U.S. patent application number 15/814711 was filed with the patent office on 2018-11-15 for carrier recovery for use in an ultra-wideband communication system.
This patent application is currently assigned to DecaWave, Ltd. The applicant listed for this patent is DecaWave, Ltd. Invention is credited to Ciaran McElroy, Michael McLaughlin, Dries Neirynck.
Application Number | 20180331718 15/814711 |
Document ID | / |
Family ID | 64098017 |
Filed Date | 2018-11-15 |
United States Patent
Application |
20180331718 |
Kind Code |
A1 |
McLaughlin; Michael ; et
al. |
November 15, 2018 |
CARRIER RECOVERY FOR USE IN AN ULTRA-WIDEBAND COMMUNICATION
SYSTEM
Abstract
In an ultra-wideband ("UWB") receiver, a received UWB signal is
periodically digitized as a series of ternary samples. During a
carrier acquisition mode, the samples are continuously correlated
with a predetermined preamble sequence to develop a correlation
value. When the value exceeds a predetermined threshold, indicating
that the preamble sequence is being received, estimates of the
channel impulse response ("CIR") are developed. When a
start-of-frame delimiter ("SFD") is detected, the best CIR estimate
is provided to a channel matched filter ("CMF"). During a data
recovery mode, the CMF filters channel-injected noise from the
sample stream. Both carrier phase errors and data timing errors are
continuously detected and corrected during both the carrier
acquisition and data recovery modes of operation.
Inventors: |
McLaughlin; Michael;
(Dublin, IE) ; Neirynck; Dries; (Dublin, IE)
; McElroy; Ciaran; (Dublin, IE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
DecaWave, Ltd |
Dublin |
|
IE |
|
|
Assignee: |
DecaWave, Ltd
Dublin
IE
|
Family ID: |
64098017 |
Appl. No.: |
15/814711 |
Filed: |
November 16, 2017 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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15310548 |
Nov 11, 2016 |
10075210 |
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PCT/EP2014/060722 |
May 23, 2014 |
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15814711 |
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13775282 |
Feb 25, 2013 |
8760334 |
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15310548 |
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13033098 |
Feb 23, 2011 |
8436758 |
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13775282 |
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12885517 |
Sep 19, 2010 |
8437432 |
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13775282 |
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61316299 |
Mar 22, 2010 |
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61316299 |
Mar 22, 2010 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G01S 3/48 20130101; G01S
3/023 20130101; H04B 1/71637 20130101; G01S 3/46 20130101 |
International
Class: |
H04B 1/7163 20060101
H04B001/7163; G01S 3/48 20060101 G01S003/48; G01S 3/02 20060101
G01S003/02; G01S 3/46 20060101 G01S003/46 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 7, 2013 |
EP |
PCT/EP2013/070851 |
Claims
1. A method for use in an ultra-wideband (UWB) communication system
in which a modulated carrier signal is transmitted via a
transmission channel having an impulse response, the method
comprising the steps of: 1.1. sequentially developing a plurality
of complex estimates of said impulse response; 1.2. developing an
accumulated estimate of said impulse response as a function of a
first set of said estimates; 1.3. correlating with said accumulated
estimate a second set of said estimates; 1.4. developing an
estimated peak of said correlation, said peak corresponding to a
selected on-time estimate in said second set of estimates; 1.5.
developing a timing phase error as a function of said on-time
estimate, a selected early estimate in said second set of estimates
preceding said on-time estimate, and a selected late estimate in
said second set of estimates following said on-time estimate.
2. The method of claim 1 wherein the early estimate comprises the
estimate in said second set immediately preceding said on-time
estimate.
3. The method of claim 1 wherein the late estimate comprises the
estimate in said second set immediately following said on-time
estimate.
4. The method of claim 1 further characterized as comprising the
step of: 1.6. resampling by said timing phase error said estimates
following said second set of estimates.
5. Apparatus for use in an ultra-wideband (UWB) communication
system in which a modulated carrier signal is transmitted via a
transmission channel, the apparatus configured to perform the
method according to claim 1.
6. A method for use in an ultra-wideband (UWB) communication system
in which a modulated carrier signal is transmitted via a
transmission channel having an impulse response, the method
comprising the steps of: 1.1. developing a first complex estimate
of said impulse response; 1.2. filtering the carrier signal as a
function of the first complex estimate to develop a filtered
carrier signal; 1.3. despreading the filtered carrier signal to
develop a set of timing error metrics comprising an early metric,
an on-time metric and a late metric; and 1.4. developing a timing
phase error as a function of at least a selected one of said
metrics.
7. The method of claim 6 further characterized as comprising the
step of: 1.5. resampling by said timing phase error said filtered
carrier signal.
8. Apparatus for use in an ultra-wideband (UWB) communication
system in which a modulated carrier signal is transmitted via a
transmission channel, the apparatus configured to perform the
method according to claim 6.
9. A method for use in an ultra-wideband (UWB) communication system
in which a modulated carrier signal is transmitted via a
transmission channel having an impulse response, the method
comprising the steps of: 1.0. in a carrier acquisition mode of
operation: 1.1. sequentially developing a plurality of complex
estimates of said impulse response; 1.2. developing an accumulated
estimate of said impulse response as a function of a selected first
set of said estimates; 1.3. correlating with said accumulated
estimate a selected second set of said estimates; 1.4. developing
an estimated peak of said correlation, said peak corresponding to a
selected on-time estimate in said second set of estimates; 1.5.
developing a timing phase error as a function of said on-time
estimate, a selected early estimate in said second set of estimates
preceding said on-time estimate, and a selected late estimate in
said second set of estimates following said on-time estimate; and
2.0. in a data recovery mode of operation: 2.1. filtering the
carrier signal as a function of said accumulated estimate to
develop a filtered carrier signal; 2.2. despreading the filtered
carrier signal to develop a set of timing error metrics comprising
an early metric, an on-time metric and a late metric; and 2.3.
developing said timing phase error as a function of at least a
selected one of said metrics.
10. The method of claim 9 further characterized as comprising the
step of: 1.6. resampling by said timing phase error said estimates
following said second set of estimates.
11. The method of claim 9 further characterized as comprising the
step of: 2.4. resampling by said timing phase error said filtered
carrier signal.
12. Apparatus for use in an ultra-wideband (UWB) communication
system in which a modulated carrier signal is transmitted via a
transmission channel, the apparatus configured to perform the
method according to claim 9.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a Divisional of application Ser. No.
15/310,584, filed 11 Nov. 2016 ("First Parent application").
[0002] The First Parent application is a Continuation-In-Part of
PCT Application Serial No. PCT/EP2013/070851, filed 7 Oct. 2013
("Second Parent application").
[0003] The First Parent application is also a Continuation-In-Part
of application Ser. No. 13/775,282, filed 25 Feb. 2013, now U.S.
