Patent Publication Number: US-2022215761-A1

Title: Wireless communication

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application claims the benefit of the International Application No. PCT/GB2020/051289, filed on May 28, 2020, and of the Great Britain patent application No. 1907510.0 filed on May 28, 2019, the entire disclosures of which are incorporated herein by way of reference. 
    
    
     FIELD OF THE INVENTION 
     The present disclosure concerns wireless communication. In particular, but not exclusively, this disclosure concerns wireless communication between a rotary wing platform and a satellite. 
     BACKGROUND OF THE INVENTION 
     A rotary wing platform is an aircraft that generates lift by use of one or more rotating wings or blades. Examples of rotary wing platforms include helicopters, unmanned aerial vehicles, and tiltrotor aircraft. 
     Rotary wing platforms may be required to communicate with or via satellites, or high-altitude pseudo-satellites (HAPS). To do so, both the satellite and the rotary wing platform comprise a radio frequency (RF) transceiver. The transceivers provide a communication link over a direct line-of-sight transmission path between the transceivers. 
     However, this transmission path typically passes through the path of the rotary wing platform&#39;s rotating blades. As the blades of the rotary wing platform rotate overhead, each blade repeatedly passes through the transmission path. Thus, in flight, the transmission path, and therefore the communication link, between the transceivers will suffer repeated periodic obstructions by the blades of the rotary wing platform. While the transmission path is obstructed by a blade, any data transmitted by one transceiver may be prevented from reaching the other, and may be lost. The obstruction of the transmission path by the blades therefore causes burst errors on the communication link resulting in loss of transmitted data. Forward error correction (FEC) coding may be applied in an attempt to overcome the data loss, but the redundancy introduced by this process results in a reduced link data rate and increases transceiver complexity. Furthermore, FEC coding has only a limited capability to correct for the burst errors which are characteristic of transmission path obstruction by the rotating blades of a rotary wing platform. 
     Furthermore, while the transmission path is obstructed, transmissions by the rotary wing platform transceiver can reflect downwards off the obstructing blade(s), presenting a radiation hazard to aircraft crew. 
     The present disclosure seeks to mitigate the above-mentioned problems. Alternatively or additionally, the present disclosure seeks to provide improved measures for communication between a rotary wing platform and a satellite. 
     SUMMARY OF THE INVENTION 
     The present disclosure provides, according to a first aspect, a method of communicating between a rotary wing platform and a ground terminal via a satellite, the method comprising: 
     receiving, at the rotary wing platform, a forward link signal transmitted by the satellite; 
     at the rotary wing platform, on the basis of the received forward link signal, estimating at least one obstruction characteristic associated with obstruction of a signal transmission path between the rotary wing platform and the satellite by one or more blades of the rotary wing platform; 
     at the rotary wing platform, determining a plurality of time periods during which the at least one obstruction characteristic indicates that the signal transmission path will not be obstructed by the one or more blades of the rotary wing platform; and 
     at the rotary wing platform, transmitting to the satellite a bursted carrier return link signal comprising a plurality of bursts, wherein each burst in the plurality of bursts is transmitted during one of the determined time periods. 
     According to a second aspect of the present disclosure, there is provided a transceiver for a rotary wing platform comprising: 
     a receiver configured to receive a forward link signal transmitted by a satellite; 
     signal processing electronics, configured to estimate, on the basis of the received forward link signal, at least one obstruction characteristic associated with obstruction of a signal transmission path between the rotary wing platform and the satellite by one or more blades of the rotary wing platform and to determine a plurality of time periods during which the at least one obstruction characteristic indicates that the signal transmission path will not be obstructed by the one or more blades of the rotary wing platform; and 
     a transmitter, configured to transmit to the satellite a bursted carrier return link signal comprising a plurality of bursts, wherein each burst in the plurality of bursts is transmitted during one of the determined time periods. 
     According to a third aspect of the present disclosure, there is provided a system for communicating between a rotary wing platform and a ground terminal via a satellite, comprising a rotary wing platform configured to: 
     receive, at the rotary wing platform, a forward link signal transmitted by the satellite; 
     on the basis of the received forward link signal, estimate at least one obstruction characteristic associated with obstruction of a signal transmission path between the rotary wing platform and the satellite by one or more blades of the rotary wing platform; 
     determine a plurality of time periods during which the at least one obstruction characteristic indicates that the signal transmission path will not be obstructed by the one or more blades of the rotary wing platform; and 
     transmit to the satellite a bursted carrier return link signal comprising a plurality of bursts, wherein each burst in the plurality of burst is transmitted during one of the determined time periods; and 
     a ground terminal configured to: 
     receive the bursted carrier signal; 
     on the basis of the received return link signal, identify whether at least a part of the received return link signal does not correspond to a burst; and 
     discard the at least part. 
