U.S. patent application number 13/584539 was filed with the patent office on 2014-02-13 for system and method for interference triggered frequency hopping.
This patent application is currently assigned to Redline Communications, Inc.. The applicant listed for this patent is Serban Cretu, Aurel Picu, Yuriy Popov, Octavian Sarca. Invention is credited to Serban Cretu, Aurel Picu, Yuriy Popov, Octavian Sarca.
Application Number | 20140044150 13/584539 |
Document ID | / |
Family ID | 48094898 |
Filed Date | 2014-02-13 |
United States Patent
Application |
20140044150 |
Kind Code |
A1 |
Sarca; Octavian ; et
al. |
February 13, 2014 |
SYSTEM AND METHOD FOR INTERFERENCE TRIGGERED FREQUENCY HOPPING
Abstract
Systems and methods of interference-triggered frequency hopping
in a wireless communication system. A master is provided in the
wireless communication system in wireless communication with slave
nodes each configured to use different preselected communication
frequencies to permit frequency hopping. A current channel is
selected from among multiple channels in the wireless communication
system in which the master and at least some of the slave nodes
send and receive wireless communications. Each of the channels uses
different ones of the preselected communication frequencies. In the
current channel, interference with communications between the
master and a selected one of the slave nodes that use the current
channel is detected. A new channel is selected only in response to
detecting the interference. The system switches from the current
channel to the new channel such that communications between the
master and the selected slave node use the new channel.
Inventors: |
Sarca; Octavian; (Aurora,
CA) ; Popov; Yuriy; (Markham, CA) ; Picu;
Aurel; (Aurora, CA) ; Cretu; Serban; (Toronto,
CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Sarca; Octavian
Popov; Yuriy
Picu; Aurel
Cretu; Serban |
Aurora
Markham
Aurora
Toronto |
|
CA
CA
CA
CA |
|
|
Assignee: |
Redline Communications,
Inc.
Markham
CA
|
Family ID: |
48094898 |
Appl. No.: |
13/584539 |
Filed: |
August 13, 2012 |
Current U.S.
Class: |
375/133 ;
375/E1.036 |
Current CPC
Class: |
H04B 1/713 20130101;
H04B 1/715 20130101; H04B 1/7183 20130101 |
Class at
Publication: |
375/133 ;
375/E01.036 |
International
Class: |
H04B 1/715 20110101
H04B001/715 |
Claims
1. A method of interference-triggered frequency hopping in a
wireless communication system, comprising: providing in the
wireless communication system a master in wireless communication
with a plurality of slave nodes configured to use different
preselected communication frequencies to permit frequency hopping;
selecting a current channel from among a plurality of channels in
the wireless communication system in which the master and at least
some of the slave nodes send and receive wireless communications,
each of the channels using different ones of the preselected
communication frequencies; detecting interference in the current
channel with communications between the master and a selected one
of the slave nodes that use the current channel; selecting a new
channel of the channels different from the current channel only in
response to detecting the interference; and switching from the
current channel to the new channel such that communications between
the master and the selected slave node use the new channel.
2. The method of claim 1, further comprising maintaining wireless
communications between the master and the selected slave node in
the current channel such that no frequency hopping occurs away from
the current channel until interference is detected in the current
channel.
3. The method of claim 1, wherein the detecting is carried out by
the master or by the selected slave node.
4. The method of claim 1, wherein the selecting the new channel is
based on a pre-defined frequency hopping algorithm or a
pre-computed frequency hop sequence.
5. The method of claim 4, further comprising, before switching to
the new channel, determining a performance associated with a next
channel of the channels different from the current channel, and if
the performance fails to satisfy a channel performance criterion,
selecting a further channel of the channels different from the
current channel as the new channel; otherwise, if the performance
satisfies the channel performance criterion, assigning the next
channel as the new channel.
6. The method of claim 5, wherein the channel performance criterion
includes whether interference associated with the next channel is
worse than the detected interference in the current channel such
that the channel performance criterion is not satisfied if the
interference associated with the next channel is worse than the
detected interference in the current channel.
7. The method of claim 5, wherein the channel performance criterion
includes whether a wireless communication link is established in
the next channel between the master and the selected slave node
such that the channel performance criterion is not satisfied if the
wireless communication link fails within a predefined timeout to be
established between the master and the selected slave node.
8. The method of claim 4, further comprising, before switching to
the new channel, repeating, for a predetermined number of times:
determining a performance associated with a next channel of the
channels different from the current channel, and if the performance
fails to satisfy a channel performance criterion, selecting a
further channel of the channels different from the current channel
as the new channel; and if, for any of the further channels
selected, the performance fails to satisfy the channel performance
criterion for the predetermined number of times, reverting to the
current channel without switching to the new channel.
9. The method of claim 4, wherein the pre-defined frequency hopping
algorithm includes a random number generator such that the new
channel is randomly selected from among the plurality of
channels.
10. The method of claim 4, wherein the pre-defined frequency
hopping algorithm produces a hop sequence based on parameters, the
parameters including a seed, a channel list, and a number of
hops.