Pat. No. 8,760,334, issued 24 Jun. 2014 ("Third Parent
Application").
[0004] The Second Parent application is a Continuation-In-Part of
the Third Parent application.
[0005] The Third Parent application is a Continuation-In-Part of
application Ser. No. 13/033,098, filed 23 Feb. 2011 ("Fourth Parent
application"), now U.S. Pat. No. 8,436,758, issued 7 May 2013,
which is in turn related to Provisional Application Ser. No.
61/316,299, filed 22 Mar. 2010 ("Parent Provisional").
[0006] The Third Parent application is also a Continuation-In-Part
of application Ser. No. 12/885,517, filed 19 Sep. 2010, now U.S.
Pat. No. 8,437,432, issued 7 May 2013 ("Parent patent"), which is
in turn also related to the Parent Provisional.
[0007] The subject matter of this application is also related to
the subject matter of PCT Application Serial No. PCT/EP2013/070851,
filed 7 Oct. 2013 ("Related application").
[0008] This application claims priority to:
[0009] 1. The First Parent application;
[0010] 2. The Second Parent application;
[0011] 3. The Third Parent application;
[0012] 4. The Fourth Parent application;
[0013] 5. The Parent patent;
[0014] 6. The Parent Provisional; and
[0015] 7. The Related application;
collectively, "Related References", and hereby claims benefit of
the filing dates thereof pursuant to 37 CFR .sctn. 1.78(a)(4).
[0016] The subject matter of the Related References, each in its
entirety, is expressly incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
[0017] The present invention relates generally to ultra-wideband
communication systems, and, in particular, to a receiver for use in
an ultra-wideband communication system adapted to determine the
angle of arrival of an RF signal.
2. Description of the Related Art
[0018] In general, in the descriptions that follow, we will
italicize the first occurrence of each special term of art which
should be familiar to those skilled in the art of ultra-wideband
("UWB") communication systems. In addition, when we first introduce
a term that we believe to be new or that we will use in a context
that we believe to be new, we will bold the term and provide the
definition that we intend to apply to that term. In addition,
throughout this description, we will sometimes use the terms assert
and negate when referring to the rendering of a signal, signal
flag, status bit, or similar apparatus into its logically true or
logically false state, respectively, and the term toggle to
indicate the logical inversion of a signal from one logical state
to the other. Alternatively, we may refer to the mutually exclusive
boolean states as logic_0 and logic_1. Of course, as is well known,
consistent system operation can be obtained by reversing the logic
sense of all such signals, such that signals described herein as
logically true become logically false and vice versa. Furthermore,
it is of no relevance in such systems which specific voltage levels
are selected to represent each of the logic states.
[0019] In general, in an ultra-wideband ("UWB") communication
system, a series of special processing steps are performed by a UWB
transmitter to prepare payload data for transmission via a
packet-based UWB channel. Upon reception, a corresponding series of
reversing steps are performed by a UWB receiver to recover the data
payload. Details of both series of processing steps are fully
described in IEEE Standards 802.15.4 ("802.15.4") and 802.15.4a
("802.15.4a"), copies of which are submitted herewith and which are
expressly incorporated herein in their entirety by reference. As is
known, these Standards describe required functions of both the
transmit and receive portions of the system, but specify
implementation details only of the transmit portion of the system,
leaving to implementers the choice of how to implement the receive
portion.
[0020] One or more of us have developed certain improvements for
use in UWB communication systems, which improvements are fully
described in the following pending applications or issued patents,
all of which are expressly incorporated herein in their
entirety:
[0021] "A Method and Apparatus for Generating Codewords", U.S. Pat.
No. 7,787,544, issued 31 Jul. 2010;
[0022] "A Method and Apparatus for Generating Codewords",
application Ser. No. 11/309,222, filed 13 Jul. 2006, now
abandoned;
[0023] "A Method and Apparatus for Transmitting and Receiving
Convolutionally Coded Data", U.S. Pat. No. 7,636,397, issued 22
Dec. 2009;
[0024] "A Method and Apparatus for Transmitting and Receiving
Convolutionally Coded Data", U.S. Pat. No. 8,358,709, issued 22
Jan. 2013; and
[0025] "Convolution Code for Use in a Communication System", U.S.
Pat. No. 8,677,224, issued 18 Mar. 2014.
[0026] One particular problem in multi-path, spread-spectrum
systems, including UWB, is channel-induced noise present in the
received signal. One common technique for significantly reducing
the noise level relative to the receive level is to develop, during
reception of a training sequence portion of the preamble of each
transmitted packet, an estimate of the channel impulse response
("CIR"). Following detection in the received packet of the
start-of-frame delimiter ("SFD"), the best CIR estimate is reversed
in time and the complex conjugate is developed. This conjugate CIR
estimate is thereafter convolved with the payload portion of the
packet using a channel matched filter ("CMF"). Shown in FIG. 1 is a
UWB receiver 10 adapted to operate in this manner. As is known, the
signal received via an antenna 12 is continuously conditioned by a
filter 14. During reception of the training sequence, channel
estimator 16 develops from the conditioned signal the conjugate CIR
estimate. During reception of the payload data, detector 18 employs
a CMF (not shown) to convolve the conditioned signal with the
conjugate CIR estimate, thereby significantly improving the
signal-to-noise ratio ("SNR") and facilitating recovery of the
payload data. See, also, "Efficient Back-End Channel Matched Filter
(CMF)", U.S. Pat. No. 7,349,461, issued 25 Mar. 2008.
[0027] As noted in 802.15.4a, .sctn. 5.5.7.1, "UWB devices that
have implemented optional ranging support are called
ranging-capable devices ("RDEVs")." (Emphasis in original.) For
certain applications, such RDEVs are commonly implemented in the
form of a relatively compact, autonomous radio-frequency
identification ("RFID") tag or the like. Due to the small form
factor and limited power supply, it is especially important to
select circuit implementations that provide maximum performance at
minimum power. Unfortunately, in known implementations of the UWB
receiver, improvements in performance usually come at the expense
of power. For example, it is known that a rake filter provides good
performance in multi-path, spread-spectrum systems such as UWB.
See, e.g., slide 21 of "The ParthusCeva Ultra Wideband PHY
Proposal", IEEE P802.15 Working Group for Wireless Personal Area
Networks, March 2003, a copy of which is submitted wherewith and
which is expressly incorporated herein in its entirety by
reference. However, known rake filter implementations tend to
consume significantly more power than other prior art
techniques.
[0028] In ranging systems, as in other RF systems, the receiver
must coordinate its internal operation to the signal being received
from the transmitter. In general, the receiver must achieve
synchronism with the received carrier signal, a process referred to
as carrier recovery. In addition, the receiver must further achieve
synchronism with the information signals superimposed on the
carrier, a process referred to as timing recovery. We submit that
prior art techniques for performing both carrier recovery and
timing recovery in the digital domain are less than optimum.
