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Demand for high speed data transmission has increased in various fields of our day to day lives. Laser inter-satellite communication has proved to be high speed phenomenon for data transmission. In this paper, we compare the maximum transmission distance with non-return-to-zero (NRZ) and return-to-zero (RZ) modulations for two different LIC scenarios. Simulation results show that for the long-range LIC system with a saturated booster optical amplifier, the RZ modulation scheme can offer a longer transmission distance than the NRZ modulation scheme. For the short-range LIC system without an optical amplifier, the RZ modulation scheme performs almost the same as the NRZ modulation scheme.
Laser intersatellite Communication, Non-Return to Zero Modulation, Return to zero Modulation.
1.1 SATELLITE COMMUNICATION
Satellite communications are comprised of 2 main components:
The satellite itself is also known as the space segment, and is composed of three separate units, namely the fuel system, the satellite and telemetry controls, and the transponder. The transponder includes the receiving antenna to pick-up signals from the ground station, a broad band receiver, an input multiplexer, and a frequency converter which is used to reroute the received signals through a high powered amplifier for downlink. The primary role of a satellite is to reflect electronic signals. In the case of a telecom satellite, the primary task is to receive signals from a ground station and send them down to another ground station located a considerable distance away from the first. This relay action can be two-way, as in the case of a long distance phone call. Another use of the satellite is when, as is the case with television broadcasts, the ground station's uplink is then down linked over a wide region, so that it may be received by many different customers possessing compatible equipment. Still another use for satellites is observation, wherein the satellite is equipped with cameras or various sensors, and it merely downlinks any information it picks up from its vantage point.
The Ground Station
This is the earth segment. The ground station's job is two-fold. In the case of an uplink, or transmitting station, terrestrial data in the form of baseband signals, is passed through a baseband processor, an up converter, a high powered amplifier, and through a parabolic dish antenna up to an orbiting satellite. In the case of a downlink, or receiving station, works in the reverse fashion as the uplink, ultimately converting signals received through the parabolic antenna to base band signal. 
DEMAND FOR HIGH-SPEED INTERSATELLITE COMMUNICATION
The Demand for high-speed inter-satellite communication systems is increasing for military and commercial applications .The reasons are follows:
Firstly, many satellites and spacecrafts have been developed for military and commercial purposes such as surveillance, weather forecasting, environment probing, and space exploration. For applications that need more accurate measurements and/or to cover a bigger geographical space, the acquired information will drastically increase with monitoring frequency, observation area, and resolution of the images.
Secondly, high-speed data interconnections between satellites are essential for providing a satellite data network with ubiquitous global coverage.
Thirdly, the development of nanosatellite clusters places new requirements on the compactness and weight of the intersatellite communication system between nanosatellites. A nanosatellite cluster, which consists of a number of collocated and interlinked nanosatellites, can replace a single large-satellite. It has attracted a great deal of attention and inspired many new space missions recently. In a nanosatellite cluster, each nanosatellite has its own limited function.
To facilitate communication and effective cooperation, there is a need for each nanosatellite to be equipped with a high-speed communication system for communication among the cluster. Spacecrafts and airplanes, high-speed Internet access that is required for scientific and leisure purposes also calls for high-speed inter-spacecraft communication links.
Therefore we require an advanced intersatellite communication systems that will be able to handle large volume of data transfer between satellites. 
1.2. DRAWBACKS OF THE CONVENTIONAL RADIO FREQUENCY INTERSATELLITE COMMUNICATION SATELLITE LIMITATIONS
Limitations of a satellite communications system are determined by the technical characteristics of the satellite and its orbital parameters. Active communications satellite systems are limited by two things. Satellite transmitter power on the down links and receiver sensitivity on the up links. Some early communications satellites have been limited by low-gain antennas.
