Montree Budsabathon

Optimum beamforming for pre-FFT OFDM adaptive antenna array

It is well known that orthogonal frequency-division multiplexing (OFDM) is robust to frequency-selective fading in wireless channels due to the exploitation of a guard interval that is inserted at the beginning of each OFDM symbol. However, once delayed signals beyond the guard interval are introduced in a channel with a large delay spread, intersymbol interference causes a severe degradation in the transmission performance. In this paper, we propose a novel pre-fast Fourier transform (FFT) OFDM adaptive antenna array, which requires only one FFT processor at a receiver, for suppressing such delayed signals. We derive the optimum weight set for beamformers based on the maximum signal-to-noise-and-interference power ratio (Max-SNIR) and the minimum mean square error (mmse) criteria, respectively. In addition, we propose a novel mmse-criterion-based commutative optimization scheme, which is more robust to the estimation error of the channel state information. Furthermore, we show the equivalence between the Max-SNIR-criterion-based scheme and the proposed commutative optimization scheme. Computer simulation results show its good performance even in channels where directions of arrival of arriving waves are randomly determined.

Robustness of OFDM signal against temporally localized impulsive noise

We present the theoretical analysis of the bit error rate (BER) performance of single-carrier modulation (SCM) and orthogonal frequency division multiplexing (OFDM) systems under two types of temporally localized impulsive noise environments, namely stationary and bursty impulsive noise. The robustness of OFDM system against impulsive noise is then discussed and compared with that of the SCM system at the same transmission rate. In addition to the common knowledge on OFDM, we show that OFDM system is sometime less robust to impulsive noise than SCM system.

Spread Spectrum‐Based Subcarrier Recovery Method for Multi‐Carrier Code Division Multiplexing System

A time domain Spread Spectrum (SS)-based subcarrier recovery method is proposed for down-link Multi-Carrier Code Division Multiplexing (MC-CDM) system, which periodically inserts SS pilot signals into data signal stream in a time division manner at the transmitter. Instantaneous channel impulse response is first estimated by correlating the received pilot signal at the receiver, and then corresponding instantaneous frequency response essential for subcarrier recovery is calculated from the Discrete Fourier Transform (DFT) of the estimated impulse response. The SS pilot-assisted method is also applicable to Direct Sequence (DS)-CDM system as a means to estimate instantaneous channel impulse response for Rake combining, and taking the same approach in both MC-CDM and DS-CDM systems, there is no large difference between them in terms of transmission efficiency and receiver complexity. The proposed MC-CDM system must be a promising down-link multiplexing protocol, because it can achieve better bit error rate (BER) performance than the DS-CDM system with the same SS pilot-assisted Rake combiner in frequency selective fast Rayleigh fading channel.

A Pre-FFT OFDM Adaptive Array Antenna with Eigenvector Combining

SUMMARY This paper proposes a novel pre-FFT type OFDM adaptive array antenna called “Eigenvector Combining.” The eigenvector combining array antenna is a realization of a post-FFT type OFDM adaptive array antenna through a pre-FFT signal processing, so it can achieve excellent performance with less computational complexity and shorter training symbols. Numerical results demonstrate that the proposed eigenvector combining array antenna shows excellent bit error rate performance close to the lower bound just with 2 OFDM symbol-long training symbols.

When does H/sub /spl infin// equalizer outperform H/sub 2/ equalizer in multipath fading environments?

We present the BER performance for fractional-tap-spaced DFEs with adaptive a priori H/sub /spl infin// filtering algorithm and with H/sub 2/ (recursive least squares, RLS) filtering algorithm in various frequency selective fading channels. Computer simulation results demonstrate the better bit error rate (BER) performance of the DFE with the adaptive a priori H/sub /spl infin// filtering algorithm when the ratio of the average energy-per-bit to the white noise spectral density (E/sub b//N/sub 0/) and channel-fading rate (f/sub d/T/sub s/) is large. Therefore, it is concluded that the adaptive a priori H/sub /spl infin// filtering algorithm is more robust and less sensitive to modeling error and channel parameter variation but more sensitive to input white noise.