U.S. patent application number 12/066882 was filed with the patent office on 2008-09-11 for method of clustering devices in wireless communication network.
This patent application is currently assigned to KONINKLIJKE PHILIPS ELECTRONICS, N.V.. Invention is credited to Dagnachew Birru, Kiran Challapali, Carlos Cordeiro.
Application Number | 20080219201 12/066882 |
Document ID | / |
Family ID | 37865359 |
Filed Date | 2008-09-11 |
United States Patent
Application |
20080219201 |
Kind Code |
A1 |
Challapali; Kiran ; et
al. |
September 11, 2008 |
Method of Clustering Devices in Wireless Communication Network
Abstract
In a wireless communication network (200) comprising a base
station (210) and a plurality of remote terminals (220), the
plurality of remote terminals (220) are divided into a plurality of
clusters (230) for communication with the base station (210), and
each of the remote terminals (220) is assigned to a cluster (230)
based on at least one characteristic, measured by one or more of
the remote terminals (220), of one or more external signals
transmitted by one or more external terrestrial transmitters (250)
not associated with the communication network (200). Accordingly: a
parameter of a communication between the base station (210) and
each remote terminal (220) may be selected according to the cluster
(230) to which each remote terminal (220) belongs; remote terminals
(220) within a cluster (230) may be enabled to communicate directly
with each other; and/or remote terminals (220) may be selected to
perform frequency spectrum profile measurements of the frequency
band used by the communication network (200) according to the
clusters (230) to which they are assigned.
Inventors: |
Challapali; Kiran; (New
City, NY) ; Birru; Dagnachew; (Yorktown Heights,
NY) ; Cordeiro; Carlos; (Ossining, NY) |
Correspondence
Address: |
PHILIPS INTELLECTUAL PROPERTY & STANDARDS
P.O. BOX 3001
BRIARCLIFF MANOR
NY
10510
US
|
Assignee: |
KONINKLIJKE PHILIPS ELECTRONICS,
N.V.
EINDHOVEN
NL
|
Family ID: |
37865359 |
Appl. No.: |
12/066882 |
Filed: |
September 16, 2005 |
PCT Filed: |
September 16, 2005 |
PCT NO: |
PCT/IB06/53298 |
371 Date: |
March 14, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60718127 |
Sep 16, 2005 |
|
|
|
Current U.S.
Class: |
370/312 |
Current CPC
Class: |
H04W 80/00 20130101;
H04W 24/10 20130101; H04W 74/00 20130101; H04W 36/30 20130101 |
Class at
Publication: |
370/312 |
International
Class: |
H04J 15/00 20060101
H04J015/00 |
Claims
1. In a wireless communication network (200) comprising a base
station (210) and a plurality of remote terminals (220), a method
of communication, comprising: dividing the plurality of remote
terminals (220) into a plurality of clusters (230) for
communication with the base station (210); assigning each of the
remote terminals (220) to one of the clusters (230) based on at
least one characteristic, measured by one or more of the remote
terminals (220), of one or more external signals transmitted by one
or more external terrestrial transmitters (250) not associated with
the wireless communication network (200); and performing at least
one of the following three operations: (1) selecting, according to
a cluster (230) to which each remote terminal (220) belongs, at
least one parameter of a communication between the base station
(210) and each remote terminal (220); (2) selecting, according to
the clusters (230) to which they are assigned, which ones among the
plurality of remote terminals (220) will perform frequency spectrum
profile measurements of a frequency band used by the external
terrestrial transmitters (250) not associated with the wireless
communication network (200); and (3) selecting, according to the
clusters (230) to which they are assigned, which ones among the
plurality of remote terminals (220) will transmit to the base
station (210) frequency spectrum profile measurements of a
frequency band used by the external terrestrial transmitters (250)
not associated with the wireless communication network (200).
2. The method of claim 1, wherein the at least one characteristic
measured by the one or more remote terminals (220) comprises a
frequency spectrum profile measured at each of the remote terminals
(220) produced by the one or more external signals transmitted by
the one or more external terrestrial transmitters (250) not
associated with the wireless communication network (200).
3. The method of claim 2, wherein the frequency spectrum profile
comprises a measurement vector, x.sub.i, of size 1*f where f is a
number of channels assigned for defining the external signals
transmitted by the one or more external terrestrial transmitters
(250), where i is an index number of a corresponding remote
terminal (220), and where j is an index number of a cluster
(230).
4. The method of claim 3, wherein the plurality of remote terminals
(220) is n, and wherein dividing the n remote terminals (220) into
a plurality of clusters (230) for communication with the base
station (210) and assigning each of the n remote terminals (220) to
one of the clusters (230) based on the measured characteristic of
the one or more external signals comprises: (610) establishing a
trial value of k=2 clusters (230); (630) assigning k of the n
measurement vectors of the n remote terminals (220) as trial mean
measurement vectors, m.sub.j, for each of the k clusters (230);
(640) for each of the n remote terminals (220), determining which
of the trial mean measurement vectors, m.sub.j, is closest to its
measurement vector x.sub.i and tentatively assigning the remote
terminal (220) to the corresponding cluster (230) j as a tentative
assignment; (650) for each cluster (230), j, calculating a new mean
measurement vector m.sub.j using the measurement vectors for all of
the remote terminals (220) tentatively assigned to the cluster
(230) j in step (640); repeating steps (640) and (650) until there
is no change in the values of the mean measurement vectors m.sub.j;
(660) computing a total vector distance, J, between the measurement
vectors x.sub.i for each of the n remote terminals (220) and the
mean vector m.sub.j for the cluster (230) j to which the remote
terminal (220) was last tentatively assigned in step (640); (670)
comparing J to a maximum allowed value J*; (680) when J is greater
than the maximum allowed value J*; incrementing k by 1 and
repeating steps (630) through (670) until J is less than or equal
to the maximum allowed value J*; and when J is less than or equal
to the maximum allowed value J*; using the trial value of k as a
number of clusters (230) into which the n remote terminals (220)
are divided, and using the tentative assignments of step (c) to
assign each of the n remote terminals (220) to one of the k
clusters (230).
