AU2021105715A4 - System and method of polarization shift keying signaling with wavelength diversity for free space optics - Google Patents

Abstract

SYSTEM AND METHOD OF POLARIZATION SHIFT KEYING SIGNALING WITH WAVELENGTH DIVERSITY FOR FREE SPACE OPTICS ABSTRACT The present invention is related to system and method of polarization shift keying signaling with wavelength diversity for free space optics. The objective of present invention is to solve the abnormalities presented in the prior art techniques related method and systems of free space optics communication. The present free space optical wireless communication system uses Polarization Shift Keying (PoLSK) signaling and the wavelength diversity scheme; using more than one transceiver to implement for the terrestrial communication system working with low transmit power, high speed and reliable communication link. 28 DRAWINGS 908 Transmitter Section (T x ; T06 ExFree Spac~ I Channel I I o 96E pl(,t FIGURE 1 29

Description

DRAWINGS

908 Transmitter Section (T

x ; T06 ExFree Spac~ I Channel

I Io96Epl(,t

FIGURE 1

SYSTEM AND METHOD OF POLARIZATION SHIFT KEYING SIGNALING WITH WAVELENGTH DIVERSITY FOR FREE SPACE OPTICS FIELD OF INVENTION

[001]. The present invention relates to the technical field of optical communication.

[002]. The present invention relates to the field of free space optical wireless communication system.

[003]. The present invention relates to the field of free space optical wireless communication system, this uses Polarization Shift Keying (PoLSK) signaling.

[004]. More particularly, the present invention is related to system and method of polarization shift keying signaling with wavelength diversity for free space optics. BACKGROUND & PRIOR ART

[005]. The subject matter discussed in the background section should not be assumed to be prior art merely as a result of its mention in the background section. Similarly, a problem mentioned in the background section or associated with the subject matter of the background section should not be assumed to have been previously recognized in the prior art. The subject matter in the background section merely represents different approaches, which in-and-of-themselves may also be inventions.

[006]. Free space optical communication is a promising candidate to meet the demands of high-speed communication systems [1]. It is a line of sight technology that can support several applications, such as inter-and intra-chip communication, satellite-to-satellite communication, and alternative to fibre optic communication, mobile network installation and FSO-based radio (RoFSO). Several other advantages are increased bandwidth and link capacity, reduced power requirement, compact design, highly resistant to eavesdropping, immunity to electromagnetic interference and no license requirement [2].

[007]. The main challenges of the FSO system is the atmospheric environment, because when the light beam propagates through an open space, its properties may be affected by the refractive index variation, which fluctuates the signal intensity at the receiver terminal and therefore the bit error rate. Several mitigation schemes such as coded modulation, aperture averaging and diversity are available. Here in this invention we have proposed wavelength diversity based system to improve the FSO links performance and make it work in each environment

[008]. Several statistical models are available for atmospheric turbulence channel analysis. However log-normal, Gamma-Gamma, Malaga distribution, K distribution and negative exponential channel models are common. Several modulation schemes, such as On-Off keying (OOK), phase-shift keying (PSK), differential phase-shift keying (DPSK), pulse position modulation (PPM) and PoLSK etc. were proposed to improve the performance of the FSO system.

[009]. Polarization Shift Keying (PoLSK) is a digital effective alternative modulation technique in FSO compared to other modulation schemes. The PoLSK have two advantages: (1) the alignment of polarization coordinates of the transmitter and the receiver is not necessary, (2) Light intensity is more uniform when propagating through atmospheric turbulences. PoLSK is a technique which uses SOP of optical signal as information carrying parameter. The reason of using this modulation scheme is that it uses constant envelope and hence provides reduced sensitivity to phase noise of Laser and also the patterning effects of Optical amplifiers used in the receivers. Therefore, PoLSK modulation is chosen be a good choice for FSO system.

[0010]. PoLSK provides immunity to atmospheric scintillation, higher data rates and lower BER compared to other modulation schemes. It has high immunity to laser phase noise and light intensity is more uniform when propagated through atmospheric turbulence. Links with PoLSK-based FSO systems have improved performance in terms of peak optical power. PoLSK is therefore, an effective method to reduce the impact of turbulence effects.

[0011]. The results have been presented in graph and discussed to describe the impact of the individual parameters on the outage probability and average BER, i.e. the reliability of the received signal in the PoLSK modulated wavelength diversity FSO links. These parameters are the Signal-to-Noise Ratio (SNR), transmission wavelength, diversity order, Link length, beam radius (w z), and atmospheric turbulence strength.

[0012]. The modulation scheme, like any other communication system, is an essential aspect of FSO communication. In case of optical communication, it is highly desirable to lessen the transmit power and enhanced communication quality; which is achievable by using the best modulation method; miniaturization and portability in practical applications.

