Satellite Communications-II

Satellite Communications-II

Dr. Nasir D. Gohar


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 channelsTDMA/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


Back



FH-Spread Spectrum


Back


DS-Spread Spectrum


Back


Exam ple 2.7

W e consider a case where 8 ch ips per b it are used to generate the W alsh functions. Specify thesefunctions, sketch them, and show that they are orthogonal to each other.

H 8 =

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

=

O 1O 2O 3O 4O 5O 6O 7O 8

+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

O 1

O 2

O 3

O 4

O 5

O 6

O 7

O 8

F igure 2.12 P lots o f W alsh functions.


Exam ple 2.8

W e consider a case where 8 chips per b it are used to generate the W alsh functions. S tations 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,respective ly. The stations use the chip sequence to send a 1 b it and use negative chip sequences tosend a 0 b it(e .g., sta tion A uses 1 0 1 0 1 0 1 0 to send the 0 b it and so on). A ll ch ip sequences arepairwise orthogonal. Th is implies that the normalized corre la tion of any two d istinct ch ip sequences is0 and the normalized corre lation of any chip sequence with itse lf is 1. W e assume that a ll sta tions aresynchronized in time; therefore, ch ip sequences begin at the same instant. W hen two or morestations transmit simultaneously, the ir b ipolar signals add linearly. For example, if in one ch ip periodthree stations output +1 and one station outputs -1 , the net result is +2. W e consider five d ifferentcases when one or more stations transmit(see tab le 2.5). W e want to show that the reciever recoversthe b it stream of station C by computing the normalized inner products of the recieved sequences withthe 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 tab le 2.5)

S 1 C

8 8

1 + 1 + 1 + 1 + 1 + 1 + 1 + 1= = 1

S 2 C

8 8

2 + 0 + 0 + 2 + 0 + 2 + 2 + 0= = 1

S 3 C

8 8

3 + 1 + 1 - 1 + 3 + 1 + 1 - 1= = 1

S 4 C

8 8

2 + 0 + 0 - 2 + 2 + 0 + 0 - 2= = 0

S 5 C

8 8

1 - 1 - 1 - 3 + 1 - 1 - 1 - 3= = -1

Thus, the receiver recovers a b it sequence of 1 1 1 - 0 for sta tion C .W e assume that a ll the chips are synchronized in time. In a rea l situation it is impossib le to do so.The sender and receiver are synchronized by having the sender transmit a long enough known chipsequence that the receiver can lock onto it. A ll o ther (unsynchronized) transmissions are then seenas random noise.

Table 2.5 F ive cases

Station a(A B C D) Transmitting Received Chip Sequesnce

- - 1 - C S 1 = (-1 +1 +1 -1 -1 +1 +1 -1)- - 1 1 C + D S 2 = (-2 0 0 -2 0 +2 0 +2 0)1 1 1 - A + B + C S 3 = (-3 +1 +1 +1 -3 +1 +1 +1)11 - - A + B S 4 = (-2 0 0 +2 -2 0 0 +2)1 1 0 - A + B + C S 5 = (-1 -1 -1 +3 -1 -1 -1 +3)

a. Note: a dash (-) m eans no transm ission by that station



SATELLITE RADIO NAVIGATION Navstar GPS

Satellite Communications-II

Documents

Transcript of Satellite Communications-II