5. The method of claim 1, wherein the at least one parameter of the
communication between the base station (210) and the remote
terminal (220) that is selected according to a cluster (230) to
which the remote terminal (220) belongs comprises at least one
selected from the group consisting of: an error correction code, a
modulation format, a guard interval between adjacent transmitted
symbols, and one or more frequency channels.
6. The method of claim 1, wherein the at least one characteristic
of the one or more external signals measured by the one or more
remote terminals (220) comprises a time of arrival of a sync signal
included in at least one of the one or more external signals
transmitted by the one or more external terrestrial transmitters
(250) not associated with the wireless communication network
(200).
7. The method of claim 6, wherein the sync signal is a field sync
sequence in a digital television (DTV) broadcast signal.
8. The method of claim 1, wherein the one or more external signals
transmitted by the one or more external terrestrial transmitters
(250) not associated with the wireless communication network (200)
includes a television broadcast signal in a frequency channel in
which the wireless communication network (200) operates.
9. In a wireless communication network (200) comprising a base
station (210) and a plurality of remote terminals (220), a method
of communication, comprising: determining a location of each of the
plurality of remote terminals (220) with respect to the base
station (210); dividing the plurality of remote terminals (220)
into a plurality of clusters (230) for communication with the base
station (210); assigning each of the remote terminals (220) to one
of the clusters (230) based on the determined location of each
remote terminal (220) so as to group remote terminals (220)
together in each cluster (230) according to their proximity to each
other; and selecting at least one parameter of a communication
between the base station (210) and each remote terminal (220)
according to a cluster (230) to which each remote terminal (220)
belongs.
10. The method of claim 9, wherein determining a location of each
of the plurality of remote terminals (220) with respect to the base
station (210), comprises, for each remote terminal (220):
determining a distance, d.sub.12, between the base station (210)
and the remote terminal (220); determining a distance, d.sub.02,
between the remote terminal (220) and a known location of a
television broadcast antenna; and solving a pair of simultaneous
equations using: (1) the distances d.sub.12 and d.sub.02, (2) the
known location of the television broadcast antenna, and (3) a known
location of the base station (210), to determine the location of
the remote terminal (220).
11. The method of claim 10, wherein determining the distance,
d.sub.12, between the base station (210) and the remote terminal
(220) comprises: measuring a turnaround time interval, t.sub.12,
for a token to be transmitted roundtrip between the base station
(210) and the remote terminal (220); and determining the distance,
d.sub.12, from the turnaround time interval, t.sub.12.
12. The method of claim 10, wherein determining the distance,
d.sub.02, between the known location of the television broadcast
antenna and the remote terminal (220), comprises: determining a
time of arrival, t.sub.1, at the base station (210) of a sync
signal included in a television signal transmitted by the
television broadcast antenna; determining a time of arrival,
t.sub.2, at the remote terminal (220) of the sync signal included
in the television signal transmitted by the television broadcast
antenna; determining a time interval, to 2, for the television
signal to travel from the television broadcast antenna to the
remote terminal (220) using the times of arrival t.sub.1 and
t.sub.2 and a known distance d.sub.01 between the base station
(210) and the television broadcast antenna; and determining
d.sub.02 according to the equation d.sub.02=t.sub.02*c, where c is
the speed of light.
13. The method of claim 12, wherein the television signal is in a
frequency channel in which the wireless communication network (200)
operates.
14. The method of claim 9, wherein the at least one parameter of
the communication between the base station (210) and the remote
terminal (220) that is selected according to a cluster (230) to
which the remote terminal (220) belongs comprises at least one
selected from the group consisting of: an error correction code, a
modulation format, a guard interval between adjacent transmitted
symbols, and one or more frequency channels.
15. In a wireless communication network (200) comprising a base
station (210) and a plurality of remote terminals (220), a method
of determining a location of each of the plurality of remote
terminals (220) with respect to the base station (210), comprising:
(a) determining a distance, d.sub.12, between the base station
(210) and the remote terminal (220), based on a turnaround time
interval, t.sub.12, for a token to be transmitted roundtrip between
the base station (210) and the remote terminal (220); (b)
determining a time of arrival, t.sub.1, at the base station (210)
of a sync signal included in an external signal transmitted by an
external terrestrial transmitter (250) located at a known location;
(c) determining a time interval, t.sub.02, for the external signal
to travel from the external terrestrial transmitter (250) to the
remote terminal (220) using: (1) a known distance d.sub.01 between
the base station (210) and the external terrestrial transmitter
(250), (2) the time of arrival t.sub.1, and (3) a time of arrival,
t.sub.2, at the remote terminal (220) of the sync signal included
in the external signal transmitted by the external terrestrial
transmitter (250); (d) determining a distance, d.sub.02, between
the remote terminal (220) and the known location of the external
terrestrial transmitter (250), based on the time interval to 2; and
(e) determining the location of the remote terminal (220) using:
(1) the distances d.sub.12 and d.sub.02, (2) the known location of
the external terrestrial transmitter (250), and (3) a known
location of the base station (210).
