WO2005040846A1

WO2005040846A1 - Line of sight signal detection

Info

Publication number: WO2005040846A1
Application number: PCT/IB2004/052175
Authority: WO
Inventors: Bernard Hunt; Martin S. Wilcox
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2003-10-25
Filing date: 2004-10-22
Publication date: 2005-05-06
Also published as: GB0324954D0

Abstract

Pulse propagation time between a transmitter station and a receiver is measured at s4 and s6. This can occur by detecting a step increase in signal strength or by correlating a locally generated replica of the pulse signal which was known to have been transmitted with the received signal. The distance between the stations is then measured at step s7. The phase shifts of the 10 transmitted pulse signals are then estimated using the propagation time t, and the difference between the phase shift estimates at this time calculated. The estimated phase difference is then compared with the output of the phase comparator time stamped at time t (step s8). If the phase difference is not substantially as estimated, the received signals are determined not to be suitable line of sight signals for distance determination and a signal indicating this is generated (step s5). If the phase difference is small, the amplitudes of the two pulse signals are compared (step s9), allowing for any known difference in the ERP of the transmitted signals. If the pulse signals are reflected or pass through an intervening obstacle, an attenuation differential will be detected. If the attenuation differential falls outside of an acceptable range, the received signals are determined not to be suitable line of sight signals for distance determination and a signal indicating this is generated (step s5).

Description

DESCRIPTION LINE OF SIGHT SIGNAL DETECTION

The present invention relates to the detection of line of sight signals.

The propagation time of a signal from a first station to a second station can be used to determine the distance between the two stations. However, the distance determined will only be accurate if the signal follows the line of sight path between the stations. Consequently, it is desirable to be able to determine whether a useable line of sight path signal is available for distance determination.

According to the present invention, there is provided a method of determining the presence of a line of sight signal path from a transmitter station to a receiver station which is useable for determining the distance between said stations, the method comprising: transmitting first and second signals from a transmitter on significantly different frequencies; receiving said signals at the receiver; determining characteristics of the received signals; comparing the difference between a determined characteristic of the received first signal and the corresponding determined characteristic of the received second signal with a reference; and determining whether a suitable line of sight signal path exists in dependence on the result of said comparison. The frequencies of the first and second signals must be sufficiently different that characteristics of the signals can be expected to be detectably differentially affected by reflection from objects. The characteristics may comprise any or all of phase, amplitude or attenuation and polarisation. Preferably, a method according to the present invention comprises: synchronising clocks at the transmitting and receiver stations; transmitting said first and second signals at one or more predetermined times; preparing to receive said signals at the receiver station at said predetermined time or times; and determining a propagation time for said signals from the time of arrival of said signals at the receiver station using the clock of the receiver station. More preferably, a method according to the present invention includes estimating a value for said difference on the basis of the determined propagation time, wherein said reference is said estimated value. Preferably, said signals are transmitted simultaneously or substantially simultaneously. However, they may be transmitted sequentially in environment that are effectively static on the time scale of the transmission of the signals. The first and second signals can modulated with different spreading codes. According to the present invention, there is provided a receiver station for use in a method according to the present invention. Such a receiver station comprises: receiving means for receiving signals on significantly different frequencies; processing means configured for: determining characteristics of signals received by the receiving means, comparing the difference between a determined characteristic of a first received signal and the corresponding determined characteristic of a second received signal with a reference, and determining whether a suitable line of sight signal path exists in dependence on the result of said comparison. Orthogonally polarised antennas may be provided so that polarisation differences can be compared, although this can be achieved instead using any other suitable type of antenna.. A receiver station according to the present invention may comprise a clock with the processing means configured to be responsive to the clock to preparing to receive signals at a predetermined time and to determine a propagation time for a signal received as a result of said preparation from its time of arrival using the clock. More preferably, the processing means is configured for estimating a value for said difference on the basis of a determined propagation time for a received signal, wherein said reference is said estimated value. Preferably the receiving means comprises first and second receiver circuits for sequentially receiving signals having significantly different centre frequencies. Alternatively, the receiving means comprises first and second receiver circuits for receiving signals modulated with different spreading codes and having significantly different centre frequencies. In either case, which the signals can have overlapping spectra. Preferably, the receiving means comprises first and second receiver circuits for concurrently receiving signals on significantly different frequencies. According to the present invention, there is provided a radio system comprising a receiver station according to the present invention and a transmitter station configured for transmitting two signals having significantly different centre frequencies. Preferably, the transmitter station includes a clock and is configured for transmitting two signals on significantly different frequencies in dependence on said clock, and the clocks of the receiver station and the transmitter station are synchronised.

Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 shows transmitting and receiver stations with a line of sight path therebetween; Figure 2 shows the transmitting and receiver stations without a line of sight path therebetween; Figure 3 is a block diagram of a first transmitter station; Figure 4 is a block diagram of a first receiver station; Figure 5 is a flowchart illustrating the operation of the processor of the first receiver station; Figure 6 is a block diagram of a second receiver station; Figure 7 is a flowchart illustrating the operation of the processor of the second receiver station;

A first embodiment of the present invention will now be described. Referring to Figure 1, a transmitter station 1 can transmit radio waves by a line of sight path to a receiver station 2. The path is considered to be light of sight because it is straight and there are no intervening obstacles having a significant effect on the propagation of the radio waves. Referring to Figure 2, in another situation, the transmitter station 1 can transmit radio waves to the receiver 1. However, there is no direct propagation path from the transmitter station 1 to the receiver 2 because of an intervening obstacle 3. Instead, signals reach the receiver 2 from the transmitter 1 by reflection from an object 4. Referring to Figure 3, the transmitter station 1 comprises an accurate clock 5, a baseband signal processing unit 6, a first rf (radio frequency) unit 7, a second rf unit 8, a combiner 9 and a dual band antenna 10. The baseband signal processing unit 6 processes information signals to produce modulating signals that are provided to the first rf unit 7. The first rf unit 7 modulates a carrier using the modulating signal from the baseband processing unit 6 and outputs the modulated carrier to the combiner 9. These components are used for the transmission of data, e.g. speech, picture data, downloadable files or any other type of data, and also for allowing distance determination to be performed. The clock 5 outputs simultaneous control signals at predetermined times, e.g. every 5 seconds, to the baseband processing unit 6 and the first and second rf units 7, 8. The baseband processing unit 6 responds to its control signal from the clock 5 by ceasing the output the modulating signal and buffering incoming information signals. The first rf unit 7 responds to its control signal from the clock 5 by interrupting the generation of the modulated carrier and outputting a short rf pulse on a predetermined measurement frequency, e.g. 870MHz. The second rf unit 8 responds to its control signal from the clock by outputting a short rf pulse to the combiner 9. The carrier generator of the second rf unit 8 is phase-locked to that of the first rf unit 7 and their pulses are synchronised. The pulse produced by the second rf unit 7 is on a significantly different test frequency, e.g. 2.61 GHz. The outputs of the first and second rf units 7, 8 are fed to the combiner 9 which combines them and feeds the combined signal to the dual band antenna 10. Referring to Figure 4, the receiver station 2 comprises a dual band antenna 11 , a diplexer 12, a first receiver unit 13, a second receiver unit 14, a local oscillator unit 15, a baseband processing unit 16, a processor 17, an accurate clock 18 and a phase comparator 19 which compares the phases of the signals received by the first and second receiver units 13, 14 and supplies the result to the processor 17. The diplexer 12 splits signals received by the antenna 1 1 according to frequency. For example, signals in a band from 868MHz to 915MHz are output to the first receiver unit 13 and signals in a band from 2.4GHz to 2.7GHz to the second receiver unit 14. The transmitted data is recovered by the baseband processing unit 16 in a conventional manner. Alternatively, if the transmitter is arranged to transmit some or all of the data at both of the transmission frequencies, the receiver station 2 can be modified to benefit from frequency diversity. Here, the baseband processing unit 16 is connected to both of the receivers 13, 14 (not shown in the Figure), allowing the transmitted data to be recovered using I and Q data from both of the receivers 13, 14, thus increasing the reliability of data transmission. The local oscillator unit 15 outputs quadrature signal pairs, i.e. in-phase (I) and quadrature (Q) signals, at different frequencies to the first and second receiver units 13, 14 respectively. The first receiver unit 13 operates as a direct conversion receiver and outputs baseband I and Q signal samples to the baseband processing unit 16 and the processor 17. The baseband processing unit 16 regenerates the original information signal from the I and Q signal samples from the first receiving unit 13. The second receiver unit 14 is similar to the first receiver unit 13. However, it only outputs I and Q signal samples to the processing unit 17. The clock 18 intermittently outputs control signals to the local oscillator unit 15 and the processor 17. The clocks 5, 18 of the transmitter and the receiver stations 1 , 2 are synchronised so that both generate their control signals at the same time. The local oscillator unit 15 responds to its control signal from the clock 18 by tuning its outputs to the predetermined measurement frequencies which are 870MHz and 2.61 GHz. The lower frequency output is fed to the first receiving unit 13 and the higher frequency output is fed to the second receiving unit 14. The processor 17 responds to its control signal from the clock 18 by performing a measuring process, which is described below. A measurement process for measuring the distance between the transmitter station 1 and the receiver station 2 is triggered when the processor 17 of the receiver station 2 receives a control signal from the associated clock 18. Referring to Figure 5, a timer is started (step s1) when the control signal from the clock 18 is received by the processor 17. Once the timer has been started, the processor 17 starts buffering samples from