A doppler compensation method using photonic modulation

Abstract

This disclosure provides a method of correcting Doppler shift in a received RF signal, comprising: receiving, by a photonic device: an optical carrier signal; a received RF signal which has undergone a Doppler shift during a transmission; a corrected local oscillator corrected for the Doppler shift; modulating a first beam of the optical carrier signal with the received RF signal; modulating a second beam of the optical carrier signal with the corrected local oscillator; and, generating, with a photodiode, an electrical signal using the modulated first and second beams of the optical carrier signals.

Description

[0001] A Doppler Compensation Method using Photonic Modulation

[0002] Technical Field of the Invention

[0003] The present disclosure relates to a Doppler compensation method of a received RF signal using a photonic device.

[0004] Background to the Invention

[0005] The phenomenon of Doppler shift on transmitted RF signals between relatively moving transmitters and receivers is well known in the art. Ordinarily, it is necessary to have an understanding of the relative movement between a transmitter and a receiver to correct for the Doppler shift.

[0006] The present invention seeks to provide an improved method and system of correcting the Doppler shift in a received RF signal. In particular, the present invention seeks to provide a method of compensation or correcting for Doppler shift without knowledge of the relative positions or movements of the transmitter and receiver.

[0007] Summary of the Invention

[0008] The present invention provides a method of and system for correcting a Doppler shift in a received RF signal according to the appended claims.

[0009] The present disclosure provides a method of correcting Doppler shift in a received RF signal, comprising: receiving a doppler shifter RF signal, providing a doppler corrected local oscillator and using the corrected local oscillator and received RF signal to generate an output which corresponds to the received RF signal with the Doppler shift corrected for.

[0010] The present disclosure provides a method of correcting a Doppler shift in a received RF signal comprising: receiving, by a photonic device: an optical carrier signal; a received RF signal which has undergone a Doppler shift during a transmission; a corrected local oscillator corrected for the Doppler shift. The method may further comprise: modulating a first beam of the optical carrier signal with the received RF signal; modulating a second beam of the optical carrier signal with the corrected local oscillator; and, generating, with a photodiode, an electrical signal using the modulated first and second beams of the optical carrier signals. The RF signal may be an RF signal band. The corrected local oscillator may be referred to as a corrected local oscillator signal. The corrected local oscillator signal may be corrected with respect to the transmission frequency of the received RF signal, prior to a Doppler shift. The uncorrected local oscillator may comprise a reference signal which is chosen as the carrier frequency or centre frequency of the band in which the RF signal is transmitted. The reference local oscillator may comprise a signal having a reference signal which is increased or decreased in keeping with the Doppler shift experienced by the received RF signal.

[0011] The corrected local oscillator may be corrected for Doppler shift by taking the difference between the reference local oscillator frequency and the received RF signal frequency, this difference corresponding to the Doppler shift. The difference may be obtained using a mixer configured to mix the received RF signal and the reference local oscillator.

[0012] The corrected local oscillator may be corrected for Doppler shift by: inputting, to a corrected local oscillator mixer, a reference local oscillator signal; and, mixing the reference local oscillator signal with the RF signal to provide the corrected local oscillator.

[0013] The difference may be obtained using any means known in the art. The difference may be obtained from a mixer. The mixer may be an electronic mixer or a photonic mixer. The mixer may be configured to obtain an output signal which comprises a frequency which is the difference between the frequency of the reference local oscillator and the received RF signal.

[0014] The reference local oscillator signal may have a frequency corresponding to a transmission frequency of the received RF signal prior to transmission and the Doppler shift. The transmission frequency of concern may be the carrier frequency or centre frequency. The centre frequency may be the centre frequency of the transmission band. Using the centre frequency of a transmission band may provide a corrected local oscillator which is suitable for correcting the Doppler shift experienced by a plurality of RF signals received by the receiver.

[0015] The carrier frequency or centre frequency of the received RF signal band prior to transmission and the Doppler shift may be known to the receiver ahead of the transmission or communicated within the transmission of the received RF signal. The carrier frequency or centre frequency may be predetermined and shared between the transmitter and the receiver. The reference local oscillator may comprise a known standard reference signal for the required transmission frequency such as the International Telecommunication Union reference frequency which corresponds to the carrier frequency or centre frequency.

[0016] In some embodiments, the photonic device may comprise at least one Dual Parallel Mach Zehnder, DPMZ, Modulator. The DPMZ modulator may be configured as a mixer or a carrier supressed, single sideband modulator. The photonic device may comprise a photonic mixer configured modulate and mix the first and second beams. The mixed first and second beams may include a beat frequency which is the difference which corresponds to the Doppler corrected received RF signal. The output signal from the mixer may be provided to the photodiode which is configured to provide an electrical RF output signal.

[0017] In some embodiments of the present disclosure, the photonic device may comprise at least one carrier suppressed single sideband, CS-SSB, modulator. At least one of: the corrected local oscillator may be CS-SSB modulated by a first CS-SSB modulator; or, the received RF signal may be CS-SSB modulated by a second CS- SSB modulator. It will be appreciated that the terms first and second CS-SSB may be referred to as a local oscillator CS-SSB and received RF signal CS-SSB. The first and second CS-SSBs may be null-null biased. The first and second CS-SSBs may be similarly configured but have different input signals. Both the corrected local oscillator and the received RF signal may be CS-SSB modulated by the respective first and second CS-SSB modulators.

[0018] The corrected local oscillator and received RF signal may be provided to the respective first and second CS-SSB modulators in quadrature. That is, the corrected local oscillator signal may be provided to the first CS-SSB in-phase and 90 degrees out of phase. The 90 degree phase difference may be leading or lagging, i.e. positive or negative, in dependence of the Doppler shift. Hence, a positive phase shift may be used when the transmitter is determined to be travelling towards the receiver, and a negative shift may be used when the transmitter is moving away from the receiver.

[0019] The CS-SSB modulation may comprise: applying a 90 degree phase shift to the corrected local oscillator inputted to a first child modulator of the first CS-SSB and applying the in-phase corrected local oscillator to a second child modulator of the first CS-SSB; and, applying a 90 degree phase shift to the received RF signal inputted to a first child modulator of the second CS-SSB and applying the in-phase RF signal to a second child modulator of the second CS-SSB.

