August, 2004
IEEE P802.15-04/427r0
IEEE P802.15
Wireless Personal Area Networks
Project
IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs)
Title
Ranging Protocols and Network Organization
Date
Submitted
10 August 2004
Source
Benoit Denis
Voice:
Fax:
E-mail:
CEA-LETI
DCIS/SMOC/LCARE
17 rue des Martyrs
F 38054 Grenoble cedex 9
FRANCE
(33)(0)4 38 78 58 11
(33)(0)4 38 78 51 59
denisbe@chartreuse.cea.fr
STMicroelectronics
Broadband Wireless LAN Group
Advanced System Technology
39, ch. du Champ des Filles
CH - 1228 Plan-les-Ouates
SWITZERLAND
Re:
TG4a Ranging Subcommittee Contribution
Abstract
This submission describes the impact of network protocols on ranging.
Purpose
In support of TG4a Ranging Subcommittee work.
Notice
This document has been prepared to assist the IEEE P802.15. It is offered as a
basis for discussion and is not binding on the contributing individual(s) or
organization(s). The material in this document is subject to change in form and
content after further study. The contributor(s) reserve(s) the right to add, amend or
withdraw material contained herein.
Release
The contributor acknowledges and accepts that this contribution becomes the
property of IEEE and may be made publicly available by P802.15.
Submission
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Benoit Denis, CEA/LETI
August, 2004
IEEE P802.15-04/427r0
Table of Contents
INTRODUCTION ________________________________________________________________________ 2
CLASSICAL RANGING TRANSACTIONS ___________________________________________________ 3
POSSIBLE EMBODIMENTS FOR UWB LOCATION SYSTEMS ________________________________ 5
RANGING TRANSACTIONS AND COMMUNICATION STANDARDS: THE IEEE 802.15.3 EXAMPLE
________________________________________________________________________________________ 8
PNC TO DEV RANGING ____________________________________________________________________ 8
General Transactions _______________________________________________________________________ 8
Ranging Errors from Relative Clock Drifts and Response Delays ____________________________________ 10
Single TOF Estimation ..................................................................................................... 11
Joint DEV’s and PNC’s TOF Estimations ........................................................................ 13
Time stamp ______________________________________________________________________________ 13
DEV TO DEV RANGING ___________________________________________________________________ 15
General Transactions ______________________________________________________________________ 15
Ranging Errors from Clock Drifts and Synchronization Offsets _____________________________________ 16
REFERENCES __________________________________________________________________________ 17
Introduction
The purpose of this document is to make a brief description of classical time-based ranging
transactions. It highlights the main challenges that must be overcome in terms of synchronization
requirements, network organization and protocols. Concepts will be illustrated with few
examples (corresponding to existing UWB localization systems related in the literature). Finally,
the adaptation of these ranging techniques to communication standards will be discussed with the
IEEE 802.15.3 example. This contribution can be viewed as a complement for the document
IEEE 802.15-04/418r0, and is specifically offered to the 802.15.4a Ranging Subcommittee as a
basis for further discussions on the protocol hooks that must be provided in support of various
ranging and location techniques.
Submission
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Benoit Denis, CEA/LETI
August, 2004
IEEE P802.15-04/427r0
Classical Ranging Transactions
Classical ranging transactions (namely the Two Way Ranging -TWR- and the One Way Ranging
-OWR- schemes), and the corresponding synchronization requirements, are highly dependant on
the communication protocol and the network topology.
The first technique enabling to measure the signal round-trip Time-Of-Flight (TOF) between two
asynchronous transceivers consists in using a classical two-way remote synchronization
technique. A pair of terminals are time-multiplexed with half-duplex packet exchanges. This
procedure relies on a typical mechanism for fused location and communication ([1][2][6][8]): a
requestor sends packets to a responder which replies after synchronizing with packets containing
synchronous timing information. The reception of this response allows the requestor to determine
the round-trip TOF information (C.f. Figure 1). This solution seems to be particularly adapted for
distributed networks (a fortiori in the lack of coordination).
