DESC:FORM 2
THE PATENTS ACT, 1970
(39 OF 1970)
&
THE PATENTS RULES, 2003

COMPLETE SPECIFICATION
[SEE SECTION 10, RULE 13]

A RADIO FREQUENCY (RF) COMMUNICATION METHOD, RF TRANSMITTER, AND RF RECEIVER THEREOF

BHARAT ELECTRONICS LIMITED
WITH ADDRESS:
OUTER RING ROAD, NAGAVARA, BANGALORE 560045, INDIA

THE FOLLOWING SPECIFICATION PARTICULARLY DESCRIBES THE INVENTION AND THE MANNER IN WHICH IT IS TO BE PERFORMED.
FIELD OF INVENTION
[0001] The present invention relates generally to radio communication system and specifically to Radio over fiber (RoF) or RF over fiber (RFoF).

BACKGROUND
[0002] Surveillance systems are classified as strategic systems and tactical systems. Further, the strategic systems are classified as Electronic Intelligence (ELINT) and Communication Intelligence (COMINT). The tactical systems are classified as Electronic Support Measures (ESM), Electronic Counter Measures (ECM) and Electronic Counter-Counter Measures (ECCM). At the simplest, electro-optics is the combination of optical and electronic technologies. As radars have transmitters so in the electro-optics there are lasers and lamps. As the radars have receivers so in the electro-optics there are detectors.
[0003] The optical technology has advanced to a state where it is acknowledged as a cost effective and vastly superior alternative to conventional electronic systems. There are numerous benefits of using optical fiber. Its inherent immunity to Radio Frequency interference makes RoF a popular choice for replacing RF links. RoF also has much smaller attenuation than RF cable which enables realizing high performance ultra-wideband systems with promising sensitivities and dynamic ranges. Fiber optics is designed to handle higher speeds which improve the processing speed of the system. Fiber is light weight and easier to physically route around space constrained structures. Since fiber optics does not radiate electromagnetic energy, emissions cannot be intercepted and physically tapping the fiber takes great skill to get undetected. Thus, the fiber is the most secure medium available for carrying sensitive data. Maintenance cost of the RoF Module is very low.
[0004] In US patent No. US8265484B2, the RF signal transport over passive optical networks is disclosed, wherein the RF signal transported is converted into digital signals using sampling signal associated with the frequency and phase reference signal and mechanism to form as packets and transmit over optical transceiver.
[0005] In another conventional disclosure, PCT WO1994023507A1 discloses that the optical beam having high radio frequency modulation is generated at output by generating in source a low frequency modulation, using it to control the optical output of a laser and further modulating the optical output in an optical modulator by a control signal having another lower frequency modulation generated by source. Either or both of the lower frequency modulations also carry an information containing modulation. The effect of the optical modulator is to up-convert the modulation carried by the optical beam by the modulation frequency of the control signal. The optical modulator may be a Mach-Zehnder interferometer. The non-linearity of such a modulator with respect to its control input may be exploited by selecting the amplitude of the control signal such that the optical output is up converted by an integer multiple of the modulation frequency generated by source. These methods avoid the need to apply the high frequency modulation to either the laser input or the control input directly.
[0006] Thus, there is a need for an effective RoF system for surveillance system and a method for optimizing RoF modules.

SUMMARY
[0007] This summary is provided to introduce concepts related to a Radio Frequency (RF) communication method. This summary is neither intended to identify essential features of the present invention nor is it intended for use in determining or limiting the scope of the present invention.
[0008] In an embodiment of the present invention, a Radio Frequency (RF) communication method is provided. The method includes receiving an ultra-wideband RF input signal from an RF source. The ultra-wideband RF input signal is split into a plurality of RF signals by an RF power splitter. The plurality of RF signals are modulated by a plurality of modulators operating in parallel to generate a plurality of modulated RF signals. The plurality of modulated RF signals are combined by an RF power combiner to form an output signal. The output signal is transmitted over an optical fibre. The optical fibre is spliced such that amplitude, frequency, and phase of the output signal matches with the optical fibre.
[0009] In another embodiment of the present invention, a Radio Frequency (RF) transmitter is provided. The RF transmitter includes an RF power splitter, a plurality of modulators, an RF power combiner, and a transmitter. The RF power splitter is configured to receive an ultra-wideband RF input signal and splits the ultra-wideband RF input signal into a plurality of RF signals. The modulators are configured to operate in parallel and modulate the plurality of RF signals to generate a plurality of modulated RF signals. The RF power combiner is configured to combine the plurality of modulated RF signals to form an output signal. Further, the transmitter is configured to transmit the output signal over an optical fibre. The optical fibre is spliced such that amplitude, frequency, and phase of the output signal matches with the optical fibre.
[0010] In yet another embodiment of the present invention, a Radio Frequency (RF) receiver is provided. The RF receiver includes an RF power splitter and a plurality of demodulators. The RF power splitter is configured to split an output signal into a plurality of reconstructed RF signals. The demodulators are configured to operate in parallel and demodulate the plurality of reconstructed RF signals to generate a plurality of reconstructed demodulated RF signals.
[0011] In an exemplary embodiment, the modulators include Mach-Zender interferometers for direct modulation of a plurality of laser signals based on the plurality of RF signals.