11. The method of claim 4, wherein the pre-computed frequency hop
sequence is based on a seed randomly generated by the master using
a characteristic unique to the master, and wherein the frequency
hop sequence is calculated by the master or by the selected slave
node using the seed and a random number generator function.
12. The method of claim 11, wherein the pre-computed frequency hop
sequence is calculated by the master and communicated to the at
least some of the slave nodes.
13. The method of claim 4, wherein the switching to the new channel
includes announcing a channel switch announcement to the selected
slave node, and responsive thereto, the selected slave node
carrying out the switching to the new channel as determined by the
pre-defined frequency hopping algorithm or in accordance with the
pre-computed frequency hop sequence.
14. The method of claim 1, further comprising removing the current
channel from a list of channels available for selection responsive
to detecting interference.
15. A non-transitory computer-readable medium encoded with
instructions that, when executed by one or more processors,
implement a method of interference-triggered frequency hopping in a
wireless communication system, the method comprising: providing in
the wireless communication system a master in wireless
communication with a plurality of slave nodes configured to use
different preselected communication frequencies to permit frequency
hopping; selecting a current channel from among a plurality of
channels in the wireless communication system in which the master
and at least some of the slave nodes send and receive wireless
communications, each of the channels using different ones of the
preselected communication frequencies; detecting interference in
the current channel with communications between the master and a
selected one of the slave nodes that use the current channel;
selecting a new channel of the channels different from the current
channel only in response to detecting the interference; and
switching from the current channel to the new channel such that
communications between the master and the selected slave node use
the new channel.
16. The computer-readable medium of claim 15, further comprising
maintaining wireless communications between the master and the
selected slave node in the current channel such that no frequency
hopping occurs away from the current channel until interference is
detected in the current channel.
17. The computer-readable medium of claim 15, wherein the detecting
is carried out by the master or by the selected slave node.
18. The computer-readable medium of claim 15, wherein the selecting
the new channel is based on a pre-defined frequency hopping
algorithm or a pre-computed frequency hop sequence.
19. The computer-readable medium of claim 18, further comprising,
before switching to the new channel, determining a performance
associated with a next channel of the channels different from the
current channel, and if the performance fails to satisfy a channel
performance criterion, selecting a further channel of the channels
different from the current channel as the new channel; otherwise,
if the performance satisfies the channel performance criterion,
assigning the next channel as the new channel.
20. The computer-readable medium of claim 19, wherein the channel
performance criterion includes whether interference associated with
the next channel is worse than the detected interference in the
current channel such that the channel performance criterion is not
satisfied if the interference associated with the next channel is
worse than the detected interference in the current channel.
21. The computer-readable medium of claim 18, further comprising,
before switching to the new channel, repeating, for a predetermined
number of times: determining a performance associated with a next
channel of the channels different from the current channel, and if
the performance fails to satisfy a channel performance criterion,
selecting a further channel of the channels different from the
current channel as the new channel; and if, for any of the further
channels selected, the performance fails to satisfy the channel
performance criterion for the predetermined number of times,
reverting to the current channel without switching to the new
channel.
22. The computer-readable medium of claim 18, wherein the
pre-defined frequency hopping algorithm includes a random number
generator such that the new channel is randomly selected from among
the plurality of channels.
23. The computer-readable medium of claim 18, wherein the
pre-defined frequency hopping algorithm produces a hop sequence
based on parameters, the parameters including a seed, a channel
list, and a number of hops.
24. The computer-readable medium of claim 18, wherein the
pre-computed frequency hop sequence is based on a seed randomly
generated by the master using a characteristic unique to the
master, and wherein the frequency hop sequence is calculated by the
master or by the selected slave node using the seed and a random
number generator function.
25. The computer-readable medium of claim 18, wherein the switching
to the new channel includes announcing a channel switch
announcement to the selected slave node, and responsive thereto,
the selected slave node carrying out the switching to the new
channel as determined by the pre-defined frequency hopping
algorithm or in accordance with the pre-computed frequency hop
sequence.
26. The computer-readable medium of claim 15, further comprising
removing the current channel from a list of channels available for
selection responsive to detecting interference.
Description
BACKGROUND OF THE INVENTION
[0001] Frequency hopping is a well-known technique used in wireless
communications systems to protect against interference or other
channel impairments. However, it has the following drawbacks:
[0002] The overhead caused by constantly switching channels, during
which time the system is unable to transmit information, increases
as the bit rate increases because for a given amount of downtime,
the amount of data that cannot be transmitted also increases. For
example, for a 100 Mb/s link and 1 ms downtime, the amount of data
which cannot be transmitted while frequency switching is going on,
is 100 000 bits or 12.5 kBytes. [0003] The interference produced
and received in frequency hopping systems expands on all channels
on which it hops. [0004] The time it takes a communication slave to
find its communication master is much longer if the master is
continuously hopping.
