1. SATELLITE ACCESS TECHNIQUES
NED UNIVERSITY OF ENGINEERING AND TECHNOLOGY
Department of Telecommunications Engineering
SATELLITE COMMUNICATION (LAB ASSIGNMENT)
SATELLITE ACCESS TECHNIQUES
Group Members:
Syeda Faiza Warsi TC – 058 (GL)
Sanam Iftikhar TC - 305
Usama Aftab TC - 054
Sukena Rizvi TC - 033
Rabia Javed TC - 046
Farheen Khan TC - 303
Submitted To:
Ms. Farzeen Iqbal
Date of Submission:
11th October 2011 1

2. SATELLITE ACCESS TECHNIQUES
Multiple Access (MA) refers to the general process used in communications systems in which system assets (circuits, channels, transponders, etc.) are allocated to users.
Goal is to allow the communications network to respond to expected changes in user demand and adapt resources to provide the desired level of performance
Satellite systems often offer benefits over terrestrial transmission alternatives for implementation of efficient MA because the inherent ground/space link architecture allows network asset optimization without the need to add additional nodes or other components to the system.
TIME DIVISION MULTIPLE ACCESS (TDMA)
TDMA is a channel access method for shared medium networks
Shares single carrier frequency with multiple users
Non-continuous transmission makes handoff simpler
Slots can be assigned on demand in dynamic TDMA
Flexible bit rate
No need for precise narrowband filters
Less stringent power control than CDMA due to reduced ICI
Requires network-wide timing synchronization
Multi-path distortion 2

3. 3 BURST TRASMISSION PLAN
A master station with visibility of the whole network maintains access discipline and scheduling.
BTP specifies the frequency; polarization of the transmissions; position: duration of each burst in the frame; name/identification of the originating station for every transponder
TDMA FRAME STRUCTURE
CTBR: Enables receiving ES to recover remainder of burst. Contains sequence of pre determined signals used for synch of bit clock
UW: bits; acts as station identifier / burst marker; Maintains Frame Synchronization
SIC: identifies Tx Station
Preamble: Allows the receiver to acquire on top of the coarse synchronization provided by the reference burst a fine estimate of timing and carrier phase. 3

4. Guard Period: Used to avoid loss and to reduce interference to the following user, caused by propagation delay. Thus a user's timeslot is protected from interference from the preceding user, by the guard interval (guard period) at the end of that preceding user's timeslot.
Longer Guard periods allow more distant echoes to be tolerated. However, longer guard intervals reduce the channel efficiency.
NETWORK SYNCHRONISATION
Open window method; Wait for UW
Once detected, close detection gate
Open detection gate before end of frame
Wait for UW
if 3 times in a row correct; start operations
NON-DETECTION FALSE DETECTION
Probability decreases, if Probability decreases, when
– bit error rate of link decreases – length of UW increases
– length of UW decreases – correlation threshold increases
– correlation threshold decreases
Pre-assigned TDMA
Every earth station is allotted a specific time slot
Demand assigned TDMA
The time slot are allotted to the earth station on request from the control station 4

5. 5 Comparison of Uplink power requirements for FDMA and TDMA
In TDMA , all station transmit bursts in the uplink producing equal power as received at the satellite, therefore requires a higher bit rate and higher EIRP to transmit.
In FDMA , there are modulated carriers from different earth station in form of digital or analog, so there is no need to maintain power levels to transmit, low bit rate and low EIRP is required
For small number of satellite FDMA should be the mode of operation, where as TDMA permit more efficient use of the satellite transponder by eliminating the need of backoff. 5

6. Satellite Switched TDMA
SS-TDMA employs sequenced beam switching to add an additional level of multiple access in a frequency translation satellite.
The switching is accomplished at RF or at an intermediate frequency (IF) and is unique to satellite based systems.
Adds antenna beam switching to provide additional MA capability to adapt to changing demand requirements.
Provides fixed switch positions that increase options and allow the network to support varied users.
3 antenna beams are used (each serving 2 ES)
3x3 satellite switch matrix is the key component that permits the antenna interconnections to be made on a switched basis.
With three beams, six modes would be required for full interconnectivity (signals carried in each beam are transferred to each of the other beams at some time in the switching sequence).
UL and DL microwave frequencies are different. 6