[0029] In the RF system topology shown in FIG. 15, it can be seen
that, because of the non-zero angle of incidence, .theta., the RF
signal will arrive at one antenna before the other. In particular,
it can be seen that the path to antenna A is greater than to
antenna B by p=d*sin(.theta.). In order to calculate .theta., the
angle of incidence, the time difference of arrival could be found.
If d is relatively large then this would provide quite an accurate
estimate of .theta.. On the other hand, if d is small the estimate
turns out to be highly error prone.
[0030] FIG. 16 shows two receivers, 70a and 70b, which are clocked
from the same crystal 72. If the same crystal 72 clocks identical
phase locked loops ("PLLs"), 74a and 74b, the generated carriers
that are supplied to the respective down converter mixers,
76ac-76as and 76bc-76as, will have the same phase. The RF signal
will arrive at a slightly later time at antenna A than antenna B,
so it will encounter a down converter carrier phase that is
different in each of the mixer s 76. If the baseband processors,
78a and 78b, are capable of calculating the complex impulse
response of the channel, that impulse response will have a
different in-phase ("I") to quadrature ("Q") ratio I/Q which is
equal to the phase delay caused by the signal travelling the extra
distance, p, before encountering the mixer 70a and being
down-converted by the carrier. If the carrier frequency is high,
e.g., 4 GHz or 6.5 GHz, then quite small distances, p, will lead to
a relatively large carrier phase difference.
sin .theta. = p d [ Eq . 1 ] .lamda. = c f [ Eq . 2 ]
##EQU00001##
[0031] where: [0032] f is the carrier frequency, [0033] c is the
speed of light, and [0034] .lamda. is the carrier wavelength.
[0034] .alpha. 2 .pi. = p .lamda. [ Eq . 3 ] ##EQU00002##
[0035] where: [0036] .alpha. is the phase difference between the
two carriers for the same point on the incident RF signal.
[0036] p = .alpha. .lamda. 2 .pi. ( from Eq . 2 and Eq . 3 ) [ Eq .
4 ] sin .theta. = .alpha. .lamda. 2 .pi. d ( from Eq . 1 and Eq . 4
) [ Eq . 5 ] .theta. = sin - 1 .alpha. .lamda. 2 .pi. d ( from Eq .
5 ) [ Eq . 6 ] ##EQU00003##
[0037] If, in Eq. 6, d is set to be a half wavelength, then FIG. 17
shows the relationship between .alpha., the phase difference of the
impulse responses, and .theta., the angle of incidence. Note that
the slope of the dark grey section is approximately 3, whereas the
slope of the lighter grey section is 0.6, i.e., 5 times worse. If,
however, d is set to be a one wavelength, then FIG. 18 shows the
relationship between .alpha. and .theta.. Note that, at this
separation, there is an ambiguity in that each phase relationship
has two possible angles of incidence. As can be seen from FIG. 19,
as the antennae are moved further apart, say to 3 wavelengths, the
ambiguity only increases.
[0038] We submit that the larger separation of one wavelength or
more is advantageous for two reasons: first, the slope of the angle
of incidence curve versus phase change curve is larger and stays
larger for longer, thereby allowing more accurate determination of
angle of incidence; and second, as the antennas get closer
together, their near fields interfere and their performance starts
to affect each other. This is particularly the case when the
separation is lower than one wavelength.
[0039] We submit that what is needed is an improved method and
apparatus for use in the receiver of a UWB communication system to
determine angle of incidence. In particular, we submit that such a
method and apparatus should provide performance generally
comparable to the best prior art techniques but more efficiently
than known implementations of such prior art techniques.
BRIEF SUMMARY OF THE INVENTION
[0040] In accordance with an embodiment of our invention, we
provide a method for use in an ultra-wideband (UWB) communication
system in which a modulated carrier signal is transmitted via a
transmission channel having an impulse response, the method
comprising the steps of: 1.1. sequentially developing a plurality
of complex estimates of said impulse response; 1.2. developing an
accumulated estimate of said impulse response as a function of a
first set of said estimates; 1.3. correlating with said accumulated
estimate a second set of said estimates; 1.4. developing an
estimated peak of said correlation, said peak corresponding to a
selected on-time estimate in said second set of estimates; 1.5.
developing a timing phase error as a function of said on-time
estimate, a selected early estimate in said second set of estimates
preceding said on-time estimate, and a selected late estimate in
said second set of estimates following said on-time estimate.
[0041] In accordance with a different embodiment of our invention,
we provide a method for use in an ultra-wideband (UWB)
communication system in which a modulated carrier signal is
transmitted via a transmission channel having an impulse response,
the method comprising the steps of: 1.1. developing a first complex
estimate of said impulse response; 1.2. filtering the carrier
signal as a function of the first complex estimate to develop a
filtered carrier signal; 1.3. despreading the filtered carrier
signal to develop a set of timing error metrics comprising an early
metric, an on-time metric and a late metric; and 1.4. developing a
timing phase error as a function of at least a selected one of said
metrics.
[0042] In accordance with yet a different embodiment of our
invention, we provide a method for use in an ultra-wideband (UWB)
communication system in which a modulated carrier signal is
transmitted via a transmission channel having an impulse response,
the method comprising the steps of: 1.0. in a carrier acquisition
mode of operation: 1.1. sequentially developing a plurality of
complex estimates of said impulse response; 1.2. developing an
accumulated estimate of said impulse response as a function of a
selected first set of said estimates; 1.3. correlating with said
accumulated estimate a selected second set of said estimates; 1.4.
developing an estimated peak of said correlation, said peak
corresponding to a selected on-time estimate in said second set of
estimates; 1.5. developing a timing phase error as a function of
said on-time estimate, a selected early estimate in said second set
of estimates preceding said on-time estimate, and a selected late
estimate in said second set of estimates following said on-time
estimate; and 2.0. in a data recovery mode of operation: 2.1.
filtering the carrier signal as a function of said accumulated
estimate to develop a filtered carrier signal; 2.2. despreading the
filtered carrier signal to develop a set of timing error metrics
comprising an early metric, an on-time metric and a late metric;
and 2.3. developing said timing phase error as a function of at
least a selected one of said metrics.