[0005] In order to avoid this overhead, but enjoy all the
advantages of frequency hopping, in the system described here,
frequency hopping only occurs if interference is detected. If no
interference is detected, no hopping occurs. Furthermore, since
frequency hopping does not occur on a regular basis, there is less
chance of interference to and from nearby systems. Since the system
operates on a fixed frequency in the absence of interference, links
can be established much faster on this system than in traditional
frequency hopping systems.
BRIEF SUMMARY OF INVENTION
[0006] According to an aspect of the present disclosure, a method
of interference-triggered frequency hopping in a wireless
communication system includes: providing in the wireless
communication system a master in wireless communication with a
plurality of slave nodes configured to use different preselected
communication frequencies to permit frequency hopping; selecting a
current channel from among a plurality of channels in the wireless
communication system in which the master and at least some of the
slave nodes send and receive wireless communications, each of the
channels using different ones of the preselected communication
frequencies; detecting interference in the current channel with
communications between the master and a selected one of the slave
nodes that use the current channel; selecting a new channel of the
channels different from the current channel only in response to
detecting the interference; and switching from the current channel
to the new channel such that communications between the master and
the selected slave node use the new channel.
[0007] The method can further include maintaining wireless
communications between the master and the selected slave node in
the current channel such that no frequency hopping occurs away from
the current channel until interference is detected in the current
channel.
[0008] The detecting can be carried out by the master or by the
selected slave node. The selecting the new channel can be based on
a pre-defined frequency hopping algorithm or a pre-computed
frequency hop sequence.
[0009] The method can further include, before switching to the new
channel, determining a performance associated with a next channel
of the channels different from the current channel, and if the
performance fails to satisfy a channel performance criterion,
selecting a further channel of the channels different from the
current channel as the new channel; otherwise, if the performance
satisfies the channel performance criterion, assigning the next
channel as the new channel.
[0010] The channel performance criterion can include whether
interference associated with the next channel is worse than the
detected interference in the current channel such that the channel
performance criterion is not satisfied if the interference
associated with the next channel is worse than the detected
interference in the current channel.
[0011] The channel performance criterion can include whether a
wireless communication link is established in the next channel
between the master and the selected slave node such that the
channel performance criterion is not satisfied if the wireless
communication link fails within a predefined timeout to be
established between the master and the selected slave node.
[0012] The method can further include, before switching to the new
channel, repeating, for a predetermined number of times:
determining a performance associated with a next channel of the
channels different from the current channel, and if the performance
fails to satisfy a channel performance criterion, selecting a
further channel of the channels different from the current channel
as the new channel; and if, for any of the further channels
selected, the performance fails to satisfy the channel performance
criterion for the predetermined number of times, reverting to the
current channel without switching to the new channel.
[0013] The pre-defined frequency hopping algorithm can include a
random number generator such that the new channel is randomly
selected from among the plurality of channels. The pre-defined
frequency hopping algorithm can produce a hop sequence based on
parameters, the parameters including a seed, a channel list, and a
number of hops. The pre-computed frequency hop sequence can be
based on a seed randomly generated by the master using a
characteristic unique to the master, and wherein the frequency hop
sequence is calculated by the master or by the selected slave node
using the seed and a random number generator function. The
pre-computed frequency hop sequence can be calculated by the master
and communicated to the at least some of the slave nodes.
[0014] The detecting interference can include comparing an actual
noise floor based on a received signal strength in the current
channel and a signal to interference and noise ratio (SNR) with an
expected noise floor. The expected noise floor can be calculated
based on a thermal noise floor for the current channel and a
receiver noise figure.
[0015] The comparing can include determining whether the actual
noise floor exceeds the expected noise floor by a detection
threshold, and if so, determining that interference is present in
the current channel. The switching to the new channel can include
announcing a channel switch announcement to the selected slave
node, and responsive thereto, the selected slave node carrying out
the switching to the new channel as determined by the pre-defined
frequency hopping algorithm or in accordance with the pre-computed
frequency hop sequence.
[0016] The switching to the new channel can include the master
selecting the new channel, causing the selected slave node to lose
communications with the master, and responsive thereto, the
selected slave node carrying out the selecting the new channel as
determined by the pre-defined frequency hopping algorithm or in
accordance with the pre-computed frequency hop sequence.
[0017] The method can further include removing the current channel
from a list of channels available for selection responsive to
detecting interference. The method can further include storing the
current channel in a list of unacceptable channels in which an
unacceptable level of interference is present.
[0018] According to another aspect of the present disclosure, a
non-transitory computer-readable medium encoded with instructions
that, when executed by one or more processors, implement a method
of interference-triggered frequency hopping in a wireless
communication system, is provided. The method can carry out any
combination of the methods described above.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1 shows a wireless communications system, including a
master and 6 slave(s).
[0020] FIG. 2A shows an example master startup routine.
[0021] FIG. 2B shows an example slave startup routine.
[0022] FIG. 2C shows an alternative master startup routine.
[0023] FIG. 3 shows an embodiment of frequency hopping.
DESCRIPTION OF THE INVENTION
[0024] The invention provides a mechanism for protecting against
interference or, in general, against other channel impairments like
flat fading. The system is equally applicable to point-to-point and
point-to-multipoint systems.
