Background to the Invention Radio-frequency (RF) based devices are widely used for wireless security or locating systems. RF identification (RFID) tags provide a way to locate or monitor the location of a target that has been appropriately tagged. A central unit, or server, is typically in communication with a number of such tags. The tags may be predominantly passive devices or may be capable of independent communication with the central unit.

The RF range of the electromagnetic spectrum has the advantage of penetration through objects which are opaque to optical wavelengths (e. g. ultra-violet, visible, infra-red), and is therefore not restricted to a direct line of sight. Of course, signal strength reduces with propagation distance and passage through objects.

Similar technology has been employed for security purposes, to activate alarm systems or communicate security violations. For example, a vehicle alarm can be armed/disarmed and an engine immobiliser activated/deactivated remotely. RF transponders are often fitted to vehicles to convert electronic coded signals into an RF signal that can be transmitted to a receiver, such as a key fob or a central monitoring server. In this way, a single central unit may monitor the location and status of a large number of vehicles. A particular vehicle may be paged to indicate its location amongst the other vehicles or else its absence from the monitored area. Typically, the RF transponder within a vehicle will be a permanent installation powered from the car battery and coupled to many of the vehicles systems. In this way, the transponder can not only communicate status information to the central server but may also activate/deactivate the vehicle systems in response to a security violation. For example, immobilising the vehicle and activating an alarm.

Despite the wide usage of RF based technology, these known systems suffer from a variety of limitations and disadvantages. One such problem relates to ensuring that the RF communications link between a transmitter unit and a receiver unit is intact. It is, of course, possible for a system to operate by the transmitter only communicating with the receiver in order to indicate that a security violation has occurred. This has the advantage

of placing only minimal demands on the transmitter power source, permitting the use of a lightweight internal battery and allowing the transmitter to be small and portable.

However, there is no guarantee that the communication link will be operable when it becomes necessary to transmit an alarm signal, whether for reasons of battery power, signal obscuration, range or device malfunction.

One approach to this issue is for the transmitter to continuously transmit some form of test signal, which is received by the receiver and an indication given that the signal either has or has not been received. In this way, the central server, or a user if carrying a portable receiver, can be sure that the RF link is in tact and will be operational when needed. The problem with this"always-on"approach is the continual power drain on the transmitter's power source, which means that either the system will suffer from a significantly reduced lifetime or else the transmitter must be connected to a high capacity source, such as a vehicle battery. The system will generally then require professional installation.

An additional problem relates to the arming or activation of the security system.

There may be no need for the alarm system to be operational when, for example, a driver is with his/her vehicle. The system may be activated manually, but this places a reliance on the operator (driver). It is therefore preferable that the system is self-arming in some way. A further consideration is that many insurers require that security systems are "always on"in order to satisfy policy requirements or to qualify for lower premiums. For example, the Thatcham security standard ratings, as set by the Thatcham research centre. Thus there are a number of issues to be considered in the provision of a reliable, compact, remote security system and which are not adequately addressed by existing systems.

Summary of the Invention According to a first aspect of the present invention, a radio-frequency (RF) communication system comprises: a transmitter module, which comprises an RF transmitter for generating and emitting an RF communications signal, in use the transmitter module transmitting a pulsed RF test signal at discrete time intervals; and, a receiver module, which comprises an RF receiver for receiving an RF communications signal and means for indicating the status of an RF communications link between the transmitter module and the receiver module in

dependence on the pulsed RF test signal, thereby, in use, verifying the integrity of the RF communications link.

In this way, the user can be assured that the RF link was in tact, and in range, up to the time the last RF test pulse was received. The receiver indication means can indicate either that the RF test signal has been successfully received during a discrete time interval or that the RF test signal has not been successfully received during a discrete time interval.

In order to ensure regular communication between transmitter and receiver, but not to drain the transmitter power source, it is preferred that the ratio of a period between consecutive RF test signals to the test signal duration (i. e. a duty cycle) is at least 100: 1.

More preferably, the ratio of a period between consecutive radio-frequency test signal to the test signal duration is at least 6000: 1.

The test signal may also be modulated with data, which may include a unique identifier relating to the transmitter module that sent the test signal. A range of modulation techniques and frequencies may be employed.

