Radar Wifi Interference Summary 2008

Summary of the study titled Efficiency of Dynamic Frequency Selection and Other Possibilities of Meteorology Radar Jamming Avoidance [9]Authors: Horvth, Zoltn Micskei, Tibor Varga, Dvid Lukovszki, Csaba Abstract Sebagai bagian dari World Weather Watch saat ini Badan Meteorologi Klimatologi dan Geofisika (BMKG) mengoperasikan 27 Radar Cuaca C-Band sebagai upaya untuk meningkatkan akurasi pelayanan deteksi dini potensi cuaca ekstrim yang membahayakan jiwa maupun materi. Semakin tingginya penetrasi perkembangan pengguna WiFi (Wireless Fidelity) akses point di Indonesia, menyebabkan tingginya resiko interferensi frekuensi yang mengganggu operasional Radar cuaca C-band di frekuensi 5 GHz akibat dari penggunaan perangkat pemancar standar IEEE 802.11a The significance of the jamming is out of question, since it may seriously affect not only the efficiency of crucial, increasingly indispensable shortterm weather forecasts, but it might cause other unpredictable effects. DFS (Dynamic Frequency Selection) has become the most widespread solution to dissolve the interference issues between meteorological radar systems and WLAN devices. However, according to our investigations, in many cases this effort is insufficient. This paper focuses on to describe the problem, its explanation and significance. To approach the solution, issues of recent related standards, the regulations are examined regarding the radar jamming. The effects on users and vendor aspects are also mentioned.

May 20, 2008. Hungary, Budapest

Contents

Contents...........................................................................................................................2 1 Executive summary.......................................................................................................3 2 Weather Radar RLAN Jamming......................................................................................4 2.1 Definition of the problem...................................................................................4 2.2 Explanation of the problem................................................................................5 2.2.1 The reason of jamming within the frequency range..........................................7 2.3 Significance of the problem...............................................................................8 3 Recent solutions..........................................................................................................10 3.1.1 Examining the theoretical efficiency...............................................................10 3.1.2 Changes from ETSI EN 301 893 v1.2.3 to v1.4.1 ............................................13 3.1.3 DFS compliance of WLAN devices on the market............................................13 3.1.4 Conclusion......................................................................................................14 3.2 Elimination of jammings...................................................................................15 3.3 Current regulation in Hungary.........................................................................15 4 Possible future steps...................................................................................................16 4.1 Technical solution ideas...................................................................................16 4.2 Regulation and prevention...............................................................................17 4.2.1 Dedicated frequency band for the meteorological radars.................................17 4.2.2 Definite actions in case of illegal use..............................................................18 4.2.3 Dissemination.................................................................................................18 5 Conclusion...................................................................................................................19 References.....................................................................................................................20

mmary of the study titled "Analysis of the Efficiency of Dynamic Frequency Selection and Other Possibilities of Meteorology Radar Jamming Avoidanc

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1 Executive summaryAt present as part of the European weather forecast system, there are three working weather radars in Hungary under the supervision of HMS (Hungarian Meteorology Service). Radiation signals of jamming RLAN (Radio Local Area Network) transmitters result unusable radar response images. The layers and sectors appearing in the images are mostly caused by IEEE 802.11a standard WiFi devices located near the earth surface and operating within the radars frequency-range. This effect will be inevitable with the penetrating WiFi transmitters in 5GHz frequency band. The significance of the jamming is out of question, since it seriously affects not only the efficiency of crucial, increasingly indispensable short-term weather forecasts, but may cause other unpredictable effects. The root of the problem is that the Hungarian regulations allow the use of RLAN and WMAN (Wireless Metropolitan Area Network) devices in the 5470 -5725 MHz frequency band, providing that they do not by any means disturb the operation of the meteorological radars. According to the recommendation, wide-band devices are not to be used within a 30 km radius of the radars. A method has been standardized to solve this problem. DFS (Dynamic Frequency Selection) has become the most widespread solution to dissolve the interference issues between meteorological radar systems and WLAN devices. In practice a device is marked DFS compatible, if it suits to the DFS tests of the actual ETSI EN 301 893 standard. On the other hand, it is questionable whether this DFS compatibility provides enough protection for meteorological radars. According to our investigations, in many cases the DFS shows insufficient performance, since it cannot be adapted to different conditions such as the wide scale of physical properties of radar signals. It may also increase the problem that those who use their devices in compliance with the regulations may also jam the operation of radar, while those using their devices illegally, causes more significant damages. Besides, the majority of the available devices comply only outdated, 4-5 year old ETSI DFS standards. Consequently, we can state that the actual, version 1.4.1 ETSI standard published in July, 2007 still does not provide sufficient protection for meteorological radar systems in practice. To urge the solution for the problem different issues should be considered while the problem intensifies by the time. First, in our opinion even an updated DFS standard was not able to provide adequate solution for the radar jamming. Secondly, regulations could not solve the problem, comprehensively. Besides, vendors conform to the new standards and regulations slowly, and last but not least the users are not well informed enough and also malicious users aggregate the problem. Unfortunately there is no trivial solution for the described problem, but one may say that since neither DFS nor other solutions are capable of dealing with the problem globally and efficiently, therefore we have to achieve the dedication of the band for meteorological purposes only by co-operation with other national authorities at European Union level by convincing the device manufacturers of the advantages of it. It is true, that this is only a long term solution for the problem, but we have to start this process immediately for a prospective future reduction of the damages. The solution decreases the number of available WiFi-channels only to a negligible extent as compared to the present situation. However, in short term we have to concentrate on the filtering of jammings, the prevention of putting to the market and the distribution of unreliable devices by way of market-control and communication means, or the withdrawal from the market of the already marketed devices by campaigns, and the effective detection of the already installed jamming-causing transmitters.


