COFDM System Research Articles

An improved channel estimation for underwater acoustic OFDM and COFDM systems

An improved channel estimation for underwater acoustic OFDM and COFDM systems

An improved channel estimation for underwater acoustic OFDM and COFDM systems

Recently, there has been a tremendous increase in research and development of underwater communication systems. Orthogonal frequency division multiplexing has been widely employed for underwater acoustic communication because of its numerous advantages. Coding as well as interleaving techniques can be used along with orthogonal frequency division multiplexing for improving the error correction capability of the receiver. For demodulating the signal with fairly acceptable accuracies, the channel parameters need to be estimated at the receiver. Channel estimation can be performed by inserting pilots into the sub-carriers of an OFDM symbol. Underwater channels exhibit sparse channel impulse response and hence, the channel can be estimated using the compressive sensing techniques for properly reconstructing the received signals. Dictionary-based sparse channel estimation is presented in this paper which outperforms the channel estimation by LS and MMSE estimations. The results have been compared for various environmental conditions as well as for various numbers of pilots. It has been observed that the number of pilots is to be chosen as a trade-off between channel estimation accuracy and bandwidth efficiency.

Performance Analysis of RSCC Based WiMax System for Different SUI Channel Conditions Using QPSK and QAM-16

In recent times, new advanced wireless systems are growing at fast rate to meet high speed applications. To meet these demands, 4G technologies are being used nowadays. Broadband wireless access (BWA) is the outcome in this direction which promises to cater these high speed and high quality applications. WiMax is an IEEE 802.16 standard-based BWA technology which employs Coded orthogonal frequency division multiplexing access (COFDM). This paper analyses bit error rate for WiMax based COFDM system under Standard University Interim channel conditions.

The Simulation of RS Codes and QC-LDPC Codes Concatenated in COFDM System

In OFDM system,error correction coding is essential to ensure the effective and reliable transmission in channels of various types of data. This paper mainly focus on cascading coding of RS codes, interweaving and QC-LDPC codes,using this encoding with OFDM system to construct new COFDM system. The paper also research about the structure of COFDM system and the performance of the concatenated code.Using Matlab to simulate the result from several aspects such as encoding, length of code, bit rate, decoding mode and different channels.The result has shown the promotion of the new concatenated coding in COFDM system compared with the traditional coding system.

Novel OFDM Based on C-Transform for Improving Multipath Transmission

In this paper, a new orthogonal-frequency-division multiplexing using the C-transform (C-OFDM) is introduced. An exact closed-form bit-error rate (BER) is derived for the proposed C-OFDM system over multipath channels and different kinds of modulation formats. The BER performance of the proposed C-OFDM is evaluated by mathematics and simulation for different channel models, modulation formats, under zero-padding (ZP) and minimum mean-square-error (MMSE) detection. The results are then compared with those of the discrete cosine transform (DCT) and discrete Fourier transform (DFT)-based OFDM, showing that over multipath channel, the new C-OFDM has better BER performance than both the DCT-OFDM and the DFT-OFDM. The proposed scheme is also found to achieve some reduction in the peak-to-average power ratio (PAPR) in comparison with the aforementioned OFDM schemes as the block-diagonal-structure (BDS) property of the C-transform minimizes the input signal superposition. The proposed system has the merits of being resilience to multipath channel dispersion and relatively has lower PAPR.

Analyzing the Performance of Coded OFDM based WiMax System with different Fading Conditions

The demands for high speed applications are growing at a faster rate. To meet theserequirements, 4G technologies are being used nowadays. COFDM based WiMax is the outcome in this direction which promises to cater these high speed and high quality applications. Worldwide Interoperability for Microwave Access (WiMax) is an IEEE 802.16 standard-based broadband wireless access (BWA) technology which employsCoded orthogonal frequency division multiplexing access (COFDM). This paper analyses Bit Error Rate for WiMax based COFDM system with QPSK modulation scheme under various channel conditions like AWGN, Rayleigh, Rician and Nakagami-m. It has been observed that performance of Nakagami fading channel is better than other fading channels.

Application of the Joint Fast Walsh Hadamard-Inverse Fast Fourier Transform in a LDPC Coded OFDM Wireless Communication System: Performance and Complexity Analysis

An efficient low complexity T transform which combines the Walsh Hadamard Transform and Discrete Fourier Transform (DFT) into a single fast orthonormal unitary transform is considered for the application in low density parity check coded orthogonal frequency division multiplexing (LDPC-OFDM) across additive white Gaussian noise channel model and multipath fading channel models. The T-transform is developed through the sparse matrices factorization method using Kronecker product scheme. T-transform based COFDM system, which is capable of reducing peak to average power ratio (PAPR) of the transmitted symbols and improving the bit error rate (BER) performance at a reasonable reduced complexity. Several signaling formats such as Quadrature phase shift keying and 16- ary Quadrature amplitude modulation (16-QAM) are considered. A T COFDM system is described which could provide an alternative to DFT-COFDM system, and is therefore a better alternative to balance the transform complexity, PAPR reduction and system performance. Simulation results are used to examine and compare the complexity, PAR and the BER performance of T-OFDM system and DFT-OFDM system. Numerical results show that the T-COFDM system outperforms the DFT-COFDM based in the multipath channel models.

