Kuei Chiang Lai

Low-complexity adaptive tree search algorithm for MIMO detection

In this paper, a reduced-complexity multiple-input multiple-output (MIMO) detector for the spatial multiplexing systems is described. It incorporates the idea of Ungerboecks set partitioning in the tree search to reduce the computational complexity. An algorithm that adapts the number of partitions based on the channel conditions is proposed to achieve a better tradeoff between complexity and performance. Simulation results are presented to demonstrate the efficacy of the proposed algorithm under the assumptions of perfect channel and noise power estimates.

Steady-state analysis of the adaptive successive interference canceler for DS/CDMA signals

The adaptive successive interference canceler (ASIC) is a multistage receiver that sequentially detects and removes cochannel users from the received signal impinging on a single antenna element. Each stage of the ASIC consists of a conventional matched filter (MF) detector and an adaptive interference canceler (AIC) that employs the least-mean-square (LMS) algorithm to recursively estimate the received amplitude of the detected signal. In this paper, we investigate the performance of the ASIC using a Wiener model of convergence for the LMS algorithm, deriving expressions for the asymptotic mean and variance of the amplitude estimate and the steady-state bit error rate (BER). The analyses and computer simulations demonstrate that the performance of the ASIC exceeds that of the conventional SIC (CSIC), which utilizes the MF output as the received amplitude estimate.

A successive cancellation algorithm for fetal heart-rate estimation using an intrauterine ECG signal

In this paper, we present a two-stage successive cancellation (SC) algorithm that sequentially separates fetal and maternal heartbeats from an intrauterine electrocardiogram (IuECG) signal containing both fetal and maternal QRS complexes. The ECG signal is modeled as a series of fetal, maternal, and noise events. Peak detection is first employed to locate the potential fetal and maternal QRS complexes, referred to as candidate events. Each stage automatically generates a template of a source from the candidate events in the initialization period, and thereafter performs classification of the remaining candidate events based on a template matching technique. The detected events of the stronger signal are subtracted from the composite ECG signal prior to initialization and classification of the weaker signal. Once the fetal and maternal complexes are successfully detected and separated, a counting mechanism is utilized to derive the corresponding heart rates. Computer simulation results on real IuECG data demonstrate the effectiveness of the SC algorithm.

Variation of the Fixed-Complexity Sphere Decoder

The fixed-complexity sphere decoder is a near-optimum tree search detector for the spatial multiplexing scheme. The search tree is statically divided into a full expansion stage and a single expansion stage. To enable the power-saving design and to achieve a lower average complexity with little performance loss, we propose an algorithm that adapts, based on the channel conditions, the switching point between the two stages. The algorithm preserves the capability of parallel processing and pipelining in tree search, and yields predictable complexities.

Fusion of Decisions Transmitted Over Flat Fading Channels Via Maximizing the Deflection Coefficient

In this paper, we consider the design of the fusion rules for binary distributed detection systems in which the decisions made by the local sensors are sent to the fusion center (FC) over parallel flat-fading channels using on-off keying. For complexity concerns, we focus on the linear-combining fusion rules whose combining weights are chosen to maximize the deflection coefficient of the fusion statistic. Both coherent and noncoherent fusion rules are considered. The proposed fusion rules account for both the knowledge of the channels, which ranges from the instantaneous channel-state information (CSI) to the statistics of the fading envelopes, as well as the detection performance of local sensors. Simulation results show a robust detection performance over a variety of signal conditions.

Performance evaluation of a generalized linear SIC for DS/CDMA signals

The linear successive interference canceler (LSIC) is a multiuser detector that separates code-division multiple-access (CDMA) signals in a multistage manner. In each stage, a user is detected, and its contribution is regenerated and canceled from the input of that stage. A users spreading sequence is employed for despreading and respreading, and the magnitude of the despreader output is used as the amplitude estimate to reconstruct that users signal. This paper describes a generalized version of the LSIC (GLSIC) that employs various types of linear filters for the despreading and respreading operations. We analyze the bit error rate (BER), asymptotic multiuser efficiency (AME), and the mean and variance of the amplitude estimates of the GLSIC. It is shown that for certain choices of linear filters, complete cancellation of a user can be achieved, irrespective of the reliability of the symbol estimates. We also demonstrate that, from a BER and AME viewpoint, it is not beneficial to use a linear canceler with a decorrelator or a minimum mean-square-error (MMSE) receiver.

Analysis of the linear SIC for DS/CDMA signals with random spreading

The linear successive interference canceler (LSIC) is a multiuser detector that employs the magnitude of the matched filter (MF) output as the received amplitude estimate of the detected user for use in signal reconstruction. This paper investigates the performance of the LSIC when random spreading sequences are employed. Specifically, the conditional mean and the signal-to-interference-plus-noise ratio (SINR) of the decision variable in each stage are derived to quantify the effects due to imperfect symbol and amplitude estimates. In addition, under the constraint that each user must achieve a certain SINR requirement, we examine the received powers needed for each of a specified number of users and the maximum number of users that a system can support when the LSIC is used in a base station. Computer simulations are presented to compare these results with those of several linear multiuser detectors.

Comparative performance of adaptive receivers for demodulating IS-95 downlink data

The performance of adaptive receiver architectures is investigated for demodulating IS-95 downlink data. The implementations include an adaptive minimum mean-square-error (MMSE) equalizer followed by a despreader, and an adaptive successive interference canceler (SIC), which are compared to a conventional matched filter (MF). By concentrating on an actual signal model (IS-95) and multipath channels, we address technical issues that other structures might not emphasize for generic CDMA (code-division multiple-access) signals. For the SIC, we are concerned with estimating the multipath channel as well as the relative user amplitude levels in order to regenerate a portion of the received signal due to a particular base station. Significant performance gains can be achieved by accurately identifying and equalizing for a base station channel using the pilot channel of the IS-95 downlink signal.

Adaptive successive interference cancellation for the IS-95 uplink

In this paper we examine an adaptive successive interference canceler (ASIC) that can separate direct-sequence code-division-multiple-access (DS/CDMA) signals for the IS-95 (Interim Standard 95) uplink under near-far conditions, i.e., without power control. The ASIC employs a multistage architecture where each stage consists of a conventional matched filter (MF) detector with equal gain combining (EGC), followed by an adaptive interference canceler (AIC) that is adjusted by the least-mean-square (LMS) algorithm to track time-varying multipath channels. Variants of the basic ASIC architecture which exploit the specific structure of the IS-95 uplink traffic channel are also discussed. The bit-error-rate (BER) performance of the ASIC is compared with that of other interference cancelers, demonstrating the effectiveness of the ASIC for near-far scenarios.

A signal separation algorithm for fetal heart-rate estimation

In this paper we describe an adaptive algorithm for separating fetal and maternal heart beats from data containing both fetal and maternal QRS complexes. The algorithm classifies the combined heart-rate data as a series of fetal maternal, and noise events using a technique of template matching. Peak detection is first employed to locate the potential fetal and maternal QRS complexes (referred to as candidate events). Fetal and maternal templates are generated automatically from the candidate events in the initialization period, and are used to classify the remaining candidate events based on certain similarity criteria. Once the fetal and maternal complexes are successfully detected and separated, a counting mechanism can be utilized to derive the corresponding heart rates. Computer simulations using real data demonstrate the effectiveness of the algorithm.