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2 Weather Radar RLAN Jamming2.1 Definition of the problemAt present as part of the European weather forecast system, there are three working weather radars in Hungary under the supervision of HMS (Hungarian Meteorological Service): in Budapest, Napkor and Pognyvr. These radars measure the atmosphere precipitation in Hungary and its neighbourhood. Based on the information and pictures provided by the HMS, we first present the influence of the strays on a rough radar image then the published radar images.

Figure 2-1. Rough image captured by the Budapest radar The radar layers shown in the rough radar image above (See Figure 2-1) appear as a result of radiation signals of jamming RLAN (Radio Local Area Network) transmitters. Each colour designates different dBZ layers. These layers correspond to the intensity of the reflected signal, that is, the larger is the numerical value represented by the colour shown in the radar image (scale can be seen on the right), the higher is the received signal strenght. Based on this, we can estimate the quantity of the precipitation to be expected. In the case of the jamming layers, the colours indicate significant quantity of rain, so their influence is rather disturbing. It is also dangerous when the values from the real clouds superponal with the ones from the jamming (see in the left bottom of Figure 2-1) and as result we may came to a false conclusion regarding the quantity of the precipitation. This may cause significant problems in the weather forecast and pre-estimations. At HMS a temporary solution was found for the problem, the advantages and drawbacks of which will be discussed in Section 3.2. A precipitation intensity-map covering the entire country is prepared from the images captured by the three Hungarian radars. One radar covers an approximately 240 km territory in radius, therefore at the sections of the circles


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we may use the redundancy for filtering the jamming to cover the blind spots caused by temporarily non-functional radars or the terrain conditions. These composite pictures (See Figure 2-2) can be found at HMS website (4), they are public and updated in every 15 minutes.

Figure 2-2. Precipitation intensity-map without jamming The permanent WiFi jamming is filtered from these images, but sometimes the effects of the jamming still appear in these images as shown in Figure 2-3.

2.2 Explanation of the problem

Figure 2-3. Noisy precipitation-intensity map

The layers and sectors appearing in the images are mostly caused by IEEE 802.11a standard WiFi-devices located near the earth surface and operating within the radars frequency range.


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Figure 2-4. Radar operation During its operation the radar goes round keeping a specific altitude angle (elevation) or scans a given sector then raises the elevation. (See Figure 2-4. describing the operation of the radar). In the meantime it receives the reflected signals of hydrometeors (raindrop, ice), and from the distance calculated from time-related returns, it determines the outdoor absorption and the secondary absorption of the hydrometeor formations until a given distance and then it produces an image reflecting the values compensated by these absorptions. If a WiFi device is working within the radars frequency domain and happens to give signals while the radar dish with its approximately 10 beamwidth, looks in that direction, this additive signal is processed as well. Since these WiFi-devices are located near the earth surface, the jamming appears typically at 00-10 elevation degree, at which the atmosphere visible at the horizon approximately 240 km distance - is examined. Due to the narrowness of the main lobe (3dB beamwidth) of the radar antenna the jamming mostly appears only at 10-20 width. Consequently, in the radar image a narrow, almost continuous band or a very narrow sector appears. Contrary to the precipitation, the intensity of the above increases by departing from the radar. This is due to the WiFi device transmitting with permanent power, but doing so almost constantly, thus the radar almost always receives its signals. Since the radar by nature of its operation, assigns a well-defined distance to the receiving time, this causes the radial direction sector. However, along with the distance, it also makes compensation in the manner described above, so it assigns greater intensity to signals received at a later point in time farther signals but of identical power. This results in the radial course discolouration in the picture. It will not be radially continuous if the WiFi-device does not transmit permanently. That is why the resulting pattern typically begins at some 10 km distance from the radar depending on the power of the WiFi (the sensitivity layer is located here along with the compensation) and it continuously intensifies up to 240 kms. However, the radiation is not continuous, since even in case of maximum transmission speed, the temporal filling factor of the WiFi channel will not be 100%. (See Figure 2-5)

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Figure 2-5. Temporal filling of WiFi channel100 110

2.2.1 The reason of jamming within the frequency range mete

In the 5600-5650 MHz frequency range the IEEE 802.11a standard devices may have jamming effects on radars. Though the relating standard [3] does not mention the transmission at the frequency in question, but does not prohibit it either. Thus, such 802.11a standard devices were manufactured and are also available in Hungary, the transmission frequency of which can be set to this frequency band by way of certain software or hardware. The jamming may intensify in case of such devices with higher than permitted output or a more strongly concentrated transmit power in a certain beamwidth. The general channel distribution of WiFi-devices operating in the 5 GHz band can be seen in Table 2-1. Note that since the channel width is 20 MHz and the distance of the transmitting frequency is the same, there is no interference among the channels.rea1 1MA

