Precise Time Transfer in a Mobile Radio Terminal (2005)

The degradation suffered by GPS installed in a mobile phone in shielded environments can be ameliorated to some extent by the use of network assistance (satellite information, GPS time information and coarse position) obtained from a fixed GPS terminal in the network, which can allow terminals to operate with GPS signals weaker by 20 dB or more. In this paper, we describe a new method of providing precise time aiding to mobile terminals used in un -synchronised communications networks such as GSM and W-CDMA based on the Matrix positioning method by which the relative timings of network signals received by the handset are used to obtain both a terminal position and a network synchronisation map. This enables the terminal to carry precise GPS time information from open to shielded areas without requiring any GPS assistance from a networkbased server. Precise time aiding within a mobile communication terminal allows for GPS implementations of lower complexity and therefore of lower cost. This can be a crucial factor in mass-market take -up of satellite positioning technology in mobile phones. INTRODUCTION There are several methods in use, or proposed, for the positioning of mobile terminals in a communications network. They divide into two categories: (a) those methods which use signals from satellites, notably GPS, and (b) those methods which use signals from the network. The satellite methods offer excellent performance in open spaces in terms of accuracy and coverage, but work poorly, if at all, inside buildings or at street level in high-rise urban settings. This is in part because of the increased scattering of the satellite signals, but is mainly because the satellite signals are weak: in open spaces at the surface of the Earth the signal strength from a GPS satellite may be around about -130 dBm. On the other hand, the methods which use signals from the network are generally less accurate in open spaces, but they work nearly everywhere since the strengths of the signals received by a mobile terminal from the network are typically 60 dB higher. Furthermore, the accuracy of network-based methods is largely insensitive to environment. One of the ways of overcoming the weakness of the satellite signals is to provide the satellite receiver in the terminal with assistance information – satellite ephemeris and almanac data, time aiding, and position aiding. This information is usually generated within the network and sent to the terminal on request, and can result in a detection gain of more than 20 dB, allowing weaker satellite signals to be detected and hence increasing the coverage of the satellite positioning service. It also decreases the time taken for a position to be obtained, which can otherwise be many tens of seconds. Some recent implementations of GPS receivers within mobile terminals have been further hampered by poor antenna performance and noisy receivers, sometimes adding a further 10 dB of attenuation. Even in excellent implementations, the orientation of the GPS antenna makes a big difference to the signal level at the receiver input terminals. Under these circumstances, the assistance information is required even to obtain reasonable positioning service in ‘normal’ conditions. Because of these limitations, an A-GPS positioning service is usually provided with a fall-back method which can provide the terminal’s position in the event that the AGPS method fails. In CDMA networks, which are synchronised, the fall-back method is typically Advanced Forward-Link Trilateration (AFLT). This uses the known positions of the base stations, known signal transmission timings, and measured signal reception timings to calculate a terminal’s position. Other mobile communication technologies, such as GSM and WCDMA, use un-synchronised networks, i.e. ones in which the transmission time offsets of the signals radiated by one base station relative to another are unknown. In this case, the fall-back method is typically Cell-ID, i.e. the terminal’s position is reckoned as the position of the serving base station, or the centroid of the serving cell. The particular problem of the Cell-ID method is that it is quite inaccurate. In well-implemented systems, the error can be as little as about one quarter of the inter-cell spacing, i.e. ranging from several hundreds of metres in dense-urban environments to ten km or more in the countryside. There is then not a good fit between the AGPS method and its fall-back, so that a user experiences a sharp degradation in accuracy on moving from an open to a shield ed environment, often exacerbated by having to wait a long time before the A-GPS method fails and the inaccurate position is reported. The problems encountered by A-GPS systems are illustrated in Table 1. Two such systems deployed commercially in Japan were tested side-by-side. One of the systems was associated with a CDMA 2000 network (which was synchronised), and was supported by time aiding. The fall-back positioning method was Advanced Forward-Link Trilateration (AFLT). The other system was associated with a PDC network, without time aiding (the network was unsynchronised), and with Cell-ID as the fall-back positioning method. The table gives the radius in metres of the circle centred on the true position containing 67 % of the results, both for an area with open sky (the PDC and CDMA based systems were equally good here) and for each system averaged over a number of dense urban outside environments. The A-GPS yield is also shown, this being the percentage of all results for Where 67% (m) A-GPS yield (%) Open Sky 15 100 Urban outside (CDMA) 68 74 Urban outside (PDC) 114 30 Table 1: A-GPS test results from a Japanese city which GPS was used rather than the fall-back method. As expected, the systems worked very well in ‘open-sky’ conditions, but suffered both in accuracy and yield in more challenging environments. The impact of time aiding is reflected in the figures for A-GPS yield. However, even with time aiding (CDMA), no GPS result was achieved within a time-out period in more than a quarter of the tests. Without time aiding (PDC), this figure rose to nearly three-quarters. It is clear that both the time aiding and the accuracy of the fall-back method were important influences on the user experience of these positioning services. Another dis advantage of A-GPS is that it is expensive to incorporate into mobile phones. As volumes rise, so the cost of implementation reduces, but even so the service is likely to be restricted to ‘high-end’ terminals. To be sufficiently cost-effective for the mass market, the silicon cost needs to be no more than about $1 per device, and the positioning service must then provide ubiquitous coverage, fast response, and reasonable accuracy everywhere with well-mannered degradation from open to shielded areas. One way of reducing the complexity of the GPS receiver is to provide it with the GPS time accurate to within several microseconds. This greatly reduces the search space for the detection of satellites so that fewer correlators are needed, and hence a smaller silicon footprint. The time to fix and battery drain are also reduced. The Matrix positioning method, a development of the standardised E-OTD (GSM) and OTDOA (3G) methods, uses signals transmitted by the network in order to find the position of the mobile terminal. Unlike its EOTD/OTDOA predecessors, it needs no additional hardware in the network (except for the SMLC server needed by all positioning systems), and in particular does not need any Location Measurement Units (LMUs) [1]. The standard version of Matrix works by determining the relative positions of two or more terminals so locating them simultaneously. However, recent improvements to Matrix provide for good positioning performance with just one terminal, the so-called ‘Solo Matrix’ feature. As a by-product, a timing map of the network is produced within the system, and we show below how this can be used provide precise GPS time aiding in unsynchronised networks. Thus Matrix can enhance satellite positioning systems by providing time and position aiding, and when combined with GPS the technology is referred to as ‘Enhanced GPS’ (E-GPS). In this paper, we describe the Matrix method, provide results of field trials, and describe two implementations showing how a terminal containing a GPS receiver can carry precise GPS time information around with it.