US20080284639A1

US20080284639A1 - Method for reducing radio ranging errors due to clock frequency offsets

Info

Publication number: US20080284639A1
Application number: US11/749,517
Authority: US
Inventors: Zafer Sahinoglu
Original assignee: Mitsubishi Electric Research Laboratories Inc
Current assignee: Mitsubishi Electric Research Laboratories Inc
Priority date: 2007-05-16
Filing date: 2007-05-16
Publication date: 2008-11-20
Anticipated expiration: 2027-05-16
Also published as: EP1992964A2; US7463194B1; JP2008286782A; EP1992964A3; EP1992964B1

Abstract

A method reduces errors in two-way ranging between transceivers due to a frequency offset. A first transceiver measures a time C_A1between, transmitting a request frame and receiving a reply frame, and a time C_A2between receiving a first information bit of the reply frame and receiving a last information bit of the reply frame. A second transceiver measures a time C_B2between receiving the request frame and transmitting the last information bit of the reply frame, and a time C_B1between receiving a first information bit of the request frame and a last information bit of the request frame. A correction factor

α_{C} = \sqrt{\frac{C_{B 1}}{C_{A 2}}}

is applied according to

{\hat{C}}_{B 2} = \frac{C_{B 2}}{α_{o}},

and the one-way time is

{\hat{T}}_{i} = \frac{C_{A 1} - {\hat{C}}_{B 2}}{2} T_{i},

where T_iis an idea period of the clocks of the transceivers.

Description

FIELD OF THE INVENTION

The present invention relates generally to radio communication systems, and more particularly to determining distances between transceivers using two-way ranging, which is known as radio ranging.

BACKGROUND OF THE INVENTION

As shown in FIG. 1, a conventional communication frame 100 for a wireless network: includes as fields: a preamble 110, a start of frame delimiter (SFD) 120, a physical layer header (PHR) 130, and a physical layer sevice data unit (PSDU) 140.
The PSDU typically includes a payload, see IEEE 802.15.4a-D7 standard, November 2006, incorporated by reference. The preamble 110 can be used for acquisition and ranging. The SFD 120 is used for frame synchronization. The detection of the SFD indicates the beginning of the PHR 130 and the PSDU 140. The frame 100 has a time duration of T ₁ 160, and a time duration of the PHR and PSDU is T ₂ 150.
FIG. 2 shows a conventional ranging method. To estimate a distance between a first transceiver A 210 and a second transceiver B 220 in a wireless communications network, the first transceiver A 210 transmits 201 a request frame to the second transceiver B 220. FIG. 2 also shows a time axis 240 for the transceiver A and a time axis 250 for the transceiver B 220. The transceiver A 210 records a time t₁that the request frame was transmitted according to a clock of the transceiver A 210. Upon receiving the request frame, the transceiver B 220 transmits 202 a reply frame to the transceiver A 210. The transceiver A 210 measures the time of arrival (TOA) t₂of the reply frame according to its clock.
A delay at the transceiver B 220 between receiving the request frame and transmitting the reply frame is T _ta ^B 250. The transceiver B 220 transmits 205 the delay 250 to the transceiver A 210 in a timestamp report. An estimate of the distance D between the transceiver A 210 and the transceiver B 220 is the time for the round trip T _round ^A 270 minus the delay at the transceiver B 220 divided by two and multiplied by the speed of light.
$\begin{matrix} D = \frac{(t_{2} - t_{1})}{2} c = \frac{(T_{round}^{A} - T_{t α}^{B})}{2} c . & (1) \end{matrix}$
A corresponding one-way flight time T_t 260 is
$\begin{matrix} T_{t} = \frac{(T_{round}^{A} - T_{ta}^{B})}{2} . & (2) \end{matrix}$
The conventional method as specified in method in the IEEE 802.15.4a Standard Draft 7 does not address errors due to a frequency offset between the clocks of the transceivers. Indeed, in practical applications, the measurements of T_round ^Aand T_ta ^Bare different from their true value due to the frequency offset. The clock tolerance of the transceiver A 210 is e_A, and the clock tolerance of the transceiver B 220 as e_B.
After factoring in the clock tolerances, the one-way time of flight estimate {acute over (T)}_tbecomes
$\begin{matrix} {\hat{T}}_{t} = \frac{T_{round}^{A} (1 + e_{A}) - T_{ab}^{B} (1 + e_{B})}{2} . & (3) \end{matrix}$
A residual error e_twis a difference between the time of flight estimate with ideal clocks and with clocks with tolerances e_Aand e_B. Then.
e _tw =T _t e _A +T _ta ^B(e _A +e _B). (4)
Generally, T_ta ^B>>T_t. Therefore, Equation (4) approximates to
e _tw ≈T _ta ^B(e _A +e _B). (5)
It is desired to reduce the residual error e_tw. Therefore, a mechanism is needed to reduce the effect: of clock frequency offset on the range error.

