and PAPR for LPOFDM UWB System

Self-interference (SI) occurs if the transmission is through a multipath channel. The channel distorts helps to break the orthogonality between the spreading codes. In addition, the amplitude of linear precoded orthogonal frequency division multiplexing (LP-OFDM). LP-OFDM signals have strong fluctuations. Thus, it is necessary to seek or to minimize these two problems by the reduction of term SI and the amplitude of fluctuations Peak-to-Average Power Ratio (PAPR). In this paper a new method of allocation of spreading code has proposed. It consists of a double selection of spreading codes minimizing joint SI and PAPR. Simulation results show that the LP-OFDM system is optimized by this proposal compared to the conventional solution where the systems don’t operate in full load.

Low Complexity Discrete Hartley Transform Precoded OFDM System over Frequency-Selective Fading Channel

Orthogonal frequency-division multiplexing (OFDM) suffers from spectral nulls of frequency-selective fading channels. Linear precoded (LP-) OFDM is an effective method that guarantees symbol detectability by spreading the frequency-domain symbols over the whole spectrum. This paper proposes a computationally efficient and low-cost implementation for discrete Hartley transform (DHT) precoded OFDM systems. Compared to conventional DHT-OFDM systems, at the transmitter, both the DHT and the inverse discrete Fourier transform are replaced by a one-level butterfly structure that involves only one addition per symbol to generate the time-domain DHT-OFDM signal. At the receiver, only the DHT is required to recover the distorted signal with a single-tap equalizer in contrast to both the DHT and the DFT in the conventional DHT-OFDM. Theoretical analysis of DHT-OFDM with linear equalizers is presented and confirmed by numerical simulation. It is shown that the proposed DHT-OFDM system achieves similar performance when compared to other LP-OFDMs but exhibits a lower implementation complexity and peak-to-average power ratio.

Cross-layer multiple-access optimization scheme for linear precoded OFDM UWB systems

With the growing demand for new wireless applications accompanied with high expectations for better QoS fulfillment especially for multimedia and real-time applications, the performance of the radio resource management in a multiuser context is ensured by the ability to provide an efficient and optimized spectrum sharing scheme that should respect the wireless channel conditions and satisfy the different users’ demands. From the physical layer perspective, metrics such as spectrum efficiency and minimum BER are the most important criteria to be considered. On the other hand, from a user perspective, QoS as well as fairness among the competing users are the main metrics because they determine how much end-users are satisfied and how efficient the available resources are shared among the existing users. Based on the use of the linear precoded orthogonal frequency division multiplexing (LP-OFDM) solution proposed as an evolution of the well-known multiband OFDM (MB-OFDM) solution supported by the WiMedia Alliance for future high-rate ultra-wideband (UWB) systems, the objective of this paper is twofold. First, we study a multiuser optimization spectrum sharing scheme for LP-OFDM systems. Second, based on the optimization study, we define a novel multiple-access solution which jointly considers the frequency resource allocation and the time scheduling for the high-rate LP-OFDM UWB systems. Simulation results demonstrate the efficiency of the use of the LP-OFDM transmission technique in the multiuser spectrum sharing scheme. Besides, the novel multiuser time–frequency sharing scheme shows its capacity to provide a high performance level for high-priority users.

Bit Rate Optimization with MMSE Detector for Multicast LP‐OFDM Systems

We propose a new resource allocation algorithm with minimum mean square error (MMSE) detector for multicast linear precoded orthogonal frequency division multiplexing (LP‐OFDM) systems. To increase the total multicast bit rate, this algorithm jointly uses the LP‐OFDM modulation technique and an adaptation of the OFDM‐based multicast approaches to exploit the transmission link diversities of users. The LP technique applied to multicast OFDM systems with zero forcing (ZF) detector has already proved its ability to increase the unirate multicast system bit rate in a power line communication (PLC) context. The new MMSE detector and the new related bit‐loading algorithm are developed to enhance the ZF detector results. To improve both the bit rate and the fairness among multicast users, the utilization of the LP component in multirate multicast systems is then investigated. Simulations are run over indoor PLC channels, and it is shown that the proposed LP‐based methods outperform the OFDM‐based methods in terms of total bit rate and fairness index for both unirate and multirate multicast systems. Additionally, it is shown that the proposed bit‐loading algorithm with MMSE detector outperforms the ZF detector and the OFDM‐based receiver in terms of total multicast bit rate and fairness among users.