     It will of course be appreciated that features described in relation to one aspect of the present disclosure may be incorporated into other aspects of the present disclosure. For example, the method of the disclosure may incorporate any of the features described with reference to the apparatus of the disclosure and vice versa. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the present disclosure will now be described by way of example only with reference to the accompanying schematic drawings of which: 
         FIG. 1  shows a block diagram of a system according to embodiments of the present disclosure; 
         FIG. 2  shows a block diagram of the FL receiver of  FIG. 1 ; 
         FIG. 3  shows a block diagram of the adaptive filter of  FIG. 2 ; 
         FIG. 4  shows a block diagram of the RL transmitter of  FIG. 1 ; 
         FIG. 5  shows a block diagram of the RL receiver of  FIG. 1 ; and 
         FIG. 6  shows a flow chart illustrating the steps of a method according to embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  shows a block diagram of a system  100  according to embodiments of the present disclosure. System  100  comprises a communications link between a ground terminal  101  and a rotary wing platform  103  via a satellite  102 . The communications link comprises a forward link (FL)  105  from ground terminal  101  to rotary wing platform  103 , and a return link (RL)  107  from rotary wing platform  103  to ground terminal  101 . FL  105  comprises a first link  105   a  from ground terminal  101  to satellite  102  and a second link  105   b  from satellite  102  to rotary wing platform  103 . RL  107  comprises a first link  107   a  from rotary wing platform  103  to satellite  102  and a second link  107   b  from satellite  102  to ground terminal  101 . 
     Ground terminal  101  comprises a FL transmitter  109  and a RL receiver  111 . Rotary wing platform  103  comprises a FL receiver  113  and a RL transmitter  115 . Satellite  102  comprises a FL transponder  108  and a RL transponder  110 . At ground terminal  101 , FL transmitter  109  is configured to transmit FL signals to rotary wing platform  103  via FL transponder  108 . FL transponder  108  is configured to forward signals received from FL transmitter  109  to rotary wing platform  103 . At rotary wing platform  103 , FL receiver  113  is configured to receive FL signals. Thus, FL transmitter  109 , FL transponder  108 , and FL receiver  113  can be said to enable FL  105 . At rotary wing platform  103 , RL transmitter  115  is configured to transmit RL signals to ground terminal  101  via RL transponder  110 . RL transponder  110  is configured to forward signals from RL transmitter  115  to ground terminal  101 . At ground terminal  101 , RL receiver  111  is configured to receive RL signals. Thus, RL transmitter  115 , RL transponder  110 , and RL receiver  111  can be said to enable RL  107 . 
     While in  FIG. 1 , FL transmitter  109  and RL receiver  111  are shown as separate units, in embodiments, FL transmitter  109  and RL receiver  111  are provided as a single satellite transceiver unit. Similarly, in embodiments, FL receiver  113  and RL transmitter  115  may be provided as a single rotary wing platform transceiver unit. In embodiments, FL transponder  108  and RL transponder  110  may be provided by a single transponder unit. 
     Signals transmitted over FL  105  or RL  107  will travel between ground terminal  101  and rotary wing platform  103  along a signal transmission path determined by the positions of ground terminal  101 , satellite  102 , and rotary wing platform  103 . FL  105  and RL  107  can therefore each be said to have a signal transmission path. It will be appreciated by the skilled person that, while in  FIG. 1  FL  105  and RL  107  are shown as separate arrows, signals travelling over FL  105  and signals travelling over RL  107  may follow substantially the same transmission path, albeit in opposite directions. The signal transmission path suffers periodic obstruction by one or more blades of rotary wing platform  103 . The obstruction of the signal transmission path can be said to have frequency, corresponding to the regularity with which the one or more blades pass through the signal transmission path. Thus, the obstruction can also be said to have a period. The obstruction can also be said to have a mark/space ratio, indicating the ratio of a period of time during which the signal transmission path is obstructed to that during which the signal transmission path is not obstructed. It will be understood by the skilled person that it is second link  105   b  of FL  105  and first link  107   a  of RL  107  which are subjected to obstruction by blades of rotary wing platform  103 . 
     FL transmitter  109  is configured to transmit a FL data stream. The FL data stream comprises data intended for transmission from ground terminal  101  to rotary wing platform  103  over FL  105 . 
     In embodiments, a FL signal comprises a signal with predictable signal to noise ratio. In embodiments, the FL signal comprises a continuous carrier signal. In embodiments, the FL signal comprises a full occupancy bursted carrier signal. Thus, the FL signal may comprise a plurality of forward link bursts. In such embodiments, the FL signal may comprise a regular and evenly bursted waveform structure. In such embodiments, it may be that each of the plurality of bursts are of equal length. Thus, in embodiments, each forward link burst in the plurality of forward link bursts is of the same length, such that each forward link burst in the plurality of forward link bursts corresponds to an equal length of time. In embodiments, the plurality of forward link bursts are transmitted at substantially regular intervals. Thus, each forward link burst in the plurality of forward link bursts may be transmitted a predetermined time period (for example, substantially corresponding to a length of a forward link burst) after the preceding forward link burst. In embodiments, the FL signal comprises a combination of continuous carrier and full occupancy bursted carrier signals. 
     In embodiments, FL  105  uses one or more of optimized synchronization, interleaving, forward error correction, and erasure detection and insertion in order to enable reliable error-free transmission of the FL data stream from ground terminal  101  to rotary wing platform  103 . These standard techniques for overcoming interference on a communication link are well known by those skilled in the art and therefore will not be discussed further here. 
