Note: Descriptions are shown in the official language in which they were submitted.
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COMPENSATING FOR OSCILLATOR DRIFT IN WIRELESS MESH NETWORKS
CROSS-REFERENCE TO RELATED APPLICATIONS
paw This application claims benefit of United States patent application serial
number 15/655,031, filed July 20, 2017, and claims benefit of United States
patent
application serial number 15/655,781, filed July 20, 2017. Each of these
applications
is hereby incorporated herein by reference.
BACKGROUND OF THE INVENTION
Field of the Invention
[0002] Embodiments of the present invention relate generally to wireless
network
communications and, more specifically, to compensating for oscillator drift in
wireless
mesh networks.
Description of the Related Art
[0003] A conventional wireless mesh network includes a plurality of nodes
configured
to communicate with one another. In certain types of heterogeneous wireless
mesh
networks, both continuously-powered nodes (CPDs) and battery-powered nodes
(BPDs) communicate and interact with one another within the mesh network.
Typically, CPDs are coupled to a power grid and have continuous access to
power
(except during power outages). BPDs, on the other hand, are battery-powered
and
therefore have only a finite supply of power.
[0004] Due to these power constraints, BPDs normally remain in a powered down
state, and then only power on at specifically timed communication intervals to
perform
data communications with one another and with CPDs. In order to power on at
similarly timed intervals, BPDs include low-frequency oscillators according to
which
.. current time is maintained. Such oscillators are usually crystals having a
particular
resonant frequency. That resonant frequency may change over time based on the
ambient temperature and/or various temperature fluctuations, a phenomenon that
is
known in the art as "temperature induced oscillator drift" or simply "clock
drift." Clock
drift may affect the timing with which BPDs power on to perform data
communications. If the clock drift of a given BPD becomes too large, then the
BPD
may power on too early or too late and, consequently, miss the predetermined
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communication interval. In such situations, the BPD can become disconnected
from
the wireless mesh network.
[0005] One solution to the above problem is to include a temperature-
compensated
crystal oscillator (TCXO) within each BPD. However, such a solution is not
practicable with battery-powered devices, like BPDs, because typical TCX0s
consume relatively large amounts of power..
[0006] As the foregoing illustrates, what is needed in the art are a more
effective
techniques for compensating for oscillator drift in a battery powered device.
SUMMARY OF THE INVENTION
[0007] One embodiment of the present invention sets forth a computer-
implemented
method for compensating for oscillator drift, including acquiring, at a first
node
residing in a wireless mesh network, a first calibration packet from a second
node
residing in the wireless mesh network, acquiring, at the first node, a second
calibration packet from the second node after a first period of time has
elapsed,
comparing the first period of time to a second period of time specified in the
first
calibration packet to determine a first drift value associated with an
oscillator included
in the first node, and adjusting the oscillator to compensate for the first
drift value.
[0oos] At least one advantage of the techniques described herein is that
battery
powered nodes can compensate for temperature induced oscillator drift based on
the
highly accurate oscillators included in continuously powered devices, without
implementing power consuming TCXO techniques.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] So that the manner in which the above recited features of the present
invention
can be understood in detail, a more particular description of the invention,
briefly
summarized above, may be had by reference to embodiments, some of which are
illustrated in the appended drawings. It is to be noted, however, that the
appended
drawings illustrate only typical embodiments of this invention and are
therefore not to
be considered limiting of its scope, for the invention may admit to other
equally
effective embodiments.
[0010] Figure 1 illustrates a network system configured to implement one or
more
aspects of the present invention;
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[0on] Figure 2 illustrates a network interface configured to transmit and
receive data
within the wireless mesh network of Figure 1, according to various embodiments
of
the present invention;
[0012] Figures 3A-3B illustrate how calibration packets are propagated between
layers of nodes within the wireless mesh network of Figure 1, according to
various
embodiments of the present invention;
[0013] Figure 4 illustrates how a node determines oscillator drift based on
calibration
packets received from another node, according to various embodiments of the
present invention;
[0014] Figure 5 is a graph of low frequency oscillator drift as a function of
temperature, according to various embodiments of the present invention;
[0015] Figures 6A-6B illustrate a flow diagram of method steps for
compensating for
oscillator drift in a low frequency oscillator, according to various
embodiments of the
present invention;
[0016] Figure 7 illustrates high frequency receiver circuitry included in the
network
interface of Figure 2, according to various embodiments of the present
invention; and
[0017] Figure 8 is a flow diagram of method steps for compensating for
oscillator drift
in a high frequency oscillator, according to various embodiments of the
present
invention.
DETAILED DESCRIPTION
[owls] In the following description, numerous specific details are set forth
to provide a
more thorough understanding of the present invention. However, it will be
apparent to
one of skill in the art that the present invention may be practiced without
one or more
of these specific details. In other instances, well-known features have not
been
described in order to avoid obscuring the present invention.
System Overview
[0019] Figure 1 illustrates a network system configured to implement one or
more
aspects of the present invention. As shown, the network system 100 includes a
wireless mesh network 102, which may include a source node 110, intermediate
nodes 130 and destination node 112. The source node 110 is able to communicate
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with certain intermediate nodes 130 via communication links 132. The
intermediate
nodes 130 communicate among themselves via communication links 134. The
intermediate nodes 130 communicate with the destination node 112 via
communication links 136. The network system 100 may also include an access
point
150, a network 152, and a server 154.
