Pulsed Interference Reduction Employing Intelligent Blanking TECHNICAL FIELD The present disclosure relates to a system and method for mitigating interference from an interference signal. In particular, but without limitation, this disclosure relates to mitigation of interference within received wireless signals through pulse blanking.
BACKGROUND
The detection of wireless signals can be affected by interference from nearby transmission systems, for instance, a co-sited pulsed interferer radio system. Minimising the impact of co-sited interferers, including pulsed interferers, is a common problem for mobile platforms having multiple radio systems.
Filtering can be used to discriminate against out-of-band interference, but filtering is of limited benefit where the interference itself is in-band (within the frequency bandwidth of the receiving channel).
Likewise, mobile platforms offer very limited scope to vary the siting of new antennas to reduce such interference. In the case of aircraft platforms, a large number of radio systems must compete for the limited number of appropriate locations on the fuselage space, with the result that separation between antennas -and hence radio frequency isolation -is limited. The example of an aircraft, however, is but one option. Similar performance degradations occur in other applications.
It is often difficult to mitigate pulsed interference from a co-sited radio system. One approach to the mitigation of co-sited pulsed interferer radio systems, particularly on military platforms, is to use a central suppression system (or bus) which receives and distributes suppression pulses supplied over cabling by each pulse emitter on that platform.
These suppression pulses (or, more accurately, bursts -since they last for the entire emission period, which may comprise a series of pulses) are sent by the transmitter to notify the receiver of transmissions. These pulses can be used to suppress the impact of the interference on other electronic systems by artificially lowering the receive sensitivity during the burst. This therefore blanks or otherwise protects the receiver inputs from the interference.
This form of blanking can affect performance, for instance, by reducing the throughput of the blanked system. Furthermore, suppression over a burst may last longer than is strictly necessary as dictated by the emitted waveform. Furthermore, such systems require that each emitter that produces pulsed interference also provides a suitable suppression pulse to allow effective blanking of the interference. This means that the emitters must be directly connected to the receiver in order to provide the require control signals for blanking. Furthermore, emitters -including those installed/retrofitted after initial commissioning -must be compatible with this common distribution system and need to provide a suitable suppression pulse.
The use of suppression buses is much less common in civilian aircraft, particularly aircraft for the air transport market, and in such cases individual receive systems requiring a suppression pulse must first detect the pulsed interferer from its radiation.
This mechanism is less reliable than the use of a suppression bus, depending on the received interferer level.
Accordingly, there is a need for an improved means of countering in-band interference from pulsed emissions.
SUMMARY
According to a first aspect there is provided a system for mitigating interference from an interference signal, wherein the system comprises blanking circuitry configured to: receive one or more signals, each signal being having been detected at a corresponding antenna, each signal comprising a desired component deriving from a desired signal; and for each received signal: detect an interference component within the received signal, the interference component deriving from an interference signal; determine a strength of the desired component relative to the interference component; in response to the strength of the desired component relative to the interference component being less than a threshold, blank the received signal and outputting a blanked signal; and in response to the strength of the desired component relative to the interference component being greater than or equal to the threshold, inhibit blanking for the received signal and thereby output the received signal in unblanked form.
By blanking based on the strength of an interference component relative to the strength of a desired component in the signal, the system is able to intelligently adapt to the properties of the received signal to avoid blanking when interference strong enough to be detrimental to performance. This helps to mitigate interference whilst increasing the throughput (efficiency) of the system but reducing the amount of time that the signal is blanked.
Performing this analysis on components of signal(s) received over antenna(s) allows the system to adapt to changing interference over time in unpredictable environments. This is particularly useful for scenarios where the signal derives from an external source that is unable to provide advanced warning of the signal or interference ahead of receipt.
Blanking can refer to the system stopping the detection or receipt of a signal over the corresponding antenna for a period of time. This may be by disconnecting the antenna from signal processing circuitry or through any other means of reducing the sensitivity of the system with respect to the antenna.
According to an embodiment, the strength of the desired component relative to the interference component is a ratio of the desired component to the interference component.
