U.S. patent number RE44,492 [Application Number 13/089,492] was granted by the patent office on 2013-09-10 for system and method for reuse of communications spectrum for fixed and mobile applications with efficient method to mitigate interference.
This patent grant is currently assigned to Shared Spectrum Company. The grantee listed for this patent is Mark Allen McHenry. Invention is credited to Mark Allen McHenry.
United States Patent |
RE44,492 |
McHenry |
September 10, 2013 |
**Please see images for:
( Certificate of Correction ) ** |
System and method for reuse of communications spectrum for fixed
and mobile applications with efficient method to mitigate
interference
Abstract
A communications system network that enables secondary use of
spectrum on a non-interference basis is disclosed. Each secondary
transceiver measures the background spectrum. The system uses a
modulation method to measure the background signals that eliminates
self-generated interference and also identifies the secondary
signal to all primary users via on/off amplitude modulation,
allowing easy resolution of interference claims. The system uses
high-processing gain probe waveforms that enable propagation
measurements to be made with minimal interference to the primary
users. The system measures background signals and identifies the
types of nearby receivers and modifies the local frequency
assignments to minimize interference caused by a secondary system
due to non-linear mixing interference and interference caused by
out-of-band transmitted signals (phase noise, harmonics, and
spurs). The system infers a secondary node's elevation and mobility
(thus, its probability to cause interference) by analysis of the
amplitude of background signals. Elevated or mobile nodes are given
more conservative frequency assignments that stationary nodes.
Inventors: |
McHenry; Mark Allen (McLean,
VA) |
Applicant: |
Name |
City |
State |
Country |
Type |
McHenry; Mark Allen |
McLean |
VA |
US |
|
|
Assignee: |
Shared Spectrum Company
(Vienna, VA)
|
Family
ID: |
26905942 |
Appl.
No.: |
13/089,492 |
Filed: |
April 19, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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12326755 |
Dec 2, 2008 |
Re. 43066 |
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60211215 |
Jun 13, 2000 |
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60264265 |
Jan 29, 2001 |
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Reissue of: |
09877087 |
Jun 11, 2001 |
7146176 |
Dec 5, 2006 |
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Current U.S.
Class: |
455/454; 455/450;
455/67.13; 455/447; 455/452.1; 455/67.11; 455/115.1 |
Current CPC
Class: |
H04W
72/08 (20130101); H04W 16/14 (20130101) |
Current International
Class: |
H04W
72/00 (20090101) |
Field of
Search: |
;455/454,447,450,452.1,67.11,67.13,71,115.1,226.1,452.2,226.2 |
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0769884 |
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JP |
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May 1992 |
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WO |
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WO 2004/054280 |
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WO |
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WO 2006-101489 |
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Sep 2006 |
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WO |
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WO 2007-034461 |
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WO |
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WO |
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WO |
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WO |
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WO |
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WO |
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WO 2007/109169 |
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Sep 2007 |
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WO |
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WO 2007/109170 |
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Sep 2007 |
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WO |
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Primary Examiner: Dao; Minh D
Attorney, Agent or Firm: Morris & Kamlay LLP
Parent Case Text
This application .Iadd.is a divisional of U.S. application Ser. No.
12/326,755, which is a reissue of U.S. Pat. No. 7,146,176, which
.Iaddend.claims priority under 35 USC 119(e) .[.based on of.].
.Iadd.to .Iaddend.U.S. Provisional Patent .[.Applications.].
.Iadd.Application .Iaddend.Ser. No. 60/211,215 dated Jun. 13, 2000
and Ser. No. 60/264,265 dated Jan. 29, 2001. Both applications are
incorporated by reference in entirety.
Claims
What is claimed is:
.[.1. A method for a network of secondary communication devices
consisting of transceivers, base stations and a central controller
sharing a radio frequency channel with existing primary users with
minimal interference to the primary users comprising the steps of:
each secondary transceiver and secondary base station measuring the
primary signal level in the channel, each secondary transceiver
communicating the signal level to the central controller, and the
central controller determining which channels each node may
potentially use by comparing the primary signal level to a
threshold value, wherein a portion of the secondary transceivers
and secondary base stations in a region distant from where the
channel is being used sequentially transmit a short duration probe
signal with a certain power level (P_probe), the secondary
transceivers and secondary base stations within a primary region
where the channel is being used measure the probe signal amplitude
value (P_received) and send these values to the central controller,
and the central controller determines the maximum power level for
each secondary transceivers and secondary base stations in the
distant region by the formula: P_transmission (dBm)=P_probe
(dBm)-P_received (dBm)+constant, with the value of the constant
depending on the maximum interference level allowed in the primary
region plus a safety margin, and the above steps are repeated at
regular intervals..].
.[.2. The method according to claim 1, further comprising the step
of: using high processing gain probe waveforms such as, but not
limited to, direct sequence waveforms, single or multiple
continuous wave (CW) tones..].
.[.3. The method of claim 2, wherein the high processing gain probe
waveform is either multiple CW waveforms or combinations of
narrowband waveforms, each with energy in a frequency zone within
the NTSC six MHz channel width and minimal energy at other
frequencies in the channel, the frequency zone being in the lower
and upper guard bands, between the video carrier and the
color-subcarrier, or between the color-subcarrier and the sound
carrier..].
.[.4. A method for a network of secondary communication devices
consisting of transceivers, base stations and a central controller
sharing a radio frequency channel with existing primary users with
minimal interference to the primary users comprising the steps of:
each secondary transceiver and secondary base station measuring the
primary signal level in the channel, each secondary transceiver
communicating the signal level to the central controller, the
central controller determining which channels each node may
potentially use by comparing the primary signal level to a
threshold value, wherein a modulation scheme where each secondary
transceiver and secondary base station transmits and receives data
for a certain time period, then simultaneously halts transmissions,
making measurements of the background signals for a time period,
and then either transmitting or receiving probe signals..].
.[.5. A method for a network of secondary communication devices
consisting of transceivers, base stations and a central controller
sharing a radio frequency channel with existing primary users with
minimal interference to the primary users comprising the steps of:
each secondary transceiver and secondary base station measuring the
primary signal level in the channel, each secondary transceiver
communicating the signal level to the central controller, the
central controller determining which channels each node may
potentially use by comparing the primary signal level to a
threshold value, wherein proximate primary receivers are identified
to each secondary transceivers and secondary base stations by
having each secondary transceiver and secondary base station
measure the strength of all strong signals within a certain range
of the spectrum, and those signals with a power level above a
threshold value declare that these are proximate nodes, and
determine the proximate radio's receive frequency using well-known
standards information, and restricting the secondary transceiver's
or secondary base station's transmit frequency list from
harmonically related values, adjacent channel values, or image
related values compared to the primary signal..].
.[.6. A method for a network of secondary communication devices
consisting of transceivers, base stations and a central controller
sharing a radio frequency channel with existing primary users with
minimal interference to the primary users comprising the steps of:
each secondary transceiver and secondary base station measuring the
primary signal level in the channel, each secondary transceiver
communicating the signal level to the central controller, and the
central controller determining which channels each node may
potentially use by comparing the primary signal level to a
threshold value, wherein proximate primary receive only radios are
identified to each secondary transceivers and secondary base
stations by having each secondary transceivers and secondary base
stations measure the strength of the primary receiver's local
oscillator leakage, and and those signals above a threshold value
declare that these is a proximate receive-only node, and determine
the proximate receiver's frequency using well-known standards
information, and restricting the secondary transceivers or
secondary base station's transmit frequency list from harmonically
related values, adjacent channel values, or image related values
compared to the primary signal..].
