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

6. Illustration of 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. Burst formation for an individual earth station
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. Example of frame structure for INTELSAT/EUTELSAT
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. Example of the burst reception process
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