According to an embodiment, the ratio is a ratio of an average or root mean square of the desired component to an average or root mean square of the interference component. This allows the system to adapt to changing interference levels whilst avoiding triggering the blanking based on very short-term fluctuations.
According to an embodiment, the interference signal is a pulsed interference signal and the blanking circuitry is further configured to generate, for each received signal, a corresponding blanking signal based on detected pulses of interference for the respective received signal for use in blanking the respective signal.
According to an embodiment, generating, for each received signal, the corresponding blanking signal comprises, for each received signal: generating an initial blanking signal including a blanking pulse for each detected pulse of interference within the respective received signal; and filtering the initial blanking signal to produce the corresponding blanking signal, wherein the filtering comprises discarding each blanking pulse or set of blanking pulses that does not conform to an expected pulse profile. This helps to avoid false positives that can result in blanking of non-interference (i.e. desired) signals.
According to an embodiment, discarding each blanking pulse or set of blanking pulses that does not conform to an expected pulse profile comprises one or more of: discarding each blanking pulse that has a length that falls outside of a predetermined range; discarding each set of blanking pulses that does not conform to any of a set of one or more expected pulse timing profiles; and discarding each set of blanking pulses that occur at a frequency that is greater than a predefined frequency.
According to an embodiment, the one or more signals includes a plurality of signals and the system is configured to generate a shared blanking signal for use in blanking each of the plurality of signals. By making use of spatial diversity, a more accurate blanking signal can be generated, thereby more effectively mitigating interference.
According to an embodiment, the shared blanking signal is generated by selecting the blanking signal that corresponds to the received signal that contains the strongest interference component out of the plurality of received signals.
According to an embodiment, the shared blanking signal is generated by performing a logical AND function on the blanking signals for the plurality of received signals.
According to a further aspect there is provided a method for mitigating interference from an interference signal, wherein the method comprises: receiving one or more signals, each signal being having been detected at a corresponding antenna, each signal comprising a desired component deriving from a desired signal; and for each received signal: detecting an interference component within the received signal, the interference component deriving from an interference signal; determining a strength of the desired component relative to the interference component; in response to the strength of the desired component relative to the interference component being less than a threshold, blanking the received signal and outputting a blanked signal; and in response to the strength of the desired component relative to the interference component being greater than or equal to the threshold, inhibiting blanking for the received signal and thereby outputting the received signal in unblanked form.
According to an embodiment, the strength of the desired component relative to the interference component is a ratio of the desired component to the interference component.
According to an embodiment, the ratio is a ratio of an average or root mean square of the desired component to an average or root mean square of the interference component.
According to an embodiment, the interference signal is a pulsed interference signal; and the method further comprises generating, for each received signal, a corresponding blanking signal based on detected pulses of interference for the respective received signal for use in blanking the respective signal.
According to an embodiment, generating, for each received signal, the corresponding blanking signal comprises, for each received signal: generating an initial blanking signal including a blanking pulse for each detected pulse of interference within the respective received signal; and filtering the initial blanking signal to produce the corresponding blanking signal, wherein the filtering comprises discarding each blanking pulse or set of blanking pulses that does not conform to an expected pulse profile.
According to an embodiment, discarding each blanking pulse or set of blanking pulses that does not conform to an expected pulse profile comprises one or more of: discarding each blanking pulse that has a length that falls outside of a predetermined range; discarding each set of blanking pulses that does not conform to any of a set of one or more expected pulse timing profiles; and discarding each set of blanking pulses that occur at a frequency that is greater than a predefined frequency.
According to an embodiment, the one or more signals includes a plurality of signals and the method comprises generating a shared blanking signal for use in blanking each of the plurality of signals.
According to an embodiment, the shared blanking signal is generated by selecting the blanking signal that corresponds to the received signal that contains the strongest interference component out of the plurality of received signals.
According to an embodiment, the shared blanking signal is generated by performing a logical AND function on the blanking signals for the plurality of received signals.