       FIG. 2  shows a block diagram of FL receiver  113 . FL receiver  113  comprises an RF front end  201 . RF front end  201  is configured to receive FL signals, perform any desired down-conversion and amplification, and perform analogue to digital conversion on the received FL signals. RF front end  201  outputs a digital signal  203  corresponding to one or more signals received over FL  105 . 
     A demodulator  205  is configured to extract a baseband signal  207  from digital signal  203 . FL receiver  113  is configured to process baseband signal  207  to estimate at least one obstruction characteristic associated with obstruction of the signal transmission path between ground terminal  101  and rotary wing platform  103 . In embodiments, the at least one obstruction characteristic comprises one or more of: a period of the obstruction, a mark/space ratio of the obstruction, and a phase of the obstruction. 
     In embodiments, FL receiver  113  is configured to process baseband signal  207  to determine a signal quality indicator  209  of the received FL signal. In embodiments, signal quality indicator  209  comprises one or more of: a received signal quality indicator, a signal to noise ratio, and an error vector magnitude (EVM). Signal quality indicator  209  provides an indication as to whether, at a given point in time, the signal transmission path is obstructed by the one or more blades of rotary wing platform  103 . For example, a drop in the signal to noise ratio of the received FL signal may indicate that the signal transmission path is obstructed. 
     In embodiments, estimating the at least one obstruction characteristic is performed on the basis of signal quality indicator  209 . In embodiments, estimating the at least one obstruction characteristic comprises comparing signal quality indicator  209  to a predetermined threshold  211 . In embodiments, predetermined threshold  211  is set at a fixed level. In alternative embodiments, predetermined threshold  211  is adaptively changed in response to changes in the state of the signal transmission path. In embodiments, the signal transmission path is considered to be obstructed when the determined signal quality indicator does not exceed predetermined threshold  211 . In embodiments, the at least one obstruction characteristic comprises one or both of: a period of signal quality indicator  209 , and a mark/space ratio of signal quality indicator  209 . 
     In embodiments, estimating the at least one obstruction characteristic comprises operating an adaptive filter  213  to model the signal transmission path. In embodiments, FL receiver  113  is configured to provide adaptive filter  213  with determined signal quality indicator  209  as an input. In embodiments, FL receiver  113  is configured to provide adaptive filter  213  with predetermined threshold  211  as an input. In embodiments, the modelling comprises predicting a current state of the signal transmission path. In embodiments, the modelling comprises predicting a future state of the signal transmission path. In embodiments, the state of the signal transmission path comprises an expected received signal strength of a signal transmitted over the signal transmission path. 
     In embodiments, FL receiver  113  is configured to determine, on the basis of the estimated at least one obstruction characteristic, a plurality of obstruction-free time periods during which the at least one obstruction characteristic indicates that the signal transmission path will not be obstructed by the one or more blades of rotary wing platform  103 . In embodiments, the plurality of obstruction-free time periods are determined by use of adaptive filter  213 . In embodiments, determining a plurality of obstruction-free time periods may comprise determining one or more RL  107  bursts for which the at least one obstruction characteristic indicates that the signal transmission path will not be obstructed by the one or more blades of rotary wing platform  103 . Thus, in embodiments, FL receiver  113  may conceptually divide time into a plurality of granular time intervals. In embodiments, the plurality of time intervals are all of equal duration. In other embodiments, the plurality of time intervals may vary in duration (for example, in response to changes in blade pitch or rotational speed). Thus, in embodiments, determining the plurality of obstruction-free time periods comprises selecting one or more of the plurality of time intervals for which the at least one obstruction characteristic indicates that the signal transmission path will not be obstructed. 
     In embodiments, the duration of the time intervals is fixed in relation to a duration of the received FL  105  bursts (where the FL  105  comprises a full occupancy bursted carrier link). For example, the duration of the time intervals may correspond to a unit fraction of the duration of the received FL  105  bursts, such that an integer number of time intervals sums to the duration of a FL  105  burst. In embodiments, the time intervals are arranged such that the start and end times of a FL  105  burst correspond to the start times of two respective time intervals. Thus, the FL  105  and RL  107  bursts can be said to be aligned. It will be appreciated that there may be multiple RL  107  bursts for each FL  105  burst, such that the FL  105  bursts are aligned with, for example, one in every five RL  107  bursts. In such embodiments, the FL  105  burst and an integer number of time intervals can be said to correspond to the same period of time. It may be that each of the time intervals corresponds to a respective RL  107  burst. Thus, FL receiver  113  can, in such cases, be said to be configured to generate and utilise a uniform bursted waveform framework on the FL  105  and the RL  107 . In embodiments, determining the at least one obstruction characteristic comprises determining, for each forward link burst in the plurality of forward link bursts, whether the forward link burst was successfully received at rotary wing platform  103 . In such embodiments, it may be that determining the plurality of time periods comprises identifying a time period corresponding to a successfully received forward link burst. 
     It will be appreciated that the granularity of the obstruction characteristic is determined, at least in part, by the duration of the FL  105  bursts. Thus, in embodiments having a uniform bursted waveform framework on the FL  105  and the RL  107 , FL receiver  113  may be configured to propagate the estimated obstruction characteristic to those RL  107  bursts corresponding to the obstructed FL  105  bursts. Such an embodiment can provide an efficient means for determining obstruction-free time periods in which to transmit over RL  107 . 