Channel 36 40 44 48 52 56 60 64 149 153 157 161

Transmission frequency (MHz) 5180 5200 5220 5240 5260 5280 5300 5320 5745 5765 5785 5805

Minimum (MHz) 5170 5190 5210 5230 5250 5270 5290 5310 5735 5755 5775 5795

Maximum (MHz) 5190 5210 5230 5250 5270 5290 5310 5330 5755 5775 5795 5805

Table 2-1. 5 GHz channels used by 802.11a In our experience the following channels are also frequently used (See Table 2-2). Although these channels cannot be found in the 802.11a standards, the local regulations (Hungarian for example) may authorize their use for wireless access systems e.g. for WLAN (Wireless Local Area Network).


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Channel 100 104 108 112 116 120 124 128 132 136 140

Transmission frequency (MHz) 5500 5520 5540 5560 5580 5600 5620 5640 5660 5680 5700

Minimum (MHz) Maximum (MHz) 5490 5510 5530 5550 5570 5590 5610 5630 5650 5670 5690 5650 5670 5690 5710 5570 5590 5610 5630 5510 5530 5550

Table 2-2. Other channels used by 802.11a The Hungarian regulations [1] allow the use of the frequency bands seen Table 2-3 while not exceeding the specified maximal EIRP (Equivalent Isotropic Radiated Power). In some bands, different EIRP may be applied by automatic TPC (Transmitted Control Power) or without it.Channel 36-48 52-64 100-140 Transmission frequency (MHz) 5150 - 5250 5250 - 5350 5470 - 5725 Maximum average EIRP 200mW with TPC: 200mW; without TPC: 100mW with TPC: 1W; without TPC: 500mW Use Indoor Indoor Outdoor

Table 2-3. Regulation in 5 GHz band The Hungarian regulations [1] allow the use of RLAN and WMAN (Wireless Metropolitan Area Network) devices in the 5470 -5725 MHz frequency band, provided that they do not by any means disturb the operation of the meteorological radars. According to the recommendation, wide-band devices are not to be used within a 30 km radius of the radars. The width of the main spectrum (3dB) is about 1.25MHz in case of 0.8s impulse length, consequently, three 802.11a channels are dangerous for the operation of a meteorological radar: channel numbers 120, 124 and 128, which in turn represent the frequency bands between 5590-5610 MHz, 5610-5630 MHz and 5630-5950 MHz (see Table 2-2, marked parts). It is apparent from these values, that the above channels are dangerous because they interfere with the frequency bands used by the radars. Thus, if these devices do not have the DFS (Dynamic Frequency Selection) ability or they have it, but it does not operate properly, they will definitely disturb the measurements of the radars. It can be helpful, if we tune the radars located on the verge of two WiFi-channels by shifting its frequency ensuring that their entire spectrum fall within the same WiFi band. By using this method the extent of the jamming could be reduced almost by half, in case of a small number of jamming transmitters. This is highly recommended for the radars at Napkor and Pognyvr.

2.3 Significance of the problemThe significance of the jamming caused by the WiFi transmitters cannot be questioned, since it seriously affects the efficiency of crucial, increasingly indispensable short-term weather forecasts. It is significant not only because it may cause permanent jammings, but because it may cause other unpredictable effects.


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Valuable information may not be collected from certain areas by the aid of the radars because of the jamming, since the natural phenomena to be observed in such territories may be concealed by the jamming, causing some blind-spots. It may also increase the problem that those who use their devices in compliance with the regulations may also jam the operation of a radar, while those using their devices illegally, cause significant damage.


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3 Recent solutions

DFS (Dynamic Frequency Selection) has become the most widespread technological solution to dissolve the interference issues between meteorological radar systems and WLAN devices. However, according to our observations, in many cases this effort is insufficient. There are two standards, documents related to DFS: the IEEE 802.11h standard and the ETSI EN 301 893 directives. The IEEE 802.11h standard [5] is an amendment to the original 802.11 standard [2] which deals with the radio spectrum and power management operations in details. It defines new processes, message types and frame types to be implemented. Although the main function of the standard is to cooperate with European radar systems, it also affords a possibility to have a uniformly used radio spectrum, and to manage coverage or power consumption with transmit power control. The standard defines horizontal (between stations) and vertical (within a station) communication protocols, but it allows l the manufacturers to choose their own implementation. It does not even define the conditions (e.g. radar signal detecting), that start the extended functions in the devices. The ETSI EN 301 393 documents [6], [7], [8] contain information regarding these conditions. The ETSI EN 301 893 standards summarize the functional requirements, that every radio access network operating in the 5 GHz band has to meet. These requirements consist of the specification of transmitted signals, but also contain methods of spectrum management, such as DFS. In practice, a device is marked DFS compatible, if it suits to the DFS tests of the actual ETSI EN 301 893 standard. On the other hand, it is questionable whether this DFS compatibility provides enough protection for meteorological radars. In the following pages we try to find a solution for this problem.