[0043] The methods of our invention may be embodied in an
apparatus, or in a computer readable code on a suitable computer
readable medium such that when a processor executes the computer
readable code, the processor executes the respective method.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
[0044] Our invention may be more fully understood by a description
of certain preferred embodiments in conjunction with the attached
drawings in which:
[0045] FIG. 1 illustrates, in block diagram form, a prior art
receiver adapted for use in a UWB communication system;
[0046] FIG. 2 illustrates, in block diagram form, one embodiment of
the receiver shown in FIG. 1, but constructed in accordance with
our invention;
[0047] FIG. 3 illustrates, in block diagram form, the portion of
the receiver shown in FIG. 2 that performs carrier and timing
recovery when the receiver is operating in a carrier acquisition
mode;
[0048] FIG. 4 illustrates, in block diagram form, the portion of
the receiver shown in FIG. 2 that performs carrier and timing
recovery when the receiver is operating in a data recovery
mode;
[0049] FIG. 5 illustrates, in block diagram form, the error angle
calculation block shown, e.g., in FIG. 3;
[0050] FIG. 6 illustrates, in relative isolation, those components
of the receiver shown in FIG. 3 adapted to perform carrier
recovery;
[0051] FIG. 7 illustrates, in block diagram form, a programmable
scaler adapted for use with our carrier loop filter shown in FIG.
6;
[0052] FIG. 8 illustrates, in signal wave form, our approach to
scale the filtered phase error estimate and apply it to the
accumulator more often than once per symbol in the case of 850
kb/s;
[0053] FIG. 9 illustrates, in signal wave form, our approach to
scale the filtered phase error estimate and apply it to the
accumulator more often than once per symbol in the case of 6.8
Mb/s;
[0054] FIG. 10 illustrates, in signal wave form, our approach to
scale the filtered phase error estimate and apply it to the
accumulator more often than once per symbol in the case of 110
kb/s;
[0055] FIG. 11 illustrates, in block diagram form, a rotator
adapted for use with our carrier loop filter shown in FIG. 6;
[0056] FIG. 12 illustrates, in relative isolation, those components
of the receiver shown in FIG. 4 adapted to perform timing
recovery;
[0057] FIG. 13 illustrates, in block diagram form, a resampler
adapted for use with our timing recovery loop shown in FIG. 12;
[0058] FIG. 14 illustrates, in flow diagram form, a process for
seeding the timing loop filter shown in FIG. 12.
[0059] FIG. 15 illustrates, generally in topographic perspective,
an RF communication system, and, in particular, illustrates the
different angles of incidence of the transmitted RF signal on two
antennas spaced apart by a distance d;
[0060] FIG. 16 illustrates, in block diagram form, the antennas of
FIG. 15, together with the respective RF receivers;
[0061] FIG. 17 illustrates, in waveform, the ratio of the phase
shift of the carrier as a function of angle of incidence with
respect to two 1/2 wavelength separated antennas;
[0062] FIG. 18 illustrates, in waveform, the ratio of the phase
shift of the carrier as a function of angle of incidence with
respect to two 1 wavelength separated antennas;
[0063] FIG. 19 illustrates, in waveform, the ratio of the phase
shift of the carrier as a function of angle of incidence with
respect to two antennas separated by 3 wavelengths;
[0064] FIG. 20 illustrates, in waveform, the ratio of the phase
shift of the carrier as a function of angle of incidence with
respect to two antennas separated by 7.5 cm at two different
carrier frequencies; and
[0065] FIG. 21 illustrates, in waveform, the resultant calculated
angle of incidence for a 100 packet test.
[0066] In the drawings, similar elements will be similarly numbered
whenever possible. However, this practice is simply for convenience
of reference and to avoid unnecessary proliferation of numbers, and
is not intended to imply or suggest that our invention requires
identity in either function or structure in the several
embodiments.
DETAILED DESCRIPTION OF THE INVENTION
[0067] Shown in FIG. 2 is a UWB receiver 10' constructed in
accordance with our invention. As in the prior art system shown in
FIG. 1, the signal received by antenna 12 is continuously
conditioned by filter 14. The conditioned signal is then
periodically sampled by an analog-to-digital converter ("ADC") 20
and provided as a continuous series of digital samples. In
accordance with a preferred embodiment of our invention, ADC 20 is
specially adapted to provide each digital sample in ternary form,
i.e., [-1, 0, +1]. In view of the difficulty of currently available
standard digital circuit technology efficiently to represent a
3-value variable in the form of a single ternary trit, we
anticipate, at least in the near term, such variables will require
representation using 2 conventional, binary bits, wherein a first
one of the bits represents the numeric component of the variable,
i.e., [0, 1], and the second bit represents the sign of the
variable, i.e., [+, -]. In this regard, it could be argued that
circuit technology has not progressed all that much since Soviet
researchers built the first (perhaps only?) documented
ternary-based computer systems. See, "A Visit to Computation
Centers in the Soviet Union," Comm. of the ACM, 1959, pp. 8-20; and
"Soviet Computer Technology 1959", Comm. of the ACM, 1960, pp.
131-166; copies of which are submitted herewith and which are
expressly incorporated herein in their entirety by reference.
[0068] In the context of our invention, our trit can be
distinguished from a conventional sign+magnitude implementation
such as that described in Amoroso83, cited above. Consider the
strategy for A/D conversion shown in FIG. 5 of Amoroso83; and,
note, especially, that there are three separate and distinct
switching thresholds: (i) a sign threshold [T.sub.0]; (ii) a
positive magnitude threshold [T.sub.0+.DELTA.]; and (iii) a
negative magnitude threshold [T.sub.0-.DELTA.]. (See, also,
Amoroso83, p. 1119, lines 21-24.) We have discovered that adapting
the ADC to use ONLY a positive magnitude threshold
[T.sub.0+.DELTA.] and a negative magnitude threshold
[T.sub.0-.DELTA.] results in only a very small loss in resolution,
while improving the performance of an impulse radio UWB receiver.
Accordingly, in our preferred embodiment, ADC 20 implements only
positive/negative magnitude thresholds [T.sub.0.+-..DELTA.],
thereby simplifying the circuit while simultaneously improving both
the conversion time of the ADC 20 and, in general, the performance
of the receiver. Such an implementation lends itself naturally to
our trit-based scheme, wherein the three defined states indicate,
for example, that: [0069] [-1]=>the input is below the negative
magnitude threshold [T.sub.0-.DELTA.]; [0070] [0]=>the input is
between the negative magnitude threshold [T.sub.0-.DELTA.] and the
positive magnitude threshold [T.sub.0+.DELTA.]; and [0071]
[+1]=>the input is above the positive magnitude threshold
[T.sub.0+.DELTA.]. In contrast to a conventional sign+magnitude
implementation, our trit-based ADC 20 can be readily adapted to
operate either at a higher sample rate (improved performance but
with more power) or at an equivalent sample rate (substantially
equivalent performance but with less complexity, thereby reducing
both circuit size and power consumption).
[0072] Additional details relating to the construction and
operation of our UWB receiver 10' can be found in the Related
References. As explained in the Related References, receiver 10'
initially operates in an acquisition mode, during which the
components are configured to detect an incoming UWB signal
transmitted by a remote UWB transmitter (not shown), and to achieve
synchronism with that transmission, a process referred to as
acquisition. Having achieved acquisition, receiver 10' transitions
into a data mode, during which the components are configured to
recover data contained within each transmitted packet, a process
referred to as demodulation or data recovery.
