Important Questions and Answers: Satellite Access

SATELLITE ACCESS

1. What is a single mode of operation?

A transponder channel abroad a satellite may be fully loaded by a single transmission from an earth station. This is referred to as a single access mode of operation.

2. What are the methods of multiple access techniques?

FDMA – Frequency Division Multiple Access Techniques

TDMA – Time Division Multiple Access Techniques

3. What is an CDMA?

CDMA – Code Division Multiple Access Techniques

In this method, each signal is associated with a particular code that is used to spread the signal in frequency and time.

4. Give the types of CDMA.

• Spread spectrum multiple access

• Pulse address multiple access

5. What is SCPC?

SCPC means Single Channel Per Carrier. In a thin route circuit, a transponder channel (36 MHz) may be occupied by a number of single carriers, each associated with its own voice circuit.

6. What is a thin route service?

SCPC systems are widely used on lightly loaded routes, this type of service

being referred to as a thin route service.

7. What is an important feature of Intelsat SCPC system?

The system is that each channel is voice activated. This means that on a two way telephone conversation only one carriers is operative at any one time.

8. What is an TDMA? What are the advantages?

TDMA – Time Division Multiple Access Techniques

Only one carrier uses the transponder at any one time, and therefore Inter modulation products, which results from the non-linear amplification of multiple carriers are absent. Advantages : The transponder traveling wave tube can be operated at maximum power output.

9. What is preamble?

Certain time slots at the beginning of each burst are used to carry timing and synchronizing information. These time slots collectively are referred to as preamble.

10. Define guard time.

It is necessary to prevent the bursts from overlapping. The guard time will vary from burst to burst depending on the accuracy with which the various bursts can be positioned within each frame.

11. What is meant by decoding quenching?

In certain phase detection systems, the phase detector must be allowed for some time to recover from one burst before the next burst is received by it. This is known as decoding quenching.

12. What is meant by direct closed loop feedback?

The timing positions are reckoned from the last bit of the unique word in the preamble. The loop method is also known as direct closed loop feedback.

13. What is meant by feedback closed loop control?

The synchronization information is transmitted back to an earth station from a distant, that is termed feedback closed loop control.

14. Define frame efficiency.

It is measure of the fraction of frame time used for the transmission of traffic.

15. What is meant by digital speech interpolation?

The point is that for a significant fraction of the time, the channel is available for other transmission and advantages are taken of this in a form of demand assignment known as digital speech interpolation.

16. What is meant by telephone load activity factor?

The fraction of time a transmission channel is active is known as the telephone load activity factor.

17. What are the types of digital speech interpolation?

• Digital time assignment speech interpolation

• Speech predictive encoded communications

18. What is meant by freeze out?

It has assumed that a free satellite channel will be found for any incoming speed spurt, but there is a finite probability that all channels will be occupied and the speech spurt lost. Losing a speech spurt in this manner is referred to as freeze out.

19. What are the advantages of SPEC method over DSI method?

Freeze out does not occur during overload conditions.

20. Define satellite switched TDMA?

Space Division Multiplexing can be realized by switching the antenna interconnections in synchronism with the TDMA frame rate, this being known as satellite switched TDMA.

21. What is SS / TDMA?

A repetitive sequence of satellite switch modes, also referred to as SS /TDMA.

22. What is processing gain?

The jamming or interference signal energy is reduced by a factor known as the processing gain.

23. What is burst code word?

It is a binary word, a copy of which is stored at each earth station.

24. What is meant by burst position acquisition?

A station just entering, or reentering after a long delay to acquire its correct slot position is known as burst position acquisition.

25. What is an single access?

A transponder channel aboard a satellite may be fully loaded by a single transmission from earth station.

26. What is an multiple access technique?

A transponder to be loaded by a number of carriers. These may originate from a number of earth station may transmit one or more of the carriers. This mode of operation known as multiple access technique.

27. What is meant by space division multiple access?

The satellite as a whole to be accessed by earth stations widely separated geographically but transmitting on the same frequency that is known as frequency reuse. This method of access known as space division multiple access..

28. What are the limitations of FDMA-satellite access?

a. If the traffic in the downlink is much heavier than that in the uplink, then FDMA is relatively inefficient.

b. Compared with TDMA, FDMA has less flexibility in reassigning channels.

c. Carrier frequency assignments are hardware controlled..

29. Write about pre-assigned TDMA satellite access.

Example for pre-assigned TDMA is CSC for the SPADE network. CSC can accommodate upto 49 earth stations in the network and 1 reference station. All bursts are of equal length. Each burst contains 128 bits. The bit rate is 128 Kb / s.

30. Write about demand assigned TDMA satellite access.

The burst length may be kept constant and the number of bursts per frame used by the given station is varied when the demand is varied.