fallsem2013-14_cp3310_19-jul-2013_rm01_reliability__redundancy.pdf

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Reliability & Redundancy• The Probability of Failure.

• The Application of reliability theory has enabled satellite engineers to build

satellites that perform as expected at acceptable construction costs.

•The Reliability of a component can be expressed in terms of the probability of

failure after time t, PF (t)

•The reliability of a device is defined as

R(t)= Ns(t) / No No. of surviving components at time t

No. of components at start of test period.

The number of components that failed in time t is Nf (t) = N0 - Ns(t)

N0

Mean Time Before Failure = 1/ N0

Σti

i=1

The average failure rate λ = 1 / MTBF

Time

Prob.

of

failure

Burn

in

End of life



Frequency Division Multiplexing Techniques.(analog )

Time Division Multiplexing Techniques.(Digital).

Watts

Power receivedWatts

dBW

EIRP =

10 log ( 4 πR /λ 2) dB

= 10 log (4πR /λ)2



(S/N) in / (S/N) out

Td = To (NF – 1) To =290K

G / T Ratio for Earth Stations

C / N =[ ][

[ ][

Pt Gt Gr

K Ts Bn

λ

4 π R]2

]2 [ ]Pt Gt

K Bn

λ

4 π R

Gr

Ts



Complete Link Design

U

D.

T

The useful carrier signal at the receiving earth station receiver input is

given by

Gs ,GT and GR are the satellite transponder gain , the satellite transmitting antenna gainand the receiving station antenna gain respectively. L – Loss on downlink.

The Noise power spectral density at the input to receiving earth station receiver would be

given by No = NoD + Nou ( Gs .GT. GR ) / L

where

[ ]T ={ } /{ }

[ ]T

-1

= [ ]u

-1

+ [ ]D

-1



carrier to noise ratio

=76 = 18.8dB



Band width of FM Signals : Carson’s Rule

Carson’s rule states that the BW required to transmit an FM signal is given byB = 2 ( ▲f pk + f max ) Hz

Where ▲fpk is the peak frequency deviation and f max is the highest frequency

present in the modulating signal.

Pre emphasis and de emphasis :

Pre emphasis and de emphasis are used with all FM transmissions because it is

possible to reduce the noise power at the output of the de-emphasis circuit in the

receiver and thus to improve the baseband S/N ratio.

De-emphasis It is a circuit that characteristics of LPF which would cut all the

high frequency content of the signal.

To counteract this effect …we need a Pre emphasis circuit in HPF characterization

The signal is unaffected by the process of both ,while the noise power in the

receiver is significantly reduced



Concepts

Multiple access vs. multiplexing Multiplexing allows several transmission sources

to share a larger transmission capacity. Oftenused in hierarchical structures.

Multiple access: two or more simultaneoustransmissions share a broadcast channel. Oftenused in access networks

Bandwidth (bps) vs. bandwidth (Hz)

bps: data rate

Hz: frequency in physical carrier

7



FDMA

8



Frequency Division Multiplexing

The bandwidth is divided intofrequency slots

Each frequency slot is allocatedto a different user

FDM was first introduced in thetelephone network

Other examples – broadcastradio and cable television

9

A CB f

C f

B f

A f

W

W

W

0

0

0

(a) Individual signals occupy W Hz

(b) Combined signal fits into channel

bandwidth



Separate bit streams are

multiplexed into a high-speed

digital transmission line

Transmission is carried out in

terms of frames which arecomposed of equal sized slots

which are assigned to users

Demultiplexing is done by reading

the data in the appropriate slot in

each frame

11

(b) Combined signal transmits 1unit every T seconds

t A1 A2

t B1 B2

t C1 C2

3T 0T 6T

3T 0T 6T

3T 0T 6T

t B1 C1 A2 C2B2A1

0T 1T 2T 3T 4T 5T 6T

(a) Each signal transmits 1unit every 3T seconds



FDMA and TDMA –

A Comparison

In TDMA, only one carrier from any of several EarthStations is present at Satellite at any time

FDMA requires each Earth Station capable of transmitting and receiving on multiple carrier frequencies.TDMA is more open to to digital transmission (storage,

processing, rate-conversion etc.) than FDMA TDMA requires precise synchronization



• Factors which limit the number of

subchannels provided within a satellite

channel via FDMA – Thermal noise

– Intermodulation noise

– Crosstalk



• Cost of digital components continues to

drop

• Advantages of digital components

– Use of error correction

• Increased efficiency of TDM

– Lack of intermodulation noise



[ ]T-1 = [ ]u

-1 + [ ]D-1



(dB)- (dB)

Sat - EIRP

Sat(dB)

- (dB)

= 44 – 205.4 +( 56.3- 10log160) – (-228.6)-75.6 – 0 – 1 = = 24.9dB

[C / No]T -1 = [C / No]u -1 + [C / No]D -1=22.8dB



3 dB coupler

3 dB coupler

f1

f2

f 3

f1 f2



1. Transmission of information over a satellite communication system always results in

some degradation in the quality of the information.

2. In Analog links the degradation takes the form of a decrease in S/N ratio.

3. In Digital links the degradation is a measure of information content of the signal in termsof the bit error rate.

4. In digital system extra redundant bits can be added to our data stream which can tell us

when an error occurs in the data and can also point to the particular bit or bits that have

been corrupted.

5. Forward error correction and detection may be switched in and out on demand,

depending on the measured bit error rate or C/N ratio at the earth terminal.6. PCM changes analog data into binary words for transmission over a digital link.

7. The efficiency of coding is a measure of the number of redundant bits that must be added

to detect or correct data with errors.

8. In some FEC systems the number of redundant bits is equal to the number of data bits

,resulting in a halving of the data rate for a given channel transmission rate. This is called

HALF rate. This loss of communication capacity is traded for a guaranteed low error rate.Employed in VSAT Systems.

9. The link will be designed with a margin of a few decibels so that the BER falls below an

acceptable level.

10. ARQ protocol is used to ensure that the probability of an un dected bit error becomes

low.



1. Determine the frequency band in which the system must

operate. Comparative designs may be required to help makethe selection.

2. Determine the communications parameters of the satellite.Estimate any values that are not known.

3. Determine the parameters of the transmitting and

receiving earth stations.

4. Start at the transmitting earth station. Establish an uplink budget and a transponder noise power budget to finduplink C/N in the transponder.

5. Find the output power of the transponder based ontransponder gain or output back-off.

The above can be found in ref. 2



downlink power and noise budget for the receiving earthstation.

7.Calculate S/N or BER in the baseband channel

result and compare with the specificationrequirements.

the propagation conditions under which the link mustoperate

Redesign the system by changing some parameters if the link margins are inadequate

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