[0013]. Polarization shift keying (PoLSK) is an alternative modulation technique to traditional on-off keying (OOK) and phase-shift keying (PSK) due to its high sensitivity to laser phase noise and does not suffer from frequency chirp. Polarization states (SOPs) are used in PoLSK to modulate binary information. While PoLSK modulated signal propagating through the atmosphere, the SOPs of light are unaffected by the atmospheric turbulence. Also, it provides immunity to atmospheric scintillation, higher data rates, and lower bit error rate (BER) compared to the conventional OOK scheme. The SOPs of PoLSK are well maintained over long distances so PoLSK can be used for long distance communications.

[0014]. The optical signal states of polarization (SOPs) are information-carrying parameters. The information is encoded using an external modulator over different SOPs. PoLSK uses intensity modulation where two orthogonal polarization directions are the transmission of 0 and 1 data bits. Thus, data bits in x and y polarizations are always complimentary. About the amplitude and phase in the actual case of laser beam propagation, the SOPs have more stable characteristics.

[0015]. Diversity can deal with the BER degradation observed by atmospheric turbulence. It uses multiple copies of the same signal to compensate the impairments induced by the atmosphere and enhance the reliability of the FSO link. It reduces the outage probability and eliminates the in-site tracking requirement due to laser misalignment. It can be temporal, spatial, and wavelength. Temporal means that we send the same data bits in different time slots, spatial involves sending the same data through multiple transmitters and receiving with multiple receivers through single wavelength transmission.

[0016]. Mathematical expressions for the estimation of BER and outage probability using wavelength diversity for FSO systems over log-normal turbulence channels have been derived in [3]. 0.85 im, 1.55 pm and 10 pm have been used to implement wavelength diversity technique under atmospheric turbulence conditions for link length equal to 1 Km and 1.5 Km and found improvement in outage probability [3].

[0017]. BPSK FSO system error performance in turbulence regimes from weak to strong has been evaluated in [8]. They used probability density function (PDF) of the received irradiance after traversing the atmosphere using the gamma-gamma distribution and negative exponential distribution to model turbulence in the saturation region and beyond. The effect of turbulence induced irradiance fluctuation using spatial diversity at the receiver and up to

12 dB gain in the electrical SNR with two direct detection PIN photodetector in strong atmospheric turbulence is reported.

[0018]. A theoretical model of a circular polarization shift keying system for free-space optical links via an atmospheric turbulence channel has been developed by [9]. It was reported that controlling the circular polarization error below normalized threshold within 8dB, 9dB, and 10dB in turbulent scenarios ranging from weak to strong can significantly reduce the likelihood of communication interruption.

[0019]. Outage performance of a polarization shift keying (PoLSK) based multichip parallel relay aided FSO system BY [10]. According to their research, the relay-assisted FSO system based on PoLSK has better efficiency than direct transmission and OOK systems. By expanding the number of relay paths, outage performance increases.

[0020]. A Wavelength Division model Multiplexed system utilizing Polarization Shift Keying modulation is proposed [11] to boost the capacity of a FSO System and reported system improves the capacity of an FSO connection by using WDM as high as 10 Gbits/s and up to 10 km under thick fog circumstances. The suggested method aided in implementing virtual classrooms in mountainous locations where wired connections are inconvenient.

[0021]. The performance of a circle polarization shift keying (CPoLSK)-based FSO system was analyzed for operating the space-to-ground channel under an unstable regime of turbulence strength [12]. They reported that the modulation scheme provides a more efficient way to compensate for scintillation effects than the on-off-keying based FSO system.

[0022]. A novel model for the hybrid Free Space Optical / Radio Frequency (FSO / RF) communication system that receives diversity by closed-form expression for outage probability is proposed in [13]. It is stated that the FSO system with receiver diversity performs better at low average Signal to Noise Ratio (SNR) than the RF system and almost comparable to the proposed system, but its efficiency degrades compared to the other two systems.

[0023]. Signal quality in FSO systems with PoLSK modulation was studied in

[14] and it was observed that the ABER reduces with increasing SNR, and the most significant system performance for weak atmospheric turbulence, and worst for strong atmospheric turbulence. The only exception is the atmospheric I-K distribution, which performs the worse under moderate atmospheric turbulence [14].

[0024]. References

[0025]. [1] Z. Ghassemlooy, W. Popoola, and S. Rajbhandari, Optical wireless communications: system and channel modelling with Matlab@. CRC press, 2019. doi: 10.1201/9781315151724.

[0026]. [2] D. Shah and D. Kothari, "Performance Analysis of Free Space Optical Link With Wavelength Diversity Under Weak and Moderate Turbulence Conditions," Sens. Lett., vol. 17, no. 2, pp. 137-143, Feb. 2019, doi: 10.1166/sl.2019.4058.

[0027]. [3] V. Xarcha et al., "Wavelength diversity/or free space optical systems: Performance evaluation over log normal turbulence channels," in 2012 19th International Conference on Microwaves, Radar & Wireless Communications, 2012, vol. 2, pp. 678-683.

