Ch 3 satellite communications ii

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


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


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


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


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


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.


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


FDMA-Examples Intelsat IV and V used FDMA/FM/FAMA system

SPADE DAMA Satellite System – SPADE ES Tx


FDMA-Examples SPADE DAMA Satellite System – Carrier Frequency

Assignment


FDMA-Examples SPADE DAMA Satellite System – Frame Structure of

Common Signaling Channel (CSC)


TIME DIVISION MULTIPLE ACCESS (TDMA)-The Basic

Concept


TIME DIVISION MULTIPLE ACCESS (TDMA)-The CEPT

Primary Multiplex Frame Block Diagram


TIME DIVISION MULTIPLE ACCESS (TDMA)-The CEPT

Primary Multiplex Frame Timing Sequence


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


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


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


Correlation and Cross-Correlation

Back

Wayne Tomasi-Ch 15 NDG Notes 19


21

Back




FH-Spread Spectrum



Back


DS-Spread Spectrum




Back




Back


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.


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





SATELLITE RADIO NAVIGATION

Navstar GPS

Engineering

Ch 3 satellite communications ii