[0020] A discovery protocol may be implemented to determine node adjacency to
one
or more adjacent nodes. For example, intermediate node 130-2 may execute the
discovery protocol to determine that nodes 110, 130-1, 130-3, and 130-5 are
adjacent
to node 130-2. Furthermore, this node adjacency indicates that communication
links
132-2, 134-2, 134-4 and 134-3 may be established between the nodes 110, 130-1,
130-3, and 130-5, respectively. Any technically feasible discovery protocol
may be
implemented without departing from the scope and spirit of embodiments of the
present invention.
[0021] The discovery protocol may also be implemented to determine the hopping
sequences of adjacent nodes, i.e. the sequence of channels across which nodes
periodically receive payload data. As is known in the art, a "channel" may
correspond
to a particular range of frequencies. Once adjacency is established between
the
source node 110 and at least one intermediate node 130, the source node 110
may
generate payload data for delivery to the destination node 112, assuming a
path is
available. The payload data may comprise an Internet protocol (IP) packet, or
any
other technically feasible unit of data. Similarly, any technically feasible
addressing
and forwarding techniques may be implemented to facilitate delivery of the
payload
data from the source node 110 to the destination node 112. For example, the
payload data may include a header field configured to include a destination
address,
such as an IP address or media access control (MAC) address.
[0022] Each intermediate node 130 may be configured to forward the payload
data
based on the destination address. Alternatively, the payload data may include
a
header field configured to include at least one switch label to define a
predetermined
path from the source node 110 to the destination node 112. A forwarding
database
may be maintained by each intermediate node 130 that indicates which
communication link 132, 134, 136 should be used and in what priority to
transmit the
payload data for delivery to the destination node 112. The forwarding database
may
represent multiple paths to the destination address, and each of the multiple
paths
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may include one or more cost values. Any technically feasible type of cost
value may
characterize a link or a path within the network system 100. In one
embodiment,
each node within the wireless mesh network 102 implements substantially
identical
functionality and each node may act as a source node, destination node or
intermediate node.
[0023] In network system 100, the access point 150 is configured to
communicate
with at least one node within the wireless mesh network 102, such as
intermediate
node 130-4. Communication may include transmission of payload data, timing
data,
or any other technically relevant data between the access point 150 and the at
least
one node within the wireless mesh network 102. For example, communications
link
140 may be established between the access point 150 and intermediate node 130-
4
to facilitate transmission of payload data between wireless mesh network 102
and
network 152. The network 152 is coupled to the server 154 via communications
link
142. The access point 150 is coupled to the network 152, which may comprise
any
wired, optical, wireless, or hybrid network configured to transmit payload
data
between the access point 150 and the server 154.
[0024] The server 154 may represent a destination for payload data originating
within
the wireless mesh network 102 and a source of payload data destined for one or
more nodes within the wireless mesh network 102. The server 154 is generally a
computing device, including a processor and memory, that executes an
application for
interacting with nodes within the wireless mesh network 102. For example,
nodes
within the wireless mesh network 102 may perform measurements to generate
measurement data, such as power consumption data. The server 154 may execute
an application to collect the measurement data and report the measurement
data. In
one embodiment, the server 154 queries nodes within the wireless mesh network
102
for certain data. Each queried node replies with requested data, such as
consumption data, system status and health data, and so forth. In an
alternative
embodiment, each node within the wireless mesh network 102 autonomously
reports
certain data, which is collected by the server 154 as the data becomes
available via
autonomous reporting.
[0025] The techniques described herein are sufficiently flexible to be
utilized within
any technically feasible network environment including, without limitation, a
wide-area
network (WAN) or a local-area network (LAN). Moreover, multiple network types
may
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exist within a given network system 100. For example, communications between
two
nodes 130 or between a node 130 and the corresponding access point 150 may
occur via a radio-frequency local-area network (RF LAN), while communications
between access points 150 and the network may be via a WAN such as a general
packet radio service (GPRS). As mentioned above, each node within wireless
mesh
network 102 includes a network interface that enables the node to communicate
wirelessly with other nodes. Each node 130 may implement any and all
embodiments
of the invention by operation of the network interface. An exemplary network
interface is described below in conjunction with Figure 2.
[0026] Figure 2 illustrates a network interface configured to transmit and
receive data
within the mesh network of Figure 1, according to various embodiments of the
present
invention. Each node 110, 112, 130 within the wireless mesh network 102 of
Figure 1
includes at least one instance of the network interface 200. The network
interface
200 may include, without limitation, a microprocessor unit (MPU) 210, a memory
212,
a digital signal processor (DSP) 214, a memory 216, digital to analog
converters
(DACs) 220, 221, analog to digital converters (ADCs) 222, 223, analog mixers
224,
225, 226, 227, a phase shifter 232, a low frequency (LF) oscillator 230, a
power
amplifier (PA) 242, a low noise amplifier (LNA) 240, an antenna switch 244, an
antenna 246, a temperature sensor 250, a high frequency (HF) oscillator 260,
and a
phase locked loop (PLL) 262.