According to a second aspect of the present invention, an RF security system comprises an RF communications according to the first aspect, the transmitter module further comprising means for generating and emitting an RF alarm signal which can be received by the receiver module over the RF communication link.

In this way, a security system is provided which uses an RF link, the integrity of which is regularly tested. The RF alarm signal may be generated in response to a security violation, as monitored by a variety of sensors, and its reception by the receiver module may be indicated by audio or visual means.

According to a third aspect of the present invention, a transmitter module for use in a system according to the first or second aspect comprises computer executable code for performing the step of automatically transmitting a pulsed RF test signal at discrete time intervals for verifying the availability of an RF communications link.

According to a fourth aspect of the present invention, a method for monitoring the status of an RF communications link between a transmitter module and a receiver module comprises the steps of: generating and emitting from the transmitter module a pulsed RF test signal at discrete time intervals ; and, indicating at the receiver module the status of the RF communications link in dependence on the test signal.

Using this method, a system according to the first aspect may verify the integrity of an RF communications link.

According to a fifth aspect of the present invention, an RF security system comprises: a transmitter module, which comprises an RF transmitter for generating and emitting an RF communications signal including an RF alarm signal which can be received by a receiver module over an RF communication link ; and, a receiver module, which comprises an RF receiver for receiving an RF communications signal including an RF alarm signal, and means for indicating that the RF alarm signal has been received; wherein, in use, the alarm indication means becomes operable when the strength of an RF signal transmitted by the transmitter has fallen below a predetermined value or has fallen by a predetermined factor at the receiver.

In this way, the security system is self-arming based on transmitter-receiver proximity, thereby removing the need for manual arming by a human operator who may be fallible. More accurately, the alarm system is actually self-inhibiting for a certain signal strength, thereby satisfying the"always-on"criterion required by many insurers.

Preferably, the RF security system comprises an RF communication system according to the first aspect, thereby allowing the integrity of the RF communications link to be verified.

Preferably, the RF signal strength is modified in dependence on the presence or absence of an external stimulus. This enables conservation of battery power when, for example, a particular stimulus such as vehicle motion is present, and signal strength is only increased when the stimulus is absent.

According to a sixth aspect of the present invention, a receiver module for use in a system according to the fourth aspect comprises computer executable code for performing the step of enabling the alarm indication means when the strength of an RF signal transmitted by a transmitter has fallen below a predetermined value or has fallen by a predetermined factor at the receiver.

According to a seventh aspect of the present invention, a transmitter module for use in a system according to the fourth aspect comprises computer executable code for performing the step of modifying the strength of an RF signal emitted by the transmitter in dependence on the presence or absence of the external stimulus.

According to a eighth aspect of the present invention, a method for remotely

enabling an RF security system comprising a transmitter module and a receiver module, the receiver module having an alarm indication means for indicating the reception of an RF alarm signal transmitted by the transmitter module, comprising the steps of: generating and emitting from the transmitter module an RF signal ; and, enabling the alarm indication means when the strength of the RF signal transmitted by the transmitter has fallen below a predetermined value or has fallen by a predetermined factor at the receiver.

Using this method, an RF security system according to the seventh aspect may be remotely enabled.

According to an ninth aspect of the present invention, a method for conserving power usage in an RF transmitter module comprising a power source, an RF transmitter for generating an RF communications signal, and a sensor for responding to an external stimulus, the method comprising the steps of: generating and emitting from the transmitter module the RF communications signal having a first RF power level in dependence on the presence, or absence, of the external stimulus ; and, generating and emitting from the transmitter module the RF communications signal having a second RF power level in dependence on the absence, or presence, respectively, of the external stimulus; the first power level being lower than the second power level, thereby reducing consumption of power from the power source when the external stimulus is present, or absent, respectively.

Using this method battery power in an RF transmitter module may be conserved when, for example, a particular stimulus such as vehicle motion is present. Signal strength would be increased when the stimulus is absent.