[0020] The outputs from the first and second CS-SSBs may be combined in a beam combiner or provided to the photodiode. The photodiode may provide an output having a beat frequency which is the difference between the CS-SSB local oscillator signal and the CS-SSB received RF signal.

[0021] The method may further comprise: determining whether the Doppler shift is positive or negative; applying either a positive 90 degree phase shift or negative 90 degree phase shift to the received RF signal and local oscillator depending on the determination of whether the Doppler shift is positive or negative.

[0022] Determining whether the Doppler shift is positive or negative may comprise receiving or determining the relative positions and / or velocities of the transmitter and receiver. In some embodiments, determining whether the Doppler shift is positive or negative may comprise: comparing the carrier frequency or centre frequency of the received RF signal to a reference signal; and, determining whether the carrier frequency or centre frequency has been upshifted or downshifted from the comparison.

[0023] At least one of the corrected local oscillator and received RF signal may be double sideband modulated. Hence, the photonic modulator may comprise a double sideband modulator. Where a double sideband modulation is applied, the method may further comprise filtering the double sideband modulated corrected local oscillator or double sideband modulated received RF signal to remove an optical sideband and the optical carrier signal.

[0024] In some embodiments, the photonic device may use a mixture of CS-SSB and DSB modulation. Hence, one of the corrected local oscillator or received RF signal may be double sideband modulated and the other of the corrected local oscillator or received RF signal may be CS-SSB modulated.

[0025] The received RF signal band may comprise a plurality of RF signals having different carrier frequencies. The received RF signal band may comprise a plurality of RF signals having different carrier frequencies within a predetermined frequency band. The correcting of the Doppler shift in each of the RF signals may be achieved using a corrected local oscillator which has been corrected for the frequency band. Hence, as noted, the corrected local oscillator may be corrected with regards to the centre frequency of the transmission band.

[0026] The present disclosure further provides a photonic system for carrying out the method disclosed herein. A photonic system of the present disclosure may comprise: a photonic modulator configured to receive an optical carrier signal, a Doppler shifted received RF signal and a Doppler corrected local oscillator and modulate optical carrier signal with the received RF signal and corrected local oscillator.

[0027] The system may comprise a photodiode which may be configured to convert an output of the photonic modulator to an electrical signal which corresponds to the received RF signal corrected for Doppler shift.

[0028] The system may comprise a local oscillator correction module. The local oscillator correction module may be configured to provide the corrected local oscillator having a frequency which is the difference between the frequency of a reference local oscillator which corresponds to the transmission frequency of the received RF signal and the frequency of the received RF signal, and configured to provide the corrected local oscillator to the photonic modulator.

[0029] The local oscillator correction module may comprises: an input configured to receive the reference local oscillator signal; an input configured to receive the received RF signal; and, a mixer configured to mix the reference local oscillator and received RF signal and output the difference between the received RF signal and reference local oscillator, wherein the output is the corrected local oscillator.

[0030] The mixer may be a photonic mixer or an electronic mixer. The photonic modulator may comprise one or more double sideband modulators, one or more CS- SSB modulators and / or one or more photonic mixers. The photonic system may further comprise a photodiode for receiving a

[0031] The photonic system may comprise a Doppler detection module for determining whether the Doppler shift of the received RF signal is positive or negative. The Doppler detection module may compare the received RF signal with corrected local oscillator. The photonic system may comprise an optical carrier source. The optical carrier source may comprise a laser. The optical carrier source may comprise a plurality of lasers to provide a first beam and second beam. The lasers may be phase locked.

[0032] The photonic system may comprise a beam splitter configured to split the optical carrier signal into a first and second beams. The beam splitter may be a polarization dependent splitter. The photonic system may comprise a beam combiner for combining the first beam and the second beam output by the photonic modulator.

[0033] The system may comprise one or more polarization rotators to rotate the first or second beam downstream of the polarization dependent beam splitter or upstream of the polarization beam combiner.

[0034] The skilled person will appreciate that except where mutually exclusive, a feature described in relation to any one of the aspects, embodiments or examples described herein may be applied mutatis mutandis to any other aspect, embodiment or example. Furthermore, except where mutually exclusive, any feature described herein may be applied to any aspect and / or combined with any other feature described herein.

[0035] Brief Description of the Drawings

[0036] In order that the invention may be more clearly understood one or more embodiments thereof will now be described, by way of example only, with reference to the accompanying drawings, of which:

[0037] Figure 1 shows a method for correcting a Doppler shifted RF signal;

[0038] Figure 2 shows an arrangement for carrying out the method of Figure 1;

[0039] Figure 3 shows a method for generating a corrected local oscillator;

[0040] Figure 4 shows an arrangement which may be used to carry out the method described in Figure 3;

[0041] Figure 5 shows a Dual-Parallel Mach Zehnder, DPMZ, modulator which may be used to as the photonic mixer in Figure 4;

[0042] Figure 6 shows a system for carrying out the method of Figure 1 using photonic mixers for generating the corrected local oscillator and corrected RF signal; Figure 7 shows a schematic arrangement of a DPMZ which may be utilised in the arrangement of Figure 6 to correct the received Doppler shifted RF signal.

[0043] Figure 8 shows an alternative system for carrying out the method of Figure 1 using an electronic mixer;

[0044] Figure 9 a schematic illustration of output light powers from a single sideband suppressed carrier arrangement in which a corrected local oscillator and a received RF signal are used to provide a Doppler corrected RF signal;

[0045] Figure 10 shows the output light powers from a carrier suppressed single sideband modulator which is configurable to select the upper sideband or lower sideband of a received RF signal;

[0046] Figure 11 shows a carrier supressed single sideband arrangement configured to carry out the method of claim 1 ;

[0047] Figure 12 shows a first carrier supressed single sideband module for use in the arrangement of Figure 11 ;

[0048] Figure 13 shows a second carrier supressed double sideband module for use in the arrangement of Figure 11 ;

[0049] Figure 14 shows a double sideband modulator arrangement in the form of a single Mach Zehnder modulator for using in the arrangement of Figure 11;

[0050] Figure 15 shows a machine learning algorithm which may be used to determine the corrected local oscillator;

[0051] Figure 16 shows a schematic representation of a collision avoidance scenario which may be use the doppler correction method according to the present disclosure; and,

[0052] Figure 17 shows a method of determining potential collisions using the doppler correction method of the present disclosure.