the first and second receiving units 13, 14 and the phase comparator 19 (step s2). The buffered samples are time stamped. The buffering continues until the timer times out after a period, which in this example corresponds to the lengths of the pulses generated by the first and second rf units 7, 8 of the transmitter station 1 (step S3). The buffered samples from the second receiving unit 14 are analysed to determine the beginning of the pulse generated by the second rf unit 8 of the transmitter station 1 (step s4). This can occur by detecting a step increase in signal strength or the crossing of a threshold signal strength. A better technique involves correlation of a locally generated replica of the pulse signal which was known to have been transmitted with the received signal. The location of the beginning of the pulse is obtained by shifting the phase of the replica with the received pulse until there is maximum correlation, and determining the amount of phase shift present at that point. If the pulse cannot be identified, there may be a fault, the stations 1 , 2 may be too far apart or there may be interference from other stations or other signal paths. Consequently, the processor 18 generates a signal indicating that the pulse signal could not be identified (step s5). If the pulse is detected, the time stamp of the first sample pair, which is recognisably part of the pulse, is taken as the propagation time of the pulse signal from the transmitter station 1 (step s6). It is axiomatic that the line of sight signal, if present, will arrive first because it will have followed the shortest path. A value for the distance between the transmitter and receiver stations 1 , 2 can then be calculated using: s = t -3xl0⁸ where t is the time indicated by the time stamp and s is the distance value (step 7). The value of twill need to be adjusted to compensate for signal delays in the transmitter station 1 and the receiver station 2 to get the true antenna to antenna propagation time. The adjustment consists of subtracting the value of the known delays in the transmitter station 1 and the receiver station 2 from the value of t before multiplying it by the speed of propagation of the signal, 3x10^Λ8. The value of the known delays could be stored in the memory of either the transmitter station 1 , the receiver station 2 or both, and could be transmitted from one to the other to aid the calculation. Furthermore, the value could be updated periodically to account for variations in the circuit delays with aging and temperature. The phase shifts of the transmitted pulse signals are then estimated using the propagation time t . The difference between the phase shift estimates at this time is then calculated. The estimated phase difference is then compared with the output of the phase comparator time stamped at time t (step s8). If the phase difference is not substantially as estimated, i.e. the difference between the estimate and measured values is not small, the received signals are determined not to be suitable line of sight signals for distance determination and a signal indicating this is generated (step s5). If the phase difference is small, the amplitudes of the two pulse signals are compared (step s9), allowing for any known difference in the effective radiated powers (ERP) of the transmitted signals. If the pulse signals are reflected or pass through an intervening obstacle, the attenuation of the signals will often be frequency dependent and an attenuation differential will be detected. If the attenuation differential falls outside of an acceptable range, the received signals are determined not to be suitable line of sight signals for distance determination and a signal indicating this is generated (step s5). If the attenuation differential is within the acceptable range, the processor 17 makes the calculated value s available to other processes as the distance between the transmitter station 1 and the receiver station 2 (step s10). A second embodiment of the present invention will now be described. In the second embodiment, the transmitter station 1 is as shown in Figure 3. Referring to Figure 6, a receiver station 202 comprises first and second dual band antennas 211a, 211b, first and second diplexers 212a, 212b, a first receiver unit 213, a second receiver unit 214, a local oscillator unit 215, a baseband processing unit 216, a processor 217 and an accurate clock 218. The first antenna 211a is vertically polarised and the second antenna 211b is horizontally polarised. Both antennas 211a, 211b are onmidirectional in this embodiment. The diplexers 212a, 212b split signals received by the antennas 211a, 211b according to frequency. For example, signals in a band from 868MHz to 915MHz are output to the first receiver unit 213 and signals in a band from 2.4GHz to 2.7GHz to the second receiver unit 214. The local oscillator unit 215 outputs quadrature signal pairs at different frequencies to the first and second receiver units 213, 214 respectively. The first receiver unit 13 operates as a direct conversion receiver and outputs baseband I and Q signal samples to the baseband processing unit 216 and the processor 17. The samples output to the baseband processor 216 are the roots of the squares of the samples of the signals from the two antennas 211a, 211b. However, separate sample pairs for each antenna 211a, 211b are output to the processor 217. The baseband processing unit 216 regenerates the original information signal from the I and Q signal samples from the first receiving unit 213. The second receiver unit 214 is similar to the first receiver unit 213. However, it only outputs I and Q signal samples to the processing unit 17. The clock 218 intermittently outputs control signals to the local oscillator unit 215 and the processor 217. The clocks 5, 218 of the transmitter and the receiver stations 1 , 202 are synchronised so that both generate their control signals at the same time. The local oscillator unit 215 responds to its control signal from the clock

