Free Space Optical (FSO) communication is an optical communication technology in which laser diode is used as a light source, photo detector is used for detecting the optical signal and air/free space acts as a medium for propagation. Further, FSO offers many features like cost effectiveness, protocol independence, high speed connectivity, ease of deployment and license free operation. It is a secure communication which can bring superior quality, wideband services to home or businesses. This has been included in US patent: Free space optics broadband home network U.S. Pat. No. 8,401,395 published on Mar. 19, 2013.
Conventionally, FSO communication finds applications in last-mile access, back-haul for wireless cellular networks, fiber backup and disaster recovery. Large bandwidth, low cost of installation and excellent security are attractive features of an FSO communication. But there is also problem of signal degradation with the usage of FSO systems because of fog, turbulence and misalignment. In order to combat the effects of turbulence, the technologies such as multiple input multiple output (MIMO) and relay networks with serial and parallel relay combinations have been used in M. Safari and M. Uysal, “Relay-assisted Free-space optical communication”, IEEE Trans. on wireless Commun., vol. 7, no. 12, pp. 5441-5449, December 2008.
Conventionally, in radio frequency (RF) systems, the nature of broadcasting of signal is used with the advantage to send the signal through different relays, thereby enhancing the system performance. Further, these relays can be ordered in a serial or parallel fashion according to the need of user. Series combination (also known as multi hop communication) is generally used to increase the range of communication whereas parallel combination (also known as cooperative communication) is meant for increasing the system performance as described in J. N. Laneman and G W. Wornell, “Energy-efficient antenna sharing and relaying for wireless networks,” in Proc. IEEE Wireless Communications and Networking Conf. (WCNC), Chicago, Ill., September 2000.
However, in the FSO network, multiple transmitters and receivers are used at each node in the mesh configuration. However, for cooperative communication, one of the transmitter-receiver can be used as relay without any extra cost. Further, relay nodes for cooperative communication are generally chosen from the existing communication network. Accordingly, it does not include any extra hardware requirement. However in the case of MIMO, extra hardware is always required to get good performance and accordingly, hardware requirement is at least double thereof in case of MIMO when compared to that of cooperative system.
Cooperative diversity technique for combating turbulence-induced fading over free-space optical (FSO) links has been demonstrated in Chadi Abou-Rjeily and Serj Haddad, “Cooperative FSO Systems: Performance Analysis and Optimal Power Allocation,” Journal of Lightwave Technology, vol. 29, No. 7, April, 2011. Accordingly, they have developed a closed-form optimal solution for transmitting the entire optical power along the strongest link between the source and the destination nodes. In C. Abou-Rjeily and A. Slim, “Cooperative diversity for free-space optical communications: Transceiver design and performance analysis,” IEEE Trans. Communication, vol. 59, No. 3, March 2011. Further, one-relay cooperative diversity scheme has been proposed and analysed thereof, for non-coherent FSO communications with intensity modulation and direct detection (IM/DD). Accordingly, the error performance is derived in semi-analytical and closed-form expressions in the presence and absence of background radiation.
Cooperative relay technique with pulse-position modulation (PPM) and optical interleave-division multiple-access for achieving spatial diversity and robust transmission performance to alleviate the degrading effects of atmospheric turbulence has been demonstrated in Jingyuan Fan, Xiaolin Zhou, and Jun Liu, “Design and Evaluation of an IDMA Cooperative Relay Free-Space Optical System,” International conference on space optical systems and applications, 2011. Still further, a three-way FSO communication setup is proposed in M. Karimi and M. Nasiri-Kenari, “BER analysis of cooperative systems in free-space optical networks,” J. of Lightw Technol., vol. 27, no. 24, pp. 5639-5647, December 2009. Accordingly, the cooperative protocol can be applied to achieve the spatial diversity without much increase in hardware. BER performance has been studied in different strategies in the presence of shot noise. They have shown comparison with direct link and MIMO.
Conventionally, diversity combining schemes have been extensively used in wireless and free space optical communication systems for cooperative and MIMO systems. There are the three diversity combining techniques which are subject of patents and publications in journals/books. These techniques are: Selection combining (SC), Equal gain combining (EGC) and Maximal ratio combining (MRC). Further, MRC gives the optimal performance; however it requires a priori knowledge of channel state conditions and adaptive weight adjustment of the diversity combiner. Still further, the diversity combining schemes for wireless communications have been patented earlier in US patent “Multi-antenna wireless communication method, multi-antenna wireless communication system, and multi-antenna wireless communication device” U.S. Pat. No. 8,385,455, published Feb. 26, 2013.
Generally, in wireless communication systems, the signal is sent through different relays to enhance system performance. When the relays are ordered in parallel combination (also known as cooperative communication), it gives the advantage of enhanced system performance. Relay nodes can be chosen in the network as per the requirement of cooperation and hence it does not include any extra hardware requirement like multiple input multiple output (MIMO) system. Cooperative communication has been discussed in “Cooperative multiple access in wireless networks” U.S. Pat. No. 8,576,772 published on Nov. 5, 2013. This US document discloses the performance on MIMO systems. The present invention discusses the system model of FSO cooperative communication system.
In U.S. Pat. No. 7,120,200 “Transmitter diversity technique for wireless communications”, AlamoutiSiavash and TarokhVahid explain a simple block coding arrangement is created with symbols transmitted over a plurality of transmit channels, in connection with coding that comprises only simple arithmetic operations, such as negation and conjugation. Accordingly, the diversity created by the transmitter utilizes space diversity and either time or frequency diversity. However, MRC which is an optimum performing diversity combining technique needs feedback and adaptive weight adjustment.
In CN101199124, “Method and apparatus for optimum selection of MIMO and interference cancellation”, QUALCOMM INC provides systems and methods for facilitating interference nulling and rank prediction in an access terminal. Further, a plurality of receiver demodulator types are implemented to the access terminal, and an interference covariance matrix is estimated at the access terminal. Still further, SNRs (Signal to Noise Ratios) are calculated for the various receiver demodulator types, and an optimum rank and associated CQI information are identified and generated thereof, respectively. Furthermore, the information is then transmitted to an access point. Here, the scheme disclosed in the document needs a priori knowledge of channel quality in formation.
WO2012144108, given by Endo Kazuomi and Hashimoto Yoichi named as “Optical reception method and optical receiver” allows the reception in which receive sensitivity does not depend upon polarization state in reception of an M-ary phase optical signal. However, in this optical reception method, an M-ary phase optical signal of a single polarization is separated into a first optical signal and further a second optical signal of mutually orthogonal polarization. Accordingly, the ratio of the power of the first optical signal to the power of the second optical signal is calculated. Furthermore, the difference between the phase of the first optical signal and the phase of the second optical signal is calculated as an amount of compensation, whereupon, on the basis of the ratio and the amount of compensation, the first optical signal and the second optical signal are combined using amaximal ratio combining method. Thence, the amount of compensation is modified on the basis of the ratio. However, the invention deals with SISO (Single Input Single Output) system whose BER performance is not optimal.
Methods and systems of diversity combining, namely, MRC, EGC and SC in free space optical communications as known in the prior art have drawbacks associated therewith including:    1. Complexity in hardware in case of MRC.    2. Prior knowledge of channel state information in case of MRC.    3. Adaptive weight adjustment in case of MRC.    4. Poor bit error rate (BER) in case of EGC and SC
Accordingly, there is a need for eliminating the drawbacks associated with the systems and methods of diversity combining in free space optical communication systems as known in the prior art. Further, there is a need for developing a method that involves less number of hardware components, simple approach and cost-effectiveness.