1. MULTIPLE ACCESS TECHNIQUES
UNIT5
MULTIPLE ACCESS TECHNIQUES
(DCA -p140-p156)
Introduction
TDMA, TDMA-Frame structure, TDMA-Burst structure
TDMA-Frame efficiency, TDMA-superframe
TDMA-Frame acquisition & Synchronization
TDMA compared to FDMA
TDMA Burst Time Plan
Multiple Beam ( Satellite switched) TDMA satellite system
Beam Hopping(Transponder Hopping) TDMA
CDMA & hybrid access techniques.

2. MULTIPLE ACCESS
Multiple access is the ability of a large number of earth stations to simultaneously interconnect their respective voice, data, teletype, facsimile and television lines through a satellite.

3. The basic problem involved in satellite communication is how to permit a changing group of earth stations to share a satellite in a way that optimizes
1) satellite capacity
2) spectrum utilization
3) satellite power
4) interconnectivity
5) flexibility
6) adaptability to different traffic mixes
7) cost
8) user acceptability

4. VARIOUS MULTIPLE ACCESS TECHNIQUES
FDMA – All users share the satellite at the same time, but each transmits in its own unique frequency band including the guard band. Commonly employed with analog modulation.
TDMA –The users transmit in their own unique time slots using the single carrier
CDMA – Many earth stations simultaneously transmit orthogonally coded spread spectrum signals that occupy the same frequency band.

5. Capacity Allocation
FDMA
FAMA-FDMA
DAMA-FDMA
TDMA
Advantages over FDMA

6. FDMA
Satellite frequency is already broken into bands, and is broken in to smaller channels in Frequency Division Multiple Access (FDMA).
Overall bandwidth within a frequency band is increased due to frequency reuse (a frequency is used by two carriers with orthogonal polarization).

7. FDMA (cont.) The number of sub-channels is limited by three factors:
Thermal noise (too weak a signal will be effected by background noise).
Intermodulation noise (too strong a signal will cause noise).
Crosstalk (cause by excessive frequency reusing).

8. FDMA (cont.) FDMA can be performed in two ways:
Fixed-assignment multiple access (FAMA): The sub-channel assignments are of a fixed allotment. Ideal for broadcast satellite communication.
Demand-assignment multiple access (DAMA): The sub-channel allotment changes based on demand. Ideal for point to point communication.

9. TDMA
TDMA (Time Division Multiple Access) breaks a transmission into multiple time slots, each one dedicated to a different transmitter.
TDMA is being used increasingly in satellite communication.
TDMA uses the same techniques (FAMA and DAMA) as FDMA does.

10. TDMA (cont.) Advantages of TDMA over FDMA.
Digital equipment used in time division multiplexing has increasingly become cheaper.
There are advantages in digital transmission techniques. Ex: error correction.
Lack of intermodulation noise means increased efficiency.

11. TDMA Frame Structure

12. Time Division Multiple Access

14. Difference between TDM and TDMA
In TDM everything comes from the same transmitter and the clock and carrier frequencies do not change , while in TDMA each frame contains a number of independent transmissions.
One advantage of TDMA is that it permits a transponder’s TWT to operate at or near saturation and thus it maximizes downlink C/N ratio.

15. In TDMA many earth stations in the satellite communication network use a single carrier for transmission via the satellite transponder on a time division basis.
TYPES OF TDMA
F-TDMA
D-TDMA
TDMA

16. Number of Voice Channels
The number of voice channels in a TDMA system is given by
n=(R-NP/T)/r
where
r= Voice channel bit rate
R=Satellite channel bit rate
N=Number of bursts in a frame
P=Number of digits in the preamble
T= Frame period

17. TDMA FRAME STRUCTURE

18. TDMA BURST STRUCTURE
TDMA BURST STRUCTURE

19. Reference Burst Structure
Carrier and clock recovery
Unique word
Order wire channel
Management Channel
Transient timing channel
Preamble(header)
Signalling channel or control bits

