Compression and Coding schemes Sat Communication

Satellite Communication

1. SATELLITE COMMUNICATION UNIT IV – SATELLITE ACCESS AND CODING METHODS KARPAGAM INSTITUTE OF TECHNOLOGY COIMBATORE - 105. DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING

2. Broadcast Center – High quality video to MPEG-4 Analyze each frame in Video and eliminate redundant or irrelevant data and extrapolate information from other frame. Frame Encoding – Three Ways Intraframe - Complete image data for that frame and provide least compression. Predicted frame – Display the frame with enough information – Medium Compression Bidirectional frame – Display information from surrounding frame – Maximum compression. Encryption and Transmission Video compression, Scrambles the data and use correct decryption algorithm and security keys. Broadcast center beams it directly to one of its satellites and picks up the signal with an onboard dish. Amplifies the signal and back to e arth. Compression - Encryption

3. Video and Audio files have large bursts. Compression Temporal – Correlation between neighbouring pixels among set of frame Spatial - Correlation between neighbouring pixels in single frame. Spectral – Correlation between color or luminance components Psycho Visual – Exploit perceptual properties of human visual system Compression – Lossless and Lossy Lossless – Compressed data can be reconstructed without loss of information Lossy - Minimize the bit rate to achieve fidelity measure. Video and Audio Compression

4. Promote system interoperability among audio and video devices. MPEG-1 : Encoding and Decoding Streaming Video and Audio files MPEG-2: Digital television; Compress files for high quality video transmission. MPEG-4: Standard for compression high definition video into smaller scale files to computers cell phone and PDA’s MPEG-2I: Referred to as multimedia framework. Interprets what digital content to which individual use. MPEG Video Coding Standards

5. Encryption Plain text, Cipher text Symmetric Key encryption – Common keys Public key encryption – Public key, Private key Encryption

6. Signal transmitted via transponder is affected by many factors. Satellite links are usually designed with low margin of link budget Low E b /N is preferred to provide reliable message decoding. Turbo codes represent an optimal technique due to unique properties. Turbo Codes Introduced in 1993. Implemented in modern communication systems due to high efficiency and code gain. Low BER and Low E b /N 0. Does not have marked limit for BER reduction – Iteration decoding to reduce BER. Disadvantages : Complicate implementation High latency due to iteration decoding and complicated decoding algorithm. No mathematical tool for turbo code design Coding Schemes

7. Compromise between turbo code complexity and efficiency is the basic task of turbo code designing. For offline systems, error free detection of message can be reached for channel with strong noise and jamming. SMS – Low Noise ration and error free detection Decoding of Turbo Codes Code gain with maximum channel capacity and high encoder complexity. Classical algorithms cannot be used for decoding. Two SISO (Soft Input Soft Output) decoders connected with iteration loop. Posteriori probability estimation Log likelihood Ratio (LLR) Turbo Codes

8. Architecture of Turbo Code

9. Telemetry turbo code – SMS. It is determined for short data blocks transmission with low noise ratio. Modification : Optimum interleaver with appropriate data block and code ratio Generation of sequences with the smallest mutual correlations. Shannon limit and efficiency of turbo code based on interleaver quality. Three types of interleavers Telemetry, Pseudorandom and Edge effect elimination interleavers ; Code ratio R = ½, R=1/3, R=1/4 & R=1/6. Block lengths : 1024,1784,3568,4096,7136,8464,9216 Code ratio = ½ and block length = 4096. Modified turbo codes attains smallest BER. BER = 1.9 x 10 -4 d E b /N =0.8 dB. Modifications of Turbo Code