BRIEF DESCRIPTION OF ACCOMPANYING DRAWINGS
[0012] The detailed description is described with reference to the accompanying figures.
[0013] Fig. 1 illustrates a schematic block diagram of a Radio over fiber (RoF) module, in accordance with an embodiment of the present invention.
[0014] Fig. 2 illustrates a schematic block diagram of a Radio Frequency (RF) transmitter, in accordance with an embodiment of the present invention.
[0015] Fig. 3 illustrates a schematic block diagram of a Radio Frequency (RF) receiver, in accordance with an embodiment of the present invention.
[0016] Fig. 4 is a flowchart illustrating a Radio Frequency (RF) communication method in accordance with an embodiment of the present invention.
[0017] It should be appreciated by those skilled in the art that any block diagram herein represents conceptual views of illustrative systems embodying the principles of the present invention. Similarly, it will be appreciated that any flow chart, flow diagram, and the like represent various processes which may be substantially represented in computer readable medium and so executed by a computer or processor, whether or not such computer or processor is explicitly shown.

DETAILED DESCRIPTION
[0018] The various embodiments of the present invention provide a Radio Frequency (RF) communication method, an RF transmitter, and an RF receiver.
[0019] In the following description, for purpose of explanation, specific details are set forth in order to provide an understanding of the present invention. It will be apparent, however, to one skilled in the art that the present invention may be practiced without these details. One skilled in the art will recognize that embodiments of the present invention, some of which are described below, may be incorporated into a number of systems.
[0020] However, the systems and methods are not limited to the specific embodiments described herein. Further, structures and devices shown in the figures are illustrative of exemplary embodiments of the present invention and are meant to avoid obscuring of the present invention.
[0021] Furthermore, connections between components and/or modules within the figures are not intended to be limited to direct connections. Rather, these components and modules may be modified, re-formatted or otherwise changed by intermediary components and modules.
[0022] References in the present invention to “embodiment” or “embodiment” mean that a particular feature, structure, characteristic, or function described in connection with the embodiment or the embodiment is included in at least one embodiment or embodiment of the invention. The appearances of the phrase “in an embodiment” in various places in the specification are not necessarily all referring to the same embodiment.
[0023] In an embodiment of the present invention, a Radio Frequency (RF) communication method is provided. The method includes receiving an ultra-wideband RF input signal from an RF source. The ultra-wideband RF input signal is split into a plurality of RF signals by an RF power splitter. The plurality of RF signals are modulated by a plurality of modulators operating in parallel to generate a plurality of modulated RF signals. The plurality of modulated RF signals are combined by an RF power combiner to form an output signal. The output signal is transmitted over an optical fibre. The optical fibre is spliced such that amplitude, frequency, and phase of the output signal matches with the optical fibre.
[0024] In another embodiment of the present invention, a Radio Frequency (RF) transmitter is provided. The RF transmitter includes an RF power splitter, a plurality of modulators, an RF power combiner, and a transmitter. The RF power splitter is configured to receive an ultra-wideband RF input signal and splits the ultra-wideband RF input signal into a plurality of RF signals. The modulators are configured to operate in parallel and modulate the plurality of RF signals to generate a plurality of modulated RF signals. The RF power combiner is configured to combine the plurality of modulated RF signals to form an output signal. Further, the transmitter is configured to transmit the output signal over an optical fibre. The optical fibre is spliced such that amplitude, frequency, and phase of the output signal matches with the optical fibre.
[0025] In yet another embodiment of the present invention, a Radio Frequency (RF) receiver is provided. The RF receiver includes an RF power splitter and a plurality of demodulators. The RF power splitter is configured to split an output signal into a plurality of reconstructed RF signals. The demodulators are configured to operate in parallel and demodulate the plurality of reconstructed RF signals to generate a plurality of reconstructed demodulated RF signals.
[0026] In an exemplary embodiment, the modulators include Mach-Zender interferometers for direct modulation of a plurality of laser signals based on the plurality of RF signals.
[0027] Fig. 1 illustrates a schematic block diagram of RoF module with Mach-Zender Modulator for wideband surveillance receiver according to an embodiment of the present invention. The RoF module (100) includes a RoF transmitter section (102), a RoF receiver section (104) and an optical cable (105) that connects the RoF transmitter (102) to the RoF receiver (104).
[0028] The RoF transmitter (102) includes a first transmitter impedance matching module (106), a first transmitter amplifier (110), a first transmitter equalizer (112), a first transmitter attenuator (114), a second transmitter amplifier (116), an equalizer (118), a second transmitter impedance matching unit (120), a Mach-Zender external modulator (124), a laser (126), and a laser driver (128).