218 by tuning its outputs to the predetermined measurement frequencies which are 870MHz and 2.61GHz in this example. The lower frequency output is fed to the first receiving unit 213 and the higher frequency output is fed to the second receiving unit 214. The processor 217 responds to its control signal from the clock 218 by performing a measuring process, which is described below. A measurement process for measuring the distance between a transmitter station 1 and the receiver station 202 is triggered when the processor 217 of the receiver station 2 receives a control signal from the associated clock 218. Referring to Figure 7, a timer is started (step s101) when the control signal from the clock 218 is received by the processor 217. Once the timer has been started, the processor 217 starts buffering samples from the first and second receiving units 212, 213 (step s102). The buffered samples are time stamped. The buffering continues until the timer times out after a period corresponding to the lengths of the pulses generated by the first and second rf units 7, 8 of the transmitter station 1 (step s103). The buffered samples from the second receiving unit 214 are analysed to determine the beginning of the pulse generated by the second rf unit 8 of the transmitter station 1 (step s104), e.g. by detecting a step increase in signal strength or the crossing of a threshold signal strength or by correlating a locally generated replica of the pulse signal which was known to have been transmitted with the received signal. If the pulse cannot be identified, there may be a fault, the stations may be too far apart or there may be interference from other stations or other paths. Consequently, the processor 18 generates a signal indicating that the pulse signal could not be identified (step s105). If the pulse is detected, the time stamp of the first sample pair, which is recognisably part of the pulse, is taken as the propagation time of the pulse signal from the transmitter station 1 (step s106). A value for the distance between the transmitter and receiver stations 1 , 202 can then be calculated using: s = t-3xl0⁸ where t is the time indicated by the time stamp and s is the distance value (step 107). The value of t may need to be adjusted to compensate for signal delays in the transmitter station 1 and the receiver station 202 to get the true antenna to antenna propagation time. If the pulses from the transmitter station 1 have followed a line of sight path, the polarisations of the two signals at the receiver station 2 should be the same and unchanged. Consequently, the processor 217 determines the polarisation of each signal from the samples of the signals from the two antennas 211a, 211 b. The polarisations are then compared (step s108). If the polarisations are not similar, a signal indicating that a useable line of sight signal does not exist is generated (step s105). However, if the polarisations are similar, the processor 17 makes the calculated value s available to other processes as the distance between the transmitter station 1 and the receiver station 202 (step s109). It is not necessary that the pulses at the different frequencies are transmitted at the same instance, since the determination of the presence of a line-of-sight can be made using sequentially transmitted pulses. The frequencies of sequentially transmitted pulses can be the same as those described in the above embodiments, or take any other suitable values. The bandwidths of the pulses is not critical to the invention; the pulses can be narrowband or wideband. If ultra-wideband pulses are used, It will be understood by the skilled person that some modification of the line-of sight determination processes might be beneficial. In particular, following the determination of a pulse propagation time, it may be easer to use a look-up table completed with appropriate information, rather than a simple algorithm as could be used in narrowband implementations. The skilled person will appreciate how to populate such a look-up table. In another embodiment, a receiver station like the station 2 of Figure 4, is used, although the diplexer 12 is omitted, pulses from the antenna instead being applied directly to the receiver units 13, 14. Wideband pulses are transmitted by the transmitter station 1 sequentially. A first transmitted pulse has a bandwidth which overlaps with the bandwidth of a subsequently transmitted second pulse, although centre frequencies of the two pulses are substantially different. The receiver station 2 is able to determine whether or not there is a line of sight with the transmitter station because of the difference in the centre frequencies. The system might be termed an ultra-wideband system. In a further embodiment, wideband or ultra-wideband pulses having overlapping spectra can be transmitted simultaneously. Here, the different pulses are differentiated by the use of different spreading codes. In this embodiment, the receiver 13 is used for the reception of a pulse modulated with a first spreading code and a first centre frequency, and the receiver 14 is used for the reception of a pulse modulated with a second spreading code and a centre frequency separated from the centre frequency of the first pulse. In this case, the signals can be separated even though they have overlapping spectra through demodulation using a different spreading code in each receiver 13, 14. Although in the above, the transmission of pulses is discussed, it will be appreciated that the pulse may form a part of a continuous stream of data, or may be the start of a data burst, or may take any other form. The scope of the invention is not intended to be limited by the embodiments described but only by the appended claims.

