System and method for distance measurement by inphase and quadrature signals in a radio system

[0001] This is a continuation of U.S. patent application Ser. No. 09/960,278 filed Sep. 21, 2001, now U.S. Pat. No. 6,674,396, which is a continuation of U.S. patent application Ser. No. 09/482,681, filed Jan. 14, 2000, now U.S. Pat. No. 6,295,019, which is a continuation of U.S. patent application Ser. No. 09/083,993, filed May 26, 1998, now U.S. Pat. No. 6,111,536.

[0002] 1. Field of the Invention

[0003] The present invention generally relates to distance measurement, and more specifically to a system and method for distance measurement by inphase and quadrature signals in a radio system.

[0004] 2. Related Art

[0005] Distance measurement systems are required in a wide variety of applications. Surveying, position determination, precision farming and mining are some example applications that require distance measurement systems.

[0006] In recent years, various systems for distance measurement systems have been developed. The global positioning satellite (GPS) system operated by the United States Department of Defense, for example, is a highly complex system of determining the position of an object by distance measurement. The GPS system depends on measuring the time-of-flight of signals from three or more orbiting satellite transmitters by a navigation receiver. According to the GPS system, each satellite broadcasts a time-stamped signal that includes the satellite's ephemeris, i.e., its own position. When the mobile unit receives a GPS signal, the mobile unit measures the transmission delay relative to its own clock and determines the distance from the transmitting satellite's position.

[0007] Another approach is that employed by the U.S. Navy's TRANSIT system. In that system, a mobile unit performs continuous doppler measurements of a signal broadcast by a low earth orbit (LEO) satellite. The measurements continue for several minutes. The system usually requires two passes of the satellite, necessitating a wait of more than 100 minutes. In addition, because the distance and position calculations are performed by the mobile unit, the satellite must broadcast information regarding its position, i.e., its ephemeris. Although the TRANSIT system is capable of high accuracy (on the order of one meter), the delay required is unacceptable for commercial applications.

[0008] Although these systems accurately measure the distance between objects, they are extremely complex, and, more importantly, expensive to implement. For example, both the GPS and TRANSIT systems require multiple satellites, sophisticated receivers and antennas that require hundreds of millions dollars of investments. Also, response times of GPS and TRANSIT systems are typically slow due to their narrow bandwidth. Furthermore, since these systems depend on orbiting satellites, they require an unimpeded view of the sky to effectively perform range measurement. For these reasons, it has been recognized that there is a need for a simple, low cost distance measurement system.

[0009] The present invention is directed to a system and a method for distance measurement using a radio system. According to the present invention, a distance is measured by determining the time it takes a pulse train to travel from a first radio transceiver to a second radio transceiver and then from the second radio transceiver back to the first radio transceiver.

[0010] The actual measurement is a two step process. In the first step, the distance is measured in coarse resolution, and in the second step, the distance is measured in fine resolution.

[0011] In accordance with the present invention, a first pulse train is transmitted using a transmit time base from the first radio transceiver. The first pulse train is received at a second radio transceiver. The second radio transceiver synchronizes its time base with the first pulse train before transmitting a second pulse train back to the first radio transceiver. The second pulse train is received at the first radio transceiver, which then synchronizes a receive time base with the second pulse train.

[0012] The time delay between the transmit time base and the receive time base can then be determined. The time delay indicates the total time of flight of the first and second pulse trains. The time delay comprises coarse and fine distance attributes.

[0013] The coarse distance between the first and second radio transceivers is determined. The coarse distance represents the distance between the first and second radio transceivers in coarse resolution. An inphase (I) signal and a quadrature (Q) signal are produced from the time delay to determine the fine distance attribute. The fine distance indicates the distance between the first and second transceivers in fine resolution. The distance between the first and second radio transceivers is then determined from the coarse distance and the fine distance attributes.

[0014] Further features and advantages of the present invention, as well as the structure and operation of various embodiments of the present invention, are described in detail below with reference to the accompanying drawings.

Overview of the Invention

[0025] The present invention is directed to a system and a method for distance measurement using a radio system. The present invention can be used to measure the distance between a plurality objects.