[0027] Memories 212 and 216 are coupled to MPU 210 and DSP 214, respectively,
for local program and data storage. Memory 212 and/or memory 216 may be used
to
store various types of data, including a forwarding database and/or routing
tables that
include primary and secondary path information, path cost values, and so
forth.
Memories 212 and 216 may also store executable program code.
[0028] In one embodiment, MPU 210 implements procedures for processing IP
packets transmitted or received as payload data by the network interface 200.
The
procedures for processing the IP packets may include, without limitation,
wireless
routing, encryption, authentication, protocol translation, and routing between
and
among different wireless and wired network ports. In one embodiment, MPU 210
implements the techniques performed by the node when MPU 210 executes a
firmware program stored in memory within network interface 200.
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[0029] The MPU 214 is coupled to DAC 220 and DAC 221. Each DAC 220, 221 is
configured to convert a stream of outbound digital values into a corresponding
analog
signal. The outbound digital values are computed by the signal processing
procedures for modulating one or more channels. DSP 214 is also coupled to ADC
.. 222 and ADC 223. Each ADC 222, 223 is configured to sample and quantize an
analog signal to generate a stream of inbound digital values. The inbound
digital
values are processed by the signal processing procedures to demodulate and
extract
payload data from the inbound digital values.
[0030] In one embodiment, MPU 210 and/or DSP 214 are configured to buffer
incoming data within memory 212 and/or memory 216. The incoming data may be
buffered in any technically feasible format, including, for example, raw soft
bits from
individual channels, demodulated bits, raw ADC samples, and so forth. MPU 210
and/or DSP 214 may buffer within memory 212 and/or memory 216 any portion of
data received across the set of channels from which antenna 246 receives data,
including all such data. MPU 210 and/or DSP 214 may then perform various
operations with the buffered data, including demodulation operations, decoding
operations, and so forth. MPU 210 and DSP 214 perform various operations based
on oscillation signals received from LF oscillator 230 and HF oscillator 260.
[0031] LF oscillator 230 may be coupled to a counter circuit (not shown)
configured to
maintain an estimate of the current time. MPU 210 is configured to update the
current
time estimate and other associated data including, for example, an uncertainty
estimate associated with the current time estimate. MPU 210 times
transmissions
based on LF oscillator 230. Memory 212 includes a calibration application 252
that,
when executed by MPU 210, performs a calibration procedure to compensate for
temperature induced drift of LF oscillator 230. In doing so, calibration
application 252
determines a relationship between drift of oscillator 230 and temperature
across a
range of temperatures measured by temperature sensor 250. Calibration
application
252 generates calibration data 254 to represent drift as a function of
temperature for
LF oscillator 230. Later, based on temperature measurements from temperature
sensor 250, calibration application 252 estimates current drift based on
current
temperature and calibration data 254. Calibration application 252 then
corrects for
this drift. This procedure is described in greater detail below in conjunction
with
Figures 3A-6.
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[0032] HF oscillator 260 is coupled to PLL 262 and configured to drive the
frequency
of the various receiver circuitry shown within network interface 200. That
receiver
circuitry is configured to sample incoming data transmissions. Calibration
application
252 is configured to perform a similar compensation process as that described
above
to compensate for temperature induced drift of HF oscillator 260. In doing so,
calibration application 252 interacts with temperature sensor 250 and certain
other
circuitry, as described in greater detail below in conjunction with Figures 7-
8.
[0033] Persons having ordinary skill in the art will recognize that network
interface 200
represents just one possible network interface that may be implemented within
wireless mesh network 102 shown in Figure 1, and that any other technically
feasible
device for transmitting and receiving data may be incorporated within any of
the
nodes within wireless mesh network 102.
[0034] Referring generally to Figures 1-2, under various circumstances, nodes
130
may transmit messages to server 154 that reflect various operating conditions
associated with those nodes 130. The operating conditions associated with a
given
node 130 could include, for example, a set of environmental conditions and/or
events
detected by the node 130, status information associated with the portion of
the
wireless mesh network 202 to which the node 130 is coupled, and status
information
associated with a utility grid the node 130 is configured to monitor. In
addition, nodes
130 may transmit messages to each other according to a transmission schedule.
To
follow the transmission schedule, each node 130 maintains a current time
estimate,
as described, that can be updated based on time beacons received from
neighboring
nodes. In addition, each node may calibrate that clock based on calibration
packets
received from upstream nodes or other neighboring nodes, as described in
greater
detail below in conjunction with Figures 3A-3B.
Compensating for Drift of a Low Frequency Oscillator
[0035] Figures 3A-3B illustrate how calibration packets are propagated between
layers of nodes within the wireless mesh network of Figure 1, according to
various
embodiments of the present invention.
[0036] As shown in Figure 3A, wireless mesh network 102 of Figure 3A is
divided into
a continuously-powered device (CPD) mesh 300 and a battery-powered device
(BPD)
mesh 310. CPD mesh 300 includes CPDs 302(0) and 302(1) as well as a network
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time protocol (NTP) server 304. CPDs 300 may include one or more nodes 130
and/or APs 150 of Figure 1. BPD mesh 310 includes BPDs 312(0) through 312(4).