20. TDMA Traffic Burst Structure
Preamble or Header
Carrier and clock recovery
Unique word
Order wire channel
Service channel
Info burst 1
Info burst 2
Info burst N
Traffic data
Signalling channel or control bit
------

21. TDMA FRAME EFFICIENCY

22. TDMA Frame Efficiency
The TDMA frame efficiency ()depends on the ratio of the time devoted to transmission of information bits in the frame to the total frame length and is thus defined as
 = Traffic bits/ total bits
or  = 1-t/TF
Where t represents the sum of all the guard times and preambles including the reference burst and is expressed as
t =(n + 2 )P/R

23. TDMA frame efficiency Where R = Satellite transmission link bit rate ,
n = Number of traffic burst in a frame considering two reference bursts per frame ,
P = Number of bits in the preamble plus guard time,
TF = Frame period
To have better frame efficiency t should be small and TF should be large.
In various digital communication satellites, Current frame efficiency is of the order 0.9

24. TDMA frame efficiency
Once the frame efficiency is known the number of telephone or voice channels () that may be supported by a TDMA system is given by
 = R/r 
where
r = Bit rate of a voice channel.
R = Satellite transmission link rate
 = TDMA frame efficiency

25. TDMA SUPERFRAME

26. TDMA Or Master Super frameTF
nTF
The orderly arrangement of sequential frames which increases the efficiency by reducing the overheads is called TDMA super frame or master frame.

27. TDMA Or Master Super frame
O identify the frames in a super frame a frame identification number is carried in the management channel or in a separate channel in the reference burst for each frame.
Usually the identification number of frame, serves as superframe marker. When the number of stations N is a variable, demand assignment is used, then a superframe short burst(SSB) is transmitted at the super frame rate.

28. TDMA FRAME ACUISITION AND
SYNCHRINIZATION

29. TDMA Frame Acquisition and Synchronization
It is the process to maintain the accuracy of TDMA during the transmission time and reception time .
These processes helps in locating and controlling the traffic stations transmit burst time phase at the transmit side , and carry out the similar process at the receiving end.

30. Various Timings in TDMA
In TDMA various timing involved at the transmit and receive side are transmit frame timing (TFT),transmit burst timing (TBT) , Receive frame timing (RFT) and the receive burst timing (RBT)
TFT = Start of station transmit frame
TBT = Start of transmission of traffic burst to the satellite
RFT = Instant of occurrence of last bit or symbol of primary reference bursts unique word
RBT= Last bit or symbol of traffic burst unique word.

31. Various acquisition and synchronization processes
Receive frame acquisition (RFA)
Transmit frame acquisition (TFA)
Receive frame synchronization (RFS)
Transmit frame synchronization (TFS)

32. TDMA FRAME ACQUISITION
The acquisition process is required when the traffic station enters or re-enters the operation.
Techniques
Closed loop acquisition
Open loop acquisition

33. Closed loop acquisition
is obtained by transmitting either a low power burst , or a pseudo noise sequence modulated burst, observing its position , correcting it to an assigned position and then transmitting the traffic burst at full power.
Open loop acquisition
It is initiated by informing the station entering the network of the time shift DN relative to the reference burst at which the station should transmit a short burst consisting of a preamble and a unique word.

34. TDMA FRAME Synchronization
The synchronization is needed due to satellite’s movement in the orbit.
Synchronization is achieved by introducing a transmit frame delay DN to mark the transmit frame timing of station N such as
DN = MTf - 2dN/C
Where M = smallest integer , value is such that DN ≥0
Tf = Frame period
dN = distance between the satellites N
C = velocity of light

35. In closed loop synchronization method or loop back method, Dn is determined by the earth station directly from monitoring its own transmission .
In this the station observes the position of its own burst relative to the reference burst by measuring time interval between the reference burst unique word and the traffic burst unique word , computes the error and determines a new value for the delay Dn.

36. In open loop synchronization
This is the simplest method of transmit timing.
A control station listens to the bursts and sends timing instructions back to each transmitting station.

