Patent Application
20060171327
This invention is generally related to wireless communications, and more particularly to coping with interference in a wireless communications network.
Certain wireless LAN (“WLAN”) products, such as products based on the IEEE 802.11 standard, operate in unregulated spectrum. One problem associated with operating in unregulated spectrum is the potential of encountering interference from other devices. Regulated spectrum is relatively free of interference because unlicensed products which operate in the regulated spectrum can be removed from the marketplace. Even in unregulated spectrum there is at least a possibility of negotiating strategies for coping with interference from standards-compliant devices via standards organizations. However, some of the potential interfering devices are not standards-compliant, and some are not even communications devices. There is therefore a need for techniques and devices for coping with interference in unregulated spectrum.
In a wireless network, a technique for coping with interference from another device that adversely effects communications includes: differentiating between the interference and normal communications; determining whether the interference originates from a known type of device; and selecting a remedial action based at least in-part on whether the interference originates from a known type of device.
One technique for differentiating between interference and normal communications is to induce a quiet interval during which normal communications are ceased. Another technique is to employ a parallel demodulation engine. The parallel demodulation engine offers the advantage of obtaining interference signal data without reducing bandwidth in order to implement a quiet interval. However, the parallel demodulation engine may be less effective in comparison with use of a quiet interval when the interference signal is similar to normal communications signals. The quiet interval may be assembled from temporally non-contiguous quiet gaps between normal communications. The assembled quiet interval offers the advantage of avoiding relatively long interruptions in normal communications, and may interleave with predefined gaps in standard communications protocols. For interference sources which exhibit a pulse waveform the interference sources are analyzed based on pulse period and duration. This format offers the advantage of being relatively compact.
The invention offers improved WLAN performance when interference is encountered. By recognizing a particular source of interference it may be possible to continue communications on the channel by implementing counter measures calculated to overcome the identified interference source. Maintaining communications may be less disruptive than breaking a communications link and re-authenticating on a different channel.
Referring to
The access point (100) is adapted to recognize and respond to interference (106) generated by a device (114) other than the end station (102). For example, the access point includes a table (108) of interference profiles in memory (110) which are indicative of particular sources of interference. The memory (110) also includes a table (112) of counter measure plans which specify actions to be taken when a particular source of interference is recognized. Each counter measure plan specifies at least one remedial action, such as altering transmission characteristics and changing to an alternate communication channel. The remedial actions may be arranged hierarchically such that multiple actions are attempted in a predefined order until a satisfactory result is obtained. Each interference profile in the table (108) is associated with at least one counter measure plan in the corresponding table (112), and multiple interference profiles may be associated with a particular counter measure plan.
The first step (200) in the technique employed by the access point (100) to cope with interference is recognizing the existence of the interference (106). The access point may recognize the interference by analyzing the signal received at the access point. For example, a quiet interval may be implemented such that the signal received at the access point does not include normal traffic (116) between the access point and end station, but rather comprises any existing interference, e.g., signal (106). An alternative to use of the quiet interval is to analyze the combination of normal traffic signal (116) and interference signal (106). For example, a parallel demodulation engine (120) may be programmed to identify, from the combined signal, types of interference that differ recognizably from actual data in the channel. Alternatively, recognition of a combined signal which has a relatively high proportion of noise or is not in a format specified by the communications protocol being utilized may be used as an indication of the presence of interference. Alternatively, some communications protocols specify use of periodic communications between an access point and end station primarily to verify that the communications link is operational. Such a protocol may also be used to recognize the existence of interference when the communications link fails for purposes of the present technique.
Once the access point or wireless end station recognizes the existence of interference it then captures a sample (118) of the interference as indicated in step (202) in order to attempt to identify the source of the interference. The sample may be captured by storing a portion of the interference signal (106) received at the access point or wireless end station. The received signal, which is analog, may then be sampled and converted to digital format for processing. Each sample measurement is associated with a time stamp indicating the relative time at which the sample was obtained. Hence, the resulting data comprises sets of energy magnitude measurements and time stamps.
Because there are different possible sources of interference, and the characteristics of the interference associated those sources may vary, the sampling rate and period are selected to capture a sufficient sample to identify all known potential sources of interference stored in the digital patterns in memory. The sample (118) is then compared with the interference profiles in table (108) to identify a match, or the absence of a match, as indicated by step (204). Alternatively, an adaptive algorithm may be employed to adjust the sampling period and rate until a match between the sample and an interference profile is located or eliminated as a possibility. If a matching interference profile is located in table (108) then the associated counter measures plan is selected as indicated by step (206). As discussed above, the counter measures plan may include one or both of changing transmission signal characteristics as indicated by step (208) and changing to an alternate operating channel as indicated by step (210). If no matching interference profile is located then the access point changes to the alternate operating channel as indicated by step (210).