       FIG. 3  shows a block diagram of adaptive filter  213 . Adaptive filter  213  is configured to receive as an input, signal quality indicator  209 . Signal quality indicator  209  is passed to an adaptive finite impulse response (FIR) filter  303  as a reference. A second instance of signal quality indicator  209  is subjected to a prediction delay  305  and provided to adaptive FIR filter  303  as an input  301 . Adaptive FIR filter  303  comprises a plurality of taps  307 , for example 250 taps. It will be appreciated by the skilled person that other numbers of taps could be also used. In embodiments, the number of taps  307  is sufficient to enable adaptive FIR filter  303  to cover a period of time corresponding to multiple obstruction periods. Adaptive FIR filter  303  is configured to minimize an error between its input and reference, by adjusting a tap configuration of adaptive FIR filter  303 . In embodiments, adaptive FIR filter  303  comprises a least mean squares (LMS) adaptive filter. In alternative embodiments, adaptive FIR filter  303  comprises a recursive least squares (RLS) adaptive filter. 
     The tap configuration of adaptive FIR filter  303  is mirrored in a prediction FIR filter  309 . Prediction FIR filter  309  is provided with signal quality indicator  209  as its input, and therefore operates to predict the signal strength at a point in time in the future corresponding to a length of prediction delay  305 . Thus, in embodiments, prediction FIR filter  309  is configured to predict a future state of the signal transmission path. In embodiments, prediction FIR filter  309  is configured to determine the allocation of data bearing bursts (for example, to the time intervals) on RL  107 . Prediction FIR filter  309  is configured to generate a blanking control signal  215 . In embodiments, prediction FIR filter  309  may be configured to generate blanking control signal  215  on the basis of receipt of non data-bearing (for example, due to obstruction by the rotor blades) FL  105  bursts. 
     Adaptive filter  213  therefore assists in determining a plurality of obstruction-free time periods during which the at least one obstruction characteristic indicates that the signal transmission path will not be obstructed by the one or more blades of rotary wing platform  103 . 
     Returning to  FIG. 2 , blanking control signal  215  controls an input selection function  217 . Input selection function  217  is configured to, in response to blanking control signal  215  indicating that the signal transmission path is not obstructed, pass baseband signal  207  through to the output  221  of input selection function  217 . In response to blanking control signal  215  indicating that the signal transmission path is obstructed, input selection function  217  instead passes erasures  219  through to output  221 . Thus, blanking control signal  215  controls input selection function  217  to insert erasures in the place of data received while the signal transmission path is obstructed. An erasure comprises a neutral soft decision signifying a lack of, for example no, confidence in the received data. Therefore, blanking control signal  215  can be said to control input selection function  217  to signal a lack of confidence in data received while the signal transmission path is obstructed (for example, by inserting neutral soft-decision data). Inserting erasures in the place of data received while the signal transmission path is obstructed prevents noisy or erroneous data from biasing later decoding stages, for example a forward error correction decoder. Output  221  of input selection function  217  therefore comprises a modified version of baseband signal  207 , in which those portions of the received signal that correspond to periods of time when the signal transmission path was obstructed are replaced by erasures. 
     Output  221  of input selection function  217  is, in embodiments, processed by a de-interleaving function  223  and/or a forward error correction (FEC) decoder  225 . It will be appreciated by the skilled person that de-interleaving function  223  and FEC decoder  225  are optional elements, and only required in embodiments incorporating those elements&#39; corresponding functionalities. The output  227  of FL receiver  113  comprises the FL data stream. 
       FIG. 4  shows a block diagram of RL transmitter  115 . RL transmitter  115  is configured to transmit to ground terminal  101  via RL transponder  110  a bursted carrier RL signal comprising a plurality of bursts. In embodiments, the plurality of bursts are all of the same duration. Thus, it may be that each of the plurality of bursts corresponds to an equal length or time, such that the plurality of bursts can be said to be even and regular. In embodiments, the plurality of bursts bear a deterministic relationship to the FL  105  bursts. For example, it may be that a duration of each of the plurality of bursts corresponds to a unit fraction of a duration of a FL  105  burst. Thus, multiple bursts in the plurality may together correspond to an associated FL  105  burst (i.e., each forward link burst corresponds to an integer number of return link bursts). In embodiments, each of the plurality of bursts corresponds to a respective one of the earlier mentioned plurality of time intervals. Each burst in the plurality of bursts is transmitted during one of the plurality of obstruction-free time periods determined by use of adaptive filter  213 . Thus, in embodiments, the transmission of the return link bursts is synchronized to the receipt of the forward link bursts. In embodiments, multiple bursts in the plurality of bursts may be transmitted during a single obstruction-free time period. In embodiments, each of the plurality of bursts is 1 ms in length. 
     RL transmitter  115  receives RL data stream  401  for transmission to ground terminal  101  from a data source. RL data stream  401  comprises data intended for transmission from rotary wing platform  103  to ground terminal  101  over RL  107 . In embodiments, data stream  401  is encoded by an FEC encoder  403  and subsequently passed to an output selection function  405 . It will be appreciated by the skilled person that FEC encoder  403  is an optional element, and only required in embodiments incorporating forward error correction functionality. In embodiments, RL transmitter  115  is configured to operate FEC encoder  403  to encode the burst. In embodiments, a burst comprises data and a code-word length of the encoder is equal to a length of the data. Aligning the code-word length of a FEC encoder to the length of data in a burst simplifies the application of adaptive coding and modulation by enabling seamless transitions. 