BPDs 312 may include one or more nodes 130 of Figure 1. As a general matter,
data
that is transmitted from CPD mesh 300 to BPD mesh 310 is referred to herein as
"outbound" data and may be described as traveling in an "outbound" or
"downlink"
direction. Similarly, data that is transmitted from BPD mesh 310 towards CPD
mesh
300 is referred to herein as "inbound" data and may be described as traveling
in an
"inbound" or "uplink" direction.
[0037] BPDs 312 of BPD mesh 310 are included in different "hop layers" based
on
hopping distance to CPD mesh 300. BPDs 312(0) and 312(1) are included in hop
layer one (H Li) because those nodes are one hop away from CPD mesh 300. BPDs
312(2) through 312(4) are included in hop layer two (HL2) because those nodes
are
two hops away from CPD mesh 300. Wireless mesh network 102 is configured to
propagate data packets across CPD mesh 300 and BPD mesh 310 in a coordinated
manner based on hop layer. Those data packets may include time beacons,
calibration packets, network packets, and so forth.
[0038] Because BPDs 312 operate with a limited power supply, a given BPD 312
may
power down for long periods of time and then power on briefly to perform data
communications with other BPDs 312. In order to coordinate the powering on of
many BPDs 312, each BPD 312 measures and compensates for temperature induced
oscillator drift using a technique described in greater detail below in
conjunction with
Figures 3B-6. By maintaining an accurate estimate of time, many BPDs 312 can
power on at approximately the same time, perform data communications, and then
power down, thereby minimizing the amount of time each BPD 312 is powered on.
Each BPD 312 is configured to measure temperature induced oscillator drift
based on
calibration packets received from upstream nodes, as described in greater
detail
below in conjunction with Figure 3B.
[0039] As shown in Figure 3B, BPD 312(0) receives calibration packets 320(0)
from
CPD 302(0), and BPD 312(3) receives calibration packets 320(1) from BPD
312(0). A
given BPD 312 determines temperature induced oscillator drift based on the
difference between receipt times of calibration packets 320. This technique is
described in greater detail below in conjunction with Figure 4. Each BPD 312
may
obtain calibration packets from an upstream node. With the exception of BPDs
312 in
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the outermost hop layer, each BPD 312 may also transmit calibration packets to
downstream nodes. Accordingly, the technique described herein can be
implemented
to determine and compensate for oscillator drift across the entire wireless
mesh
network 102.
[0040] Figure 4 illustrates how a node determines oscillator drift based on
calibration
packets received from another node, according to various embodiments of the
present invention. As shown, CPD 302 transmits calibration packets 400 and 402
to
BPD 312, with transmissions times determined based oscillator 404. CPD 302
transmits calibration packet 400 first, then transmits calibration packet 402
after a
short time delta measured by oscillator 404. CPD 302 includes within
calibration
packet 400 an indication of this time delta, referred to herein as the
transmit time delta
and shown as Att. BPD 312 receives calibration packets 400 and 402 and then
measures, via an oscillator 406, the time delta between receipt of calibration
400 and
receipt of calibration packet 402. This time delta is referred to herein as
the receive
time delta or Atr.
[0041] BPD 312 determines the difference between the transmit time delta Att
and the
receive time delta Atr. Under ideal operating conditions, BPD 312 may
determine that
the difference between Att and Atr is zero, indicating that oscillator 404
within CPD
302 is substantially synchronized with oscillator 406 within BPD 312. However,
under
typical operating conditions, oscillators 404 and 406 within CPD 302 and BPD
312,
respectively, may not be synchronized for various reasons, including
temperature-
induced oscillator drift. In some cases, CPD 302 may implement oscillator 404
as a
temperature compensated crystal oscillator (TCXO) that is capable of
maintaining
reasonably accurate time despite temperature fluctuations. BPD 312, on the
other
hand, generally cannot implement a TCXO due to power limitations. Accordingly,
oscillator 406 within BPD 312 may be subject to greater temperature induced
oscillator drift compared to oscillator 404 within CPD 302.
[0042] BPD 312 is configured to determine this relative oscillator drift based
on the
difference between Att and Atr. For example, if Att is greater than Atr, then
oscillator
406 oscillates at a slightly lower frequency compared to oscillator 404. In
this
example, BPD 312 does not count as many clock edges of oscillator 406 when
measuring Atr compared to the number of clock edges counted by oscillator 404
when measuring Att. Conversely, if Att is less than Atr, then oscillator 406
oscillates
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at a slightly higher frequency compared to oscillator 404. Specifically, BPD
312
counts more clock edges when measuring Atr than the number of clock edges
counted by oscillator 404 when measuring Att.
[0043] BPD 312 determines the drift of oscillator 406 based on computing Att -
Atr and
then performs two operations. First, BPD 312 compensates for this drift so
that future
data communications occur based on a more accurate time estimate. In doing so,
BPD 312 could, for example, correct a number of counted clock edges to account
for
the determined drift. Second, BPD 312 records the determined drift along with
a
measurement of the current temperature in order to generate and/or expand a
mapping of temperature values to drift values. This particular operation is
discussed
in greater detail below in conjunction with Figure 5.