[0029] The RoF receiver (104) includes a photo diode (130), a receiver impedance matching module (132), a first receiver amplifier (134), a first receiver equalizer (136), a first receiver attenuator (138), a second receiver amplifier (142), a second receiver equalizer (144), a second receiver attenuator (146), and a third receiver amplifier (148).
[0030] The RF signal received by the antenna is fed to RoF Transmitter module located near the antenna. The RF signal is conditioned and amplified, which modulates with light using External Modulation. The resultant optical signals are transmitted over single mode Optical Fiber cable with lengths beyond 50 meters at 1550 nm wavelength to the RoF Receiver module. At the Receiver end high frequency broadband planar PIN photodiode is used to detect this light signal. The output of the receiver module is conditioned and converted into Intermediate Frequency signals (IF). These IF signals are further processed after sampling at the receiver.
[0031] The RoF modules are developed with multi-channel and phase matched capabilities. Hence 4 channels of RF-Optical and Optical-RF conversions can be simultaneously achieved. This is very useful in surveillance receivers where a technique like BLI (Base Line Interferometry) is used for broadband Direction finding.
[0032] In the general ROF modules, usage of Direct modulation by LASER diode in RoF Tx section puts an upper limit on Frequency of operation i.e., up to 4 GHz. Hence the Direct modulation-based RF-Op conversion cannot be used in wide band surveillance receivers. This challenge can be addressed by replacing the LASER based direct modulation stage with “Mach-Zender Interferometer” based External Modulation (Fig. 1). Using these technique higher RF bandwidths can be achieved.
[0033] The RF signal picked up by the antenna is fed to RoF Transmitter module present in the AHU, which modulates RF signal with light using External Modulation. These optical signals are transmitted over single mode Optical Fiber cable of 50 m length at 1550 nm wavelength to the RoF Receiver module at processing unit located at far of distance. At the Receiver end high frequency broadband planar PIN photodiode is used to detect this light signal. The technical specifications achieved using the special hardware is given below Table 1.
Sr. No. Parameters Specification
1 Operating Frequency 2 GHz -18 GHz
2 Gain 4 dB min
3 Gain flatness ? 4 dB
4 Sensitivity @ 0 dB SNR -75 dBm Min
5 Instantaneous Dynamic range 40 dB Min
6 Extended dynamic range 70 dB Max (-70dBm to 0dBm)
7 Input at 1dB compression Without attenuator : -35 dBm
With attenuator : +5 dBm
8 Noise figure 10 dB Max without attenuator
27 dB Max with attenuator
9 VSWR 2:1 Max
10 Phase noise @100Hz offset 100 dBc Max
11 Impedance 50 ohms
12 Pulse Width Handling 50 nsec to CW
13 Interface RS 422
14 Phase Matching ? 15?Max
15 Amplitude Matching ? 2.0 dB Max
16 Voltage & Current +12V @ 3.2A, +5V @ 8A, -5V @ 0.8A
17 Operating Temperature -20°C to +55°C
18 Storage Temperature -40°C to +85°C
Table-1
[0034] The developed technology is tested for various parameters to ensure the developed technologies are suitable for ESM/ELINT applications.
[0035] For measuring Sensitivity, Dynamic Range, Gain, 1dB compression point, Harmonics and Spurious of ROF module, the ROF Tx and Rx module are excited with a DC supply of 12V and ±5V. RF signal is fed from the signal generator to RoF Transmitter module and the received RF signals is monitored on the spectrum analyzer at the output of RoF receiver module.
[0036] Table 2 showing Noise Figure of four channel RF over Fiber Module is presented below:
Table-2
[0037] Table 3 showing Sensitivity of four channel RF over Fiber Module is presented below:
Table-3
[0038] ESM system for Direction Finding (DF) for the complete 360° coverage, 4 antenna arrays are used (each covering an angle of ±45°). The 4 RF’s from antenna are fed to a channel tuned receiver module for down conversion to the IF then it is given to Digital Receiver for Radar parameter measurement. Here depending upon the DF technology used, phase and amplitude of radar signals received by the antenna, Direction is calculated using phase / amplitude comparison Algorithms. Hence, all components present in the front end of antenna hardware should be amplitude and Phase Matched. In need of this, Phase and Amplitude matching of Modules is critical. Hence the four channel RoF modules are tested and evaluated. The practical ESM system will have 4 set of antennas in each quadrant, the output of these antenna is fed to ROF transmitter module, the optical signals are then transmitted from the AHU to the deck where the receiver ROF module will convert them back to RF signals and is fed to the input of the Super Heterodyne Receiver module. The SHR module will have 4 RF channels which further down converts to IF. The measured Amplitude matching and phase matching results of four channel RoF module with Channel-4 taken as reference are tabulated in the Table 4 presented below.
[0039] Table 4 showing Amplitude matching results of four channel RoF module is presented below:
Table-4
[0040] An apparatus having capability of converting high dynamic range ultra-wide band RF signals to laser signals and back to ultra-wideband RF signals with very low noise figure and insertion is provided in an embodiment of the present invention. The method provided by the present invention includes conversion of ultra-wide band RF signals in to laser signals with low noise figure with RF module and a laser diode, transportation of multiple laser signals on a combined fiber line, separating of the combined laser signal in to multiple laser signals and conversion of laser signals into RF signals with photodiode, and achieving amplitude and phase matching over n fiber lines with optical splicing. The hardware includes the RF, equalizer, laser and photodiode in the chain to achieve the specifications of the apparatus. The equalizer achieves flatness over entire ultra-wide band frequency ranges.