SUMMARY OF THE INVENTION

The invention provides a method for reducing ranging errors due to a clock frequency offset in two-way ranging between radio transceivers, which minimizes impacts of clock frequency offsets on range estimation between two devices using two-way time of arrival measurements.
A first transceiver measures a time C_A1between transmitting a request frame and receiving a reply frame, and a time C_A2between receiving a first information bit of the reply frame and receiving a last information bit of the reply frame.
A second transceiver measures a time C_B2between receiving the request frame and transmitting the last information bit of the reply frame, and a time C_B1between receiving a first information bit: of the request frame and a last information bit of the request frame.
A correction factor
$α_{c} = \sqrt{\frac{C_{B 1}}{C_{A 2}}}$
is applied according to
${\hat{C}}_{B 2} = \frac{C_{B 2}}{α_{a}},$
and the one-way time of flight is
${\hat{T}}_{t} = \frac{C_{A 1} - {\hat{C}}_{B 2}}{2} T_{i},$
where T_iis an ideal period of the clocks of the transceivers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a conventional frame used in wireless communications between transceivers;

FIG. 2 is a timing diagram, of a conventional two-way time of arrival based ranging method;

FIG. 3 is a block diagram of a structure of a request frame 300 according to an embodiment of the invention;

FIG. 4 is a block diagram of a structure of a reply frame 400 according to an embodiment of the invention;

FIG. 5 is a block diagram of a two-way ranging method according to art embodiment of the invention;

FIG. 6 is a block diagram of an operation of timing counters at transceivers according to an embodiment of the invention; and