16. The method of claim 15, wherein the external terrestrial
transmitter (250) is a television broadcast transmitter (250) and
the external signal is a television signal in a frequency channel
in which the wireless communication network (200) operates.
17. The method of claim 15, further comprising: (f) determining a
time of arrival, t.sub.3, at the base station (210) of a sync
signal included in a second external signal transmitted by a second
external terrestrial transmitter (250) not associated with the
wireless communication network (200) located at a known location;
(g) determining a time interval, t.sub.23, for the external signal
to travel from the second external terrestrial transmitter (250) to
the remote terminal (220) using: (1) the known distance d.sub.13
between the base station (210) and the second external terrestrial
transmitter (250), (2) the time of arrival t.sub.3, and (3) a time
of arrival, t.sub.4, at the remote terminal (220) of the sync
signal included in the second external signal transmitted by the
second external terrestrial transmitter (250) not associated with
the wireless communication network (200); (h) determining a
distance, d.sub.23, between the remote terminal (220) and a known
location of the second external terrestrial transmitter (250),
based on the time interval t.sub.23; (i) determining the location
of the remote terminal (220) using: (1) the distances d.sub.12 and
d.sub.23, (2) the known location of the second external terrestrial
transmitter (250), and (3) the known location of the base station
(210); and (j) averaging the locations produced in steps (e) and
(i) using the first and second external signals transmitted by the
first and second external terrestrial transmitters (250) to more
accurately determine the location of the remote terminal (220).
18. The method of claim 15, further comprising: determining a time
of arrival, t.sub.3, at the base station (210) of a sync signal
included in a second external signal transmitted by a second
external terrestrial transmitter (250) not associated with the
wireless communication network (200), located at a second known
location; determining a time interval, t.sub.23, for the external
signal to travel from the second external terrestrial transmitter
(250) to the remote terminal (220) using: (1) the known distance
d.sub.13 between the base station (210) and the second external
terrestrial transmitter (250), (2) the time of arrival t.sub.3, and
(3) a time of arrival, t.sub.4, at the remote terminal (220) of the
sync signal included in the second external signal transmitted by
the second external terrestrial transmitter (250) not associated
with the wireless communication network (200); and determining a
distance, d.sub.23, from the known location of second external
terrestrial transmitter (250) to the remote terminal (220) based on
the time interval t.sub.13, wherein determining the location of the
remote terminal (220) using: (1) the distances d.sub.12 and
d.sub.02, (2) the known location of the external terrestrial
transmitter (250); and (3) the known location of the base station
(210), further comprises determining the location of the remote
terminal (220) in three dimensions by further using (4) the known
location of the second external terrestrial transmitter (250); and
(5) the distance d.sub.23.
19. In a wireless communication network (200) comprising a base
station (210) and a plurality of remote terminals (220), a method
of communication, comprising: dividing the plurality of remote
terminals (220) into a plurality of clusters (230) for
communication with the base station (210); assigning each of the
remote terminals (220) to one of the clusters (230) based on at
least one characteristic, measured by one or more of the remote
terminals (220), of one or more external signals transmitted by one
or more external terrestrial transmitters (250) not associated with
the wireless communication network (200); and enabling each remote
terminal (220) to communicate data directly with other remote
terminals (220) in its assigned cluster (230) without passing the
data through the base station (210).
20. In a wireless communication network (200) comprising a base
station (210) and a plurality of remote terminals (220), a method
of communication, comprising: dividing the plurality of remote
terminals (220) into a plurality of clusters (230) for
communication with the base station (210); assigning each of the
remote terminals (220) to one of the clusters (230) based on at
least one characteristic, measured by one or more of the remote
terminals (220), of one or more external signals transmitted by one
or more external terrestrial transmitters (250) not associated with
the wireless communication network (200); and selecting which ones
among the plurality of remote terminals (220) will perform
frequency spectrum profile measurements of a frequency band used by
the external terrestrial transmitters (250) not associated with the
wireless communication network (200), according to the clusters
(230) to which they are assigned.
Description
[0001] This application claims the benefit of U.S. provisional
application Ser. No. 60/718,127 filed Sep. 16, 2005, which is
incorporated herein in whole by reference.
[0002] This invention pertains to the field of wireless
communication networks, and more particularly to a method of
clustering devices in a wireless communication network.