.[.7. A method for a network of secondary communication devices to
share the analog TV spectrum consisting of the steps of, each
secondary transceivers and secondary base stations measuring the
strength of the background TV signal strength, and if the primary
TV signal strength is greater than a certain level above the noise
level but less than another higher level, then the secondary system
will use a waveform with energy between 1.5 MHz above the channel
start frequency and 4.5 MHz above the channel start frequency to
avoid interference caused by the analog video and sound
carriers..].
.[.8. A method for a network of secondary communication devices
consisting of transceivers, base stations and a central controller
to identify which device is causing Interference to a primary user
comprising of the steps of, a method to unambiguously marking the
secondary system's signal when received by the primary receiver
such as, but not limited to, amplitude modulating the secondary
signal, and provide a method for the affected primary user to
communicate with the secondary system's central controller and
communicate the primary receiver's location and the channel
frequency, and the central controller determine the closest
secondary transceiver or secondary base station to the primary node
and the likely frequencies being transmitted that might cause the
interference, and command the secondary transceiver or secondary
base station to transmit data, and sequentially reducing the power
of the closet secondary transceiver or base station until the
primary user reports that the problem is resolved, and if the
interference to the primary receiver continues, determine the next
closest secondary transceiver or secondary base station to the
primary node and repeating the previous step until the secondary
node causing the Interference is located..].
.[.9. A method for a network of secondary communication devices
consisting of transceivers, base stations and a central controller
sharing a radio frequency channel with existing primary users with
minimal interference to the primary users comprising the steps of:
each secondary transceiver and secondary base station measuring the
primary signal level in the channel, each secondary transceiver
communicating the signal level to the central controller, and the
central controller determining which channels each node may
potentially use by comparing the primary signal level to a
threshold value, wherein each secondary transceivers arid secondary
base stations measures the strength of multiple signals from
several other stationary transmitters and by analysis of these
signal level amplitudes and if there is significant co-channel
interference determines if the secondary transceiver or secondary
base station is moving or elevated, and if the secondary
transceiver or secondary base station is moving or elevated, then
the node will use more conservative spectrum assignments that
include one or more of the following: reducing the node's maximum
transmitted power, Increasing the repetition rate of the node's
probing and primary signal level measurements, and use of another
channel..].
.Iadd.10. A method of allocating channels in a wireless
communication system, the method comprising: coordinating a
measurement interval with a plurality of transceivers during which
each of the plurality of transceivers halts transmissions;
receiving a signal strength measurement made during the measurement
interval from each of the plurality of transceivers; and allocating
a channel to at least one of the plurality of transceivers based at
least in part on the signal strength measurements. .Iaddend.
.Iadd.11. The method of claim 10, further comprising the step of
coordinating a test interval for each of the plurality of
transceivers, during which each of the plurality of transceivers
transmits a predetermined test signal. .Iaddend.
.Iadd.12. The method of claim 11, wherein the test signal is a
probe signal. .Iaddend.
.Iadd.13. The method of claim 10, further comprising the step of
receiving a measurement of the amplitude of at least one probe
signal from each of the plurality of transceivers, and wherein the
step of allocating the channel is further based in part on the
measurements of the at least one probe signal amplitude.
.Iaddend.
.Iadd.14. The method of claim 13, further comprising the steps of:
determining a maximum transmit power associated with the allocated
channel based on the measurements of the at least one signal
amplitude; and communicating the maximum transmit power to the at
least one of the plurality of transceivers for which the channel is
allocated. .Iaddend.
.Iadd.15. The method of claim 10, further comprising the step of:
determining whether at least one of the plurality of transceivers
is mobile, and wherein allocating the channel is based in part on
the mobility of the transceiver. .Iaddend.
.Iadd.16. The method of claim 10, further comprising the step of:
determining whether the at least one of the plurality of
transceivers is elevated, and wherein the step of allocating the
channel is based in part on whether the at least one of the
plurality of transceivers is elevated. .Iaddend.
.Iadd.17. The method of claim 10, wherein the step of coordinating
the measurement interval comprises synchronizing the measurement
interval to substantially a same time period. .Iaddend.
.Iadd.18. The method of claim 10, wherein the step of coordinating
the measurement interval comprises coordinating a duration of the
measurement interval such that each of the plurality of
transceivers operates within the measurement interval for not more
than one percent of operating time. .Iaddend.
.Iadd.19. The method of claim 10, wherein the step of receiving the
signal strength measurement comprises receiving a signal strength
measurement of a signal from a network distinct from the wireless
communication system. .Iaddend.
.Iadd.20. The method of claim 10, wherein the step of receiving the
signal strength measurement comprises receiving a signal strength
measurement of a television signal. .Iaddend.
.Iadd.21. The method of claim 10, wherein the step of receiving the
signal strength measurement comprises: providing a list of proposed
channels to a first transceiver; and receiving the signal strength
measurement of a channel from the list of proposed channels from
the first transceiver. .Iaddend.
.Iadd.22. The method of claim 21, wherein the step of allocating
the channel to at least one of the plurality of transceivers
comprises allocating at least one channel from the list of proposed
channels to the first transceiver. .Iaddend.
.Iadd.23. The method of claim 10, wherein the step of allocating
the channel to at least one of the plurality of transceivers
comprises: comparing each of the signal strength measurements to a
predetermined threshold; determining an allocation list based in
part on the comparisons; and allocating a channel from the
allocation list. .Iaddend.
.Iadd.24. The method of claim 23, wherein the allocation list is
determined based at least in part on a regulatory database of
emitters. .Iaddend.
.Iadd.25. A method of accessing channels in a wireless
communication system, the method comprising: synchronizing a
measurement interval with a plurality of transceivers during which
each of the plurality of transceivers halts transmissions;
measuring a signal strength of a signal from a network distinct
from the wireless communication system during the measurement
interval; and receiving a channel allocation based at least in part
on the signal strength. .Iaddend.
.Iadd.26. The method of claim 25, further comprising the step of
communicating the signal strength to a central controller, wherein
the channel allocation is received from the central controller.
.Iaddend.
.Iadd.27. The method of claim 25, further comprising the step of
receiving a channel allocation list from a central controller, and
wherein the step of measuring the signal strength comprises
measuring the signal strength in each channel of the channel
allocation list. .Iaddend.
.Iadd.28. The method of claim 25, wherein the step of measuring the
signal strength comprises measuring a signal strength in a channel
outside of a bandwidth of the channel. .Iaddend.
.Iadd.29. The method of claim 25, further comprising receiving a
test interval assignment from the central controller. .Iaddend.
.Iadd.30. The method of claim 29, further comprising transmitting a
predetermined probe signal during the test interval assignment.
.Iaddend.
.Iadd.31. The method of claim 30, wherein the predetermined probe
signal comprises at least one continuous wave (CW) signal.
.Iaddend.
.Iadd.32. The method of claim 30, wherein the predetermined probe
signal comprises a BPSK waveform. .Iaddend.
.Iadd.33. The method of claim 29, further comprising: receiving a
test signal transmitted by one of the plurality of transceivers
during the test interval; determining a metric value based on the
received test signal; and communicating the metric value to the
central controller. .Iaddend.