FIG. 7 is a flow diagram of the method according to an embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The embodiments of my invention provide two-way ranging methods for estimating a distance between a first transceiver A and a second transceiver B in a wireless communications network. The method reduces errors due to frequency offsets in clocks of the transceivers.
As shown in FIG. 3, a request frame 300 includes as fields: a preamble 301, a start of frame delimiter (SFD) 302, a physical layer header (PHR) 303, and request physical layer service data unit (PSDU_request) 304. The PHR 303 and PSDU_request 304 can carry data or information bits of the request frame 300. A time duration of the data, or information bit fields 303 and 304 is denoted by T _frame1 350.
As shown in FIG. 4, a reply frame 400 includes the preamble 303, the start of frame delimiter (SFD) 302, a physical layer header (PHR) 403 and a reply physical layer service data unit (PSDU_reply) 404. A time duration of the data or information bit fields 403 and 404 of the reply frame is T _frame2 450. The time durations T _frame1 350 and T _frame2 450 for the information bits can be different. However, for simplicity of this description, the duration for both T _frame1 350 and T _frame2 450 can be generally denoted by T_frame.
FIG. 5 shows a timing diagram of a ranging method according to an embodiment of the invention. The block diagram of the method is shown in FIG. 6. FIG. 7 shows the method steps. The transceiver A 510 includes a transmit clock 511, and the transceiver B 520 includes a receive clock B 512.
An ideal clock period (cycle) is T_i. The clock period is determined by a frequency of a crystal of the clock. The ideal clock frequency is specified by the manufacturer. In real systems, however, the clock frequency can be offset from the ideal frequency due to, e.g., manufacturing inconsistencies or environmental conditions. Therefore, the actual clock period of the transceiver A 510 is T_a, and the actual clock period of the transceiver B 520 is T_b. It is desired to minimize errors due to this frequency offset.
As shown in FIG. 6, a transceiver A 510 transmits the request frame, see inset 300 for start and end of frame fields, to a transceiver B 520. Transceiver A 510 starts 740 a first transmit counter A₁when the SFD 302 of the request frame 300 is emitted by an antenna 700 of the first transceiver.
The request frame is received in the second transceiver B 520. Transceiver B 520 starts 770 first and second receive counters B₁and B₂when detecting the end of the SFD 302 of the request frame 300 at the antenna 720 of the second transceiver B 520.
Tranceiver B 520 stops 730 its first receive counter B₁when receiving a last information bit: of the request frame 300, which is at the end of the request PSDU 304.
Transceiver B 520 determines a difference between the start and stop values of the first receive counter B₁The first: receive counter B₁measures a duration C _B1 703 where
$\begin{matrix} C_{B 1} = \frac{T_{frame}}{T_{i}} \frac{T_{a}}{T_{b}}, & (6) \end{matrix}$
The transceiver B 520 prepares the reply frame 400 and transmits the reply frame to transceiver A 510.
Transceiver B 520 stops 760 the second receive counter B₂when the end of the SFD 302 of the reply frame 400 is emitted by its antenna 720. The difference between the start and stop values of the second receive counter B₂is denoted as C _B2 704, and it measures a turn around time
$\begin{matrix} C_{B 2} = \frac{T_{ta}^{B}}{T_{b}}, & (7) \end{matrix}$
where T_ta ^Bis the true turn around time according to the ideal clock period.
In step 750, transceiver A 510 stops its first transmit counter A₁and starts a second transmit counter A₂when it detects the end of the SFD 302 of the received reply frame 400.
A difference C _A1 701 between the start and stop values of the first receive counter A₁corresponds to
$\begin{matrix} C_{A 1} = \frac{2 T_{t} + T_{ta}^{B}}{T_{a}}, & (8) \end{matrix}$
where T₁is the one-way flight time of the frame from transceiver A 510 to transceiver B 520.
Transceiver A 510 stops 780 the second transmit counter A₂when it receives the last bit of the PSDU _reply 404 of the received reply frame 400 at the antenna 700. The difference between the start and stop values of the second transmit clock A₂is denoted as C _A1 702 and it measures the time
$\begin{matrix} C_{A 2} = \frac{T_{frame 2}}{T_{i}} \frac{T_{b}}{T_{a}} & (9) \end{matrix}$
Transceiver B 520 transmits the counter values C _B1 703 and C _B2 704 to transceiver A 510 in a timestamp report 500.
Transceiver A 510 determines a correction factor
$α_{a} = \frac{T_{a}}{T_{b}} 705$
as a square root of the ratio of C_B1and C_A2. It can be seen after dividing the expression in Equation (6) by the expression in Equation (9) that
${[\frac{T_{a}}{T_{b}}]}^{2} = \frac{C_{B 1}}{C_{A 2}} .$
Transceiver A 510 divides C _B2 704 by the correction factor α_c 705 to obtain an estimate Ć _B2 707.
The time of flight estimate {acute over (T)}_t 708 is determined by the transceiver A 510 as
$\begin{matrix} {\hat{T}}_{i} = \frac{C_{A 1} - {\hat{C}}_{B 2}}{2} T_{i} & (10) \end{matrix}$
Note that the ideal time period T _i 706 is known to both transceivers, e.g., it is the time specified by the manufacturer.

Ranging Example

A ranging system has the following example values;
T_frame=100μs;

- ideal clock frequency is 500 MHz, i.e., the ideal period T_i− 1/500 MHz;
- clock: frequency of transceiver A 530 is 495 MHz, that is, T_a− 1/495 MHz;
- clock frequency of transceiver B 520 is 495 MHz, i.e., T_b= 1/495 MHz:

distance between transceivers A and B is 10 m, i.e.,

- true one way time of flight is T_t=30 ns; and
- true turn around time is T_ta ^B=1 ms.