BRIEF DESCRIPTION OF THE DRAWINGS
Arrangements of the present invention will be understood and appreciated more fully from the following detailed description, made by way of example only and taken in conjunction with drawings in which: Figure 1 shows an example of coupling paths between air traffic control transponder antennas and air to ground antenna arrays on a commercial aircraft; Figure 2 shows the Air Traffic Control (ATC) interference and Air-To-Ground (A2G) signal coupling for the underside of the aircraft of Figure 1; Figure 3 shows a pulse blanking system according to an embodiment; and Figure 4 shows a flow chart for a method of detecting and blanking interference according to an embodiment.
DETAILED DESCRIPTION
The embodiments described herein pertain to methods aimed at minimising the impact on the performance of a radio receiver system of non-predictable pulsed interference emissions in the same frequency band from co-sited radio systems. Interference from such emissions can be reduced by intelligently blanking the emissions only during each pulse, rather than across the entirety of the period over which the pulses are detected. Embodiments achieve this effectively by exploiting signal differences derived from multiple, spatially separated antennas. Having said this, it should be noted that some of these techniques can also offer some reduction in the impact of pulsed interference emissions when the receiver has access to only one antenna. For instance, throughput can be improved, regardless of the number of antennas, if blanking is applied only when the signal to interference ratio is less than a threshold level.
Blanking, as discussed herein, is where the receive sensitivity is raised for a period of time to avoid the receipt of interference signals. This may be achieved by zeroing the signal or labelling the signal as unreliable, such that the signal is not output by the system.
Whilst the methods described herein are applicable to any wireless interference scenario, a specific example that encounters problems with co-sited pulsed interferers is an air-to-ground (A2G) downlink. That is, transmissions from a base-station to an aircraft terminal. An A2G communications link may be based on the 3GPP Long Term Evolution (LTE) standard operating as part of a Complementary Ground Component (CGC) service within the S-band Mobile Satellite Service (MSS) frequency allocation (2170-2200 MHz receive). This can be affected by pulsed interference resulting from the second harmonic of the aircraft's high-power Air Traffic Control Radio Beacon System (ATCRBS) / Mode-S transponder (2180MHz).
It should be noted that, the abbreviation ATC is used herein to relate to both an air traffic control radar beacon system (ATCRBS -typically mode-A/C) and/or Mode-S operation, including use as part of the Traffic Collision Avoidance System (TCAS).
As discussed above, issues with pulsed interferers can be found in A2G wireless transmissions operating in the 2170-2200 MHz MSS (receive) band. These problems can be caused by the aircraft's air traffic control (ATC) transponder. Whilst ATC transponders have nominal emissions at 1090MHz, the second harmonic of these transmissions lies within the A2G receive band at 2180MHz.
Figure 1 shows an example of coupling paths between air traffic control (ATC) transponder antennas and air to ground (A2G) antenna arrays on a commercial aircraft. The aircraft comprises two A2G antennas 110, 112 for wirelessly communicating with base-stations on the ground. The A2G antennas 110 are located on the underbelly of the aircraft. A first A2G antenna 110 is located towards the front of the aircraft and a second A2G antenna 112 is located towards the rear.
The aircraft further comprises two ATC transponder antennas 120, 122 for transmitting to air traffic control systems. A first ATC transponder 122 is located on the underbelly of the aircraft and a second ATC transponder 120 is located on the top of the aircraft.
Coupling paths are formed between each of the ATC transponders 120, 122 and each of the A2G antennas 110, 112.
For a Class 1 ATC transponder (i.e. for aircraft operating at altitudes above 15000ft, or at a maximum cruising true airspeed >324 km/h), the minimum ATC transmit power is 125W (51dBm) and maximum transmit power is 500W (+57dBm).
The minimum radio-frequency (RF) attenuation at the 2nd harmonic frequency (2180 MHz) is specified to be at least 50dB for Mode A/C transponders, and 60dB for Mode-S transponders.
Assuming a minimum separation between ATC and A2G antennas of 2m, the free space path loss at 2180MHz is approximately 39dB. Thus, for OdBi nominal antenna gains, the received power of the ATC Mode-S second harmonic at the A2G antenna port may be as high as -42dBm. By contrast, the reference sensitivity of an LTE waveform is typically much lower (in the region of -90 to -100dBm).
