Doppler Frequency Estimation and Nullification of the ...

Doppler Frequency Estimation and Nullification of the Satellite Receiver Subhankar Shome, Samarendra Nath Sur, Debjyoti Ghosh, Debasish Bhaskar Sikkim Manipal Institute of Technology, Sikkim Manipal University Majitar, Rangpo, East-Sikkim -737132, India. [email protected], [email protected], [email protected] Abstract— In today’s world DSSS (Direct Sequence Speard Spectrum) is a well established technology for wireless communication, whereas some satellite based technology Like: GPS, GNSS [1] are also using the same, but it is still not too much popular in the commercial RF link Like: DTH. Today Direct to Home (DTH) is a popular commercial satellite communication by which millions of people enjoying their daily life worldwide. In high-speed multimedia satellite communication systems, it is essential to provide high-quality, economical services by using efficient transmission schemes which can overcome channel impairments appearing in the satellite link.[2] This paper introduces a new technique at the receiver end to compensate for rain attenuation and the Doppler shift in the satellite communication link. Extensive simulation results show that the proposed algorithm can provide a great enhanced performance compared to conventional algorithms.

and rain attenuation. The Doppler effect in satellite communications is the change in frequency of an electromagnetic signal that results from the relative speed of the satellite and the Earth terminal. This effect is an important consideration in satellite communications. In this paper authors have built an advanced spread spectrum receiver system in MATLAB SIMULINK [4] environment and got some interesting results showing the improved performance in terms of high resistance against Doppler shift. The basic block diagram of the modified receiver is shown in figure below-

Keywords— DSSS, Doppler, Correlation, satellite, SIMULINK.

I. INTRODUCTION To simulate this type of intelligent satellite receiver the authors are use the DSSS Technology. The implementation of the DSSS communication system involved the analysis and design of unique synchronization loops for code tracking and carrier estimation, as a result of the use of complex spreading sequences. The SS Communications are widely used today for Military, Industrial, Avionics, Scientific, and Civil uses. The advantages of using SS include the following [3]: • Low power spectral density. o As the signal is spread over a large frequency-band, the Power Spectral Density is getting very small, so other communication systems do not suffer from this kind of communication. o The ability to utilize the Satellite payload channels, which is achievable as the transmitted signal is spread in such a way that it become noise-like and thus would not interfere with the payload traffic. • Interference limited operation. • Privacy due to unknown random codes. • Applying spread spectrum implies the reduction of multipath effects. • Random access possibilities. As users can start their transmission at any arbitrary time. • Good anti-jam performance. Those unique features of spread spectrum help authors to build an advanced receiver that is capable of mitigating Doppler shift

Fig: 1 Basic block diagram for modified satellite receiver.

II.

SYSTEM MODELING

A. Satellite Downlink Transmitter

Fig:2 MATLAB model for satellite downlink Transmitter.

Figure 2 shows the simulated block diagram of the designed satellite transmitter. Random Integer Generator creates a random data stream as information source. Rectangular QAM modulator baseband maps the data stream to 16-QAM constellation. Here spread spreading is introduced as second

level of modulation in order to achieve high security and intelligence in receiver side as in GPS and GNSS. Raised Cosine Transmit Filter is used to up-samples and shapes the modulated signal using the square root raised cosine pulse shape. After pulse shaping authors are used Saleh Model as traveling wave tube amplifier (High Power Amplifier). Gain (Tx. Dish Antenna Gain) block introduced the gain of the transmitter parabolic dish antenna on the satellite. B. Satellite Downlink Path

Fig: 3 Downlink satellite path

Figure 3 shows the simulated block diagram of the designed satellite downlink path. Free Space Path Loss block attenuates the signal by the free space path loss. In general downlink path loss is of around 180dB. The path loss in a satellite downlink path can be calculated by the equation given below Down link loss=10 log (4d/(C/f))2-------------(1) , Where ’d’ is the distance between satellite & ground station ‘f’ is the frequency of the down link wave ‘C’ is the velocity of the light = 3 x108 m/Sec Phase/Frequency Offset (Doppler and Phase Error) rotates the signal to model phase and Doppler error on the link. C. Satellite Downlink Receiver