Tround
TOF
A
B
TOF
Response Delay
Preamble Acquisition
Header
Channel
Communication
Acquisition
Payload
Synchro H
Transmitted packets
Received packets
Elapsed times measured by
the system
Figure 1: Two Way Ranging (TWR) transaction enabling to estimate the round-trip Time-OF-Flight between two
asynchronous terminals (feeding TOA-based positioning algorithms)
Now, if two terminals are synchronized to a common clock (i.e. sharing the same time reference
and time base), it is clear that the TOF information can be directly obtained from a simple OWR
transaction (not detailed here).
The two previous transactions provide so-called TOA location metrics corresponding to the
relative distance between terminals.
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Benoit Denis, CEA/LETI
August, 2004
IEEE P802.15-04/427r0
Another possible transaction consists in forming a difference of TOAs at a couple or reference
terminals. The resulting TDOA can be obtained through OWR transactions (C.f. Figure 2). In
this scheme, a pair of isochronous terminals (viewed as anchors) preliminary detect the TOA
associated with packets emitted by the mobile. The TOAs are estimated relative to a common
reference time (shared among references) but independent on the actual transmission time.
Anchors have to be re-synchronized with an external clock or by a beacon signal periodically
broadcasting packets to all the fixed references [8][11][12]. This beacon signal may come from a
coordinator or a dedicated terminal. Note that “synchronization” means “absolute
synchronization” here, and implies that anchors know their relative distance to the beacon
provider.
Reference
Time
Mobile Terminal
A
TOF(A-B)
B
TOAB
Isochronous Terminals
Transmitted packets
TOF(A-C)
Received packets
C
Elapsed times measured by
the system
TOAC
Figure 2: One Way Ranging (OWR) Protocol allowing to estimate differential Time of Arrival at a couple of two
isochronous terminals (feeding TDOA-based positioning algorithms)
For all the considered scenarios (TOA/TWR, TOA/OWR and TDOA/OWR), slot lengths and
procedures fixed by a pre-existing communication standard represent drastic constraints for the
maximum and minimum (blind distances) relative measurable distances, and the ranging
accuracy.
Finally, the approach taken to calculate the position of a mobile terminal depends on whether
TOA or TDOA location metrics are used. A straightforward approach uses a geometric
interpretation to calculate the intersection of circles for TOA-based algorithms and hyperbolas
for TDOA-based algorithms. Indeed, if three TOA are measured between a mobile terminal and
three (or more) distinct anchors (note that anchors should be considered as nodes doted with a
prior knowledge of their relative positions) on the one hand, or if two TDOA (or more) can be
formed at a set of three distinct anchors (or more), the mobile position can be easily computed in
the 2-D plane. These solutions (geometrical solutions or solutions based on optimization
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Benoit Denis, CEA/LETI
August, 2004
IEEE P802.15-04/427r0
procedures) correspond to popular radiolocation methods, but it is not the purpose of the very
document to focus on these positioning algorithms.
Possible Embodiments for UWB Location Systems
The principles described in the previous section have already adopted specific embodiments for
UWB location assets. The solutions listed hereafter do not pretend to be regarded as an
exhaustive overview.
Time Domain [8] demonstrated that both TOA/TWR transactions (Half-duplex ranging and
TCP/IP protocol) for ranging, and TDOA/OWR transactions for positioning, can be viable when
fusing location and communications abilities.
In a more general framework, methods had been described for UWB location systems [3]. In a
first one, two asynchronous transceivers use a TWR duplex protocol to achieve synchronization
and estimate the range. This configuration provides TOA through TWR transactions, that is to
say only the relative distance information (although additional AOA measurement could enable
to find the position) (C.f. 1-Figure 3). In a second solution, a transmitter and a receiver are
synchronized with a universal external clock and the range is determined through a OWR
transaction. This configuration provides TOA through a OWR transaction, that is to say only the
relative distance information (although additional AOA measurement could enable to find the
position) (C.f. 2-Figure 3). In another solution, three relative distance measurements from a
mobile to three references are performed through three distinct full duplex TWR links. This
configuration leads to the estimation of three TOAs through three distinct TWR transactions. The
position is then calculated with classical triangulation methods (C.f. 3-Figure 3). A variant relies
on the pre-synchronization (achieved with a universal external clock) of a mobile transmitter
with anchors acting as receivers. This configuration leads to the estimation of three TOAs
through three distinct OWR transactions. According to another solution, three pre-synchronized
anchors (with a universal external beacon signal) act as receivers (C.f. 4), Figure 3). This
configuration leads to the estimation of two TDOAs through three distinct OWR transactions.