.Iadd.34. The method of claim 33, wherein the metric value
comprises an amplitude of the received test signal. .Iaddend.
.Iadd.35. The method of claim 25, further comprising the steps of:
receiving a test interval assignment from the central controller;
determining a test channel frequency; and transmitting a
predetermined test signal during the test interval assignment and
at the test channel frequency. .Iaddend.
.Iadd.36. The method of claim 25, further comprising the step of
performing at least one transmitting or receiving information over
the allocated channel. .Iaddend.
.Iadd.37. The method of claim 25, further comprising the step of
transmitting a signal of a predetermined waveform type over the
allocated channel. .Iaddend.
.Iadd.38. The method of claim 37, wherein the predetermined
waveform type comprises an orthogonal frequency division multiplex
(OFDM) signal. .Iaddend.
.Iadd.39. The method of claim 37, further comprising the step of
amplitude modulating the signal of a predetermined waveform type.
.Iaddend.
.Iadd.40. A method of accessing channels in a wireless
communication system, the method comprising: coordinating a test
interval with a plurality of transceivers; receiving a first test
signal transmitted by one of the plurality of transceivers during
at least a first portion of the test interval; determining a metric
based on the received test signal; and receiving a channel
allocation based at least in part on the metric. .Iaddend.
.Iadd.41. The method of claim 40, wherein the test signal is a
probe signal. .Iaddend.
.Iadd.42. The method of claim 40, further comprising: determining a
test channel frequency; and receiving the first test signal during
the first portion of the test interval and at the test channel
frequency. .Iaddend.
.Iadd.43. The method of claim 40, further comprising transmitting a
second test signal during at least a second portion of the test
interval. .Iaddend.
.Iadd.44. The method of claim 40, further comprising: determining a
test channel frequency; and transmitting a second test signal
during at least a second of the test interval and at the test
channel frequency. .Iaddend.
.Iadd.45. The method of claim 40, wherein receiving the first test
signal comprises receiving a plurality of continuous wave (CW)
tones. .Iaddend.
.Iadd.46. The method of claim 40, wherein receiving the first test
signal comprises receiving a BPSK waveform. .Iaddend.
.Iadd.47. The method of claim 46, wherein the BPSK waveform
comprises a pseudo random sequence. .Iaddend.
.Iadd.48. The method of claim 46, wherein the BPSK waveform
comprises a signal having a bandwidth that is approximately equal
to a channel allocation bandwidth. .Iaddend.
.Iadd.49. The method of claim 40, wherein the step of determining
the metric comprises determining an amplitude. .Iaddend.
.Iadd.50. The method of claim 40, wherein determining the metric
comprises the steps of: sampling the received first test signal to
generate a plurality of samples; and performing FFT processing on
the samples. .Iaddend.
.Iadd.51. The method of claim 40, wherein determining the metric
comprises the steps of: sampling the received first test signal to
generate a plurality of samples; and coherently integrating the
samples over a coherence time. .Iaddend.
.Iadd.52. A method of accessing channels in a wireless
communication system, the method comprising: receiving a channel
allocation list; synchronizing a measurement interval with a
plurality of transceivers during which each of the plurality of
transceivers halts transmissions; measuring a received signal
metric during the measurement interval; associating the received
signal metric with a channel from the channel allocation list; and
determining a channel allocation from the channel allocation list
based at least in part on the received signal metric. .Iaddend.
.Iadd.53. The method of claim 52, wherein measuring the received
signal metric comprises: determining a channel from the channel
allocation list; and determining the received signal metric based
in part on a signal received outside of a bandwidth of the channel.
.Iaddend.
.Iadd.54. The method of claim 53, wherein the signal received
outside of the bandwidth of the channel comprises a signal received
at a harmonic of the channel. .Iaddend.
.Iadd.55. The method of claim 53, wherein the signal received
outside of the bandwidth of the channel comprises a signal received
at frequency determined based on a cross product of a primary
signal with a secondary signal. .Iaddend.
.Iadd.56. The method of claim 53, wherein signal received outside
of the bandwidth of the channel comprises a signal received at a
predetermined frequency offset from the channel. .Iaddend.
.Iadd.57. The method of claim 56, further comprising the step of
restricting transmitted power based at least in part on the
received signal metric. .Iaddend.
.Iadd.58. The method of claim 56, further comprising the step of
changing to another frequency. .Iaddend.
.Iadd.59. The method of claim 56, wherein the predetermined
frequency offset is a harmonically-related frequency offset.
.Iaddend.
.Iadd.60. The method of claim 56, wherein the predetermined
frequency offset comprises an adjacent channel offset.
.Iaddend.
.Iadd.61. The method of claim 56, wherein the predetermined
frequency offset comprises a local oscillator frequency offset.
.Iaddend.
.Iadd.62. The method of claim 56, wherein the predetermined
frequency offset comprises an IF image related offset.
.Iaddend.
.Iadd.63. The method of claim 56, wherein the predetermined
frequency offset comprises a transmit/receive pair frequency
offset. .Iaddend.
.Iadd.64. A system comprising: a plurality of transceivers, each of
the plurality of transceivers configured to halt transmissions
during a measurement interval; and a controller configured to
receive a signal strength measurement made during the measurement
interval from each of the plurality of transceivers; the controller
further configured to allocate a channel to at least one of the
plurality of transceivers based at least in part on the signal
strength measurements. .Iaddend.
.Iadd.65. The system of claim 64, wherein the controller is one of
the plurality of transceivers. .Iaddend.
.Iadd.66. The system of claim 64, said controller further
configured to coordinate a test interval for each of the plurality
of transceivers, during which each of the plurality of transceivers
transmits a predetermined test signal. .Iaddend.
.Iadd.67. The system of claim 64, said controller further
configured to receive a measurement of the amplitude of at least
one probe signal from each of the plurality of transceivers, and to
allocate the channel based in part on the measurements of the at
least one probe signal amplitude. .Iaddend.
.Iadd.68. The system of claim 67, said controller further
configured to: determine a maximum transmit power associated with
the allocated channel based on the measurements of the at least one
signal amplitude; and communicate the maximum transmit power to the
at least one of the plurality of transceivers for which the channel
is allocated. .Iaddend.
.Iadd.69. The system of claim 64, wherein the step of coordinating
the measurement interval comprises synchronizing the measurement
interval to substantially a same time period. .Iaddend.
.Iadd.70. The system of claim 64, wherein the signal strength
measurement comprises a measurement of a signal from a network
distinct from the wireless communication system. .Iaddend.
.Iadd.71. The system of claim 64, said controller further
configured to allocate the channel based at least in part on a
regulatory database of emitters. .Iaddend.
.Iadd.72. A transceiver configured to: coordinate a measurement
interval with at least one other transceiver during which each of
the transceivers halts transmissions; receive a signal strength
measurement made during the measurement interval from the at least
one other transceiver; and allocate a channel to the at least one
other transceiver based at least in part on the signal strength
measurements. .Iaddend.
.Iadd.73. The transceiver of claim 72, further configured to
communicate the signal strength to a central controller, wherein
the channel allocation is received from the central controller.
.Iaddend.
.Iadd.74. The transceiver of claim 72, further configured to
measure the signal strength in a channel outside of a bandwidth of
the channel. .Iaddend.
.Iadd.75. The system of claim 72, further configured to perform at
least one transmitting or receiving information over the allocated
channel. .Iaddend.