     Output selection function  405  is controlled by blanking control signal  215 . In embodiments, output selection function  405  is configured to, in response to blanking control signal  215  indicating that the signal transmission path will be obstructed, store bursts in a burst delay memory  407  (from which bursts are allocated). In embodiments, output selection function  405  is configured to, in response to blanking control signal  215  indicating that the signal transmission path will not be obstructed, pass bursts from burst delay memory  407  to a burst modulator  409 , and subsequently on to an RF front end  411  for transmission. If burst delay memory  407  is empty, output selection function  405  passes bursts directly from its input to burst modulator  409 . In embodiments, RF front end  411  comprises a separate unit to RF front end  201 . In alternative embodiments, RF front end  411  and RF front end  201  comprise a single unit. In embodiments, burst delay memory  407  can be considered to store a queue of bursts. Bursts are passed to burst modulator  409  from a head of the queue, such that the first burst stored in burst delay memory  407  is the first burst to be passed to burst modulator  409 . Burst delay memory can therefore be said to operate on a first-in first-out (FIFO) basis. Therefore, in embodiments, transmitting a bursted carrier RL signal comprises, in response to the at least one obstruction characteristic indicating that the transmission path is obstructed, storing a burst in a queue. In embodiments, transmitting a bursted carrier return link signal comprises, in response to the at least one obstruction characteristic indicating that the transmission path is not obstructed, transmitting a burst from a head of the queue. In other embodiments, burst delay memory  407  does not receive and store bursts, but instead acts as a standard data buffer, with the allocation of data to RL  107  bursts performed on the basis of the output of burst delay memory  407 . 
     Thus, in embodiments, RL transmitter  115  is configured to transmit bursts only when the signal transmission path is not obstructed. Time intervals during which a burst is not transmitted can be said to be radio frequency blanked. In embodiments, the time intervals are all a fixed uniform length of time. Thus, in such embodiments, the time intervals can be said to be “atomic” and may correspond to a predetermined number of bits or symbols, rather than packets or frames. Such a burst (i.e., the length of which is fixed in terms of the number of bits/symbols and therefore also corresponds to a fixed period of time) can be referred to as a physical layer burst. It will be appreciated that a data packet generally comprises a fixed number of bits. The time period associated with the transmission of that data packet will therefore vary according to the data rate. By performing blanking of physical layer bursts rather than higher level packets or frames, it is possible to maintain a synchronized uniform bursted waveform framework on FL  105  and RL  107 . Thus, such embodiments can maintain synchronization between and alignment of the FL  105  bursts and RL  107  bursts. In embodiments, the time intervals correspond to a minimum independent unit of modulation and demodulation. It may be that an obstruction period covers more than one time interval. Thus, in such cases, it may be that multiple consecutive time intervals are blanked. 
     Thus, in embodiments, RL transmitter  115  is configured to transmit physical layer bursts. In such embodiments, RL transmitter  115  may be configured to perform RF blanking of physical layer bursts. The use of physical layer bursts enables the decoupling of the means of managing the obstruction of the signal transmission path from the data rates and higher-layer protocols of RL  107  (e.g. Network Layer Packets and/or Transport Layer Frames). Thus, in such embodiments, burst allocation aspects such as lead-in and lead-out times are not data rate dependent. The use of physical layer bursts therefore enables an RF blanking scheme which is data rate agnostic (because the scheme operates on the basis of the time intervals rather than data packets). This can provide finer granularity blanking and an associated increase in transmission efficiency. Furthermore, such an RF blanking scheme can blank time intervals independently of one another, allowing the blanking to be localized to only those bursts affected by an obstruction (i.e., the synchronization and recovery of neighboring unblanked bursts is unaffected). 
     In embodiments, transmitting the bursted carrier return link signal comprises transmitting a burst when the at least one obstruction characteristic indicates that a pre-determined period of time has elapsed since the transmission path was last obstructed, and that transmission of the data bearing bursts will be completed a pre-determined period of time before the transmission path is next obstructed. Such embodiments can be said to impose a buffer period around the time period during which the transmission path is obstructed by one or more blades of rotary wing platform  103 . Imposing a buffer period around a period of obstruction ensures that bursts are only transmitted when there is a high degree of certainty that transmission of the burst will not be interfered with by an obstructing blade. This helps to prevent data loss and minimizes the likelihood of RF energy being reflected downwards off the rotor blades. Furthermore, imposing such a buffer period reduces the likelihood of partially blocked bursts, which can be difficult for RL receiver  111  to determine whether to retain or discard. 
       FIG. 5  shows a block diagram of RL receiver  111  according to embodiments of the present disclosure. RL receiver  111  comprises an RF front end  501 . RF front end  501  is configured to receive RL signals, perform any desired down-conversion and amplification, and perform analogue to digital conversion of signals received over RL  107 . RF front end  501  outputs a digital signal  503  corresponding to the received signals. 