Claims

1. A method of determining the presence of a line of sight signal path from a transmitter station (1) to a receiver station (2) which is useable for determining the distance between said stations, the method comprising: transmitting first and second signals from a transmitter on significantly different frequencies; receiving said signals at the receiver; determining characteristics of the received signals; comparing (sδ, s9) the difference between a determined characteristic of the received first signal and the corresponding determined characteristic of the received second signal with a reference; and determining whether a suitable line of sight signal path exists in dependence on the result of said comparison.

2. A method according to claim 1 , wherein said characteristics comprise phase.

3. A method according to claim 1 or 2, wherein said characteristics comprise amplitude or attenuation.

4. A method according to claim 1 , 2 or 3, wherein said characteristics comprise polarisation.

5. A method according to any preceding claim, comprising: synchronising clocks 5, 18) at the transmitting and receiver stations; transmitting said first and second signals at one or more predetermined times; preparing to receive said signals at the receiver station at said predetermined time or times; and determining a propagation time for said signals from the time of arrival of said signals at the receiver station using the clock of the receiver station.

6. A method according to claim 5, comprising estimating a value for said difference on the basis of the determined propagation time, wherein said reference is said estimated value.

7. A method according to any preceding claim, wherein said signals are transmitted sequentially.

8. A method according to any of claims 1 to 6, wherein said signals are transmitted simultaneously or substantially simultaneously.

9. A method according to claim 8, in which the first and second signals are modulated with different spreading codes.

10. A receiver station for use in a method according to any preceding claim, the receiver station comprising: receiving means (13, 14) for receiving signals on significantly different frequencies; processing means (16, 17) configured for: determining characteristics of signals received by the receiving means, comparing the difference between a determined characteristic of a first received signal and the corresponding determined characteristic of a second received signal with a reference, and determining whether a suitable line of sight signal path exists in dependence on the result of said comparison.

11. A receiver station according to claim 10, wherein said characteristics comprise phase.

12. A receiver station according to claim 10 or 11 , wherein said characteristics comprise amplitude or attenuation.

13. A receiver station according to claim 10, 11 or 12, including polarisation sensitive antennas (211a, 211 b), wherein said characteristics comprise polarisation.

14. A receiver station according to any one of claims 10 to 13, comprising a clock (18), wherein the processing means (17) is configured to be responsive to the clock to preparing to receive signals at a predetermined time and to determine a propagation time for a signal received as a result of said preparation from its time of arrival using the clock.

15. A receiver station according to claim 14, wherein the processing means (17) is configured for estimating a value for said difference on the basis of a determined propagation time for a received signal, wherein said reference is said estimated value.

16. A receiver station according to any one of claims 10 to 15, wherein the receiving means comprises first and second receiver circuits (13, 14) for concurrently receiving signals on significantly different frequencies.

17. A receiver station according to any one of claims 10 to 15, wherein the receiving means comprises first and second receiver circuits (13,

14) for sequentially receiving signals having significantly different centre frequencies.

18. A receiver station according to claim 17, wherein the receiving means comprises first and second receiver circuits (13, 14) for receiving signals modulated with different spreading codes and having significantly different centre frequencies.

19. A receiver station as claimed in any of claims 16 to 18, in which the signals have overlapping spectra.

20. A radio system comprising a receiver station (2) according to any one of claims 10 to 19 and a transmitter station (1) configured for transmitting two signals having significantly different centre frequencies.

21. A radio system comprising a receiver station (1) according to any one of claims 14 or 15 and a transmitter station (2) including a clock and configured for transmitting two signals on significantly different frequencies in dependence on said clock, wherein the clocks of the receiver station and the transmitter station are synchronised.