This invention relates to a method and apparatus for determining distance to a radio transponder. A particular, but not exclusive, application of the invention is in keyless entry systems, such as passive keyless entry (PKE) systems used in the automotive market.

PKE systems are becoming increasingly popular in the automotive market. These systems typically comprise a radio transponder housed in a key fob or such like and a vehicle security system mounted in a vehicle that is able to interrogate the radio transponder using radio signals. When a user carrying the transponder approaches the vehicle, the security system transmits a challenge signal to the transponder. When the transponder receives the challenge signal it automatically transmits a response signal back to the security system. The response signal is based in part on the challenge signal and in part on a unique identification (ID) stored in the transponder. The security system verifies that the response signal it receives from the transponder is as expected and, if so, unlocks the vehicle's doors or, in some cases, authorises starting of the vehicle's engine, e.g. at the press of a button. Much effort has been expended improving the security and utility of the PKE systems. In particular, it has been suggested to monitor the distance of the radio transponder from the vehicle. This has a number of advantages. For example, PKE systems can be prone to so-called "relay attacks" by criminals. A relay attack involves a criminal using one or more radio repeaters to extend the range of communication between a vehicle's security system and the user's radio transponder to gain access to the vehicle while the user is out of normal transmission range, e.g. out of sight of the vehicle. Monitoring the distance of the radio transponder from the vehicle allows the security system to guard against relay attacks, e.g. by preventing entry when the transponder is more than a certain distance away from the vehicle. Another advantage of monitoring the distance of the radio transponder from the vehicle is that the vehicle can respond in a more sophisticated manner to a user's approach with knowledge of the user's position. For example, when the user is a few metres away, the vehicle's indicators may flash to acknowledge the users approach; when the user is around 1 m away, the vehicle's interior light may be switched on; when the user is next to the vehicle, the doors may be unlocked; and/or when the user is inside the vehicle, the vehicles engine may be permitted to start. Whilst a number of methods of monitoring the distance of the radio transponder from the vehicle have been considered, the best method is generally considered to be measuring the time of flight (TOF) of the signals between the security system and the transponder. For example, the time between the security system transmitting the challenge signal and receiving the response signal can be measured and, with knowledge of the time taken between the transponder receiving the challenge signal and transmitting the response signal, the time of flight of the radio signals can be deduced. However, in order to accurately measure distance using TOF, the challenge signal and the response signal must comprise wide band radio frequency (RF) signals. If not, it may not be possible to measure TOF accurately (e.g. to within a few ns). Due to regulatory considerations, wideband RF signals can only generally be used at low power. PKE systems using TOF distance measurements generally only therefore operate at low power. The low power of PKE systems makes path loss a significant problem. For example, the radio transponder is typically carried by a user in their pocket, wallet or briefcase. Transmission of the response signal by the transponder is often therefore inhibited by the user's body. Indeed, the human body strongly attenuates radio signals at ultra high frequencies (UHF), typically used by PKE systems. Similarly, reception of the response signal by the security system may be inhibited by the proximity of other objects to the security system's antenna. For example, when the vehicle is parked, the antenna may be close to or virtually touching other metal objects such as nudge barriers or other vehicles in a car park. This can significantly attenuate the signal received at the antenna. The applicants have therefore recognised that the PKE systems can have difficulty in successfully receiving the response signal from the transponder. If the signal is not received with sufficient signal strength, making a TOF measurement may be impossible or the measurement may be inaccurate. Interference can also cause problems. The frequency bands used by PKE systems are typically Industrial, Scientific and Medical (ISM) bands commonly used in other communication systems such as Bluetooth®, Wi-Fi® and such like. Communication devices operating close to the PKE system, and in particular the antenna of the vehicle security system, may therefore interfere with the response signal. Multipath interference is particular problem. For example, the response signal may arrive at the security system over a line of sight (LOS) propagation path and over a reflected propagation path created by reflection from a nearby object, e.g. another vehicle in a car park, with a phase difference created by the different path lengths. In the worst case, the phase difference may be 180° and the signal may be received over each path with the same strength. This can cause 100% destructive interference, with the result that no response signal is received by the security system. Of course, in other more common scenarios, the response signal may suffer partial destructive interference for similar reasons. Overall, it is very likely that multipath interference will cause some deterioration in received signal strength and a corresponding degradation in TOF measurement accuracy. Multipath interference can also result in successful but incorrect TOF measurements being made. For example, a user's body may almost totally block the LOS signal propagation path between the transponder and the security system, but not a reflected propagation path. This can lead to the response signal being received over the reflected path more strongly than over the LOS path. The TOF measurement may therefore be based on the response signal travelling over the (longer) reflected path and give an incorrect estimation of the distance of the transponder from the vehicle. In particular, where the system guards against relay attacks as described above, entry to the vehicle may be prevented even when the user is close to the vehicle. The present invention seeks to over come these problems.