3.1.1 Examining the theoretical efficiencyIn the following section the theoretical requirements defined in the newest ETSI standard [8] are revised for their capability to protect radar systems from WLAN interference.

3.1.1.1 Interference Threshold values

We have done some research to see whether the ETSI defined Interference Threshold values are sufficient practically enough to sense radar signals without causing interference at the radar. The calculations cover three common real life situations. In the first case the end-user operates a DFS compatible device with its default factory antenna, in accordance with the relevant the regulations [1]. In the second case the user attaches an antenna with higher gain, which still not exceeds the maximum allowed by the EIRP. The third case covers the situation when the user raises EIRP above the maximum permissible level. In our calculations we used the actual, measured receive sensitivity for WLAN signals of the radar (measured -95 dBm), the ETSI defined -64 dBm Interference Threshold value for Master devices with more than 200 mW maximum transmit power, and the maximum allowed 30 dBm EIRP in the pertinent frequency band. The results showed a minimum 23 dB reserve for the radar when using WLAN with maximum allowed EIRP. This means, that if the signal does not proceed equally in both directions, open-air attenuation can even be 23 dB lower towards the radar without suffering WLAN interference. To see whether this reserve is enough, field tests were made. During the tests the difference between the attenuation in the two directions never exceeded 7 dB, which could also have been caused by the minimal time difference between the measurements. In conclusion, the 23 dB reserve seems to be enough.


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If the user sets EIRP above the allowed maximum, using a common 10 dB antenna, the transmit power must be at least 43 dBm (which is 20 W) to cause interference at the radar. As a result, it can be stated, that power levels were chosen correctly in the ETSI standard.

3.1.1.2 Parameters of DFS test signalsHereby we summarize how the radar test signals defined by the ETSI correspond with the actual radar signals used by Hungarian radar stations. We have done the comparison based on the following attributes of the radar signals: pulse width pulse repetition frequency burst length Comparing the required signals for testing and the radar signals used in practice we found that only #2 test signals show high similarity with those radar signals which are used in Hungary. The attributes, which made the other test signals inadequate, were the following: too short burst length too high pulse width too high pulse repetition frequency All-in-all it can be stated, that real radar pulses are shorter, and their repetition frequency is lower, therefore it supposedly makes it more complicated to detect them, than the radar signals in the standard. Radar systems also exist with frequency hopping radar signals. The detection of these signals cannot be tested by the application of the ETSI standard. These facts can easily result in a device compatible with ETSI standards [8], but they will still not be able to avoid interference with real life radars.

3.1.1.3 Channel Availability Check TimeThe actual ETSI standard [8] defines the Channel Availability Check Time for DFS Master devices to check for radar signals during power up in a minimum of 60 seconds. In our opinion this check time is insufficient. For Hungarian radar systems a full measuring period takes 15 minutes during normal operation. This has the consequence that the radar only transmits to an exact direction on a vertical angle with full power just once every 15 minutes. On every other turn, vertical angle changes, so the received power at the WLAN reduces. This is caused by the 1 degree beam width of the radar. From this we can come to the conclusion, that during the 1 minute Channel Availability Check the DFS capable WLAN device will not surely receive radar signals with full power. As a result, it may not detect the radar operating in the selected channel. During our tests we discovered that a DFS compliant device can detect radar signal with chance when there is not any traffic in the channel in question. Radars usually operate for a long time (years) with unmodified main parameters like frequency, transmit power, location. Thus there is a high possibility that a radar system will operate on the same frequency at the time of powering up a WLAN device and during its lifetime. From these two statements it follows that the most important tests should be run during power up to detect radar signals in a WLAN device. To provide a more successful detection, the actual 1 minute long Channel Availability Check Time should be raised to 10-15 minutes. As a side effect, this modification could raise a new issue. DFS capable WLAN devices can not only change channels when detecting a radar, but also interference from other WLAN networks on the same channel can make them to do so. If a DFS Master device


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chose a channel from the 5250-5350 MHz or the 5470-5725 MHz band during channel switch, it would have to wait another 10-15 minutes before the effective transmission could start. This situation could paralyze wireless traffic too often and for too long in areas with high 5 GHz RLAN density (which is, we must say, not quite typical yet).

3.1.1.4 Channel Closing Transmission TimeThere is a part of the ETSI standard that discusses Channel Closing Transmission Time measuring, which is the summarized duration of the frames transmitted after a radar signal has been detected. We experienced that the measurement is too difficult according to its importance. The 260 ms limit defined by the standard is much higher than the duration of a radar burst received upon a radar turns, thus it does not provide protection for the radar at the first turn. Practically, to measure Channel Move Time would be sufficient, which is the time of the last transmission after detecting a radar signal. If this time is short enough, the radar will not be disturbed by the WLAN signal upon its next turn. By our opinion, measuring Channel Closing Transmission Time should be dismissed from the ETSI standard.