.Iadd.76. The system of claim 72, further configured to transmit a
signal of a predetermined waveform type over the allocated channel.
.Iaddend.
.Iadd.77. A device configured to: receive a channel allocation
list; synchronize a measurement interval with a plurality of
transceivers during which each of the plurality of transceivers
halts transmissions; measure a received signal metric during the
measurement interval; associate the received signal metric with a
channel from the channel allocation list; and determine a channel
allocation from the channel allocation list based at least in part
on the received signal metric. .Iaddend.
.Iadd.78. The device of claim 77, further configure to measure the
received signal metric by: determining a channel from the channel
allocation list; and determining the received signal metric based
in part on a signal received outside of a bandwidth of the channel.
.Iaddend.
.Iadd.79. The device of claim 78, wherein the signal received
outside of the bandwidth of the channel comprises a type selected
from the group consisting of: a signal received at a harmonic of
the channel, a signal received at frequency determined based on a
cross product of a primary signal with a secondary signal, and a
signal received at a predetermined frequency offset from the
channel. .Iaddend.
.Iadd.80. The device of claim 78, wherein the signal received
outside of the bandwidth of the channel comprises a signal received
at a predetermined frequency offset from the channel, wherein the
predetermined frequency offset is selected from the group
consisting of: a harmonically-related frequency offset, an adjacent
channel offset, a local oscillator frequency offset, an IF image
related offset, and a transmit/receive pair frequency offset.
.Iaddend.
Description
BACKGROUND
1. Field of Invention
This invention relates to communications spectrum allocation and
reuse on a non-interference basis in bands which have pre-existing
spectrum users (both transmit/receive type and receive-only
type).
2. Description of Prior Art
Communication systems commonly use methods to optimize the use of
the spectrum. There are several approaches involving radio networks
where channels are selected to optimize system capacity.
Cellular phone and other types of systems use low power
transmissions and a cellular architecture that enables spectrum to
be reused many times in a metropolitan area. These systems assume
that within the allocated frequency band, the system is the primary
user and that there is a control or signaling channel between all
nodes. The goal of these systems is to maximize the number of calls
system wide given a fixed amount of bandwidth. This problem is
complex because of the nearly innumerable choices of
frequency/channel combinations possible, the time varying nature of
the calls, and the unpredictable propagation loses between all of
the nodes. While global optimization schemes would give the highest
capacities, limited communications capacity between the nodes,
finite channel measuring capabilities in some of the nodes, and
short decisions times require that distributed non-optimal methods
be used. Examples are disclosed in U.S. Pat. Nos. 4,672,657 (1987),
4,736,453 (1988), 4,783,780 (1988), 4,878,238 (1989), 4,881,271
(1989), 4,977,612 (1990), 5,093,927 (1992), 5,203,012 (1993),
5,179,722 (1993), 5,239,676 (1993), 5,276,908 (1994), 5,375,123
(1994), 5,497,505 (1996), 5,608,727 (1997), 5,822,686 (1998),
5,828,948 (1998), 5,850,605 (1998), 5,943,622 (1999), 6,044090
(2000), and 6,049,717 (2000).
The above patents describe methods where current channel
measurements (noise level, carrier-to-interference ratio (C/I)),
previous channel measurement statistics, and traffic loading are
used in different ways to optimize capacity while minimizing
latency in channel assignment, equipment requirements, and dropped
calls. All of these methods assume that the system is the primary
spectrum user. This would allow the primary system to select
channels where it was jammed, but it would create significant
interference to another system.
Several methods to enable a system to operate as the secondary
spectrum user with minimal impact to the primary user have been
disclosed. The first type assume that there are predetermined
spatial "exclusions zones" where if the secondary user avoids
transmission while located in these areas, then there will be no
interference to the primary user. U.S. Pat. No. 5,422,930 (1995)
uses a telephone circuit based keying method where the telephone's
location is known and when the secondary user is connected to the
specific phone line, authorization is given for operation using a
set of frequencies. U.S. Pat. No. 5,511,233 (1996) is similar
method where an undefined position location system is used. U.S.
Pat. No. 5,794,1511 (1998) uses a GPS (global positioning system)
to locate the secondary user.
This geolocation exclusion method has significant short-falls. To
determine the exclusion zones, propagation estimates or propagation
methods would have to be made. There would be large uncertainties
in the antenna type, antenna orientation, antenna height, and power
level used by the secondary user. There would be uncertainties in
the local propagation conditions between the secondary user and the
primary user, and these propagation conditions might change because
of ducting or other temporary atmospheric conditions. To mitigate
these problems, the exclusion zones would have to have very large
margins, which would greatly reduce system capacity, or some
unintended interference would be created. These schemes do not
address how the interference caused by one specific secondary user
would be quickly and economically identified and eliminated.
A second type of secondary spectrum allocation method uses detailed
propagation modeling of the primary and secondary communication
systems and channel occupancy measurements made by the secondary
system (U.S. Pat. No. 5,410,737 (1995) and U.S. Pat. No. 5,752,164
(1998)). The channel measurements are use to validate and improve
the propagation modeling estimates. Using this information, the
spectrum is allocated so that the primary user is not impacted.
Because of the large uncertainties in propagation estimates, the
above method must use large margins to insure minimal interference.
Using measurements of the propagation losses between the primary
and secondary user can be directly used to reduce these margins
only if the primary system transmits and receives using the same
antenna, at the same frequency and at a known power level. In this
case the secondary radio directly estimates it's impact on the
primary system and can select its frequency and power level to
avoid interference. However, most communication systems use
different transmit and receive frequencies and often use different
transmit and receive antennas. Hence, the measurements of the
primary signal received by the secondary don't provide direct
information on the impact the secondary transmitter has on the
primary receiver. This method also doesn't describe how
unintentional interference would be identified and mitigated.
A third approach insurers that the measurements of the primary
signals made by the secondary user can be used to determine the
available spectrum is to add a narrow bandwidth "marker" signal to
every primary receiver antenna system (U.S. Pat. No. 5,412,658
(1995)). This approach has significant cost impact to the primary
user and because the CW marker transmitter is collocated to the
primary receiver, it will cause significant interference to the
primary user.
A fourth method has the primary and secondary users sharing a
spectrum band between the primary and secondary users to reserve
bandwidth (U.S. Pat. No. 5,428,819 (1995)). An "etiquette" is
observed between the users and each user makes measurements of the
open channels to determine priority usage. This method has the
disadvantage that the primary system must be modified to
communicate with the secondary system, which is cost prohibitive if
the primary user is already established. Also, the method will fail
in many cases because of the well known "hidden node problem". This
occurs when the secondary nodes are unable to receive transmissions
from a primary node because of the particular propagation
conditions. Thus, the secondary user incorrectly believes the
channel is available and his transmissions cause interference.
A fifth method assumes that the primary and secondary systems are
controlled by a central controller (U.S. Pat. Nos. 5,040,238
(1991), 5,093,927 (1992), 5,142,691 (1992), and 5,247,701 (1993)).
When interference occurs, the secondary system's power level and/or
frequency list is adjusted. Some of the methods use channel
measurements at the secondary system to detect changes in the
frequency usage that would require a re-prioritization of channels.
This method has obvious problems because the primary system would
have to be highly modified to interact with the secondary system
and to be able to make the required spectrum measurements. The
spectrum is now fully allocated and there are primary users in
every band. What is needed is a method that enables secondary
operation without any modification to the existing primary
user.