[0053] Detailed Description of the Invention

[0054] In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of various embodiments and inventive concept. However, those skilled in the art will understand that the present invention may be practiced without these specific details, that the present invention is not limited to the described embodiments, and that the present invention may be practiced in a variety of alternate embodiments. In other instances, well known methods, procedures, components, and systems have not been described in detail.

[0055] The present disclosure may find particular application in communication or navigations systems which experience Doppler shift between a transmitter and receiver. The communication systems may be any communication system which experiences a Doppler shift and may include, amongst others, mobile communications such as 4G or 5G or satellite communication systems, for example. The navigation system and method may relate to one in which a path of travel of an approaching object is required, particularly where there is no line of sight, for collision avoidance. The methods may also find some employment in RADAR or LIDAR systems.

[0056] The system and method are generally concerned with correcting a received RF signal which has undergone some Doppler shift. The received RF signal may be transmitted by any transmitter which is experiencing a shift relative to the receiver. The transmitter and / or receiver may be provided on mobile equipment. For example, the transmitter and / or receiver may be provided on one or more of a satellite, aircraft, train, or automobile. The transmitter and / or receiver may be provided on stationary equipment. For example, the transmitter and / or receiver may be provided by a ground station (which may be referred to as a base station) or other form of stationary receiver. Generally, a transmitter may be configured to transmit to a receiver which is stationary relative to the transmitter in which either or both of the transmitter or receiver are geographically stationary or moving.

[0057] Due to the relative movement between the transmitter and receiver, the transmitted signal may undergo a Doppler shift in accordance with well-known principles as understood in the art. It is of note that the system and method may process a received RF signal with no knowledge of whether there has been a Doppler shift, and may process received RF signals which have not experienced any Doppler shift.

[0058] It is well known to use a local oscillator in photonic devices, such as phonic mixers. In photonic and electronic mixers, a local oscillator may be mixed with a received RF signal and output an intermediate frequency which is the difference between the local oscillator and the received RF. This is well known in the art. In the case of a photonic mixer, the output may then be provided to a photodiode to provide an electrical output. The choice of the local oscillator frequency is made on the basis of the received RF signal frequency and the desired output frequency, i.e. the intermediate frequency. The local oscillator may be selected to provide a down conversion or up conversion of the original received RF signal as desired.

[0059] It will be appreciated that, where a received RF signal is subjected to a Doppler shift, the Doppler shift will be present in the output from the mixer which can be and often is undesirable.

[0060] The present disclosure may correct for a Doppler shift of a received RF signal by using a local oscillator which has been corrected to account the Doppler shift in a received RF signal. The local oscillator may be corrected using the Doppler shifted received RF signal and a reference RF signal which corresponds to the transmitted RF signal, prior to the Doppler shift.

[0061] The present disclosure provides embodiments for obtaining the corrected local oscillator and also embodiments for using the corrected local oscillator in photonic modulators to correct a received RF signal.

[0062] Figures 1 and 2 disclose a general method and system for compensating for a Doppler shift experienced by a transmitted RF signal. The method 1 comprises: receiving 2, at a photonic device, a Doppler shifted RF signal from a transmitter; providing 3, at the photonic device, a corrected local oscillator having a frequency which has been corrected for the Doppler shift experienced by the received RF signal; and, using 4 the corrected local oscillator and received RF signal to generate an output from a photodiode which corresponds to the transmitted RF signal in which the Doppler has been corrected for. It will be appreciated that the term ‘local oscillator’ used herein may refer to a local oscillator signal, rather than the local oscillator configured to provide the signal.

[0063] The method 1 of compensating the received RF signal in the present disclosure is achieved with the use of a corrected local oscillator. The corrected local oscillator is corrected such that, when mixed or otherwise modulated with the received RF signal, the resultant signal, e.g. an intermediate signal outputted by a mixer, corresponds to the transmitted RF signal (prior to a Doppler shift). That is, the received RF signal with the Doppler shift reduced or removed.

[0064] The reduction or removal of the Doppler shift may be achieved in some circumstances where the received RF signal is transmitted on a single carrier wave, for example, and the local oscillator is corrected for the Doppler shift at that carrier frequency. In other embodiments, the RF signal may comprise a plurality of RF signals on different carrier frequencies within a predetermined band. In such cases, the corrected local oscillator may be corrected according to the Doppler shift experienced by a frequency within band, for example, the centre of the band frequency (or some other pre-determined frequency within the band). In doing so, a plurality of received RF signals may be corrected using a common corrected local oscillator such that the effect of the Doppler shift is reduced on the plurality of RF signals, if not entirely removed. This may be referred to as an approximate Doppler shift correction and may be applicable where the effective Doppler shift is approximated to be the same across each of the transmission frequencies due to the relative size of the Doppler shift vs the differences between the various transmission frequencies.

[0065] The generation of the corrected local oscillator may be achieved in the absence of the knowledge of the relative movement, e.g. relative speed or angle of travel, experienced between the transmitter and receiver. Instead, the present disclosure may use a reference local oscillator having a reference frequency which is chosen to correspond with the transmitted RF signal, prior to experiencing the Doppler shift. This reference local oscillator may be mixed with the received Doppler shifted RF signal to provide a local oscillator which is representative of the Doppler shift experienced by the RF signal.

[0066] The mixer used to generate the corrected local oscillator may be any suitable mixer. For example, the mixer may be an electronic mixer or a photonic mixer. The output of the mixer may be the difference between the reference local oscillator and the received Doppler shifted RF signal which will correspond to the Doppler shift. This is described in further detail below in connection with Figure 3. Remaining with Figure 2, the system used to carry out the method may include one or more photonic modulators 21 which are configured to modulate an optical carrier signal 22, e.g. a beam, with the received RF signal 23 which is subjected to a Doppler shift. In the example provided in Figure 2, the modulator 21 may be a photonic mixer 21 in the form a Dual Parallel Mach Zehnder, DPMZ, modulator which is configured to modulate corresponding dual parallel branches of the optical carrier signal 22 with the Doppler shifted RF signal 23 and the corrected local oscillator signal 24 and recombined to provide an intermediate frequency for outputting to a photodiode 25.