[0026] Briefly stated, the key to measuring a distance according to the present invention is to precisely measure the time it takes for a single bit to travel from a first radio transceiver to a second radio transceiver. Since in reality it is difficult to transmit and receive a single bit, the distance is measured by determining the time it takes a pulse train to travel from a first radio transceiver to a second radio transceiver and then from the second radio transceiver back to the first radio transceiver. Stated in other terms, the distance is measured by determining the travel time for a complete exchange of data between the first and second radio transceivers.

[0027] The actual distance measurement is a two step process. In the first step, the distance is measured in coarse resolution, also referred to as a coarse measurement. In the second step, the distance is measured in fine resolution, also referred to as a fine measurement. Alternatively, the coarse distance and the fine distance can be measured in parallel, or the fine distance can be measured before the coarse distance is measured.

[0028] The radios utilized in the present invention may be any one of several types of radios that are capable of transmitting and receiving coded or modulated signals. The modulation can be time or phase modulation, frequency modulation, amplitude modulation, code division multiplex or time division multiplex schemes. It will be apparent to persons skilled in this art that other types of modulation schemes can be easily implemented.

[0029] In the following description, an impulse radio system comprising a plurality of transceivers are used for distance measurement. However, it should be understood that impulse radios are chosen as an example only. It will be apparent to persons skilled in the art that the present invention can be implemented easily by other types of radio systems. Furthermore, the present invention also be implemented using optical signals from a laser, light emitting diode (LED) or other types of optical source.

[0030] Impulse radio has been fully described in a series of patents, including U.S. Pat. No. 4,641,317 (issued Feb. 3, 1987), U.S. Pat. No. 4,813,057 (issued Mar. 14, 1989), U.S. Pat. No. 4,979,186 (issued Dec. 18, 1990) and U.S. Pat. No. 5,363,108 (issued Nov. 8, 1994) to Larry W. Fullerton. A second generation of impulse radio patents include U.S. Pat. No. 5,677,927 (issued Oct. 14, 1997), U.S. Pat. No. 5,687,169 (issued Nov. 11, 1997) and co-pending application Ser. No. 08/761,602 (filed Dec. 6, 1996) and Ser. No. 09/045,929 (filed Mar. 23, 1998) to Fullerton et al. These patent documents are incorporated herein by reference.

The Present Invention

[0031] The preferred embodiments of the invention are discussed in detail below. While specific steps, configurations and arrangements are discussed, it should be understood that this is done for illustrative purposes only. A person skilled in the relevant art will recognize that other steps, configurations and arrangements can be used without departing from the spirit and scope of the present invention.

[0032] FIG. 1 illustrates two radio transceivers 104 and 108 that can be used for distance measurement according to one embodiment of the present invention. The first radio transceiver 104 is spaced at a distance d from the second radio transceiver 108. The first radio transceiver 104 has two time bases, a transmit time base and a receive time base. The second radio transceiver 108 requires only a time base, although, it can also have two time bases. Other transceiver arrangements in which the present invention can be used for position determination are described in a co-pending application Ser. No. 09/045,929 (filed Mar. 23, 1998), titled: System and Method for Position Determination by Impulse Radio.

[0033] Referring now to FIGS. 1 and 2, the first radio transceiver 104 transmits a pulse train S₁ using the transmit time base. The pulse train S₁ has N pulses, where N is an integer. The second radio transceiver 108 receives the pulse train S₁ and synchronizes its time base with the pulse train S₁. The second radio transceiver 108 then transmits a pulse train S₂ having N pulses. The pulse train S₂ is received by the first radio transceiver 104. The transceiver 104 synchronizes its receive time base with the pulse train S₂. The time delay between the transmit time base and the receive time base is then determined. The time delay represents the total time taken for the round trip flight from the first radio transceiver 104 to the second radio transceiver 108 and back to the first radio transceiver 104.

[0034] FIG. 2 illustrates an example pulse train S₁ (or S₂) according to one embodiment. The pulse train S₁ (or S₂) comprises 100 pulses. Each individual pulse has an exemplary pulse width of 50 nano seconds. The total width of the pulse train S₁ (or S2) is 10 micro seconds. The pulse train S₁ (or S₂) is periodic, having a period of 30 micro seconds. Stated in other terms, the pulse train S₁ or (S₂) is repeated at every 30 micro seconds interval. The duty cycle of the pulse train S₁ (or S₂) is 0.333.