A sixth method uses field monitors the measure the secondary signal
strength at specific locations. One sub-method is intended to
enable secondary usage inside buildings (U.S. Pat. Nos. 5,548,809
(1996) and 5,655,217 (1997)). Field monitors are located
surrounding the secondary system nodes which determine what
channels are not used by nearby primary systems or if the channels
are in use, if the coupling between the primary to them where the
coupling to detected. The second sub-method is intended to enable
adjacent cellular based mobile communication systems (U.S. Pat.
Nos. 5,862,487 (1999)).
OBJECTIVES AND ADVANTAGES
Accordingly, several objects or advantages of my invention are: (a)
to provide a method to determine what channels a newly installed
secondary transceiver can use without causing interference to the
primary system while the other secondary transceivers are using the
same channels; (b) to provide a method to determine what channels a
newly installed secondary transceiver can use without causing
interference to the primary system that has minimal impact to the
capacity of the secondary system; (c) to provide a method to
determine what channels a secondary transceiver can use without
causing interference to the primary system while the primary system
is operating; (d) to provide a method to determine if a primary
receiver is in close proximity to a secondary transceiver which
greatly reduces that probability of adjacent channel or "IF image"
interference to the proximate primary receiver; (e) to provide a
method to measure propagation losses using a unique waveform that
causes minimal interference to TV signals; (f) to provide a method
to measure propagation losses using a unique waveform that causes
minimal interference to data signals; (g) to provide a method to
vary the secondary waveform type that improves the capacity of the
secondary system while creating minimum interference to the primary
system; (h) to provide a method to modulate the secondary signal so
the primary user can quickly and positively .Iadd.determine
.Iaddend.if the reception problems are caused by the secondary
signal or by other causes; (i) to provide a method to identify what
secondary user is causing interference to a primary user; and (j)
to provide a method to precisely and efficiently reduce the
transmitter power level of a secondary user that is causing
interference to a primary user to a level which doesn't cause
interference; (k) to provide a method to determine if the secondary
node is moving indicating that its frequency allocations needs to
be checked more frequently or with a different method; (m) to
provide a method to determine if the secondary node is at an
elevated position indicating that it is more likely to cause
interference to distant primary users and indicating that the very
conservative frequency allocation methods should be used;
Further objects and advantages of my invention will become apparent
from a consideration of the drawings and ensuing description.
DRAWING FIGURES
FIG. 1 shows the arrangement of the nodes and illustrates the
secondary spectrum usage concept.
FIG. 2 shows the method to test for potential interference.
FIG. 3 is a flowchart describing the actions of the secondary node
and the central controller to determine which channels are
available.
FIG. 4 shows the spectrum of the four-tone probe waveform and the
spectrum of an NTSC TV signal.
FIG. 5 is a graph that shows the primary and secondary signal
strengths versus time at the secondary receiver.
FIG. 6 is a graph that shows the primary and secondary signal
strengths versus time at the primary receiver.
FIG. 7 is a graph that shows the secondary signal modulation phase
in different channels.
FIG. 8 is a graph of the nominal receiver timeline.
FIG. 9 illustrates the method to detect nearby primary receivers
via local oscillator leakage measurements.
FIG. 10 is a graph the spectrum of the secondary signal and the
spectrum of an NTSC TV signal when the TV signal may potentially
interfere with the secondary signal
FIG. 11 is a graph the spectrum of the secondary signal and the
spectrum of an NTSC TV signal when the TV signal does not interfere
with the secondary signal.
FIG. 12 is a table the shows the waveforms to be used in various
conditions.
FIG. 13 is a block diagram of the secondary system transceiver.
FIG. 14 shows the configuration used to determine which secondary
node is causing interference.
FIG. 15 is a flowchart describing the method used to determine
which secondary node is causing interference.
FIG. 16 illustrates the method to determine a secondary node's
approximate altitude.
FIG. 17 shows the method to determine if a secondary node is moving
or stationary.
REFERENCE NUMERALS IN DRAWINGS
10 primary receiver 12 primary transmitter 20 secondary transceiver
21 new secondary transceiver 22 secondary base station 24 secondary
service area 26 primary service area A 28 primary service area B 30
secondary central controller 40 obstacle 50 antenna 52 amplifier 54
tuner 56 controller 58 programmable modem 60 user device 62
variable attenuator 64 preselect filter
DESCRIPTION
This invention allows a secondary user to efficiently use the
spectrum on a non-interference basis with an existing primary user.
FIG. 1 shows a primary transmitter 12 sending signals to one or
more primary receivers 10. Separated by a large distance there is a
network of secondary wireless transceivers 20 and secondary base
stations 22. The secondary base stations 22 are connected by high
capacity wire line or microwave links to a secondary central
controller 30. The secondary users that are located within a
secondary service area 24 also uses the primary channel, but they
don't cause interference to the primary user because the distance
and obstacles 40 between sufficiently attenuate the secondary
signals radiated to the primary receivers 10. Thus, if the
secondary transceivers 20 and 22 always transmit below certain
power levels (which are different for each node), then the primary
user will not be affected and the spectrum can be re-used.
Determining the secondary transceiver's maximum power level is very
difficult since it depends on antennas, cable losses, locations,
radio frequency (RF) propagation, and other factors which can't
economically be reliably predicted. In the preferred embodiment, a
combination of primary signal strength measurements, measurements
of signals from nearby primary receivers, and
secondary-to-secondary node coupling measurements are made to
determine this power level.
FIG. 2 shows a new secondary transceiver 21 that is to be added to
the secondary network. To establish connectivity with the secondary
network, the new secondary transceiver 21 initially uses a startup
channel, which is a primary allocation for the network and is
reliable. This may be in the ISM unlicensed band, cellular
telephone band, or any other band. The central controller provides
the new secondary node 21 a list of channels that are potentially
useful based on propagation calculations and channels surrounding
secondary transceivers 20 have found don't cause interference.
The new secondary node 21 then measures the primary signal strength
in each of the proposed channels. As will be described later, this
measurement is coordinated with the secondary signals in the
secondary service area 24. During the measurement interval the
secondary signals are switched off to prevent the secondary signals
from affecting the primary signal measurement. If the primary
signal is below a certain value, then the new secondary node 21 is
assumed to be located in a region where the channel is potentially
available for spectrum reuse. If the primary signal is above
another certain value, then the new secondary node 21 is assumed to
be located in the primary service region B 28, the channel is not
available for spectrum reuse by this node, and this node can be
used to received signal probes.
FIG. 2 shows the method to estimate the secondary system's
interference to the primary system. In the preferred embodiment of
this invention, the vast majority of the primary and secondary
users sharing the same channel will be geographically separated by
10's of km and will have low antenna heights (10 m or less). The
vast majority of paths between the secondary and primary nodes will
not allow line of sight propagation and will have 30 dB to 50 dB of
excess propagation loss compared to free space losses. Because of
these large losses, the secondary users will not interfere with the
primary users and significant reuse of the spectrum is
practical.
However, there are a variety of factors which may reduce the
propagation losses and create interference: (1) The primary or
secondary users may have elevated antennas (100 m or more), (2)
incorrect information on the secondary user's location, and (3)
unusual propagation due to atmospheric conditions. These conditions
are rare but exist often enough that the secondary system must
mitigate them in order to operate on a non-interference basis. The
conditions also vary with time so they must be mitigated on a
regular basis.