[0028]. [4] A. A. Farid and S. Hranilovic, "Outage Capacity Optimization for Free-Space Optical Links With Pointing Errors," J. Light. Technol., vol. 25, no. 7, pp. 1702-1710, Jul. 2007, doi: 10.1109/JLT.2007.899174.

[0029]. [5] L. C. Andrews, R. L. Phillips, C. Y. Hopen, and M. A. Al Habash, "Theory of optical scintillation," JOSA A, vol. 16, no. 6, pp. 1417 1429, 1999.

[0030]. [6] V. S. Adamchik and 0. I. Marichev, "The algorithm for calculating integrals of hypergeometric type functions and its realization in REDUCE system," in Proceedings of the international symposium on Symbolic and algebraic computation - ISSAC '90, Tokyo, Japan, 1990, pp. 212-224. doi: 10.1145/96877.96930.

[0031]. [7] S. Navidpour, M. Uysal, and M. Kavehrad, "BER Performance of Free-Space Optical Transmission with Spatial Diversity," IEEE Trans. Wirel. Commun., vol. 6, no. 8, pp. 2813-2819, Aug. 2007, doi: 10.1109/TWC.2007.06109.

[0032]. [8] W. 0. Popoola and Z. Ghassemlooy, "BPSK Subcarrier Intensity Modulated Free-Space Optical Communications in Atmospheric Turbulence," J. Light. Technol., vol. 27, no. 8, pp. 967-973, Apr. 2009, [Online]. Available: http://jlt.osa.org/abstract.cfmn?URI=jlt-27-8-967

[0033]. [9] Z. Wang, W.-D. Zhong, S. Fu, and C. Lin, "Performance comparison of different modulation formats over free-space optical (FSO) turbulence links with space diversity reception technique," IEEE Photonics J., vol. 1, no. 6, pp. 277-285, 2009.

[0034]. [10] K. Prabu and D. Sriram Kumar, "Polarization shift keying based relay-assisted free space optical communication over strong turbulence with misalignment," Opt. Laser Technol., vol. 76, pp. 58-63, Jan. 2016, doi: 10.1016/j.optlastec.2015.07.012.

[0035]. [11] J. Jeyarani, D. Sriramkumar, and B. E. Caroline, "Performance analysis of free space optical communication system employing WDM PolSK under turbulent weather conditions," J. Optoelectron. Adv. Mater., vol. 20, no. September-October 2018, pp. 506-514, 2018.

[0036]. [12] Y. Su and T. Sato, "Analysis of CPolSK-based FSO system working in space-to-ground channel," Opt. Commun., vol. 410, pp. 660-667, Mar. 2018, doi: 10.1016/j.optcom.2017.11.023.

[0037]. [13] M. A. Amirabadi and V. Tabataba Vakili, "A novel hybrid FSO /RF communication system with receiver diversity," Optik, vol. 184, pp. 293-298, May 2019,doi: 10.1016/j.ijleo.2019.03.037.

[0038]. [14] J. Todorovic, B. Jaksic, P. Spalevic, D. Bandjur, and M. Bandjur, "Analysis of signal quality in FSO systems with PolSK modulation," Serbian J. Electr. Eng., vol. 17, no. 2, pp. 171-186, 2020, doi: 10.2298/SJEE2002171T.

[0039]. Some of the work listed herewith:

[0040]. 17237360PTICAL MODULATION CONVERTER AND METHOD FOR CONVERTING THE MODULATION FORMAT OF AN OPTICAL SIGNAL EP - 22.11.2006 Int.Class G02F 2/OOAppl.No 5729762Applicant MARCONI COMM SPAInventor D ERRICO ANTONIO An optical modulation converter (10) for converting the modulation format of an optical input signal is characterized by a birefringent medium (14), polarization maintaining fibre, with a selected differential group delay between its two main axes of symmetry through which the optical input signal is passed to be separated into two optical components each travelling along one of the main axes of the medium at a different group velocity to thereby convert the modulation format of the input signal. By appropriate selection of the differential group delay imparted by the medium relative to the bit rate of the input signal and by appropriately presenting the input signal relative to the main axes of the medium conversion between different modulation formats can be achieved. These include direct conversion from optical DPSK (Differential Phase Shift Keying) to POLSK (POLarization Shift Keying), DPSK to IM (Intensity-Modulated) through an intermediated conversion to POLSK, POLSK to IMDD (Intensity Modulation Direct Detection), and IM to POLSK.