[0067] In another embodiment, the modulator 21 may comprise one or more carrier supressed single sideband, CS-SSB, modulators. A CS-SSB modulation may be applied to both the received Doppler shifted received RF signal 23 and the corrected local oscillator 24 to provide two modulate optical signals for recombining prior to being passed to the photodiode 25.

[0068] In some embodiments, the photonic modulator 21 may comprise a double sideband modulator and filter arrangement as an alternative to the carrier suppressed single sideband modulation, as described below in relation to Figure 14.

[0069] It will be appreciated that other photonic modulation schemes which employ a corrected local oscillator may be possible within the present disclosure.

[0070] The output of the photonic modulator 21 may be provided to a photodiode 25 which converts the modulated optical signal into a Doppler corrected electrical RF signal 26.

[0071] It is known that a DPMZ mixer can be used as an RF mixer for both up and down conversion. To provide a mixer, an RF signal is fed into one of the child Mach Zehnder interferometers and an electrical local oscillator, LO, is fed into the other. Both of the child Mach Zehnder interferometers are held in the null-null bias condition. The outputs of the child Mach Zehnders are recombined to provide an output which corresponds to the difference between the RF signal and the local oscillator.

[0072] As already noted, the output of the DPMZ is typically provided to a photodiode which converts the modulated optical output into an electrical signal. The electrical signal may be used as required, for example to generate a digital signal using an analogue to digital converter. Upon being converted into a digital signal, the signal may be processed using known techniques to extract or use the data provided in the RF signal. The processing of the output of the photodiode is generally beyond the scope of this disclosure and will not be described further.

[0073] A DPMZ mixer in the null-null condition has a particular advantage over an electrical mixer in that it produces no local oscillations or RF sideband on the optical carrier and also isolates the local oscillator signal from the desired output of the up or down converted intermediate signal, typically by more than 80dB. On the other hand, electronic mixers typically produce unwanted frequencies from sidebands which can be problematic and require further signal processing, such as filtering, to address.

[0074] Figures 3 and 4 respectively show a method 30 and apparatus (which may be referred to as a local oscillator generation module 40) for generating the Doppler correcting local oscillator 24 according to an embodiment of the present disclosure. In a first step 31, the Doppler shifted RF signal is received by a receiver. It will be appreciated that the receiver and receiving of the Doppler shifted RF signal may be the same as that described in connection with Figure 1. Hence, the received RF signal 31 may be split and used in different modules of the system. Similarly, the optical carrier signal 22 may be provided from a common optical source, e.g. a laser, the beam of which is split into a first and second beam for use in different modules of the system.

[0075] In a second step 32, a reference local oscillator 44 is provided with the received RF signal 23 to a photonic mixer 41 which mixes the received RF signal 23 and reference local oscillator 44 to provide an output which corresponds to the Doppler shift. That is, the mixer 41 may be configured to provide an output which is the difference between the received RF signal 23 and the reference local oscillator 44 and which corresponds to the Doppler shift experienced by the received RF signal 23.

[0076] The optical output from the mixer 41 may be provided to photodiode 45 which converts the optical signal into the corrected local oscillator electrical signal 24. The corrected local oscillator 24 can then be used as an input to correct the received RF signal 23, as shown in Figure 2. As noted above, the mixer 41 may be photonic or electronic. Figure 4 shows an example of a photonic mixer 41 which requires a light source for an optical carrier signal 22 and which outputs an optical signal into the photodiode 45, as known in the art, and it will be appreciated that the optical carrier signal 22 and photodiode 45 would not be required where the mixer 41 is a known electronic mixer.

[0077] As noted above, the reference local oscillator 44 frequency may be determined on the basis of the carrier or centre frequency of the transmitted RF signal. The reference local oscillator 44 may be an International Telecommunication Union reference frequency which corresponds to the carrier frequency or centre frequency, for example. As noted above, the selection of the reference RF signal may be achieved with the knowledge of the transmitted RF signal carrier frequency or centre frequency of the transmission band which may be predetermined or shared between the transmitting and receiving entities prior to transmission. The sharing or predetermining of the carrier frequency or frequency band may be carried out as often as required. The sharing of the frequencies of interest may be carried out during an initial hand shake between the transmitter or receiver and / or at routine intervals, for example.

[0078] An example of a photonic mixer 41 which may be used to generate the corrected local oscillator is provided in Figure 5. Figure 5 shows a DPMZ modulator 41 which receives an optical carrier signal from a light source 52, for example a laser, which is split into two parallel beams 52’, 52”. A first beam 52’ of the two beams is modulated with the received Doppler shifter RF signal, RFd 23. A second beam 52” of the two beams is modulated with the reference local oscillator LOr44. The first and second modulated beams 52m’, 52m” are subsequently re-combined to provide an intermediate frequency output which has a beat frequency which is the difference between LOrand RFd and thus corresponds to the Doppler shift experienced by the received RF signal 23. The intermediate signal 52i may be converted to an electrical signal using the photodiode 45 to provide an electrical signal which corresponds to the corrected (or correcting) local oscillator LOd 24.

[0079] Figure 6 shows an embodiment of a system 60 which can be used to provide a corrected Doppler shifted RF signal 26. The system 60 may comprise an optical carrier signal in the form of a laser 22, a beam splitter 61, which may be a polarisation dependent beam splitter, a local oscillator generation module 40, a photonic modulator 21 in the form of a DPMZ mixer 62, and a photodiode 25. The inputs include the Doppler shifted received RF signal 23 which is provided as an input to the DPMZ mixer 62 and local oscillator generation module 62. The output is provided by photodiode 25 and corresponds to the Doppler corrected RF signal 26.

[0080] The laser may be any suitable laser, the types of which are well known in the art. The laser may be, for example, a distributed feedback laser. It will be appreciated that the laser 22 and splitter 61 are conventional and operated in a conventional manner and so not described further.

[0081] The local oscillator generation module 62 may be the same as that described in connection with Figure 5 but with the light source being provided from the splitter rather than directly from a laser 52.