According to the first aspect of the present invention, there is provided an apparatus for determining distance to a radio transponder, the apparatus comprising: at least two antennas for receiving a signal from the transponder; a circuit for combining the output of the antennas generated by the signal; and a signal processor for deriving the time of flight of the signal from the combined output of the antennas to determine the distance to the transponder. Also, according to a second aspect of the present invention, there is provided a method of determining distance to a radio transponder, the method comprising: receiving a signal from the transponder via at least two antennas; combining the outputs of the antennas generated by the signal; and deriving the time of flight of the signal from the combined outputs of the antennae to determine the distance to the transponder. For example, a receiver, such as a vehicle mounted receiving unit of PKE system, may have two or more antennas for receiving signals from a radio transponder. The outputs of the antennas can be combined by the receiver and a TOF measurement made based on the combined output. The TOF measurement is indicative of the distance from the antennas to the radio transponder, e.g. the range of the transponder. By using two or more antennas, the signal is received from the radio transponder more reliably. In particular, the problems of signal attenuation and interference mentioned above are mitigated. For example, when one of the antennas is inadvertently located near to a metallic object, such as another vehicle in a car park, and the signal to that antenna is strongly attenuated, there remains a good chance that the other antenna(s) do(es) not experience this difficulty. Likewise, if LOS path between the transponder and one of the antennas is blocked, e.g. by a user's body, there is a good chance the LOS path(s) to the other antenna(s) is/are not blocked. So, at least one antenna should always be able to receive the signal without significant attenuation. In other words, the apparatus and method of the invention have antenna diversity. Likewise, if a communication device operating at a similar frequency to the invention is operating close to one antenna and causing significant interference, it is unlikely that the other antenna(s) will experience interference as strong as at that antenna. Similarly, if one antenna experiences multipath interference that reduces the signal strength of the received signal, it is unlikely that the other antenna(s) will experience multipath interference that reduces the received signal strength to the same extent, as the other antenna(s) inevitably receive the signal over slightly different paths. So, even when an antenna experiences interference, it should be possible to derive the time of flight of the signal from the combined outputs of the two or more antennas. The outputs of the antennas may be combined in a variety of ways. The result should be that the signal processor is able to derive the time of flight of the signal from a single signal comprising the joint output of the antennas. This can be achieved by adding the outputs of the antennas together. For example, the combining circuit may comprise an adder for adding the outputs of the antennas together. Likewise, the method may comprise adding the outputs of the antennas together. In other words, the outputs of the antennas may be cumulatively combined. However, one problem with this is that, as the outputs of the antennas are added together, they may interfere with one another. This can be mitigated to some extent by the path lengths between the antennas and the combining circuit being equal. One way of doing this is for the wires between the antennas and the combining circuit to be equal in length. Another way is for a delay to be introduced in one or more of the paths to make the signal travel times from the antennas to combining circuit equal. This avoids the possibility of the paths between the antennas and the combining circuit introducing a phase difference in the antenna outputs that causes (destructive) interference. However, any phase difference in the signal at the respective antennas may still be present in the outputs arriving at the combining circuit. It is therefore preferred that the combining circuit comprises a switch for switching between the outputs of the antennas. Likewise, it is preferred that the method comprises switching between the outputs of the antennas. The combined output may therefore comprise a sequence of the outputs of the antennas. This eliminates the possibility of interference between the different outputs. The rate of switching depends on the length of time it takes to receive sufficient information in a single antenna output successfully to derive the time of flight of the signal received via that antenna. In a typical PKE system for example, this length of time might be a minimum of around 0.1 ms. On the other hand, it is desired to switch between the antennas as quickly as possible to be sure that the signal received via all the antennas is included in the combined output. (The signal will be received at the different antennas very close together in time.) So, it is desired to switch between all the antennas within say a few ms. It is therefore preferred to switch between the output of one antenna and the output of another antenna between around every 0.1 ms and 1 ms. TOF measurement is a well established technique. However, it is intended to be performed on a signal received at a single antenna. The results of performing a TOF measurement on the combined output of the invention are less straightforward. Not only may the signal travel over several different propagation paths, but it may also have arrived via more than one antenna. It is therefore likely that the signal is received at several slightly different times. This means that several different flight times are likely to be derivable