After transmission of the request and reply frames, the corresponding example counter values are:

- C_B1=51.515 clock periods according the transceiver B clock;
- C_A2=48529 clock periods according to transceiver A clock;

$α_{c} = \sqrt{\frac{C_{B 1}}{C_{A 2}}} = 1.0303;$

- C_B2=510,000 clock periods according to transceiver B clock;
- C_A1=495,030 clock periods according to transceiver A clock;
- Ć_B2/α_c=495,000; and

${\hat{T}}_{t} = \frac{C_{A 1} - {\hat{C}}_{B 2}}{2} T_{i} = 30 ns .$
This is equal to the true time of flight for a 10 meter separation between the transceivers A and B.

Method Overview

As shown in FIG. 7, the embodiments of the invention provide a method for reducing ranging errors due to the effect of clock frequency offsets between clocks of two transceivers. In practice, a transceiver clock frequency can differ from its ideal clock frequency as specified by the manufacturer due to any of a number of conditions, including but: not limited to uncontrollable fabrication processes and environmental conditions, if two transceivers 510 and 520 are performing ranging, the transceivers can have different clock periods or frequencies.
For example, a one nanosecond timing error causes a 30 cm range error. Therefore, an accurate measurement of time is important for precision ranging in the order of centimeters.
According to the embodiments of the invention, if the ranging transceivers know their relative clock: frequency offset, the frequency offset related range error can be reduced. To achieve this, each transceiver uses two counters or two instants of a single counter to measure various time periods as described below and shown in the Figures.
The first transceiver 510 measures the number of clock cycles or periods, that is, timing information C _A1 701, between the transmission of the request frame 300 and the reception of the reply frame 400.
The first transceiver 510 also measures the number of clock cycles between the reception of the first data or information bit of the reply frame, i.e., the beginning of PHY header, and the last information bit of the reply frame, i.e., at the end of data pay load of the PSDU_reply. This counter value is denoted as C _A2 702.
The responding second transceiver 520 measures the number of clock cycles between the reception of the request frame and the completion of transmission of the reply frame. This counter value is denoted as C _B2 704.
The second transceiver 520 also measures the number of cycles between the reception of the first information bit of the request frame, i.e., the beginning of PHY header, and the last information bit of the request frame, i.e., the end of the data payload of the PSDU_request. This counter value is denoted as C _B1 703.
The second transceiver 520 transmits C_B1and C_B2to the first transceiver 510.
The first transceiver 510 determines the correction factor 705 as
$α_{c} = \sqrt{\frac{C_{B 1}}{C_{A 2}}} .$
Then, the first receiver 510 applies the correction factor to C_B2to obtain a best estimate 707
${\hat{C}}_{B 2} = \frac{C_{B 2}}{α_{c}} .$
Then, the first transceiver can determine the one-way time of flight 708 as
${\hat{T}}_{i} = \frac{C_{A 1} - {\hat{C}}_{B 2}}{2} T_{i} .$
This time of flight corresponds to the range or distance between the two transceivers.
It should be noted that the first transceiver could transmit: its timing information to the second receiver, in which the second transceiver can perform the ranging.
Although the invention has been described by way of examples of preferred embodiments, it is to be understood that various other adaptations and modifications can be made within the spirit and scope of the invention. Therefore, it is the object of the appended claims to cover all such variations and modifications as come within the true spirit and scope of the invention.

Claims

1. A method for reducing errors in two-way ranging between two transceivers due to a clock frequency offset, comprising:

measuring, in a first transceiver, a time C_A1between transmitting a request frame and receiving a reply frame, and a time C_A2between receiving a first information bit of the reply frame and receiving a last information bit of the reply frame;

measuring, in a second transceiver, a time C_B2between receiving the request frame and transmitting the last information bit of the reply frame, and a time C_B1between receiving a first information bit of the request frame and a last information bit of the request frame;

applying a correction factor

α_{c} = \sqrt{\frac{C_{B 1}}{C_{A 2}}}

to the time C_B2to obtain an estimated time

{\hat{C}}_{B 2} = \frac{C_{B 2}}{α_{c}}; and

determining a one-way time of flight according to

{\hat{T}}_{i} = \frac{C_{A 1} - {\hat{C}}_{B 2}}{2} T_{i},

in which T_iis an ideal period of the clocks of the first and second transceiver and the one-way time of flight corresponds to a range between the two transceivers.