[0003] In the United States, the Federal Communications Commission
(FCC) has recently released a proposed rulemaking to allow
unlicensed wireless communication networks to operate on certain
bands presently utilized by other, existing ("incumbent"), radio
services, such as broadcast television. The FCC proposed standards
to prevent the unlicensed wireless network's transmitting devices
from interfering with the incumbent radio services. For example,
these unlicensed transmitting devices are required to vacate any
channel within a short time period (e.g., a few seconds) after an
incumbent transmitter begins operating. One method of insuring that
a transmitting device of an unlicensed wireless network vacates a
channel when required to do so is for the device to periodically
stop transmitting and to "listen" for incumbent transmitters by
checking all channels within their operating band(s) for the
presence of any transmissions from incumbent transmitters. If the
device detects the presence of any incumbent radio transmissions,
the device is then required to take appropriate measures (e.g.,
change channels; reduce power; shut down; etc.) to insure that it
does not interfere with the incumbent signal(s).
[0004] FIG. 1 shows an exemplary unlicensed wireless communication
network 100 comprising a base station (BS) 110, and a plurality of
remote terminals (RTs) 120. In one embodiment, wireless
communication network 100 may be a Wireless Regional Area Networks
(WRAN). One typical application is broadband service where RTs 120
are on the consumer side (e.g., broadband modems) while BS 110
belongs to the service provider and services many RTs 120.
[0005] RTs 120 may be fixed or mobile devices. Typically, wireless
communication network 100 may have as many as 100 or more RTs
operating with BS 110. As shown in FIG. 1, in general there may be
one or more external transmitters 150 (e.g., incumbent television
transmitters) not associated with communication network 100
transmitting radio signal(s) in the same general geographical area
as wireless communication network 100. Furthermore, new external
transmitters 150 also may begin transmitting at any time, and these
new transmitters are also considered incumbents so that their
signals must be protected from interference by transmissions from
any of RTs 120 or BS 110.
[0006] If BS 110 and all of the RTs 120 are required to
periodically stop transmitting and listen for incumbent
transmitters on every possible channel in order to meet the channel
vacation requirements, the time required for this checking may be
considerable and the frequency may be often, and this can
significantly decrease the availability of wireless communication
network 100.
[0007] Furthermore, wireless communication network 100 may operate
over an area with a diameter on the order of tens of miles. So it
is possible that a first group of RTs 120 may be located many miles
closer to an incumbent external transmitter 150 than a second group
of RTs 120. In that case, communication on one or more channels may
be forbidden for the RTs 120 in the first group in order to protect
the signal of the incumbent external transmitter 150, but
communication on these same channels may be permissible for the
second group of RTs 120 that are located many miles away from
incumbent external transmitter 150. Conversely, the second group of
RTs 120 may be located many miles closer than the first group of
RTs 120 to a different, second incumbent external transmitter 150,
so that communication on one or more different channels may be
permissible for the first group of RTs 120, but forbidden for the
second group of RTs 120. Even though BS 110 may know the locations
and frequencies of all of the incumbent external transmitters 150
in its operating area, in general BS 110 has no convenient way of
knowing which RTs 120 are located near which incumbent external
transmitter 150. In that case, it may be forced to disable
communication with all of the RTs 120 on all of the channels on
which any of the incumbent external transmitters 150 are operating.
This reduces the efficiency and data capacity of the network.
[0008] Additionally, in some cases it would be desirable, and would
increase communication efficiency, for RTs 120 that are located in
close proximity to each other to be able to communicate with each
other directly, without passing data or messages through BS 110.
However, if the BS 110 and RTs 120 have no convenient way of
knowing which RTs 120 are located in close proximity, it is not
practical to enable such direct communications.
[0009] Accordingly, it would be desirable to provide a method and
means of grouping together remote terminals in a communication
network that permits efficient assignment of resources for
measuring the frequency spectrum profile of a frequency band used
by the communication network. It would also be desirable to provide
a method and means of grouping together remote terminals in a
communication network that permits a base station to select and
tailor one or more parameters of its communication with a remote
terminal based on one or more common characteristics of the group
to which the remote terminal belongs. It would further be desirable
to provide a method and means of grouping together remote terminals
in a communication network that facilitates direct communication
between remote terminals that are in close geographical proximity
to each other. It would still further be desirable to provide a
system and method of determining the locations of fixed and mobile
remote terminals in a wireless communication network.
[0010] In one aspect of the invention, in a wireless communication
network comprising a base station and a plurality of remote
terminals, a method of communication comprises dividing the
plurality of remote terminals into a plurality of clusters for
communication with the base station; assigning each of the remote
terminals to one of the clusters based on at least one
characteristic, measured by one or more of the remote terminals, of
one or more external signals transmitted by one or more external
terrestrial transmitters not associated with the wireless
communication network; and selecting at least one parameter of a
communication between the base station and each remote terminal
according to a cluster to which each remote terminal belongs.
[0011] In another aspect of the invention, in a wireless
communication network comprising a base station and a plurality of
remote terminals, a method of communication comprises determining a
location of each of the plurality of remote terminals with respect
to the base station; dividing the plurality of remote terminals
into a plurality of clusters for communication with the base
station; assigning each of the remote terminals to one of the
clusters based on the determined location of each remote terminal
so as to group remote terminals together in each cluster according
to their proximity to each other; and selecting at least one
parameter of a communication between the base station and each
remote terminal according to a cluster to which each remote
terminal belongs.