Symbol Synchronization Technique in COFDM Systems

In this paper we focus our research on the symbol timing synchronization technique in COFDM systems. A new method utilizing pilots to do coarse symbol timing is proposed. It overcomes the problem of fluctuation of the estimated symbol start position with cyclic prefix correlation method. The symbol timing error with the proposed method is within only /spl plusmn/10 samples. Different from previous algorithms in , we utilize the known pilot information to estimate the residual symbol timing offset with low system complexity. This paper also proposes a new control model for the sampling clock adjustment, different from the phase-locked loop (PLL), or delay-locked loop (DLL) method. The simulation and correspondent Field Programmable Gate Array (FPGA) circuit through test in HDTV prototype in Team of Engineering Expert Group (TEEG) proves its feasibility and availability. The proposed method is also suitable for burst mode transmission systems such as Wireless Local Area Network (WLAN) and Fixed-Broadband Wireless Access (F-BWA).

A soft decision decoding scheme for wireless COFDM with application to DVB-T

This paper investigates the soft decision decoding techniques for wireless COFDM systems. A new method to estimate the channel state information (CSI) for the Viterbi decoder is proposed, which overcomes the problem of reliability metric of transmitted signals in wireless as well as mobile channels. The CSI for each subcarrier is calculated as a properly defined squared Euclidean distance. On the basis of which, a soft decision decoding scheme to improve reception performance under multipath fading interference and co-channel interference (CCI) from existing analog broadcasting signals is also proposed. The simulation results show that the scheme can provide a compatible improvement to the terrestrial digital video broadcasting (DVB-T) system. The proposed scheme is also suitable for other wireless COFDM transmission systems.

Optimal antenna diversity combining for IEEE 802.11a system

IEEE 802.11a/g devices promise to accommodate the increasing consumer demands for high data rate wireless communications. A novel optimal decoding method for receiver antenna diversity combining for a COFDM system is discussed in this paper to provide robust wireless connections for applications such as wireless multimedia communications. The proposed optimal diversity combining integrates the channel equalization, demodulation (bit metrics calculation) and diversity combining in the decoder, which is the maximum likelihood decoding method for the diversity combining for the COFDM system. The proposed method can have about 4-6 dB gain over a single antenna receiver and 2-4 dB gain over a traditional symbol level diversity combining receiver without increasing the implementation complexity.

Comments on: "Reduction of peak-to-average power ratio of OFDM system using a companding technique"

The authors comment on the work of Xianbin Wang, Tjhung, and and Ng (see ibid., vol.45, no.3, 1999). They add that Wang et al. present a very elegant analysis of the symbol error rate in a companded COFDM system. A comment that they make is that it seems that the noise power was constant, while the transmitted signal's power was increased as it was companded. It would in our opinion be more meaningful to compare the performance of the companded signal to that of an uncompanded signal of equal power.

Adaptive bit loading scheme using COFDM at lowelevation overLEO satellite communication channel

Much interest has been shown in employing LEO satellite communication systems. A novel algorithm is proposed for adaptive bit loading (multilevel modulation assignment) between COFDM subcarriers in a frequency selective fading channel. Two-dimensional alignment has been carried out. The system performance is greatly improved at low elevation angles compared to that using fixed bit loading. Furthermore, the comparison shows that this adaptive bit loading COFDM system is suitable for frequency selective slow fading channels, which have time-varying deep nulls in the frequency response.

Performance analysis of a COFDM/FM in-band digital audio broadcasting system

In this paper, we present a performance analysis of a coded orthogonal frequency division multiplexing DAB system that is power multiplexed over a commercial FM station. We model the mobile channel with a time-frequency scattering function. Results show that the performance of the FM demodulator is crucial, since the level of the residual FM signal has a strong influence on the BER performance of the COFDM system. As with other mobile communication systems, the use of interleaving greatly enhances the performances.

A fast single-chip implementation of 8192 complex point FFT

Large-scale single-frequency networks are now being considered in Europe as very promising network topologies to achieve drastic savings in spectrum usage for digital terrestrial television transmission. Such networks are possible using the COFDM system, with large guard intervals (more than 200 /spl mu/s) to absorb long echoes. In order to limit the spectral efficiency loss to about 20%, very long size fast Fourier transforms (up to 8 K complex points) have to be performed in real time for the demodulation of every COFDM symbol (every 1 ms). This paper presents the first VLSI single chip dedicated to the computation of direct or inverse fast Fourier transforms of up to 8192 complex points. Due to its pipelined architecture, it can perform an 8 K FFT every 400 /spl mu/s and a 1 K FFT every 50 /spl mu/s. All the storage is onchip, so that no external memories are required. A new internal result scaling technique, called convergent block floating point, has been introduced in order to minimize the required storage for a given quantization noise, The chip, 1 cm/sup 2/ large with 1.5 million transistors, has been designed in a 3.3 V-0.5 /spl mu/m triple-level metal CMOS process and is fully functional. The 8 K complex FFT function could therefore be introduced in the coming years in digital terrestrial TV receivers at low cost.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>