[0044] By determining and compensating for drift in the manner described, BPD
312
may maintain a more accurate estimate of the current time and therefore be
capable
of powering on at precisely timed communication intervals. With precise
timing,
these communication intervals may be very short, thereby conserving power. In
addition, BPD 312 may also then perform the calibration procedure described
above
with BPDs in downstream hop layers, as described above in conjunction with
Figure
3B. In general, any downstream BPD 312 may request calibration packets from
any
upstream BPD 312 (or CPD 302) and then perform the two-packet calibration
procedure to determine and compensate for clock drift.
[0045] An advantage of this approach is that all BPDs 312 within wireless mesh
network 102 may compensate for drift based on the highly accurate oscillators
within
CPDs 302. Another advantage of this approach is that the determined drift
value may
represent not only the drift of the oscillator, but also any temperature
induced
frequency variations caused by other components, including capacitors, and
other
temperature sensitive components.
[0046] Figure 5 is a graph of low frequency oscillator drift as a function of
temperature, according to various embodiments of the present invention. As
mentioned, when a given BPD 312 determines the current drift via the two
packet
calibration procedure, the BPD 312 records the current drift along with the
current
temperature. By recording many such pairs of current drift and current
temperature,
BPD 312 generates a dataset similar to that shown in exemplary form in Figure
5.
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[0047] As shown, graph 500 includes a temperature axis 510, a drift axis 520,
and a
plot 530 of drift as a function of temperature. Plot 530 is generally
constructed as a
collection of discrete coordinate pairs of the form (Ti, Di) where Ti is a
temperature
and Di is a drift value measured at that temperature via the two packet
calibration
technique. Plot 530 includes discrete coordinate pairs for temperatures TO,
Ti, T2,
T3, and T4.
[0048] BPD 312 is configured to interpolate between coordinate pairs in order
to
estimate the drift at a specific temperature that has not be recorded by BPD
312. For
example, to estimate the drift value for a temperature T5, BPD 312 performs a
linear
interpolation between the coordinate pairs associated with temperatures T3 and
T4,
and then based on this linear interpolation maps the temperature T5 to an
estimated
drift value D5. Based on this drift value D5, BPD 312 may then perform
oscillator
compensation.
[0049] BPD 312 is configured to request calibration packets from an upstream
node at
any given time and based on any set of conditions. In one embodiment, BPD 312
request calibration packets periodically. In practice, however, BPD 312
requests
calibration packets from an upstream node upon detecting that insufficient
data points
exist to generate a reliably estimate of drift as a function of the current
temperature.
[0050] For example, suppose BPD 312 only stores one coordinate pair associated
with temperature TO. If the current temperature exceeds beyond a threshold 512
from
temperature TO, then BPD 312 may determine that insufficient coordinate pairs
exist
to determine the current drift at, say, temperature Ti. In this situation, BPD
312
would request calibration packets from an upstream node and then determine the
current oscillator drift at temperature Ti. BPD 312 would then expand the
mapping of
temperature to drift values based on this newly acquired drift measurement.
[0051] In short, whenever the current temperature is greater than or less than
the
temperature associated with a previously determined coordinate pair by a given
threshold 512, BPD 312 may request a set of calibration packets and expand the
dataset. In this manner, BPD 312 continuously improves the mapping of
temperature
to drift values based on changing environmental conditions.
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[0052] Figures 6A-6B illustrate a flow diagram of method steps for
compensating for
low frequency oscillator drift, according to various embodiments of the
present
invention. Although the method steps are described in conjunction with the
systems
of Figures 1-5, persons skilled in the art will understand that any system
configured to
perform the method steps, in any order, is within the scope of the present
invention.
[0053] As shown in Figure 6A, a method 600(A) begins at step 602, where a
first node
within wireless mesh network 102 generates a first temperature measurement at
time
tO. The first node may be a BPD 312 such as those shown in Figures 3A-4. The
first
node may implement temperature sensor 250 to generate the first temperature
measurement. At step 604, the first node determines that the first temperature
measurement is outside of a tolerance window associated with a first reference
point.
The tolerance window could be, for example, threshold 512 shown in Figure 5.
At
step 606, the first node requests calibration packets from a second node. The
second
node may be an adjacent BPD 312 in the same or different hop layer as the
first
node, or the second node could be a CPD 302 within an adjacent hop layer.
[0054] At step 608, the first node receives a first calibration packet from
the second
node at time t1. At step 610, the first node parses the first calibration
packet to
extract Att. At step 612, the first node receives a second calibration packet
from the
second node at time t2 and computes Atr = It2 - 01. At step 614, the first
node
determines a first drift value associated with an LF oscillator within the
first node by
computing the difference between Att and Atr. To compensate for this drift, at
step
616, the first node adjusts that LF oscillator or associated circuitry. The
first node
may also perform additional steps to expand a mapping between temperature and
drift, as discussed below in conjunction with Figure 6B.
[0055] As shown in Figure 6B, the method 600(A) proceeds to the method 600(B).
At
step 618, the first node generates a second reference point based on the first
temperature measurement and based on the first drift value. At step 620, the
first
node generates a second temperature measurement at time t3. At step 622, the
first
node determines that the second temperature measurement falls between the
first
reference point and the second reference point. At step 624, the first node
linearly
interpolates between the first reference point and the second reference point
to
generate an estimated reference point. At step 626, the first node extracts a
second
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drift value from estimated reference point. At 628, the first node adjusts the
LF
oscillator to compensate for this second drift value.