Submission
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Benoit Denis, CEA/LETI
August, 2004
1)
IEEE P802.15-04/427r0
TX/RX
RX/TX
A
B
RX
TX
2)
TX/RX
TX
4)
3)
B
A
A
A
RX/TX
RX/TX
B
D(B-D)
D
RX
RX
B
D(B-C)
RX/TX
D(B-C)
C
D(B-D)
D(C-D)
D
D(C-D)
RX
C
OWR Ranging Protocol
Mobile Terminals
TWR Ranging Protocol
Anchors / Fixed terminals
A priori known distances
Pre-synchronized Terminals
Figure 3: Possible generic configurations for UWB location Assets [3]: 1) & 2) provide ranging information between
distinct mobile terminals (relative distance) and 3) & 4) provide position of a particular mobile terminals relatively
to anchors or fixed terminals
According to AEther Wire & Location [6][9], range information is obtained from TWR halfduplex protocols between distributed UWB transceivers. This solution is equivalent to the first
solution previously described in [3] (C.f. Figure 3). Each localizer acquires as many contacts as
possible and the range information is shared within the whole network. As local groups of nodes
form into clusters, nodes in one cluster link with nodes from another cluster, forming bridges.
Beyond this, a suggested positioning approach would be distributed and the architectural network
model would be ad hoc.
In the PAL (Precision Asset Location) system proposed by MSSI [12], active tags (to be located),
periodically broadcast short packet bursts (including synchronization preamble and tag ID) to a
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Benoit Denis, CEA/LETI
August, 2004
IEEE P802.15-04/427r0
set of wired passive synchronized receivers which form TDOAs from TOA measurements. Then,
a centralized calculation of tag positions is lead with a non-linear optimization algorithm. This
solution is equivalent to the fourth solution proposed in [3] (C.f. 4- Figure 3).
According to another proposal by MMSI [11], a UWB mobile rover initiates a sequence of
packet bursts including synchronization preamble and rover’s ID toward fixed UWB beacons.
Then the latter transpond after a fixed time offset to the rover (avoiding collisions) which
determines the corresponding round trip delays and finally calculates its position by minimizing
an error functional via Newton-Raphson algorithm. This solution is equivalent to the third
solution proposed in [3] (C.f. 3-Figure 3).
In a third MSSI’s embodiment [4] (C.f. Figure 4), UWB transceivers are set at known fixed
locations and a mobile terminal determines its own location by solving a set of equations
according to measured TOFs. At local level, the mobile terminal forms TDOA from TOA
measurements. In order to eliminate a clock distribution system, self-synchronization of pulse
timing is achieved by generating a start pulse at one of the transceivers. One of the transceivers
(namely A) broadcasts a specific signal towards the others (including fixed and unknown
positions), so that the fixed anchors (namely B, C, D) can determine the transmission time from
A (with a priori known relative distances and TOA information). Then B transmits its own signal
after a pre-arranged delay to avoid pulse collision at the mobile terminal E, received by both
fixed and unknown positions. In order to avoid that B is considered as A’s signal, additional data
(like source ID) are used among communicated packets. The position calculation, which is easily
obtained with classical LMS optimization from the set of TDOA measurements formed at the
mobile, can be lead at any location (performed by the mobile terminal or by the anchors after
propagating the set of equations). One of the main advantages of the proposed system is that
absolute self re-synchronization is achieved at each new transmitted burst.
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Benoit Denis, CEA/LETI
August, 2004
IEEE P802.15-04/427r0
TX
D(A-B)
Calculated from
D(A-B) and TrecB
to avoid collisions
at the mobile
Time Slot
A
Step 1
A
RX/TX
TOF(A-B)
B
B
RX
E
TrecB
TOF(B-C)
C
Step 2
TOF(A-E)
D(B-C)
RX/TX
RX/TX
C
TOF(B-E)
E
D
Step 3
Estimated TDOA at
the mobile terminal
D(C-D)
Transmitted packets
Received packets
Figure 4 : Self-synchronized UWB Assets location [4]
Ranging Transactions and Communication Standards: the IEEE
802.15.3 example
PNC to DEV Ranging
General Transactions
In order to estimate the relative distance between the PNC and the DEV, a classical Two-Way
Ranging (TWR) would obviously be required since the entities are a priori asynchronous, for
instance if the DEV intended to join the PicoNet for the first time, or if the propagation
environment had changed within the superframe duration. The beacon synchronization would
obviously provide relative synchronization reference time for the DEV. However, ranging
transactions between the PNC and the DEV could benefit from the MAC resources which are
naturally available at the beginning of each superframe.