[0025] FIG. 1 shows a wireless system with master 111, and slaves
101, 102, 103, 104, 105 and 106 connected to master 111 over links
121, 122, 123, 124, 125 and 126. The master could be, for example,
a base station or an access point. The slave could be, for example,
a subscriber unit, a remote unit or a client.
Initialization
[0026] FIG. 2A shows the startup routine for the master. The master
has a list of channels which can be, for example, automatically
generated or manually inputted by a user. This list can be, for
example [5150; 5300; 5270; 5120; 5550] MHz. The master then sorts
the list of channels using a criterion. The criterion for example,
can include, but is not limited to, ascending or descending order
of frequency. The list is then indexed from 0 to (nchannels--1);
where nchannels is the number of channels used in the system.
[0027] For the example used with ascending order, the channels can
be indexed as follows: [0028] 0=5120 [0029] 1=5150 [0030] 2=5270
[0031] 3=5300 [0032] 4=5550 with nchannels=5.
[0033] The master generates a seed randomly in step 202. Masters on
adjacent systems will attempt to select different seeds to reduce
the possibility of interference. To enable this, seed generation
can be tied to a characteristic unique to a given master such as
Media Access Control (MAC) address, serial number, or other unique
identifying features associated with the master.
[0034] In step 203, the master selects a channel from the list.
This selection can be performed using a variety of techniques,
including but not limited to: [0035] Random selection [0036] Manual
selection by a user [0037] Scanning to find the best channel The
master sets the current channel index, cur_chan_idx to the index of
the selected channel.
[0038] In step 204, the master then begins to operate on the
channel indexed by cur_chan_idx.
[0039] FIG. 2B shows an example slave startup routine. In step 211,
the slave scans or uses other ways to find the channel on which the
master operates. In one embodiment, the slave(s) have a list of
supported channels that is much larger than the list of channels
available to the master. In yet another embodiment, the slave(s)
have a list of supported channels which is exactly the same as that
available to the master.
[0040] In step 212, after it finds the master, the slave connects
to it.
[0041] In step 213, once the slave connects to the master, the
master communicates synchronization information to the slave. The
synchronization information can include, but is not limited to, for
example: [0042] A seed; [0043] A list of channels. For example, if
the list of channels supported by the slave(s) is larger than that
available to the master, then the master transmits a list of
available channels to the slave(s). [0044] A compressed list of
channels, which could be, for example, a range of frequencies or a
list of ranges of frequencies used by the master. [0045] A
pre-computed frequency hop sequence (further explanation on the
generation of the frequency hop sequence is given below and in the
section titled "Frequency hopping process when interference is
detected") [0046] A maximum number of jumps Nchchg [0047]
Information to allow the slave to synchronize its clock to the
master
[0048] In step 214, once the slave(s) receive the synchronization
information from the master, the slave(s) perform synchronization
processing using this information. Synchronization processing can
involve different steps. For example, if the slave(s) receive a
list of channels from the master, the slave(s) order the list of
channels using the same criterion as the master, and derive the
channel indexes in a similar manner to that of the master. In
another example, if a compressed list of channels such as a range
of frequencies is transmitted to a slave, the slave uses a
decompression algorithm to obtain the list of channels. The
slave(s) set cur_chan_idx to the same value as that used by the
master. Other steps can be performed as part of the synchronization
processing, such as slave clock synchronization.
[0049] In step 215, once the synchronization processing is
complete, the slave(s) send an acknowledgement to the master to
confirm that the slave(s) are synchronized to the master. The
master keeps track of the slaves that are synchronized using the
acknowledgment sent by each slave.
[0050] In another embodiment, the master startup routine includes
the following additional step 203A as shown in FIG. 2C: After the
seed is generated, the master calculates a frequency hop sequence
of channel indexes of length Nchchg using the seed. Further details
of this calculation are provided below in the section titled
"Frequency hopping process when interference is detected". This
pre-computed frequency hop sequence is stored for use in case of
interference. In an embodiment, the master alone computes the
frequency hop sequence, and then transmits the frequency hop
sequence to the slave(s) as part of the synchronization
information. In another embodiment, the slave(s) receive the seed
from the master as part of the synchronization information, and as
part of the synchronization processing, the slave(s) compute the
frequency hop sequence before sending the acknowledgement to
confirm that the slave(s) are synchronized to the master.
[0051] If a new slave joins the system, the master and new slave
must synchronize with each other using the same process outlined
above. In an embodiment, the master and slave(s) communicate the
synchronization information securely using techniques such as
public key encryption.
Detection of Interference
[0052] Interference may be detected on cur_chan_idx through a
variety of ways.
[0053] For example, in an embodiment, the master and slave(s)
measure the actual noise floor by measuring the power of the
received signal during the communication gaps when no master or
slave is transmitting, and make decisions about interference based
on the noise floor. For example, if the master and slave(s) find
that the actual noise floor has increased sufficiently over an
expected noise floor, then the master may indicate to the slaves
that there is interference present in the channel.