Brief Description of the Drawings Examples of the present invention will now be described in detail with reference to the accompanying drawings, in which: Figure 1 shows RF communications link between a transmitter module and a receiver module; Figure 2 shows a periodic KIT test signal according to the present invention; Figure 3 shows a KIT pulse with a ramped leading and trailing edge; Figure 4 shows an encoded KIT pulse and the corresponding internal receiver KIT

signal ; Figure 5 shows a periodic KIT pulse with alarm pulse during the dwell period; Figure 6 shows the internal architecture of a transmitter module; Figure 7 shows 16-bit Manchester encoding; Figure 8 shows the internal architecture of a receiver module; Figure 9 shows an operational flow chart for a vehicle alarm transmitter module ; Figure 10 shows an operational flow chart for a vehicle alarm receiver module; Figure 11 shows RF signal strength monitoring in a receiver module; Figure 12 shows the RF signal strength trigger level for disabling the receiver alarm indicators; Figure 13 shows the internal architecture of another transmitter module ; Figure 14 illustrates the external form of the transmitter module ; Figure 15 shows the internal architecture of another receiver module; Figure 16 illustrates typical microprocessor connections within the receiver (or transmitter) module ; Figure 17 illustrates the external form of the receiver module ; and, Figure 18 shows a docking station with receiver module connections.

Detailed Description The present invention is directed towards the provision of a reliable radio- frequency (RF) communications link 10 between a transmitter module 11 and a receiver module 12, as shown in Figure 1, and its application to a remote security alarm system.

A key aspect of the invention is the emission of a pulsed RF test signal 13 by the transmitter module at discrete time intervals to check the integrity of the RF communications link between transmitter and receiver. This signal is known as the"keep- in-touch" (or KIT) signal. Figure 2 illustrates the RF timing diagram for a typical KIT signal.

The KIT signal comprises a short burst of RF radiation 20 (carrier wave active period) followed by a comparatively long dwell period 21. This signal will usually be repeated in a periodic manner 22, although it does not have to be periodic.

As shown in more detail in Figure 3, for reasons of practicality and efficiency, the RF pulse 30 will generally be ramped up 31 to reach a desired power level 32, which will persist for much of the duration of the pulse, prior to ramping the signal strength back down 33 to zero. The use of this technique can considerably reduce the spurs on the associated RF spectrum, thereby minimizing the strength of any harmonics present. The

KIT signal may operate with a wide range a duty cycle, i. e. the ratio of the period between consecutive RF test signals to the test signal duration. For example, a duty cycle of 100: 1 ensures a test pulse is active for 1/100 of the time. However, such a duration is unnecessary and places a larger drain on the transmitter power supply. A duty cycle of around 6000: 1 is a good compromise, permitting the RF link to be tested every minute with a short 10ms burst.

The RF link may be operated on a range of carrier frequency, including both specifically allocated frequencies and license exempt bands. The frequency of choice will generally be a compromise between an available band and a frequency that is characterized by significant physical penetration through obstacles, in order to ensure maximum range of operation. One such long wave RF frequency, occupying a license exempt band, is centered at 869.4 MHz. Furthermore, as indicated in Figure 4, during the RF burst the carrier wave may be modulated with data (encoded KIT signal) 40, which may represent an address or other information. Any conventional modulation technique may be employed, including amplitude, phase and frequency modulation. However, a preferred technique is frequency shift key (FSK) modulation.

The receiver module checks for successful reception of a KIT signal within each time period and then indicates either the presence or absence of the signal. In this way, the RF link is tested at frequent regular intervals, but without excessively draining the power supply. In one embodiment, as illustrated in Figure 4, the receiver module has an internal KIT signal 41, which is kept high 42 as long as a KIT test signal has been received within the previous test period. This internal KIT signal only goes low 43 when a KIT test signal has not been received within the previous test period. Of course, alternatively the internal signal may switch from low to high. In dependence on this internal signal the receiver module then indicates either the presence or absence of the signal. This indication may be in the form of a visible or audible signal directed at the user and may be continuous or discrete. However, to save power, the receiver module will preferably indicate briefly that a KIT test signal has been successfully received. In this way, it need not matter to the user why the test signal has not been received, but simply that for some reason the integrity of the RF link has been compromised. The actual reason may be a component or power failure, or else the receiver module is out of range or significantly obscured from the transmitter module.