[0082] Figure 7 shows an example of a DPMZ arrangement which may be implemented in the DPMZ mixer 62 of Figure 6. The DPMZ may be similar to that used to generate the corrected local oscillator in that it comprises a pair of parallel child Mach Zehnder interferometers 71, 72 driven in the null-null bias condition. The Doppler shifted received RF signal 23 is fed into one of the child Mach Zehnder interferometers 71 and the corrected local oscillator, LOd, 24 is fed into the second 72. The outputs of the child Mach Zehnders 71, 72 are recombined to provide an optical output which corresponds to the difference between the RF signal and the corrected local oscillator. The output may be referred to as an intermediate frequency signal or intermediate signal. The optical intermediate signal may be provided to the photodiode 25 which converts the optical signal into an electrical signal which can be subsequently processed in accordance with an end requirement.

[0083] Figure 8 shows a second example of a system which can be used to provide a corrected Doppler shifted RF signal 26. The system is generally similar to that of Figure 6 described above and common reference numerals and the previous description relied upon. However, the system of Figure 8 utilises an electronic mixer 40’ in place of the photonic mixer to provide the corrected local oscillator for use in the DPMZ. Electronic mixers of the type required for use here are well known and not described further for the sake of brevity. As noted above, the present disclosure contemplates a system which utilises one or more single sideband carrier suppressed modulators in which the received RF signal and corrected local oscillator may be single sideband modulated with the optical carrier supressed and the outputs combined to provide an optical signal with a beat frequency which is provided to the photodiode which converts the signal into an electrical output signal corresponding to the corrected RF signal.

[0084] The optical outputs of an CS-SSB according to the present disclosure is shown in Figure 9 in which the light power is shown in the Y axis and the frequency is shown in the X-axis. The plot demonstrates the operating principle of the Doppler correction with the original optical carrier frequency of the modulator as the dashed line which has been supressed. The solid lines LOrand RFtare the sidebands corresponding to the local oscillator and transmitted RF signal which would be present in the case where there is no Doppler shift occurring during the transmission. The dashed sidebands LOd and RFd correspond to the corrected local oscillator and received RF signal when there has been a Doppler shift. It is of note that, due to the local oscillator being corrected, i.e. frequency shifted by an amount corresponding to the Doppler shift, the separation between the local oscillator and respective RF signal in the Doppler shifted and nonshifted cases are the same. Hence, when the optical sideband signals are provided at the photodiode, the beat signal is the same and corresponds to the RF signal with no Doppler shift.

[0085] Figure 10 builds on this principle by showing the light signals of Figure 9 on the right-hand side, and the downshifted signals on the left-hand side. It will be appreciated that the photodiodes shown in Figure 10 may be the same photodiode as the photonic modulator may be configured to up-convert or down-convert by selecting the sideband of interest.

[0086] As noted the present disclosure provides a photonic modulator in the form of a carrier suppressed single sideband, CS-SSB, modulator. Photonic CS-SSBs are known in the art and involve the modulation of an optical carrier with quadrature phased RF inputs using a DPMZs. Hence, one child modulator of a DPMZ is provided with an in- phase RF signal, with the other provided with the same RF signal with a 90degree phase difference. The output from the CS-SSB is a single sideband with the carrier suppressed. The choice of the 90degree phase difference, i.e. leading or lagging, determines whether it is upper sideband or lower sideband which is provided at the output. The upper sideband is associated with a +90degree phase shift, the lower sideband is associated with a -90degree phase shift.

[0087] In the present disclosure, the quadrature RF inputs are provided by in-phase and 90degree phase shifted waveforms of the respective RF signal, i.e. the corrected local oscillator 24 or the received RF signal 23, in two separate CS-SSB modulators. This is shown in Figure 11 which provides a schematic representation of a system 110 in which the photonic modulator 21 of Figure 2 is provided by apair of CS-SSB modulators 111, 112 with which a Doppler shift in a received RF signal can be corrected. Common reference numerals are used to denote common features which have been previously described and these features will not be described further.

[0088] Figure 12 shows an implementation of the first CS-SSB modulator 111, which may be referred to as the local oscillator SSB, and Figure 13 shows an implementation of the second CS-SSB 112 which may be referred to as the received RF signal SSB.

[0089] Returning to Figure 11, the general arrangement comprises an optical carrier source in the form of a laser 22 which feeds an optical carrier signal into a beam splitter 61. The beam splitter 61 is configured to provide a first output to the first CS-SSB module 111 for modulation by the corrected local oscillator 24 and a second output to the second CS-SSB module 112 for modulation by the received RF signal 23. The outputs from the local oscillator SSB 111 and received RF signal SSB 112 are recombined in a combiner 114 which beats the respective signals before providing an output to the photodiode and outputs the Doppler corrected RF signal. The term “beats” as used herein should be taken to have its conventional meaning within the field of photonics which is to superimpose the respective light signals to provide a combined signal which is the difference of the optical frequencies between the two signals.

[0090] It will be noted that the splitter 61 and combiner 114 are polarization dependent which allows the recombination of the respective light waves to be more straightforward as well-known in the art. Due to the orthogonal nature of the outputs from the beam splitter 61, one of the SSB paths may include polarisation rotators 113 to allow for the correct modulation in the SSB. Although not shown, it will be appreciated that the corrected local oscillator LOd may be generated using the photonic mixer 41 shown in Figures 4 and 5. As such, the light beam originating from the laser 22 may be further split to allow an optical carrier to be provided to the corrected local oscillator photonic mixer.

[0091] With reference to Figure 12 there is shown an embodiment of the local oscillator SSB comprising dual parallel Mach-Zehnder modulators in a null-null bias condition in which a first child modulator 111-1 receives the corrected local oscillator signal LOd and the second child modulator 111-2 receives the 90degree phase shifted corrected local oscillator signal + / -90 LOd. Figure 13 shows similar but for the received Doppler shifted RF signal in which the first child modulator 112-1 receives the received RF signal, and the second child modulator 112-2 receives the phase shifted RF signal + / - 90 Doppler Shifted RF.

