Friday, April 27, 2012

How to Improve Forward Link Positioning for Cellular Networks? III. Hearability and Accuracy

How to Improve Forward Link Positioning ... ? I. Introduction
1x HDP Enhancements
Enhanced Location Based Services Support in cdma2000
Enhance Downlink Positioning in WiMAX/16m
How Wide A Widband Channel Should Be?
IEEE ICC 2008 Tutorial, Location Based Services for Mobiles
Location Based Services for Mobiles: I. Introduction

Hearability Issue

Hearability of a forward-link positioning system usually is quantified by how many reference signals a terminal may utilize to make a positioning fix in a pre-defined positioning duration.  In theory, a terminal need measure parameters of only 4 different reference signals for a precise three-dimension fix.  However, more reference signals a terminal can use, more diversity benefits a terminal may use for a more accurate positioning fix.

A hearability issue of a cellular positioning network generally is a dimension limitation issue.  It mostly is due to limitations of network geometry and network deployment.  In other words, it is a network issue.  For example, for a given cellular network, say a CDMA2000 1x RTT network or a WCDMA network, its hearability mainly depends on a network topology of the cellular network and a frequency reuse factor of the cellular network.  The network topology including network sectorization may affect achievable DoP values for positioning.  The frequency reuse factor may have a significant impact on co-channel interference to a terminal, which in turn relates to the positioning accuracy achievable by the terminal.  Hearability of an exemplary CDMA2000 1x RTT network is shown in Figure 1.

Figure 1. The hearability of CDMA2000 1x Pilots for AFLT, IEEE ICC 2008 "cdma2000 Highly Detectable Pilot" 
On the other hand, since major considerations for an actual deployment of cellular network base stations are voice and data service capacity, environmental impact and financial limitations, etc.,  a mobile phone network usually is not optimized for mobile positioning in nature.

Accuracy Issue

Positioning accuracy of cellular network forward link positioning is a dimension limitation issue.  Mainly the accuracy is limited by a frequency reuse factor and available bandwidth.  In general, given a certain positioning duration, wider bandwidth received reference signals have, more uncorrelated signal samples a terminal can obtain.  On the other hand, it is known that an achievable SNR highly depends on the frequency reuse factor of a cellular network.  More particularly, a CRLB of the achievable positioning accuracy is asymptotically linear to the number of uncorrelated signal samples and SNR value in dB.

In addition, from a signal processing or a receiver design perspective, a correlation between received signal samples largely depends on a sampling frequency on the received signals and achievable multipath resolution.  Multipath resolution is a function of both a channel delay profile and a bandwidth of received signals.  For example, a statistic delay profile of an exemplary cellular network is shown in Figure 2.  Additional discussions on the statistic delay profile can be found in another blog, "How Wider A Wideband Channel Should be?".   In general, wider the bandwidth of a transmit signal is and  higher the multipath resolution of a channel is achievable.

Figure 2. A statistic model of delay spread. 

Additional Reference

[1] E. Sousa, V. Jovanovic, C. Daigneault, “Delay spread measurements for the digital cellular channel in Toronto”, IEEE Trans. on Vehicular Technology, Nov 1994
[2] J. Ling, D. Chizhik, D. Samardzija, R. Valenzuela, “Wideband and MIMO measurements in wooded and open areas”, Lucent Bell Laboratories,
[3] K. Baum, “Frequency-Domain-Oriented Approaches for MBWA: Overview and Field Experiments”, Motorola Labs, IEEE C802.20-03/19, March 2003
[4] L. Greenstein, V. Erceg, Y. S. Yeh, M. V. Clark, “A New Path-Gain/Delay-Spread Propagation Model for Digital Cellular Channels,” IEEE Transactions on Vehicular Technology, VOL. 46, NO.2, May 1997, pp.477-485.
[5] A. Algans, K. I. Pedersen, P. Mogensen, “Experimental Analysis of the Joint Statistical Properties of Azimuth Spread, Delay Spread, and Shadow Fading,” IEEE Journal on Selected Areas in Communications, Vol. 20, No. 3, April 2002, pp. 523-531.
[6] Spatial Channel Model AHG (Combined ad-hoc from 3GPP & 3GPP2), “Spatial Channel Model Text Description ”, 3GPP, 2003

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