Project Members: Ken Pesyna, Zak Kassas, Todd Humphreys

Summary:Femtocells are low-power base stations operated by cellular providers typically installed inside homes, businesses, and in weak-signal environments [1]. Cellular air interface standards impose strict time and frequency requirements on femtocell synchronization [2]. There exist today a number of synchronization solutions which claim to meet some or all of these requirements, including Cellular Network Listen [3], solutions based on the Precision Time Protocol (PTP/IEEE 1588) [4], and solutions based on the Global Positioning System (GPS) [5].

In addition to synchronization requirements, designers of femtocells must consider location requirements. Regulations on femtocell localization are not currently well defined. Phase I of the U.S. Federal Communications Commission (FCC) E-911 mandate states that a carrier must be able to provide “the location of the base station or cell site receiving the 911 call” [6]. The FCC has not yet specified what this requirement entails for femtocells. However, a recent FCC request for comments [7] suggests that, quite understandably, the FCC would like femtocell location accuracy to be roughly equivalent to macrocell location accuracy, or within about 10 meters. If such a strict location requirement is eventually adopted, then the location by-product of the Cellular Network Listen and PTP synchronization technologies would prove inadequate, as these struggle to deliver location accuracies under 100 meters [8]. In contrast, GPS provides sub-10 meter location accuracy in addition to excellent timing and frequency synchronization [5]. Thus, despite the difficulty of exploiting its signals indoors, the GPS’s global, freely-available signals and promise of accurate timing and positioning make GPS a hard habit to break.

State-of-the-art GPS technology has enabled receivers using assisted GPS, or A-GPS, to acquire and track GPS signals with a carrier-to-noise ratio, C/N0, as low as 14 dB-Hz [9]. Unfortunately, this is not good enough to enable broad indoor femtocell deployment. Indoor attenuation models and empirical studies indicate that a significant percentage of deployed femtocells would see GPS signals below C/N0 = 14 dB-Hz [10]. A realistic goal for indoor GPS might be to acquire and track signals with C/N0 as low as 5 dB-Hz, approximately a 9-dB improvement over the state-of-the-art. Such sensitivity would allow GPS-synchronized femtocells to be deployed in 90% of residences [10].

This paper proposes a new technique called Tightly-Coupled Opportunistic Navigation (TCON) to close this 9-dB sensitivity gap in GPS-assisted femtocells. TCON exploits non-GPS “signals of opportunity” (SOPs) to assist and enhance conventional GPS navigation techniques in weak signal environments [11]. Compared with the most advanced incarnations of A-GPS [12], TCON is superior because it fuses GPS and SOP observables at the carrier phase level instead of at the so-called group delay level. The resulting increased aiding precision reduces uncertainty in the GPS acquisition search and enables extended coherent integration, both of which lead to dramatic improvements in GPS sensitivity.


[1] R. Kim, J. Kwak, and K. Etemad, “WiMAX femtocell: requirements, challenges, and solutions,” Communications Magazine, IEEE, vol. 47, no. 9, pp. 84–91, 2009.
[2] Rakon Corporation, “Rakon femtocell timing solutions,” 2008, http://www.rakon.com/resources/Documents/Rakon%20Fe Fact%20Sheet.pdf.
[3] J. Edwards, “Implementation of network listen modem for WCDMA femtocell,” in Cognitive Radio and Software Defined Radios: Technologies and Techniques, 2008 IET Seminar on. IET, pp. 1–4.[4] J. Eidson and K. Lee, “IEEE 1588 standard for a precision clock synchronization protocol for networked measurement and control systems,” in Sensors for Industry Conference, 2002. 2nd ISA/IEEE. IEEE, 2002, pp. 98–105.
[5] W. Lewandowski, J. Azoubib, and W. Klepczynski, “GPS: Primary tool for time transfer,” Proceedings of the IEEE, vol. 87, no. 1, pp. 163–172, 1999.
[6] F.C.C. Docket No., “94-102,” Revision of the commission’s rules to ensure compatibility with enhanced 911.
[7] ——, “07-114,” In the Matter of Wireless E911 Location Accuracy Requirements.
[8] “Femtocell synchronization and location, a Femto Forum topic brief,” Femto Forum Whitepapers, vol. 1, no. 15, June 2010, http://www.femtoforum.org/femto/pdfs01.php. [9] “SiRFstarIV GSD4t datasheet,” CSR, Cambridge, UK.
[10] Rosum Corporation, “In-building location, timing, and frequency coverage analysis of A-GPS and TV-GPS for femtocell applications,” Whitepaper, 2009.
[11] K. Pesyna, Z. Kassas, J. Bhatti, and T. E. Humphreys, “Tightly-coupled opportunistic navigation for deep urban and indoor positioning,” in Proceedings of the ION GNSS 2011, 2011.
[12] R. Rowe, P. Duffett-Smith, M. Jarvis, and N. Graube, “Enhanced GPS: The tight integration of received cellular timing signals and GNSS receivers for ubiquitous positioning,” in Position, Location and Navigation Symposium. IEEE/ION, 2008, pp. 838–845.

Related Publications:

Extending the Reach of GPS-assisted Femtocell Synchronization and Localization Through Tightly-Coupled Opportunistic Navigation

GPS-assisted Femtocell Synchronization and Localization Through Tightly-Coupled Opportunistic Navigation