[0041]. 1038282680PTICAL TRANSMISSION AND RECEPTION WITH HIGH SENSITIVITY USING M-PPM COMBINED WITH ADDITIONAL

MODULATION FORMATS N - 28.05.20141nt.Class H04B 0/516Appl.No 201280011817.8 Applicant ALCATEL-LUCENT Inventor LIU XIANG. An apparatus transmits data using a format where information bits intended for transmission are mapped into symbols each carrying a plurality of bits, some of which are encoded through pulse position modulation (PPM) format and the rest of which are encoded through an additional modulation format on each PPM pulse. The additional modulation format for the PPM pulse may be at least one of a polarization-division-multiplexed (PDM) modulation, phase shift keying (PSK) modulation, polarization shift keying (PolSK) modulation, amplitude modulation (AM), quadrature-amplitude modulation (QAM) modulation, or a combination thereof. In one embodiment, the additional modulation of the PPM pulses is done through polarization-division multiplexed quadrature-phase-shift keying (PDM-QPSK). The unique combined use of PDM-QPSK and PPM produces much higher receiver sensitivity than either PPM or PDM-QPSK alone.

[0042]. 20070274732OPTICAL MODULATION CONVERTER AND METHOD FOR CONVERTING THE MODULATION FORMAT OF AN OPTICAL SIGNAL US - 29.11.20071nt.Class H04B 10/06Appl.No 0598701Applicant D ERRICO ANTONIO Inventor D'Errico Antonio An optical modulation converter (10) for converting the modulation format of an optical input signal is characterized by a birefringent medium (14), polarization maintaining fibre, with a selected differential group delay between its two main axes of symmetry through which the optical input signal is passed to be separated into two optical components each travelling along one of the main axes of the medium at a different group velocity to thereby convert the modulation format of the input signal. By appropriate selection of the differential group delay imparted by the medium relative to the bit rate of the input signal and by appropriately presenting the input signal relative to the main axes of the medium conversion between different modulation formats can be achieved. These include direct conversion from optical DPSK

(Differential Phase Shift Keying) to POLSK (Polarization Shift Keying), DPSK to IM (Intensity-Modulated) through an intermediated conversion to POLSK, POLSK to IMDD (Intensity Modulation Direct Detection), and IM to POLSK.

[0043]. 20120224852SYSTEM, METHOD, AND APPARATUS FOR HIGH SENSITIVITY OPTICAL DETECTION S 6.09.2012 Int.Class H04B 10/04Appl.No 13041384Applicant Liu Xiang Inventor Liu Xiang An apparatus transmits data using a format where information bits intended for transmission are mapped into symbols each carrying a plurality of bits, some of which are encoded through pulse position modulation (PPM) format and the rest of which are encoded through an additional modulation format on each PPM pulse. The additional modulation format for the PPM pulse may be at least one of a polarization-division-multiplexed (PDM) modulation, phase shift keying (PSK) modulation, polarization shift keying (PolSK) modulation, amplitude modulation (AM), quadrature-amplitude modulation (QAM) modulation, or a combination thereof. In one embodiment, the additional modulation of the PPM pulses is through polarization-division-multiplexed quadrature-phase-shift keying (PDM-QPSK). The unique combined use of PDM-QPSK and PPM produces much higher receiver sensitivity than either PPM or PDM-QPSK alone.

[0044]. 104253653 QUATERNARY LIGHT POLARIZATION ENCODING AND SELF-CALIBRATION WIRELESS OPTICAL COMMUNICATION SYSTEM AND METHOD CN - 31.12.20141nt.Class H04B 10/532Appl.No 201410480797.XApplicant HARBIN INSTITUTE OF TECHNOLOGY SHENZHEN RADUATE SCHOOL Inventor YAO YONG. The invention provides a quaternary light polarization encoding and self-calibration wireless optical communication system. The transmitting end of the system is used for sending a quaternary polarization shift keying modulation signal by a quaternary modulation assembly, and the receiving end of the system is used for performing adaptive correction on the polarization of a polarization shaft by a polarization shaft correction assembly. The invention also discloses a quaternary light polarization encoding and self-calibration wireless optical communication method. The system and the method have the advantages that the received error signal is processed to obtain the correction angle of the polarization shaft, and the receiving end is controlled to automatically adjust, so the judgment accuracy is improved; by utilizing a quaternary polarization shift keying modulation technology, the communication rate is increased.

[0045]. WO/2013/0158590PTICAL RECEIVER CONFIGURABLE TO ACCOMMODATE A VARIETY OF MODULATION FORMATS WO 31.01.2013Int.Class H04B 10/158Appl.No PCT/US2012/036164Applicant MASSACHUSETTS INSTITUTE OF TECHNOLOGY Inventor CAPLAN, avid, 0. The present invention provides a simple means of demodulating optical signals, e.g. wideband M-ary orthogonal. The demodulator comprises an optical processor and a comparison module. The optical processor transforms M input optical signals into 21og2(M) intermediary optical signals and the comparison module determines the logical representation of the input data based on log2(M) binary comparisons of the optical power of the intermediary signals. Example embodiments may be reconfigurable to receive optical signals using M-FSK, M-PPM, M-PolSK, and hybrid M-ary orthogonal modulation formats. Example embodiments also offer small size, weight and power consumption for both free-space and fiber optic environments as well as improved receiver sensitivity and reduced electron bandwidth requirements.