[0035] The pulse train can be envisioned in both time and space by considering the speed of radio waves in the atmosphere and the frequencies involved. The pulse train is generated by a 10 MHz signal source (described below in detail). Since electromagnetic wave travels at a speed of approximately 100 ft/nano second, a 10 MHz signal source will generate a pulse for each 100 ft of travel by the wave. Consequently, if a 10 MHz oscillator is used to trigger a counter to count the distance traveled by a pulse train, the counter can measure a distance within approximately 50 ft resolution by detecting the rising edge or the falling edge of the clock.

[0036] FIG. 3 is an example timing diagram illustrating the pulse trains exchanged between the first radio transceiver 104 and the second radio transceiver 108. In order to simplify the timing diagram, each pulse train is represented by a single pulse. In FIG. 3, time units in the horizontal axis are represented in micro seconds.

[0037] Referring now to FIG. 3, at time t=0, the first radio transceiver 104 transmits the pulse train S₁ (indicated by a reference numeral 304) using its transmit time base. The pulse train 304 is a periodic pulse train having a time period T. In one embodiment, the pulse train has a period T=30 micro seconds. The pulse train 304 comprises 100 pulses, each pulse having a pulse width of 50 ns. The total width of the pulse train 304 is 10 micro seconds.

[0038] At time t=t₁, the second radio transceiver 108 begins to receive the pulse train S₁ (indicated by a reference numeral 308). In other words, it takes the pulse train S₁ a time t₁ to travel from the first radio transceiver 104 to the second radio transceiver 108. The second radio transceiver 108 completes the reception of the pulse train S₁ at time t=t₂, where t₂=(t₁+10) micro seconds. The second radio transceiver 108 synchronizes its time base with the pulse train 308. At time t=t₃, the second radio transceiver 108 begins transmission of the pulse train S₂ (indicated by a reference numeral 312). At time t=t₄, the first radio transceiver 104 begins the reception of the pulse train S₂ (indicated by a reference numeral 316). The first radio transceiver 104 completes the reception of the pulse train 316 at time t=t₅, where t₅=(t₄+10) micro seconds.

[0039] In the example of FIG. 3, t₄=16 micro seconds. The total time of flight for the pulse train S₁ to travel from the first radio transceiver 104 to the second radio transceiver 108 and for the pulse train S₂ to travel from the second radio transceiver 108 to the first radio transceiver 104 is 6 micro seconds, which is also referred to as the time delay. In the present invention, the time delay is used to calculate the coarse distance and the fine distance.

[0040] FIG. 4 illustrates a phase locked loop (PLL) circuit that can be used to generate timing signals at the first radio transceiver 104 and the second radio transceiver 108. The function of the PLL circuit is to synchronize a high frequency oscillator to a stable lower frequency signal source to thereby provide a stable timing signal. The PLL circuit is shown for illustrative purposes only, and it should be apparent to persons skilled in the art that other types of circuits can also be used to provide stable timing signals.

[0041] Referring now to FIG. 4, a voltage controlled oscillator 404 generates a base clock signal 406. The base clock signal 406 is divided using a counter 408. The output of the counter (indicated by a reference numeral 410) is received at a phase detector circuit 412. The phase detector circuit 412 also receives a reference signal 414 from a reference signal generator 416. The phase detector circuit 412 outputs a phase difference signal 418. A PLL loop filter 420 receives the phase difference signal 418 and outputs an error signal 422. The error signal 422 is used to adjust the voltage controlled oscillator 404 to thereby provide a stable base clock signal 406. Depending on the application, the base clock signal 406 or the output of the counter 408 can be used as a timing signal.

Coarse Distance Measurement

[0042] FIG. 5 illustrates a block diagram of a system for determining the coarse distance from the time delay according to one embodiment. Simply stated, the coarse distance indicates the number of pulses counted by a counter during the time delay. In one embodiment, each pulse corresponds to a distance of 100 ft. Thus, for example, if a counter counts 16 pulses during the time delay, the distance traveled by the pulse train is 1600 ft.