[0054] In another example, the actual noise floor can be evaluated
from the received signal strength (RSSI) and from the signal to
interference and noise ratio (SNR) during transmission as
follows:
Actual Noise Floor (dB)=RSSI (dB)-SNR (dB) The expected noise floor
can be calculated as: Expected Noise Floor (dB)=TNF+NF where
TNF=thermal noise floor for the channel in dB
[0055] NF=receiver noise figure (dB)
Then, the noise floor increase can be calculated as: Noise Floor
Increase (dB)=max{(Actual Noise Floor-Expected Noise Floor),0} The
Receiver Noise Figure of a receiver is the difference in dB between
the noise at the output of the actual receiver and the noise at the
output of an ideal receiver with the same bandwidth, where the
noise at the output of the actual receiver is the amplified thermal
noise floor combined with extra noise introduced by the receiver
itself while the noise at the output of the ideal receiver is just
the amplified thermal noise floor.
[0056] In another embodiment, as part of the interference detection
process, the noise floor increase is compared to detection
threshold(s). The detection threshold(s) can be fixed, or set
adaptively using a variety of techniques, including historical data
analysis and adaptive techniques. Detection threshold(s) can be set
manually by an operator, via automated techniques, or through a
combination of automated and manual techniques, for example an
operator can manually override detection threshold(s) set via an
automated mechanism
Frequency Hopping Process when Interference is Detected
[0057] FIG. 3 is a flowchart to illustrate an example
frequency-hopping system according to aspects of the present
disclosure.
[0058] In step 301, interference is detected on cur_chan_idx.
Interference detection has previously been explained. Interference
can be detected on the channel indexed by cur_chan_idx by the
master or the slave(s). If interference is detected by the
slave(s), the slave(s) will notify the master.
[0059] The master can initiate a channel switch as follows:
[0060] In step 302, the master then selects a new channel index
new_chan_idx.
[0061] In an embodiment, the system performs this selection
"on-the-fly", that is, a new_chan_idx is selected when interference
is detected, using a pre-defined frequency hopping algorithm. In an
example frequency hopping algorithm, the master inputs the random
seed, nchannels and cur_chan_idx into a generating function
randomfn( ) to generate the new channel index new_chan_idx,
e.g.:
new_chan_idx=randomfn(cur_chan_idx, seed, n-channels) An example of
randomfn( ) could be: new_chan_idx=(cur_chan_idx+rand(
)*(nchannels-1)/RND_MAX) % n-channels; where rand( ) is a random
number generator function which generates numbers in the range
between [0,RND_MAX], and is initialized using seed.
[0062] In another embodiment, as explained in the section titled
"Initialization", the master, or the master and slave(s), calculate
a frequency hop sequence of channel indexes of length Nchchg using
the seed. In an embodiment, the master, or the master and slave(s)
can calculate this sequence by repeatedly performing randomfn( )
Nchchg times, and store the result of the calculation every time.
As explained above, if the master alone calculates this sequence,
the master will transmit the frequency hop sequence to the slave(s)
as part of the synchronization information. Alternatively, if the
master and the slave(s) calculate the frequency hop sequence, the
seed is transmitted as part of the synchronization information, and
the slave(s) calculate the frequency hop sequence in the same way
as the master as part of the synchronization processing. If
interference is detected, the master selects new_chan_idx from the
pre-computed frequency hop sequence.
[0063] In step 303, after the master selects new_chan_idx, the
master and the slave(s) try to establish better performing links on
new_chan_idx. There are different possible embodiments to achieve
this.
[0064] In an embodiment, the master announces several times to the
slaves that a channel change will occur at time T.sub.ch. At time
T.sub.ch, the master switches to the channel indexed by
new_chan_idx. A synchronized slave that receives a channel switch
announcement from the master will know that it has to switch
channel at time T.sub.ch. In an embodiment, the synchronized
slave(s) select(s) the new_chan_idx either using the same
pre-defined frequency hopping algorithm as the one used in the
master, or from the same pre-computed frequency hop sequence as the
master. This embodiment requires that the master and slave clocks
must be synchronized to each other, so that the jump will occur
simultaneously for the slaves that received a channel switch
announcement from the master. Announcing the channel switch helps
reduce the system transmission interruption time and increase
throughput. If the master sends out a channel switch announcement
and a synchronized slave does not receive the channel switch
announcement, then at time T.sub.ch, when the master switches
channels, the slave will lose its wireless link to the master. The
slave selects new_chan_idx using one of the methods outlined above,
and switches to the channel indexed by new_chan_indx.