     A burst demodulator  505  operates to extract a baseband signal  507  from digital signal  503 . In embodiments, RL receiver  111  is configured to process baseband signal  507  to identify, on the basis of the received RL signal, whether at least a part of the received RL signal does not correspond to a data bearing burst. In embodiments, identifying whether the at least part corresponds to a data bearing burst comprises determining a signal quality indicator  509  of the received RL signal. In embodiments, signal quality indicator  509  comprises one or more of: a received signal strength indicator, a signal to noise ratio, and an error vector magnitude. Signal quality indicator  509  provides an indication as to whether, at a given point in time, the at least part corresponds to a data bearing burst. For example, a drop in the signal to noise ratio of the received RL signal may indicate that the at least part does not correspond to a data bearing burst. In embodiments, identifying whether the at least part corresponds to a data bearing burst comprises comparing determined signal quality indicator  509  of the received RL signal to a further predetermined threshold  511 . In embodiments, the at least part is considered to correspond to a burst when the determined signal quality indicator  509  of the received RL signal exceeds the further predetermined threshold  511 . In embodiments, a time duration of the at least part is equal to a time duration of a burst. 
     In embodiments, identifying whether at least a part of the received RL signal does not correspond to a data bearing burst comprises operating a threshold comparison  513 . In embodiments, RL receiver  111  is configured to provide threshold comparison  513  with the determined signal quality indicator  509  of the received RL signal as an input. In embodiments, RL receiver  111  is configured to provide threshold comparison  513  with the further predetermined threshold  511  as an input. Threshold comparison  513  is configured to identify whether at least a part of the received RL signal does not correspond to a data bearing burst and, on the basis of the identifying, generate a burst deletion control signal  515 . 
     Burst deletion control signal  515  controls an output selection function  517 . Output selection function  517  is configured to, in response to burst deletion control signal  515  indicating that the at least part of the received RL signal does not correspond to a data bearing burst, delete the burst. In response to burst deletion control signal  515  indicating that the at least part of the received RL signal corresponds to a data bearing burst, output selection function  517  instead passes the burst to an optional FEC decoder  519 . The output of FEC decoder  519  comprises a received data stream. Thus, burst deletion control signal  515  controls output selection function  517  to delete those portions of the received RL signal that do not correspond to a data bearing burst. Deleting portions of the received RL signal that do not correspond to a data bearing burst can improve the performance of FEC decoder  519  by reducing an output bit error rate of FEC decoder  519  and can improve the latency of RL  107  by removing any need for interleaving. 
     Thus, in embodiments, RL receiver  111  is configured to, in response to identifying that at least a part of the received return link signal does not correspond to a data bearing burst, discard the at least part. 
     In embodiments, transmitting the bursted carrier signal comprises operating a frequency hopping mechanism to modify the transmission frequency (or frequency channel) between successive bursts. It will be appreciated that the transmission frequency, in this context, refers to the center frequency. In embodiments, RL transmitter  115  is configured to modify the transmission frequency after every burst. In alternative embodiments, RL transmitter  115  is configured to modify the transmission frequency after transmission of a predetermined number of bursts. In embodiments, RL transmitter  115  is configured to seamlessly transition between hopped and non-hopped modes of operation. Thus, RL transmitter  115  may be configured to transition between hopped and non-hopped modes “on the fly” In embodiments, the seamless transitions may be facilitated by the use of physical layer bursts. In such embodiments, the two modes of operation use common modulation, coding, and interleaving techniques and share an identical approach to obstruction detection and RF blanking Thus, in such cases, the two modes of operation can be said to have identical time domain approaches to RF blanking in which the RF blanking is independent of center frequency. 
     The overall operation of system  100  can be summarized as follows. FL transmitter  109  transmits a signal over FL  105 , which is received by FL receiver  113 . At FL receiver  113 , RF front end  201  performs any desired down-conversion and amplification, digitizes the received FL signal, and passes the digitized signal  203  to demodulator  205 . Demodulator  205  extracts a baseband signal  207  corresponding to the received FL signal. On the basis of extracted baseband signal  207 , FL receiver  113  determines signal quality indicator  209 . Adaptive filter  213  operates to estimate, on the basis of determined signal quality indicator  209  and predetermined threshold  211 , at least one obstruction characteristic associated with obstruction of the signal transmission path between ground terminal  101  and rotary wing platform  103 . Adaptive filter  213  determines a plurality of time periods during which the at least one obstruction characteristic indicates that the signal transmission path will not be obstructed by the one or more blades of rotary wing platform  103 . Adaptive filter  213  generates, on the basis of the determined plurality of time periods, blanking signal  215  to control input selection function  217 . Blanking signal  215  controls input selection function to accept data received while the signal transmission path is not obstructed, and to erase data received while the signal transmission path is obstructed. De-interleaving function  223  and FEC decoder  225  process the output of input selection function  217  to decode the received data stream and extract the received data. 
     Meanwhile, RL transmitter  115  receives data stream  401  for transmission to ground terminal  101  from a data source. FEC encoder  403  encodes data stream  401  and passes it to output selection function  405 . Controlled by blanking control signal  215 , output selection function  405  stores data in burst delay memory  407  when the signal transmission path will be obstructed, and, when the signal transmission path will not be obstructed, allocates data (for example, retrieved from burst delay memory  407 ) to RL  107  bursts and transmits those bursts. In embodiments, RL transmitter  115  is further configured to, on the basis of blanking control signal  215 , perform RF blanking of RL  107 . Performing RF blanking of RL  107  reduces reflection and scattering of transmitted RF radiation by the rotor blades. 
