ECE 4371, Fall, 2015 Introduction to Telecommunication Engineering/Telecommunication Laboratory Zhu Han Department of Electrical and Computer Engineering.
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ECE 4371, Fall, 2015
Introduction to Telecommunication Engineering/Telecommunication Laboratory
Zhu Han
Department of Electrical and Computer Engineering
Class 6
Sep. 10th, 2015
OverviewOverview
Cochannel interference
Analysis of Noise– Math model (good thing, not required)
– Noise shape, preemphasis/deemphasis
– FM threshold effects
Security basics
Good news: This is the last class for exam 1
Co-channel InterferenceCo-channel Interference Source: Acoswct, Interference: Icos(wc+w)t
r(t)= Acoswct+Icos(wc+w)t=E(t)cos(wct+)
=tan^-1(Isinwt/(A+Isinwt))~=(I/A)sinwt
PM: y=(I/A)sinwt, FM =(Iw/A)coswt
When A is large, suppress weak interference better than AM.
Capture effect– Winner takes all
– 35dB for AM
– 6 dB for FM/PM
White Gaussian noise– Noise increases linearly with frequency in FM.
System Model and Noise ModelSystem Model and Noise ModelDiscriminator consists of a slope network and an envelope detector.
Signal after bandpass filterSignal after bandpass filter The incoming FM signal s(t) is
defined by
At the bandpass filter output
Discriminator OutputDiscriminator Output Note that the envelope of x(t) is of no interest to us (limiter)
(2.141) )()( sin)( 2
1)(
where
tttrdt
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2.40) (Fig isoutput tor discrimina The
tntmkdt
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Noise After DiscriminatorNoise After Discriminator
(2.142) )(sin)(2
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Noise After Discriminator cont.Noise After Discriminator cont. The average output signal power = kf
2P
Recall fjdt
d TF
2.
nQ(t) nd(t)
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d
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)( fSQN )( fS
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(2.145) )()(2
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Noise After Discriminator cont.Noise After Discriminator cont. Assume that nQ(t) has ideal low-pass characteristic with
bandwidth BT(2.146)
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cN
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output receiver At the
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FM Preemphsis and DeemphasisFM Preemphsis and Deemphasis
FM Preemphsis and DeemphasisFM Preemphsis and Deemphasis
FM Threshold EffectFM Threshold Effect
2
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nI(t)
ExampleExample Illustrating impulse ike components in (t) d(t)/dt
produced by changes of 2 in (t); (a) and (b) are graphs of (t) and (t), respectively.
Threshold EffectThreshold Effect Dependence of output signal-
to-noise ratio on input carrier-to-noise ratio for FM receiver. In curve I, the average output noise power is calculated assuming an unmodulated carrier. In curve II, the average output noise power is calculated assuming a sinusoidally modulated carrier. Both curves I and II are calculated from theory.
Comparison of modulation systemsComparison of modulation systems
Comparison of the noise Comparison of the noise performance of various performance of various CW modulation systems. CW modulation systems. Curve I: Full AM, Curve I: Full AM, = 1. = 1. Curve II: DSB-SC, SSB. Curve II: DSB-SC, SSB. Curve III: FM, Curve III: FM, = 2. = 2. Curve IV: FM, Curve IV: FM, = 5. = 5. (Curves III and IV include (Curves III and IV include 13-dB pre-emphasis, de-13-dB pre-emphasis, de-emphasis improvement..)emphasis improvement..)
EncryptionEncryption Encryption is a translation of data into a secret code. Encryption is the
most effective way to achieve data security. To read an encrypted file, you must have access to a secret key that enables you to decrypt it. Unencrypted data is called plain text; encrypted data is referred to as cipher (text).
Encryption can be used to ensure secrecy, but other techniques are still needed to make communications secure: authentication, authorization, and message integrity.– Message integrity - both parties will always wish to be confident that
a message has not been altered during transmission. The encryption makes it difficult for a third party to read a message, but that third party may still be able to alter it in a useful way.
– Authentication is a way to ensure users are who they say they are - that the user who attempts to perform functions in a system is in fact the user who is authorized to do so.
– Authorization protects computer resources (data, files, programs, devices) by allowing those resources to be used by resource consumers having been granted authority to use them.
– Digital rights management etc.
Encryption – cipher taxonomyEncryption – cipher taxonomy
CIPHERS
MODERNCIPHERS
CLASSICALCIPHERS
PUBLIC KEY
PRIVATE KEY
SUPERPOSITION
TRANSPOSITION
ROTORMACHINES
QuantumCIPHERS
Transposition MethodTransposition Method
Da Vinci’s code
Ex.
I am a student
I m s u e t
a a t d n
Substitution MethodSubstitution Method
Shift Cipher (Caesar’s Cipher)
I CAME I SAW I CONQUERED
H BZLD H TZV H BNMPTDSDC
Julius Caesar to communicate with his army
Language, wind talker
Rotor MachineRotor Machine The primary component is a set of rotors, also termed wheels or drums,
which are rotating disks with an array of electrical contacts on either side. The wiring between the contacts implements a fixed substitution of letters, scrambling them in some complex fashion. On its own, this would offer little security; however, after encrypting each letter, the rotors advance positions, changing the substitution. By this means, a rotor machine produces a complex polyalphabetic substitution cipher.
German Enigma machine used
during World War II for submarine.
Movie U571, Italian Job
KeyKey
Public Key System - RSAPublic Key System - RSA
Named after its inventors Ron Rivest, Adi Shamir and Len Adleman
Base on Number Theory
y=ex (mod N) => x=??
If the size of N is 100, it takes 100 billion years to decipher with 1GHz computer.
Applications– Digital Signatures
– Digital Cash: Movie, swordfish
– Timestamping Services: Movie, entrapment
– Election
Movie, mercury rising
Encryption – cipher taxonomyEncryption – cipher taxonomy
Historical pen and paper ciphers used in the past are sometimes known as classical ciphers. They include substitution ciphers and transposition ciphers.
During the early 20th century, more sophisticated machines for encryption were used, rotor machines, which were more complex than previous schemes.
Encryption methods can be divided into symmetric key algorithms and asymmetric key algorithms. In a symmetric key algorithm (DES, AES), the sender and receiver must have a shared key set up in advance and kept secret from all other parties; the sender uses this key for encryption, and the receiver uses the same key for decryption.
In an asymmetric key algorithm (RSA), there are two separate keys: a public key is published and enables any sender to perform encryption, while a private key is kept secret by the receiver and enables him to perform decryption.
Quantum CryptographyQuantum Cryptography
Use physics law, if the signal is measured (eavesdropped), the receiver can always detected.
Mission is really impossibleMission is really impossible
When you see it, the information has been already changed
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