Design and Simulation

The system consists of a modulating and a demodulating block and an advantage of filters added to improve the performance and optimize errors like noise.

The best suited modulator for FSP is Mach-Gender modulator which is tuned for high performance. The Lining demodulator demodulates the signal which in turn passed to a filter to attenuate the demodulated output and then through an amplifier to Increase the signal strength. Digital approach for Implementation of QPS Modulation Is attempted here and compared with existing systems. Simulation and characterization is done to freeze design ramset's.The system mainly concentrates on parameter like performance, security, minimum size and cost saving which will be the future of satellite communications.

In satellite communication, modulation is necessary to transfer the Information between satellite and the earth. Free-space optical (FSP) communications Is a optical communications is primarily interstellar links (Sills). The usual parameters that system designers want to optimize drive the desire to utilize optical communications onboard a satellite: size, weight and power and of course, cost.FSP yester uses a laser beam as a wireless connectivity between transmitter and receiver, free-space as propagation medium for carrying information. The performance of the system is greatly influenced by the propagation medium. Thus, the selection of modulation technique is a vital role in the design process of the system.

The most common modulation scheme used in satellite communication is BASK. This project illustrates that QPS can be give better results . Below are a quick summary of the bullets points as why QPS is superior.QPS requires half the bandwidth of ASK and hence is quite effective in optical communication. Tapes and Boras analyzed the exact density evolution and Gaussian approximation for both log- normal and gamma-gamma turbulence channel models using QPS modulation technique for a hybrid Optical/RFC channel during the year 2008. They observed a very good agreement between density evolution and Gaussian approximation for both the channel models (Tapes and Boras, 2008).

Hence here we have designed and analyzed the FSP system in detail for QPS technique.This gives an overview of proposed optical SSL system. Then, the selection of suitable optical wavelength which is used or deep space links. Then we revise the SSL link performance and freeze the system parameters focusing on transmitted bit rate between LEO satellites at bit error rate (BEER) ranged from 10-6 tool-9 employing PAD detector and QPS modulation at desired wavelength maximizing the quality of the system as well.

Fig 1: SSL System The optical system of SSL is consisting of two transceivers communicating by means of emitting and receiving optical signals.The proposed SSL uses lasers in free space channel as a signal carrier as well a wireless connectivity between transmitter and achieve as in Fig. 1. Using this FSP channel is considered a key technology for realizing an ultrahigh speed and long-haul communication system.

This laser source output is transferred, and the laser beam formed by collimation lenses system, and emitted using an optical antenna. The optical path loss depends on the laser wavelength and distance between transmitter and receiver platforms.At the receiver, the process is reversed and optical signal is converted into a bit stream. These bit streams modulated using Non Return to Zero (NOR) modulation because it s superlative scheme for obtaining maximum coverage distance of the link . Alt becomes extremely difficult to modulate the laser directly; therefore Mach Gender (MS) modulator used as an external and alternative optical modulator as seen in Fig. 1 2.

LITERATURE REVIEW From the previous section it is clear that QPS modulation technique has a improved performance than other techniques.Following are some of the already completed work in this area which forms the basis for current proposed work. Aziza, M. [1] Observed that advance modulation techniques have better performance ender controlled FSP turbulence channel. This paper experimentally investigates different modulation schemes, on-off keying (ASK) with non-return-to-zero (NOR) and return-to-zero (RAZZ) and the binary phase shift keying (BASK) operating under the turbulent atmosphere.

The received average signal is measured and used to characterize the strength of the turbulence.The received signal eye diagram and power histograms are presented and analyzed for performance evaluation of the selected modulation schemes in the weak turbulence model. Assam Kaki[2] focused on the impact of wavelength variation on proposed SSL teens two satellites network at LEO distance using PAD detector and NOR modulation. Space communication systems as employed in satellite to satellite links are traditionally performed using microwaves. Since, the future of satellite systems will permit more efficient and more reliable operation these (Inter Satellite Links) Sills will be optical links.

This paper proposes a link model between satellites in free space over Low Earth Orbit (LEO) in order to recognize maximum bit rate. The performance of such a link is affected by various parameters, as in depending on the propagating wavelength. Moreover, the maximum bit rate is calculated and analyzed employing Avalanche Photodiode (PAD) and Non Return to Zero (NOR) modulation at different wavelengths improving the quality of the system. Hashish, A.

H. [3] Observed that use of optical wireless communication system in space communications has better performance than lengthy fibers.As the number of satellites orbiting Earth increase year by year, a network between the satellites provides a method for them to communicate with each other. This is important for satellites to send information to one another and also to relay the information from en satellite to another satellite and then to the ground stations. By using laser satellite communication, the satellites can be connected with data rates up to several Gaps. The paper will present the optical wireless communication (SCOW) link performance focusing on data transfer between Low Earth Orbit satellites.

The system performance including bit rates, input power, wavelength and distance on an inter-satellite link were analyzed. M. Serbia [4] Observed the high speed optical differential quaternary phase shift keying (SKIPS). By excluding external feedback paths, limitations due to circuit and signal propagation delays is overcome .

The waveforms of the prerecorded input and output signals are plotted for a short PROS to verify correct function of the prerecorded. The receiver sensitivity was measured for different PROS lengths with and without using prerecorded.Melanie Tot. [5] Focused on the reliability of commercially available optical fiber modulators for space flight environments. General failure modes covered by the Dielectric standards are discussed as well as mitigation techniques for ground based systems. The details for testing such device for space flight are included and innovative testing methods for making long term reliability assessments for these devices.

From the literature review, it is very clear that advanced modulation techniques like QPS are well suited for free space optical communications.It Justifies the chosen targeted link specifications that these parameters must be considered together to make the system well efficient and significantly improved performance.