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Mitigating 3G interferrence to GPS due to coexistence in 3G handset

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2 Author(s)
AlSharabati, T. ; Electr. Eng. Dept., Univ. of South Carolina, Columbia, SC, USA ; Yinchao Chen

Co existence of Global Positioning System services (GPS) with cellular and other services and features in a mobile handset presents challenges to GPS performance. Particularly challenging is the over the air (OTA) interface to the GPS receiver that results from carriers like the Global System for Mobile (GSM) and Wideband Code Division Multiple Access (WCDMA) where the transmit (TX) section of the latter could interfere with the performance of the GPS due to TX band noise injected into the GPS band. For the case of WCDMA, the problem is the out of band interference due to the TX wide band noise which has a spectrum that matches the GPS spectrum; both use spread spectrum technology to encode the data. Unlike WCDMA, GSM is built on the Time Division Multiple Access (TDMA) technology where the user occupies one of eight time slots in the multiple access domain. Also, unlike WCDMA where the transmission is continuous, GSM transmission is pulsed and hence the GPS has to cope with and suffer the degradation due to pulsed interference when it is on in the form of TX noise. In this paper, we introduce an analysis of the WCDMA and GSM TX noise (based on actual hardware scenarios) and their effects on GPS sensitivity. It is worthwhile noting that the analysis and scenarios given in this paper represent real life hardware line up issues based on products that go in the hands of the consumers. In addition fundamental line up characterization will be carried out e. g. lineup noise figure, insertion and mismatch losses. It will also be shown that the severity of the degradation is frequency band dependent i.e. the closer the frequency band to the GPS band, the more severely the degradation is. The degradation will be in the form of an increase in the total system noise figure (~5.5 dB). The total system noise figure will contribute a dB for dB in sensitivity degradation i.e. ~5.5 loss in received signal strength. Loss in the received signal power leads to loss of reliable and acc- urate service and most importantly loss of service in areas where it is needed most. The paper is organized as follows: section I is the introduction where the types of interference and their nature is presented. Section II gives theory and analysis of the sources of noise figure contributions that apply to both WCDMA and GSM. In section III we introduce a generic measurement setup that can be used to characterize the GPS received signal power that incorporates both types of noise. The results of the measurement will be graphed. A comparison in received signal strength will be made between the ideal case, where there is no noise figure at all, the case where the noise figure will be due to the hardware lineup (WCDMA TX is off) and the case where the noise figure is due to the hardware line up and WCDMA TX noise (WCDMA TX is on) in the GPS band. The last curve will be for inserting external LNA in the line up. In section IV we propose mitigation techniques that can either partially or completely eliminate both sources of interference. Some of the techniques will require changes in hardware in the front end line up. Finally, in section V and based on the results, we will provide a conclusion and solution scenarios to help in making early-stage decisions, by choosing between less product cost with less performance or more cost and more product complexity with better performance.

Published in:

Aerospace & Electronics Conference (NAECON), Proceedings of the IEEE 2009 National

Date of Conference:

21-23 July 2009

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