Thus, without mitigation, the high peak power and potentially high repetition rate of the ATC transponder interference under typical ATC interrogation scenarios (interrogation rate, ATC mode mixture, etc.) can result in severe degradation of the radio receiver performance (e.g. increased error rate, reduced throughput, etc.).
A first embodiment of the present method in wireless communications involves the improvement of data throughput in a broadband Air To Ground (A2G) communications link, where the link between the ground station and aircraft is affected by pulsed interference emissions generated by other avionics systems on the same aircraft or from ground stations. In a more specific example, the pulsed interference emissions are harmonics of the nominal emission frequencies of the other aviation systems caused by nonlinear elements in the relevant transmission chain (including antenna).
Conventional suppression systems suppress entire emission 'bursts' of pulses, where individual bursts may comprise a sequence of shorter on/off pulses. ATC emissions are examples of bursts of on/off pulses. Blanking across the entirety of the period over which the pulses are received reduces the throughput of the system by preventing reception of data, even during the intervening periods between each pulse, where no interference is present.
In light of the above, embodiments of the invention aim to only blank for each 'on' pulse in the burst rather than the whole burst (the whole sequence of on/off pulses). This offers significant improvement in data throughput. In addition, blanking over shorter periods can reduce the negative affect on the receiving system. For instance, digital radio receivers utilising Forward Error Correction (FEC) codes are typically better able to accommodate a short loss of input signal than a longer one.
Furthermore, to provide further improvement, embodiments described herein recognise that blanking of every interferer pulse may not always the best option, since the impact of any interference will depend on the signal to interferer ratio at the particular antenna port. Accordingly, embodiments described herein use blanking intelligently, applying blanking only where it results in an improved performance (that is, generally, at lower signal to interference ratios).
It should be noted that this method of intelligent blanking of pulses will provide benefit for the single antenna case; however, use of two or more antennas, each with different signal to interference levels due to their spatial diversity, provides additional improvement. For instance, where multiple antennas are utilised, one received signal may be blanked whilst another might not. The unblanked signal can then be utilised during detected interference to increase the throughput of the system.
Figure 2 shows the ATC interference and A2G signal coupling for the underside of the aircraft of Figure 1. This illustrates the advantages of blanking using multiple antennas.
Two A2G remote radio heads (RRHs) are located on the underside of the aircraft.
Each RRH is a transceiver that is connected to a respective set of A2G antenna elements. A front RRH has two antenna elements Al, A2 and a rear RRH also has two antenna elements A3, A4. The antenna element polarisations in each pair are orthogonal to each other. The front RRH is located closer to the ATC antenna than the rear RRH.
It can be seen that the received ATC interferer power at each antenna array/RRH is different but their amplitude ratio and phase difference are essentially constant, being predetermined by the airframe geometry and antenna siting. The same may also be true for each individual antenna element in MIMO antennas, after accounting for any polarisation losses for the case of the dual-polarised arrays. However, the A2G powers are essentially the same at each array, but the A2G signals will experience differing delays, depending on the relative direction (azimuth and elevation) of the serving ground station.
The example embodiment of the invention makes use of the spatial diversity provided by multiple antennas to more effectively detect and blank pulses of interference.
This embodiment is described with reference to the antenna arrangement shown in Figure 1. The plane has two A2G antennas 110. A first antenna (antenna 1) is located closer to the ATC antenna than a second antenna (antenna 2). Each antenna is connected to a respective receive branch.
Whilst Figure 1 has multiple ATC antennas, Figure 2 shows a single ATC antenna for simplicity. In the present embodiment this relates to the first ATC antenna 122 located on the underside of the plane in Figure 1. It will be appreciated that a similar coupling arrangement will be established between the second ATC antenna 120 (located on the roof of the aircraft, above the first ATC antenna 122) and the two A2G antennas.
Antenna 1 will detect a signal (S1) which is made up of two components. A first component represents signals from the ATC antenna (ATC1) and a second component represents LTE air to ground (A2G) communication signals from a base station (LTE1): S1 = ATC1 + LTE1 where ATC1 and LTE1 represent the complex amplitudes of the respective signal components.
