GB2247128A - Method of, and system for, transmitting beacon signals for use in an in-vehicle navigation system - Google Patents

Abstract

In order to avoid having to transmit low power, short range, high bit rate beacon signals for use in an in-vehicle navigation system (14) in the microwave frequency range, the beacon signals from e.g. antenna (16) are transmitted in the guard band between two already allocated radio system frequency allocations. One of these already allocated frequency allocations is the GSM cellular radio telephone band. The rf power of the beacon signals would not exceed the power normally permitted in the guard band. Also disclosed is a method of determining the position of a beacon by spatially filtering the received beacon signal. <IMAGE>

Description

DESCRIPTION METHOD OF, AND SYSTEM FOR, TRANSMITTING BEACON SIGNALS FOR USE IN AN IN-VEHICLE NAVIGATION SYSTEM The present invention relates to a method of transmitting low power, short range (100 to 200 metres), high bit rate signals and more particularly to a method of transmitting beacon signals for use in an in-vehicle navigation system.

There are several different in-vehicle navigation systems under development in different countries. For navigation purposes a typical system such as CARIN, described for example in the Philips Technical Review, Vol. 43, No. 11/12, December, 1987, pages 317 to 329. "CARIN, a car information and navigation system" by M.L.G. Thoone, comprises a map stored digitally on a CD-ROM and the vehicle's position on the map is determined by vehicle mounted equipment including a computer, a direction sensor and a distance sensor. However, such sensors are prone to inaccuracies and the cumulated errors make it difficult for the system to provide correct positioning. One technique to mitigate the effects of these cumulated errors is to use map matching but even this technique cannot ensure a one hundred per cent positioning accuracy at any one time.Alternatively or additionally these cumulated errors can be compensated for by additional positional information provided by location beacons positioned at specified locations to provide location coordinates which relate to the digital map. Communications for performing this function are essentially one-way and the information can be relayed by infra-red or radio links.

In more sophisticated systems it is proposed that road traffic information be provided by and collected by way of beacon signals. As road traffic information is a dynamic information service then an information and traffic control centre is provided and information is relayed to and from the beacons by a suitable link, for example a landline link and thence to a vehicle by a one-way or two-way radio link.

An ever present problem with any new radio communications system is spectrum congestion. As a result it means that in many countries, newly introduced radio communication systems are being allocated frequencies in the microwave part of the spectrum which means that certain components cannot be shared with equipment operating at lower frequencies for example a separate antenna may have to be provided. Ideally the in-vehicle navigation system should be at least nationwide to provide comprehensive coverage.

In many countries there already exists a nationwide communications network which is the cellular radio telephone network. Whilst it would be ideal for an in-vehicle navigation system to operate its one- or two-way beacon signals in the same part of the radio spectrum as the cellular radio telephone network, there are no spare frequency channels so that unless the cellular radio telephone network operators are prepared to release one of their frequency channels for beacon signalling, there is little alternative but to operate the beacons in the microwave frequency range.

It is an object of the present invention to avoid the necessity for the beacon signals having to be operated in a frequency band which is incompatible with existing data information services.

According to one aspect of the present invention there is provided a method of transmitting beacon signals in an in-vehicle navigation system, comprising transmitting the beacon signals as low power signals in the guard band between adjacent allocated radio system frequency allocations.

The present invention is based on the realisation that with a high bit rate radio system, for example the GSM digital cellular radio telephone system, the guard bands are relatively wide, for example of the order of 250 kHz in the case of GSM, and that although the guard bands serve a useful purpose of preventing mutual interference between two frequency adjacent radio services, they nevertheless represent an under-utilisation of a useful part of the radio spectrum. Transmitting low power, short range radio services in the guard band not only makes a more effective use of the spectrum but also it means that the antenna and other receiving equipment can be shared with that operating in one of the adjacent frequency allocations.

The time and frequency envelopes of the modulated rf carrier of the beacon signal may be substantially the same as those of a cellular radio telephone signal in a nearby frequency channel.

In GSM the time and frequency envelopes are specified in order to avoid transient sidebands and interference on adjacent channels.

The beacon signal as transmitted may have a minimum signal to noise and interference level substantially 10dB greater than the interference and noise level normally expected in the guard band but would not exceed the power normally permitted in the guard band. Thus the risk of transient sidebands and interference resulting from the transmission of the beacon signals is very low and will not exceed radiation levels already permitted in a guard band. As a result the original purpose of the guard band is maintained.

The protocol used for transmitting beacon signals may be the same as that used by an adjacent communication service with which it is desired to share equipment.