The self-testing remote RF link described above may be used in a variety of applications, but a key application according to the present invention is in a remote

security system. In such a system, the transmitter module is capable of transmitting an alarm signal to the receiver module over the self-testing RF communication link. The sending of an alarm signal will typically be in dependence on an external stimulus detected by a sensor that is either internal or external to the transmitter module.

Advantageously, the alarm signal may take the same form as the KIT test signal and therefore be generated in the same manner. Figure 5 shows the RF timing diagram (not to scale) in which an RF alarm pulse 50 is transmitted over the link during the dwell period 51 between consecutive KIT test signals 52 and 53. The alarm pulse may be encoded with data and is received by the receiver module, which immediately indicates reception of the alarm signal. As a result of the regularly self-testing remote RF link, a user may have a high degree of confidence that the link is in tact for the transmission and reception of an alarm signal, if required.

We will now consider the operation and architecture of the system components in more detail. Figure 6 shows one possible configuration for the internal components of the transmitter module 600. The key hardware elements are the RF transmitter 601 for generating an RF signal and the aerial 602 for emitting the RF signal. A modulator and code generator 603 provide the means for generating data and modulating the RF signal with it. Power for the transmitter and control circuitry is derived from an internal battery 604. This will preferably be a compact rechargeable type, such as Lithium (Li) ion. In some circumstances it may be possible to operate with an RF power level below 10mW, but in practice a level of 10mW (10dBm) or higher will be used to extend the range of the remote RF link according to the demands of the operational environment. Depending on battery capacity, an effective radiated power (ERP) of between10mW and 500mW (10- 27dBm) can be achieved, giving the RF link a range of between 80 meters and 1.5 kilometers. Another consideration is the limit placed on the ERP of a device operating in a particular band. For example, the license exempt 869.4 MHz frequency is restricted to an ERP of +27dBm (500mW).

Due to the short duration, high peak power nature of the RF pulses to be generated, conventional batteries generally have too high an internal impedance to drive the transmitter directly at the power level required. Therefore, as shown in Figure 6, it is more effective and efficient to drive the transmitter via the controlled discharge of a capacitor 605. The capacitor will typically require a large capacitance, and therefore a supercap type capacitor is particularly suitable. The charging and discharging of the capacitor is controlled via a high-value series resistor 606 and initiated by means of an

internal switch 607, the keep-in-touch switch, which is controlled by the same control logic 608 that controls the modulator and code generator 603. This control logic 608 will typically comprise a simple microprocessor-based timing and control system, powered directly from the battery 604. A particularly efficient way of operating the RF power source is by charging the supercap 605 during the dwell time between consecutive KIT pulses, thereby ensuring the supercap is fully charged and ready for the next KIT signal.

An RF alarm pulse can also be controlled and generated by the same transmitter module components, although ideally separate power circuitry is required to satisfy the associated power demands. The alarm could be raised via a KIT pulse, by encoding alarm data on the KIT pulse after the KIT data, although some deterioration in alarm response is then to be expected. Therefore, to ensure that power is readily available to generate a separate RF alarm pulse, it is preferred that the module comprises a separate supercap capacitor 609, series resistor 610 and internal switch (alarm switch) 611 for this purpose, as shown in Figure 6. The transmission of an alarm signal will be triggered in response to some external stimulus, as monitored by either a sensor internal to the module or an external sensor in communication with the module. The module shown in Figure 6 is provided with an internal detector 612. Of course, a whole array of commercially available sensors may be used, depending upon the precise application of the alarm system. Examples include general motion sensors and more specialized vibration detectors, such as glass-breaking harmonic sensors, magnetic, thermal and smoke sensors, simple circuit break sensors and passive infra-red (PIR) sensors. In the application of the system to vehicle or property security, the sensor (s) will usually indicate a security violation and the alarm pulse will be transmitted. The remote alarm system can thus operate independent of, and alongside, a local static alarm or control system, orelse can be coupled to it.

