CRC-Based Message Detection/Demodulation Apparatus and Method for Radio Frequency Signals

• US 20090075674A1
• Filed: 04/23/2007
• Published: 03/19/2009
• Est. Priority Date: 04/22/2006
• Status: Active Grant

• Current Assignee: Locus Location Systems, LLC
• Original Assignee: Locus Location Systems, LLC
• Inventors: Qahwash, Murad
• Granted Patent: US 8,019,360 B2
• Time in Patent Office: Days
• US Class Current: 455/456.200
• CPC Class Codes: H04B 17/27 for locating or positioning the transmitter
• CPC Class Codes: H04W 64/00 Locating users or terminals or network equipment for network management purposes, e.g. mobility management

Abstract

A method for determining a location of a wireless transmitting device transmitting a digitally modulated transmitted radio frequency signal. The method comprises, at spaced-apart receivers: generating a received radio frequency signal and including time tags therein responsive to the transmitted radio frequency signal, detecting a destination message signal responsive to the received radio frequency signal, comparing the destination message signal and the received radio frequency signal and determining a time of arrival of the destination message signal responsive to the step of comparing and responsive to the time tags. The method further comprises determining a pairwise time difference of arrival of the received radio frequency signal for two pairs of receivers and determining the location of the device responsive to the pairwise time difference of arrival.

Claims

1. A method for determining a location of a wireless transmitting device transmitting a digitally modulated transmitted radio frequency signal, the method comprising:

at spaced-apart receivers;

generating a received radio frequency signal and including time tags therein responsive to the transmitted radio frequency signal;

detecting a destination message signal responsive to the received radio frequency signal;

comparing the destination message signal and the received radio frequency signal;

determining a time of arrival of the destination message signal responsive to the step of comparing and responsive to the time tags;

determining a pairwise time difference of arrival of the received radio frequency signal for two pairs of receivers; and

determining the location of the device responsive to the pairwise time difference of arrival.

2. The method of claim 1 wherein the transmitted radio frequency signal comprises a radio frequency carrier modulated by a source message signal, and wherein the source message signal is generated by subjecting an information signal to scrambling, encoding and interleaving processes, and wherein the step of detecting the destination message signal further comprises subjecting the received radio frequency signal to inverse processes of the scrambling, encoding and interleaving processes to detect the destination message signal.

3. The method of claim 1 wherein the transmitted radio frequency signal comprises a radio frequency carrier modulated by a source message signal, and wherein the source message signal lacks a preamble and synchronization information.

4. The method of claim 1 wherein die transmitted radio frequency signal comprises a radio frequency carrier modulated by a source message signal further comprising an inbound control signal in a trunked radio system, and wherein the source message signal further comprises an identifier of the transmitting device.

5. The method of claim 1 wherein the transmitted radio frequency signal comprises a GFSK radio frequency signal.

6. The method of claim 1 wherein the destination message signal comprises destination message signal samples and the received radio frequency signal comprises received radio frequency signal samples, and wherein the step of detecting further comprises determining the destination message signal samples from the received radio frequency signal samples.

7. The method of claim 6 wherein the received radio frequency signal samples comprise sample groups each sample group comprising an equal number of samples, and wherein the time tags are present between two juxtaposed sample groups.

8. The method of claim 6 wherein a number of destination message signal samples is less than a number of received radio frequency signal samples.

9. The method of claim 6 wherein the step of comparing further comprises shifting the received radio frequency signal samples relative to the destination message signal samples until a substantial match is detected, and wherein the step of determining a time of arrival is further responsive to a combination of the time tag and a number of shifts until the substantial match is detected.

10. The method of claim 6 wherein the step of comparing further comprises shifting the received radio frequency signal samples relative to the destination message signal samples, determining a correlation value for a time internal defined by each shift and determining a peak of the correlation values and the time interval during which the peak occurred, and wherein the step of determining the time of arrival is responsive to the time interval during which the peak occurred.