[0043] Referring now to FIG. 5, a transmit time base 504 is coupled to a counter 508. The counter 508 can be a phase locked loop counter that receives an output 506 from the transmit time base 504 and provides a stable signal referred to as a transmit timing signal 510. In one embodiment, the transmit timing signal 510 is a 10 MHz square wave with a 50% duty cycle.

[0044] The transmit timing signal 510 is received at a counter 512, where the transmit timing signal 510 is divided by N, where N is an integer. The counter 512 outputs a first timing signal 514. In one embodiment, N is 2, and consequently, the first timing signal 514 is a 5 MHz square wave.

[0045] The first timing signal 514 is received at a transmit code position counter 516, which counts the number of pulses in the first timing signal 514. In reality, the transmit code position counter 516 counts the number of pulses from the start of the transmission of the pulse train S₁ from the first radio transceiver 104. The output of the counter 516 is referred to as a first count value 518, which is a binary number.

[0046] Likewise, a receive time base 520 is coupled to a counter 524. The counter 524 receives an output 522 from the receive time base 520 and outputs a receive timing signal 526. In one embodiment, the receive timing signal 526 is a 10 MHZ square wave with a 50% duty cycle.

[0047] The receive timing signal 526 is received at a counter 528, where it is divided by N, where N is an integer. The counter 528 outputs a second timing signal 530. In one embodiment, N is 2, and consequently, the second timing signal 530 is a 5 MHz signal.

[0048] The second timing signal 530 is received at a receive code position counter 532. The receive code position counter 532 outputs a second count value 534 that indicates the number of pulses counted at the time the transceiver 104 starts to receive the pulse train S₂.

[0049] The first and second count values (518 and 534, respectively) are received at a latch 540. In one embodiment, the first count value 518 is received at the data input port of the latch 540, while the second count value 534 is received at the “enable” port of the latch 540. The latch 540 outputs a code delay value 542. The code delay value 542 indicates the number of pulses counted between the time the first radio transceiver 104 begins the transmission of the pulse train S₁ and the time the first radio transceiver 104 starts to receive the pulse train S₂. In other words, the code delay value 542 indicates the sum of the pulses in a pulse train S₁ (or S₂) and the number of pulses in the time delay. Thus, the number of pulses in the time delay, i.e., the coarse distance, can be calculated by subtracting the number of pulses in a pulse train from the code delay value.

Fine Distance Measurement

[0050] FIG. 6A illustrates a fine distance measurement circuit according to one embodiment of the present invention. The fine distance measurement circuit receives three inputs: the transmit timing signal 510, the first timing signal 514 and the second timing signal 530. These signals have been described in connection with the coarse distance determination in FIG. 5.

[0051] The transmit timing signal 510 and the first timing signal 514 are received at a first gate 604. In one embodiment, the first gate is an equivalence gate or an inverted exclusive-or (XOR) gate. Also, in one embodiment, the transmit timing signal 519 is a 10 MHz signal, while the first timing signal 514 is a 5 MHz signal. The first timing signal 514 is also referred to as the TX(I) signal.

[0052] The first gate 604 compares the phase of the transmit timing signal 510 to the phase of the TX(I) signal 514 and outputs a TX(Q) signal 606. The duty cycle of the TX(Q) signal 606 is proportional to the phase difference between the transmit timing signal 510 and the TX(I) signal 514.

[0053] FIG. 6B illustrates an alternate circuit comprising an inverter and two latches for producing the TX(I) signal 514 and the TX(Q) signal 606. The transmit timing signal 510 is received at the latch 650 and the inverter 654. The latch 650 outputs the TX(I) signal 514. The inverter 654 inverts the transmit timing signal 510, which is received by the latch 658. The latch 658 outputs the TX(Q) signal 606. It will become apparent to persons skilled in the art that these signals can be produced by other types of circuits.

[0054] Turning again to FIG. 6A, the second timing signal 530 and the TX(I) signal 514 are received at a second gate 608. The second timing 530 signal is also referred to as the RX signal 530. In one embodiment, the second gate 608 is an exclusive-or gate. The second gate 608 compares the phase of the RX signal 530 to the phase of the TX(I) signal 514 and outputs an I signal 610. The I signal 610 has a duty cycle proportional to the phase difference between the TX(I) signal 514 and the RX signal 530.