[0012] In a further aspect of the invention, in a wireless
communication network comprising a base station and a plurality of
remote terminals, a method of determining a location of each of the
plurality of remote terminals with respect to the base station
comprises: (a) determining a distance, d12, between the base
station and the remote terminal, based on a turnaround time
interval, t12, for a token to be transmitted roundtrip between the
base station and the remote terminal; (b) determining a time of
arrival, t1, at the base station of a sync signal included in an
external signal transmitted by an external terrestrial transmitter
not associated with the wireless communication network and located
at a known location; (c) determining a time interval, t.sub.02, for
the external signal to travel from the external terrestrial
transmitter to the remote terminal using: (1) a known distance
d.sub.01 between the base station and the external terrestrial
transmitter, (2) the time of arrival t1, and (3) a time of arrival,
t.sub.2, at the remote terminal of the sync signal included in the
external signal transmitted by the external terrestrial transmitter
not associated with the wireless communication network; (d)
determining a distance, d.sub.02, between the remote terminal and
the known location of the external terrestrial transmitter, based
on the time interval t.sub.02; and (e) determining the location of
the remote terminal using: (1) the distances d.sub.12 and d.sub.02,
(2) the known location of the external terrestrial transmitter, and
(3) a known location of the base station.
[0013] In still another aspect of the invention, in a wireless
communication network comprising a base station and a plurality of
remote terminals, a method of communication comprises dividing the
plurality of remote terminals into a plurality of clusters for
communication with the base station; assigning each of the remote
terminals to one of the clusters based on at least one
characteristic, measured by one or more of the remote terminals, of
one or more external signals transmitted by one or more external
terrestrial transmitters not associated with the wireless
communication network; and enabling each remote terminal to
communicate data directly with other remote terminals in its
assigned cluster without passing the data through the base
station.
[0014] In yet another aspect of the invention, in a wireless
communication network comprising a base station and a plurality of
remote terminals, a method of communication comprises dividing the
plurality of remote terminals into a plurality of clusters for
communication with the base station; assigning each of the remote
terminals to one of the clusters based on at least one
characteristic, measured by one or more of the remote terminals, of
one or more external signals transmitted by one or more external
terrestrial transmitters not associated with the wireless
communication network; and selecting which ones among the plurality
of remote terminals will perform frequency spectrum profile
measurements of a frequency band used by the external terrestrial
transmitters not associated with the wireless communication
network, according to the clusters to which they are assigned.
[0015] FIG. 1 shows a wireless communication network;
[0016] FIG. 2 illustrates a wireless communication network
including remote terminals divided into clusters;
[0017] FIG. 3 shows a diagram for explaining a method of
determining the location of a remote terminal in a wireless
communication network;
[0018] FIG. 4 illustrates a wireless communication network where
remote terminals are divided into clusters based on geographical
proximity to each other;
[0019] FIG. 5 illustrates a frequency spectrum profile measurement
of incumbent transmissions in a frequency band used by a
communication network; and
[0020] FIG. 6 shows a flowchart of a method of dividing remote
terminals into clusters, and assigning the remote terminals to the
clusters, in a wireless communication network.
[0021] While various principles and features of the methods and
systems described below can be applied to a variety of
communication systems, for illustration purposes the exemplary
embodiments below will be described in the context of an unlicensed
wireless communication network, such as that described above, that
operates in one or more frequency bands that are populated with
incumbent transmitters. Of course, the scope of the invention is
defined by the claims appended hereto, and is not limited by the
particular embodiments described below. Furthermore, as used
herein, the term "an external terrestrial transmitter not
associated with the wireless communication network" refers to any
terrestrial radio transmitter that transmits its signal
independently of the operation of the wireless communication
network, for example: a terrestrial analog or digital television
broadcast transmitter; a television relay transmitter; a
terrestrial commercial radio broadcast transmitter; a radio
repeater in the public service or amateur radio bands; etc.
[0022] Disclosed herein is a method of communication for a wireless
communication network comprising a base station and a plurality of
remote terminals. The method divides the plurality of remote
terminals into a plurality of clusters for communication with the
base station, and assigns each of the remote terminals to one of
the clusters.
[0023] FIG. 2 illustrates a wireless communication network 200
including a base station (BS) 210 and a plurality of remote
terminals (RTs) 220 divided into clusters 230.
[0024] As described in further detail below, each of the RTs 220 is
assigned to one of the clusters 230 based on at least one
characteristic, measured by one or more of the RTs 220, of one or
more external signals transmitted by one or more external
terrestrial transmitters 250 not associated with the wireless
communication network 200.
[0025] In one embodiment, the measured characteristic is a time of
arrival at an RT 220 of a sync signal included in an external
signal transmitted by the external terrestrial transmitter 250. For
example, where the external terrestrial transmitter 250 is a
digital television (DTV) transmitter, the sync signal may be a
field sync sequence in the DTV broadcast signal. In that case, the
measured time of arrival of the sync sequence at RT 220 is used to
calculate the location of the RT 220, which is in turn used to
assign RT 220 to a particular cluster 230.
[0026] In another embodiment, the measured characteristic is
"profile" of incumbent transmissions from all of the external
terrestrial transmitters 250 that are received at each of the RTs
220. Beneficially, the incumbent profile may be a frequency
spectrum profile, measured at each of the RTs 220, produced by the
external signals from the external terrestrial transmitters 250. In
that case, RTs 220 are assigned to clusters 230 in order to group
together in each cluster 230 RTs 220 having similar incumbent
(e.g., frequency spectrum) profiles.
