Performance Evaluation of VSAT-QPSK System

Transcription

1 Performance Evaluation of VSAT-QPSK System T.P. Surekha 1, T. Ananthapadmanabha 2, C. Puttamadappa 3 1 Associate Professor, Dept. of E&CE, Vidyavardhaka College of Engineering, Mysore, India. 2 Professor, Dept. of E&EE, National Institute of Engineering, Mysore, India and Chairman Of IEI, Mysore local center, Mysore, India. 3 Professor, Dept of E&CE, and Principal of Sapthagiri college of Engineering, Bengaluru Abstract The objective of this paper is to develop Simulation model for Very small aperture terminal- Quadrature phase shift keying modulation System (VSAT- QPSK System) to analyse Bit Error Rate (BER) which is feasible for Data transmission with BER 1oe-7. Data messaging network is operating in India at Karnataka with extended C-band. VSATs in Karnataka of KPTCL use VSATS G Hz uplinks and GHz downlinks. These frequencies are dedicated to fixed services. The Satellite is Intelsat -3A, the hub has a 7.2 m diameter antenna and uses travelling wave tube amplifier at the transponder. The VSAT s are 1.2 m with RF power of 1W or 2W depending on their position in the uplink beam with data rate of 64 or 128 K bit/s. The performance of the system is analysed by the error probability called BER (Bit Error Rate) and results are derived from earth station to hub and hub to earth station using satellite transponder as the media of communication channel. The Link budgets are evaluated using the Data s of Karnataka Power Transmission Corporation Limited (KPTCL). Keywords BER, Convolutional codes, Link budget, Satellite Communication, VSAT. I. INTRODUCTION This paper is concerned with Very Small Aperture Terminal (VSAT), VSAT is a main communication media for Karnataka Power Transmission Corporation Limited (KPTCL)/ ESCOM (Electric supply companies) Supervisory control and data acquisition (SCADA) network and it also provides voice communication to all KPTCL, ESCOMS stations, and major generating stations with load dispatch centre (LDC). VSATs are designed for data transmission and distribution over a wide geographical area amongst a large number of locations. The small size and low transmit power of a VSAT station are the factors that keep the price of the earth station at a level that makes a VSAT network an economic alternative to a terrestrial data network using telephone lines and modems. The hub usually houses a central host computer, which can act as a data switching centre. The links from the hub to the VSAT are called outbound links. The links from the VSAT to the hub are called inbound links. Both inbound and outbound links consist of two parts, uplink and downlink. Implementation deals with the modeling and simulation of RF communications link involving satellite transponder with QPSK Modem technique. Integration of QPSK modem with satellite transponder can deliver better data rates with minimum error compared to QAM modem system. Here BER achieved is 10e-7 where it is 10e-4 in QAM system. II. SIMULATION MODEL Figure 1 shows the complete model of VSAT QPSK modem system consisting of three major sections as explained below. Transmitter Section Source: The data source is a random bit generator. Modulator: The modulator is a QPSK modulator. QPSK modulation helps to visualize the constellation diagram. The scatter diagram allows us to visualise the real and imaginary components of the complex signal. Transmitter model: The transmitter from the output of the modulator to the output of the antenna is modelled as shown, the sequence of models represents a saturating up-converter and a power amplifier also operated at saturation is further followed by RRC filter and transmitting antenna. Transponder section The transponder from the input of the receiving antenna to the output of the transmitting antenna is modelled as a memory-less non linearity sandwiched by two filters. 726

2 Receiver antenna: here the gain of an antenna is proportional to f 2 Receiver thermal noise: this block adds noise that represents the effective system temperature of the receiver, noise temperature T s : 45 K I/Q imbalance: In-phase / Quadrature (I/Q) processing receivers, matching the amplitudes and phases of the I and Q branches is of major concern. This block creates a complex base band model of signal impairments caused by imbalances between in-phase and quardrature receiver. Automatic gain control: The purpose of this circuit is to provide relatively constant output amplitude so that circuits following the AGC block require less dynamic range. Phase and frequency offset: Rotates the signal to represent correction of phase and Doppler error on the link. Data filter: The filter labelled RRC in the receiver acts a matched filter to de-modulate signal using the raised cosine pulse shape QPSK demodulator: This block demodulates a signal that was modulated using QPSK modulation with a constellation on a rectangular lattice Fig 1: Complete model of VSAT QPSK System Non linearity: The non linear behaviour of the transmit amplifiers, both uplink and downlink is implemented according to the serial memory-less model. That is, the model consist of a pair of cascaded memory-less transfer characteristics, AM/PM followed by AM/AM. Receiver section The receiver model from the input of the antenna to the input of the demodulator is modelled as a single transfer function. Other functions housed in the ground station are described below. III. SIMULATION ANALYSIS All signal in the signal processing and communications can generate frame based data. Frame - based takes an advantage of Simulink matrix processing capabilities to reduce overhead. Complex modulations are best viewed using a scatter diagrams. The scatter diagram allows us to visualize the real and imaginary components of the complex signal. Thus Fig 2 shows the scatter plot of VSAT QPSK Transmitter with free errors and Fig 3 shows the scatter plot of VSAT QPSK Receiver with slight phase tilt, which can be corrected by Phase offset block. Fig 4 shows the spectrum of transmitter/receiver signal. Both spectrums are almost similar, but some effect of noise can be seen in the receiver spectrum as shown in blue colour. Fig 5 shows the Bit error rate of simulated model compared with BERtool. Finally Fig 6 shows the power Characteristic of travelling wave tube Amplifier showing the saturation of transponder power. 727