[0041] In another embodiment, the present invention provides a method to convert RF signals into laser signals with laser diode and externally modulate using Mach-Zender Interferometer to achieve high dynamic range. The method achieves low noise figure with placement of low noise RF amplifier before the laser diode. The method achieves the correct level of RF power to laser with RF limiter. The method achieves high dynamic range (60 dB) over frequency range (2-18 GHz). The extended dynamic range is enhanced (80dB) through a programmable digital attenuator, controlled using Ethernet interface. The method achieves amplitude matching over multiple transmission lines. The method achieves phase matching over multiple transmission lines.
[0042] Fig. 2 illustrates a schematic block diagram of an RF transmitter (200) in accordance with an embodiment of the present invention. The RF transmitter includes an RF source (202), an RF power splitter (204), a plurality of modulators including 1st through Nth (206a-206n), and an RF power combiner (208).
[0043] The RF source (202) generates an ultra-wideband RF input signal. The RF power splitter (204) receives the ultra-wideband RF input signal and splits into 1st through Nth RF signals. The 1st through Nth modulators (206a-206n) receive the 1st through Nth RF signals respectively and parallelly modulate the 1st through Nth RF signals to generate 1st through Nth modulated RF signals. Examples of the 1st through Nth modulators (206a-206n) include Mach-Zender interferometers for direct modulation of a plurality of laser signals based on the plurality of RF signals. The 1st through Nth modulated RF signals are fed to the RF power combiner (208) which combines the 1st through Nth modulated RF signals to generate the output signal. The output signal is transmitted over the optical fiber. The optical fiber is spliced such that amplitude, frequency, and phase of the output signal matches with the optical fiber.
[0044] Fig. 3 illustrates a schematic block diagram of an RF receiver (300) in accordance with an embodiment of the present invention. The RF receiver (300) includes power dividers (302, 304a, 304b, 306a, 306b), frequency discriminators (308a and 308b), printed microstrip delays (310a and 310b), and phase discriminators (312a and 312b).
[0045] The power divider (302) receives the output signal transmitted by the RF transmitter (200) over the optical fiber. The power divider (302) splits the aforesaid signal into a plurality of signals which are demodulated parallelly. The Fig. 3 provides two such branches of parallel demodulators. Both the demodulator branches operate in parallel and are structurally and functionally similar. The power divider (304a) splits the first signal into two. One of the signals is delayed by the printed delay on microchip (310a). Both the signals (delayed and not delayed) are fed to the phase discriminator (312a) which generates the demodulated signal.
[0046] The present invention also provides a method of RF communication. The method includes transmission and reception of the RF signals over an optical fiber.
[0047] Referring now to Fig. 4, a flowchart illustrating an RF communication method is shown in accordance with an embodiment of the present invention.
[0048] At step 402, the RF power splitter (204) receives the ultra-wideband RF input signal from the RF source (202).
[0049] At step 404, the RF power splitter (204) splits the ultra-wideband RF input signal into the 1st through Nth RF signals
[0050] At step 406, the 1st through Nth modulators (206a-206n) parallelly modulate the 1st through Nth RF signals
[0051] At step 408, the RF power combiner (208) combines the 1st through Nth modulated RF signals to form an output signal.
[0052] At step 410, the output signal is transmitted over the optical signal.
[0053] The method of transmitting RF signals over the optical fiber includes generating the ultra-wideband RF input signal by the RF source (202). The RF power splitter (204) receives the ultra-wideband RF input signal and splits into 1st through Nth RF signals. The 1st through Nth modulators (206a-206n) receive the 1st through Nth RF signals respectively and parallelly modulate the 1st through Nth RF signals to generate 1st through Nth modulated RF signals. The 1st through Nth modulated RF signals are fed to the RF power combiner (208) which combines the 1st through Nth modulated RF signals to generate the output signal.
[0054] The method of reception of RF signals over the optical fiber includes receiving, by the power divider (302), the output signal transmitted by the RF transmitter (200) over the optical fiber. The power divider (302) splits the aforesaid signal into a plurality of signals which are demodulated parallelly.
[0055] The present invention brings out criticality of RoF technology for surveillance system, which is a vital element in the field of surveillance.
[0056] The foregoing description of the invention has been set merely to illustrate the invention and is not intended to be limiting. Since modifications of the disclosed embodiments incorporating the spirit and substance of the invention may occur to person skilled in the art, the invention should be construed to include everything within the scope of the invention.
,CLAIMS:We claim:
1. A Radio Frequency (RF) communication method, comprising:
receiving from an RF source, an ultra-wideband RF input signal;
splitting, by an RF power splitter, the ultra-wideband RF input signal into a plurality of RF signals;
modulating, by a plurality of modulators operating in parallel, the plurality of RF signals to generate a plurality of modulated RF signals;
combining, by an RF power combiner, the plurality of modulated RF signals to form an output signal; and
transmitting the output signal over an optical fibre, wherein the optical fibre is spliced such that amplitude, frequency, and phase of the output signal matches with the optical fibre.