[0073] Shown in FIG. 3 are the components of our receiver 10' that
perform carrier and timing recovery when the receiver 10' is
operating in the acquisition mode. As explained in our Related
References, during operation, ADC 20 develops trit-valued samples
of both the in-phase, Sr[5:0], and quadrature, Si[5:0], components
of the received signals. In the acquisition mode, these samples are
passed to the correlator 24, where they are correlated against the
predefined preamble code. If a valid preamble is present, the
output of the correlator 24 comprises a noisy estimate of the
channel impulse response ("CIR"). This noisy CIR estimate is
passed, without carrier and timing recovery, to accumulator 26. If
a preamble is present, then, as accumulator 26 adds symbols
together, the CIR estimate will grow faster than the noise floor.
By comparing successive accumulated groups of CIR estimates, the
acquisition control logic (described in our Related References) can
determine if there is a valid preamble present.
[0074] Once a preamble has been identified, the carrier recovery
logic is activated to correct carrier error in the received data.
Logic 46 computes an instantaneous phase error estimate of the
received UWB signal by performing an inverse tangent operation on
the in-phase, i.e., real, and quadrature, i.e., imaginary,
components of the signal phasor. A carrier loop filter 48 uses this
estimate to compute a correction angle to be applied to the current
input to the accumulator 26. This correction angle is developed as
a carrier recovery phase signal (7-bits unsigned) wherein the
output range is 0.0 to almost 2.0, and where the value 2.0 is
equivalent to one revolution. A look-up-table ("LUT") 50 converts
the correction angle to a corresponding pair of cosine (5-bits,
including a sign bit) and sine (5-bits, including a sign bit)
values. Using these sine and cosine values, a rotator 52 rotates
the correlated samples by implementing a complex multiplication for
each sample, followed by rounding to return the output real and
imaginary samples to 7-bit signed values. The rotated correlated
samples are then resampled by resample 54 for use by the
accumulator 26.
[0075] There is a finite probability that a detected preamble is
not valid. Accordingly, our accumulator 26 spends some time
assessing the quality of the incoming signal. If the quality is
found to be poor, the preamble is rejected and accumulator 26
resumes searching for a preamble. If the quality is found to be
sufficiently high, the next task is to search for a
start-of-frame-delimiter ("SFD") by comparing the incoming
correlator samples against the accumulated CIR estimate. This is
performed for each symbol, and the result quantised to a 2-bit
signed value. This 2-bit signed value should be +1 throughout the
preamble, but then, once the SFD pattern is received, it should
follow the pattern of the SFD. For example, for a short SFD, this
would be [0, +1, 0, -1, +1, 0, 0, -1]. A corresponding search
pattern is used to find this sequence on the 2-bit quantised
values. This allows the SFD pattern to be identified, and the time
to transition to data mode determined.
[0076] Shown in FIG. 4 are the components of our receiver 10' that
perform carrier and timing recovery when the receiver 10' is
operating in the data mode. No later than the end of the
acquisition mode, the CIR estimate developed by accumulator 26 is
loaded into the channel-matched: filter ("CMF") 36. Once receiver
10' switches to data mode, the ADC 20 samples are passed to the CMF
36, which gathers the energy of the received UWB pulses together,
thereby compensating for the smearing effect of the multi path
channel. The resulting output pulses are passed through the timing
and carrier recovery loops to compensate for any offset, and then
into despreader 40. Despreader 40 applies the same pseudo-random
noise ("PN") spreading sequence used in the transmitter (not shown)
to generate the burst, and integrates over the burst. As despreader
40 does not know if a logic_1 or a logic_0 is being transmitted, it
despreads at both possible locations to produce two estimates.
These two estimates provide the soft input to the Viterbi decoder
42, which produces a maximum likelihood estimate of the bit
sequence. The first section of the packet to be demodulated is the
PHY header ("PHR"), which contains information about the data rate
and the number of octets transmitted. As is known, this is
protected by a single-error-correct-double-error detect ("SECDED")
code. Once the PHR is decoded, the relevant information is
extracted to allow the demodulation, i.e., recovery, of the data
payload. The remaining payload is then passed through the Viterbi
decoder 42 to Reed-Solomon ("RS") decoder 44 to correct any errors
or report errors that cannot be corrected.
[0077] As shown in FIG. 3, in the acquisition mode, the
instantaneous phase error estimate is derived from the output of
correlator 24 as follows:
1. The accumulated samples, Sr[14:0] and Sr[14:0], are quantized
and the conjugate calculated in logic block 56; 2. The complex
correlated samples from resampler 54 are multiplied by the
conjugate by multiplier 58; and 3. The products are summed by
summer 60 to produce the complex instantaneous phase error estimate
phasor, comprising Sr[15:0] and Si[15:0].
[0078] As shown in FIG. 4, in the data mode, the complex
instantaneous phase error estimate phasor simply comprises the
outputs of despreader 40, comprising Sr[15:0] and Si[15:0].
[0079] The instantaneous phase error estimate phasor is then
converted to a corresponding angle by logic block 62. As shown in
FIG. 5, the complex error estimate phasor consists of two 17-bit
signed numbers. The angle of the phasor is determined by dividing
the imaginary component of the phasor, Si[15:0], by the real
component, Sr[15:0]. Ideally the resulting quotient would then be
passed to an inverse tangent function to compute the exact angle;
however, for the purposes of our carrier recovery algorithm, we
have determined that the quotient per se is a sufficient
approximation to the angle. To simplify the division computation,
we first identify the sign and quadrant of the phasor, and then
compute the absolute value of the real and imaginary parts. These
absolute values are then passed to the division computation, and
the resulting angle is post-processed to map it into the
appropriate quadrant/sign. The remapped estimate is then subjected
to rounding and saturation to produce the signed 7-bit
instantaneous phase error estimate, S[0:-5], in radians.
[0080] The structure of carrier loop filter 48 can be seen in
greater detail in FIG. 6. As shown, the instantaneous phase error
estimate is passed through a proportional gain arm 48a and an
integral gain arm 48b. The gains, K.sub.p and K.sub.i, are
controlled by a gear-shifting algorithm to allow the loop to lock
quickly, and then to rapidly tighten to a narrow bandwidth to
minimise the impact of noise on the carrier recovery algorithm. The
gear shifting is controlled by a counter (e.g., see, FIG. 7) that
counts the number of phase error estimates supplied to the loop,
and selects a respective scaling factor. (The default gear shifting
table is described below.) The filtered phase error estimate is
accumulated in a carrier phase accumulator 48c to track the
over-all phase error, and to adjust the instantaneous phase error
to track frequency errors of up to +/-40 ppm.