7. Pre-Assigned FDMA
Frequency slots pre assigned to analog and digital signals
Pre-assignment also may be made on the basis of a single channel per carrier (SPSC). An important feature of the SPSC system is that on a 2-way channel, only 1 carrier is operatable at a time.
All three earth stations (as shown) access a single satellite transponder channel simultaneously, and each communicates with both of the others. Thus it is assumed that the satellite receive and transmit antenna beams are global, encompassing all three earth stations. Each earth station transmits one uplink carrier modulated with a 60-channel super group and receives two similar downlink carriers.
Demand-Assigned FDMA
Transponder frequency bandwidth is subdivided into a number of channels
Polling method : A master earth station continuously polls all the earth stations in sequence, and if a call request is encountered, frequency slots are assigned from the pool of available frequencies. The polling delay with such a system tends to become excessive as the number of participating earth stations increases. 7

8. b. Centrally controlled random access: earth stations may request calls through the master earth station as the need arises. Frequencies are assigned, if available, by the master station, and when the call is completed, the frequencies are returned to the pool. If no frequencies are available, the blocked call requests may be placed in a queue, or a second call attempt may be initiated by the requesting station.
c. Distributed control Random Access: Control is assigned at each earth station. An example is the Spade system operated by INTELSAT
SPADE SYSTEM
Spade is a loose acronym for SCPC pulse-code-modulated multiple-access demand-assignment equipment.
Spade was developed by Comsat for use on the INTELSAT satellites and is compatible with the INTELSAT SCPC pre-assigned system
All the earth stations are permanently connected through the CSC.
Each ES has a memory of the list of frequencies available and the list
is constantly updated through the CSC.
The CSC also passes the signaling info from the calling to destination station.
Each earth station has a DASS( demand assignment signaling and switching)
equipment for this function. 8

9. 9 Bandwidth limited and power limited TWTA operation:
Carriers can access transponder (BW=BTR) in FDMA mode
If there are K carriers of band B, then best achievement would be K= BTR /B.
By increasing EIRP, this cant be improved, so it is band limited and the system is said to be bandwidth-limited. Likewise, for digital systems, the bit rate is determined by the bandwidth, which again will be limited to some maximum value by B TR.
The signal bandwidth will be approximately equal to the noise bandwidth, and if the EIRP is below a certain level, the bandwidth will have to be correspondingly reduced to maintain the [ C/ N] at the required value.
C/N=EIRP+(GR)-Losses-k-TS -BN
FDMA Downlink Analysis
Consider the overall carrier-to-noise ratio, in terms of noise power instead of noise power density.
(C/N)REQ<(C/N)D
Ignoring the inter-modulation noise since it is reduced by FDMA back off
Consider the effect of power limitation; the output power for each FDMA carrier is (if there are K carriers): EIRPD=EIRPS-BO0-K
The back off is needed here to be equal to the following to achieve the best (C/N):-
BO0 =K(B/BTR) 9

10. 10 CODE DIVISION MULTIPLE ACCESS (CDMA)
Channel access method used by various radio communication technologies
Allows several transmitters to send information over a single communication channel
CDMA also known as Spread Spectrum Multiple Access (SSMA)
Steps
Spreads the bandwidth of the data uniformly for the same transmitted power
Spreading Code is a pseudo-random code
Locally generated code runs at a much higher rate than the data to be transmitted i.e Rch>>Rb
FH-CDMA
It is the repeated switching of frequencies during radio transmission, often to minimize the effectiveness of the unauthorized interception or jamming of telecommunications.
Transmits radio signals by rapidly switching a carrier among many frequency channels, using a pseudorandom sequence known to both transmitter and receiver. 10

11. DS-CDMA Modulation 11 DS-CDMA
DSSS phase-modulates a sine wave pseudo-randomly with a continuous string of pseudo-noise (PN) code symbols called "chips", each of which has a much shorter duration than an information bit.
DSSS uses a signal structure in which the sequence of chips produced by the transmitter is known a priori by the receiver. The receiver can then use the same PN sequence to counteract the effect of the PN sequence on the received signal in order to reconstruct the information signal.
DS-CDMA Modulation 11

12. DS-CDMA De-Modulation 12
Spreading and De-Spreading
Multiply the data being transmitted by a "noise" signal at a frequency much higher than that of the original signal, thereby spreading the energy of the original signal into a much wider band.
For de-spreading to work correctly, the transmit and receive sequences must be synchronized.
The resulting effect of enhancing signal to noise ratio on the channel is called process gain. This effect can be made larger by employing a longer PN sequence and more chips per bit, but physical devices used to generate the PN sequence impose practical limits on attainable processing gain.
DSSS allows multiple transmitters to share the same channel within the limits of the cross-correlation properties of their PN sequences.
DS-CDMA
De-Modulation 12