Yingming Tsai

Wideband Spectrum Sensing Based on Sub-Nyquist Sampling

In this paper, we consider the problem of locating multiple active spectrum subbands in a wide range of frequency bands. A major challenge associated with such wideband spectrum sensing is that it is either infeasible or too expensive to perform Nyquist sampling on the wideband signal. In this paper, we propose a sensing scheme based on a sub-Nyquist sampling method called multicoset sampling, which is similar to the polyphase implementation of the Nyquist sampling, but requires less A/D converters. In contrast to the traditional sub-Nquist approaches where the wideband signal is first reconstructed from the sub-Nyquist samples, we develop a method that directly estimates the power spectrum of the wideband signal of interest using the sub-Nyquist samples, by exploiting its statistical properties. We also characterize the statistical distribution of the proposed power spectrum estimator, based on which we obtain a constant-false-alarm energy detector for the frequency bins. Simulation results are provided to demonstrate the effectiveness of the proposed multiband spectrum sensing method based on sub-Nyquist sampling.

Blind Frequency-Dependent I/Q Imbalance Compensation for Direct-Conversion Receivers

Frequency-dependent I/Q imbalance is one of the major impairments in the direct-conversion receivers (DCR) for high-speed wideband wireless systems. We propose two new blind methods for compensating frequency-dependent I/Q imbalance. The first one is a time-domain approach. Specifically we develop a blind identifiability condition based on which a cost function and a gradient descent search algorithm are proposed for blind I/Q imbalance compensation. The second blind method is a frequency-domain approach for OFDM systems. Here we provide blind estimators for the frequency-selective I/Q imbalance parameters, which once obtained, the I/Q imbalance can then be compensated by a simple single-tap matrix filter inversion. We provide extensive simulation results to demonstrate the performance of the proposed algorithms.

Polyphase codes for uplink OFDM-CDMA systems

We propose a OFDM-CDMA system that employs polyphase codes to support variable spreading factors. A systematic approach for constructing the polyphase code sequences of variable spreading factors is developed. Polyphase codes exhibit better auto- and cross-correlation properties than Hadamard codes. When employed in OFDM-CDMA systems, polyphase codes result in certain structured multiple-access interference (MAI) caused by multipath. Analytical and numerical results show that OFDM-CDMA systems employing polyphase codes have better PAPR performance than those using Hadamard codes. The BER performance of the OFDM-CDMA system using polyphase codes is evaluated by numerical results and compared to that of the OFDM-CDMA system using Hadamard codes with and without clipping.

Blind Estimation and Compensation of Frequency-Flat I/Q Imbalance Using Cyclostationarity

I/Q imbalance is one of the major concerns in the design of direct-conversion front-end receivers in high data rate wireless networks. To address the challenge, various I/Q imbalance estimation and compensation algorithms have been proposed in the literature. In this paper, we propose a blind cyclostationary based estimation and compensation of frequency-flat I/Q imbalance. The proposed blind estimation algorithm uses second-order statistics to compensate I/Q imbalance without estimating the mismatch parameters and is an unbiased estimator when a DC offset exists at the receiver. The performance of our approach is evaluated and compared to other existing blind I/Q imbalance estimation algorithms.

Successive Maximum Likelihood Estimation for Time and Frequency Synchronization

A new repetitive synchronization signal structure is proposed for wireless communication systems. A corresponding successive maximum likelihood detection algorithm that uses the proposed synchronization signals is provided. The successive maximum likelihood detection algorithm uses reliably estimated frequency offset in a successive way to refine the estimated timing offset. The performance of our approach is evaluated and compared to other existing algorithms.

Variable spreading factor orthogonal polyphase codes for constant envelope OFDM-CDMA system

Novel orthogonal polyphase codes with constant envelope design for OFDM-CDMA system are proposed in the paper. The proposed codes can support large number of users and variable spreading factor like Hadamard codes. Proposed orthogonal polyphase codes not only have better periodic auto- and cross correlations than Hadamard codes but also have constant envelope properties which is preferred for uplink/reverse link transmission. New transmitter and receiver architecture and MMSE receiver are proposed for the OFDM-CDMA system using the proposed polyphase codes. Simulation results show that OFDM-CDMA system using proposed polyphase codes has better PAPR and BER performance than OFDM-CDMA system using Hadamard codes and OFDMA system