FALLSEM2013-14_CP3310_19-Jul-2013_RM01_Reliability__Redundancy.pdf
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Transcript of 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
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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
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(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
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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
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carrier to noise ratio
=76 = 18.8dB
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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
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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
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FDMA
8
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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
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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
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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
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• Factors which limit the number of
subchannels provided within a satellite
channel via FDMA – Thermal noise
– Intermodulation noise
– Crosstalk
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• Cost of digital components continues to
drop
• Advantages of digital components
– Use of error correction
• Increased efficiency of TDM
– Lack of intermodulation noise
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[ ]T-1 = [ ]u
-1 + [ ]D-1
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(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
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3 dB coupler
3 dB coupler
f1
f2
f 3
f1 f2
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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.
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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
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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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