2. The method of claim 1, in which the request frame includes a preamble, a start of frame delimiter (SFD), a physical layer header (PHR), and a request physical layer service data unit (PSDU_request), and in which a duration of the SFD and the PSDU_requestis T_frame1, and the reply frame includes the preamble, the start of frame delimiter (SFD), the physical layer header (PHR), and a reply physical layer service data unit (PSDU_reply), and in which a duration of the SFD and the PSDU_replyis T_frame2.

3. The method of claim 1, in which the ideal time period T_iis known to the first and second transceivers.

4. The method of claim 2, in which the durations T_frame1and T_frame2are different.

5. The method of claim 1, in which the clocks of the first and second transceivers have a frequency offset.

6. The method of claim 2, in which the PHR and PSDU carry information bits.

7. The method of claim 2, in which

C_{B 1} = \frac{T_{frame}}{T_{i}} \frac{T_{a}}{T_{b}},

and in which T_aand T_bare actual periods of the clocks of the first and second transceiver respectively.

8. The method of claim 7, in which

C_{B 2} = \frac{T_{ta}^{B}}{T_{b}},

where T_ta ^Bis a true turn around time according to the ideal clock period.

9. The method of claim 7,

C_{A 1} = \frac{2 T_{i} + T_{ta}^{B}}{T_{a}},

where T_tis the one-way time of flight.

10. The method of claim 7, in which

C_{A 2} = \frac{T_{frame 2}}{T_{i}} \frac{T_{b}}{T_{a}} .

11. The method of claim 1, further comprising:

transmitting C_B1and C_B2to the first transceiver in a timestamp report.

12. (canceled)

13. The method of claim 1, further comprising:

transmitting the request frame from the first transceiver to a second transceiver, in which the request frame includes a preamble, a start of frame delimiter (SFD), a physical layer header (PHR), and request physical layer service data unit (PSDU_request), and in which a duration of the SFD and the PSDU_requestis T_frame1;

starting a first transmit counter A₁of the first transceiver when the SFD of the request frame is emitted by an antenna of the first transceiver, in which periods of the first transmit counter is determined according to a transmit clock having a period T_a;

starting a first receive counter B₁and a second receive counter B₂of the second transceiver when detecting an end of the SFD of the request frame at an antenna of the second transceiver, in which periods of the first and second receive counters are determined according to a transmit clock having a period T_b;

stopping the first receive counter B₁when receiving a last bit of the request frame;

determining a difference between the start and stop values of the first receive counter B₁;

measuring a duration

C_{B 1} = \frac{T_{frame}}{T_{i}} \frac{T_{a}}{T_{b}},

in which T_iis an ideal clock period;

transmitting, in response to receiving the request frame, the reply frame from the second transceiver to the first transceiver, in which the reply frame include the preamble, the start of frame delimiter (SFD), the physical layer header (PHR), and a reply physical layer service data unit (PSDU_reply), and in which a duration of the SFD and the PSDU_replyis T_frame2;

stopping the second receive counter B₂when an end of the SFD of the reply frame is emitted by the antenna of the second transceiver;

measuring a duration

C_{B 2} = \frac{T_{ta}^{B}}{T_{b}},

where T_ta ^Bis a true turn around time according to the ideal clock period T_t;

starting a second transmit counter A₂when the first transceiver detects an end of the SFD of the reply frame, and stopping the first transmit counter A₁;

measuring a difference between the start and stop values of the first receive counter A₁correspond to

C_{A 1} = \frac{2 T_{i} + T_{ta}^{B}}{T_{a}},

where T_tis a one-way flight time of the frame from the first transceiver to the second;

stopping the second transmit counter A₂when a last bit of the PSDU_replyof the received reply frame at the antenna of the first transceiver;

measuring

C_{A 2} = \frac{T_{frame 2}}{T_{i}} \frac{T_{b}}{T_{a}};

transmitting the first and second receive values C_B1and C_B2to the first transceiver;

determining in the first transceiver a correction factor

α_{c} = \frac{T_{a}}{T_{b}}

as a square root of the ratio of C_B1and C_A2;

dividing C_B2by the correction factor α_cto obtain an estimate Ć_B2; and

estimating a time of flight estimate {acute over (T)}_tin the first transceiver as

{\hat{T}}_{i} = \frac{C_{A 1} - {\hat{C}}_{B 2}}{2} T_{i}