Unfortunately, the signal level from each secondary transceiver 20
at each primary receiver 10 can't be measured directly because of
the expense in deploying the measurement equipment and the location
of the primary receivers 10 may be unknown. Simulations and
analysis could be used to estimate these effects, they would
require extensive detailed knowledge of all primary users, terrain
features and atmospheric data, which is impractical to obtain.
Instead, the secondary signal level at the primary receivers 10 is
estimated by the use of propagation models and measuring the
secondary signal level at secondary transceiver 20 and secondary
base stations 22 surrounding the primary receivers 10. In the
example shown in FIG. 2, the new secondary transceiver 21 desires
to transmit using channel B without interfering with primary users
in service area B 28. Using propagation models and the FCC emitter
database, the maximum transmit power that the new secondary
transceiver 21 can use without interference to primary receivers 10
in service area B 28 is calculated. The transmit power level is
reduced by a certain value (10 dB-20 dB) to account for modeling
uncertainties.
The secondary central controller 30 then tasks the new secondary
transceiver 21 to transmit a probe signal for a brief period
(several milliseconds). The secondary central controller 30
previously coordinates with the secondary transceivers 20 and
secondary base stations 22 in service area B 28 so that they
measure the probe signal amplitude. The central controller
identifies which nodes are within service area B 28 by comparing
the primary signal level measurements to a threshold value as
previously described. These amplitude values are sent to the
secondary central controller 30. If any of the probe signal
amplitudes exceed a threshold value, then the maximum transmit
power level that the new secondary transceiver 21 can use on
channel B is reduced by the amount the maximum measurement exceeded
the threshold. The value of the maximum transmission power level is
thus equal to the following formula: P_transmission (dBm)=P_probe
(dBm)-P_received (dBm)+"constant", with "P_probe" the probe
transmission power level, "P_received" the maximum received probe
power level, and the value of the "constant" depending on the
maximum interference level allowed in the "primary region" plus a
safety margin.
These measurements are repeated at a regular interval (10's of
minutes to a few hours) and the probe signal amplitudes are
compared to previous values. If there is a significant change due
to changes in the secondary equipment (new location, antenna
rotations, changes to the system cabling . . .) or due to unusual
propagation conditions, the maximum transmit power level that the
new secondary transceiver 21 can use on channel B is changed so
that the maximum measurement value equals the threshold value.
If the secondary equipment is mobile, than the measurements are
made more frequently and the threshold value is set higher to
account for lags in transmitting the data to the secondary central
controller 30 and other system delays. The probe duration is
adjusted to balance the probe measurement time versus probe
waveform detection probability and depends on the number of
secondary nodes and the node dynamics. In a secondary service area
26 or 28 with 10,000 users, 10% of the capacity allocated to
probing, and probing done every hour, the probe duration is
approximately 2 ms.
To decrease the amount of time spent probing, groups of secondary
transceiver 20 and secondary base stations 22 can transmit the
probe signals simultaneously. If the secondary transceivers 20 and
secondary base stations 22 in service area B 28 measure a probe
signal amplitude greater than the threshold value, then each of the
secondary transceiver 20 and secondary base stations 22 can
individually re-transmit the probe signal to determine which link
will cause interference.
FIG. 3 is a flow chart showing the above procedure used to
determine the maximum transmit power level that each secondary
transceiver 20 and secondary base station 22 can use.
To minimize the interference to the primary system, the probe
waveform is not the same as used to transmit data. The waveform is
designed to have minimal effect on the primary waveform, to be
easily and quickly acquired by the secondary system, and to have
sufficient bandwidth across the channel of interest so that
frequency selective fading doesn't introduce large errors. In the
preferred embodiment of this invention, one of the following
waveforms is used depending of the primary signal modulation.
FIG. 4 shows the probe signal waveform spectrum used with NTSC TV
video signals. The signal uses nominally four (with a range from
one to twenty) CW tones distributed in four frequency zones in the
6 MHz TV channel. Two of the zones are near the channel frequency
start and end values. The third zone frequency limits are 1.5 MHz
to 4.5 MHz above the channel start frequency Oust below the color
subcarrier frequency). The fourth zone is from 5 MHz to 5.5 MHz
above the channel start frequency (between the color subcarrier
frequency and aural carrier frequency). Signals in these zone
regions can experimentally be shown to: (1) have much less impact
to the TV reception than tones at other frequencies and (2) are at
frequencies that the NTSC signal spectrum is at minimum values. The
tones in each zone can be transmitted at the same time to reduce
the probe measurement time or can be transmitted one at a time to
minimize the receiver processing requirements.
The value of this waveform is that .Iadd.it .Iaddend.has
approximately the same level of impact to the TV signal as a
broadband waveform used to send data, but this waveform can be
received with a narrow bandwidth (.about.10 Hz) receiver compared
to a wide bandwidth (several MHz) broadband receiver, thus it can
be transmitted at much lower (.about.50 dB) amplitude and will have
minimal impact to the primary signal.
The relative amplitudes of the CW tones in each zone are shown in
FIG. 4 and are set to cause nearly the same level of TV
interference. Experimentally it can be shown that signals .[.in
the.]. in the zones near .[.are.]. the channel frequency start and
end values cause approximately the same degradation of the TV
signal. The zone from 1.5 MHz to 4.5 MHz above the channel start
frequency has signals nominally 30 dB (20 dB to 40 dB range)
reduced in amplitude compared to the start and end zone signals.
The zone from 5 MHz to 5.5 MHz above the channel start frequency
has signals nominally 10 dB (0 dB to 20 dB range) reduced in
amplitude compared to the start and end zone signals.
To receive this waveform, standard FFT processing techniques are
used to measure the amplitude of each CW tone and the amplitudes
are normalized by the 30 dB and 10 dB amounts described above.
Selective fading will cause the relative amplitude of each tone to
vary just as would occur with a data waveform and must be accounted
for to estimate the interference caused by a data waveform. To
account for fading, the largest of the four CW tone amplitudes is
used to estimate the worse case channel conditions. The probability
that all four tones are faded causing the propagation losses to be
over estimated is very low.
If the primary signal is other than NTSC TV video signals, the
probe signal is a conventional BPSK waveform with bandwidth
approximately equal to the channel bandwidth. This sets the chip
rate at approximately the inverse of the bandwidth (a 10 MHz
bandwidth would have a chip rate of 10 Mcps). The waveform
transmits a pseudo random sequence with the maximum length that can
be coherently integrated when limited by channel conditions or
receiver hardware complexity. In non-line-of-sight (LOS)
propagation conditions, the maximum channel coherence time is
approximately 100 ms. Current low cost receiver hardware is limited
to sampling and processing approximately 10,000 samples. Assuming 2
samples per chip, the maximum sequence is approximately 5,000
samples. Thus, the sequence length is set to the minimum of the
chip rate (symbols per second) times 100 ms (the maximum sequence
duration) and 5,000.
To receive the BPSK probe signal, the secondary receiver samples
the signal for a period equal to the transmit period and using a
non-linear technique to measure the amplitude of probe signal. Each
sample value is squared and the resulting series analyzed using an
FFT. At the frequency corresponding to twice the chip rate, a
narrow bandwidth spectral line will exist with amplitude that is
related to the received probe signal amplitude. It is well known to
those familiar in the art that this technique is able identify BPSK
signals with amplitude well below the noise level and provides
nearly optimal signal detection performance. Thus, the probe signal
can be transmitted at a much lower power level than a regular data
signal (which reduces interference to the primary signal) and can
still be detected.