Referring
The duration of the quiet interval and the sampling rate within the quiet interval are selected to enable identification of interference that matches any of the stored interference source profiles in table (108). In the case of an interference signal exhibiting a pulse waveform the sampling rate may be selected such that there is a minimum of 2 samples within the shortest expected pulse duration. The shortest expected interference pulse duration for some Bluetooth devices for a 2.4 GHz WLAN band is 300 microseconds, which corresponds to a maximum sample period of 150 microseconds. In other words, both the sampling period and the sampling duration would be 150 microseconds maximum. The accuracy of the time stamps also effects the sampling period requirement. For example, if the time stamps have an accuracy of +/−40 microseconds then the sampling period should be selected to be no greater than 110 microseconds so as to provide the sample period at 150 microseconds maximum. In terms of a specific example, a microwave oven might emit a >2.5 millisecond pulse which repeats every 16 milliseconds, i.e., has a 16 millisecond period. In this example searching only for pulses longer then 2.5 milliseconds by creating 2.6 millisecond quiet periods has a relatively small probability of intercepting the interference, but creating a 17 millisecond quiet interval has a 100% probability of intercepting a 2.5 millisecond pulse.
Techniques may be employed to mitigate loss of bandwidth due to implementation of the quiet interval. In particular, the quiet interval may be dynamically adjusted or interrupted based on network conditions. For example, recurring scheduled long duration measurements may be interrupted when network bandwidth utilization is high due to valid traffic requirements. Similarly, measurements may be adjusted or interrupted when the access point has a high load value, or when memory depth is filling and depleting rapidly, or when transmit and retries errors are high or low.
Referring to
The technique described above for representing pulse interference sources will now be described with respect to a specific example. Given a microwave oven at 2 meters distance, with peak energy in the channel at −24 dBm, a peak energy point P1 occurs at a time T1 (time=0 Sec). The energy attributable to the microwave drops below a noise floor of −81 dBm between successive energy peaks. Having collected data for a predefined window, the energy values are compared in order to identify the highest value, P1, T1. The samples preceding P1, T1 are then parsed until a sample at P0, T0 (time=−3.7 mSec) with energy value below the noise floor is located. The samples following P1, T1 are also parsed until a sample P2, T2 (time=3 mSec) with energy value below the noise floor is located. The pulse duration is determined by calculating the time between T0 and T2, which is 6.7 mSec. The accuracy of the technique may be modified by interpolation or dithering. For example, because the samples at T0 and T2 may not have the exact energy values as the noise floor, as interpolation between sample on either side of the noise floor can be employed to enhance accuracy. The pulse period is determined by calculating the time between consecutive energy peaks, i.e., T12-T11. Consecutive peaks may be identified by searching the collected samples for samples having higher energy values than the samples immediately preceding and following. Spurious samples and secondary interference sources may be filtered by using only consecutive peaks within a predetermined range. For example, if a first detected peak has energy value xdBm then only other peaks having energy value X +/−10 dBm are considered to be related peaks. Alternatively, or in addition to the energy level comparison, more than two consecutive peaks may be compared to determine that the pulse period is constant. Any peaks which fall outside the pulse period constant by greater than a predetermined value are discarded. Again, interpolation and dithering techniques may be employed to increase accuracy.
In view of the techniques for filtering peaks described above, it will be apparent that multiple, different sources of interference that occur simultaneously on the channel can be detected and identified. In other words, the inventive technique can differentiate between the interference sources by filtering peaks, and hence generate from the received signal representative pulse period and pulse duration data sets describing each individual interference source.
While the invention is described through the above exemplary embodiments, it will be understood by those of ordinary skill in the art that modification to and variation of the illustrated embodiments may be made without departing from the inventive concepts herein disclosed. Moreover, while the preferred embodiments are described in connection with various illustrative structures, one skilled in the art will recognize that the system may be embodied using a variety of specific structures. Accordingly, the invention should not be viewed as limited except by the scope and spirit of the appended claims.
A claim of priority is made to U.S. Provisional Patent Application Ser. No. 60/649,799, entitled Interference Counter Measures for Wireless LANs, filed Feb. 3, 2005, which is incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 60649799 | Feb 2005 | US |