Similarly, antenna 2 will detect signal (S2) comprising a first component associated with air traffic control (ATC2) and a second component representing LTE air to ground communication signals (LTE2): S2 = ATC2 + LTE2 where ATC2 and LTE2 represent the complex amplitudes of the respective signal components.
Given that the distance between antenna 1 and antenna 2 will be minimal compared to the distance to the serving base station, the LTE signals from each antenna will be approximately the same (LTE = LTE1 LTE2).
Having said this, the ATC signal at antenna 1 will be much greater than the ATC signal at antenna 2, as it is proportionally much closer to the ATC antennas than antenna 2. That is, ATC2 = a * ATC1 where a is a ratio with a « 1. This means that the signal to interferer ratio will be greater at antenna 1 than at antenna 2.
In a first implementation, which is generally applicable to any in-band pulsed interferer, the impact of the interferer on A2G receiver performance is reduced by only blanking the 'on' pulses of the burst of pulses and then secondly employing intelligent blanking of the receiver inputs during interferer pulses, with blanking implemented differently for each receiver antenna branch, dependent on the particular signal to interferer ratio at that branch.
At low wanted signal levels the strength of the interference pulse (e.g. the ATC 2nd harmonic pulse) will be much higher than the wanted signal and the pulse will be relatively easy to detect. However, detection will fail at high wanted signal levels where the wanted signal is comparable to or larger than the interference signal. This embodiment uses the fact that in an A2G system, multiple antenna locations are often provided to mitigate shadowing by the airframe, as shown in Figures 1 and 2.
These multiple antennas can be used to improve interference suppression as multiple versions of the signal plus interferer plus noise are present in the receiver system, thus the level at which the interference detection fails is different for each antenna element.
Figure 3 shows a pulse blanking system according to an embodiment.
The system includes two antennas 310, 315. Each antenna 310, 315 is connected to a respective receive line (antenna branch). Each receive line includes a respective band pass filter 320, 325, a respective signal converter (a downconverter and analogue to digital converter) 330, 335, a respective delay line 340, 345 and a respective intelligent blanking module 350, 355.
The second half of each receive line (from the output of each signal converter 330, 335) is connected in parallel to an interference detector 360 that is shared between receive lines. The interference detector 360 provides control signals (blanking signals) for controlling each intelligent blanking module 350, 355.
The output of each intelligent blanking module 350, 355 is connected to a modem 370.
Each intelligent blanking module 350, 355 operates as an on/off switch that is controlled by the output of the interference detector 360. This allows each receive line to be blanked during pulsed interference. When a receive line is blanked the respective intelligent blanking module 350, 355 isolates the antenna 310 from the modem 370 (e.g. by opening a switch).
Each band pass filter 320, 325 filters out high and low frequency signals so that only signals within the required frequency band for the communication channel are allowed through the filter. In the present embodiment the band pass filters 320, 325 are LTE receive (RX) band pass filters including 2180 MHz. More precisely, each band pass filter has a passband of between 2170 MHz and 2185 MHz, corresponding to a single spectral allocation in the MSS band. Having said this, alternative communication schemes to LTE may be utilised and alternative frequencies may be filtered (i.e. alternative pass-bands might be used).
Each signal converter 330, 335 comprises signal processing circuitry forming part of the respective receive chain. This includes a downconverter to convert the respective received signal from the carrier frequency to a lower frequency; this can either be at a low intermediate frequency or at baseband. One or both of the signal converters 330, 335 may be receivers, such as LTE receivers. Accordingly, the signal converters 330, 335 may be configured to convert a signal received at a carrier frequency in the region of 2170-2185 MHz to baseband. Having said this, the signal converters 330, 335 may operate according to an alternative communication scheme.
The modem 370 is configured to modulate and demodulate signals in addition to acquiring and tracking signals. In the present embodiment, the modem 370 refers to the receive part of such a modem. Accordingly modem 370 is configured to filter and demodulate the baseband signals converted by the signal converters 330, 335. In the present case, the modem 370 is an LTE modem configured to work with LTE signals. Having said this, the modem 370 may operate according to an alternative communication scheme.