Power: The amount of power available in an active satellite is limited by the weight restrictions imposed on the satellite. Early communications satellites were limited to a few hundred pounds because of launch- vehicle payload restraints. The only feasible power source is the inefficient solar cell. (Total power generation in the earlier satellites was less than 50 watts.) As you can see, the power output is severely limited; therefore, a relatively weak signal is transmitted by the satellite on the down link. The weak transmitted signal is often reduced by propagation losses. This results in a very weak signal being available at the earth terminals. The level of signals received from a satellite is comparable to the combination of external atmospheric noise and internal noise of standard receivers. Special techniques must be used to extract the desired information from the received signal. Large, high-gain antennas and special types of preamplifiers solve this problem but add complexity and size to the earth terminal. (The smallest terminal in the defense communication systems network has effectively an 18-foot antenna and weighs 19,500 pounds.) Development of more efficient power sources and relaxation of weight restrictions have permitted improved satellite performance and increased capacity.
Receiver Sensitivity: Powerful transmitters with highly directional antennas are used at earth stations. Even with these large transmitters, a lot of signal loss occurs at the satellite. The satellite antenna receives only a small amount of the transmitted signal power. A relatively weak signal is received at the satellite receiver. This presents little problem as the strength of the signal received on the up link is not as critical as that received on the down link. The down-link signal is critical because the signal transmitted from the satellite is very low in power. Development of high-gain antennas and highly sensitive receivers has helped to solve the down-link problem.
Availability: The availability of a satellite to act as a relay station between two earth terminals depends on the locations of the earth terminals and the orbit of the satellite. All satellites, except those in a synchronous orbit, will be in view of any given pair of earth stations only part of the time. The length of time that a non-synchronous satellite in a circular orbit will be in the ZONE OF MUTUAL VISIBILITY (the satellite can be seen from both terminals) depends upon the height at which the satellite is circling. Elliptical orbits cause the satellite zone of mutual visibility between any two earth terminals to vary from orbit to orbit. These times of mutual visibility are predictable.
Cost perspective: launch cost is a rapidly increasing function of the mass and volume of a satellite; hence, lightweight and compact intersatellite communication terminals are desirable to reduce the launch cost. Both the high speed and small size requirements make laser intersatellite communication (LIC) systems attractive as compared to conventional radio frequency (RF) intersatellite communication systems. Since the frequency of laser is very high (hundreds of THz) compared with that of RF signal (few to tens of GHz), it is much easier to carry high data rates.
Highly directional: A laser beam can be designed to maintain a small footprint and high energy density even after it travels a long distance. This characteristic allows a small-sized antenna (telescope) to be used to collect enough optical signal power, and enable a compact, lightweight and low power laser intersatellite communication system to be designed. Furthermore, it is immune to interference, interception and jamming and offers more secured operation due to the narrow beam spread and receiver field-of-view (FOV) whereas its RF counterpart is subjected to those factors. 
1.3 ADVANTAGES OF NANOSATELLITES
Nanosatellites, also called "nanosats", are a relatively recent term used to describe artificial satellites with a mass between 1 and 10 kg (2.2ââ‚¬"22 lb). Larger satellites are often called microsatellites, while smaller satellites are called pico satellites. The term "nanosatellite" appears to have been introduced by NASA sometime around 2004. It is still in the process of adoption, as many satellites of this size are simply called "small satellites."
The idea of a nanosatellite has absolutely nothing to do with nanotechnology, a term that refers the precise engineering of materials on atomic and molecular scales. From a nanoscale perspective, a 5-kg satellite looks like Mt. Everest. Nanosatellites are appealing because their small size makes them affordable and opens up the potential for a swarm of the satellites. They can piggyback on larger launches, avoiding the need for a dedicated launch. From a military perspective, a nanosatellite may be useful for the redundancy it could offer. Its small size might also help it avoid detection.