[0046]. WO/2001/084748RECEIVER FOR AN OPTICAL INFORMATION TRANSMISSION WO - 08.11.2001Int.Class H04B 10/2569Appl.No PCT/DE2001/001428Applicant SIEMENS KTIENGESELLSCHAFT Inventor NOt, Reinhold. A receiver for optical signals (OSI, OS2) with polarization multiplex or polarization shift keying contains a polarization mode dispersion compensator (PMDC) which is connected upstream of a polarizing element and which can also serve as a polarization transformer

(PT), in order to compensate depolarization caused by polarization mode dispersion.

[0047]. 2005160065DIFFERENTIAL POLARIZATION SHIFT-KEYING OPTICAL TRANSMISSION SYSTEM JP - 6.06.20051nt.Class H04B 10/04Appl.No 2004327916Applicant SAMSUNG ELECTRONICS CO LTD Inventor KIN KUNPROBLEM TO BE SOLVED: To provide a polarization shift-keying optical transmission system using a polarization-modulated optical signal. SOLUTION: The differential polarization shift-keying optical transmission system includes a transmitter unit for precoding inputted data to generate and output a polarization-modulated optical signal using a pre-coded signal, and a receiver unit connected with the transmitter unit via an optical fiber for optical transmission. The receiver unit includes a one-bit delay interferometer for splitting the optical signal inputted from the transmitter unit into first and second split optical signals, generating a delayed optical signal by causing the second split signal to be delayed, generating an optical signal by inverting the phase of a part of the first split signal, and generating constructive interferential and destructive-interferential optical signals by causing respective parts of the delayed optical signal to interfere with the other part of the first split optical signal and with the phase-inverted signal, and a balanced receiver for outputting a differential signal between the constructive interferential and destructive-interferential optical signals.

[0048]. 200201016390PTICAL DATA TRANSMITTING APPARATUS AND METHODUS - 01.08.2002 Int.Class H04B 10/04Appl.No 10103863. Applicant NEC Corporation. Inventor Yano Yutaka. An optical data transmitting apparatus and method allows data transmission over a distance that surpasses a limit, which has been imposed due to group velocity dispersion (GVD) and self phase modulation (SPM) effect within optical fiber. This has been accomplished by allocating trinal duobinary symbols to optical 1, P, -1 symbols. These optical symbols have the same intensity, +1 and -1 symbols have inverted optical phases each other, and orthogonally polarized each other between ±1 and P symbols. At the receiver, conventional polarization shift keying receiver can be used to restore original binary data stream.

[0049]. 536578DIFFERENTIAL POLARIZATION SHIFT-KEYING OPTICAL TRANSMISSION SYSTEMEP - 1.06.2005. Int.Class H04B 10/516Appl.No 04027100Applicant SAMSUNG ELECTRONICS CO LTD. Inventor KIM HOONA differential polarization shift-keying optical transmission system includes a transmitter unit for precoding inputted data to form a precoded signal and generate a polarization-modulated optical signal using the precoded signal. The system further includes a receiver unit connected with the transmitter unit via an optical fiber for optical transmission. The receiver unit includes a delay interferometer for splitting the optical signal into first and second split optical signals, generating a delayed optical signal by causing the second split signal to be delayed 1 bit, generating a phase-inverted optical signal by inverting the phase of a part of the first split signal, and generating generate constructive-interferential and destructive-interferential optical signals by causing respective parts of the delayed signal to interfere with another part of the first split signal and with the phase-inverted signal. A balanced receiver in the receiver unit outputs a differential signal corresponding to a difference between the constructive-interferential and destructive-interferential optical signals.

[0050]. Groupings of alternative elements or embodiments of the invention disclosed herein are not to be construed as limitations. Each group member can be referred to and claimed individually or in any combination with other members of the group or other elements found herein. One or more members of a group can be included in, or deleted from, a group for reasons of convenience and/or patentability. When any such inclusion or deletion occurs, the specification is herein deemed to contain the group as modified thus fulfilling the written description of all Markus groups used in the appended claims.

[0051]. As used in the description herein and throughout the claims that follow, the meaning of "a," "an," and "the" includes plural reference unless the context clearly dictates otherwise. Also, as used in the description herein, the meaning of "in" includes "in" and "on" unless the context clearly dictates otherwise.

[0052]. The recitation of ranges of values herein is merely intended to serve as a shorthand method of referring individually to each separate value falling within the range. Unless otherwise indicated herein, each individual value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context.

[0053]. The use of any and all examples, or exemplary language (e.g. "such as") provided with respect to certain embodiments herein is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention otherwise claimed. No language in the specification should be construed as indicating any non-claimed element essential to the practice of the invention.

[0054]. The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art. SUMMARY

[0055]. The present invention mainly cures and solves the technical problems existing in the prior art. In response to these problems, the present invention provides system and method of polarization shift keying signaling with wavelength diversity for free space optics.