The modem 370 is configured to output demodulated data via an output. The modem 370 is configured to employ spatial diversity by selecting between the signals from the receive lines. This may be based on rules, for instance, selecting the signal with the highest signal to noise plus interferer ratio (SINR or SNIR) or selecting the strongest signal.
The interference detector 360 receives a respective output signal from each of the signal converters 330, 335. The interference detector 360 is configured to detect when interference is present in each of the input signals and, if it is present, output a respective blanking signal for the duration of the detected interference. This can be based on a threshold applied to signal envelope strength and/or power of the interference signal (e.g. the magnitude of the strength or power of the interference signal).
When the interference signal exceeds the threshold, the detector 360 outputs a signal indicating that a pulse has been detected (e.g. a blanking signal). When the interference signal does not exceed the threshold, the detector 360 outputs a signal indicating that a pulse is not detected (e.g. turns off the blanking signal). This decision may be achieved via a comparator.
The interference detector 360 is also configured to determine the relative strength of the interference to the desired signal. The interference detector 360 is able to determine whether to blank a given signal depending on the relative strength of the interference within the signal. This ensures that the signal is only blanked when necessary, thereby increasing the throughput of the system over alternative blanking systems. In addition, the interference detector 360 may be further configured to filter out blanking pulses that do not meet a predetermined pulse profile. This helps to avoid false positives that can result in blanking of non-interference (i.e. desired) signals. This filtering shall be discussed in more detail below.
Respective blanking signals output by the detector 360 control the each interference module 350, 355 to blank the receive line for the duration of each detected pulse. That is, for each signal, a respective blanking signal is provided to the respective intelligent blanking module 350 to blank the respective signal. The respective blanking signal for each receive line may be based on detected interference within the specific receive line, or may be formed as a shared blanking signal that is utilised to control each receive line in the system. The shared blanking signal may be selected based on the strength of the interference component within each receive line, for instance, the blanking signal deriving from the receive line experiencing the strongest interference may be selected. Alternatively, a combined blanking signal may be formed from the respective blanking signals from each receive line, for instance, by performing a logical AND function on the blanking signals.
In the present embodiment, the interference detector 360 is configured to detect pulsed interference from air traffic control (ATC) signals. Accordingly, the interference detector 360 in the present embodiment is an ATC pulse detector. Having said this, the method described herein may be applied to any form of pulsed interference.
A delay line 340, 345 is incorporated into each receive line to compensate for processing and transmission delays within the interference detector 360 and thereby ensure optimum pulse blanking.
In the present embodiment, the intelligent blanking modules 350, 355 are located after the signal converters 330, 335 but before the modem 228. This can help provide improved blanking. Potential dispersion of interference pulses across the receive chain can be mitigated by reducing the delay in the receive line.
In an alternative embodiment, the intelligent blanking modules 350, 355 are located ahead of the band filters 320, 325, signal converters 330, 335 and modem 370. By putting the intelligent blanking module 320, 325 at the input additional protection is provided to the receive chain. Furthermore, this avoids dispersion of the ATC pulse through the receive chain circuitry.
Whilst the embodiment of Figure 3 has an interference detector 360 that is shared between multiple receive lines, alternative embodiments exist where multiple interference detectors 360 are provided, one for each antenna/receive line.
Accordingly, in this embodiment, each interference detector 360 generates a respective blanking signal. These signals can be output to a shared blanking signal generator that generates a shared blanking signal for the system. In contrast, all of these functions are implemented by the interference detector 360 of Figure 3.
Whilst the embodiment of Figure 3 shows only two antennas and two receive lines, any number of antennas and receive lines may be implemented, with pulse blanking being applied to each receive line.
By only blanking on detected pulses, rather than across the entirety of the period of the pulse-train, throughput is improved. Whereas LTE terminals can be severely disrupted by ATC pulse interference, the LTE protocols themselves are relatively tolerant to signal interruptions (blanking) equal to ATC pulse durations. Furthermore, throughput can be improved by only blanking on receive lines for which the relative strength of the interference signal is above a predefined level.
Figure 4 shows a flow chart for a method of detecting and blanking interference according to an embodiment. This method may be performed by the pulse detector 360 and one of the intelligent blanking modules 350, 355 of Figure 3.