3.1.1.5 Non-Occupancy PeriodNon-Occupancy Period is the time after detecting a radar signal while no transmission is allowed on that channel. The standard defines 30 minutes for Non-Occupancy period, which does not seem necessary. If radar operates on a given frequency, it will operate on the same frequency after 30 minutes, for a week, or even for months or years later, anyway. Therefore, it would make more sense if after detecting and clearly identifying a radar signal, the WLAN device could not reuse that channel at all. The lock of it could be dissolved only by a software reset or reboot. Another, maybe more useful solution would be to prevent transmitting on the channel where radar signals (or other disturbing traffic) were detected, until the WLAN device has used every other available channel at least once. Consequently, we do not see the reason for defining the Non-Occupancy Period as the amount of time.

3.1.1.6 Problems with Slave devices without Radar Interference Detection FunctionThere can be a real life situation, when the radar signals can only be heard by a Slave device without Radar Interference Detection Function, In this case the Salve transmission generates interference at the radar. In this scenario the antenna of the DFS Slave device and the radar are in line-of-sight and the radar falls in the main beam of the antenna. The opposite of this situation is, when the antenna of the Master device, e.g. is placed on an outdoor wall of a building. The radar falls absolutely out of the Master devices main beam, and because of the building theyre not in line of sight. In this case it is possible that the DFS Master device receives the radar signal with even 50-60 dB less power than the DFS Slave does. The maximum allowed EIRP for Slave devices without Radar Interference Detection Function is 200 mW, 23 dBm. This raises the reserve calculated prior with 30 dBm EIRP to 30 dBm. This means, that, if the received strength of the radar signal would be 30 dBm less in the Master than in the Slave, the WLAN would still switch channels before the radar senses it. Since the received radar signal can be weaker by even 50-60 dBm at the Master with Radar Interference Detection Function, the signal of the Slave might appear at the radar, and the WLAN will not switch to another channel.

ummary of the study titled "Analysis of the Efficiency of Dynamic Frequency Selection and Other Possibilities of Meteorology Radar Jamming Avoidan

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3.1.2 Changes from ETSI EN 301 893 v1.2.3 to v1.4.1Changes after version 1.2.3 have significantly affected the DFS part of the ETSI EN 301 893 standard. According to our experiments, previously some sections were not finished or could be misconceived. The new parts and changes referring to DFS of the actual 1.4.1 version [8] are summarized below. During the comparison of the consecutive versions we came to the conclusion, that due to the changes in the new version, a device which has passed the tests of the older version would not necessarily pass the new one.

3.1.2.1 Test transmission sequences

The previous version specified a test transmission sequence that consists of regularly transmitted packets with a given interval (e.g. 2ms). The packets have fixed length (e.g. 0,2 ms), therefore, the transmission should exceed a minimum activity ratio of 10 %. The new version defines a slightly different test transmission for the DFS tests. The test transmission sequence shall consist of packet transmissions that together exceed the transmitter minimum activity ratio of 30 % measured over an interval of 100 ms. This change makes the test sequence generation easier, because of its wider interval. The other effect of the change is that due to the higher activity ratio DFS compliant devices need to detect radar signals in worse circumstances.

3.1.2.2 Manufacturer requisites

The manufacturers need to care about the following when creating DFS compliant devices: The DFS functions which the devices support have to be detailed. DFS controls (hardware or software) related to radar detection shall not be accessible to the user, so that the DFS requirements can neither be disabled nor altered. At first, it is important to know which DFS functions should be tested in the device. The second issue is also an important one. We have met devices in which DFS radar scanning was an option to choose from the users interface.

3.1.2.3 More radar test signals

The older standard defined only three radar signals for the tests, while the actual standard has more than 20 different predefined radar signals which are required to use for the tests. Detecting radar signals is a very difficult issue from hardware side. One cannot solve the problem by changing the software on a device, since changing the software on a device can not make it detect other types of radar signals. The extended radar test signal set contains radar signals that were not available in previous standards. Devices that complied with the older standards have not been tested with these recent signals.

3.1.2.4 Determining whether the tests were successful

There has been a change in the way the results should be rated. According to the new standard, each test has to be repeated by using 20 different radar test signals. The device passes a test if it detects at least 60 % of the test signals and then halts the transmission. Based on the previous standard the periodically repeating radar bursts had to be generated, while based on the actual standard a single test applies to detect single radar burst. This highly eases the rating of the results, since Was the test successful, if the device did only detect the third radar burst? is not an issue any more.

3.1.3 DFS compliance of WLAN devices on the marketAs part of the study we tried to figure out whether the 802.11a 5GHz WLAN products which were sold in Hungary comply with ETSI standards. As much certificates as possible were


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collected from the public places and from vendors. The documents certify which standards or normative the device did comply during the factory tests. Regarding to some devices, if these certificates were missing, we tried to find some information about them on the homepage of their vendors or in the user manuals. We managed to collect more than 50 certificates of different devices (APs, client stations). Analyzing the collected pieces of data it can be stated, that in altogether only two devices comply with the ETSI EN 301 893 v1.3.1 standard [7], which contains the more recent, corrected DFS tests. All the other devices complied with the previous standards only. During our experiments we also found one device that only became DFS compliant after a firmware upgrade. Even then DFS was only an option; it could be activated on the management web pages. Figure 3-1 shows the effects when DFS option is switched off by a certain user. In the Figure, besides the default WLAN interference, the effect of the generated WLAN traffic is shown at the bottom right corner when DFS is disabled. The majority of the available devices comply with the out-of-date, 4-5 year old ETSI DFS standards only. This is a serious issue, since such a device which complies with older standards would not necessarily passes the tests of the recent ones.