[0056]. As one aspect of the present invention relates to A free space optical wireless communication system, wherein the free space optical wireless communication system uses Polarization Shift Keying

(PoLSK) signaling and a wavelength diversity scheme using more than one transceiver to implement for the terrestrial communication system working with low transmit power, wherein the system comprises A transmitter section (908), which is configured to receive data from Pseudo-Random Sequence Generator or the data source (901), the received data is then line encoded through Non return to zero (NRZ) coder (902), wherein The optical beam is generated through CW laser (903), and the information is transmitted by switching the polarization of the optical beam between two linear orthogonal states of polarization (SOPs) separated using polarization beam splitter (904), wherein PoLSK signal is then modulated using lithium niobate (LiNbO3) based Mach-Zehnder interferometer (MZI) modulator (906), comprising a beam splitter followed by a waveguide-based wavelength-dependent phase shift, wherein the modulated signal is then combined using the polarization beam combiner (905) to obtain a resultant modulated optical signal, wherein the resultant signal is then passed through the transmitter optics (907) which comprises of collimator lens, beam focusing optics and transmitted over the free space channel; and A receiver, designed to detect the optical signal transmitted from the transmitter and converts it from optical to electrical, wherein each receiver optics (909) comprises of telescope, fine pointing system, collimator, photo-detector and filters.

OBJECTIVE OF THE INVENTION

[0057]. The principal objective of the present invention is to provide system and method of polarization shift keying signaling with wavelength diversity for free space optics. BRIEF DESCRIPTION OF DRAWINGS

[0058]. Further clarify various aspects of some example embodiments of the present invention, a more particular description of the invention will be rendered by reference to specific embodiments thereof which are illustrated in the appended drawings. It is appreciated that these drawings depict only illustrated embodiments of the invention and are therefore not to be considered limiting of its scope. The invention will be described and explained with additional specificity and detail through the use of the accompanying drawings.

[0059]. In order that the advantages of the present invention will be easily understood, a detailed description of the invention is discussed below in conjunction with the appended drawings, which, however, should not be considered to limit the scope of the invention to the accompanying drawings, in which:

[0060]. Figure 1 shows a FSO transmitter section for system and method of polarization shift keying signaling with wavelength diversity for free space optics, according to the present invention.

[0061]. Figure 2 shows Wavelength diversity based FSO system block schematic for system and method of polarization shift keying signaling with wavelength diversity for free space optics, according to the present invention.

DETAIL DESCRIPTION

[0062]. The present invention discloses system and method of polarization shift keying signaling with wavelength diversity for free space optics.

[0063]. This present invention of ternary multiplier, if compared with existing designs based on power consumption, delay, and other parameters. The noise analysis based on average noise threshold energy is done. The proposed multiplier design outperformed the existing designs in every aspect of performance.

[0064]. The present invention relates to a FSO system employing wavelength diversity with PoLSK signaling is implemented, wherein the system uses different wavelengths to transmit data over same channel and provide an advantage of low transmit power at over the existing single input single is that transceiver pairs are placed between the transmitter and receiver in the existing FSO system design.

[0065]. Wavelength diversity is a technique in which multiple LASERs with different wavelengths transmit the same signal. Varying atmospheric conditions have different effects on the different optical wavelengths. At the receiver's end, based on optimum combining technique for e.g. Selective Gain Combining technique (SGC), Maximum Ratio Combining (MRC) and Equal Gain Combining (EGC), the signal with least attenuation, and better ABER will be chosen.

[0066]. In order to eliminate the imperfections, the present invention investigates a PoLSK along with wavelength diversity techniques. The performance of modulation schemes is susceptible to turbulence fluctuation. Thus, the adaptive detection technique at the Rx is needed to improve performance [1].

[0067]. The optical beam is generated through CW laser (903), and the information is transmitted by switching the polarization of the optical beam between two linear orthogonal states of polarization (SOPs) separated using polarization beam splitter (904). The PoLSK signal is then modulated using lithium niobate (LiNbO3) based Mach-Zehnder interferometer (MZI) modulator (906), comprising a beam splitter followed by a waveguide-based wavelength-dependent phase shift. The modulated signal is then combined using the polarization beam combiner (905) to obtain the resultant modulated optical signal (E-pol (0,t)). The resultant signal is then passed through the transmitter optics (907) which comprises of collimator lens, beam focusing optics and transmitted over the free space channel.

[0068]. Fig. 2 shows the block diagram of receiver designed to detect the optical signal transmitted from the transmitter and converts it from optical to electrical. Each receiver optics (909) consists of telescope, fine pointing system, collimator, photo-detector and filters.

[0069]. The resultant optical signals are allowed to flow through optimum combining techniques (910), thereafter, the best possible signal out of optimal combining is chosen as a received signal.

[0070]. The proposed system employs PoLSK which is a form of digital optical modulation. Information is transmitted using the state of polarization of a fully polarized light wave. In this scheme the received signal is demodulated and detected using stokes parameters. A detailed theoretical treatment is presented in [1].