In order for a vehicle's computer to be able to correct for cumulated errors, it is necessary for it to be known when the vehicle is actually passing by a beacon bearing in mind that the beacon signal is received as the vehicle approaches and passes the beacon and that the vehicle's speed may be high, low or variable. A method of overcoming the effects of multipath in the beacon signal as received which is independent of the vehicle's speed is to spatially filter the received signal and then determine the peak value in the spatially filtered signal.

According to another aspect of the present invention there is provided an in-vehicle navigation system including a plurality of location beacons, each said location beacon having a transmitter for transmitting beacon information as a low power signal in the guard band between adjacent allocated radio system frequency allocations.

The present invention will now be described, by way of example, with reference to the accompanying drawings1 wherein: Figure 1 is a sketch diagram of a beacon sending a signal to a vehicle and a part of the vehicle's dashboard showing some components of an in-vehicle navigation system, Figure 2 illustrates an example of a time mask of the envelope of an RF waveform, Figure 3 is a graph illustrating in full lines an example of a frequency spectrum specification of a GSM digital cellular radio telephone, in broken lines the GMSK modulation with a modulation index BT=0.3 and in chain-dot lines a frequency spectrum specification of a beacon signal transmission, the ordinate represents the relative power in dB and the abscissa the frequency (F) in kilohertz relative to the centre frequency Fc, Figure 4 shows graphs of time (T) versus signal strength (dB) of beacon signals as received at 15kph and at 150kph, Figure 5 shows graphs of the waveforms shown in Figure 4 after they have been filtered with respect to time, Figure 6 shows pulse waveforms derived from wheel sensors at 15kph and at 150kph, Figure 7 illustrates in diagrams A, B and C obtaining a position fix on a beacon using a spatial sampling technique, and Figure 8 is a block schematic diagram of an arrangement for obtaining a position fix on a beacon by spatial sampling.

Referring to Figure 1, the vehicle mounted equipment includes a reproducer 10 for reproducing images on a display screen 12, such images may include a map the details of which are stored digitally on a CD-ROM or direction information displayed as a stylised junction layout with a direction indicating arrow.

A navigation device 14 which includes a computer determines the location of the vehicle on the reproduced map by processing signals derived from a direction sensor (not shown) which may comprise a global positioning device and a distance sensor (not shown) which may be coupled to the undriven wheels of the vehicle and comprise a wheel sensing device used in an anti-lock braking system.

The navigation device 14 includes a radio receiver or transceiver (not shown) for receiving beacon signals which are relayed from an antenna 16 mounted on a kerbside post 18 to an antenna 20 carried by the car, which signals are used amongst other things to correct for cumulative errors resulting from errors in the direction and speed sensors. The beacon signals are low powered signals having a typical range of 100 to 200 metres.

In the simplest arrangement of the system the beacon signals comprise positional information signals which are stored in a storage means, which, together with a transmitter (not shown), are located in a protective box 22 carried on the post 18. In a more elaborate arrangement of the system the beacon signals include traffic flow information derived from an information control centre (not shown) by way of a communications link, for example a landline link, and relayed to the vehicle by the transmitter.

In accordance with the method of the present invention, the beacon signals are low power, high bit rate (270 Kb/s) signals which have a range of 100 to 200m. The centre frequency of the beacon signals is selected so that it lies in the guard band between two known allocated radio system frequency allocations.

For convenience of description one of the allocated frequency allocations is the GSM digital cellular radio telephone system having 40 nationwide frequency channels. The width of the guard band is of the order of 250 kHz. Since it is convenient for the beacon signals to have the same framing structure as a GSM signal then it is equally convenient for them to lie in the band of, or closely adjacent the frequency band allocated to, GSM. In so doing the in-car navigation system and the GSM digital cellular radio telephone system can permit commonality of hardware and to some extent software.

As at presently specified in GSM, between the guard band and a first traffic channel GMSK there is an unused channel which is reserved for other, unspecified purposes. The existence of this unused channel is useful in that as shown in Figure 3, if the channel GMSK is centred on Fc then its frequency envelope has fallen by at least 60dB at the occurrence of the guard band GB and continues to drop in amplitude to a minimum level 24 corresponding to the interference and noise level.

The beacon signal BS is shown in chain-dot lines in Figure 3. This signal BS has substantially the same time mask of the envelope of the R.F. waveform at the transmitter output of a GSM signal as is shown in Figure 2 which means that it has substantially the same frequency spectrum as the GMSK spectrum shown in Figure 3. The beacon signal BS as transmitted may have a minimum signal to noise ratio and interference level substantially lOdB greater than the interference and noise level normally expected in the guard band but in any event the output level would not exceed the power normally permitted in the guard band. Also since it has substantially the same shaped signal spectrum then any transient sidebands and interference on adjacent channels are below the interference and noise level 24.