POWER CONSUMPTION-AN IMPORTANT ISSUE FOR LIC SYSTEMS
Power consumption is an important issue for LIC Systems because the energy is limited on satellites. Therefore, the power efficient modulation formats have attracted high attention for LIC systems. In optical fiber communication systems, non-return-to-zero (NRZ) and return-to-zero (RZ) are widely used for their easy implementation and cost effectiveness. Fiber dispersion is defined as spreading out of a light pulse in time as it propagates down the fiber. Fiber nonlinearity is defined as movement of light is not straight from the source to the destination. The RZ format is preferably used to improve transmission performance of high capacity due to the smaller inter-symbol interference (ISI) induced by fiber dispersion and the lower fiber nonlinearity with the same peak optical power, optical fiber communication systems, compared with the NRZ format . Without the fiber dispersion and fiber nonlinearity, performance comparison between NRZ and RZ formats is necessary to determine which format can achieve better performance for LIC systems. RZ modulation performs better than the NRZ modulation for intersatellite and free space laser communication systems. However, when compared from the viewpoint of the receiver sensitivity instead of the transmission distance. In an LIC system, the transmission distance is greatly limited because of the un-availability of optical in-line amplifiers. 
2. SYSTEM CONFIGURATION
2.1 INTRODUCTION TO NRZ AND RZ MODULATION SCHEMES
2.1.1 RETURN-TO-ZERO (RZ)
Return-to-zero (RZ) describes a line code used in telecommunications signals in which the signal drops (returns) to zero between each pulse. This takes place even if a number of consecutive 0's or 1's occur in the signal. The signal is self-clocking. This means that a separate clock does not need to be sent alongside the signal, but suffers from using twice the bandwidth to achieve the same data-rate as compared to non-return-to-zero format .
The "zero" between each bit is a neutral or rest condition, such as a zero amplitude in pulse amplitude modulation (PAM), zero phase shift in phase-shift keying (PSK), or mid-frequency in frequency-shift keying (FSK). That "zero" condition is typically halfway between the significant condition representing a 1 bit and the other significant condition representing a 0 bit.
2.1.2 NON-RETURN-TO-ZERO (NRZ)
In telecommunication, a non-return-to-zero (NRZ) line code is a binary code in which 1's are represented by one significant condition (usually a positive voltage) and 0's are represented by some other significant condition (usually a negative voltage), with no other neutral or rest condition. The pulses have more energy than a RZ code. Unlike RZ, NRZ does not have a rest state. NRZ is not inherently a self-synchronizing code, so some additional synchronization technique (for example a run length limited constraint , or a parallel synchronization signal) must be used to avoid bit slip.
For a given data signaling rate, i.e., bit rate, the NRZ code requires only half the bandwidth required by the Manchester code.
NRZ-Level itself is not a synchronous system but rather an encoding that can be used in either a synchronous or asynchronous transmission environment, that is, with or without an explicit clock signal involved. Because of this, it is not strictly necessary to discuss how the NRZ-Level encoding acts "on a clock edge" or "during a clock cycle" since all transitions happen in the given amount of time representing the actual or implied integral clock cycle. The real question is that of sampling--the high or low state will be received correctly provided the transmission line has stabilized for that bit when the physical line level is sampled at the receiving end
2.2 SYSTEM CONFIGURATION
2.2.1 LIC SYSTEM
(LASER INTER SATELLITE COMMUNICATION SYSTEM)
An LIC system consists of two subsystems: pointing, acquisition and tracking (PAT) subsystem and communication subsystem. Fig. 2 shows a simplified block diagram of the LIC system. The acquisition/tracking detector determines the orientation of the partner satellite by measuring the incoming laser beam. With the coarse and fine steering mechanisms, the PAT subsystem ensures both satellitesââ‚¬â„¢ apertures (telescopes) to point towards each other so that a laser communication channel can be established between two satellites.
Fig 2. Block diagram of LIC system.