[0081] When receiver 10' transitions from acquisition to data mode,
the rate of update of the carrier recovery loop 48 will change
(from the preamble symbol interval to the data symbol interval);
this requires that the integral term in the carrier loop filter 48b
be scaled to compensate for this change as follows:
TABLE-US-00001 TABLE 1 Carrier Loop Filter Scaling Table Preamble
Data Scale Symbol Symbol Factor 992 1024 33/32 1016 10124 129/128
992 8192 8 * (33/32) 1016 8192 8 * (129/128)
[0082] In data mode, we have determined that the application of the
phase rotation in a single "lump" at the end of each symbol has a
negative impact on the performance of the receiver. For the 6.8
Mb/s case, the phase is applied at the end of a group of 8 symbols,
so the symbols towards the end of this group suffer from increased
phase error as compared to those at the start. Similarly for the
850 kb/s case, the symbols representing a logic_1 will have a
higher phase error than those representing a logic_0. Worst of all
is the 110 kb/s case, which will suffer from a phase error increase
throughout a symbol and, depending on the hop position of the
symbol, will have an effectively random phase error to start with
if the carrier offset is high enough. To compensate for this, our
preferred embodiment will smooth the phase rotation during the data
demodulation phase.
[0083] As shown in FIG. 7, our approach is to scale the filtered
phase error estimate and apply it to the accumulator more often
than once per symbol. In the case of 850 kb/s, the filtered phase
error will be scaled down by a factor of 2 and accumulated twice
(i.e., every 64 clock cycles) during the phase update interval.
(See, e.g., FIG. 8.) This means that instead of the phase
adjustment being applied in a single lump at the end of the
interval, it will be distributed over the course of the symbol,
thereby allowing the despread data towards the end of the interval
to have a more accurate phase correction. In the case of 6.8 Mb/s,
the filtered phase error will be scaled down by a factor of 8 and
accumulated 8 times (i.e., every 16 clock cycles) during the phase
update interval. (See, e.g., FIG. 9.) In the case of the 110 kb/s,
the filtered phase error estimate will be scaled down by a factor
of 64 and accumulated 64 times (i.e., every 16 clock cycles) during
the phase update interval. (See, e.g., FIG. 10.) The despreader 40
will control when the phase rotation is applied, so that the phase
rotation is updated just before it is needed for demodulation of a
burst.
[0084] In our preferred embodiment, we implement a register-based
field programmable gear shifting mechanism. Ten gears may be
configured; one is reserved for demodulation mode, allowing nine
acquisition gears. Each gear is assigned: a count at which it is
activated; a K.sub.p value; and a K.sub.i value. Writing a value of
logic_0 as the count for a gear other than the first gear
terminates the gear shifting table; whilst still switching to the
demodulation gear when the acquisition phase is over. Note that two
sets of demodulation coefficients must be specified, one for the
110 Kbps data rate case, and one for the 850K and 6.81 Mbps cases.
The default values for each of the available programmable registers
are given in the following table:
TABLE-US-00002 TABLE 2 Carrier Recovery Loop Gear Shifting Table
Register Count K.sub.i K.sub.p CR0 0 0x8: 3 * 2.sup.-6 0x7:
2.sup.-3 CR1 12 0x7: 3 * 2.sup.-7 0x7: 2.sup.-3 CR2 20 0x6: 3 *
2.sup.-8 0x6: 2.sup.-4 CR3 32 0x6: 3 * 2.sup.-8 0x6: 2.sup.-4 CR4
40 0x5: 3 * 2.sup.-9 0x5: 2.sup.-5 CR5 64 0x4: 3 * 2.sup.-10 0x5:
2.sup.-5 CR6 128 0x3: 3 * 2.sup.-11 0x4: 2.sup.-6 CR7 192 0x2: 3 *
2.sup.-12 0x4: 2.sup.-6 CR8 256 0x1: 3 * 2.sup.-13 0x3: 2.sup.-7
CR9 1023 0x0: 3 * 2.sup.-14 0x3: 2.sup.-7
The K factors are coded as follows:
TABLE-US-00003 Minimum Maximum Decode Count 1 1023 Sample count on
which to apply gearing values K.sub.i 0x0 0xA 0x0 = 2.sup.-11
.times. (2.sup.-2 + 2.sup.-3) 0x1 = 2.sup.-10 .times. (2.sup.-2 +
2.sup.-3) 0x2 = 2.sup.-9 .times. (2.sup.-2 + 2.sup.-3) 0x3 =
2.sup.-8 .times. (2.sup.-2 + 2.sup.-3) 0x4 = 2.sup.-7 .times.
(2.sup.-2 + 2.sup.-3) 0x5 = 2.sup.-6 .times. (2.sup.-2 + 2.sup.-3)
0x6 = 2.sup.-5 .times. (2.sup.-2 + 2.sup.-3) 0x7 = 2.sup.-4 .times.
(2.sup.-2 + 2.sup.-3) 0x8 = 2.sup.-3 .times. (2.sup.-2 + 2.sup.-3)
0x9 = 2.sup.-2 .times. (2.sup.-2 + 2.sup.-3) 0xA = 2.sup.-1 .times.
(2.sup.-2 + 2.sup.-3) 0xB-0xF - invalid K.sub.p 0x0 0x7 0x0 = 00
0x1 = 2.sup.-9 0x2 = 2.sup.-8 0x3 = 2.sup.-7 0x4 = 2.sup.-6 0x5 =
2.sup.-5 0x6 = 2.sup.-4 0x7 = 2.sup.-3
[0085] Under very noisy conditions, the carrier recovery loop may
fail to lock correctly. This will result in a preamble rejection in
the accumulator 26 (if this mode is enabled), effectively giving
the carrier recovery loop another shot. The timing recovery loop
can still fail to lock, however, and it does not get another chance
since by the time this has an effect the preamble will likely be
confirmed.
[0086] Preferably, LUT 50 updates the SIN (5-bits signed) and COS
(5-bits signed) values under the control of accumulator 26 during
acquisition mode and by despreader 40 during data mode. This is in
order to prevent phase changes being applied at times when the data
is important to the algorithm in question, so it must be applied
outside of the impulse response during acquisition, and outside of
a burst position during demodulation.
[0087] Rotator 52 takes the SIN and COS values from LUT 50 and
applies them to the incoming data vectors. This rotation is applied
to the correlator 24 outputs during acquisition, and to the CMF 36
outputs during data demodulation. As shown in FIG. 12, the first
stage, T0, selects between the registered outputs of correlator 24
and CMF 36 depending on the mode. The second stage, T1, performs
the 4 multiplications that are required for a full complex
multiplication, these multiplication results are then registered.
The third stage, T2, sums the real components and the imaginary
components of the complex multiplication, applies rounding to
return the rotated samples to a precision of 7-bits (signed) and
registers them for the output. Therefore the total latency of the
rotator 52 is four clock cycles, C0-C3.