[0071]. Let us assume a fully polarized light wave, which propagates in the z direction. The electric field E can be decomposed into two orthogonal components, E. and Ey as defind by the Eq. (1) and (2) as:

[0072]. Where,

E,, = a., (t)eet+ Tx(t)2 = Ei2 (1)

Ey = ay(t)e(&t+PY (0,9 Eyf9 (2)

[0073]. Where o is the angular frequency anda, ay are the magnitudes

and pX, cpy are the phase of

x and y components respectively.

[0074]. The stokes parameters are given as the Eqs. (3-6) S, = a. - ay, (3)

S2 = 2a,,a, cos(S) (4)

S3 = 2aay sin(S) (5)

S = <px - <py (6)

[0075]. Refering to Fig. 1, Where, 903 is the CW laser source, 904 is the polarization beam splitter, 901 is the pseudo random bit sequence generator, 902 is the NRZ source coders, 906 is the LiNbO3 MZM, 905 is the polarization beam combiner and 907 is the transmitter optics. The expression for combined beam is expressed by Eq: (7):

Epl (0,t) = Pteltm(t)i+ [1 - m(t)]f} (7)

[0076]. Where, Pt is the transmitted power, o is the angular frequency and

m (t) is the input binary data.

[0077]. Further, the received signal after wavelength diversity from channel is modelled as:

Epo (L, t) 2e * m(t)! + [1

[0078]. Where Pr is the received power and m(t) is the input binary data.

The received optical signal Ey 0 (L, t) is split into two polarized fields

by using a beam splitter as E, (t) and Ey (t) and

[0079]. expressed as Eq. (9) and (10) as p, (t) = RA, cos(ot)[1 - m(t)] + n,(t) (9)

P2 (t) = RAr cos(&t)m(t) + ny(t) (10)

[0080]. Where R is the responsively of PIN which is photo detector n"(t)

and ny(t) represents the noise and A, is the amplitude of the received

signal.

[0081]. FSO communication uses a composite transmitter, atmosphere as a channel and a photodetector. In case of wavelength diversity there is a necessity to broadcast data over multiple wavelengths simultaneously. The reason behind this approach is that different wavelength feel different atmospheric fading [2], [3]. The selection of detector is also specific to particular wavelength. Suppose the FSO link established consists different transceiver namely, w = 1,2,3 . . W. Each transmitter

transmits the same signal via W different wavelengths at the same instant. So wthcopy of the broadcasted signal will be detected by wth

receiver. So, the signal received by w'receiver is given by Eq. (11):

y, = hwYx+N (11)

[0082]. Here y, represents the channel state, Y, represents the associated responsivity of detector, x as transmitted signal and noise by various sources as N.

[0083]. The channel state h, basically models the losses due to atmospheric turbulence and path loss. It explores the overall fluctuations in intensity of transmitted optical signal [4]. h, = hha h, (12)

[0084]. Where h, signal attenuation due to path loss is, h, occurs due to scintillation effects and hp, represents loss due to geometric spread and

pointing errors [4],[5].

[0085]. The expression in Eq. (13) represents the probability density function of Gamma-Gamma channel model, it is used to estimate the impairments added due to atmospheric turbulence and has a resemblance with the experimental data. 2(a§i)(a+P)/2 (13) fA() = -Ig[a+,8/2j- I K._g (2_ _#), I > 0 U(a)T(fi)

[0086]. Where, K(.) is the modified Bessel function of the second kind of order n, and 1(.) is the Gamma function. The positive parameters represent the large scale and small-scale intensity fluctuations, which are given as Eq. (14):

(4 0.490-R a ={exp[(1 + 1.110a2/s) 7 i1( expI 0.51

/ (1 +0.69o-,2/s)5

[0087]. Then the combined expression for probability density function including ha, hi and hp is derived to obtain the closed form expression given by Eq: (15): (h)2awb G, a,bh_ 2 (15 (A,,h,,)FajFb, , Arhw (2 - 1, a. - 1, biy 1)

[0088]. The cumulative density function (CDF) of instantaneous electrical Signal to Noise Ratio (SNR) is obtained by integrating the PDF and using property [6] in Eq. (16):

fy 'G m < a > \ (16 x"-iG < by >1x~d

) yaGp+l+1. (b1, b 2 , ... bm, -a, bm+1, ... bq IY)

[0089]. Then closed form expression in terms of CDF can be expressed as Eq. (17):

f2 33 fabh, 1 ,1+ 2 (17) F(h,) = Fa.rb, G24 A A 0 h, b.,0) ,2,a,

[0090]. Where, G"'(|) represents the Meijer G Function.

[0091]. Outage probability:

[0092]. It is an important system design parameter which is given by: Pot,, = P(SNR(h) ) SNRt) (18)

[0093]. The SNR for PoLSK based FSO system is given by: yzhp (19) SNR(h) = Y2p(9 2 MOu

2

[0094]. Where, the noise variance of channel is presented by , number

of transceivers as M and p as the power.