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A possible solution relies on the half-duplex link that is available when the DEV intends to join
the PicoNet. This approach consists in using the expected association MCTA’s frame structure
for channel time request with Imm-ACKs (or extended to Dly-ACK cases if possible). The main
idea is that Association MCTA provides Two Way Ranging on its own, when considering the
Association Request (DEV to PNC Link) and the ACK (PNC to DEV Link). More precisely,
MCTA is composed of well defined slots. Inside each slot, it is assumed that there is enough time
to perform a complete TWR scheme, send a request and get the response (an ACK). Typically, a
DEV that wants to send a ranging request to the PNC will access a MCTA slot if slotted Aloha is
used. Upon reception of the request, the PNC will respond to the DEV with an ACK frame after
SIFS, i.e. still in the same MCTA slot (See Figure 5).
F1
DEV to
PNC
SIFS
ACK
SIFS
F2
SIFS
ACK
SIFS
PNC to
DEV
Figure 5 : MCTA’s frame structure in the 802.15.3 MAC
An additional enhancement to this basic scheme consists in combining MCTA resources and
Beacon synchronization, which obviously provides another natural Two Way link (See Figure 6).
This additional Two Way Link leads to a much more accurate range estimation, and provides
drift estimation. In this new scheme, the PNC initiates the first Two Way Ranging transaction
and performs a preliminary distance estimation by measuring the elapsed time between the
emission of the Beacon signal and the reception of a request (medium access) formulated by the
DEV at the beginning of a MCTA slot. In the second TWR transaction, the DEV performs its
own estimation by measuring the elapsed time between the emission of its request and the
reception of the ACK from the PNC during the Association MCTA. Note that the PNC can
transmit its estimate within the communication payload of the ACK, so that the DEV will have
two estimates of its relative distance to the PNC. This approach is inspired by the UWB ranging
application described in [6].
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RefPNC RefDEV
IEEE P802.15-04/427r0
Transmitting to
DEV PNC’s TOF
Estimate
PNC’s TOF
Estimate
Available
T1
T2
PNC
 PNC
TOF
TOF
TOF
DEV
 PNC  
T1
PicoNet
Synchronization
Beacon
T2
Request for a
Medium Access
Acknowledgement
DEV’s TOF
Estimate
Available
Receiving
PNC’s TOF
Estimate
MCTA
Figure 6 : PNC to DEV double TWR scheme at the beginning of the superframe
On Figure 6, for the purpose of simplification, synchronization events have been represented as
detection events associated with the first pulse arrival (in the training sequence). A more realistic
representation would consider frame delineation events (detection of the first arriving pulse in the
last training sequence, at the end of the channel estimation header). Moreover, note that the
proposed representation corresponds to the simplified case when retransmission times are
conditioned by previous estimated TOAs. But, in a more general framework, time stamp must be
taken into account.
Ranging Errors from Relative Clock Drifts and Response Delays
When referring to Figure 6, one could easily obtain the following expressions:
~
2TOF PNC
T1
T1
2TOF 


1   PNC   1   PNC 1   PNC
and
~
2TOF DEV
T2
T2
2TOF 


1   PNC 1   PNC   1   PNC  
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~
Where T1 , T2 represent response delays, TOF and TOF respectively the real and estimated TOFs,
 PNC    f 0 and  PNC f 0 the frequency offsets of DEV’s and PNC’s clocks relative to an ideal
frequency f 0 .