[0065] In an alternate embodiment, the master announces that a
channel change will occur in X seconds, and after X seconds have
elapsed, the master switches to the channel indexed by
new_chan_idx. In another embodiment, the master makes several
announcements, but each time it decrements the time until the
channel change. For example: [0066] Announcement 1 at time t=0
seconds: Master announces that a change will take place in X
seconds [0067] Announcement 2 at time t=.DELTA..sub.1 seconds:
Master announces that a change will take place in (X-.DELTA..sub.1)
seconds [0068] Announcement 3 at time t=.DELTA..sub.2 seconds:
Master announces that a change will take place in (X-.DELTA..sub.2)
seconds and so on. Therefore, in this example, if a synchronized
slave does not receive announcement 1 but receives announcement 3,
it will know it has to make a switch in (X-.DELTA..sub.2) seconds
time. This embodiment has the advantage that even if the master and
slave clocks are not synchronized to each other, the slave(s) will
know when the master will switch channels. Then, similar to as
explained above, the synchronized slave(s) select(s) the
new_chan_idx using one of the methods explained above. If the
master sends out the channel switch announcements and a
synchronized slave does not receive any of the channel switch
announcements, when the master switches channels, the slave will
lose its wireless link to the master. The slave will select
new_chan_idx using one of the methods outlined above, and switch to
the channel indexed by new_chan_indx.
[0069] In yet another embodiment, the master switches channel after
making N announcements. A synchronized slave receiving any of the N
announcements will know that it has to make a channel switch. This
embodiment also has the advantage that even if the master and slave
clocks are not synchronized to each other, a synchronized slave
receiving at least one of the announcements will know that a switch
will occur. Then, as explained above, the synchronized slave(s)
select(s) the new_chan_idx using one of the methods explained
above. As explained previously, if the master sends out the channel
switch announcements and a synchronized slave does not receive any
of the channel switch announcements, then when the master switches
channels, the slave will lose its wireless link to the master. The
slave will then select new_chan_idx using one of the methods
outlined above, and switch to the channel indexed by
new_chan_indx.
[0070] In another possible embodiment, after the master selects
new_chan_idx, the master switches channel at time T.sub.ch without
announcement. All synchronized slaves will lose their wireless link
to the master. The slaves will then select new_chan_idx using one
of the methods outlined above, and switch to the channel indexed by
new_chan_indx.
[0071] The master and all the synchronized slave(s) try to
establish link(s) on the channel indexed by new_chan_idx before a
predefined timeout expires. All the synchronized slaves will hop
using the same pseudorandom sequence of channels as the master.
Slaves that are unsynchronized will lose connectivity and will have
to begin scanning for the new channel.
[0072] The master checks to ensure that all of the synchronized
slave(s) which have sent acknowledgements to the master have been
reconnected. (step 304)
[0073] If the master and all of the acknowledged synchronized
slave(s) successfully establish better-performing link(s) within
the predefined timeout, (step 305) then cur_chan_idx is set to
new_chan_idx. (step 308)
[0074] If any of the links between the master and the acknowledged
synchronized slaves fails to establish on the channel indexed by
new_chan_idx within a predefined timeout (step 306), or the channel
performance is worse than the channel corresponding to cur_chan_idx
(step 307): First, the master will check to ensure that the number
of channel changes performed is less than Nchchg (step 308). If
yes, the master will return to step 302, that is the master will
select new_chan_idx using the same predefined frequency hopping
algorithm or the same pre-computed frequency hop sequence, and try
to establish a better performing link on the channel indexed by
new_chan_idx. The master and the acknowledged synchronized slave(s)
repeat this process until they successfully establish a
better-performing link than the one on the channel indexed by
cur_chan_idx, (step 308) or the number of channel changes is equal
to Nchchg (step 306). The latter can happen: [0075] if the
predefined frequency hopping algorithm is used, they reach the
maximum number of channel changes Nchchg, or [0076] if the
pre-computed frequency hop sequence is used, they reach the end of
the pre-computed frequency hop sequence of length Nchchg.
[0077] If the number of channel changes is equal to Nchchg (step
307), the master and the acknowledged synchronized slaves will
return to the channel indexed by cur_chan_idx to attempt to
re-establish the link.
[0078] In an embodiment, if the master and all of the acknowledged
synchronized slave(s) successfully establish better-performing
link(s) within the predefined timeout, a new random seed is
generated at the master and transmitted to the slave(s). By doing
so, the system enjoys further immunity to jamming and interception,
as the pseudo-random channel sequence is being continually updated
and refreshed. If the master and acknowledged synchronized slave(s)
are configured to pre-compute frequency hop sequences, the master,
or the master and slave(s) will use this new random seed to
pre-compute a new frequency hop sequence of length Nchchg, erase
the old pre-computed frequency hop sequence and store this new
pre-computed frequency hop sequence for use in case of
interference.
[0079] In another embodiment, if the master and the acknowledged
synchronized slaves return to the channel indexed by cur_chan_idx
to attempt to re-establish the link, and if one or more of the
acknowledged synchronized slaves fail to reconnect on the original
channel they will be de-registered and will use auto-scanning to
find the master. In an embodiment, once the master and acknowledged
synchronized slave(s) re-establish a link on the channel indexed by
cur_chan_idx, then after a pre-set time interval T.sub.set, the
master and acknowledged synchronized slave(s) will perform
frequency hopping again, using the same seed as before.