[0027] In accordance with a first embodiment, FIG. 3 shows a
diagram for explaining a method of determining the location of a
remote terminal in a wireless communication network based on a time
of arrival of an external signal transmitted by one or more
external terrestrial transmitters not associated with the wireless
communication network. FIG. 3 shows a base station (BS) 210, a
remote terminal (RT) 220, and an external terrestrial transmitter
250 (e.g., a terrestrial broadcast television (TV) transmitter) not
associated with the wireless communication network 200.
[0028] The location (x.sub.1, y.sub.1) of BS 210 is assumed to be
known. The location (x.sub.0, y.sub.0) of external terrestrial
transmitter 250 is also assumed to be known (a record of the
location of TV transmitters in the United States is maintained by
the FCC). Thus, the distance d.sub.01 between TV transmitter 250
and BS 210 can be calculated and stored in BS 210. Additionally,
the locations of BS 210 and TV transmitter 250 can be separately
stored in BS 210.
[0029] Additionally, BS 210 may determine the distance d.sub.12
between the RT 220 and itself in the following way. First, BS 210
transmits a token to RT 220 and requests that RT 220 respond back
to BS 210. The turnaround time, t.sub.RT, to receive the response
from RT 220, minus any processing time, can be used to calculate
the distance d.sub.12 between the BS 210 and RT 220 according to
the following equation:
d 12 = c * t RT 2 ( 1 ) ##EQU00001##
[0030] where c is the speed of light.
[0031] Next, the distance d.sub.02 between the TV transmitter 250
and RT 220 is determined as follows.
[0032] A terrestrial television broadcast signal typically contains
a known synchronization signal. For example, in the United States a
terrestrial digital television (DTV) broadcast signal has a certain
repetitive structure. A terrestrial DTV transmitter in the United
States transmits a known signal (called a "frame sync") every 24.2
ms.
[0033] This known signal can be used to compute the distance
d.sub.02 between TV station 250 and RT 220, as follows. First, BS
210 instructs ST 220 to search for the sync sequence in a
television signal transmitted by TV transmitter 250. The time of
arrival, t.sub.2, of the sync sequence at RT 220 is determined.
Meanwhile, BS 210 also searches for the sync sequence in the TV
signal transmitted by TV transmitter 250, and records the time of
arrival, t.sub.1, of the sync sequence at its location. In that
case, the time interval, t.sub.02, needed for the TV signal to
travel from TV transmitter 250 to RT 220, can be calculated as:
t.sub.02=t.sub.2-(t.sub.1-d.sub.01/c) (2)
[0034] Once t.sub.02 is known, then d.sub.02 can be calculated
as:
d.sub.02=c*t.sub.02 (3)
[0035] Now that d.sub.02 and d.sub.12 have been calculated as
described above, one can determine the location (x.sub.2, y.sub.2)
of RT 220 by solving the following pair of simultaneous
equations:
d.sub.02.sup.2=(x.sub.0-x.sub.2).sup.2+(y.sub.0-y.sub.2).sup.2
d.sub.12.sup.2=(x.sub.1-x.sub.2)+(y.sub.1-y.sub.2) (4)
Except for x.sub.2 and y.sub.2, all of the other variables in the
equation pair (4) are known. So by simultaneously solving the
equation pair, the location (x.sub.2, y.sub.2) of RT 220 can be
found.
[0036] Meanwhile, a number of factors may negatively impact the
accuracy of the location determination method described above. For
example, multipath and clock mismatches may affect the accuracy of
the time-of-arrival measurements. Fortunately, for broadband
wireless communication network applications, a high degree of
accuracy is not required. In such an application, BS 210 only needs
to know the approximate location of RT 220 so that it can group RTs
220 accordingly. In those cases, the method described above is
typically satisfactory.
[0037] In those circumstances where a more accurate determination
of the location of RT 220 is needed, the accuracy can also be
greatly improved by repeating the above-described procedure for two
or more different external terrestrial transmitters 250 (e.g., TV
transmitters) not associated with the wireless communication
network 200, and then averaging the results to more accurately
determine the location of RT 220.
[0038] Furthermore, if signals transmitted by two or more different
external terrestrial transmitters 250 (e.g., TV transmitters) not
associated with wireless communication network 200 are available,
then the location of RT 220 in three-dimensional space (x.sub.2,
y.sub.2, z.sub.2) can also be calculated by solving the following
equation set:
d.sub.02.sup.2=(x.sub.0-x.sub.2).sup.2+(y.sub.0-y.sub.2).sup.2+(z.sub.0--
z.sub.2).sup.2
d.sub.12.sup.2=(x.sub.1-x.sub.2).sup.2+(y.sub.1-y.sub.2).sup.2+(z.sub.1--
z.sub.2).sup.2
d.sub.23.sup.2=(x.sub.3-x.sub.2).sup.2+(y.sub.3-y.sub.2).sup.2+(z.sub.3--
z.sub.2).sup.2 (5)
[0039] where d.sub.23 is the distance between RT 220 and a second
TV transmitter 250 determined using the procedure described above,
(x.sub.1, y.sub.1, z.sub.1) is the location of BS 210 in
three-dimensional space, (x.sub.0, y.sub.0, z.sub.0) is the
location of the first TV transmitter 250 in three-dimensional
space, and (x.sub.3, y.sub.3, z.sub.3) is the location of the
second TV transmitter 250 in three-dimensional space.