11. The method of claim 10 further comprising interpolating the tune interval during which the peak occurred to determine a time subinterval of an interpolated peak, and wherein the step of determining the time of arrival is responsive to the time subinterval of the interpolated peak.

12. The method of claim 6 wherein the step of comparing further comprises shifting the destination message signal samples relative to the received radio frequency signal samples, determining a correlation value for a time internal defined by each shift and determining a peak of the correlation value and the time interval during which the peak occurred, and further comprises interpolating the time interval during which the peak occurred to determine a time subinterval of an interpolated peak, and wherein the step of determining the time of arrival is responsive to a combination of a time tag immediately preceding the time interval during which the peak occurred, a number of shifts to the peak of the correlation value and the time subinterval of the interpolated peak.

13. The method of claim 1 wherein the step of determining the time of arrival further comprises conveying the time of arrival from each receiver to a processing site, and wherein the step of determining the pairwise time difference of arrival further comprises determining, at the processing site, the pairwise time difference of arrival for a plurality of receiver pairs, and the step of determining the location further comprises determining the location at the processing site.

14. The method of claim 1 wherein the transmitted radio frequency signal is modulated by a single block message signal or by a double block message signal.

15. The method of claim 1 wherein the step of generating the received radio frequency signal further comprises generating received radio frequency signal samples and inserting a time stamp into the received radio frequency samples at equal intervals

16. The method of claim 15 wherein each one of the equal intervals comprises 3072 received radio frequency signal samples.

17. The method of claim 1 further comprising generating received radio frequency signal samples responsive to the received radio frequency signal, and wherein the step of detecting the received message signal further comprises:

(a) incrementally loading the received radio frequency signal samples to successive memory locations of a buffer;

(b) selecting received radio frequency signal samples stored in selected memory locations;

(c) for each selected sample, determining a detected sample responsive to the selected sample;

(d) loading the detected samples into a memory device, wherein a capacity of the memory device is sufficient to store one detected sample for each symbol of the destination message signal;

(e) analyzing the detected samples to determine if the detected samples form a valid destination message signal;

(f) repeating the steps (a) through (e) until a result at the step (e) indicates the detected samples form a valid destination message signal.

18. The method of claim 17 wherein a step (e) further comprises applying a cyclical redundancy code check to the detected samples to determine if the detected samples form a valid destination message signal.

19. The method of claim 17 wherein responsive to the detected samples forming a valid destination message signal, the method further comprises:

(j) encoding the detected samples using an error detection code to produce encoded samples;

(k) comparing the encoded samples with the detected samples;

(l) determining a bit error rate responsive to the step (k);

(m) if the bit error rate is less than a predetermined threshold, declaring the detected samples to be valid; and

(n) if the bit error rate is greater than the predetermined threshold, returning to the step (a) for loading another received radio frequency signal sample to the next successive memoir location.

20. The method of claim 17 wherein responsive to incrementally loading another received radio frequency signal sample to a successive memory location according to the step (a), the selected memory locations of the step (b) are incremented to the next successive memory location.

21. The method of claim 17 wherein a number of memory locations between two consecutive memory locations comprises a number of samples in each symbol of the destination message signal.

22. The method of claim 17 wherein the buffer comprises a circular buffer.

23. The method of claim 17 wherein the buffer comprises a number of memory locations equal to 1+(N−

1)(M), where M is a number of samples in each symbol of the destination message signal and N is a number of symbols in the destination message signal

24. The method of claim 17 wherein the detected samples will form a valid destination message signal after processing only one sample of the last symbol of the destination message signal.

25. The method of claim 1 wherein the received message signal comprises received message signal symbols, and wherein the step of detecting further comprises determining the received message signal symbols responsive to the received radio frequency signal.

26. The method of claim 1 wherein the step of generating the received radio frequency signal comprises generating baseband I and Q signals responsive to the transmitted radio frequency signal and inserting the time tags in the I and Q signals.