The encoding of digitized data on the KIT and alarm signals permits other information to be conveyed to the receiver module, including status information and data from sensors. This facility is particularly useful where a plurality of transmitter modules are in communication with a single receiver module. In this case, the data may comprise a unique address associated with the transmitter module that sent it, thereby identifying which transmitter modules are in KIT communication with the receiver and precisely which transmitter module (s) has sent an alarm signal. The data may further comprise an address element that is common to all transmitter modules in a particular cluster and/or is unique to a particular receiver. Figure 7 shows the timing diagram for an example of

pulse coding, where a 16 bit Manchester coding has been employed. Each bit is represented by a short binary pulse, with an overall repeat period of 333, us. In this example, the first 13 bits are used for addressing, permitting up to 8192 addresses, bit 14 is used for the KIT signal, bit 15 for the alarm signal and bit 16 is used as a low battery indicator.

Figure 8 shows one possible configuration for the internal components of the receiver module 80. Here again, the key hardware components are the receiving aerial 81 and the RF receiver 82 itself. Any data present on the RF carrier signal is demodulated and decoded by a demodulator and decoder 83 and then passed to controlling electronics 84, which activate the KIT and alarm signal indicators. The indicators may include visual, audible and other types, such as vibration. The receiver module 80 contains its own power supply, preferably in the form of a lightweight compact rechargeable battery 85. As the demand placed on the receiver power supply is lower than those of the transmitter module, a more compact battery may be used and the overall dimensions for the receiver module be reduced by comparison with the transmitter module. In one embodiment, the receiver module would be the size of a key fob.

Having discussed the core components of the system, we now consider aspects of its operation in more detail, together with additional features and embellishments. One such variant is directed at the application to vehicle security, particularly for courier vehicles. In daily use, it is desirable that the system is operational all of the time, and indeed this may be a requirement to satisfy certain insurance standards, such as the Thatcham security standard rating. Clearly, switching on or arming the device manually may be undesirable and open to user neglect. Also, undesirable periodic transmissions are to be limited wherever possible.

To address these issues, a system based on signal strength is provided, which both reduces power consumption and inhibits the alarm function when it is not required, whilst still satisfying the"always on"criterion. Figure 9 shows a flow chart which describes the operation of the transmitter module. The system depends on a sensor internal to the module (e. g. the detector 612 in Figure 6), which monitors some external stimulus. For a vehicle security system, a motion switch is employed that can distinguish between the situation where the vehicle in which the module is located is stationary and one where it is in motion. As illustrated, if motion is detected by the sensor, a signal 91 is sent to a programmable integrated circuit (PIC) control unit, which switches the RF power for the KIT signal 96 to a lower level 92 of 10dBm (10mW). This conserves power while the

driver is in the vehicle and the transmitter and receiver modules are in close proximity.

If no motion is detected by the sensor 93 then, after a suitable delay 94, the RF power for the KIT signal 96 is switched to a higher level 95 of 25dBm (>300mW). This power level is then sufficient to ensure operation of the remote link over a large transmitter-receiver module separation (e. g. 0.8km), when the driver is no longer in the vehicle. The power level is only returned to 10mW when the vehicle is moving again. The time delay 94 allows for the occasions where the vehicle is stationary temporarily, for example at traffic lights or during a brief delivery stop. A delay of approximately 10 seconds might suffice for this.

The transmitter power is also monitored at the receiver module by a received signal strength indicator (RSSI). As shown in the flow chart of Figure 10, on successfully receiving a KIT signal 100, an indication of signal strength is determined 101 from the RSSI and the alarm (warning) indicator is enabled 102 or disabled 103 according to whether the RSSI level is low or high. The receiver module also checks for reception of a valid alarm signal 104. If an alarm signal is received 105, and the alarm warning indicator is enabled 106, then the alarm warning is activated 107.

As shown in Figure 11, the strength of the signal received by the receiver 111 via the aerial 110 is determined by an RSSI and the level is compared by an internal voltage comparator 114 to a pre-set level 115. The result 116, along with the received data signal 113 is fed to a programmable integrated circuit chip 117 that controls the receiver audible 118 and visual 119 alarm indicators. Thus, the receiver alarm indicator is only enabled when a low signal strength is detected. Otherwise, when the signal strength is high, the receiver alarm indicator is disabled, corresponding to the situation where the receiver module (and vehicle driver) is in close proximity to the transmitter module (and vehicle).

