Satellite Communications-II
Dr. Nasir D. Gohar
Satellite Communications-II
WHY MULTIPLE ACCESS?
 Users/Earth Stations Share the Transmission Resource
i.e. Radio Spectrum
 Aim is to develop Efficient Techniques that Maximize System
Capacity thru Dynamic Resource Allocation and Spectrum
Reuse
 Simple FDM/FM Satellite Systems become Inefficient is BW
Utilization and Economically Impractical
 Pre-Assigned or Demand-Assigned Channel Allocation
 In case of Pre-Assigned System, a given number of available
voice-band channels from each earth station are assigned to a
dedicated destination….Some-times wastage of Precious BW
Resource
 In case of Demand-Assigned System, Resources allocation is on
need basis, versatile and efficient usages of Radio Spectrum, but a
Complex Mechanism is required at all Earth Stations/Users
Satellite Communications-II
A PRE-ASSIGNED/DEDICATED SYSTEM
• Each earth station
requires two
dedicated pairs of
Tx/Rx frequencies
to communicate with
any other station
• As many
communication
partners, same
number of
transponders (RFRF duplex
translator/repeater)
• Transponder BW 36
MHz which is mostly
wasted
Satellite Communications-II
ANIK-E FREQUENCY & POLARIZATION
PLAN
• Domsat operated by
Telsat, Canada
• Group A (12 Radio Ch)
use H Polarization
• Group B (12 Radio Ch)
use V Polarization
• Radio Ch. BW=36
MHz
• Inter-Channel Guard
band =4MHz
• 10 MHz band on each
side extra to avoid
Inter-System
Interference
• Total BW = 500 MHz
Satellite Communications-II
TWO TYPES OF DUPLEXING
A Duplex Link allows simultaneous transmission of information in
both directions
 Frequency Division Duplex (FDD) – two frequency channels for
each up/down link i.e. one frequency channel for Tx and other for Rx
 Time Division Duplex (TDD) – a single frequency channel shared
by both Tx and Rx
Satellite Communications-II
THREE MULTIPLE ACCESS
TECHNIQUES
 Satellite Multiple Accessing/Destination means more than one users/earth
stations can access to one or more Radio Channels (Transponders) on board
 FDMA
 TDMA
 CDMA
 FH-CDMA
 DS-CDMA
Satellite Communications-II
CATEGORIZATION OF MA TECHNIQUES
Narrow-band Systems – Total system BW is divided into a large
number of narrow-band radio channels
 FDMA/FDD – Each user is assigned two narrow-band radio channels, one for
up-link and other for down-link
 TDMA – When each narrow-band radio channel is divided into number of time
slots, and each user is assigned two time slots, one for Tx and other for Rx.
 Hybrid TDMA/FDMA or TDMA/FDD – when two slots {same position in
time) of the user are allocated in two different narrow-band radio channels
TDMA/TDD – when two slots of the user are allocated in the same narrowband radio channel
Wide-band Systems – Total spectrum/BW is shared by all users all the
time
 Wide-band TDMA, each user is allocated two time slots to use the entire
spectrum. TDMA/FDD and TDMA/TDD both configurations are possible.
 Wide-band CDMA, entire spectrum is used by each user all the time but with
use of orthogonal codes. CDMA/FDD and CDMA/TDMA both configurations are
possible.
Satellite Communications-II
FREQUENCY DIVISION MULTIPLE ACCESS (FDMA)THE CONCEPT
 Given Radio Spectrum (RF BW) is divided into a large number of narrow-band radio
channels called sub-divisions
 Each sub-division has its own sub-carrier called IF Carrier
 A control mechanism is required to ensure that each user/earth station uses only its
own assigned sub-division at any time
 SCPC- a system where each sub-division carries only one 4-kHz voice channel
 MCPC-a system where several speech/voice band channels are frequency-division
multiplexed to form a group, super-group or even master-group
 FDM/FM/FAMA- a system using a fixed MCPC format over a long period of time
 DAMA- a system that allows all users continuous and equal access to the entire
transponder BW by assigning carrier frequencies on a temporary basis as per demand
Satellite Communications-II
FDMA-Examples
 Intelsat IV and V used FDMA/FM/FAMA system
 SPADE DAMA Satellite System – SPADE ES Tx
Satellite Communications-II
FDMA-Examples
 SPADE DAMA Satellite System – Carrier Frequency
Assignment
Satellite Communications-II
FDMA-Examples
 SPADE DAMA Satellite System – Frame Structure of
Common Signaling Channel (CSC)
Satellite Communications-II
TIME DIVISION MULTIPLE ACCESS (TDMA)-The Basic
Concept
Satellite Communications-II
TIME DIVISION MULTIPLE ACCESS (TDMA)-The CEPT
Primary Multiplex Frame Block Diagram
Satellite Communications-II
TIME DIVISION MULTIPLE ACCESS (TDMA)-The CEPT
Primary Multiplex Frame Timing Sequence
Satellite Communications-II
FDMA and TDMA – A Comparison
 In TDMA, only one carrier from any of several Earth Stations is
present at Satellite at any time
 FDMA requires each Earth Station capable of transmitting and
receiving on multitude of carrier frequencies (FDMA/DAMA)
 TDMA is more amenable to digital transmission (storage, processing,
rate-conversion etc.) than FDMA
 TDMA requires precise synchronization
Satellite Communications-II
THREE MULTIPLE ACCESS
TECHNIQUES
 Code Division Multiple Access (CDMA)-The Concept
 No restrictions on any user/earth station on time and frequency slots
usages, rather any user can use allocated BW or all system BW at any
time, however, using a special chip code to spread its low-bandwidth
signal over the entire allocated spectrum… Spread Spectrum Multiple
Access
Satellite Communications-II
 Code Division Multiple Access (CDMA)-The
Concept (Cont’d)
 Types Of CDMA
 Orthogonal Codes
 Correlation and Cross-Correlation
 How Spreading and De-Spreading is done?
