Jen-Wei Liang

Two stage CCI/ISI reduction with space-time processing in TDMA cellular networks

We propose a two-stage, separate cochannel interference (CCI) and intersymbol interference (ISI) reduction technique using space-time processing. CCI and ISI are two major interference sources in wireless communication, and they are the limiting factors in the performance and capacity of a cellular network. However because of the different characteristics between CCI and ISI, algorithms designed to simultaneously reduce both interferences tend to combat ISI more and ignore CCI, which causes performance degradation of the equalizer. Further more, MLSE-type algorithms can effectively reduce ISI while MMSE-type algorithms are more robust to CCI. These considerations motivate us to derive two-stage interference reduction: the first stage uses a space-time MMSE equalizer to reduce CCI while preserving the ISI structure of the desired signal; the second stage uses a space-time Viterbi equalizer to reduce ISI. Space-time processing assures spatial and temporal diversities are fully captured and the difference of spatial and temporal properties between signals and interferences are fully explored. Simulation results show the proposed approach leads to good performance in a GSM scenario even at low carrier-to-interference ratio.

A two-stage hybrid approach for CCI/ISI reduction with space-time processing

We present a hybrid approach for separate cochannel interference (CCI) reduction and intersymbol interference (ISI) equalization in a slow Rayleigh fading channel. In this hybrid approach, a space-time filter is designed to maximize signal-to-interference-plus-noise ratio (SINR) by jointly optimizing the weight vector and the modified channel vector. A Viterbi equalizer then follows to equalize ISI and demodulate data symbols without noise enhancement. We derive an eigenvector solution for the joint optimization of the weight vector and the modified channel vector. Simulation results show good performance even at low carrier-to-interference-ratio (CIR).

Forward link antenna diversity using feedback for indoor communication systems

An approach to mitigate forward link signal fading in a FDD communication system by using an adaptive transmit antenna array and feedback on the reverse link is presented. In a personal communication system environment, multipath propagations can lead to severe space selective fading. Cordless phones and similar devices which cannot conveniently provide multiple antennas at the receiver can suffer from long term fading and therefore have unacceptable quality. Using multiple adaptive transmit antennas, we can adjust the transmission weights to ensure the user is kept out of deep fades. This is achieved by using feedback of the received signal level on the reverse link and adapting the transmission weights. Simulation shows that significant gain against the fading characteristics can be achieved.

Multi-channel MLSE equalizer with parametric FIR channel identification

We propose a parametric finite impulse response (FIR) channel identification algorithm, apply the algorithm to a multichannel maximum likelihood sequential estimation (MLSE) equalizer using multiple antennas, and investigate the improvement in the overall bit error rate (BER) performance. By exploring the structure of the specular multipath channels, we are able to reduce the number of channel parameters to provide a better channel estimate for the MLSE equalizer. The analytic BER lower bounds of the proposed algorithm as well as those of several other conventional MLSE algorithms in the specular multipath Rayleigh-fading channels are derived. In the derivation, we consider the channel mismatch caused by the additive Gaussian noise and the finite-length channel approximation error. A handy-to-use simplified BER lower bound is also derived. Simulation results that illustrate the BER performance of the proposed algorithm in the global system for mobile communications (GSM) system are presented and compared to the analytic lower bounds.

GMSK linearization and structured channel estimate for GSM signals

GMSK is a spectrum-efficient modulation scheme, and it is adopted as the modulation standard of GSM systems. However, because of its phase modulation, Gaussian filtering, and partial response signaling properties, GMSK is not a linear modulation. We present a linear approximation of GMSK signals at T/2 fractional spacing. The partial response channel with a memory length of 3 symbol periods, which is the pulse-shaping function inherent in the GMSK modulation, can be clearly identified in this linear approximation. Further more, we use this partial response channel information in the estimation of the wireless channel, which includes the physical propagation channel and the modulation pulse-shaping function. Conventional channel estimators, e.g., correlator or least-squares estimator approaches, do not exploit the information of the pulse-shaping function which is known and available. Our proposed structured channel estimate is more accurate since we incorporate the knowledge of the channel structure in the estimation procedure. We compared the BER (bit-error-probability) performance of a GSM system with the structured and unstructured channel estimate, and simulation results show the improvement of a Viterbi equalizer in different test channels when a structured channel estimate is used.