[0080] In an alternative embodiment, once the master and
acknowledged synchronized slave(s) re-establish a link on the
channel indexed by cur_chan_idx, a new random seed is generated at
the master and transmitted to the slave(s). In yet another
alternative embodiment, either the master calculates a frequency
hop sequence of channel indexes of length Nchchg using the seed and
transmits the frequency hop sequence to the slave(s); or the master
transmits the seed to the slave(s) and computes the frequency hop
sequence. Then, after a pre-set time interval T.sub.set, the master
and acknowledged synchronized slave(s) will perform frequency
hopping again, either using the same pre-defined frequency hopping
algorithm as before; or using the new pre-computed frequency hop
sequence. This has the advantage, in the event that a malicious
jammer either intercepts or guesses the previous seed, the sequence
can be started afresh, and the jammer is less likely to be able to
discern the frequency sequence being used.
[0081] In another embodiment, if interference was detected on a
channel indexed by cur_chan_idx by either the master or one or more
of the slave(s), the master is notified, and the channel is marked
by the master as "bad" or unacceptable. The master then removes
this channel from the list of channels, and notifies the slave(s)
of the removal by either broadcasting the new list of available
channels or broadcasting that a channel should be removed from the
list. The slaves update their respective lists of channels.
[0082] In another embodiment, if an attempt to establish a link on
the channel indexed by new_chan_idx was unsuccessful or if the link
was worse than the channel indexed by cur_chan_idx, the channel
indexed by new_chan_idx is marked as "bad" or unacceptable. The
master removes this channel from the list of channels, and notifies
the slave(s) of the removal by either broadcasting the new list of
channels or broadcasting that this channel should be removed from
the list. The master can also add this channel to a list of "bad"
channels. The slaves will then update their list of channels. If a
"bad" or unacceptable channel is selected, either "on the fly" or
from a pre-computed frequency hop sequence, both the master and the
slave(s) will reject this choice. Optionally, if the master and
slave(s) are using a pre-computed frequency hop sequence, the
master and slave(s) may remove this channel from the frequency hop
sequence.
[0083] In an embodiment, after a fixed period of time the channel
is returned to the list of channels. If the master maintains a list
of "bad" or unacceptable channels, the channel is removed by the
master from the "bad" or unacceptable channel list. The master
notifies the slave(s) of the return to the list of channels, either
by broadcasting the new list of channels or broadcasting the return
of the channel. The slaves will update their list of channels. This
channel can now be selected. Optionally, if the master and slave(s)
are using a pre-computed frequency hop sequence, and this channel
was previously removed from the frequency hop sequence, the master
and slave(s) restore the channel to the frequency hop sequence.
[0084] In another embodiment, the system alternates between data
transmission and scanning to determine which channels are
performing well. Channel scanning is only carried out during a
small portion of the time so as to not limit system throughput.
[0085] In another embodiment, the master and the slave(s) are
capable of performing simultaneous scanning and data transmission
using multiple radios, and some of these radios will scan in the
background on a different channel to that indexed by cur_chan_idx.
The background scanning can be hardware or software
implemented.
[0086] In an embodiment, the master and slave(s) select the scan
channel index scan_chan_idx using the pre-determined frequency
hopping algorithm, and scan the channel indexed by scan_chan_idx.
If it is likely that the system could establish a link which
performs above a threshold on the channel indexed by scan_chan_idx,
scan_chan_idx is stored. If there is future interference on
cur_chan_idx, new_chan_idx can be set to scan_chan_idx. If however,
the channel indexed by scan_chan_idx performs below the threshold,
the channel indexed by scan_chan_idx is marked as "bad" or
unacceptable by the master. The master will either transmit the new
list to the slaves, or transmit that the channel indexed by
scan_chan_idx has been removed from the list. The master can also
add this channel to a list of unacceptable channels. The master and
slave(s) use the random seed to select a new scan_chan_idx using
the pre-determined frequency hopping algorithm, and scan the
channel indexed by scan_chan_idx. The master and slave(s) repeat
this process until a channel that is likely to perform above
threshold is found. If a channel is not found before interference
occurs on the channel indexed by cur_chan_idx, the seed is used to
compute a new_chan_idx using the pre-defined frequency hopping
algorithm and frequency hopping occurs as previously explained.
[0087] In another embodiment, if there is a pre-computed frequency
hop sequence, the master and slave(s) will set scan_chan_idx to the
next entry on the sequence while transmission on channel
cur_chan_idx is occurring. If the channel indexed by scan_chan_idx
performs above threshold, then scan_chan_idx is stored. If there is
future interference on cur_chan_idx, new_chan_idx can be set to
scan_chan_idx. If the channel indexed by scan_chan_idx performs
below threshold, the master marks this channel as acceptable and
removes it from the list of channels. The master may also add this
channel to a list of unacceptable channels. The master will either
transmit the new list to the slaves, or transmit that the channel
indexed by scan_chan_idx has been removed from the list. The slaves
will update their lists and the sequence accordingly. The master
and the slave(s) set scan_chan_idx to the next entry in the
sequence, and repeat the process. In an embodiment, the master
additionally uses the random seed to generate new entries for the
frequency hop sequence, and adds this to the sequence. The master
will also transmit the new entries to the slave(s) so that they can
update their sequences as well. If a channel is not found before
interference occurs on the channel indexed by cur_chan_idx, the
seed is used to compute a new_chan_idx and frequency hopping occurs
as previously explained.