     RF front end  501  of RL receiver  111  receives signals transmitted over RL  107 , performs any desired down-conversion and amplification, and digitizes the received RL signals. Burst demodulator  505  extracts a baseband signal  507  from digitized signal  503 . On the basis of extracted baseband signal  507 , RL receiver  111  determines signal quality indicator  509 . Threshold comparison  513  operates to estimate, on the basis of determined signal quality indicator  509  and further predetermined threshold  511 , whether at least a part of the received RL signal does not correspond to a data bearing burst. On the basis of the estimation, threshold comparison  513  generates a burst deletion control signal  515 . Under the control of burst deletion control signal  515 , output selection function  517  deletes parts of the received signal that are estimated not to correspond to a data bearing burst, and passes those parts of the received signal that are estimated to correspond to data bearing bursts to FEC decoder  519 . FEC decoder  519  decodes the received bursts and passes the extracted data to a data sink. 
     It will be appreciated by the skilled person that, in embodiments of the present disclosure, data passes across FL  105  and across RL  107  concurrently, such that data is simultaneously received by FL receiver  113  and transmitted by RL transmitter  115 . 
     Embodiments of the present disclosure also provide a transceiver for a rotary wing platform. The transceiver comprises a receiver, signal processing electronics, and a transmitter. The receiver is configured to receive a forward link signal transmitted by a satellite. The signal processing electronics are configured to estimate, on the basis of the received forward link signal, at least one obstruction characteristic associated with obstruction of a signal transmission path between the rotary wing platform and the satellite by one or more blades of the rotary wing platform and to determine a plurality of obstruction-free time periods during which the at least one obstruction characteristic indicates that the signal transmission path will not be obstructed by the one or more blades of the rotary wing platform. The transmitter is configured to transmit to the satellite a bursted carrier return link signal comprising a plurality of bursts, wherein each burst in the plurality of bursts is transmitted during one of the determined obstruction-free time periods. 
       FIG. 6  shows a flow chart illustrating the steps of a method according to embodiments of the present disclosure. A first step of the method, represented by item  601 , comprises receiving, at a rotary wing platform, a forward link signal transmitted by a ground terminal via a satellite. In embodiments, the received forward link signal comprises a continuous carrier signal. In embodiments, the received forward link signal comprises a full occupancy bursted carrier signal. 
     An optional second step of the method, represented by item  603 , comprises determining a signal quality indicator of the received forward link signal. In embodiments, the signal quality indicator comprises one or more of: received signal strength indicator, signal to noise ratio, and an error vector magnitude. 
     A third step of the method, represented by item  605 , comprises, at the rotary wing platform, on the basis of the received forward link signal, estimating at least one obstruction characteristic associated with obstruction of a signal transmission path between the rotary wing platform and the satellite by one or more blades of the rotary wing platform. In embodiments, estimating the at least one obstruction characteristic is performed on the basis of the determined signal quality indicator. In embodiments, estimating the at least one obstruction characteristic comprises comparing the determined signal quality indicator to a predetermined threshold. In embodiments, the signal transmission path is considered to be obstructed when the determined signal quality indicator does not exceed the predetermined threshold. In embodiments, the at least one obstruction characteristic comprises one or more of: a period, a mark/space ratio, and a phase. 
     Embodiments may comprise a step of providing an adaptive filter with the determined signal quality indicator as an input. Embodiments may comprise a step of providing the adaptive filter with the predetermined threshold as an input. In embodiments, estimating the at least one obstruction characteristic comprises operating the adaptive filter to model the signal transmission path. In embodiments, the modelling comprises predicting a current state of the signal transmission path. In embodiments, the modelling comprises predicting a future state of the signal transmission path. 
     A fourth step of the method, represented by item  607 , comprises, at the rotary wing platform, determining a plurality of obstruction-free time periods during which the at least one obstruction characteristic indicates that the signal transmission path will not be obstructed by the one or more blades of the rotary wing platform. 
     Embodiments may comprise a step of, at the rotary wing platform, operating a forward error correction encoder to encode each burst in the plurality of bursts. In embodiments, a burst comprises data and a code-word length of the encoder is equal to a length of the data. 
     A fifth step of the method, represented by item  609 , comprises, at the rotary wing platform, transmitting to the ground terminal via the satellite a bursted carrier return link signal comprising a plurality of bursts. Each burst in the plurality of bursts is transmitted during one of the determined obstruction-free time periods. In embodiments, transmitting the bursted carrier return link signal comprises, in response to the at least one obstruction characteristic indicating that the transmission path is obstructed, storing a burst in a queue for later transmission. In embodiments, storing a burst in the queue comprises allocating the burst to a later time interval for transmission. In embodiments, transmitting the bursted carrier return link signal comprises, in response to the at least one obstruction characteristic indicating that the transmission path is not obstructed, allocating data to bursts (for example, retrieved from the head of the queue) and transmitting those bursts during an obstruction-free period of time. In embodiments, transmitting the bursted carrier return link signal comprises transmitting data bearing bursts when the at least one obstruction characteristic indicates that a pre-determined period of time has elapsed since the transmission path was last obstructed, and that transmission of the burst will be completed a pre-determined period of time (for example, a predetermined number of time intervals, or a predetermined number of RL  107  burst durations) before the transmission path is next obstructed. In embodiments, transmitting the bursted carrier signal comprises operating a frequency hopping mechanism to modify the transmission frequency between successive bursts. 