[0040] The procedures described above can be performed for all RTs
220 in wireless communication network 200 so that BS 210 learns the
locations of all of the RTs 220.
[0041] The performance of an unlicensed wireless communication
network operating in a frequency band utilized by one or more
incumbent transmitters can be enhanced if the locations of the
remote terminals of the wireless communication network are known.
When the locations are known, a base station can divide the remote
terminals into a plurality of clusters, and assign the remote
terminals to the clusters so as to group remote terminals together
in each cluster according to their proximity to each other. In that
case, techniques such as group scheduling or multiple antenna
diversity can be employed. Remote terminals in the same
geographical area can be made to share the same directionality
thereby improving capacity as well as performance.
[0042] FIG. 4 illustrates a wireless communication network 200
comprising BS 210 and RTs 220, where RTs 220 have been divided into
clusters 230, and each RT 220 is assigned to one of the clusters
230 so as to group RTs 220 together in each cluster 2430 according
to their proximity to each other.
[0043] By clustering RTs 220 together according to their
geographical proximity, BS 210 can do one or more of the following.
[0044] BS 210 can select at least one parameter of communication
between BS 210 and each RT 220 according to the particular cluster
230 to which that RT 220 belongs. For example, BS 210 may select
different modulation and/or error correction coding formats for
different clusters 230 of RTs 220 depending upon the general
location of the cluster 230. That is, BS 210 may select a more
robust coding/modulation format for clusters 230 of RTs 220 that
are distant from BS 210, or for clusters 230 of RTs 220 that are
located close to an external terrestrial transmitter 250 not
associated with the wireless communication network 200, and which
therefore experience increased interference. Furthermore, BS 210
may optimize the guard interval when a multi-carrier scheme such as
orthogonal frequency division multiplexing (OFDM) is employed,
according to the expected multipath delay spread of a particular
cluster 230. Thus, clustering allows BS 210 to tailor one or more
parameters of its communication with an RT 220 based on one or more
common characteristics of the cluster 230 to which the RT 220
belongs. [0045] BS 210 can use a directional antenna in combination
with techniques such as space division multiplexing between
clusters 230. This can increase the overall capacity of the
wireless communication network 200, since RTs 220 that are not in
the same cluster 230 can transmit and receive at the same time with
little interference. Also, BS 210 may use different frequency
channels to communicate with different clusters 230 of RTs 220
depending upon the relative locations of incumbent transmitters
250. That is, it may be possible for BS 210 to use a first
frequency channel for communication with a first cluster 230, while
it is not permitted to use that same first frequency channel for
communication with a second cluster 230 because of the proximity of
the second cluster 230 to an incumbent transmitter 250 operating on
the first frequency channel. At the same time, BS 210 may be able
to use a second frequency channel to communicate with the second
cluster 230, while it is not permitted to use that channel for
communication with the first cluster 230 because of the proximity
of the first cluster 230 to a second incumbent transmitter 250
operating on the second frequency channel. Thus, clustering allows
BS 210 to more efficiently utilize its communication resources in
communicating with a plurality of RTs 220. [0046] BS 210 can
schedule RTs 220 in a cluster 230 to communicate directly with each
other, without having to pass messages or data through BS 210. This
can produce a multi-sensor network that can be used for
applications other than broadband service.
[0047] Although for ease of explanation, in the discussion above
the external terrestrial transmitter 250 was described in terms of
a terrestrial broadcast television (TV) transmitter, in practice
external terrestrial transmitter 250 can be any external
terrestrial transmitter that transmits a signal including some sync
or other feature of pattern that is amenable to time-of-arrival
detection and whose location is known to BS 210. In one embodiment,
external terrestrial transmitter 250 comprises a dedicated beacon
transmitter transmitting a signal which can be used for clustering
together RTs 220 in wireless communication network 200.
[0048] Although a process of clustering remote terminals in a
wireless communication network has been described above based on
determining a geographical location of each remote terminal, in
another embodiment remote terminals are assigned to clusters
according to an incumbent profile measured at each of the remote
terminals produced by one or more external signals transmitted by
one or more external terrestrial transmitters not associated with
the wireless communication network. In that case, the location of
the external over-the-air transmitter need not be known, and remote
terminals are assigned to clusters in order to group together in
each cluster remote terminals having similar incumbent
profiles.
[0049] According to this embodiment, each RT 220 makes measurements
in each incumbent (e.g., TV) channel of external signals (e.g., TV
signals) transmitted by one or more external terrestrial
transmitters 250 not associated with the wireless communication
network 200. The incumbent profile measurement can be a simple RF
signal strength measurement of the frequency spectrum used by
wireless communication network 200. Alternatively, more
sophisticated measurements may be made based on the detection of a
feature of each external signal to provide greater robustness to
multipath. In the latter case, beneficially the strength of the
detected feature is used. For example, if the incumbent transmitter
250 is nearby (or transmitting at high power), its value will be
high, and vice versa. Based on these measurements, each RT 220
constructs an incumbent profile. This incumbent profile is then
disseminated to BS 210 (or its proxy) for clustering, as described
in further detail below. This process can be repeated
periodically.
[0050] FIG. 5 illustrates a frequency spectrum profile measurement,
made by an RT 220, of incumbent transmissions in a frequency band
used by wireless communication network. 200.
