Richard E. Cagley

GEN05-4: Carrier Offset and Channel Estimation for Cooperative MIMO Sensor Networks

A cooperative MIMO network is considered with Ns sensors and a collector node with Mc antennas. In a practical implementation of this network, the sensor carriers have relative frequency offsets which must be estimated along with the MIMO channel. Generalized successive interference cancellation (GSIC) is proposed for this joint estimation problem. The primary operations in GSIC are correlation, FFT and cancellation. A reconfigurable hardware (FPGA) implementation of these GSIC primitives is described. A hybrid analysis/simulation for bit error rate (BER) is presented with results for GSIC using Alamouti and G4 c codes.

A multistage interference rejection system for GPS

The Global Positioning System (GPS) is a one-way satellite communication system that is widely used for both civilian and military applications. Because of the relatively low power of the received signals, GPS is highly susceptible to a variety of intentional and unintentional interferers. In this paper we examine a multistage space-time adaptive-processor (STAP) that is used to improve the detection capability of a GPS receiver for a wide range of jamming scenarios. Depending on the type of interference, the stages incorporate structures such as a notch filter, a beamformer and a blind linear interference canceler. Computer simulations are provided to illustrate the performance of the multistage receiver for wideband and narrowband interferers.

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.

Channel Estimation and Carrier Offset Control for Cooperative MIMO Sensor Networks

A sensor network is considered with a multiple antenna element collector node and single element sensor nodes. The sensors form a cooperative MIMO network employing eigencoding on the uplink with variable-rate transmission. The individual sensors have differing frequency offsets which must be estimated at the collector along with the flat-fading channel and propagation delays. Offset and timing control commands are transmitted on the downlink with the eigencode and water-filling powers. A generalized successive interference cancellation (GSIC) algorithm for collector-based channel/offset/delay estimation is presented with quadratic complexity in the number of sensors. Analytical and simulation results demonstrating the efficacy of GSIC and performance degradation due to carrier offset/channel/timing estimation errors are given

Performance of ML rate determination for an IS-95 downlink SIC receiver

Maximum likelihood (ML) decision rules, such as that presented by Gutierrez, Lee and Mandyam (see. Proc. IEEE 49th Vehicular Technology Conf., Houston, TX, p.417-21, 1999), can be used for rate determination in an IS-95 system to reduce the complexity of the Viterbi decoder. We extend this previous work by determining whether it is beneficial to use rate determination for the successive interference canceller (SIC) in an IS-95 downlink receiver. Assuming certain conditions are met regarding the signal-to-noise ratio (SNR) and the number of received symbols used for rate determination, employing knowledge of bit repetition can decrease the bit error rare (BER) for the corresponding Walsh channels. To further increase the effectiveness of interference cancellation, soft decision decoding can be used instead of a conventional hard decision device. The devices considered here are similar to those of Frey and Reinhardt (see Proc. IEEE 47th Vehicular Technology Conf., Phoenix, AZ, p.155-159, 1997), but they use the knowledge that there can be repeated bits in the IS-95 downlink.

SINR enhancement in an IS-95 downlink receiver

We examine the performance of methods that increase the accuracy of demodulating and reconstructing a strong base station signal such that it can be canceled prior to demodulating weaker base station signals. This successive interference cancellation (SIC) process is used to increase the signal-to-interference-plus-noise ratio (SINR) of a weaker base station being demodulated above the threshold needed to achieve a desired bit error rate. However the parameters of the stronger base station must be estimated with enough accuracy for successful signal reconstruction, otherwise, the SINR may not be sufficiently enhanced. We provide an analysis that determines how much interference can be removed from the received signal, in the form of the stronger base station, given a specific SINR. The methods we address to increase the effective SINR include features unique to the downlink of Interim Standard 95 (IS-95), such as bit repetition and properties of the power control bits.

Performance evaluation of power control bit detection for an adaptive SIC receiver and the IS-95 downlink

In adaptive successive interference cancellation (ASIC), a code-division multiple-access (CDMA) receiver attempts to reconstruct and cancel strong base station signals for which the long spreading codes may not be known. One technique that can lead to more accurate base station signal reconstruction locates the power control bits (PCB) within a power control group (PCG) and scales the signal amplitude according to the rate being used for a particular Walsh channel. In this paper, we investigate the performance of this technique by determining under what conditions PCB detection provides improved signal cancellation. Based on an average interference criterion, the probability of correctly determining the PCB locations are derived for the downlink specified in Interim Standard 95 (IS-95). Computer simulations are presented to verify the analytical results.

Pilot signal cancellation for an IS-95 downlink SIC receiver

During the last several years, numerous multiuser detectors have been proposed for code-division multiple-access (CDMA) systems. The successive interference canceler (SIC) is one such detector that is relatively simple to implement. The basic principle behind the SIC is that a weaker user can be detected with improved performance when stronger users are successively detected, reconstructed, and canceled from the received signal. In this paper, we seek to increase the detection accuracy of the SIC for all users by performing pilot signal cancellation using a parallel interference canceler (PIC). This PIC is incorporated in the initial stage of the SIC, resulting in a hybrid interference canceler (HIC) architecture. Because the pilot signal data are known, the pilot signal can be accurately reconstructed for a wide range of channel conditions. Computer simulations are presented to demonstrate the advantages of the HIC architecture compared to a conventional SIC receiver.

Nonlinear Channel Estimation Issues in Cooperative Sensor MIMO

A cooperative MIMO system for range extension in sensor networks is considered. A local sensor group forms a consensus and seeks to transmit a common pool of data to a stand-off multi-element collector. Each sensor then transmits one column of an orthogonal space-time block code (OSTBC). The resulting increased effective power and diversity can yield substantial range increases for moderate numbers of sensors. The major problem is tracking the individual sensor frequency offsets, delays and sensor-to-collector channels under high mobility. The unscented Kalman filter (UKF) is presented as a state of the art solution to the cooperative MIMO channel estimation problem, and its performance is evaluated via a hybrid analysis/simulation of bit-error rate. A hardware implementation of the collector is also discussed based on simplified correlation and homodyne estimation strategies. The homodyne estimator performance is finally compared to that of previous generalized successive interference cancellation (GSIC) and correlation-based algorithms via simulation.

SINR Enhancement Techniques for an IS-95 Downlink SIC Receiver

Due to near-far effects and multiple-access interference, several types of multiuser detectors have been developed in recent years to reliably demodulate user signals in a code-division multiple-access (CDMA) system. The downlink of Interim Standard 95 (IS-95) is particularly well suited to one such detector known as the successive interference canceler (SIC). In order to keep the receiver complexity low, entire base station signals are typically canceled in the receiver in a sequential manner. However, for the SIC to operate effectively, a base station signal that is being canceled must be reconstructed with enough accuracy such that sufficient interference power is removed for the subsequent base station to be reliably detected. If this is not possible, it may be necessary to employ techniques, specific to the signal format specified in the IS-95 downlink, which enhance the received signal-to-interference-plus-noise ratio (SINR). In this paper, we explore the performance gains achieved with several SINR enhancement techniques, and present computer simulations to demonstrate this improvement for example cochannel signal scenarios.