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ECE 5233 Satellite Communications

Prepared by: Dr . Ivica Kostanic Lecture 17: Multiple Access Schemes (2) (Section 6.2 and 6.3 ). ECE 5233 Satellite Communications. Spring 2014. Outline . IM in FDMA Calculation of C/N with IM TDMA.

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ECE 5233 Satellite Communications

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  1. Prepared by: Dr. Ivica Kostanic Lecture 17: Multiple Access Schemes (2) (Section 6.2 and 6.3 ) ECE 5233 Satellite Communications Spring 2014

  2. Outline • IM in FDMA • Calculation of C/N with IM • TDMA Important note: Slides present summary of the results. Detailed derivations are given in notes.

  3. IM products between wideband signals • Satellite signals are broadband • IM distortion is wideband as well • As the amplifier is driven into saturation distortion components grow faster (3dB for 1dB) • When there are more than two signals, there are many IM components • IM distortion raises the noise floor for the satellite signals Spectrum analyzer plot for showing IM distortion with 3 signals Note: IM distortion is mitigated by forcing the operation in the linear part of the PA (back off) IM distortion spectrum for 2 wideband signals

  4. C/N calculation with IM • Path through transponder – another link degrading C/N • The overall C/N has three components (uplink, downlink and transponder) • The overall performance is limited by the smallest of the three • Optimum reached when all of them are the same Change of C/N at the output as a function of input power Note: In practice analysis of IM becomes complex and tools are used to determine optimum operation of the transponder

  5. Examples • Intermodulation example (page 228) • Example 6.2.1 • Example 6.2.2

  6. TDMA principle • Signals from various earth stations are interlaced in time • Each signal uses the entire transponder bandwidth – no IM • Modulation must be digital to accommodate intermittent nature of transmission • Signals are wideband – not well suited for large number of narrowband signals • Transmission of the earth stations must be synchronized • Each earth station transmits one burst per frame • Duration of frame from 125us to few ms • Earth station received all the transmissions and just de-multiplexes the desired one – issue of synchronization Illustration of TDMA principle

  7. TDMA burst generation • TDMA satellite access works well with landline TDMA systems • Rate adjustment accomplished through buffering • Burst of a satellite earth station consists of preamble (overhead) and user data • Preamble – contains data necessary for system operation (synchronization and signaling) • User data - payload Burst formation for an individual earth station Note: Data rate on the satellite link is much larger than data rate of incoming streams

  8. TDMA frame structure • Bursts are organized into frames • Guard times are inserted between burst – ensures that there is no overlap between bursts • One earth station - in charge of providing reference burst (no user data) • Traffic burst – carries user data • Longer frames – less overhead Example of frame structure for INTELSAT/EUTELSAT

  9. Burst reception • On the downlink each station receives all the bursts in the frame • Beginning of the frame – reference burst • Beginning of the burst – preamble • Synchronization with the burst – unique word • Each station extract the portion of the burst • Data received discontinuously – to provide rate matching there is a buffer at the receiver Example of the burst reception process

  10. Example • Example 6.3.1

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