[0088] Our timing estimation is based on an early-late gating
algorithm. As illustrated in FIG. 3 and FIG. 4, the source of data
for this algorithm is dependent upon the phase of reception: during
acquisition mode, the necessary timing information is derived from
the output of CMF 36; and, during data mode, the despreader 40
provides the necessary timing information. In both cases the
information is used to generate an instantaneous timing estimate.
This estimate is conditioned and passed into a timing loop filter
64 typical of a second order loop. The timing loop filter 64 output
provides sample-level timing adjustment information to the
resampler 54 during acquisition mode, and clock cycle level
adjustments to the accumulator 26 and despreader 40 during data
mode.
[0089] As shown in FIG. 12, during acquisition mode, the timing
estimate is derived from early/on-time/late samples from the output
of the CMF 36. Once the preamble has been detected, the accumulator
26 will start writing CIR coefficients to the CMF 36. As noted
before, the output of the correlator 24 provides an instantaneous
noisy estimate of the CIR. As the timing offset error accumulates
in the accumulator 26, the position of this estimate within the
symbol will slowly vary. Quantizing the correlator 24 output and
feeding it into the CMF 36 result in a correlation peak at the
output of the CMF 36 corresponding to the position of the noisy
impulse response estimate in the correlator 24 output symbol. In
our preferred embodiment, the functions of quantisation of the
correlator 24 output and routing to CMF 36 are performed by
resampler 54.
[0090] As the timing of the incoming signal changes, the position
of this correlation peak in the output of the CMF 36 will move. In
general, this movement will be gradual relative to one preamble
symbol duration and therefore can be tracked. The accumulator 26
provides a timing flag to indicate when the correlation peak is
expected at the output of the CMF 36. This flag is based on the
latency of the accumulator 26 and CMF 36 relative to the estimated
impulse response location. Initially, this will be very accurate as
the timing phase error will be negligible, but, as the phase error
accumulates, the peak will move, thus providing the desired timing
information. The correlation peak is ideally the on-time sample,
with the early and late samples being the one immediately preceding
and following the on-time sample respectively. These three samples
are passed to a phase detector 66 (see, FIG. 3) that computes an
instantaneous timing phase error for the timing loop filter 64.
[0091] During demodulation, the despreader 40 provides dedicated
early and late outputs in addition to the normal on-time output
used as part of the demodulation process. These outputs are
provided for both possible burst positions (depending on if the
data is a logic_0 or a logic_1); thus, an early instantaneous
decision is needed in order to identify which of the two sets of
early/on-time/late samples to use in computing the instantaneous
timing phase error estimate. Once this decision is made, the
computation is performed and the error passed to the the timing
loop filter 64.
[0092] The phase error estimation is based on the previously
described early/on-time/late samples. A difference computation is
performed by computational block 68. The difference between the
early and late values is computed and checked against the on-time
(on-time should be greater). If the on-time value is negative or
zero (after conditioning), then the data is unreliable and the
phase estimate zeroed. The difference is divided by twice the peak
and the result checked to be less than 0.75 (otherwise it is
considered unreliable) and passed out as the instantaneous timing
error estimate in the format S[-1:-4]. This instantaneous timing
error estimate is then passed on to the timing loop filter 64.
[0093] As shown in FIG. 12, timing loop filter 64 comprises a
proportional gain arm 64a and an integral gain arm 64b. The gains,
Kp and Ki are controlled by a gear-shifting algorithm to allow the
loop to lock quickly and then rapidly to tighten to a narrow
bandwidth to minimise the impact of noise on the timing recovery
algorithm. The gear shifting is controlled by a counter (e.g., see,
FIG. 7) that counts the number of timing error estimates supplied
to the loop and operates, in one example, as follows:
TABLE-US-00004 TABLE 3 Timing Estimation Loop Gear Shifting Table
Number of samples K.sub.p K.sub.i 0-5 0 0 5-28 31/(2.sup.-7) 0
28-80 31/(2.sup.-7) 20/(2.sup.-11) 80-120 15/(2.sup.-7)
5/(2.sup.-11) 120 onwards 11/(2.sup.-7) 1/(2.sup.-11)
[0094] Timing loop filter accumulator 64c accumulates the lower
noise estimate of the timing phase error to track the timing phase
error and adjust the sampling phase error between 0 and +15.875
samples. Timing loop filter 64 also develops phase_increment
("Inc") and phase_decrement ("Dec") signals that are used to adjust
the timing in units of 16 samples because they either drop or add a
clock cycle delay in the accumulator 26 (during acquisition) or
despreader 40 (during demodulation). Therefore, if an adjustment of
-2.5 samples is required, then phase_inc is used to introduce an
offset of -16 samples, while the phase error driving resampler 40
will apply a correction of +13.5 samples, giving the overall
required phase adjustment of -2.5 samples. Similarly, for example,
an adjustment of +19.125 samples may be achieved by applying a +16
sample adjustment with the phase_dec signal, followed by an
additional +3.125 sample correction in the resampler 54. By way of
illustration, a suitable embodiment of resample 54 is illustrated
in FIG. 13.
[0095] The carrier recovery loop needs to lock quickly in order to
successfully receive the signal, whereas the timing recovery loop
can take longer. As a result, if the carrier recovery loop fails to
lock soon after the preamble is found, then the preamble will be
rejected, allowing the carrier recovery loop another chance to
lock. The timing recovery loop, however, does not get a second
chance, so, to improve the chances of lock, the timing recovery
loop can be seeded with an estimate based on the carrier loop
integrator. Preferably, seeding is enabled via the use of a control
signal: if the state of this signal is, e.g., logic_0, then timing
seeding does not take place and the gearing table (see, below) must
be set up to allow for this (initially wide bandwidth to allow
acquisition, then narrowing as the lock improves); but, if the
state of this signal is, e.g., logic_1, then timing seeding is
enabled and the loop is assumed close to lock from the outset, and
a more aggressive gearing table can be used. In our preferred
embodiment, seeding is enabled by default.
[0096] We have determined that the value of the loop integrator in
the carrier recovery loop can be used to seed the loop integrator
in the timing recovery loop, thereby giving the timing recovery
loop a jump start and enhancing the chances of it locking. The
formula we prefer to use to compute the seed value is:
I.sub.tim=(Scale)(I.sub.car)
[0097] where: [0098] I.sub.tim=timing loop integrator; [0099]
I.sub.car=carrier loop integrator;
[0099] Scale = F s 2 F c ##EQU00004## [0100] F.sub.s=sampling rate;
and [0101] F.sub.c=carrier frequency. Since F.sub.s and F.sub.c are
related this is simpler in practice:
TABLE-US-00005 [0101] F.sub.c (MHz) Channel Scale Factor 3494.4 1
1/7 3993.6 2, 4 1/8 4492.8 3 1/9 6489.6 5, 7 1/13
As shown in FIG. 14, we prefer to allow the carrier recovery loop
to settle on a good quality estimate before seeding the timing
recovery loop; we define the delay in terms of the carrier recovery
loop gearing counter and prefer to make this threshold value
programmable. Once this gearing counter threshold value is reached,
the value held in the carrier recover loop integrator is scaled
according to the table above (depending on the channel setting) to
produce a timing seed value with the precision S[-1:-15]. Rounding
does not need to be applied to this computation, just truncation,
because the computation is a one-time event in the receiver 10' and
does not take part in a recursive loop--therefore the bias
introduced by not rounding will not accumulate to cause significant
inaccuracy.
