WO1999060763A2

WO1999060763A2 - Telematics device comprising the combination of a cellular telephone and an electronic navigation apparatus

Info

Publication number: WO1999060763A2
Application number: PCT/IB1999/000816
Authority: WO
Inventors: Michael E. Barnard; Andrew T. Yule
Original assignee: Koninklijke Philips Electronics N.V.; Philips Ab
Priority date: 1998-05-19
Filing date: 1999-05-06
Publication date: 1999-11-25
Also published as: GB9810639D0; WO1999060763A3

Abstract

A telematics device comprises the combination of a cellular telephone and an electronic navigation apparatus. The cellular telephone includes a reference oscillator (122) which is also coupled to the electronic navigation apparatus, thereby reducing the cost of the telematics device by avoiding the need for two reference oscillators and also improving the performance of the device by reducing the time to first fix when the electronic navigation apparatus is switched on as the reference oscillator is controlled to a tighter specification than is normal for a stand-alone electronic navigation apparatus.

Description

DESCRIPTION

TELEMATICS DEVICE

Technical Field

The present invention relates to a telematics device comprising the combination of a cellular telephone and an electronic navigation apparatus.

Background Art A variety of electronic navigation systems are known. All work by having a plurality of radio transmitters, each transmitting information about its respective position, this information enabling a receiver to determine its position from the received signals. Currently available systems include GPS (Global Positioning System), a US satellite-based system, and GLONASS (GLObal NAvigation Satellite System), the Russian equivalent.

GPS is a widely used system and comprises a constellation of typically 24 satellites in six inclined, approximately 12-hour circular orbits around the earth. Each satellite carries extremely accurate atomic clocks, and transmits a uniquely coded spread spectrum signal on a carrier frequency centred at 1.575GHz which provides information about the current time and location of the satellite.

Receivers for electronic navigation systems are used in a variety of applications, including boats and aircraft. An application in which there is growing interest is combining an electronic navigation apparatus with a cellular telephone. Such a combination will be referred to as a telematics device in this specification. Examples of proposed telematics devices include: published PCT application WO 96/19891 , which describes a position enhanced cellular telephone system including a GPS receiver; published PCT application WO 95/14275, which describes a communication device to integrate a personal digital assistant (PDA) with a cellular telephone and a GPS receiver; US-A-5, 119,102, which describes a vehicle location system where GPS signals are received and stored, then transmitted to a remote base station where they are processed to determine the location of the vehicle; and published French patent application FR-A-2 736 789, which describes an electronic vehicle location module including a GPS receiver and a cellular telephone transceiver operating to the Global System for Mobile

Communication (GSM) standard.

Two major problems involved in integrating an electronic navigation system with a cellular telephone are minimising the cost of the equipment and reducing the power consumption. The majority of the circuitry required for such a telematics device can be produced in the form of integrated circuits (ICs) the major factors affecting cost are the number of ICs required, together with the number of off-chip components and their cost. At present a GPS receiver consumes up to 1W when operating, similar to the power consumption of a GSM phone when talking. Hence, the GPS circuitry cannot be permanently active in a portable telematics device without drastically reducing the battery lifetime, which would make such a device unattractive to potential consumers.

Another factor which affects power consumption is that a GPS receiver requires a significant amount of time (known as the time to first fix) to calculate its position when it is first turned on.

Disclosure of Invention

An object of the present invention is to reduce the cost of a telematics device and to minimise its power consumption. According to the present invention there is provided a telematics device comprising the combination of a cellular telephone having a transceiver including frequency generation means and an electronic navigation apparatus, characterised in that the frequency generation means includes a reference oscillator which is coupled to the electronic navigation apparatus to provide timing information.

The present invention is based upon the recognition, not present in the prior art, that a single reference oscillator can be shared by a cellular telephone and an electronic navigation apparatus, thereby saving costs. Further, by the navigation apparatus using the relatively high quality reference oscillator of the cellular telephone the time to first fix is reduced, thereby economising on the battery power consumption.

Brief Description of Drawings

Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, wherein: Figure 1 is a block diagram of a GSM cellular telephone; Figure 2 is a block diagram of a typical electronic navigation receiver; and

Figure 3 is a block diagram of a telematics device in accordance with the present invention.