[0092] The choice of whether to select a positive or negative phase shift for the CS- SSB modulator 111 will depend on whether the Doppler shift is positive and negative. The choice of whether to select a positive of negative phase shift for the CS-SSB modulator 112 will depend on whether the upper or lower sideband is required at the output. Hence, if the upper sideband is required then a positive 90degree phase shift will be applied to the second child modulator 112-2 of the received RF SSB 112, with a negative phase shift if the lower sideband is required. If the Doppler shift is positive, a positive phase shift is applied to the corrected local oscillator signal at the second child modulator 111-2 of the corrected local oscillator modulator 111. If the Doppler shift is negative, then a negative phase is applied to the corrected local oscillator.

[0093] The selection of the corrected local oscillator to coincide with the direction of the Doppler shift means that the sideband of the corrected local oscillator is shifted in the same direction and by the same amount as the shift experienced in the received RF signal sideband, thereby compensating for the shift and maintaining the spacing between the two sidebands when they are recombined.

[0094] It will be appreciated that, although the selection of a positive or negative phase shift for the LO may be advantageous, in some embodiments, the Doppler correcting local oscillator may be shifted to account for a negative Doppler shift in other ways. Hence, for example, an additional frequency may be added to both LOd and RFd to shift LOd and RFd to above (or below) the carrier frequency by a similar amount. The additional frequency may be greater than any possible Doppler shift such that the relative separation between LOd and RFd is maintained, but the need to flip the phase in the LOd SSB modulator 111 is removed. The additional frequency can later be subtracted from corrected RF signal. In some embodiments, a convenient additional frequency, LOadd, may comprise the corrected Doppler shift frequency LOd itself. The additional frequency may be provided by an additional local oscillator (not shown).

[0095] Returning to Figure 11, the outputs of the local oscillator SSB and the Doppler shifted RF SSB are shown at 115 and 116 respectively. In this example, the upper sideband is required at the output so a positive 90degree phase shift was chosen for the received RF SSB 112, thereby providing the RF upper sideband 116RFd to the right of the supressed carrier frequency CF. As can be seen from the output 116, the sideband 116RFd is downshifted from the transmitted frequency 116RFT which is represented by the dashed line so the Doppler shift is negative. As such, the phase shift applied to the corrected local oscillator modulator 111 is negative which downshifts the corrected local oscillator sideband 115LOd from the carrier frequency CF by an amount which corresponds to the Doppler shift. In doing so, the separation between the sidebands is maintained as the same as would be experienced between the carrier frequency and sideband if there were no Doppler shift. Recombining the two outputs of the respective SSBs results in a difference frequency which is corrected for Doppler shift.

[0096] It will be appreciated from the above explanation that if the Doppler shift were positive, the received RF sideband 116RFd would move to the right of the transmission frequency 116RFT and a corresponding positive shift would be provided to the corrected local oscillator sideband 115LO<I putting it to the right of the suppressed carrier. Similar logic follows if the lower sideband were required at the output.

[0097] The selection of the correct phase shift for the SSBs may be made using a simple logic circuit to provide a + / -90 degree phase difference in the CS-SSB upon a determination of the positive or negative Doppler shift.

[0098] In order to determine whether a positive of negative phase shift is required for the corrected local oscillator SSB modulator, it may be necessary to ascertain whether there is a positive or negative Doppler shift. As noted above, some embodiments of the present disclosure are configured to correct Doppler shift in the absence of knowledge of the relative positions and velocities of the transmitter and receiver. The detection of the relative Doppler shift, that is, whether it is upshifted or down shifted, may be achieved with conventional logic circuits which compare the received RF signal to the expected carrier frequency or centre frequency.

[0099] Figure 14 shows a double sideband modulator arrangement in the form of a single Mach Zehnder modulator 141 and filter 142 in place of the received RF SSB 112. The use of the double sideband modulation and filtering may be advantageous for higher frequencies and be simpler to implement than an CS-SSB. The double sideband modulation may provide the upper and lower sidebands of the received RF signal, with the filter removing the unwanted sideband and carrier frequency. A similar arrangement may also be used in place of the corrected local oscillator SSB 111.

[0100] Many embodiments of the present invention will be focussed on communication systems. However, the present invention for compensating for a Doppler shift in an RF signal may find utilisation in other areas. One such area is in moving objects, such as vehicles for example, in which the Doppler shifted received RF signal may be used to predict a potential collision course for two vehicles which do not have line of sight of each other.

[0101] Figure 16 shows a scenario in which a first car, car 1, and a second car, car 2 are approaching a junction with a potential collision point indicated at X. Each of the vehicles do not have line of sight of the other vehicle and so the potential for a collision is not evident until the cars are potentially too close to take evading actions.

[0102] The or each vehicle may be able to use a radar or lidar system to transmit and receive electromagnetic reflections from moving bodies in the vicinity of the vehicles. So, for example, car 2 may transmit an electromagnetic wave “1” which is reflected off surrounding objects such as buildings etc, towards car 1 of which there is no line of sight. Car 1 will reflect the electromagnetic wave which will return to a receiver in the transmitting vehicle, car 2, via pathways 3 and 4. As cars 1 and 2 both have velocity and direction relative to one another, the received signal will have undergone a Doppler shift by the time it has returned to the transmitting vehicle.

[0103] Using the Doppler shift it may be possible to determine a velocity of the vehicle in any particular time interval and select any velocity which will result in the collision of the vehicles.

[0104] It will be appreciated from the above that the collision avoidance system may be configured to recognise ‘reflection opportunities’ and focus selectively on the ‘reflection of interest’ applying Doppler shift compensation and accurately predicting the location of the desired object moving. In other words, in a vehicle display will detect every object / human moving due to electromagnetic field reflections. However, the system may be configured to focus only on objects such as vehicles that are on the road and on a potential collision course. The detection of the collision course may be based on the speed, distance and frequency, and include the Doppler shift compensation according to the present disclosure. This will be particularly useful for detecting collisions where there is no line of sight.

[0105] It will be appreciated that, in a typical operating environment, there are many RF signals which may be received by a vehicle and each of these may relate to a potential collision vehicle and may have a varying Doppler shift associated with it as the relative vector velocity changes with respect to the RF sources and the vector velocity of any reflection.