 Processing Gain, G = Chip Rate/Date Rate
Next
Satellite Communications-II
Correlation and Cross-Correlation
Back
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Back
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Satellite Communications-II
 FH-Spread Spectrum
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Satellite Communications-II
 DS-Spread Spectrum
Back
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Back
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Back
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Example
2.7
We consider a case where 8 chips per bit are used to generate the Walsh functions. Specify these
functions, sketch them, and show that they are orthogonal to each other.
H8 =
0
0
0
0
0
0
0
0
0
1
0
1
0
1
0
1
0
0
1
1
0
0
1
1
T/4
0
1
1
0
0
1
1
0
0
0
0
0
1
1
1
1
0
1
0
1
1
0
1
0
0
0
1
1
1
1
0
0
T/2
0
1
1
0
1
0
0
1
=
3T/4
O1
O2
O3
O4
O5
O6
O7
O8
T
T/4
+1
T/2
3T/4
T
+1
O5
O1
-1
-1
+1
T/4
T/2
3T/4
T
+1
T/4
T/2
3T/4
T
O2
O6
-1
-1
T/4
T/2
3T/4
T
T/4
+1
T/2
3T/4
T
+1
O7
O3
-1
+1
-1
T/4
T/2
3T/4
T
+1
T/2
3T/4
T
O8
O4
-1
T/4
-1
Figure 2.12 Plots of Walsh functions.
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Example
2.8
We consider a case where 8 chips per bit are used to generate the Walsh functions. Stations A, B, C,
and D are assigned the chip sequence 0 1 0 1 0 1 0 1, 0 0 1 1 0 0 1 1, 0 1 1 0 0 1 1 0, 0 0 0 0 1 1 1 1,
respectively. The stations use the chip sequence to send a 1 bit and use negative chip sequences to
send a 0 bit(e.g., station A uses 1 0 1 0 1 0 1 0 to send the 0 bit and so on). All chip sequences are
pairwise orthogonal. This implies that the normalized correlation of any two distinct chip sequences is
0 and the normalized correlation of any chip sequence with itself is 1. We assume that all stations are
synchronized in time; therefore, chip sequences begin at the same instant. When two or more
stations transmit simultaneously, their bipolar signals add linearly. For example, if in one chip period
three stations output +1 and one station outputs -1, the net result is +2. We consider five different
cases when one or more stations transmit(see table 2.5). We want to show that the reciever recovers
the bit stream of station C by computing the normalized inner products of the recieved sequences with
the chip sequence of station C.
Chip Sequence
A:
B:
C:
D:
0
0
0
0
1
0
1
0
0
1
1
0
1
1
0
0
0
0
0
1
1
0
1
1
0
1
1
1
Binary Values of Chip Sequence
1
1
0
1
A:
B:
C:
D:
(-1
(-1
(-1
(-1
+1 -1 +1 -1 +1 -1 +1)
-1 +1 +1 -1 -1 +1 +1)
+1 +1 -1 -1 +1 +1 -1)
-1 -1 -1 +1 +1 +1 +1)
The normalized inner products are (see table 2.5)
S1
C
=
8
S2
C
=
8
S3
C
=
8
S4
C
=
8
S5
C
=
1 + 1 + 1 + 1 + 1 + 1 + 1 + 1
8
2 + 0 + 0 + 2 + 0 + 2 + 2 + 0
8
3 + 1 + 1 - 1 + 3 + 1 + 1 - 1
8
2 + 0 + 0 - 2 + 2 + 0 + 0 - 2
8
1 - 1 - 1 - 3 + 1 - 1 - 1 - 3
8
=
1
=
1
=
1
=
0
=
-1
8
Thus, the receiver recovers a bit sequence of 1 1 1 - 0 for station C.
We assume that all the chips are synchronized in time. In a real situation it is impossible to do so.
The sender and receiver are synchronized by having the sender transmit a long enough known chip
sequence that the receiver can lock onto it. All other (unsynchronized) transmissions are then seen
as random noise.
Table 2.5 Five cases
Stationa(A B C D)
Transmitting
Received Chip Sequesnce
- - 1 - - 1 1
1 1 1 11 - 1 1 0 -
C
C
A
A
A
S1
S2
S3
S4
S5
+
+
+
+
D
B + C
B
B + C
=
=
=
=
=
(-1
(-2
(-3
(-2
(-1
+1 +1 -1 -1 +1 +1 -1)
0 0 -2 0 +2 0 +2 0)
+1 +1 +1 -3 +1 +1 +1)
0 0 +2 -2 0 0 +2)
-1 -1 +3 -1 -1 -1 +3)
a. Note: a dash (-) means no transmission by that station
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Satellite Communications-II
 SATELLITE RADIO NAVIGATION
 Navstar GPS
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