[0056] Persons skilled in the art will recognize that the approach discussed
above
may be implemented, in whole or in part, to perform compensation for other
types of
oscillators that serve different purposes within a node 130. In particular,
the
technique for determining the relationship between temperature and drift may
also be
applied to perform ongoing compensation of high frequency oscillators, as
described
in greater detail below in conjunction with Figures 7-8.
Compensating for Drift of a High Frequency Oscillator
[0057] Figure 7 illustrates high frequency receiver circuitry included in the
network
interface of Figure 2, according to various embodiments of the present
invention. As
shown, receiver circuitry 700 includes antenna 246, LNA 240, analog mixer 706,
intermediate frequency (IF) filter 708, frequency correlator 710, clock (clk)
/ data
recovery 712, automatic frequency controller (AFC) 716, drift compensator 720,
HF
oscillator 260, and PLL 262.
[0058] Some elements shown in Figure 7 are also shown in Figure 2, including
LNA
240, antenna 246, HF oscillator 260, and PLL 262. In addition, certain
elements in
Figure 7 may be included within DSP 214 of Figure 2, such as IF filter 708,
frequency
correlator 710, clk / data recovery 712, AFC 716, and in some embodiments
drift
compensator 720. Drift compensator 720 may also be implemented as a software
module within calibration application 252. Drift compensator 720 is configured
to
interoperate with AFC 716 in order to compensate for temperature induced
oscillator
drift of HF oscillator 260.
[0059] In operation, antenna 246 receives radio signals associated with
incoming
transmissions sent from neighboring or upstream nodes. Those radio signals are
typically encoded via frequency-shift keying (FSK), as is known in the art.
LNA 240
amplifies incoming signals and then transmits the amplified signals to analog
mixer
706. Analog mixer 706 may include analog mixers 226 and 227 configured to
generate phase shifted versions of incoming signals for performing quadrature
oriented operations. For simplicity, however, only one analog mixer 706 is
shown.
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[0060] Analog mixer 706 combines the amplified signal received from LNA 240
with a
high frequency signal received from PLL 262 and then transmits the combined
signal
to IF filter 708. IF filter 708 processes the combined signal to determine an
intermediate signal frequency associated with the received transmission. This
intermediate signal frequency may, on average, be similar or equivalent to a
frequency associated with an HF oscillator included within the upstream node.
[0061] Frequency correlator 710 correlates the intermediate signal frequency
to FSK
values ft+ and ft-. As known to those familiar with FSK, transmission on
either ft+
or ft- conveys either a binary "0" or a binary "1." Frequency correlator 710
determines whether the intermediate signal frequency correlates more strongly
to ft+
or ft-, and then interoperates with clk / data recovery 712 to decode either a
"0" or a
"1." Clk / data recovery 712 may accumulate many such decoded values to
reconstruct a data packet transmitted from the upstream node.
[0062] Frequency correlator 710 is also configured to determine the degree to
which
measured values of ft+ and ft- diverge from expected values for these
frequencies.
The expected values may be derived from configuration settings or statistical
measurements of ft+ and ft-. Divergence may occur due to temperature induced
drift of HF oscillator 260, referred to herein as fdrift. For example, at
higher
temperatures, HF oscillator 260 may oscillate more rapidly, causing the
intermediate
.. signal frequency to have an elevated value. In turn, ft+ and ft- may appear
to be
positively biased by an amount fthift. Alternatively, at lower temperatures,
HF oscillator
260 may oscillate more slowly, causing the intermediate signal frequency to
have a
diminished value. In turn, ft+ and ft- may appear to be negatively biased by
fdrift.
[0063] For a limited range of temperature variations, frequency correlator 710
may be
able to detect the value of fthift. As a general matter, so long as fthift is
small enough
that ft+ and ft- remain within the upper and lower frequency bounds of the
current
receive channel, respectively, frequency correlator 710 may be able to
determine fthift.
Frequency correlator 710 transmits the value of fthift to AFC 716, and AFC 716
may
then adjust HF oscillator 260 and/or PLL 262 in order to compensate for that
drift.
Accordingly, for this limited range of temperature variations and associated
range of
fdrift values, AFC 716 maintains relative synchronization between the high
frequency
signal output by PLL 262 and the radio signal received from the upstream node.
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[0064] However, with excessive drift, one or more of fL,+ and to,- may reside
outside
of the upper or lower frequency bounds for the current channel. In this
situation,
frequency correlator 710 is "saturated" and cannot reliably determine the
current drift
fdrift. This may prevent AFC 716 from performing the drift compensation
discussed
above. Consequently, the high frequency signal output by PLL 262 may lose
relative
synchronization with the radio signals received from the upstream node,
potentially
interfering with the decoding of binary values and the reconstruction of
transmitted
packets. This, in turn, may lead to repetitive packet loss.