A signal is received from a respective antenna 410. For simplicity, the method of Figure 4 shows the processing of a single signal; however, multiple signals may be processed in parallel, as discussed above.
The presence of the pulsed interferer on/off pulses is then detected 415. As discussed above, the presence of the pulsed interferer on/off pulses is detected in the interference detector 360 which is connected to each antenna port (see Figure 3). Accordingly, the signal may not be received directly from the antenna, but may undergo some filtering and modulation before pulse detection occurs. The detection can be achieved at baseband in digital circuitry, but it is also possible for detection to be performed in analogue or hybrid circuitry. The detection method not only detects the presence (or probability) of pulsed interference, but also estimates both the Root Mean Squared (RMS) signal and the RMS of peak interferer levels (see below).
An interference pulse may be detected each time the received signal exceeds a predetermined threshold. The predetermined threshold may be based on an average signal level or peak signal level. For instance, the threshold may be a fraction of the peak interference (e.g. half of the peak ATC signal). The peak interference (and therefore the threshold) can be determined during calibration of the system.
In the case of ATC second harmonic pulsed interference on an aircraft, interferer levels are not expected to vary significantly between bursts except, where successive bursts originate from different ATC antennas (normally two such antennas are provided on an aircraft -again to mitigate shadowing effects). Hence the detector may be adapted to consider two interferer levels at each A2G antenna port independently. This may be achieved by applying two or more different threshold ranges for detecting interference.
If no interference is detected, then the signal from the antenna is not blanked 420, and therefore the signal is output without alteration. In the embodiment of Figure 3, the signal would be output to the modem 370.
If interference is detected, then the strength of the interference signal and the strength of the desired signal are determined 425. These values may be determined as by determining the Root Mean Squared (RMS) of the signal (RMSs) and the RMS of peak interferer levels (RMS;).
A blanking signal for the signal is then formed 430. This is generated based on the detected interference at the respective antenna. The blanking signal is Ion' for as long as interference is detected within the signal. The blanking signal is set to 'off' when no interference is detected.
Where multiple antennas are utilised, each signal is assigned its own blanking signal.
A robust composite blanking signal can be formed based on the individually detected interference for each antenna port. For instance, a composite blanking signal may be formed by combining the individual blanking signals via a logical 'AND' operation. This reduces the chances of a false positive in the event that interference is only detected on a small number of the antennas.
In an alternative embodiment, a shared blanking signal used for blanking the system is selected from the set of blanking signals from the antenna ports. For instance, the signal with the strongest interference can be utilised for the detection of pulses and generation of blanking pulses, as this offers more reliable detection.
As shown in Figure 1, the A2G front antenna on an aircraft may typically have a much larger ATC interference level due to being located closer to the ATC antennas.
Accordingly, this antenna may be selected for use in detecting the interference pulses. In a dynamic system that blanks interference from different sources, the signal for interference detection may be selected dynamically, in real time.
Next a decision is made as to whether to apply this blanking to the received signal for each antenna port based on the current signal to interferer ratio estimate at that port.
That is, the ratio of the strength of the signal to the strength of the interference signal is determined and, if the ratio is not less than a threshold level, then the signal is not blanked (the signal is output 420 without blanking). If the ratio is less than the threshold, then the signal is blanked 440. The blanked signal is then output 450.
This novel approach ensures that blanking itself is not the cause of unnecessary performance degradation when the wanted signal is comparable or greater than the interference signal -which may be the case at one or more of the antenna ports.
The method of Figure 4 can be applied at multiple antenna ports simultaneously. A decision can be made for each antenna port (for each received signal) with regard to whether blanking is required for the respective signal. A shared blanking signal may be applied to each of the signals selected for blanking. The shared blanking signal may be chosen based on the relative strength of interference at each antenna port (e.g. the blanking signal from the antenna port with the strongest interference is shared between all antenna ports) or a composite blanking pulse may be formed by performing a logical AND function on all blanking signals.
A further novel method is the use a priori knowledge of the features of the pulses and bursts of pulses to further enhance the blanking method. This can ensure that interference detection and blanking signal generation is more robust.
