N-tone sigma-delta UWB-OFDM transmitter and receiver
ABSTRACT A new method for generating and detecting the UWB-OFDM signal using a modified sigma-delta modulator is proposed. Unlike narrowband OFDM, the UWB-OFDM spectrum can have gaps between subcarriers. The modified sigma-delta modulator, dubbed N-Tone sigma-delta, introduces N zeros at the frequencies in the quantization noise spectrum. These zeros match the locations of frequencies used by the OFDM system and the quantization noise spectrum fills the gaps in the spectrum of the UWB-OFDM signal. In fact this new structure could be used in other UWB systems anytime we have gaps in the spectrum of the transmitted signal. We describe both the transmitter and receiver structures for UWB-OFDM. We also study the spectrum of the underlying system.
Conference Paper: Generating UWB-OFDM signal using sigma-delta modulator[Show abstract] [Hide abstract]
ABSTRACT: In this paper we study the implementation issues of ultrawideband orthogonal frequency division multiplexing (UWB-OFDM) communication systems. Like narrowband OFDM, it is desirable to accomplish all modulation and demodulation process digitally in the base band. Designing such a transmitter and receiver for UWB-OFDM signal requires very fast and high-resolution digital-to-analog (D/A) and analog-to-digital (A/D) converters that operate on a very large frequency band. A modified version of sigma-delta modulator, which we call an N-tone sigma-delta, can be used for this purpose. This new structure introduces N zeros at N properly selected frequencies in the quantization noise spectrum and can be used anytime there are gaps in the spectrum of the transmitted signal. A digital transmitter and receiver for UWB-OFDM signal is proposed using this structure and its performance is studied in multipath fading channel.Vehicular Technology Conference, 2003. VTC 2003-Spring. The 57th IEEE Semiannual; 05/2003
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ABSTRACT: Historically, the Gaussian assumption for noise and interferences has dominated signal processing. The justification for this assumption is usually given by the well known central limit theorem which allows an elegant, manageable, and meaningful mathematical description of noise. Nevertheless, some communication scenarios are characterized by impulsive properties on behalf of Gaussian noise, so they are non-Gaussian distributed. Channels which are corrupted with impulsive noise can be found in several domains like for example powerline communications, ethernet traffic, underwater signal processing, or as proven in this project also the multiuser interferences of ultra wideband (UWB) communication systems. After describing and discussing impulsive noise models in detail, a comparison of their distributions has been carried out showing the suitability of the different models for solving selected problems. For example, it is well known that the channel capacity with additive impulsive noise channels is always greater than the one of additive white Gaussian noise (AWGN) channels supposed that the signal to noise ratio (SNR) remains unchanged. In this project the capacity of channels corrupted by impulsive noise has been investigated. An example shows the negative effect of the widely used hard limiter (for impulsive noise clipping) and suggests the use of an alternative one called soft limiter allowing no loss of information. Furthermore, UWB systems have been investigated regarding multiuser interferences. It is proven that the multiuser interferences of impulse radio systems are impulsive. Their probability density function is determined analytically and confirmed by simulation results. Taking this result into account a new receiver structure has been developed allowing the suppression of impulsive noise without use of a limiter. The performance of this receiver is proven by simulation.
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ABSTRACT: Orthogonal frequency division multiplexing (OFDM) has been proposed for use as the physical layer of ultra-wideband (UWB) systems for high-rate, short-range personal area networking (PAN). For ultra-wideband systems, there is a constraint on the maximum power spectral density for the transmitted signal. Therefore, the bandwidth of the transmitted spectrum must be spread widely by a bandwidth expansion scheme so that the transmitted power spectral density can be kept as low as possible. In this paper, frequency expansion of the UWB system is achieved by using a simple frequency-diversity coding scheme. A major issue for the frequency-diversity coding scheme is that the receiver must sample the baseband received signal using high-sampling-rate analog-to-digital converters (ADCs) for discrete signal processing (DSP). However, such high-sampling-rate ADCs and DSP are expensive and have high power consumption. One advantage of the proposed frequency-diversity coding scheme is that the sampling rate of the baseband ADCs and DSP can be less then the Nyquist rate. The aliasing phenomenon occurs due to the reduced sampling rate, yet it, however, appears as transmission diversity to the receiver. The performance of the frequency-diversity coded OFDM system with an under-sampling-rate receiver is analyzed by evaluating the pairwise error probability. From the analysis of the pairwise error probability, design criteria for the frequency-diversity coded OFDM are obtained. A practical construction of frequency-diversity codes is proposed based on linear block codes. Simulation and analytical results for frequency-diversity coded OFDM systems are presented. The results show that a significant diversity/coding gain can be achieved with the under-sampling-rate receiver.Communications, 2005. ICC 2005. 2005 IEEE International Conference on; 06/2005