3.1.4 Conclusion

Figure 3-1. Effects of the manually enabled or disabled DFS

While analyzing the actual 1.4.1 version of the ETSI EN 301 893 standard, the following were formed: Interference Threshold Values were chosen appropriate (see 3.1.1.1). The radar test signals defined by the standard should be more alike the actually used ones, which detection is more difficult (see 3.1.1.2). Channel Availability Check Time is too short (see 3.1.1.3). Measuring Channel Closing Transmission Time after successful radar detection is too difficult compared to its importance. It could be neglected form ETSI standards (see 3.1.1.4). The 30 minutes set for a minimum of Non-Occupancy period does not bear a no relation with radar operation. It must be redefined (see 3.1.1.5).


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Certain WLAN DFS Slave devices are not required to detect radar signals, which may cause constant interference at the radar (see 3.1.1.6). In Summary, we can state that the actual version 1.4.1 ETSI standard published in July, 2007 still does not provide sufficient protection for the meteorological radar systems. The evolution of the ETSI standards has also been examined, and the actual version 1.4.1 was compared to the most supported version 1.2.3 of the ETSI standards (see Section 3.1.2). At present, the mass of the devices, available on the market comply with the outdated version 1.2.3 of the ETSI standard. Because of the extended requirements in the version 1.4.1 standard older devices would not always pass the tests. There are devices which are not DFS compliant out-of-the-box, and they only become that after a firmware upgrade. Some devices allow the user to enable or disable DFS compatibility. ETSI standards in themselves never can make new devices compatible. Old devices will never be DFS compatible, and they may cause interference any time. A more sophisticated solution is needed to settle the situation.

3.2 Elimination of jammingsThe HMS experts developed a program, with the aid of which the jammed bands can be automatically cut from the images from a pre-given direction. The result can miss important information, which can be corrected only visually. This information can be also supplied by the program by interpolation from the image pixels located next to the concealed sector. If we try to filter these jamming signals artificially, there is the risk that we filter not only jammings, but some real phenomena, as well. With this correction the majority of the jamming can be eliminated, but we pay a stiff price for it: we may lose useful information. A jamming transmitter causes approximately 20-100 loss, which in case of 5-10 transmitters constitutes a highly significant part of the entire 3600. This indicates that the problem cannot be solved in the way described above, since it is only the matter of time for the number of the transmitters to increase so much that due to the critical level of jamming we will not be able to gain useful information by using the radar. There is an extremely high risk of the above in case of smaller, but suddenly breaking storms. This matter gains even more significance if we take into account that air transport navigation also uses this information to a certain extent, thus such a mistake may claim hundreds of lives.

3.3 Current regulation in Hungary

The information material titled Wide-Band Data-Transmission by Radio Access Devices published by the Hungarian National Communications Authority (NCA) in 2005 sets out in detail the rules relating to radio systems which operate in the 5.6 GHz band. The purpose of this regulation is to avoid the interferences of radars and WLAN devices. The document emphasizes that the Jamming of the radars is strictly prohibited! It recommends that no WLAN devices should be used within a 30 km radius of a radar in the 5600-5650 MHz frequency band. Since the document contains only recommendations, it does not provide a legal ground for official proceedings.


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4 Possible future stepsWe strived to present an approach to the possibilities of averting radar-jammings both from technical and legal aspects, which is set forth below. It is to be made clear in advance that the outlined technical proposals are only ideas, providing solutions in theory and they are not confirmed in practice. Furthermore, it is to be noted that we could not prevent the problem by using technical solutions, but could find only subsequent solutions for already existing (and because of the development of radar technology consequently recurring) problems. Moreover, some technical ideas are not supported by any legal background.