37. TDMA BURSTTIME PLAN

38. TDMA Burst Time Plan Message data Burst time plan identification
Traffic station identification
Number of transmit Bursts
Number of receive bursts

39. TDMA Burst Time Plan Burst Data Burst identification
Transmit-receive Flag
Transponder identification
Burst Position
Number of transmit-receive sub bursts

40. TDMA Burst Time Plan Sub burst Data
Transmit receive sub burst identification
Sub burst position
Sub burst Length

41. MULTIPLE BEAM (SATELLITE SWITCHED TDMA SATELLITE SYSTEM

42. BEAM HOPPING (TRANSPONDER HOPPING) TDMA
In beam hopping TDMA the switching is achieved by each station hopping from one freq to another according to the destination of the transmitted burst.
Each earth station selects its transmission time on the corresponding frequency so as to transmit its burst within correct time slot.
It should be noted that the beam hopping is also carried out along with the on-board signal regeneration.
This helps in avoiding the inter burst interferences associated with conventional TDMA.

43. Satellite switched TDMA

44. FDMA

45. FDMA FDMA allocates a single channel to one user at a time
It is a technique in which the carrier bandwidth is divided into sub channels of different frequency width, each carrying a signal at the same time in parallel.

46. FDMA FDMA can be performed in two ways:
Fixed-assignment multiple access (FAMA): The sub-channel assignments are of a fixed allotment. Ideal for broadcast satellite communication.
Demand-assignment multiple access (DAMA): The sub-channel allotment changes based on demand. Ideal for point to point communication

48. TDMA
In Time Division Multiple Access (TDMA), it makes use of the same frequency spectrum but allows more users on the same band of frequencies by dividing the time into “slots” and shares the channel between users by assigning them different time slots.
TDMA is utilized by Digital-Advanced Mobile Phone System (D-AMPS) and Global System for Mobile communications(GSM). However , each of these systems implements TDMA in a somewhat different and incompatible way.

49. TDMA COMPARED TO FDMA The advantages of TDMA over the FDMA are:-
The bit capacity of the TDMA system is independent of the number of accesses.
The power amplifiers in the transponder of a TDMA system can be operated in the saturation mode as opposed to that of a FDMA system. This increases the capacity of multiple accesses.
The duty cycle of the Earth station in a TDMA system is low.
TDMA systems allow the use of digital techniques like digital speech interpolation, satellite onboard switching and so on.
TDMA system offer more flexibility due to use of high speed logic circuits and processors witch offer high data rates.

50. TDMA systems are more economical as compared to FDMA systems because they are easy to multiplex, independent of distance and can be easily interfaced with terrestrial services.
TDMA systems can tolerate higher level of interference noise.
The disadvantages of TDMA over FDMA are as follows:-
The peak power of the amplifier in TDMS system is always large.
TDMA system is more complex as compared to an FDMA system. The earth station of a TDMA system requires ADC, clock recovery, synchronization, burst control and data processing before transmission.
TDMA system use high speed PSK/FSK circuits.

52. CDMA
Code Division Multiple access is a technique in which every communicator will be allocated the entire spectrum for all the time.
CDMA uses codes to identify connections.
This technology is used in ultra high frequency(UHF) cellular telephone systems in the 800 MHz and 1.9 GHz bands.

53. COMPARISION
TDMA/FDMA/CDMA

54. S. No
Parameter
FDMA
TDMA
CDMA 1
Spectrum utilization for a single station
It does not use full bandwidth
It uses full bandwidth within allocated time slot.
It uses full bandwidth throughout operation 2
Analog/Digital
Generally analog
Digital 3
Interference
Interference may occur
Interference is eliminated 4.
Synchronization
Difficult in demand assigned
Easy

55. S. No Parameter FDMA TDMA CDMA 5 Intermodulation distortion Present
Not present 6
Secrecy (Interception)
Almost insecure
Better than FDMA
Fully secure 7
Jamming
Exist
Does not exist 8
Bit rate
Medium
High
low 9
Flexibility
Poor
Better
Best 10
Cost
Low
Lowest