Satellite Communications
Vinod T. Kumar
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 (RF-
RF 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 TECHNIQUESNarrow-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 narrow-
band 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
Wayne Tomasi-Ch 15 NDG Notes 19
Wayne Tomasi-Ch 15 NDG Notes 20
21
Back
Wayne Tomasi-Ch 15 NDG Notes 22
Wayne Tomasi-Ch 15 NDG Notes 23
Satellite Communications-II
FH-Spread Spectrum
Wayne Tomasi-Ch 15 NDG Notes 24
Wayne Tomasi-Ch 15 NDG Notes 25
Back
Satellite Communications-II
DS-Spread Spectrum
Wayne Tomasi-Ch 15 NDG Notes 26
Wayne Tomasi-Ch 15 NDG Notes 27
Wayne Tomasi-Ch 15 NDG Notes 28
Back
Wayne Tomasi-Ch 15 NDG Notes 29
Wayne Tomasi-Ch 15 NDG Notes 30
Wayne Tomasi-Ch 15 NDG Notes 31
Back
Wayne Tomasi-Ch 15 NDG Notes 32
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 00 1 0 1 0 1 0 10 0 1 1 0 0 1 10 1 1 0 0 1 1 00 0 0 0 1 1 1 10 1 0 1 1 0 1 00 0 1 1 1 1 0 00 1 1 0 1 0 0 1
=
O1
O2
O3
O4
O5
O6
O7
O8
+1
-1
T/4 T/2 3T/4 T
+1
-1
T/4 T/2 3T/4 T
+1
-1
T/4 T/2 3T/4 T
+1
-1
T/4 T/2 3T/4 T
+1
-1
T/4 T/2 3T/4 T
+1
-1
T/4 T/2 3T/4 T
+1
-1
T/4 T/2 3T/4 T
+1
-1
T/4 T/2 3T/4 T
O1
O2
O3
O4
O5
O6
O7
O8
Figure 2.12 Plots of Walsh functions.
Wayne Tomasi-Ch 15 NDG Notes 33
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 Binary Values of Chip Sequence
A: 0 1 0 1 0 1 0 1 A: (-1 +1 -1 +1 -1 +1 -1 +1)
B: 0 0 1 1 0 0 1 1 B: (-1 -1 +1 +1 -1 -1 +1 +1)
C: 0 1 1 0 0 1 1 0 C: (-1 +1 +1 -1 -1 +1 +1 -1)
D: 0 0 0 0 1 1 1 1 D: (-1 -1 -1 -1 +1 +1 +1 +1)
The normalized inner products are (see table 2.5)
S1 C
8 8
1 + 1 + 1 + 1 + 1 + 1 + 1 + 1= = 1
S2 C
8 8
2 + 0 + 0 + 2 + 0 + 2 + 2 + 0= = 1
S3 C
8 8
3 + 1 + 1 - 1 + 3 + 1 + 1 - 1= = 1
S4 C
8 8
2 + 0 + 0 - 2 + 2 + 0 + 0 - 2= = 0
S5 C
8 8
1 - 1 - 1 - 3 + 1 - 1 - 1 - 3= = -1
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 - C S1 = (-1 +1 +1 -1 -1 +1 +1 -1)
- - 1 1 C + D S2 = (-2 0 0 -2 0 +2 0 +2 0)
1 1 1 - A + B + C S3 = (-3 +1 +1 +1 -3 +1 +1 +1)
11 - - A + B S4 = (-2 0 0 +2 -2 0 0 +2)
1 1 0 - A + B + C S5 = (-1 -1 -1 +3 -1 -1 -1 +3)
a. Note: a dash (-) means no transmission by that station
Wayne Tomasi-Ch 15 NDG Notes 34
Wayne Tomasi-Ch 15 NDG Notes 35
Wayne Tomasi-Ch 15 NDG Notes 36
Satellite Communications-II
SATELLITE RADIO NAVIGATION
Navstar GPS