4.1 Technical solution ideas

We may attempt to eliminate the problem either actively or passively. By way of passive methods we try to filter the existing jammings more efficient without interfering with the operation of WLAN devices. The detailed description of the methods is included in Section 3.2 of the original study [9]. Those radars which operate on the verge of the two WiFi-channels can be tuned to such frequency that their entire spectrum is within one WiFi band. By this method, in case of only a few jamming transmitters the extent of the jamming can be reduced almost by half. The detection of the jamming can be solved with the help of a simple band-pass filter. The spectrum of the radar (1.25 MHz) is narrower than the spectrum of the jamming (20 MHz), so it does not conceal it, thus it is possible to find out whether a WLAN signal arrived at the very instant of the receipt. It is advisable to indicate the backscattering radar signal as unreliable if it is received simultaneously with the jamming. With the timely filtering of the returning signals to the radars the probability of the jamming effects can be reduced to a small extent, or the presence of the jamming can be easily established. If we are aware of the parameters of the measurements (e.g. elevation angle) and the other environmental variables (e.g. the height of the antenna) we may determine such a short time-window in which the reflection of the radar signals is expectable. If a signal arrives outside of this time-window, it must be a jamming. In case of using the complementary secondary channels of the radar measurements (e.g. satellite snapshots) the jamming signal of WLAN does not appear, so the fact of the jamming can be verified or the jamming can be filtered. However, it may happen that in the course of the filtering with a secondary channel we may detect a nonexisting incident falsely or do not detect an existing one. When using data from different sources, it may cause difficulties to transform the pictures as well as the use of very complicated picture-recognition algorithms. From the radar signals we may produce three-dimension pictures in which the jammings are clearly separable. By the use of more radars the detection of the jammings or the possibility of filtering them can be increased, thus the possibility of making mistakes can be reduced. If we compare several successive radar images we can find out which signals are jammings and which ones represent real precipitation. Although we can filter the jamming this way, the effectiveness of this method depends on the pictureprocessing algorithm, so its reliability can be only moderate. The above mentioned passive methods serve the subsequent reduction of jamming effects, but they do not eliminate them, thus they provide only a temporary solution for the problem. The other group of the solutions is the active interference with the operation of the devices in such a way, that the jamming does not appear at the radars. These methods are discussed in detail in Section 3.3 of the original study [9].


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By sending out certain types of WLAN-packets we may have an influence on the operation of jamming devices. With the help of these methods it might be possible to eliminate the interference caused by those WiFi-devices which do not support DFS or may support it, but are unable to sense the radar signals. The number of jamming sources can be reduced statistically with band allocation by using their general more common than DFS - frequency-selection function. The jamming generation method can be implemented without significant investment, however to select the useful signals from the jamming signals generated by them may cause problems at the radar or slow down its operation. The main advantage of the active methods is that we may gain room for the radar signals by using them in such a way that the jamming WLAN signals appear in significantly fewer numbers. However, the methods are not perfect and they need significant interference with the operation of the radar or they might change its operation.

4.2 Regulation and prevention

This Section briefly summarizes the legal and dissemination means available to mitigate the problem. In general we may say that legal regulation in itself does not mean a solution and it exerts its effect only in the long run, however, it may provide sufficient legal ground for official proceedings and it may have compelling effects on the manufacturers of WiFi devices. It is advisable to support the regulations not only by legal, but by communication means, which may reach their goals within a short term and may directly influence the users. Further on we list those options which might help to solve the problem.

4.2.1 Dedicated frequency band for the meteorological radars

Unfortunately there is no trivial solution for the described problem; it can be handled by applying several methods at the same time. We must also face the fact that the situation is growing worse, since more and more devices are getting installed, thus the band is becoming more saturated, which further intensifies the problem. This phenomenon now occurs not only in Hungary, but in almost every European country which uses modern meteorological radar system. Therefore the introduction of a legal regulation capable of radically solving the problem should be considered. It seems reasonable that we handle the matter where it had occurred upon the use of the same frequency (see Section 2.21). Sharing the frequency can be terminated by appropriate legal regulations, excluding the WLAN-devices from the frequency band used by the radars (5600-5650 MHz). However, the implementation of the above regulation is influenced by several factors: International approximation of laws; Hungary, as a member of the European Union has an obligation to fully approximate laws, hence this problem cannot be regulated domestically, but it is necessary to solve it at EU level. The pressure by foreign authorities may also further this matter, so it is necessary to closely cooperate with them and act together. The interests of WiFi-device vendors; Profit-orientated manufacturers are not interested in the changes, since they have to implement the changes in each of their devices. Their burden may be reduced due to the fact that they would not have to extend the difficult DFS-function to the radars, it would be sufficient if their products could cooperate with other WiFi-devices properly. Slow effect; Unfortunately, it causes further difficulties that the effects of the legal regulations do not become evident immediately. The legislation and the preliminary discussions, the switching time for the manufacturers of the WiFi-devices is a 2-3 year long period in itself. However, for already existing devices the conditions specified by the regulations may not be performed subsequently, thus in this case we would have to wait for a firmware-update, or a replacement of the device by the user.


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It seems likely that there will be a lot of opposition from the industry and community, who will object to the reduction of the already small number of freely available channels. However, we may easily verify that from the 3 un-joint channels in the 2,4 GHz band and from the approximately 26 channels in the 5 GHz band only 3 would be eliminated, which is only 10% (see Section 2.2.1). A prohibition of the use of a certain channel for other purposes is also necessary because in short term the saturation of the 5GHz band is predictable, similarly to the tendency observed in the past years in the 2.4 GHz band. On the other hand in this case the jamming-causing interference will grow also in the common bands. The above phenomenon is intensified by licensing of the outdoor use of the 5 GHz band. One may say that since neither DFS nor other solutions are capable of dealing with the problem globally and efficiently, we have to achieve the dedication of the band for meteorological purposes only by co-operation with other national authorities at European Union level by convincing the device manufacturers of the advantages. It is true, that this is only a long term solution for the problem, but we have to start this process immediately for a prospective future reduction. The solution decreases the number of available WiFi-channels only to a negligible extent as compared to the present situation.