[0106] The present disclosure contemplates the use of a machine learning algorithm to determine potential RF signals from vehicles which would cause a collision. It will be appreciated that this set may be a very small number of signals compared to the total number of RF signals in a typical operating environment which may include signals from sources such as: Wi-Fi, mobile phones, anti-collision car radar, automatic doors and speed cameras etc.

[0107] The criteria for objects to be included as potential collision threat object set may include those with a vector speed and distance that intersects the timeline position of the receiver vehicle. Such collision signals will in general be weak compared to the majority of RF signals. Hence, the system may comprise signal processing to lock-on and filter the inputs. The signal processing may include photonic signal processing where applicable.

[0108] The system may be configured to pick out potential collision vehicles by repeatedly calculating the distance and vector speed of all RF signals to see if they intercept the track of the receiver object at different points in time. The results of this vector and speed and distance are stored and used to predict the likely measurement of the object in the next time period. In this way the system may continually reduce the number of possible fits so less calculations are required over time. Storing the results for each time period may create a path of travel and add to the reliability of fit. The system may further comprise access to the layout of the local environment such that only feasible paths of travel, i.e. those within the boundaries of possible paths of travels as defined by roads, for example, are considered.

[0109] Additionally, a knowledge of the likely performance of the RF sources will be required, since the variation in the frequency, phase and intensity of an RF source will depend on its application. The simplest RF sources may comprise an unswept single frequency. Automatic door sensors and police speed cameras are examples of such simple sources. For these simple sources, the system may place in the high-risk set, those who’s vector speed, distance to intercept and no frequency change is observed once the Doppler shift has been corrected for. Such a Doppler correction may be readily done using Single Side Band (SSB) photonic methods as disclosed herein. Similar photonic Doppler correction can be applied to other RF sources once the sources are recognised and their performance known.

[0110] By way of an example, it is known that if a fixed angle is maintained between moving objects they will collide. Thus, once the expected Doppler corrective frequency is calculated, say for a reflection of such a source, in the next time period of a path of travel for an approaching vehicle, this Doppler correcting frequency could be applied via SSB photonic frequency shifter as disclosed herein. Once the photonic shift is applied, and the received frequency is found, it may be assumed that the system has made a correct assessment of the situation. Furthermore, if the model finds no corrected change of frequency and no change of angle with respect to the receiver, it may be indicative that the vehicle is on a collision course.

[0111] It will be appreciated that photonic RF pre-processing methods may aid the exploitation of ubiquitous RF sources because of their superior performance. Photonic pre-processing is more agile, has a larger instantaneous bandwidth, yet has low cross talk and can have narrow dynamic multiple notch filters and selective RF gain thereby able to address more RF source in a given time frame. The photonic RF pre-processing methods will have huge impact in future driverless cars because there will be huge traffic and complex communication between Vehicle to Vehicle communication (V2V), V2X ...Vehicle to all (sensors... etc), or Machine to machine (M2M) communications or device to device communication, IOT etc, where huge bandwidth, high precision, high-speed, safety, low CO2 emission are critical.

[0112] The system is required to track the objects whose vector velocity puts them on a collision course with the receiver at a time the receiver will be there. In order to do this, the system has to identify high risk paths of travel multiple potential RF sources and reflections and, ideally, a digital model of its surroundings. The Doppler shift of the RF sources will depend on the direction and speed of the source, and the reflection angle off buildings (and the speed and direction of the receiver). Disentangling such information might best be done using predicted track solutions for the next time period and calculating the expected Doppler shift, and then looking for that Doppler shifted signal. If such a signal is found then that track is more likely to be real. The above photonic frequency shifting method will aid the search of signals for the Doppler shifted the system has calculated. To aid such tracking solutions, particularly in an urban environment, the Al should be initially programmed with GPS and electronic maps to allow the algorithm to come to more rapid and reliable tracking solutions.

[0113] Along with the ability to Doppler correction as illustrated above, Photonics will allow Al to take in more information, from more RF signals because its ultra-high instantaneous bandwidth and it ability to down convert them more cleanly to lower frequencies where higher resolution ADC electronics can work. Photonics can also apply multiple narrow notch filtering and gain to enhance these signals. The output from the Al model of physical world, will create predictions of the Doppler shift which can be applied to each proposed track in the paragraph above. Such proposed shifts can rapidly be applied by photonics, and the signal thereby searched for, in order to create the next point on the track of an object (or otherwise). Another additional advantage is that such smart Photonics modules are lightweight, can be fit in any location of the vehicle / device and can be design to sustain vibration, heat and radioactive environmental surroundings.

[0114] In some embodiments the corrected local oscillator may be generated using a machine learning algorithm. Thus, as shown in Figure 15, the received RF input may be pre-processed to provide the received RF signal in a form that can be inputted to a machine learning algorithm. For example, the received RF signal may be filtered to remove any undesirable components such as noise, etc. The RF signal may also be converted to provide a digital signal for inputting into a computer. Other pre-processing steps may include feature scaling depending on strength of signal, that is, amplification or attenuation.

[0115] The RF signal may also undergo some feature extraction function to improve the system. For example, the received RF signal will be in the time domain and may be required to be converted into the frequency domain in order to extract the pertinent information for a particular machine learning algorithm. Further, the received RF signal may undergo some dimensional reduction to increase the processing speed in the neural network.

[0116] The machine learning algorithm may comprise a neural network model which is trained using a supervised learning. The neural network could be a convolutional neural network or recurrent neural network. Long- short term memory network.

[0117] Data outputted from the neural network may post-processed to determine a useful frequency for the corrected local oscillator. For example, the predicted value will be a continuous number such as the estimated frequency which represents a regression problem and a regression model to determine the required frequency. In some embodiments, the post-processing may require a classification function to determine the required local oscillator. Although not shown, the machine learning algorithm may comprise a feedback loop.

[0118] The collision detection system may be implemented with a machine learning algorithm as shown schematically in Figure 17. In a first step a received RF signal which consists of a range of frequencies as seen by the receiver and as a result of the signal transmitted by the vehicle. The received RF signal may undergo pre-processing to reduce noise in the signal using known techniques with a view to making the feature extraction process easier. The feature extraction will reformat the received RF signal and perform various transforms to the data and identify key features which can be used in a generative model. This stage has potential to involve some machine learning if needed to get some of the features. This could include a simple classification model to pick out whether an object is moving, or if it is stationary. A second stage of preprocessing may be used to do some data formatting, for example, to reshape the data into a matrix and perform feature scaling, etc.