[0088] In another embodiment, the master and slave(s) scan the
channels in the list of unacceptable channels to determine whether
the performance of channels in the list has improved. In an
embodiment, the master and slave(s) scan each channel in the
unacceptable list periodically. If the performance of a channel has
improved, the channel is restored to the list of channels by the
master. The master notifies the slave(s) either by transmitting the
new list to the slaves, or by transmitting the removal of the
channel from the unacceptable list and the restoration to the list
of channels to the slave(s).
[0089] In another embodiment, the master and slave(s) alternate
between scanning channels in the list of unacceptable channels to
determine if channel performance has improved, and scanning the
next channel in the sequence to determine whether it is a suitable
candidate to hop to.
[0090] In another embodiment, if the system cannot establish a new
link within Nchchg jumps or before the end of a pre-computed
frequency hop sequence, and the system has returned to
cur_chan_idx: The radios assigned to background scanning will scan
channels set according to the pre-defined frequency hopping
algorithm while the system is waiting for the pre-set time interval
T.sub.set to end. If the radios used to perform background scanning
are able to find a suitable channel, the master will transmit this
result to the slave(s), and the system can jump to this
channel.
[0091] In addition to generating a pre-computed sequence using a
seed, it is also contemplated to generate a pre-computed sequence
of channels based on performance rankings of the channels. Rankings
can be assigned based on their past performance, or based on
results obtained by background scanning.
[0092] In an embodiment, the master generates the pre-computed
sequence of channels based on performance rankings of the channels.
Then, a pre-computed sequence based on channel rankings is
transmitted to the slaves.
[0093] Ranking can also be combined with random selection. In an
embodiment, weighted random selection can be used to select
channels. Higher weights are assigned to better performing
channels, so that there is a higher likelihood that these channels
be selected.
[0094] In another embodiment, a tiered list of the channels can be
created. To achieve this, the master keeps a running count of the
time that a channel spends on the unacceptable list within a given
duration, for example, the 24 hours preceding the current time. If
the running count for a given channel exceeds a threshold, the
master tags this channel.
[0095] In the case of a two-tiered list, the master partitions the
list of available channels into the following 2 tiers [0096] Tier
1: Channels where the running count has not exceeded the threshold
[0097] Tier 2: Channels where the running count has exceeded the
threshold
[0098] When the channel re-enters the list of available channels,
it is considered a Tier 2 channel. If the running count drops below
the threshold, the master removes this tag, and notifies the
slave(s) of the tag removal. The channel is moved up into Tier
1.
[0099] When a sequence is pre-computed, after generation the
sequence is reordered so that the Tier 2 channels are moved towards
the end of the sequence, and the Tier 1 channels are listed at the
start of the sequence.
[0100] In the case of a three-tiered list, there are two
thresholds, an upper threshold and a lower threshold. Tier 1
includes the best performing channels, Tier 2 includes the next
best performing channels, and Tier 3 includes the worst performing
channels. If, for a given channel, the running count is below the
lower threshold, it is placed in Tier 1. If the running count is
above the lower threshold but below the upper threshold, it is
placed in Tier 2. If the running count is above the upper
threshold, it is placed in Tier 3.
[0101] The sequence can be reordered so that Tier 1 channels are
the earliest within the sequence, Tier 2 are next earliest, and
Tier 3 channels are the last channels in the sequence. Then, when
the master and slave(s) perform frequency hopping, it makes it more
likely that the system will hop into a channel with better
performance than channel cur_chan_idx within the maximum number of
jumps Nchchg.
[0102] It is possible that a given channel can move, for example
from Tier 1 to Tier 2 if the running count worsens. Conversely, it
is possible that a given channel can improve, for example, from
Tier 3 to Tier 2, if the running count improves.
[0103] It can be seen that this can be generalized to an S-tiered
list with (S-1) thresholds, where S is greater than or equal to 2.
Channels will be placed into the appropriate tier depending on
their running count.
[0104] The threshold(s) can be determined by analyzing historical
records, or by automated methods, for example, using adaptive
algorithms. These thresholds can be set automatically or manually
by an operator, or through a combination of automatic and manual
methods, for example, an operator can manually override
threshold(s) set via an automated mechanism.
[0105] In another embodiment, an operator has access to the system
via a user interface. The operator can use the interface for many
purposes, for example, to decide whether the system pre-computes a
sequence of channels, and chooses new_chan_idx from this sequence;
or chooses new_chan_idx on the fly. The operator can use the
interface to adjust the detection threshold, as explained
previously. In addition to the automated processes specified above,
the operator can then manually specify, for example, which channels
which should be marked as "bad" and removed from the list of
available channels using the user interface. The operator can also
rank channels according to their performance using the
interface.
* * * * *