2.2.2 COMMUNICATION SUBSYSTEM
Fig. 3 illustrates the schematic of the communication subsystem. At the transmitter side, the pseudorandom binary sequence (PRBS) data with NRZ or RZ formats are directly modulated to a laser diode (for the data rate of no more than 10 Gb/s, direct modulation is applicable and the external modulator may induce additional power loss). The output light couples into a transmitter telescope directly for short-range applications. For long-range applications, a booster amplifier such as an erbium doped fiber amplifier (EDFA) will pump the laser to a high output power and then the light is coupled into a transmitter telescope. The telescope collimates the laser beam for transmission over the space. At the receiver side, a receiver telescope gathers light to a focused size. The received light is then fed into an optical band-pass filter (OBPF) which can remove the background noise and the amplified spontaneous emission (ASE) noise arising from EDFA. The light is then converted into electrical signal by a photo-detector (PD) followed by an electrical low-pass filter (ELPF). The bit-error-rate tester (BERT) is used to measure BER performance. Note that the booster amplifier is used in long-range LIC systems, but it is not employed in short-range LIC system.
Fig 3. Schematic for communication subsystem. 
3. RESULTS AND DISCUSSIONS
3.1. EXPERIMENTS & RESULTS
Plotting the simulated BER against the received optical power for NRZ and RZ modulations. The figure 4 shows the results for short-range LIC system and the next figure shows the results for long-range one. For the short-range LIC system, the sensitivity of RZ modulation at BER is about 1.3-dB better than the one of NRZ for the same receiver configuration. Similarly, for the long-range LIC system, the sensitivity of RZ format at BER of is about 1.1-dB better than the one of NRZ.
Fig 4. BER versus received optical power for NRZ and RZ modulations; (a) short-range LIC system; (b) long-range LIC system.
We have also plotted the BER against the transmission distance for both NRZ and RZ modulation formats for (a) short-range and (b) long-range LIC system. For the long-range LIC system, the RZ modulation performs better than the NRZ modulation. The maximum achievable transmission distance of RZ format is about 90 km longer than that of NRZ format at BER . However, for the short-range LIC system, the NRZ modulation can reach slightly longer (1.3 km) than the RZ modulation at BER. We have also conducted simulations for different receiver bandwidth by adjusting the ELPF bandwidth for RZ modulation. The results show that 1.75 GHz is the optimum RZ receiver bandwidth for our system configuration which can achieve the longest transmission distance.
The disappearance of RZ format is an advantage for short-range applications can be explained as the following reasons.
1.The maximum achievable transmission distance is greatly determined by the peak output optical power for the LIC systems with the same receiver configuration.2.For the long-range LIC system with a saturated booster EDFA, the gain saturation limits the maximum average output power. Because RZ format has smaller duty cycle than the NRZ format, it could acquire higher peak power for the same average output power. Hence, RZ format achieves longer transmission distance than NRZ. There is no saturated booster EDFA in the short-range LIC system, both optical signals with RZ and NRZ formats have the same peak power which depends on the peak output power of the direct modulated laser.
Consequently, the RZ format performs similar with NRZ format in the short-range LIC system. Comparing with NRZ format, the RZ signal has a smaller average power for the same peak power, which leads to a lower received optical power for the same BER
Fig.10 BER versus transmission distance for NRZ and RZ modulations; (a) short-range LIC system; (b) long range LIC System 
We have compared the transmission performance of NRZ and RZ modulations for two different LIC scenarios.
One scenario is the LIC system with a booster optical amplifier which is applied in the long-range intersatellite communication.
The other is the LIC system without any optical amplifier for communication within the cluster of small satellites whose crosslink distance is normally within several kilometers.
The simulated maximum transmission distance shows that for the long-range LIC system with a saturated booster optical amplifier, the RZ modulation scheme could offer a longer transmission distance than the NRZ modulation scheme. For the short-range LIC system without an optical amplifier, the RZ modulation performs almost the same as the NRZ modulation scheme.