27. The method of claim 1 wherein the destination message signal detected at each receiver includes an identification of the transmitting device transmitting the modulated transmitted radio frequency signal from which the destination message signal is detected, and wherein the step of determining the pairwise time difference of arrival comprises determining the pairwise time difference of arrival of the received signal for two pairs of receivers when the destination message signal associated with each received signal for the two pairs of receivers s includes an identification of the same transmitting device.

28. A method for detecting a modulated signal, the modulated signal received at a receiver and representing a digital message comprising message symbols configured in an information portion and an error portion, the method comprising:

(a) sampling the modulated signal at a sampling rate sufficient to generate at least two message samples for each message symbol;

(b) loading modulated signal samples to incremental memory locations of a first buffer while overwriting a sample previously stored in the memory location;

(c) selecting modulated signal samples stored in selected memory locations of the first buffer, wherein the selected memory locations are incremented responsive to loading one modulated signal sample to a memory location of the first buffer;

(d) for each selected sample, determining a detected symbol represented by the selected sample;

(e) loading the detected symbols into a second buffer, wherein a capacity of the second buffer is sufficient to store at least one sample for each message symbol;

(f) determining whether the detected symbols form a valid message; and

(g) repeating the steps (b) through (f) until a result at the step (f) indicates the detected symbols form a valid message.

29. The method of claim 28 wherein the error portion comprises a cyclic redundancy code portion.

30. The method of claim 28 wherein the step of sampling further comprises producing I and Q signals responsive to the modulated signal, sampling the I and Q signals to produce respective I and Q samples and generating the modulated signal samples responsive to the I and Q samples.

31. The method of claim 28 wherein the second buffer comprises L+1 memory locations, wherein L (−

1)(M), wherein N is the number of samples per message symbol and M is the number of symbols per message.

32. The method of claim 28 wherein a step (d) further comprises for each selected sample, comparing the selected sample to a threshold to determine the detected symbol represented by the selected sample.

33. The method of claim 32 wherein the message symbols comprise message bits and the threshold comprises a zero value, and wherein if the selected sample is greater than zero the detected symbol is a one and if the selected sample is less than zero the selected sample is a zero.

34. The method of claim 28 wherein the detected symbols comprise an information portion and an error portion, and wherein the step (o) further comprises processing the error portion to determine if the detected symbols comprise a valid message.

35. The method of claim 34 wherein the error portion comprises a cyclic redundancy code.

36. The method of claim 28 wherein a distance between two successive selected memory locations of the first buffer is equivalent to a number of samples of a message symbol.

37. The method of claim 28 further comprising:

(h) determining the information bits from the valid message;

(i) encoding the information bits to form an encoded message;

(j) comparing the encoded message and the valid message; and

(k) determining a bit error rate based on the results of step Hi).

38. The method of claim 28 wherein the digital message is formed from the message symbols by convolutionally encoding and interleaving the message symbols, wherein a step (C further comprises:

(f1) deinterleaving the detected symbols;

(f2) convolutionally decoding the results of a step (f1);

(f3) performing a cyclic redundancy code check on the results of a step (f2) to determine whether the results of the step (f2) form a valid message.

39. A method for determining a time of arrival of a message at a receiver, a transmitter transmitting a transmitted signal comprising a carrier signal modulated by the message, the method comprising:

receiving the transmitted signal at the receiver to produce a received signal;

sampling the received signal to produce received signal samples and including time stamps at intervals within the received signal samples;

processing the received signal samples to detect the message, wherein the message comprises message samples; and

correlating the received signal samples and the message samples to determine the time of arrival of the message.

40. The method of claim 39 wherein a received signal sample frequency in samples per second is greater than a message sample frequency in samples per second.

41. The method of claim 39 wherein the receiver receives a plurality of transmitted signals from a plurality of transmitters, each of the plurality of transmitted signals serially subjected to the step of sampling to produce a stream of signal samples, and wherein the stream of signal samples is subjected to the step of processing to detect messages, the method further comprising selecting a desired message from the messages, and wherein the step of correlating correlates the message samples of the desired message with the stream of signal samples to determine the time of arrival of the message signal from among the plurality of signals from which the desired message was detected.