Receiver System Temp) adds white Gaussian noise that represents the effective system temperature of the receiver. Gain block introduce (Rx. Dish Antenna Gain) gain of the receiver parabolic dish antenna at the ground station. Phase Noise and I/Q Imbalance introduces random phase perturbations and DC offset, amplitude imbalance, or phase imbalance to the signal. The DC removal block Estimates and removes the DC offset from the signal. Compensates for the DC offset in the I/Q Imbalance block. The Magnitude AGC I and Q AGC block Compensates the gain of both in-phase and quadrature components of the signal, either jointly or independently. Raised Cosine Receive Filter applies a matched filter to the modulated signal using the square root raised cosine pulse shape. In receiver side authors introduced dispreading and correlation block in order to retrieve base band data from spreaded data. A spread spectrum receiver uses a locally generated replica of the pseudonoise code and a receiver correlator to separate out the desired coded information from all possible signals with proper time synchronization. A spread spectrum correlator can be thought of as a very special matched filter -- it responds only to signals that are encoded with a pseudonoise code that matches its own code. Thus, an SS correlator can be "tuned" to different codes simply by changing its local code. This correlator does not respond to manmade, natural or artificial noise or interference. It responds only to SS signals with identical matched signal characteristics and pseudonoise code.[5] The algorithm implemented to estimate the Doppler Frequency is based in a delay-and-multiply scheme, wherein the frequency offset is estimated from the product of signal sample. This particular algorithm operates in a non-data-aided and clock-aided fashion and has open loop topology. If received sample sequence {x(kTs)}is obtained from the received signal through filtering and sampling operation, the Doppler shift is estimated by the formula given below[6]

---- (2) Where L0 is the observation length, N is the oversampling factor, D is corresponds to loop delay and z (kTs) is given by

---- (3) The details block diagram for the estimation of the Doppler shift using above equation is shown in the figure 5.

Fig: 4 MATLAB model for satellite downlink Receiver.

Figure 4 shows the simulated block diagram of the designed satellite receiver. The receiver thermal noise (Satellite

Fig: 7 With Zero Estimated Doppler value Fig: 5 Implementation of Doppler frequency recovery algorithms

Phase/Frequency Offset block is used to compensate the phase and Doppler error on the link. This block compensates the Doppler shift with the help of the estimated Doppler shift value from the Doppler frequency recovery block. The dispreading block dispread the in coming signal with the help of an identical code that is in transmitter side to retrieve the basic information data which is again demodulated by using rectangular QAM demodulator. III. RESULTS As we discussed earlier, 16 QAM modulation have been used in satellite transmitter, we are getting 16 constellations points in the transmitter side (Fig no. 6) and receiving 16 constellations points (Fig no. 8) in the receiver also with zero Doppler effect (Fig no.7). But whenever we are introduced 2000 Hz Doppler (Fig 9) in the frequency offset block, the received constellation becoming noisy (Fig no. 10) and start to rotate due to Doppler Effect. Generally in this condition people will not able to retrieve exact information at the receiver. Here our Doppler frequency recovery algorithms block (Fig No. 5) is able to estimate the Doppler frequency of 2000 Hz (Fig No. 11) which is introduced by the channel. This block helps us to nullify the Doppler shift and de-rotate the constellation at the receiver end. By this technique, we are getting a stable constellation (Fig no. 12) at the receiver and we are able to retrieve the original signal in rainy condition.

Fig: 6 Constellations of transmitted signal.

Fig: 8 Constellations of the received signal for Zero Doppler

Fig: 9 2000 Hz Doppler is introduced in channel.

Fig: 10 Constellation of received distorted signal with 2000 HZ Doppler.

Fig: 11 Estimated Doppler value of 2000 Hz at the receiver .

Fig: 12 Constellations of received signal after Doppler Estimation & Nullification.

IV. CONCLUSIONS Doppler shift is a systematic change in frequency of a carrier wave and results when transmitter and receiver are moving relative to each other. This effect is an important consideration in satellite communications. In this paper authors utilizes the unique feature of spread spectrum technique along with the Doppler shift estimation algorithm to estimate and nullify the Doppler effect. Authors have got interesting result and firmly believe that this Doppler problem in satellite communication can be mitigated by this new approach. REFERENCES [1] [2] [3] [4] [5] [6]

Understanding GPS : principles and applications. Elliott D. Kaplan, editor. Boston : Artech House, 1996. Direct sequence spread spectrum technique with residue numbeer syatem, M. I. Youssef, A. E. Emam, and M. Abd Elghany, 2009 Overview of the DSSS Communication System, University of Pretoria etd- Marx, F E, 1995 www.mathworks.com http://sss-mag.com ‘Synchronization techniques for digital receivers’ by Umberto Mengali, Aldo N. D'Andrea