     An optional sixth step of the method, represented by item  611 , comprises, at the ground terminal, receiving the return link signal. 
     An optional seventh step of the method, represented by item  613 , comprises, at the ground terminal, on the basis of the received return link signal, identifying that at least a part of the received return link signal does not correspond to a data bearing burst. In embodiments, the identifying comprises determining a signal quality indicator of the received return link signal. In embodiments, the identifying comprises comparing the determined signal quality indicator of the received return link signal to a further predetermined threshold. In embodiments, the at least part is considered to correspond to be a burst when the determined signal quality indicator of the received return link signal exceeds the further predetermined threshold. 
     An optional eighth step of the method, represented by item  615 , comprises, at the ground terminal, discarding the at least part. In embodiments, a time duration of the at least part is equal to a time duration of a burst. 
     While the present disclosure has been described and illustrated with reference to particular embodiments, it will be appreciated by those of ordinary skill in the art that the disclosure lends itself to many different variations not specifically illustrated herein. By way of example only, certain possible variations will now be described. 
     While embodiments of the present disclosure have been described in relation to communication between a rotary wing platform and a ground terminal via a satellite, the skilled person will appreciate that embodiments of the present disclosure are equally applicable to other applications in which a communication link suffers repeated, periodic or aperiodic obstruction. For example, embodiments of the present disclosure may be applied to communication by a rotary wing platform with a ground terminal via any high altitude platform, for example a high altitude pseudo satellite (HAPS), unmanned aerial vehicle (UAV), or balloon. Similarly, embodiments of the present disclosure may be applied to communication via any of the above listed platforms by, for example, a train, which may suffer periodic or aperiodic interference as it travels under overhead gantries. Embodiments of the present disclosure may find application in any communication link that suffers from periodic interruption, for example a communication link that is targeted by a pulse jammer. 
     It will be appreciated by the skilled person that ground terminal  101  need not necessarily be located on the ground. For example, in alternative embodiments, ground terminal  101  may be mounted on a fixed-wing aircraft or a ship. 
     Embodiments comprise a method of communicating between a rotary wing platform and a ground terminal via a high altitude platform, the method comprising: 
     receiving, at the rotary wing platform, a forward link signal transmitted by the high altitude platform; 
     at the rotary wing platform, on the basis of the received forward link signal, estimating at least one obstruction characteristic associated with obstruction of a signal transmission path between the rotary wing platform and the high altitude platform by one or more blades of the rotary wing platform; 
     at the rotary wing platform, determining a plurality of obstruction-free time periods during which the at least one obstruction characteristic indicates that the signal transmission path will not be obstructed by the one or more blades of the rotary wing platform; and 
     at the rotary wing platform, transmitting to the high altitude platform a bursted carrier return link signal comprising a plurality of bursts, wherein each burst in the plurality of bursts is transmitted during one of the determined time periods. 
     Embodiments comprise a method of communicating with a high altitude platform (for example a satellite), the method comprising: 
     receiving a forward link signal transmitted by the high altitude platform; 
     on the basis of the received forward link signal, estimating at least one obstruction characteristic associated with obstruction of a signal transmission path to the high altitude platform; 
     determining a plurality of obstruction-free time periods during which the at least one obstruction characteristic indicates that the signal transmission path will not be obstructed; and 
     transmitting to the high altitude platform a bursted carrier return link signal comprising a plurality of bursts, wherein each burst in the plurality of bursts is transmitted during one of the determined time periods. 
     Where in the foregoing description, integers or elements are mentioned which have known, obvious or foreseeable equivalents, then such equivalents are herein incorporated as if individually set forth. Reference should be made to the claims for determining the true scope of the present disclosure, which should be construed so as to encompass any such equivalents. It will also be appreciated by the reader that integers or features of the present disclosure that are described as preferable, advantageous, convenient or the like are optional and do not limit the scope of the independent claims. Moreover, it is to be understood that such optional integers or features, while of possible benefit in some embodiments of the disclosure, may not be desirable, and may therefore be absent, in other embodiments. 
     While at least one exemplary embodiment of the present invention(s) is disclosed herein, it should be understood that modifications, substitutions and alternatives may be apparent to one of ordinary skill in the art and can be made without departing from the scope of this disclosure. This disclosure is intended to cover any adaptations or variations of the exemplary embodiment(s). In addition, in this disclosure, the terms “comprise” or “comprising” do not exclude other elements or steps, the terms “a” or “one” do not exclude a plural number, and the term “or” means either or both. Furthermore, characteristics or steps which have been described may also be used in combination with other characteristics or steps and in any order unless the disclosure or context suggests otherwise. This disclosure hereby incorporates by reference the complete disclosure of any patent or application from which it claims benefit or priority.