42. The method of claim 41 wherein the step of selecting the desired message further comprises selecting the desired message responsive to identification information within the message.

43. The method of claim 42 wherein the identification information comprises an identification of a transmitter from the plurality of transmitters.

44. The method of claim 43 further comprising determining a time of arrival of the message transmitted from a desired transmitter responsive to an identification of the desired transmitter within the message.

45. An apparatus for detecting a valid message from among received signal samples at a receiving site, the message comprising symbols, the apparatus comprising:

first memory locations for storing the received signal samples, wherein successive received signal samples are incrementally input to successive first memory locations;

bit detectors, each bit detector associated with one of the first memory locations, wherein the sample value stored in the one of the first memory locations is input to the associated bit detector for producing a bit value;

a memory buffer having a number of second memory locations equal to the number of bit detectors, each bit detector supplying the bit value to the associated second memory location; and

a first module operating on the bit values in the memory buffer for determining if the bit values present a valid message, and wherein if a valid message is not presented, a next received signal sample is input to a next one of the first memory locations, overwriting a sample previously stored therein and the bit detectors are associated with a next one of the first memory locations, the first module operating on bit values until a valid message is presented in the memory buffer.

46. The apparatus of claim 45 wherein the first module comprises an error detecting module for determining the presence of errors in the bit values, wherein at least one error indicates that the bit values do not comprise a valid message.

47. The apparatus of claim 45 wherein the first memory locations associated with the bit detectors are spaced apart a number of first memory locations equal to a number of samples in a message symbol.

48. The apparatus of claim 45 wherein the first memory locations are configured in a circular orientation.

49. The apparatus of claim 45 wherein the valid message comprises an information portion and an error detection portion, the apparatus further comprising a second module operating on the information portion to generate a candidate valid message, the second module further comparing the valid message bits and the candidate valid message to determine any bits not in agreement.

50. The apparatus of claim 49 wherein responsive to one or more bits not in agreement, a next received signal sample is input to a next one of the first memory locations, overwriting a sample previously stored therein and the bit detectors are associated with a next one of die first memory locations, the first module operating on bit values until a valid message is presented in the memory buffer.

51. An apparatus for determining a location of a wireless transmitting device transmitting a transmitted RF signal, the apparatus comprising:

at spaced-apart receivers;

a first module for generating a received RF signal and including time tags therein responsive to the transmitted RF signal, the time tags synchronized at the receivers;

a second module for determining a valid message responsive to the received RF signal;

a third module for comparing the valid message and the received RF signal to determine a time of arrival of the message further responsive to the time tags;

determining a pairwise time difference of arrival of the message at two pairs of receivers; and

determining the location of the device responsive to the pairwise time difference of arrival.

52. The apparatus of claim 51 wherein the second module produces a plurality of candidate messages responsive to the received RF signal and determines die valid message responsive to an error detection operation on the candidate messages.

53. The apparatus of claim 51 wherein the third module comprises a correlator for shifting samples of the message relative to samples of the received RF signal, wherein a correlation peak indicates a time of arrival of the message further responsive to the time tags.

54. The apparatus of claim 53 the third module further comprising an interpolator for interpolating a time of arrival between successive time tags.

55. The apparatus of claim 51 wherein the second module further comprises:

first memory locations for storing samples of the received RF signal, wherein successive samples of the received RF signal are incrementally input to successive first memory locations;

bit detectors, each bit detector associated with one of the first memory locations, wherein the sample stored in the one of the first memory locations is input to the associated bit detector for producing a bit value;

a memory buffer having a number of second memory locations equal to the number of bit detectors, each bit detector supplying the bit value to the associated second memory location; and

a fourth module operating on the bit values in the memory buffer for determining if the bit values present a valid message, and wherein if a valid message is not presented, a next sample of the received RF signal is input to a next one of the first memory locations, overwriting a sample previously stored therein and the bit detectors are associated with a next one of the first memory locations, the fourth module operating on bit values until a valid message is presented in the memory buffer.