[0065] However, drift compensator 720 performs specific techniques to address
the
above problems. Drift compensator 720 monitors readback path 718 of AFC 716
over
a timespan and also records temperature measurements gathered via temperature
sensor 250 over the timespan. AFC 716 may output the current drift fthift
measured by
frequency correlator 710 via readback path 718. AFC 716 may also output
compensatory frequency adjustments applied to oscillator 260 and/or PLL 262
via
readback path 718. By monitoring readback path 718, drift compensator 720
generates a mapping between temperature and drift for the limited range of
temperature variations within which AFC 716 may effectively operate. This
mapping
may be similar to calibration data 254 of Figure 2 and/or the dataset
discussed above
in conjunction with Figure 5.
[0066] Based on this dataset, drift compensator 720 establishes saturation
boundaries
beyond which AFC 716 may not effectively operate. These saturation boundaries
may represent constraints on temperature or drift. For example, temperature T5
shown in Figure 5 could represent a saturation boundary that limits the
temperature
range within which AFC 716 can effectively operate. Likewise, drift D5 shown
in
Figure 5 could represent a saturation boundary that limits the drift range
within which
AFC 716 can effectively operate.
[0067] When the current drift fthift exceeds beyond a saturation boundary, or
when the
current temperature exceeds beyond a saturation boundary (indicating that
fthift
exceeds a saturation boundary), drift compensator 720 applies adjustments to
HF
oscillator 260 and/or PLL 262. Drift compensator could, for example, apply
changes
to a capacitor network (not shown) coupled to PLL 262 in order to adjust the
high
frequency output of PLL 262. These adjustments operate to bring the high
frequency
output of PLL 262 back to the range of drift values for which frequency
correlator 710
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and AFC 716 may effectively interoperate for oscillator compensation purposes.
Once drift compensator 720 has returned fdrift to that effective range,
frequency
correlator 710 may again receive values of fA+ and to,- that permit accurate
computation of fdrift, and AFC 716 may again compensate for that drift in the
manner
described.
[0068] In one embodiment, drift compensator 720 may extrapolate drift values
outside
of the effective temperature and/or drift range of AFC 716 based on the
dataset
described above. Accordingly, drift compensator 720 can determine whether the
high
frequency output of PLL 262 should be increased or decreased, despite
potentially
lacking a precise prediction of actual drift. For example, referring to Figure
5, drift
compensator 720 could extrapolate a drift value associated with temperature T2
by
extrapolating plot 530 based on data points associated with temperatures TO
and Ti.
Drift compensator 720 may also update the dataset based on these predicted
values.
In this manner drift compensator 720 may perform a "coarse" frequency
adjustment to
compensate for a potentially imprecise drift value. Then, AFC 716 may perform
a
"fine" frequency adjustment once that current drift is returned to a range
where AFC
716 can effectively operate.
[0069] With the above approach, a node 130 is capable of synchronizing high
frequency oscillation signals to those associated with an upstream node over a
wider
range of temperature values than possible with conventional approaches.
Accordingly, the node 130 may receive incoming data transmissions from the
upstream node with a greater success rate and a lower rate of data loss. The
node
130 may therefore limit the amount of time required to power on and receive
data
transmissions from the upstream node, thereby conserving power.
[0070] Figure 8 is a flow diagram of method steps for compensating for
oscillator drift
in a high frequency oscillator, according to various embodiments of the
present
invention. Although the method steps are described in conjunction with the
systems
of Figures 1-5, persons skilled in the art will understand that any system
configured to
perform the method steps, in any order, is within the scope of the present
invention.
[0071] As shown, a method 800 begins at step 802, where a node 130 receives a
radio signal from an upstream node. The node 130 is BPD 312 residing in BPD
mesh
310. The upstream node may be another BPD 312 residing in an upstream hop
layer
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of BPD mesh 310, or a CPD 302 residing in CPD mesh 300. At step 804, analog
mixer 706 within receiver circuitry 700 of the node 130 combines the received
signal
with a high frequency oscillation signal generated by PLL 262. At step 806, IF
filter
708 within receiver circuitry 700 extracts an intermediate frequency signal
from the
combined signal. At step 808, frequency correlator 710 correlates the
intermediate
signal frequency with expected ft+ and ft- values to generate measurements of
ft+
and ft-. Based on these measurements, frequency correlator 710 may determine a
first relative drift, referred to above as fthift. The first relative drift
value may vary in
accuracy depending on the degree to which ft+ and ft- fall within the
bandwidth
boundaries associated with the current receive channel.
[0072] At step 810, drift compensator 720 determines that AFC 716 cannot fully
compensate for the first relative drift. Drift compensator 720 may evaluate
the first
relative drift based on a historical dataset that indicates drift as a
function of
temperature such as that shown in Figure 5. The dataset may also indicate
saturation
boundaries specifying particular temperature values and/or drift values beyond
which
AFC 716 cannot effectively operate. If the current drift value resides at or
beyond a
saturation boundary, drift compensator 720 triggers additional compensatory
actions
in the manner described below.
[0073] At step 812, drift compensator 720 generates a first temperature
measurement
via interaction with temperature sensor 250. At step 814, drift compensator
720
determines a first drift value based on the first temperature measurement and
based
on the historical dataset mentioned above. Again, the historical dataset
correlates
temperature values to drift measurements. Drift compensator 720 may
extrapolate
the dataset based on the first relative drift measurement and potentially
based on
other recent drift measurements. For example, drift compensator 720 could
establish
an approximate direction and magnitude associated with changes in the current
drift
value, and then estimate the first drift value based on the first temperature
measurement. At step 816, drift compensator 720 adjusts the high frequency
output
associated with HF oscillator 260 and/or PLL 262 to better match the frequency
associated with the received radio signal. This high frequency output may then
better
match the frequency associated with an HF oscillator included in the upstream
node.