from the combined output. However, it is preferred that the distance is determined from a single derived flight time. So, the derived flight times might be averaged to allow the distance to be determined. However, in a preferred example, the distance is therefore determined from the strongest derived flight time. This is typically the time of flight of the signal as it is received most strongly. More specifically, it is usually the highest correlation peak in a TOF measurement. In another preferred example, the distance is determined from the earliest derived flight time. This is typically the time of flight of the signal as it is received earliest. More specifically, it is the earliest correlation peak in a TOF measurement. Advantageously, in both of the preferred examples, the derived time of flight used for the distance determination is usually based on the signal as it travels over the LOS path to the nearest antenna to the transponder. Whilst the antennas may be positioned as desired, it can be appreciated from the above that it is preferred that the antennas are spaced apart from one another. In other words, the antennas are typically mounted at mutually remote positions. This makes it more likely that they receive the signal from the transponder over significantly different signal propagation paths. It is also preferred that there are several antennas, e.g. three, four or even six antennas. The more antennas used, the less the problems of multipath and internal interference described above and the more likely a strong signal is received over a LOS path. Most typical uses of the invention involve security systems for vehicles. The antennas may therefore be mounted on a vehicle. Indeed, in order that they are suitably remote from one another, the antennas are typically substantially at the front, rear or sides of the vehicle. Use of the terms "circuit" and "signal processor" above is intended to be general rather than specific. Whilst the invention may be carried out using an actual circuit and processor, the functionality of these features is not limited to this. For example, whilst the processor could be a digital signal processor (DSP) or central processing unit (CPU) for example, it could equally well be implemented using other components, including a circuit. Likewise, whilst the circuit could be an application specific integrated circuit (ASIC), field programmable gate array (FPGA) or a composite metal oxide semiconductor (CMOS) circuit for example, it could equally well be implemented using other components, including a processor. Indeed, the processor and circuit could be implemented as a single component if desired. Likewise, use of the term "radio" above is intended to be general rather than specific. It refers to the use of electromagnetic waves for communication and not to any particular frequency band or wavelength of the electromagnetic spectrum, unless otherwise specified.

Preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a schematic illustration of a passive keyless entry (PKE) system according to the invention; and Figure 2 is an illustration of the PKE system of Figure 1 in operation.

Referring to Figure 1 , a vehicle security system 1 is mounted in a vehicle 2 and comprises a transceiver 3 operable to communicate with a transponder 4 using radio signals. In this embodiment, the vehicle security system 1 and transponder 4 form a passive keyless entry (PKE) system. The transponder 4 is designed to be carried by a user, e.g. the driver of the vehicle, and is housed in a key fob. Of course, in other embodiments, the transponder 4 can be housed in a credit card sized housing or such like, which is convenient for carrying in a wallet or purse. The vehicle security system 1 is able to lock and unlock the vehicle's doors and authorise starting of the vehicle's engine, e.g. at the press of a button, on the basis of communication between the transceiver 3 and the transponder 4. In the illustrated embodiment, the transceiver 3 is connected to two antennas 5, 6 of the security system 1. One of the antennas 5 is mounted in a front bumper of the vehicle 2 and the other of the antennas 6 is mounted in the boot or tailgate of the vehicle 2. In another embodiment, two further antennas are provided, one in each wing mirror of the vehicle 2. In another embodiment, six antennas are provided, one in each wing mirror and one substantially at each of the four corners of the vehicle 2, e.g. front driver side, front passenger side, rear driver side and rear passenger side. In yet another embodiment, the vehicle's radio and/or telephone antenna(s) is/are connected to the transceiver 3 for use as (an) antenna(s) of the system. Of course, these embodiments are just illustrations of how the invention may be implemented and the number and location of the antennas 5, 6 is subject to almost limitless variation. The security system 1 includes a signal combining circuit 7 for combining the outputs of the antennas 5, 6. In one embodiment, the combining circuit 7 comprises an adder for adding the output's of the antennas 5, 6. In another embodiment, the combining circuit 7 comprises a switch for switching between the outputs of the antennas 5, 6. For simplicity and to avoid interference when the outputs are combined, the wired path lengths between the antennas 5, 6 and the combining circuit 7 are equal. A signal processor 8 is also provided in the security system 1 and connected to receive the combined output from the combining circuit 7 and to process the combined output to derive the time of flight of signals between the security system 1 and the transponder 4, as described in more detail below. The transponder 4 has an antenna 9 and a response circuit 10. In this embodiment, the response circuit 10 comprises a memory for storing an identification (ID) code. The response circuit is operable to alter a challenge signal received from the transceiver 3 of the security system 