[0119] A generative model may be used to take the features extracted from the RF input and create a representation of the area. This will be a simple view of the area and highlight where the key objects are. This may be overlayed with a map of the area to create a more complete view of the surroundings such as roads and buildings etc.

[0120] A pathway estimation model may be used to take a set of the generative reconstructions over a short period of time. This could include some of the parts of the feature extraction and may predict the pathway of the moving objects.

[0121] A classification algorithm may then be used to look at the pathways of the moving objects and the current pathway of the RF receiver and identify which of the objects it believes will collide with itself.

[0122] The one or more embodiments are described above by way of example only. Many variations are possible without departing from the scope of protection afforded by the appended claims. The detailed description includes detailed embodiments with

Claims

CLAIMS1. A method of correcting Doppler shift in a received RF signal band, comprising: receiving, by a photonic device: an optical carrier signal; a received RF signal band which has undergone a Doppler shift during a transmission; a corrected local oscillator corrected for the Doppler shift; modulating a first beam of the optical carrier signal with the received RF signal band; modulating a second beam of the optical carrier signal with the corrected local oscillator; and, generating, with a photodiode, an electrical signal using the modulated first and second beams of the optical carrier signals; and wherein: the RF signal band comprises a plurality of RF signals on different carrier frequencies; the corrected local oscillator is corrected for Doppler shift by: inputting, to a corrected local oscillator mixer, a reference local oscillator signal; and mixing the reference local oscillator signal with a centre frequency of the received RF signal band to provide the difference between the reference local oscillator and the centre frequency of the received RF signal band, the difference being the corrected local oscillator; and the reference local oscillator signal has a frequency corresponding to the centre frequency of the RF signal band prior to transmission and the Doppler shift; and the centre frequency of the RF signal band prior to transmission and the Doppler shift is known to the receiver ahead of the transmission or communicated within the transmission of the received RF signal.

2. The method of claim 1, wherein the corrected local oscillator mixer is a photonic mixer.

3. The method of either claim 1 or claim 2, wherein the photonic device comprises at least one Dual Parallel Mach Zehnder Modulator.

4. The method of any preceding claim, wherein the photonic device comprises a photonic mixer configured modulate and mix the first and second beams, wherein an output signal of the mixer is provided to the photodiode.

5. The method of any of claims 1 to 3, wherein the photonic device comprises at least one carrier suppressed single sideband, CS-SSB, modulator.

6. The method claim 5, wherein at least one of: the corrected local oscillator is CS-SSB modulated by a first CS-SSB modulator; or, each RF signal of the received RF signal band is CS-SSB modulated by a second CS-SSB modulator.

7. The method of claim 6, wherein both the corrected local oscillator and each RF signal of the received RF signal are CS-SSB modulated by the respective first and second CS-SSB modulators.

8. The method of claims 6 or 7, wherein the corrected local oscillator and each RF signal of the received RF signal band are provided to the respective first and second CS-SSB modulators in quadrature.

9. The method of claim 8, wherein the CS-SSB modulation comprises: applying a 90 degree phase shift to the corrected local oscillator inputted to a first child modulator of the first CS-SSB and applying the in-phase corrected local oscillator to a second child modulator of the first CS-SSB; and, applying a 90 degree phase shift to each RF signal of the received RF signal band inputted to a first child modulator of the second CS-SSB and applying the in- phase RF signal to a second child modulator of the second CS-SSB.

10. The method of claim 9, further comprising: determining whether the Doppler shift is positive or negative; applying either a positive 90 degree phase shift or negative 90 degree phase shift to the corrected local oscillator depending on the determination of whether the Doppler shift is positive or negative.

11. The method of claim 10, wherein determining whether the Doppler shift is positive or negative comprises: comparing the carrier frequency or centre frequency of the received RF signal to a reference; and, determining whether the carrier frequency or centre frequency has been upshifted or downshifted from the comparison.

12. The method of any of claims 1 to 3 wherein at least one of the corrected local oscillator and received RF signal band is double sideband modulated.

13. The method of claim 12 further comprising filtering the double sideband modulated corrected local oscillator or received RF signal band to remove an optical sideband and carrier signal.

14. The method claims 12 or 13, wherein one of the corrected local oscillator or received RF signal band is double sideband modulated and the other of the corrected local oscillator or received RF signal is CS-SSB modulated.

15. A photonic system, comprising: a photonic modulator configured to receive an optical carrier signal, a Doppler shifted received RF signal band, the RF signal band comprising a plurality of RF signals on different carrier frequencies, and a Doppler corrected local oscillator and modulate respective beams of the optical carrier signal the received RF signal band and corrected local oscillator;a local oscillator correction module configured to provide the corrected local oscillator having a frequency which is the difference between the frequency of a reference local oscillator and the centre frequency of the received RF signal band, and configured to provide the corrected local oscillator to the photonic modulator; and, a photodiode to convert an output of the photonic modulator to an electrical signal which corresponds to the received RF signal corrected for Doppler shift; and wherein: the reference local oscillator signal has a frequency corresponding to the centre frequency of the RF signal band prior to transmission and the Doppler shift; the centre frequency of the RF signal band prior to transmission and the Doppler shift is known to the receiver ahead of the transmission or communicated within the transmission of the received RF signal.

16. The photonic system of claim 15, wherein the local oscillator correction module comprises: an input configured to receive the reference local oscillator signal; an input configured to receive the centre frequency of the received RF signal band; and, a mixer configured to mix the reference local oscillator and the centre frequency of the received RF signal band and output the difference between the centre frequency of the received RF signal band and reference local oscillator, wherein the output is the corrected local oscillator.

17. The photonic system of claim 16, wherein the mixer is a photonic mixer or an electronic mixer.

18. The photonic system of any of claims 15 to 17, wherein the photonic modulator comprises one or more double sideband modulators, one or more CS-SSB modulators and / or one or more photonic mixers.

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