Once the probe signal amplitudes are measured at the secondary
transceivers 20 and secondary base stations 22 in service area B
28, the values are sent to the secondary central controller 30 who
then decides what the maximum power level each secondary
transceiver 20 and secondary base station 22 can use with this
channel as is described above.
FIG. 5 shows the method used to amplitude modulate the secondary
signals. Amplitude modulation is critical because: (1) The primary
signal strength must be measured by the secondary system and the
primary signal strength will often be lower amplitude than the
secondary signal, and (2) the interference caused by the secondary
signal must be clearly discernible compared to other causes of
reception problems experienced by the primary users.
FIG. 6 shows the signal level measured at the primary receiver. The
primary signal dominates since the secondary signal is very weak
because the secondary transceivers 20 and secondary base stations
22 are a significant distance away. However, if the secondary
signal amplitude were sufficient to cause interference, the primary
user would immediately know the cause because the impairments would
periodically cease. In contrast, interference caused by other
sources (such as amateur radios, CB radios, the user's equipment
degrading, weather conditions, lightning, primary system
transmission failures, misadjustment of the primary receiver, etc.)
would not have this pattern. It is an extremely critical property
that the primary user can immediately and reliably decide if the
secondary system is the cause of reception problems. Otherwise, the
secondary service provider will be liable for all reception
difficulties the primary users encounters that would have severe
economic implications.
FIG. 7 shows how the amplitude modulation between different
channels is organized. The off periods between channels are
staggered in time so that a single receiver at each secondary
transceiver 20 or each secondary base station 22 can monitor any or
all channels of interest. A unified off period would be highly
inefficient since the off period for each channel would have to
occur more frequently to allow the multiple channels to be
measured. The timing of the off periods is determined by the
secondary central controller 30 which periodically sends timing
information, a schedule of channel off periods and measurement
tasking to the secondary transceivers 20 and secondary base
stations 22.
In addition to measuring the primary background signal, each
secondary transceiver 20 and secondary base station 22 will send
data, receive probe signals and transmit probe signals. This
information is sent to the central controller 30 via the high
capacity network connecting the base stations 22. The notional time
line for a transceiver is shown in FIG. 8. For approximately 90% of
the time (899 ms), the transceivers will either transmit or receive
data using conventional media access protocols. In the next
interval, all secondary transceivers in the region go to a
receive-only mode for one millisecond, and receive primary signals
either in the channel they are using or on other channels. Then for
100 ms, the secondary transceivers will either transmit or receive
a probe signal at frequencies that the node is reserving for future
use or at frequencies the other nodes need. These times are the
nominal values and can be reduced for latency critical applications
or increased for highly mobile applications.
An additional innovation is a technique where the secondary
transceivers 20 and base stations 22 modify their behavior when
there are nearby primary receivers 10 or transmitters 12. Closely
spaced (10's of meters) radios are susceptible to significant
interference caused by non-linear mixing interference and
interference caused by unintended out-of-band transmitted signals
(phase noise, harmonics, and spurs). In the preferred approach, the
secondary transceiver and base station (20 and 22) measure the
spectrum and identify strong signals that indicate proximate
primary transceivers. Each secondary node (20 and 22) will then
avoid transmitting on frequencies likely to cause interference to
that specific radio. The frequencies to avoid can be determined
using a simple model that includes harmonically related signals and
cross products of the primary signal with the secondary signal. For
example, if a strong cell phone transmission is detected at 890
MHz, it can be inferred that a receiver is nearby tuned to 935 MHz
(cell phone channels are paired). The secondary system may have a
significant harmonic at 935 MHz when it transmits at 233.75 MHz
(4.sup.th harmonic is 935 MHz) and at 467.5 MHz (2.sup.nd harmonic
is 935 MHz). To avoid causing interference, this specific secondary
node would restrict its transmitted power at these frequencies to
low values or change to another frequency.
In broadcast bands (i.e. TV), the primary receiver's 10 local
oscillator leakage will be detected to determine if there is a
nearby receiver as shown in FIG. 9. These signals radiate from the
primary receiver's 10 antenna and have a power level typically -80
dBm to -100 dBm and can be detected at a range of approximately 10
m to 100 m. This is a well-known technique to detect TV
receivers.sup.1 but has never been applied to spectrum management
systems before. FIG. 9 indicates how the new secondary node 21
determines if there any primary receivers in close proximity to
reduce the chance of adjacent channel interference. A primary
receiver 10 located this close will receive the secondary signal
with a large amplitude and will have increased probability of
adjacent interference. Proximity is determined by measuring the
amplitude of continuous wave (CW) signals at frequencies associated
with leakage from receiver local oscillators (LO) set to receive
signals at the channels of interest. LO signals radiate from the
primary receiver's 10 antenna and have power level typically of -80
dBm to -100 dBm and can be detected at a range of approximately 10
m to 100 m. The frequency of the LO signals are standardized and
well known. The value is the channel frequency plus the primary
receiver's IF frequency. For broadcast NTSC TV the LO signals occur
at 45.75 MHz above the video carrier frequency. .sup.1U.S. Pat. No.
4577220, Laxton et al, Mar., 1986 and other patents.
To measure the LO signal amplitude, fast Fourier transform (FFT)
methods are used to create a narrow (.about.10 Hz) bandwidth
receiver. The LO signals are detected by searching for stable,
narrow bandwidth, continuous wave (CW) signals.
FIG. 10 and FIG. 11 show the secondary signal spectrum and how it
adapts to the noise level, which includes the primary signal when
the primary signal is an NTSC TV signal or another waveform, which
doesn't fill the spectrum uniformly. FIG. 10 illustrates how in
many cases the primary signal level will be too low for the primary
receiver 10 to use, but the signal level will be much higher than
the thermal noise level. If the secondary system desires to use
this channel it will have to increase the transmitted secondary
signal level so that the received signal has the requisite signal
to noise ratio for the secondary modulation type. However,
increasing the signal power will increase the probability of
interference to the primary user and may limit the secondary usage
of the channel. FIG. 11 illustrates when the primary signal level
is very low and the noise level is effectively that of thermal
noise.
In the preferred embodiment of this invention, the secondary signal
waveform is selected based on the interference measurements made by
the secondary transceivers 20 and secondary base stations 22. If
the interference measurements indicate that the primary signal is
below the threshold value used to declare the channel open for use
and the primary signal level is well above the noise level, then
the secondary signal spectrum is reduced to fit into gaps of the
primary spectrum (from 1.5 MHz above the channel start frequency to
5.5 MHz above the channel start frequency) as shown in FIG. 10. If
the interference measurements indicate that the primary signal is a
threshold value near thermal noise, then the secondary signal
spectrum is to fit the entire channel width shown in FIG. 11.
FIG. 12 shows the rules used to select the secondary waveform type.