[0051] Next, an algorithm is described for clustering together RTs
220 having similar incumbent profiles will be described with
respect to the flowchart of FIG. 6.
[0052] At the outset, we define a number of variables as
follows:
[0053] n=the number of RTs 220 in wireless communication network
200;
[0054] f=total number of frequency channels used by wireless
communication network 200 that may include an external signal
transmitted by an external terrestrial transmitter 250;
[0055] k=number of clusters 230 into which the RTs 220 are
divided;
[0056] i=an index for each RT 220, where 1.ltoreq.i.ltoreq.n;
and
[0057] j=an index of each cluster 230, where
1.ltoreq.i.ltoreq.k;
[0058] x.sub.i=a measurement vector for RT 2201, of size 1*f;
[0059] J=a scalar objective function to be minimized;
[0060] J*=maximum allowed value for scalar objective function (an
input value);
[0061] k*=minimum number of clusters 230 required for J.ltoreq.J*
(an output value).
[0062] Turning again to FIG. 6, the algorithm proceeds as
follows.
[0063] In a step 610, the number of clusters 230 is set to two (2)
(k=2).
[0064] Meanwhile, in a step 620 each of the n RTs 220 measures a
frequency spectrum profile at its location, as described above, to
produce a measurement vector, x.sub.1.
[0065] Then, in a step 630, k of the measurement vectors x.sub.i of
the RTs 220 are randomly assigned as trial mean measurement
vectors, m.sub.j, for the k clusters 230. These k trial mean
measurement vectors m.sub.j serve as initial guesses as to the
actual mean measurement vectors for the k clusters 230.
[0066] Next, in a step 640, for each RT 2201, it is determined
which one of the mean measurement vectors m.sub.j is closest to its
measurement vector x.sub.i, and the RT 2201 is then assigned to the
cluster, j, as a trial assignment.
[0067] After all of the RTs 220 have been assigned to one of the k
clusters 230, in a step 650 an "updated" mean measurement vector
m.sub.j is calculated for each cluster 230 j using the measurement
vectors x.sub.i.sup.(j) for all of the RTs 220 i in that cluster
230 j.
[0068] Steps 640 and 650 are repeated until there is no further
change in the values of the mean measurement vectors m.sub.j.
[0069] Next, in a step 660, the scalar objective function to be
minimized, J, is calculated using the mean measurement vectors
m.sub.j for each cluster 230 j and all of the measurement vectors
x.sub.i.sup.(j).
[0070] In a step 670, the scalar objective function to be
minimized, J, is compared to the maximum allowed value for the
scalar objective function, J*. J* is a pre-selected value based on
target performance criteria for the wireless communication network
200, and may be determined through operational experience.
[0071] If J>J*, then in a step 680, the algorithm increments k
by one, and returns to step 630 above, and steps 630-670 are
repeated.
[0072] If J.ltoreq.J*, then the algorithm ends. At that point, k is
equal to k*, and the RTs 220 are assigned to the k* clusters 230 so
as to group together in each cluster 230 remote terminals 220
having similar incumbent profiles.
[0073] Mathematically, one beneficial selection for the scalar
function, J, is:
J = j = 1 k i = 1 n x i ( j ) _ - m j _ 2 ( 6 ) ##EQU00002##
[0074] where .parallel. x.sub.i.sup.(j)- m.sub.j.parallel..sup.2
indicates the distance between the measurement vector
x.sub.i.sup.(j) of RT 220 i, tentatively assigned to cluster 230 j,
and its cluster mean m.sub.j, in feature space.
[0075] There are a number of advantages of clustering remote
terminals. Some of these advantages relate to sharing the spectrum
measurement responsibilities within the wireless communication
network, and/or to more efficient dissemination of measurement
information. If all the remote terminals measure all the channels
and disseminate this information over the wireless communication
network, the load on the network could be significant. By
decimating the number of measurements made, the dissemination
overhead is significantly reduced.
[0076] In this regard, it is noted that the frequency with which a
given channel must be measured for occupation by an incumbent
transmitter depends not on the duty cycle of the incumbent
transmitter (which may be of the order of a day), but rather on the
vacation time period, which may be of the order of a few seconds.
The vacation time period is defined as the time period by which the
wireless communication network must vacate a channel after an
incumbent transmitter begins transmitting on that channel. When the
vacation time period is small, unless information dissemination
overhead is efficiently managed, it could become significant part
of the total available radio resources. This is especially true if
contention-based access mechanisms are used to disseminate
measurement information.
[0077] However, once remote terminals are clustered together based
on similar incumbent profiles, each RT does not have to make
repeated measurement of the entire available spectrum. The base
station (or its proxy) can make the optimal distribution of
measurements within a network, which involves the following trading
off. If too few remote terminals in the wireless communication
network make measurements, an incumbent transmitter might be
missed. On the other hand, if each remote terminal searches every
channel once each vacation time period, the total amount of time it
takes to determine which channels are available could be very
large. The above-described approach of clustering provides an
intelligent tool to make such a trade-off.
[0078] While preferred embodiments are disclosed herein, many
variations are possible which remain within the concept and scope
of the invention. Such variations would become clear to one of
ordinary skill in the art after inspection of the specification,
drawings and claims herein. The invention therefore is not to be
restricted except within the spirit and scope of the appended
claims.
* * * * *