The paper addresses a crucial point in ultra-wideband (UWB) radio wave propagation, which is the spatial-temporal resolution of scattering objects into multiple frequency-dependent scattering centers. The effect contributes to the widely observed temporal dispersion of pulse-shaped transmit signals and their distortion, respectively. Particularly the latter is explained by (multiple) diffraction of the incident wave, leading to (multiple) band-limited impulse responses with characteristic frequency content, which in turn causes signal distortion and a degradation of the signal-to-noise ratio in a correlation receiver. We presented a new approach on UWB propagation modeling and optimum design of correlation receivers.

A study of the ultra-wideband wireless propagation channel and optimum UWB receiver design

Multiple ray paths are resolved using high-resolution digital signal processing algorithms. The Cramer-Rao (CR) bound is used as a benchmark where a combination of the singular value decomposition method and the eigen-matrix pencil method is proven to be most successful. The conventional complex channel model for wireless propagation is extended to include the frequency-dependent feature of rays which can be used to classify the ray arrivals and provide physical insight of the channel. A novel complex-time model is used to approximate the suggested model. This approach is important to various applications such as equalizers, RAKE receivers, etc., in wireless communication systems. Five key features (noise immunity, robustness, resolution, accuracy, and physical insight) of the proposed algorithm are studied using numerical examples.

Multipath resolving with frequency dependence for wide-band wireless channel modeling

This paper has established a new theoretical framework that allows us to analyze the ultra-wideband system performance using the closed form formulations. A new generalized time-domain multipath channel based on the geometric theory of diffraction/uniform theory of diffraction (GTD/UTD) framework is incorporated into the system bit-error rate and signal-to-noise ratio by modifying the existing system model. Closed form expressions for important geometric configurations are derived in the time domain for the first time. Some interesting insights into the physical mechanisms and system analysis are observed through explicit expressions and numerical results.

A generalized time domain multipath channel and its application in ultra-wideband (UWB) wireless optimal receiver design-Part II: physics-based system analysis

A receive filter receives signals from a communication channel. The received signals correspond to original Walsh covered chip sequences transmitted by a transmit filter through the communication channel to the receive filter. The received signals are processed by an equalizer to generate a soft estimate of chip sequences corresponding to the original Walsh covered chip sequences. An N chip Walsh decover is then utilized to generate a soft estimate of code symbols corresponding to the soft estimate of the chip sequences. A number of symbol slicers are then used in parallel to produce a hard estimate of the code symbols corresponding to the soft estimate of code symbols generated by the N chip Walsh decover. Thereafter an N chip Walsh cover is used as part of a scheme to generate a hard estimate of chip sequences corresponding to the hard estimate of the code symbols generated by the symbol slicers. The hard estimate of the chip sequences generated with the aid of the N chip Walsh cover, and the soft estimate of the chip sequences generated by the equalizer, are used to generate a tracking mode error signal to adapt the response of the equalizer to the received signals.

Method and apparatus for adaptive linear equalization for walsh covered modulation

This paper deals with the time domain characteristics of UWB multipath channel while considering UWB pulse shape distortion and its impact on system performance as well as optimal receiver design. It appears that no deterministic and analytical channel model in the time domain has ever appeared in the literature, to our best knowledge. A generalized channel model and some related exact solutions for some specific geometric configurations are discussed, based on the pulse propagation mechanisms of reflection and diffraction. In addition, a system performance framework integrated with the close-form channel model expressions is also provided, corresponding to the optimal receiver structure and the concrete geometric configurations of a UWB channel. This work is very important in the design of the optimal UWB systems

Generalized time domain multipath channel and its application in ultra-wideband (UWB) wireless optimal receiver - part III: system performance analysis

The principal aim of this work is to estimate, or to approximate, the complex k-space spectrum of the wave field arriving on a linear array. First, using linear approximation, the location-dependent effect of the wave field magnitudes is modeled as an extra “loss” factor in the complex spectral variable. This complex spectrum model may provide a better description of the physical process and require less sensor elements than the real spectrum model because of the additional degree of freedom provided by the “loss” factor. A high-resolution algorithm combining the singular value decomposition method and the eigen-matrix pencil method is then employed to find the complex spectra representing the incoming real spectrum and the location dependent factors of multipath and multimode arrivals. Five key features (noise immunity, robustness, resolution, accuracy, and physical insight) of the proposed algorithm are studied using numerical examples.

A high-resolution algorithm for complex spectrum search

Blind adaptive space-time processing techniques are developed for both uplink reception and downlink transmission in multiuser TDD DS-CDMA systems to overcome multipath fading effects and multiple access interference. The system consists of a space-time array, a decorrelator, and a weight-adapter. For uplink reception, a hybrid combination of the constant modulus algorithm and decision-directed algorithm approach is employed for blind adaptation. For multi-input-multi-output channel estimation, minimum mean squared error estimation is made using re-spread sequences of the decision symbols as references. For downlink multiuser transmission, the generalized zero-forcing approach is developed for pre-equalization and pre-interference cancellation.

Blind adaptive space-time receiving and transmitting diversities for multiuser DS-CDMA systems

A novel blind 2D RAKE receiver for multiuser detection with interference rejection in DS-CDMA cellular communication systems is presented. The receiver consists of an array of receiving antennas and each antenna is connected to K filters where K is the number of users. The filter outputs are combined and despread to suppress both multiuser interference and intersymbol interference. The outputs are subsequently fed into a decorrelator to further suppress the multiuser interference. The complex weighting factors of the tap delays of these filters are adaptively determined by a hybrid combination of the constant modulus algorithm and the decision-directed algorithm. No training sequence is required, and beamforming and equalization are performed simultaneously. The blind adaptive scheme converges approximately after 100 symbol periods when the input SNR of the desired signal is 10 dB. A 30 dB reduction of the output mean square error or, equivalently, an 30 dB improvement of the output SINR is obtained even when the maximum difference of the received signal power is up to 9 dB.

Jaeyoung Kwak

Papers

A high-resolution algorithm for complex spectrum search

Blind adaptive space-time receiving and transmitting diversities for multiuser DS-CDMA systems

Adaptive blind 2D RAKE receiver for multiuser detection