Lattice-reduction aided equalization for OFDM systems

Orthogonal frequency division multiplexing (OFDM) is an effective technique to deal with frequency-selective channels since it facilitates low complexity equalization and decoding. Many existing OFDM designs successfully exploit the multipath diversity offered by frequency-selective channels. However, most of them require maximum likelihood (ML) or near-ML detection at the receiver, which is of high complexity. On the other hand, empirical results have shown that linear detectors have low complexity but offer inferior performance. In this paper, we analytically quantify the diversity orders of linear equalizers for linear precoded OFDM systems, and prove that they are unable to collect full diversity. To improve the performance of linear equalizers, we further propose to use a lattice reduction (LR) technique to help collect diversity. The LR-aided linear equalizers are shown to achieve maximum diversity order (i.e., the one collected by the ML detector), but with low complexity that is comparable to that of conventional linear equalizers. The theoretical findings are corroborated by simulation results.

Range improvement of UWB systems using adaptive multicarrier spread‐spectrum and MIMO techniques

AbstractIn this paper, we investigate the use of multiple‐input multiple‐output (MIMO) techniques with a linear precoded orthogonal frequency division multiplexing (LP‐OFDM) waveform, known as spread‐spectrum multicarrier multiple access (SS‐MC‐MA), for high data rate ultra‐wideband (UWB) systems. This scheme is an evolution of the well‐known multiband OFDM (MB‐OFDM) solution supported by the WiMedia Alliance. The aim of this paper is to improve the UWB system range while not significantly increasing the system complexity compared to the WiMedia solution. Firstly, an analytical study of the single‐input single‐output (SISO) LP‐OFDM waveform allows to find how to efficiently precode an OFDM signal in order to improve the system range. To go further and exploit the available spatial diversity, a MIMO component is added and a new geometric statistic MIMO channel model is considered. Secondly, a global MIMO LP‐OFDM system study is led to highlight the benefits of combining MIMO and LP techniques for UWB applications. The analytical and simulation results show that the joint use of MIMO and LP‐OFDM leads to a significant range improvement compared to the WiMedia solution. Copyright © 2008 John Wiley & Sons, Ltd.

Optimization of Linear Precoded OFDM for High-Data-Rate UWB Systems

We investigate the use of a linear precoded orthogonal frequency division multiplexing (LP-OFDM) waveform for high-data-rate ultra-wideband (UWB) systems. This waveform applied for the first time to UWB applications is an evolution of the multiband OFDM (MB-OFDM) solution supported by WiMedia-MBOA (MultiBand OFDM Alliance). The aim of this paper is twofold. Firstly, an analytical study of the LP-OFDM waveform allows to find how to efficiently precode an OFDM signal in order to improve the robustness of the system. Secondly, a global system study is led to highlight the benefits of adding a precoding function to an OFDM signal in the UWB context. Different system choices and parameterization strategies are thus proposed. In both analytical and global system studies, the LP component is optimized without channel state information (CSI) at the transmitter side, as in the MBOA solution. To go further, the MBOA constraints are relaxed, and an additional optimization is performed, with a CSI at the transmitter. The analytical and simulation results show that the joint use of linear precoding and OFDM leads to a significant performance increase compared to the MBOA solution. This improvement is due to the precoding scheme that provides better exploitation of the channel diversity.

Power line enhanced cooperative wireless communications

In this paper, we investigate the use of power line communication (PLC) to assist cooperative wireless relaying. We consider a communication scheme that uses the power line to initialize and synchronize wireless amplify-and-forward relays and to broadcast information between the relays. Starting from an analysis of transfer functions and noise measurements of PLC channels in office and residential environments, we propose a power line transmission scheme for the inter-relay-communication and assess the influence of this scheme on wireless relaying. This scheme is based on linear precoded orthogonal frequency-division multiplexing; it is designed to optimally exploit the frequency diversity available on PLC channels. The use of PLC leads to a very flexible way of enhancing wireless communications by plugging in additional relays where they are needed-without additional wiring.