3 Fig 2: Scatter plot of VSAT QPSK Transmitter Fig 4: Spectrum Scope of VSAT QPSK System Fig 3: Scatter plot of VSAT QPSK Receiver Fig 5: BER as a function of Eb/No for VSAT QPSK System 728

4 Where P r is power received by the receiving antenna: P t is the power applied to the transmitting antenna: G t is the gain of the transmitting antenna, G r is gain of the receiving antenna, C is the speed of light (c = 3 x 10 8 m/s), R is the range (path length) in meters, and f is the frequency in hertz. Almost all link calculations are performed after converting from products and ratios to decibels. This uses the unit popular unit of decibels, thus converting the equation 1.1 into decibels. It has the form of a power balance as P r = P t + G t + G r Path-loss (Lp) (1.2) All link budgets require knowledge of the free space path loss between the earth station and the satellite and the noise powers in the operating bandwidth. Free space Path loss: Lp = 20 log (4 R / ) (1.3) Fig 6: Power In/Out Curve for VSAT QPSK System IV. CONCLUSION The simulated results shows the improvement of BER in VSAT QPSK system compared to VSAT QAM System with respect to figures as discussed above. Fig 5 shows the Bit error rate of Simulated model with the value of 10e-6 and the same is compared with the Ber-tool observations which varies from 10e-1 to 10e-8, indicating the BER is less in QPSK VSAT System and the system performs better data transmission. The same can be observed with scatter plots as shown in Fig 2 and Fig 3 which gives the data transmission with error free. Fig 6 shows the power characteristics of a TWTA showing the saturation.. VSAT with QPSK system can perform better data transmission with small loss in transponder capacity and the system can quickly respond to traffic variation. Efficiency can be further increased by demand assignment. By using link budget analysis, uplinks and downlinks are calculated for both minimum and maximum frequency of extended C-band. Here the uplink free space path-loss is 221db and downlink is 217db as shown in the link budget table calculations. Link budget calculations The link between the satellite and Earth station is governed by the basic microwave radio link equation: The Parameters: Distance = km Uplink Frequency = 6946 MHz Downlink frequency = 4721 MHz Table 1 gives the data s of KPTCL which is used in developing the model. TABLE 1 Earth station Transmitter antenna Gain Satellite Transmitter antenna gain Earth station Receiver antenna gain Satellite Receiver antenna gain Transponder bandwidth Up-link frequency band Downlink frequency band 52.48db 31db 36.85db 38.2db 36MHz GHz GHz Up-link loss and Down-link loss 221db and 217db P r = (1.1) 729

5 Acknowledgement The authors are very grateful to the Management of Vidyavardhaka College of Engineering, Mysore, Karnataka, India. The National Institute of Engineering, Mysore, Karnataka, India and Sapthagiri College of Engineering. Bengaluru, Karnataka, India for their constant encouragement and motivation during their work. REFERENCES [1] XiaolongLi, Simulink based Simulation of quadrature Amplitude Modulation (QAM) System Proceedings of the 2008 IAJC IJME Internationl Conference.ISBN [2] T. Pratt, C. Bostian, J.Allnutt, "Satellite Communication John Wiley and Sons, 2 nd Edition. [3] Theodore S. Rappaport, Wireless Communications, Principles and Practice, Prentice Hall of India, Private Limited, 2 nd Edition. [4] Sanjay Sharma Wireless and Cellular Communications, S.K. Kataria and Sons, 2 nd Edition. [5] Bruce Elbert, M, Schiff, Simulating the performance of Communication Links with SatelliteTransponders. Application Technology Strategy, Inc. [6] Michel C.Jeruchim, Philip Balaban, and K.Sam Shanmugan, Simulation of Communication Systems, Kluwer Academic Publisher. [7] T.P.Surekha,T.Ananthapadmanabha,C.Puttamadappa, Performance Analysis of QPSK system with Different BER Tools IJETSE, vol 4, No 1, April 2001, pp1-6. [8] T.P. Surekha, T. Ananthapadmanabha, C. Puttamadappa Modeling and simulation Analysis of QPSK system with channel coding WIMO / CONeCO 2011,ccis 162, pp 57-64, Published in Springer Proceedings. 730