In the drawings the same reference numerals have been used to indicate corresponding features.

Modes for Carrying Out the Invention

The implementation of a GSM cellular telephone shown in Figure 1 is based on the current Philips digital cellular chipset for GSM, as described in the 1997 edition of Philips Semiconductors Data Handbook IC17: Semiconductors for Wireless Communications. However, the same principles apply to other GSM implementations and in large part to other cellular telephony standards. Consider first the receiver part of the circuitry operating on a voice telephone call. An antenna 102 receives signals from a remote base station at a frequency of between 890 and 915MHz. These signals pass through a diplexer filter 104, the purpose of which is to prevent strong transmitted signals from leaking into and overloading receiver circuitry. The signals then pass into an radio frequency transceiver block (RF) 108, for example a Philips IC SA1620. The receive path in this IC comprises two low noise amplifiers and a mixer to down-convert the RF signal to an intermediate frequency (IF) of between 70 and 500MHz. The mixing process uses a local oscillator signal of between 1290 and

1360MHz provided by a frequency synthesiser 120. This may comprise for example a Philips IC UMA1019 in conjunction with an external low pass filter and a voltage-controlled oscillator (VCO) to form a phase-locked loop (PLL) generating the required output frequency. A temperature-controlled crystal oscillator 122 is used to provide a reference frequency.

In practical GSM applications the oscillator 122 has a frequency of 13MHz. The basic accuracy of the oscillator 122 is 1 to 2ppm, which is the best accuracy that can be obtained at present in a mass-produced oscillator of reasonable price suitable for use in a cellular telephone. This accuracy is further improved, to within 0.1 ppm, by monitoring and adjusting the oscillator using signals received from a base station as a reference. The IF signals pass to an intermediate frequency block (IF) 110, for example a Philips IC SA1638. The receive path in this IC comprises an IF amplifier, a pair of quadrature down-mixers, and a pair of baseband filters and amplifiers. The mixers down convert the IF signal to in phase (I) and quadrature (Q) signals at baseband, using a local oscillator signal of 800 MHz. This is generated by a frequency synthesiser included in the IC together with an external low pass filter and VCO to form a PLL. The required reference frequency of 13MHz is provided by the oscillator 122.

The baseband signals pass to an interface block (D/A) 112, for example a Philips IC PCF5072, where the I and Q channels are sampled at the GSM bit clock rate of 279kHz. The sampled signals are passed to a digital baseband processor (BB) 114, for example a Philips IC PCF5083. This performs the signal processing tasks required for GSM, which tasks include speech decoding, channel decoding and deinterleaving. Received audio signals are returned to the interface block 112 for conversion back to analogue signals for reproduction on a loudspeaker 116 or other suitable output device. Both the interface block 112 and baseband processor 114 require a 13MHz reference frequency, which is again provided by the oscillator 122.

Now consider the transmission side of the circuitry. Voice signals are received by a microphone 118, or other suitable input device, and passed to the interface block 112 which digitises them and passes them to the baseband processing block 114. This block encodes the speech and also performs channel coding and interleaving to reduce the received bit error rate. The resultant signal for transmission is returned to the interface block 112 where it is modulated by a GMSK modulator, converted to analogue I and Q signals and passed to the IF block 110. Here the quadrature baseband signals are transposed up to a fixed IF frequency of 400MHz. The IF signal is passed to the RF transceiver block 108 where it is mixed up to the RF transmission frequency of between 935 and 960MHz and amplified to the required power by a power amplifier (PA) 106. It is then passed through the diplexer filter 104 and transmitted by the antenna 102.