2. The RF communication method as claimed in claim 1, wherein the plurality of modulators include Mach-Zender interferometers for direct modulation of a plurality of laser signals based on the plurality of RF signals.

3. The RF communication method as claimed in claim 1, further comprising:
receiving the modulated output signal;
splitting, by an RF power splitter, the output signal into a plurality of reconstructed RF signals; and
demodulating, by a plurality of demodulators operating in parallel, the plurality of reconstructed RF signals to generate a plurality of reconstructed demodulated RF signals.

4. A Radio Frequency (RF) transmitter, comprising:
an RF power splitter configured to receive an ultra-wideband RF input signal and split the ultra-wideband RF input signal into a plurality of RF signals;
a plurality of modulators operating in parallel and configured to modulate the plurality of RF signals to generate a plurality of modulated RF signals;
an RF power combiner configured to combine the plurality of modulated RF signals to form an output signal;
a transmitter, configured to transmit the output signal over an optical fibre, wherein the optical fibre is spliced such that amplitude, frequency, and phase of the output signal matches with the optical fibre.

5. The RF transmitter as claimed in claim 4, wherein the plurality of modulators include Mach-Zender interferometers for direct modulation of a plurality of laser signals based on the plurality of RF signals.

6. A Radio Frequency (RF) receiver, comprising:
splitting, by an RF power splitter, an output signal into a plurality of reconstructed RF signals; and
a plurality of demodulators operating in parallel and configured to demodulate the plurality of reconstructed RF signals to generate a plurality of reconstructed demodulated RF signals.

Dated this 15th day of March, 2019

For BHARAT ELECTRONICS LIMITED,
By their Agent,

(D. Manoj Kumar)
Patent Agent No.: IN/PA-2110
KRISHNA & SAURASTRI ASSOCIATES LLP