In the event of the unused channel between the first GMSK channel and the guard band being allocated for a particular application, then it is anticipated that the maximum permitted power will be less than the maximum permitted power in the adjacent GMSK channel but greater than the interference and noise level in the guard band. If this proves to be the case, then the amplitude of the beacon signal BS will have to be such that it can be received satisfactorily over a short range but not so large that it upsets the intended purpose of the guard band.

Irrespective of the frequency of the beacon signal, a problem which occurs with the detection of the beacon signal is to determine the actual point of passing (or fix of) the beacon bearing in mind that the beacon signal is detected over a range of distances by a vehicle moving at an arbitrary speed which may be fixed or variable. Figure 4 illustrates the reception of a beacon signal at 15kph and 150kph. It is assumed that when the received signal is at a peak value then the vehicle is actually passing the beacon. However, the signals as received do not have clearly defined peak values due to multipath effects.

If the signals shown in Figure 4 are filtered with respect to time in a fixed filter, then, as shown in Figure 5, although the multipath effects are largely removed at low speeds, the signal itself is almost completely suppressed at the higher speeds.

A method which overcomes this problem is to spatially sample the beacon signal strength by sampling the signal at intervals corresponding to a vehicle having travelled substantially equal increments of distance. This method of sampling can be implemented by obtaining pulses from wheel sensors operating on the non-driven wheels. Thus, as shown in Figure 6, at 15kph the frequency of these pulses is low compared to the frequency at 150kph.

Figure 8 illustrates block schematicaly the arrangement for deriving a position fix for a beacon signal. A received signal on an input 30 is applied to a spatially sampling device 32 which receives pulses from wheel sensors 34. The spatial samples are applied to a fixed filter 36 which integrates the samples to produce a signal strength waveform which is substantially free of multipath effects. This waveform is applied to an adaptive threshold device 38 which is able to select the peak in the waveform indicating the maximum signal strength, the distance corresponding to the occurrence of this peak is treated as a positional fix of the beacon itself and is relayed to the computer (not shown) to correct for any errors which have cumulated in the system.

Diagram A in Figure 7 shows the spatial sampling of the received signal strength of the beacon signal over a range of distances D. For the sake of clarity, the sampling instants are indicated as short lines on the abscissa. Diagram B in Figure 7 shows the waveform after being filtered in the fixed filter 36 (Figure 8). Diagram C in Figure 7 shows the clearly defined pulse 40 corresponding to the peak value in output from the filter 36. The distance D of the pulse from a reference point is taken as the position fix of the beacon itself.

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 the design, manufacture and use of in-vehicle navigation systems and devices and component parts thereof and which may be used instead of or in addition to features already described herein. Although claims have been formulated in this application to particular combinations of features, it should be understood that the scope of the disclosure of the present application also includes any novel feature or any novel combination of features disclosed herein either explicitly or implicitly or any generalisation thereof, whether or not it relates to the same invention as presently claimed in any claim and whether or not it mitigates any or all of the same technical problems as does the present invention. The applicants hereby give notice that new claims may be formulated to such features and/or combinations of such features during the prosecution of the present application or of any further application derived therefrom.

Claims (10)

1. A method of transmitting beacon signals in an in-vehicle navigation system, comprising transmitting the beacon signals as low power signals in the guard band between adjacent allocated radio system frequency allocations.

2. A method as claimed in Claim 1, characterised in that at least one of the allocated radio system frequency allocations is assigned for cellular radio telephone services.

3. A method as claimed in Claim 2, characterised in that a time and frequency envelopes of the rf waveform of the beacon signal are substantially the same as that of the rf waveform of a cellular radio telephone signal.

4. A method as claimed in Claim 1, 2 or 3, characterised in that the beacon signal as transmitted has a minimum signal to noise and interference level greater than the interference and noise level normally required for receiver operation but lower than the power level permitted in the guard band.

5. A method as claimed in any one of Claims 1 to 4, characterised in that a positional fix on a beacon is derived by spatially sampling the beacon signal as received by a vehicle.

6. A method as claimed in Claim 5, characterised in that the spatially sampled beacon signals are filtered to produce a signal strength waveform substantial free of multipath effects, and in that a peak in the signal strength waveform is identified using an adaptive threshold technique.

7. A method of transmitting beacon signals in an in-vehicle navigation system, substantially as hereinbefore described with reference to the accompanying drawings.

8. An in-vehicle navigation system including a plurality of location beacons, each said location beacon having a transmitter for transmitting beacon information as a low power signal in the guard band between adjacent allocated radio system frequency allocations.

9. A system as claimed in Claim 8, characterised in that a vehicle comprises spatial sampling means for determining a positional fix on a beacon from the beacon signal as received.

10. An in-vehicle navigation system substantially as hereinbefore described with reference to and as shown in the accompanying drawings.