On optimizing base station antenna array topology for coverage extension in cellular radio networks

Use of higher frequencies (1.8 GHz) for the US upper tier PCS cellular service and the FCC regulations on the network build out have resulted in significant interest in improving coverage of cellular networks. Networks whose coverage is limited imply that thermal noise is the limiting factor. Also, since the forward link (base station to mobile) has higher power than the reverse link, cell coverage is usually limited by the reverse link. This coverage can be extended by improving the reverse link budget. Use of receive antenna arrays for boosting array gain on the reverse link is therefore of great interest. When receive antenna arrays are used at the base station, several conflicting choices affect system performance and cost. Some of these aspects are: the number of antenna elements (and channels) improves coverage but also increases system cost; the maximum span of the array increases diversity hut must be limited for convenient deployment on a tower; large inter-element spacing can increase diversity but cause grating lobes at the same time. These conflicting requirements mean that a careful design of the array topology can minimize the cost. In this paper, we study performance of linear and circular base station antenna arrays with different topologies, angle spread, and the number of elements. We compare alternate topologies using maximal ratio combining for narrowband systems such as AMPS and IS-54.

Joint MLSE receiver with dynamic channel description

In this paper, we address the problem of receiving a digitally modulated signal in the presence of another identically modulated cochannel interfering (CCI) signal. We propose a joint MLSE algorithm that uses a dynamic channel description (J-MLSE/DCD). This algorithm deals with CCI more effectively while keeping the overall complexity low. In the proposed algorithm, we adaptively truncate the channels of both the desired signal and the CCI according to their individual power, thereby greatly reducing the complexity. We also describe the channel by using only a small number of channel parameters to reduce the channel mismatch before the Viterbi algorithm is applied. Analytic bit-error rate (BER) lower bounds for the J-MLSE algorithms in the time-varying specular multipath Rayleigh-fading channels are derived. Simulation results of the GMSK modulated signals used in the GSM system with path delay spread up to three symbols are presented. We find that the proposed algorithm performs significantly better than the spatial-whitened MLSE and is comparable to the high-complexity joint MLSE.

Low-complexity joint MLSE receiver in the presence of CCI

We address the problem of receiving a digitally modulated signal in the presence of another identically modulated cochannel interference (CCI) signal. We propose a joint maximum-likelihood sequential estimation (MLSE) algorithm that uses a dynamic channel description (J-MLSE/DCD). This algorithm deals with CCI more effectively while keeping the overall complexity low. In the proposed algorithm, we adaptively truncate the channels of both the desired signal and the CCI according to their individual power, thereby greatly reducing the complexity. Simulation results for the GMSK modulated signals used in the Global System for Mobile Communications (GSM) system are presented. The proposed algorithm performs much better than the spatial-whitened MLSE and is comparable to the high-complexity joint MLSE.

Phase-shifting masks: automated design and mask requirements

In this paper we present a computationally viable algorithm for the rapid design of phase- shifting masks for arbitrary two-dimensional patterns. Our approach is based on the construction of a class of optimal coherent approximations to partially coherent imaging systems described by the Hopkins model. We show that for partially coherent imaging systems with coherence factor (sigma) <EQ 0.5, the associated approximation error in the image is quite small (< 10%). A fast iterative algorithm is used to generate (suboptimal) phase- shifting masks using the approximate imaging system model. The computational effort required per iteration is O(N log N), where N is the number of discrete image points considered. Analytical results related to practical requirements for phase-shifting masks are also presented. These results address questions related to the number of discrete phase levels required for arbitrary patterns, and provide some insight into alternative phase-shifting strategies. A number of phase-shifting mask design examples are also discussed.

A space-time-filtered Viterbi receiver for CCI/ISI reduction in TDMA systems

In this paper, we present a hybrid space-time-filtered Viterbi receiver using multiple antennas for co-channel interference (CCI) reduction and intersymbol interference (ISI) equalization in a slow Rayleigh fading channel. In this approach, a space-time filter is first applied at the antenna outputs to maximize the signal-to-interference-plus-noise ratio (SINR), and the scalar output is then sent to a Viterbi equalizer. We propose a closed-form solution to jointly determine the weight vector for the space-time filter and the channel vector for the Viterbi equalizer. We also examine the need for a whitening filter prior to the Viterbi equalizer and show that it only marginally improves the performance. Simulation results are provided to validate our approach and to compare the performance of our receiver with that of different existing receivers.