The frequency tolerance specification for the radio aspects of the European Telecommunication Standards Institute (ETSI) GSM standard is specified in Part 05.10. Section 5.1 of this Part specifies that a base station shall use a single frequency source with an absolute accuracy of better than O.Oδppm. Section 6.1 specifies that for a mobile station (i.e. a cellular telephone) the carrier frequency shall be accurate to within 0.1 ppm, or accurate to within 0.1 ppm with respect to signals received from the base station (averaged over a sufficient time to allow for the effects of errors due to noise or interference). It is also specified that the mobile station shall use the same frequency source for RF frequency generation and clocking the timebase in the baseband processing. Other details of a practical GSM implementation are not shown as they are not required for an understanding of the present invention. For example, all the blocks described above operate under the control of a microprocessor, and there are additional blocks for controlling a display on a handset and interfacing with a SIM card. Architectures for GPS and other electronic navigation systems are well known, for example as described in US-A-4,754,465 and US-A-4,970,523. Figure 2 illustrates schematically one such architecture based on the current Philips EXACT chipset for GPS. An antenna 202 collects signals from a plurality of remote transmitters (for GPS these are right-hand circularly polarised signals on a carrier at 1.575GHz). The received signals are fed into an RF front end 204, for example a Philips IC UAA1570. This comprises a comparatively simple analogue section which mixes the received signal down to a much lower intermediate frequency. A crystal oscillator 210 provides a reference signal for use in the mixing process.

The IF output from the RF front end 204 is fed into a baseband processing block (BB) 206, for example a Philips IC SAA1575. This contains the digital circuitry necessary to decode the spread spectrum signals from the different remote transmitters and process the information to calculate the position and velocity of the user. The oscillator 210 is used to provide timing information for use in the decoding process. The output from the baseband processing block 206 is information about the position and velocity of the receiver, which information is passed to a user interface block (Ul) 208 for display.

The RF front end 204 (using the IC UAA1579) and baseband processing block 206 (using the IC SAA1575) can be programmed to use a range of reference oscillator frequencies. A common choice of frequency is 14.4MHz, but a 13MHz reference can also be used.

The main factors governing the time to first fix are: the time taken to acquire signals from enough (at least four) GPS satellites; the time taken to download enough data from the acquired satellites to characterise their orbits (ephemeris data) and clock discrepancies; and the time taken to process the raw measurements and calculate the position.

The last of these is the easiest to characterise as it involves a fairly well defined set of calculations and thus is dependent on the processing power of the microprocessor in the baseband processing block 206. Current GPS chip sets use a processor powerful enough to carry out this step in less than a second.

The first two factors are rather different in nature as they are issues that are normally not too important in stand-alone GPS receivers. This is because such receivers are normally left powered on, so these tasks only have to be carried out at start up and then occasionally as a background task.

Downloading suitable ephemeris data from the satellites takes a minimum of 30 seconds, because the data is repeatedly transmitted in five sub-frames each of which takes six seconds to transmit. In practice a receiver may acquire the signal at any point during the transmission cycle, so a particular instance may waste nearly a whole six second sub-frame whilst trying to find a recognisable preamble. Such ephemeris data remains in RAM while the GPS receiver section of the telematics device is inactive, although it only has a valid lifetime of a few hours since the orbits of the satellites are not constant.

The process of a GPS receiver attempting to acquire a satellite involves the receiver trying to tune into both the carrier frequency and coding of the signal transmitted by the satellite. The nature of GPS signals is such that if both parts of the signal are not replicated to a reasonable degree of accuracy, no resultant signal can be detected.

The carrier frequency transmitted by a satellite is 1.575GHz, but this can be Doppler-shifted by ±4kHz because of the relative motion of the satellite and the receiver. Each satellite uses a unique (and known) pseudo-random code of 1023 chips to spread its signal, which repeats every 1ms. To acquire the GPS signal from a satellite a receiver must identify both the Doppler shift on the signal and the phase of the satellite code to a high degree of accuracy. This is done by making a guess at the signal frequency and code phase. The receiver then generates a code with the selected phase modulated onto a carrier with the selected Doppler shift, mixes this with the incoming signal and passes the resultant signal through a correlator, which integrates the combined signal over a particular period, commonly 1ms. If the output from the correlator exceeds a threshold, the satellite signal has been acquired. If the signal is not acquired immediately, a search must be performed through code-frequency space until it is. In a simple serial search this is done by searching code space first at the selected frequency. If the signal is not found after all 1023 code phases have been attempted the search is repeated at a new frequency about 200Hz different from the previous frequency. A variety of more sophisticated search strategies are known. For the signal to be acquired, the phase of the code needs to be correct to within one chip width relative to that of the incoming signal. Similarly, for an integration period of 1ms, the frequency difference between the received signal and locally-generated signal needs to be much less than 500Hz for the signal to be acquired. An error of 150Hz in frequency corresponds to a crystal oscillator having a stability of approximately 0.1 ppm. The time taken to acquire at least four satellites is highly dependent on how much prior knowledge the system has. If it has no information whatsoever, acquisition might take an hour as a large part of the code-frequency space will need to be searched for each signal. Conversely, if the receiver knows its location and the current time accurately, and has reasonably up-to-date ephemeris data, the expected Doppler shift can be calculated accurately from the ephemeris data. If the local oscillator error is also characterised to within 0.1 ppm the signals could be acquired almost instantly.