In addition to changing the waveform based on the level of
interference, the waveform is also varied depending on the level of
multipath. In high multipath propagation conditions, it is well
known that inter-symbol interference severely degrades signal
transmission and forces certain waveforms and error correction
codes to be used. These waveforms are much less efficient
spectrally and transmit much fewer bits per second in a given
bandwidth of spectrum. In the preferred embodiment of this
invention, the waveform selection is based on the amount of
multipath encountered on the specific secondary link between the
secondary transceiver 20 and secondary base station 22. If the link
can be closed with a more spectrally efficient waveform, then that
waveform is used. Otherwise, a more robust but spectrally
inefficient waveform is used. In the prior art, the same waveform
is used for all links. Because the difference in capacity between
these waveforms can exceed a factor of 10, the secondary system
capacity can significantly be improved if a large fraction of the
links don't have severe multipath.
There are many types of waveforms that could be used to optimize
performance in a high multipath link or in high quality
(line-of-sight) link. FIG. 12 indicates certain waveform types
(OFDM/QPSK, rate 1/2 and OFDM/64QAM, rate 3/4) that are robust
against multipath. The invention disclosed here is not dependent on
these specific waveform types and others could be used.
FIG. 13 illustrates the secondary transceiver 20 and secondary base
station 22 radio architecture. A programmable modem 58 is used that
can rapidly switch between waveforms. The secondary transceiver 20
modem 58 is able to generate the probe waveform and the waveforms
in FIG. 12 can change between them in a few milliseconds. The modem
58 can digitize the intermediate frequency (IF) with at least 5,000
samples and perform an FFT to demodulate the probe signal. A tuner
54 is used that has a range of 54 MHz to 890 MHz when the secondary
channels are the TV broadcast bands. The invention disclosed here
is not limited to this band and is applicable to anywhere in the
spectrum. A controller 56 is used to control the modem 58, the
tuner 54, and the transmitter variable attenuator 62. The antenna
50, amplifier 52, and preselect filter 64 are multi-band devices.
The user device 60 accommodates voice, data or both.
FIG. 14 shows how the present invention mitigates inadvertent
interference and FIG. 15 provides a flowchart of the activities. A
primary user 10 experiences reception problems and because of the
secondary signal's amplitude modulation he or she immediately
identifies the problem source. Using a telephone or another rapid
electronic method (such as the Internet), he contacts a well-known
interference mitigation agent (either a person, a voice recognition
computer system, or an fully automated system) that provides
information to the secondary central controller 30.
The primary user reports his location, the channel with
interference and the time of the interference. The central
controller identifies all secondary transceivers 20 and secondary
base stations 22 within a distance X of the primary user active
within the time period in question, and identifies what additional
channels may have caused the interference due to adjacent channel
or image rejection problems. Using propagation and interference
models, the maximum power each secondary transceiver 20 and
secondary base station 22 is allowed to transmit, the probability
of each secondary node is calculated. The secondary nodes are
sorted by this probability. If the interference is still present, a
secondary central controller 30 tasks the most probable secondary
node to temporarily cease transmitting and then asks the primary
user if the problem has cleared. If not, the secondary central
controller 30 goes to the next probable node and repeats this
process (expanding the distance X as required) until the offending
secondary node is identified.
If the primary user had reported the interference as intermittent
(due to variations in the secondary traffic loading), the secondary
central controller 30 commands the secondary nodes to transmit for
each of the above tests instead of ceasing to transmit.
Once the secondary node causing the interference is identified, the
maximum transmit power level that node can transmit in that channel
is reduced until there is no interference. This is accomplished by
the secondary central controller 30 iteratively tasking the
secondary node to transmit signal at varying power levels until the
primary user reports no interference.
Secondary transceivers 20 and base stations 22 that are highly
elevated compared to the surrounding terrain have line-of-sight to
a large area and will have much lower propagation losses to the
surround primary nodes compared to secondary nodes that are at low
altitude. Because they are more likely to cause interference, they
are assigned frequencies that are the least likely to cause
interference as determined by the probe measurements described
above. To determine if a secondary node is elevated, the node
measures the strength of several primary signals (at different
frequencies) in the area as shown in FIG. 16. The primary signals
can be any fixed signal with high duty cycle and constant amplitude
received over a large area such as TV or FM broadcast signals. If
there are many signals above a certain threshold, then the node has
line-of-sight to a large region and is elevated. The exact
elevation distance is not determined nor is it required.
In some system applications, the frequency range of the secondary
system will not include the standard broadcast bands. The elevation
of a secondary node can still be inferred using signals from
primary cellular, PCS, or other systems (that are not constant
amplitude). These systems use frequency re-use schemes where
channels are assigned to different cell towers. If the node is
elevated, it will receive strong amplitude signals at many
frequencies within the frequency re-use scheme. If the node is not
elevated, it will receive strong amplitude signals at only one or
two frequencies within the frequency re-use scheme.
As mentioned above, the system will use a slightly different scheme
to allocate frequencies for mobile nodes. To determine if a node is
stationary or mobile, the system will periodically (approximately
once per second) measure the amplitude of background primary
signals. As shown in FIG. 17, the background signal amplitudes vary
significantly with position. Motions of a fraction of a wavelength
cause changes in background signals of several to up to 10's of
decibels. The secondary transceiver 20 periodically (approximately
every second) measures the amplitude of several background signals
from fixed, constant amplitude signals such as TV or FM broadcast
signals. If these amplitudes vary more a threshold amount, the
secondary transceiver 20 is declared to be mobile and higher
probing and measurements rates are made to more rapidly check that
the secondary frequency is available. This part of the invention
plus the feature to detect node elevation described above enables
the invention to continuously monitor the spectrum allocation
decisions at a rate suitable for mobile applications.
SUMMARY, RAMIFICATIONS, AND SCOPE
Accordingly, the reader will see that the method described above
allows efficient secondary use of spectrum while causing minimum
interference to the primary user. The method has minimal impact to
the choices of the secondary system could be added as an applique
to existing or planned communication systems. It requires no
modification to the existing primary user. The technology can be
economically built with existing component technology.
The invention will provide 100's of megahertz of spectrum to be
used which before was unavailable to new uses and will provide this
spectrum below 2 GHz which is the most useful portion for mobile
and non-line-of-sight applications. Because the method has minimal
effect on the present primary users, it allows a gradual transition
from the present fixed frequency based, broadcast use of the
spectrum set-up in the 1930's to the computer controlled, fully
digital, packet based, frequency agile systems coming in the near
future. With the advent of the Internet and the need for high-speed
connectivity to rural and mobile users, the present spectrum use
methods are inadequate and will not be able to meet this need. This
invention will provide spectrum for the new Internet driven demand
while not significantly impacting the present spectrum users.
The invention described here has many advantages. The technique
used by each secondary node uses multiple effective ways
(propagation models, measuring the primary signal level and
probing) to identify what channels are available. The technique of
amplitude modulating the secondary signals allows accurate
measurement of the primary signal levels while the secondary system
is operating. Using the special probe waveforms allows these
measurements to me made with minimal impact to the primary system.
Varying the secondary waveform greatly reduces the impact to the
primary system while increasing the capacity of the secondary
system. The methods to detect node elevation and node motion allow
for rapid checking and adjustment of spectrum allocations making
this technique applicable to mobile applications.
Although the description above contains many specifications, these
should not be construed as limiting the scope of the invention but
as merely providing illustrations of some of the presently
preferred embodiments of this invention. For example, the primary
system could be the present broadcast TV system. However, the
methods described here would be equally effective with sharing
between commercial and military systems, with sharing between radar
and communications systems and others.
Thus the scope of the invention should be determined by the
appended claims and their legal equivalents, rather than by the
examples given.
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