In the proposed ranging procedure (under the assumption of a correct detection), it can be shown
that:
T1
~
TOF PNC  1   PNC TOF 
21   PNC   
and that
T2
~
TOF DEV  1   PNC   TOF 
21   PNC 
Single TOF Estimation
~
~
If ranging is based on a single estimation TOF DEV or TOF PNC (available with single TWR
transactions), the error on the range estimate due to clock drifts and protocol response delays is
on the order of:
~
E DEV  TOF DEV  TOF   PNC   TOF 
and
~
E PNC  TOF PNC  TOF   PNC TOF 
T2
21   PNC 
T1
21   PNC   
Depending on  PNC , T1 , T2 and  , these errors can be significant. Now, considering a usual range
lower than 15m, or equivalently that the maximum time of flight TOF is lower than 50ns, and
that the absolute drift is on the order of 10-5, it is clear that the first terms involved in the
previous expressions are much lower than 50ps, and hence, can be neglected.
So, generally speaking and in first approximation, we can say that the error committed on the
range estimate is:
E DEV  
T2
21   PNC 
and
E PNC  
T1
21   PNC   
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At this point, several parameters should be discussed:
- The value of the pre-convinced reply delay T2, depending on the PNC’s ACK
- The value of  , depending on a preliminary drift compensation, or initial drift conditions
at the initiation of the PicoNet; in other words.
A preliminary drift correction obviously implies a preliminary drift estimation between the PNC
and the DEV. In other words, before performing the whole ranging procedure, the DEV could
wait for beacon synchronization over a sufficient number of Superframes. This will impact the
initial value of the term  .
Ranging Error EDEV(s) / Single DEV's T
OF
-7
Estimate / 
=10-5
PNC
10
-8
10
-9
10
-10
10
-11
10
T = 1.00e+001 s
2
T = 1.00e+002 s
-12
10
2
T = 1.00e+003 s
2
T = 1.00e+004 s
2
-13
10
-7
10
-6
10
=  -
A
-5
10
PNC
Figure 7 : Ranging Error with a Single DEV’s T OF Estimation (single TWR between the DEV and the PNC)
Actually, frequency offsets are random values. However, for the purpose of providing coarse but
realistic specifications concerning  and T2, we can arbitrary set  PNC to a pessimistic value of
10-5. As shown on Figure 7, the ranging error for the single DEV’s TOF estimation is maintained
below 50ps for  up to 10-5 if the ACK occurs within a duration less than 10μs. This corresponds
to uncompensated drift situations when the value of  is very large. Otherwise, when
considering traditional values of  = 10-6 (resp. 10-7), the constraints on the ACK collapses down
to 100μs (resp. 1000μs).
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Joint DEV’s and PNC’s TOF Estimations
~
So, a possible enhancement for the ranging procedure consists in using both TOF DEV and
~
TOF PNC estimates, so that:
- The DEV computes the clock rate (See [6]) relative to the PNC:
~
T1  T2  2TOF PNC
1 
R

~
1     T1  T2  2TOF DEV
- The DEV corrects the estimated TOF with clock rate information:
T  1
~
TOF  TOF 1       TOF DEV  2 1   . Note that the available estimate actually
2  R
corresponds to the actual radio distance, and that  is removed out of the expression, so
that the expected error on range estimate could substantially decrease
- the DEV can additionally make an approximation of its own remaining drift relatively to
the PNC and update its digital drift correction within the current superframe:
~ 
1  R   1  R 1     
R
R
As a conclusion, if a perfect detection is assumed and if the observation window is correctly
positioned (in other words, if the path corresponding to the geometrical distance is available and
detected), the ranging precision could be high since a joint estimation algorithm allows one to
take into account the relative drift between the DEV and the PNC.
Time stamp
In this section, we will to consider the use of time stamp in classical TWR transactions and the
general impact on the analytical expressions for the estimated round trip time of flight.
TReply
B
B
TOF
A
A
Transmitting to A
estimated TOF + TS
TOF
TS
Demodulating TOF +
TS estimated by B
Figure 8: General use of time stamp in a TWR transaction between two terminals
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So, in a typical Two Way Ranging scheme involving two distinct entities (namely DEV-A and
DEV-B), retransmission times, i.e. the response from DEV-B to DEV-A's request, should be
conditioned by the beginning of DEV-B's observation window, and not by the estimated time of
arrival of the request from DEV-A determined by DEV-B. Traditionally, in radiolocation systems
based on TWR transactions, time stamp is used to reduce the global measured elapsed time
(relative to the time of flight), and hence, to reduce the uncertainty on the estimated range. In
particular, specific processing dedicated to ranging (complex path detection algorithm) is
postponed after the retransmission of the answer. Then, further communications from DEV-B to
DEV-A should contain the estimated delay between the beginning of the observation window
and the estimated time of arrival, so that DEV-A could compensate the actual response delay out
of its own measurement (measured delay between the emission of the request and the reception
of the answer).
