Nima Khajehnouri

Network-based wireless location: challenges faced in developing techniques for accurate wireless location information

Wireless location refers to the geographic coordinates of a mobile subscriber in cellular or wireless local area network (WLAN) environments. Wireless location finding has emerged as an essential public safety feature of cellular systems in response to an order issued by the Federal Communications Commission (FCC) in 1996. The FCC mandate aims to solve a serious public safety problem caused by the fact that, at present, a large proportion of all 911 calls originate from mobile phones, the location of which cannot be determined with the existing technology. However, many difficulties intrinsic to the wireless environment make meeting the FCC objective challenging. These challenges include channel fading, low signal-to-noise ratios (SNRs), multiuser interference, and multipath conditions. In addition to emergency services, there are many other applications for wireless location technology, including monitoring and tracking for security reasons, location sensitive billing, fraud protection, asset tracking, fleet management, intelligent transportation systems, mobile yellow pages, and even cellular system design and management. This article provides an overview of wireless location challenges and techniques with a special focus on network-based technologies and applications.

Distributed MMSE Relay Strategies for Wireless Sensor Networks

A fundamental task in a wireless sensor network is to broadcast some measured data from an origin sensor to a destination sensor. Since the sensors are typically small, power limited, and low cost, they are only able to broadcast low-power signals. As a result, the propagation loss from the origin to the destination nodes can attenuate the signals beyond detection. One way to deal with this problem is to pass the transmitted signal through relay nodes. In this paper we propose and study two-hop multisensor relay strategies that achieve minimum mean-square-error (MSE) performance subject to either local or global power constraints. The capacity of the resulting relay link and its diversity order are studied. The effect of channel uncertainties on system performance is examined and a modified relay scheme is proposed.

A non-line-of-sight equalization scheme for wireless cellular location

Among the challenging issues that affect the performance of wireless location techniques is the temporal and spatial variations of the channel, and the distribution of the scatterers, which introduce non-line-of sight errors at the base station. This paper develops a technique for addressing this problem by exploiting the topology of the cellular network.

Improved wireless location accuracy using antenna arrays and interference cancellation

Location finding for CDMA subscribers using an antenna array at the base station is studied. Two methods are considered for radio location that are based on time of arrival (TOA) and angle of arrival (AOA) measurements. For TOA estimates, a maximum likelihood algorithm and a practical structure are proposed when an antenna array is available. For AOA estimates, a least-squares (LS) algorithm is used. The LS module is combined with a multiple access interference (MAI) cancellation technique to improve the AOA estimation accuracy in multiuser multipath scenarios.

A distributed broadcasting time-synchronization scheme for wireless sensor networks

This paper proposes a globally distributed synchronization algorithm for wireless sensor networks. In the proposed scheme, there is a master node that controls the network synchronization and the other sensors periodically broadcast synchronization pulses; they also monitor different frequency channels in an effort to overcome the effect of channel fading.

A distributed MMSE relay strategy for wireless sensor networks

In this paper we propose a multi-sensor relay strategy that achieves path-loss saving and improved power efficiency. In the proposed distributed scheme, the relay sensors do not need to share information about the received signals. An mean-square error design is pursued and the performance is shown to improve as the number of relay sensors (N) increases. Specifically, it is shown that the average power usage per sensor and the total average power drop as O(1/N/sup 2/) and O(1/N), respectively.

Detection of fading overlapping multipath components

Overlapping multipath propagation is one of the main sources of mobile-positioning errors, especially in fast channel fading situations. In this paper we present a technique for detecting the existence of overlapping fading multipath components. Such information is vital for accurate resolving of overlapping multipath components as well as avoiding unnecessary computations and errors in single-path propagation cases. The proposed algorithm exploits the property that fading multipath components fade independently. An analysis of the proposed technique is performed to validate its functionality and to select the algorithm parameters. The paper also presents simulation results that show a high ability of the proposed technique to detect overlapping multipath components along with its impact on the accuracy of multipath resolving techniques.

CDMA location using multiple antennas and interference cancellation

Location finding for CDMA subscribers using an antenna array at the base station and deploying multiple base stations is studied in this paper. Two methods are considered for radio location: measured time-of-arrival (TOA) and angle-of-arrival (AOA), and algorithms to estimate them are presented. The estimated TOA and AOA from different base stations are combined via data fusion to estimate the users interference location by exploiting the topology of the cellular network.

A Minimum Co-User Interference Approach for Multi-User MIMO Downlink Precoding

This paper proposes multi-user precoding schemes for MIMO wireless networks. We design precoders that minimize the interference power caused by a user on all other users, as opposed to forcefully nulling the interference. The resulting scheme relaxes the traditional constraint on the number of transmit and receive antennas

Adaptive subchip multipath resolving for wireless location systems

Reliable positioning of cellular users in a mobile environment requires accurate resolving of overlapping multipath components. However, this task is difficult due to fast channel fading conditions and data ill-conditioning, which limit the performance of least-squares-based techniques. This paper develops two overlapping multipath resolving methods (adaptive and nonadaptive), and shows how the adaptive solution can be made robust to the above limitations by extracting and exploiting a priori information about the fading channel. Also the proposed techniques are extended when there are antenna arrays at the base station. Simulation results illustrate the performance of the proposed techniques.