In practice the reference oscillator 210 will usually be a temperature controlled crystal oscillator having a frequency stability of 1 to 2ppm, since more accurate oscillators cannot be obtained at a reasonable cost. Even so, the cost of the oscillator, at around US$3, is some 10% of the total cost of the circuitry for a typical GPS receiver. As a result, even if the Doppler shift of the signal from a satellite is known precisely the locally-generated signal can only be generated to within 1.5 to 3kHz of the target frequency. Therefore a search through a number of frequencies is required until the signal is acquired. Once a satellite signal has been acquired it can be used to determine the error in the reference oscillator 210, enabling further satellites to be acquired more rapidly. Nevertheless, the stability of practical reference oscillators limits the performance of a GPS receiver by increasing the time to first fix. A block diagram of a telematics device according to the present invention, comprising a GSM cellular telephone and a GPS receiver, is shown in Figure 3. The GSM part of the device is identical to that shown in Figure 1. The GPS receiver is similar to that shown in Figure 2, but the RF front end 204 and baseband processing block 206 use a reference frequency produced by the GSM reference oscillator 122. Further, the user interface block 208 is adapted to interface with a display controller (not shown) shared between the cellular telephone and GPS receiver for displaying information about the receiver's position and velocity.

As noted above, the GSM reference oscillator 122 is controlled to within 0.1 ppm by reference to signals received from a base station. This provides a much more stable reference than is normal in GPS receivers, enabling the locally-generated frequency to be within 150Hz of the required frequency. This is accurate enough to acquire a satellite without having to search a range of frequencies if the Doppler shift of the signal can be predicted accurately, giving a significant advantage over stand-alone GPS receivers.

Since accurate oscillators are expensive components, using the reference oscillator 122 for both applications in a telematics device enables a significant cost saving in production, whilst at the same time reducing the time to first fix of the telematics device, thereby minimising power consumption by reducing the length of time for which the GPS receiver must remain active.

Further advantages can be obtained by obtaining an estimate of the position of the telematics device, by knowing which GSM cell it is in, and the current time, by reference to the GSM base station's transmissions.

Although a telematics device was described above in relation to the combination of a GSM cellular telephone and a GPS receiver, it will be apparent that it could equally well be used with any cellular telephone system which requires an accurate reference oscillator, and with any electronic navigation apparatus requiring an accurate frequency reference.

From reading the present disclosure, other modifications will be apparent to persons skilled in the art. Such modifications may involve other features which are already known in telematics devices, and which may be used instead of or in addition to features already described herein.

Industrial Applicability

The present invention has a wide range of industrial applications in the fields of cellular telephony and electronic navigation.

Claims

1. A telematics device comprising the combination of a cellular telephone having a transceiver including frequency generation means and an electronic navigation apparatus, characterised in that the frequency generation means includes a reference oscillator which is coupled to the electronic navigation apparatus to provide timing information.

2. A device as claimed in claim 1 , wherein the cellular telephone operates in accordance with the GSM standard and the navigation apparatus operates in accordance with the GPS system, characterised in that the reference oscillator has a frequency of 13MHz.

3. A device as claimed in claim 1 , characterised in that the reference oscillator has its frequency tolerance enhanced by frequency control circuitry using signals received from a cellular telephone base station.

4. A device as claimed in claim 1 , wherein the cellular telephone operates in accordance with the GSM standard and the navigation apparatus operates in accordance with the GPS system, characterised in that the reference oscillator has a frequency of 13MHz and in that the reference oscillator has its frequency tolerance enhanced by frequency control circuitry using signals received from a cellular telephone base station.