4.2.2 Definite actions in case of illegal useResults can be reached also by severe official proceedings and communication. The main point is that in cases when someone violates the regulations the Authority is to take definite measures against such person. The matter must be considered as a precedent, and must be communicated that way. It can be done intensively e.g. involving the press but the effect can be reached without such measures as well, since the rumours may spread among the infringing users, thus everybody will buy, install and operate devices more cautiously, and fewer people will decide on the risky, illegal operation of a certain device. Naturally, it is advisable to take the above measures along with the provision of appropriate information, indicating how someone can install and operate the suitable devices.

4.2.3 DisseminationSignificant results can be reached by informing the users. Certainly the success of this depends on the efficiency of the related campaign and the user-supporting solutions. These solutions can be the following: Repertory of devices; The Authority should handle a white- and blacklist on which the tested devices are listed. Domestic emblem of certification; The campaign can be further strengthened if the devices which appear on the white-list are marked with distinctive marks. It would be expedient to use a logo which is displayed next to the description of the specific device. Guidance for installation; It would also be useful to assist the users with installing their devices in such way that they do not cause jamming or do not violate any regulations. This could include a 1-2 page long guide-book containing the DFS and radio settings for the WiFi devices and the instructions and restrictions for locating the antennas. Campaign; It is essential that the above elements be properly communicated within the framework of a campaign. For this purpose it would be advisable to use distributors, the media and the website of the domestic Authorities. Without publicity the effects of these initiatives might be only marginal. In summary we may say that dissemination of the problem and possible solutions could be a suitable tool for mitigating the problem, either by way of the implementation of white- and blacklists, the introduction of the emblem certifying suitability, or the printed or on-line guidebooks for installation.


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5 Conclusion

In summary of the aforementioned in this paper, we may state that DFS in its present form is unsuitable to fulfil its tasks, therefore a more precise and severe standard is required, however the effects of the above may appear in the long term only and it probably will not provide a perfect solution for the problem. The only reassuring long term solution would be the exclusive use of the band in question for meteorological purposes. Although, the capabilities of WiFi devices would be only slightly reduced by the prohibition from such exclusive band, yet we are to face significant opposition. We believe that we must implement this solution with international cooperation, and the first steps should be taken here and now. However, in short term we have to concentrate on the filtering of jammings, the prevention of putting to the market and the distribution of unreliable devices by way of market-control and communication means, or the withdrawal from the market of the already marketed devices by campaigns, and the effective detection of the already installed jamming-causing transmitters.


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References1] IN HUNGARIAN, National Communications Authority (NCA), Directory of Frequency Allocations Szlessv Adattvitel Rdis Hozzfrsi Eszkzkkel, Wide-band data transmission using radio-based access equipments. Budapest, 2005. jlius 1. http://www.nhh.hu/menu4/m4 2/szeles tajek 20050705.pdf 2] ANSI/IEEE Std 802.11, 1999 Edition (R2003) - Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications http://standards.ieee.org/getieee802/802.11.html 3] IEEE Std 802.11a-1999(R2003) - Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications; High-speed Physical Layer in the 5 GHz Band http://standards.ieee.org/getieee802/802.11.html 4] Hungarian Meteorological Service (HMS) Radar snapshots http://www.met.hu/omsz.php?almenu id=weather&pid=kepek&mpx=0&kps=1&pri=3 5] IEEE Std 802.11hTM-2003 - Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications; Amendment 5: Spectrum and Transmit Power Management Extensions in the 5 GHz band in Europe http://standards.ieee.org/getieee802/802.11.html 6] ETSI EN 301 893 V1.2.3: Broadband Radio Access Networks (BRAN); 5 GHz high performance RLAN; Harmonized EN covering essential requirements of article 3.2 of the R&TTE Directive http://pda.etsi.org/pda/home.asp?wki id=fp74snqhjgLMNRMUsd1Mj 7] ETSI EN 301 893 V1.3.1: Broadband Radio Access Networks (BRAN); 5 GHz high performance RLAN; Harmonized EN covering essential requirements of article 3.2 of the R&TTE Directive http://pda.etsi.org/pda/home.asp?wki id=S tIK@6'xemnvosvFNifO 8] ETSI EN 301 893 V1.4.1: Broadband Radio Access Networks (BRAN); 5 GHz high performance RLAN; Harmonized EN covering essential requirements of article 3.2 of the R&TTE Directive http://pda.etsi.org/pda/home.asp?wki id=.mNAFQ'bWh46687Ba5Sby 9] IN HUNGARIAN, Horvth Zoltn, Micskei Tibor, Dr. Seller Rudolf, Varga Dvid, Lukovszki Csaba, Lukovszki Lszl, A WiFi DFS-megolds hatkonysgnak elemzse s a radarzavartats elhrtsnak lehetsgei, Efficiency of Dynamic Frequency Selection and Other Possibilities of Weather radar Jamming Avoidance Study in Hungarian, National Communications Authority (NCA), Hungary, 2007


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Radar Wifi Interference Summary 2008