According to this embodiment, the pattern of detected interference pulses is compared to one or more expected interference patterns and a blanking signal is formed based on whether the detected interference pulse matches an expected pulse profile. This may occur during step 430, blanking signal generation. This method may apply one or more of the following rules: * pulses that do not fall within an expected range of pulse lengths (e.g. too short or too long) can be flagged as non-interference (e.g. non-ATC) pulses, * pulses that do not conform to any expected pulse burst timing profiles can be rejected, * the frequency of detected interference pulses can be used (e.g. via a detection rate monitor) to inhibit blanking signal generation if the rate of interference is too high.
This pulse filtering method can be applied as a two-step process, where an initial blanking signal is formed for the respective receive line before the blanking signal is filtered in order to remove pulses or sets of pulses that do not conform to an expected pulse profile. This then produces a blanking signal for that receive line. The blanking signals for multiple receive lines may be filtered or combined in order to determine the shared blanking pulse for application across multiple receive lines.
Once the blanking pulse is determined, adjustments may be made to adjust the start and end timing of the pulse detections to compensate for any internal path latencies and differences in the propagation delay between the different antennas.
Finally, conventional diversity combining at the baseband receiver of the modem is used to obtain the best overall signal to interference plus noise. For instance, where the radio selects the best antenna port (or ports for a MIMO system) taking into consideration both signal to noise and signal to interference (e.g. via the signal to noise plus interferer ratio, SNIR) or based on a performance metric such as CQI (channel quality indicator).
Blanking during the interferer bursts improves performance by preventing the receiver from entering saturation -which could result in slow recovery times. Furthermore, the blanking methods described herein are particularly effective when applied to the signals conforming to LTE protocols. These protocols are relatively tolerant to signal interruptions (blanking), particularly those equal to ATC pulse durations. This is in part due to coding gain and hybrid automatic repeat request (HARQ) processing. Having said this, large interference pulses affect LTE significantly due to the effect of signal peaks on the modulation and coding scheme. Accordingly, individual pulse blanking is suited to application in LTE communication for mitigating ATC pulses.
It is observed that for simple blanking (blanking for every detected pulse) throughput is always reduced, whereas for intelligent blanking (as described herein) peak throughput is restored at higher SNIR by inhibiting blanking where it is not needed. The methods described herein therefore provide an improved signal blanking scheme which improves throughput relative to alternative blanking schemes, particularly at higher SN I R. Whilst the embodiment of Figure 3 shows detection of interference within the receiver frequency band, alternative embodiments may detect interference over a different frequency band. For instance, where interference derives from a harmonic of an interference pulse (e.g. from the 2" harmonic of an ATC transponder signal), the interference can be more effectively detected based on the fundamental frequency signal. This can be achieved through an independent pulse detection line that runs parallel to the receive line and that has its own filters and signal converters for preparing the signal for interference detection over a different frequency range.
Although the embodiments are described herein in terms of their application in wireless communications, it will be apparent to a person skilled in the art that the same techniques are equally applicable to other applications, such as radar and sensor systems. Equally, the methods are applicable in other (i.e. non-radio) frequency ranges.
In the implementations described herein, pulse detection, RMS level estimation, intelligent blanking and pulse cancellation functions can be implemented digitally, following the digitisation, decimation and filtering of the signal(s) from the antenna(s). In such implementations the mitigation functions may be implemented within: a) digital firmware using a Field Programmable Gate Array; b) digital hardware using an Application Specific Integrated Circuit (ASIC); or c) digital software located on a Digital Signal Processor (DSP), or even on a General Purpose Processor (GPP), for low to medium sampling rates.
It is also possible to implement these functions wholly or partially using analogue circuitry. Indeed, this approach may offer benefits for some systems where the dynamic range is too high for digital conversion.
While certain arrangements have been described, the arrangements have been presented by way of example only, and are not intended to limit the scope of protection. The inventive concepts described herein may be implemented in a variety of other forms. In addition, various omissions, substitutions and changes to the specific implementations described herein may be made without departing from the scope of protection defined in the following claims.