But in the very context, time stamp could not specifically be imposed by the necessity to avoid
large processing times, since the DEVs have to wait for the end of a transmitted frame (a fortiori
the channel estimation preamble) before transmitting the answer. Moreover, the range estimation
usually relies on channel estimation, which is a task imposed by the communication. In other
words, taking into account the necessity to achieve synchronization and to demodulate data, a
terminal must wait for the end of a complete frame before retransmitting its response. So, this
duration is absolutely incompressible for the reply delay.
On the one hand, the main motivation for time stamp corresponds to the necessity of transmitting
signals at prescribed instants linked to the available system temporal granularity. In other words,
it is obviously not possible to emit pulses on a “continuous” temporal scale, but at discrete times.
On the other hand, retransmission times are conditioned by the availability of DEV’s resources.
One terminal would have to wait for a minimum time before its internal resources (baseband
and/or MAC HW, etc…) get free.
In the following, for the purpose of generality, we are to take into account a potential offset Ts
between the actual instant for the emission of the request and the beginning of the integration
process (or equivalently, the beginning of the observation window). It is assumed on Figure 8
that the DEV-A opens an observation window that allows to observe the arrival of the answer in
spite of the uncertainty on TS.
So, from the observation of Figure 8, it comes that:

 1
1
~
TOF  1   A  2TOF  
2 
1 B

Submission



TReply  TS  TOF   TReply  1   B TS  TOF 




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This expression can simplified into:
2   A   B  TReply  A   B  
~
TOF  TOF

 TS  B   A 
2
2
1 B
Since TS in the worst case corresponds to the integrality of the observation window, and since the
relative clock drift between terminals is usually lower than 10ppm (10-5), this term can be
neglected.
2   A   B  can be neglected as well, taking into account usual
Moreover, the first term TOF
2
ranges and classical values for the absolute clock drifts. Finally, this leads to the expression:
TReply  A   B 
~
TOF 
2
1 B
So, when comparing the last expression to the expressions obtained for general TWR
transactions (i.e. without time stamp), it is evident that in first approximation, the use of time
stamp will not have any additional influence on performances.
DEV to DEV Ranging
General Transactions
In this section, we assume that all the DEV have preliminarily compensated their propagation
delays relative to the PNC out of their own clock before the beginning of the peer-to-peer
communications in the CTA (for instance, thanks to preliminary double TWR transactions with
the PNC), so that all the terminals in the PicoNet can be considered as synchronous in the CTA.
This represents a particularly strong hypothesis, and it is evident that in realistic cases, the DEV
can not be strictly synchronized to the PNC’s clock.
If terminals are synchronized to a common clock (PNC’s one in our case), it is well known that
direct OWR can be used for ranging. The main idea behind this is that the OWR ranging scheme
is simple and less restricting when compared to TWR schemes.
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RefA
RefPNC RefB
RefPNC
DEV-A
 PNC   A
TOF
DEV-B
 PNC   B
TSyncA TSyncB
CTA Slot
Figure 9: DEV to DEV OWR scheme within the CTA
Ranging Errors from Clock Drifts and Synchronization Offsets
When considering the Figure 9, it comes that:
~
TOF B  1   PNC   B TOF  TSyncA  TSyncB 
and the corresponding ranging error can be approximated by
~
E DEVB  TOF B  TOF  1   PNC   B TSyncA  TSyncB   TSyncA  TSyncB 
So, it is clear that the ranging error in the OWR scheme is principally due to synchronization
offsets at the DEV and one could easily tie the OWR ranging accuracy to network
synchronization performances.
Submission
Page 16
Benoit Denis, CEA/LETI
August, 2004
IEEE P802.15-04/427r0
References
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Submission
Page 17
Benoit Denis, CEA/LETI