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.

Description

This application it 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 (˜10 Hz) receiver compared to a wide bandwidth (several MHz) broadband receiver, thus it can be transmitted at much lower (˜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^th harmonic is 935 MHz) and at 467.5 MHz (2^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 receivers¹ 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. 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 (˜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 ½ and OFDM/64QAM, rate ¾) 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.

Claims

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,

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

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.

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.

12. The method of claim 11, wherein the test signal is a probe signal.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

24. The method of claim 23, wherein the allocation list is determined based at least in part on a regulatory database of emitters.

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.

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.

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.

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.

29. The method of claim 25, further comprising receiving a test interval assignment from the central controller.

30. The method of claim 29, further comprising transmitting a predetermined probe signal during the test interval assignment.

31. The method of claim 30, wherein the predetermined probe signal comprises at least one continuous wave (CW) signal.

32. The method of claim 30, wherein the predetermined probe signal comprises a BPSK waveform.

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.

34. The method of claim 33, wherein the metric value comprises an amplitude of the received test signal.

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.

36. The method of claim 25, further comprising the step of performing at least one transmitting or receiving information over the allocated channel.

37. The method of claim 25, further comprising the step of transmitting a signal of a predetermined waveform type over the allocated channel.

38. The method of claim 37, wherein the predetermined waveform type comprises an orthogonal frequency division multiplex (OFDM) signal.

39. The method of claim 37, further comprising the step of amplitude modulating the signal of a predetermined waveform type.

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.

41. The method of claim 40, wherein the test signal is a probe signal.

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.

43. The method of claim 40, further comprising transmitting a second test signal during at least a second portion of the test interval.

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.

45. The method of claim 40, wherein receiving the first test signal comprises receiving a plurality of continuous wave (CW) tones.

46. The method of claim 40, wherein receiving the first test signal comprises receiving a BPSK waveform.

47. The method of claim 46, wherein the BPSK waveform comprises a pseudo random sequence.

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.

49. The method of claim 40, wherein the step of determining the metric comprises determining an amplitude.

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.

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.

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.

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.

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.

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.

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.

57. The method of claim 56, further comprising the step of restricting transmitted power based at least in part on the received signal metric.

58. The method of claim 56, further comprising the step of changing to another frequency.

59. The method of claim 56, wherein the predetermined frequency offset is a harmonically-related frequency offset.

60. The method of claim 56, wherein the predetermined frequency offset comprises an adjacent channel offset.

61. The method of claim 56, wherein the predetermined frequency offset comprises a local oscillator frequency offset.

62. The method of claim 56, wherein the predetermined frequency offset comprises an IF image related offset.

63. The method of claim 56, wherein the predetermined frequency offset comprises a transmit/receive pair frequency offset.

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.

65. The system of claim 64, wherein the controller is one of the plurality of transceivers.

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.

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.

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.

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.

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.

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.

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.

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.

74. The transceiver of claim 72, further configured to measure the signal strength in a channel outside of a bandwidth of the channel.

75. The system of claim 72, further configured to perform at least one transmitting or receiving information over the allocated channel.

76. The system of claim 72, further configured to transmit a signal of a predetermined waveform type over the allocated channel.

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.

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.

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.

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.