[0095]. Then, the outage probability of single channel PoLSK based FSO system is given by:

{2 3,1 a,b,,u2SNRth 1,1 + 0120 Poutw = Fn,,(h,) =a b( G2,4 AnhwyP 1 a , 2,0

[0096]. Assuming that the outage probability of each one of the W optical channels Pntw is Independent and then the total outage probability of

all the W links can be expressed as: W (21) Pont = 1Pnt'w W=1

[0097]. By substituting, we obtained the total outage probability of PoLSK based FSO system with wavelength diversity Eq. (22) as: w f2 ab 2 SN Ret 1,1 + { (22) ot 11arb w=1 2^ Aohy PI gaw,b,,0)

[0098]. Average BER (ABER):

[0099]. The conditional BER for PoLSK signalled FSO system incorporated with wavelength diversity is:

(w (23)

BERo(hW)=Q2 Wa Ih

[00100]. Where, yw is the responsivity of the wth photo detector, 0 2

is the variance of the channel noise and Q() is the gaussian Q function.

[00101]. Now, the average BER with optimal combining is given by

[7]:

ABER(h,)= BERoc(hw)h,(h.)dh (24)

[00102]. The closed form expression for average BER becomes: ABERO,(hw) (25 1 { y P.A 0 h1 1-{i,1-aw1-b

) W 2

1 J 3 Lr.2PAa, hi, 1 - {, 1 - a, 1 - b atF b- G32 3WO2b 0,

f.2 22N R hA, 2 - aw 1 - b,,

w=1

12Il Fa,3b,G22W ab , ,

41rab, G3,2 b, 3Waw 0,

[00104]. Theabove expressions inEq. (25) and Eq. (26)represent theclosed formexpressionforaverageBERofPoLSKbasedFSO system.

[00105]. As an alternative to the standard modulation techniques, modulation schemes exploit the vector characteristics of the propagating optical beam. This scheme relies on the state of polarization (SOP) of a fully polarized optical beam as the information bearing parameter, thus exploiting the two orthogonal channels available in a single-mode optical fiber as well as free space propagation.

[00106]. The figures and the foregoing description give examples of embodiments. Those skilled in the art will appreciate that one or more of the described elements may well be combined into a single functional element. Alternatively, certain elements may be split into multiple functional elements. Elements from one embodiment may be added to another embodiment.

[00107]. For example, order of processes described herein may be changed and are not limited to the manner described herein. Moreover, the actions of any block diagram need not be implemented in the order shown; nor do all of the acts need to be necessarily performed. Also, those acts that are not dependent on other acts may be performed in parallel with the other acts. The scope of embodiments is by no means limited by these specific examples.

[00108]. Although implementations of the invention have been described in a language specific to structural features and/or methods, it is to be understood that the appended claims are not necessarily limited to the specific features or methods described. Rather, the specific features and methods are disclosed as examples of implementations of the invention.

Claims (4)

CLAIMS I/We claim:

1. A free space optical wireless communication system, wherein the free space optical wireless communication system uses Polarization Shift Keying (PoLSK) signaling and a wavelength diversity scheme using more than one transceiver to implement for the terrestrial communication system working with low transmit power, wherein the system comprises:

A transmitter section (908), which is configured to receive data from Pseudo-Random Sequence Generator or the data source (901), the received data is then line encoded through Non return to zero (NRZ) coder (902), wherein The optical beam is generated through CW laser (903), and the information is transmitted by switching the polarization of the optical beam between two linear orthogonal states of polarization (SOPs) separated using polarization beam splitter (904), wherein PoLSK signal is then modulated using lithium niobate (LiNbO3) based Mach-Zehnder interferometer (MZI) modulator (906), comprising a beam splitter followed by a waveguide-based wavelength-dependent phase shift, wherein the modulated signal is then combined using the polarization beam combiner (905) to obtain a resultant modulated optical signal, wherein the resultant signal is then passed through the transmitter optics (907) which comprises of collimator lens, beam focusing optics and transmitted over the free space channel; and

A receiver, designed to detect the optical signal transmitted from the transmitter and converts it from optical to electrical, wherein each of receiver optics (909) comprises of telescope, fine pointing system, collimator, photo-detector and filters.

2. The free space optical wireless communication system as claimed in claim 1, wherein uses different wavelengths along with PoLSK modulation to for data transmission at high data rate.

3. The free space optical wireless communication system as claimed in claim 1, wherein the system uses more than one transceiver to mitigate turbulence impairment. Due to wavelength diversity and PoLSK signaling the present system provided a way for the highly reliable and power efficient FSO system.

4. The free space optical wireless communication system as claimed in claim 1, wherein the system uses different wavelengths to implement diversity and can reduce the demand of high transmit power to achieve the required BER..