1 using the ID code to produce a response signal for transmission back to the security system. In use, the transceiver 3 of the security system transmits a challenge signal to the transponder 10 via a transmit antenna (not shown). The antenna 9 of the transponder 4 receives the challenge signal and outputs it to the response circuit 10. The response circuit 10 generates a response signal based on the challenge signal and the stored ID code. The transponder 4 then transmits the response signal back to the security system 1. In the example illustrated in figure 1 , the response signal travels over two signal propagations paths A, B to the respective antennas 5, 6 of the security system 1. These propagation paths A, B are both line of sight (LOS) signal propagation paths, but have different lengths, path A being longer than path B. As the signal is received by the antennas 5, 6, it is output to the combining circuit 7. In one embodiment, the combining circuit adds the outputs of the antennas 5, 6 together. In another embodiment, the combining circuit switches between the outputs of the antennas 5, 6. The combining circuit 7 outputs the combined outputs to the transceiver 3, which carries out any appropriate filtering and/or amplification of the received signal in a conventional manner. The combined output is then passed to the signal processor 8. The signal processor 8 correlates the combined output with a correlation sequence expected to be produced by the response circuit 10 of the transponder 4. This can be used to verify that the received response signal has been generated using a required ID and to authorise access to the vehicle 2. It also generates a correlation signal within the signal processor 8 that contains peaks at the times of successful correlation. With knowledge of the time between the signal being received at the antennas 5, 6 and the correlation signal being generated, the time of the correlation peaks can be used to derive the time at which the signals were received at the antennas 5, 6. Furthermore, with knowledge of the time at which the challenge signal was transmitted by the transceiver 3 and the time between the transponder 4 receiving the challenge signal and transmitting the response signal, the combined time of flight of the challenge and response signals can be deduced by the processor 8. In the illustrated example, the response signal is received over two signal propagation paths A, B of different lengths. The correlation signal will therefore include two correlation peaks, one corresponding to the output of the antenna 5 receiving the signal over path A and the other corresponding to the output of the antenna 6 receiving the signal over the path B. In this embodiment, the processor 8 selects the earliest correlation peak in the correlation signal from which to derive the time of flight of the challenge and response signals. This corresponds to the shortest signal propagation path B. In another embodiment, the processor 8 selects the strongest correlation peak from which to derive the time of flight of the challenge and response signals. In the illustrated embodiment, this again corresponds to the shortest signal propagation path B. The selected correlation peak can be used to determine the distance between the security system 1 and the transponder 4. However, it is very likely that the response signal will be received over reflected propagation paths as well as the two LOS propagation paths A, B illustrated in figure 1. Indeed, referring to figure 2, another vehicle 11 may be parked nearby the vehicle 2 in which the security system 1 is used. In this illustration, the response signal is reflected by the nearby vehicle 11 over reflected propagation paths C, D to the antennas 5, 6. So each antenna receives the response signal over both a LOS propagation path A, B and a reflected propagation path C, D. In this illustration, the response signal is received at one of the antennas 5 with a phase difference of around 90° between the LOS path A and the reflected path C; and the signal is received over the reflected path C with around 50% of the strength of the signal received over the LOS path A. This results in around 25% destructive interference at this antenna 5. Similarly, the response signal is received at the other of the antennas 6 with a phase difference of around 180° between the LOS path B and the reflected path D; and in each of the paths B, D with around the same signal strength. This results in substantially 100% destructive interference at this antenna 6. So, the combined output received by the signal processor 8 does not contain any contribution from the antenna 6 that experiences 100% destructive interference. However, the combined output does include a contribution from the antenna 5 experiencing only 25% destructive interference. Whilst no correlation peaks are generated for the signal travelling over paths B and D to the antenna 6 experiencing 100% destructive interference, a correlation peak should be generated for the signal travelling over each of the paths A and C to the antenna 5 experiencing 25% destructive interference. Taking either the strongest or the earliest correlation peak should therefore enable the time of flight of the signal arriving over path A to be derived. The distance to the transponder 4 can therefore be estimated. Furthermore, although this is not the shortest path, it provides a more reliable estimation of the distance between the security system 1 and the transponder 4 than a TOF measurement based on a reflected propagation path. The described embodiments of the invention are only examples of how the invention may be implemented. Modifications, variations and changes to the described embodiments will occur to those having appropriate skills and knowledge. These modifications, variations and changes may be made without departure from the spirit and scope of the invention defined in the claims and its equivalents.