[0074] By implementing the method 800, the node 130 may receive incoming data
transmissions in a more robust manner compared to conventional techniques.
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Accordingly, the node 130 may implement more precisely timed communication
windows and therefore conserve power.
[0075] In sum, a battery powered node within a wireless mesh network maintains
a
mapping between temperature and oscillator drift and compensates for
oscillator drift
.. based on this mapping. When the mapping includes insufficient data points
to map
the current temperature to an oscillator drift value, the battery powered node
requests
calibration packets from an adjacent node in the network. The adjacent node
transmits two calibration packets with a transmit time delta and also
indicates this
time delta in the first calibration packet. The battery powered node receives
the two
calibration packets and measures the receive time delta. The battery powered
node
compares the transmit time delta to the receive time delta to determine
oscillator drift
compared to an oscillator in the adjacent node. The battery powered node then
updates the mapping based on the current temperature and determined oscillator
drift.
[0076] At least one advantage of the techniques described herein is that
battery
powered nodes can compensate for temperature induced oscillator drift based on
the
highly accurate oscillators included in continuously powered devices, without
implementing power consuming TCXO techniques. Accordingly, battery powered
nodes can coordinate specific time intervals to power on and perform data
.. communications with high precision, thereby conserving power.
[0077] The descriptions of the various embodiments have been presented for
purposes of illustration, but are not intended to be exhaustive or limited to
the
embodiments disclosed. Many modifications and variations will be apparent to
those
of ordinary skill in the art without departing from the scope and spirit of
the described
embodiments.
[0078] Aspects of the present embodiments may be embodied as a system, method
or computer program product. Accordingly, aspects of the present disclosure
may
take the form of an entirely hardware embodiment, an entirely software
embodiment
(including firmware, resident software, micro-code, etc.) or an embodiment
combining
software and hardware aspects that may all generally be referred to herein as
a
"module" or "system." Furthermore, aspects of the present disclosure may take
the
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form of a computer program product embodied in one or more computer readable
medium(s) having computer readable program code embodied thereon.
[0079] Any combination of one or more computer readable medium(s) may be
utilized.
The computer readable medium may be a computer readable signal medium or a
computer readable storage medium. A computer readable storage medium may be,
for example, but not limited to, an electronic, magnetic, optical,
electromagnetic,
infrared, or semiconductor system, apparatus, or device, or any suitable
combination
of the foregoing. More specific examples (a non-exhaustive list) of the
computer
readable storage medium would include the following: an electrical connection
having
one or more wires, a portable computer diskette, a hard disk, a random access
memory (RAM), a read-only memory (ROM), an erasable programmable read-only
memory (EPROM or Flash memory), an optical fiber, a portable compact disc read-
only memory (CD-ROM), an optical storage device, a magnetic storage device, or
any
suitable combination of the foregoing. In the context of this document, a
computer
readable storage medium may be any tangible medium that can contain, or store
a
program for use by or in connection with an instruction execution system,
apparatus,
or device.
[ono] Aspects of the present disclosure are described above with reference to
flowchart illustrations and/or block diagrams of methods, apparatus (systems)
and
computer program products according to embodiments of the disclosure. It will
be
understood that each block of the flowchart illustrations and/or block
diagrams, and
combinations of blocks in the flowchart illustrations and/or block diagrams,
can be
implemented by computer program instructions. These computer program
instructions may be provided to a processor of a general purpose computer,
special
purpose computer, or other programmable data processing apparatus to produce a
machine, such that the instructions, which execute via the processor of the
computer
or other programmable data processing apparatus, enable the implementation of
the
functions/acts specified in the flowchart and/or block diagram block or
blocks. Such
processors may be, without limitation, general purpose processors, special-
purpose
processors, application-specific processors, or field-programmable processors.
[am] The flowchart and block diagrams in the Figures illustrate the
architecture,
functionality, and operation of possible implementations of systems, methods
and
computer program products according to various embodiments of the present
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disclosure. In this regard, each block in the flowchart or block diagrams may
represent a module, segment, or portion of code, which comprises one or more
executable instructions for implementing the specified logical function(s). It
should
also be noted that, in some alternative implementations, the functions noted
in the
block may occur out of the order noted in the figures. For example, two blocks
shown
in succession may, in fact, be executed substantially concurrently, or the
blocks may
sometimes be executed in the reverse order, depending upon the functionality
involved. It will also be noted that each block of the block diagrams and/or
flowchart
illustration, and combinations of blocks in the block diagrams and/or
flowchart
illustration, can be implemented by special purpose hardware-based systems
that
perform the specified functions or acts, or combinations of special purpose
hardware
and computer instructions.
[0082] While the preceding is directed to embodiments of the present
disclosure,
other and further embodiments of the disclosure may be devised without
departing
from the basic scope thereof, and the scope thereof is determined by the
claims that
follow.
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