[0055] The TX(Q) signal 606 and the RX signal 530 are received at a third gate 612. In one embodiment, the third gate 612 is an X-or gate. The third gate 612 outputs a Q signal 614. The Q signal 614 has a duty cycle proportional to the phase difference between the TX(Q) signal 606 and the RX signal 530.

[0056] The I and Q signals (610 and 614, respectively) are received at a first and a second low pass filter 616 and 620, respectively. The first and second low pass filters can each comprise a resistor and a capacitor. Other types of low pass filters can also be used. The first low pass filter 616 removes ac components from the I signal 610 and outputs an I_dc signal 622. The I_dc signal 622 represents the average dc value of the I signal 610. Likewise, the second low pass filter 620 removes ac components from the Q signal 614 and outputs a Q_dc signal 624. The Q_dc signal 624 represents the average dc value of the Q signal 614.

[0057] FIG. 7 is a timing diagram that illustrates the signals referred to in connection with the fine distance measurement circuit of FIG. 6A. The transmit timing signal (indicated by a reference numeral 704) and the TX(I) signal (indicated by a reference numeral 708) are received at the first gate 604. The first gate 604 outputs the TX(Q) signal (indicated by a reference numeral 712). The TX(I) signal and the RX signal (indicated by a reference numeral 716) are received at the second gate 608. The second gate 608 outputs the I signal (indicated by a reference numeral 720). The TX(Q) signal and the RX signal received at the third gate 612. The third gate 612 outputs the Q signal (inidcated by a reference numeral 724). The first low pass filter 616 removes the ac components from the I signal and outputs the I_dc signal (indicated by a reference numeral 728). Likewise, the second low pass filter 620 removes the ac components from the Q signal and outputs the Q_dc signal (indicated by a reference numeral 732). As described before, the I_dc signal 728 and the Q_dc signal 732 represent the average dc value of the I signal 720 and the Q signal 724, respectively.

[0058] In the present invention, the I_dc signal 728 and the Q_dc signal 732 are measured as a function of the distance between the first radio transceiver 104 and the second radio transceiver. Stated in other words, a plurality of the I_dc signal 728 and the Q_dc signal 732 readings are taken as the distance between the first and second radio transceivers (104 and 108, respectively) is varied.

[0059] The I_dc signal 728 and the Q_dc signal 732 are further processed to thereby measure the fine distance. The further processing of the I_dc and Q_dc signals (728 and 732, respectively) are shown in FIGS. 8 and 9.

[0060] Referring now to FIG. 8, the I_dc signal 728 and the Q_dc signal 732 are received at analog to digital (A/D) converters 804 and 808, respectively. The A/D converter 804 outputs I₁ (indicated by a reference numeral 812) and the A/D converter 808 outputs Q₁ (indicated by a reference numeral 816).

[0061] Then, the following values are determined:

• • [0062] I_max=maximum value of I₁;
  • [0063] I_min=minimum value of I₁;
  • [0064] Q_max=maximum value of Q₁;
  • [0065] Q_min=minimum value of Q₁;
  • [0066] I_ctr=center value of I₁; and
  • [0067] Q_ctr=center value of Q₁.
    Then, I_z and Q_z variables are computed:
    I_z=I₁−I_ctr; and
    Q_z=(Q₁−Q_ctr)*(I_max−I_min)/(Q_max−Q_min).
    Then, the I_z and Q_z variables are plotted as a function of distance. The above computations shifts the waveforms representing I_z and Q_z to thereby make their center points zero. Also, the above computations equalize the slopes of I_z and Q_z.

[0068] FIG. 9 illustrates the plots for the I_z and Q_z variables. The I_z variable is represented by a triangular wave 904 and the Q_z variable is represented by a triangular wave 908. The triangular waves 904 and 908 are divided into four quadrants. The ambiguity with respect to the distance given by an actual I_z value is resolved by considering a corresponding Q_z value from FIG. 9.

[0069] Then, the octant is determined from Table I.

[0070]

TABLE I
Octant	Iz	Qz
0	+	+
1	+	−
2	+	−
3	+	+
4	−	+
5	−	−
6	−	−
7	−	+

Then, an Offset value O_off is determined from Table II.