This situation is shown in Figure 12, where the RF signal strength is continuously monitored and when the signal strength exceeds the predetermined trigger level 120, the alarm is disabled. If, as a result of a vehicle security violation, a valid alarm signal is received then, providing the warning indicator is enabled, the warning indicator will be activated. In the meantime, the integrity of the RF link is regularly tested by receiving a valid KIT signal, the strength of which is continuously monitored. Thus, the system provides a low-power, always-on, self-testing, remote vehicle alarm system, which is self- disarming when the driver (receiver) and vehicle (transmitter) are in close proximity.

In other applications, the transmitter and receiver modules may not need to be so small and so can have increased functionality. Figure 13 shows the architecture of one

such transmitter module 130. Here, the transmitter function is actually performed by a smart transceiver 131 and power amplifier 132, which drives the aerial 133. The same type of smart transceiver may also be used in the receiver module to perform the receiver function, thereby offering some simplification of the manufacture process. A keyboard 134 permits the entry of data and the manual activation of the device, whilst a display 135 provides for messaging and confirmation of manually entered data. Processing and control is effected by a microprocessor 136, which is also coupled to interval 137 and external 138 sensors, of the type described previously. The module is powered by an internal battery 139. However, the module may be configured to allow connection to external security systems and associated sensors and power supply. Figure 14 gives an illustration of what the external fascia of the packaged transmitter module might look like.

Dimensions of 10mm x 55mm x20mm would be typical, similar to those of a mobile phone.

Figure 15 shows the architecture of the corresponding receiver module 150, with the receiver function performed by a smart transceiver 151 and aerial 152. Again, a keyboard 153 permits the entry of data and the manual activation, whilst a display 154 provides for messaging and confirmation of manually entered data. Processing and control is effected by a microprocessor 155, which is also coupled to interval 156 and external 57 alarm indicators, and the module has an integral battery 158 for power.

Figure 16 shows an example of the typical connections between microprocessor and other components within the module in more detail. The same basic arrangement will apply to both the transmitter and receiver module. Figure 17 gives an illustration of what the external fascia of the packaged receiver module might look like. Dimensions could be identical to those of the transmitter module, to simplify manufacture, or else somewhat smaller. As shown in Figure 17, the receiver module may be configured to communicate with several transmitter modules, each of which is monitoring the security of different items of property (e. g. shed, gate, car 1, car 2 and barn, as shown).

A further addition to the system is the provision of a docking station 159 for co- operating and communicating with a receiver module, as indicated in Figure 15. An example of such a docking station 180 and its connections 181 to the receiver module are shown in Figure 18. The aim of the docking station is to allow the receiver module to act as a much extended module with enhanced functionality. In this way the receiver module itself can remain small and portable, but can be connected to the docking station, for example by locating it in a matching cradle, to achieve the enhanced capability. Specific

features may include a battery charger 182 to recharge the internal battery of the receiver module, external sensors 183,184 with interface 187, and indicators (siren 185, strobe light 186) with driver 188, and an auto-dialer 189 to initiate a land-line or mobile phone connection. These features may be particularly useful when the receiver module in question is in communication with a large number of transmitter modules, each of which are monitoring a different object. For example, monitoring a whole vehicle lot or caravan park. The docking station allows data to be collected and processed from all the sources and can send status information or alarm message to a wide range of targets. connections to a personal computer, personal digital assistant or network (e. g. the internet). The docking station itself may be equipped with a display 190 and keyboard 191, allowing the entry of data and control instructions and the display of status information.

In conclusion, the present invention provides a comprehensive and integrated remote security system. The heart of the system is a self-testing pulsed RF communications link between transmitter and receiver modules. The use of a periodic "keep-in-touch" (KIT) signal results in minimal power drain on the interval battery, whilst a high-value supercap capacitor ensures the availability of peak power as required. The KIT signal may be encoded with identification or other data. The security element is achieved by the generation and transmission over the RF link of a pulsed alarm signal, in response to a stimulus detected by an internal or external sensor. On reception of an alarm signal, the receiver module activates internal or external alarm indicators. Further power conservation is achieved by modifying the transmitted power level in dependence on an internal sensor, and proximity self-enabling of the alarm indicator is achieved by employing received signal strength indicator technology. Greater functionality, and easy monitoring of multiple transmitters, is provided in the form of an optional docking station, which connects to a receiver module, thereby permitting the receiver module to remain small and compact.