[0102] In our preferred embodiment, we implement a register-based
field programmable gear shifting mechanism. Ten gears may be
configured; one is reserved for demodulation mode, allowing nine
acquisition gears. Each gear is assigned: a count at which it is
activated; a K.sub.p value; and a K.sub.i value. Writing a value of
logic_0 as the count for a gear other than the first gear
terminates the gear shifting table; whilst still switching to the
demodulation gear when the acquisition phase is over. Note that two
sets of demodulation coefficients must be specified, one for the
110 Kbps data rate case, and one for the 850K and 6.81 Mbps cases.
The default values for each of the available programmable registers
are given in the following table:
TABLE-US-00006 TABLE 4 Timing Estimator Default Programmable Gear
Shifting Register Values Default Register Value Count [9:0] K.sub.i
[14:10] K.sub.p [19:15] TR0 0XF8000 0 0 31 TR1 0x8141E 30 5 16 TR2
0X58428 40 1 11 TR3 0X00000 0 0 0 TR3 0X00000 0 0 0 TR5 0X00000 0 0
0 TR6 0X00000 0 0 0 TR7 0X00000 0 0 0 TR8 0X00000 0 0 0 TR9 0X5A161
N/A 1/8(110 Kbps) 11 where: Value comprises a 20-bit variable
expressed in hexadecimal format; Count comprises bits [9:0] of the
Value; K.sub.i comprises bits [14:10] of the Value; and K.sub.p
comprises bits [19:15] of the Value.
The K factors are coded as follows:
TABLE-US-00007 TABLE 5 Gear Shifting Register Value Decode Minimum
Maximum Decode Count 1 1023 Sample count on which to apply gearing
values K.sub.i 0x00 0x1F 0x00 = 00 0x01 = 1 .times. 2.sup.-7 0x1F =
31 .times. 2.sup.-7 K.sub.p 0x00 0x1F 0x00 = 00 0x01 = 1 .times.
2.sup.-7 0x1F = 31 .times. 2.sup.-7
Computing Angle of Incidence:
[0103] In a practical coherent receiver, it is necessary to track
the carrier of the transmitter. For example, in the system of FIG.
16, both receivers 70 will be doing this, but because of different
noise input mixed with the received signal, they will not
necessarily do it identically. In, for example, the clock tracking
loop shown in FIG. 3, the correlator output is accumulated to
identify the channel impulse response, but before it is accumulated
it is rotated by a carrier correction. Because this rotation is
likely to be different in each of the receivers 70, it must be
undone in order to calculate the phase difference between the two
carriers. For example, with reference to FIG. 3, during an angle of
incidence calculation mode, the rotator 52 can be rendered
inoperative (or, alternatively, LUT 50 can be configured to output
a fixed rotation of 0.quadrature.); otherwise, the logic 46 works
as described above. The phase difference between the two carriers
is therefore the difference between the angles of the first paths
in the accumulators minus the individual phase corrections that
have been applied at the time the accumulation of the channel
impulse response stops and is measured. FIG. 21 shows the
calculated angle of arrival when repeated, using this method, on
100 separate packets. In the test, the actual angle of arrival was
-5.quadrature. and the antennas were separated by one wavelength.
The carrier frequency used in this test was 4 GHz, and the standard
deviation of the error was 2.1.quadrature..
[0104] We propose two ways to solve the ambiguity in solutions that
occurs at an antenna separation of more than 1/2 a wavelength.
First, we measure the time of arrival of the packet at each
antenna. The angle of incidence that is most consistent with the
measured time of arrival differences is the one chosen. Take the
example shown in FIG. 18. If a is measured as -125.degree. then
there are two possible values for .theta. either -20.degree. or
+40.degree.. If the signal arrived at antenna A first, then
-20.degree. is the correct but if the signal arrives at antenna B
first, then +40.degree. is correct. Second, we resolve the
ambiguity is by sending two packets but at different carrier
frequencies. FIG. 20 shows an example of the relationship between
.alpha. and .theta. for two different carrier frequencies. Because
the curves are different, only one of the possible solutions occurs
at both carrier frequencies. For example, if .theta., the angle of
incidence was -50.degree. then a at 4 GHz would be measured as
+90.degree. which could correspond to an a of either -50.degree. or
of +15.degree.. At 6.5 GHz would be measured at -90.degree. which
could correspond to an a of either -50.degree., -10.degree. or
+24.degree.. Since -50.degree. is the only solution in common, it
must be the correct one. Of course in practice, noise in the system
means that the common solution will not be exactly the same so it
will be necessary to choose the solution which has the smallest
difference in the two sets of possible solutions.
[0105] Even if the two receivers 70a and 70b are fed from the same
clock, it may happen that the delay of this clock to one receiver
is different to the delay to the other receiver. In this case there
will be a fixed phase difference between the carriers. However,
this phase difference can be calibrated, e.g., by measuring a at a
known angle of arrival and subtracted from a, before applying the
formula of Eq. 6.
[0106] Rather than supplying the two different PLLs, 76a and 76b,
with the common crystal 72, there are other ways to synchronize the
receivers 70, e.g., the two receivers 70a and 70b could be
synchronized by supplying both with a clock from a single PLL,
e.g., the PLL 76a.
[0107] Although we have described our invention in the context of
particular embodiments, one of ordinary skill in this art will
readily realize that many modifications may be made in such
embodiments to adapt either to specific implementations. By way of
example, it will take but little effort to adapt our invention for
use with a different ADC scheme when it can be anticipated that the
target application will not be subject to significant levels of
in-channel CW interference. Further, the several elements described
above may be implemented using any of the various known
semiconductor manufacturing methodologies, and, in general, be
adapted so as to be operable under either hardware or software
control or some combination thereof, as is known in this art.
Alternatively, the several methods of our invention as disclosed
herein in the context of special purpose receiver apparatus may be
embodied in computer readable code on a suitable computer readable
medium such that when a general or special purpose computer
processor executes the computer readable code, the processor
executes the respective method.
[0108] Thus it is apparent that we have provided an improved method
and apparatus for use in the receiver of a UWB communication system
to determine angle of incidence. In particular, we submit that such
a method and apparatus should provide performance generally
comparable to the best prior art techniques but more efficiently
than known implementations of such prior art techniques.
* * * * *