[0071]

TABLE II
Octant	Offset Value Ooff
0	Qz * S
1	H − Iz * S
2	−Iz * S
3	H − Qz * S
4	−Qz * S
5	H + Iz * S
6	Iz * S
7	H − Qz * S

where H is the distance value of one octant, and S is the slope of the I_z waveform. The offset value spans from 0 to H for each octant as the distance scans across the octant. The fine distance is given by X*H+O_off, where X is the octant number. Then the coarse distance is adjusted as follows.

• [0072] (a) In octant 1, if the least significant bit (LSB) of the code delay value is 1, then the coarse value is incremented by 1.
• [0073] (b) In octant 7, if the LSB of the code delay value is zero, then the coarse value is decremented by 1.

[0074] Finally, the fine distance is added to the adjusted coarse distance to thereby calculate the total distance.

Applications of the Present Invention

[0075] The following applications are described as a few selected examples. It will become apparent to persons skilled in the art that the present invention can be utilized in many other applications.

(1) Position Determination

[0076] The present invention can be used to determine the position of one or more objects. The objects have fixed locations or can be moving with respect to each other. In one embodiment, a plurality of radios can be combined to provide the position of a desired object (or one or more radios). The actual position can be calculated by triangulation, direction finding antennas or other techniques that are well known in the art. A position determination system based on the present invention can be installed in a mobile phone system (e.g., cellular phone, PCS phone) to thereby provide position information to mobile phone users.

[0077] A position determination system built in accordance with the present invention would be more reliable than similar systems based on global positioning satellites (GPS), because the radios in the present invention can be better controlled to provide the desired coverage. Also, such a system based on radios will be more accurate because of the inherent sub-centimeter link accuracy for each link. Also, a position determination system according to the present invention would be superior over an optical system due to its omni-directional antenna pattern and its greater dust and rain resistance.

(2) Distance or Range Measurement for Surveying

[0078] The present invention can be utilized for accurate range measurements with sub-centimeter resolution, thereby making the present invention a low cost and less complex alternative to the GPS systems used for high resolution range measurement. Furthermore, the present invention can be used where GPS is unavailable or intermittent, such as in urban areas, in forests or in canyons. Also, the concurrent voice channel available can be used for simultaneous distance measurement and voice communications.

(3) Precision Farming

[0079] The present invention can be used to automatically or semi-automatically guide farm equipment with sub-centimeter repeatability and precision. It would not be dependent on satellite constellation as GPS and would be more immune to weather than optical systems. The concurrent data channel can be used for control commands or voice communications.

(4) Mining

[0080] Open and underground mining involves operations where GPS is intermittent or not available at all. The present invention can provide reliable and continuous service in these situations. In addition, the multipath immunity of the ultra-wideband (UWB) radio system has been demonstrated to provide excellent communications in underground tunnels where carrier based communication systems fail.

(5) Marine Navigation

[0081] The present invention can provide ship to shore link as well as precise full duplex communications. Also, two radios, one at each end of a ship can provide precision docking information.

(6) Asset Tracking

[0082] Large shipyards and warehouses have a need for dynamic asset tracking. A system based on the present invention can transmit ID tags and provide link distance as part of an asset position tracking system.

(7) Aircraft Landing

[0083] The high precision and rapid measurement rate available from the present invention makes it an ideal sensor for an aircraft precision landing device. Its independence from GPS and immunity to weather make it extremely reliable. Also, multipath immunity makes the present invention insensitive to local propagation conditions and thus extremely portable. As a result, a precision aircraft landing device built in accordance with the present invention can be set up at remote fields in a matter of hours.

(8) Aircraft Collision Avoidance

[0084] A collision avoidance system could be built using the present invention. In one embodiment, low cost transponders built according to the present invention can be installed in aircrafts. A simple link between two aircrafts that indicates a steady closing velocity suggests a collision course. A course between two aircraft at a steady velocity that is not a collision course will show a decreasing closing velocity. If additional sensor data is provided on the link, such as altitude, compass, or GPS coordinates, then faster and more accurate predictions of potential collision could be made. The system can operate with or without GPS or interface with ground equipment.

Conclusion

[0085] While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example, and not limitation. Thus the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.