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Communication Channel

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Communication Channel Outline Information Transmission Attenuation: dB Equivalent Noise Temperature Communication Limits Broadband Channel BER Frequency Response All ... – PowerPoint PPT presentation

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Title: Communication Channel


1
Communication Channel
2
Outline
  • Information Transmission
  • Attenuation dB
  • Equivalent Noise Temperature
  • Communication Limits
  • Broadband Channel
  • BER

3
Frequency Response
  • All communication channels modify/ distort
    signals transmitted.
  • A linear, time-invariant channel is characterized
    in frequency domain by its transfer function
    (frequency response or frequency
    characteristics) H(?) Y(?) / X(?)
  • Valid for fixed (or moving slowly) systems
    (otherwise other effects have to be taken into
    account, e.g. Doppler frequency shift)

Input signal, frequency domain (amplitude
spectrum)
Output signal, frequency domain (amplitude
spectrum)
4
Frequency Response Measurement
Signal Generator
Transmission Channel
Receiver/ Spectrum Analyzer
Synchronized
5
Time Response
  • The time domain and frequency domain are uniquely
    linked by the Fourier transform

Channel impulse response
Output signal (time domain)
An example of (analogy to) impulse response a
bell rings when hit by clapper
6
Time Response Measurement
Impulse Generator
Transmission Channel
Oscilloscope
Synchronized
7
T-Domain F-Domain
8
Nonlinearity BDR
P1dB-out
1dB
MDS Minimum Detectable Signal (Output Noise
Floor)
P1dB-in
Input power (dBm)
MDS
BDR (Blocking Dynamic Range)
9
Nonlinarity SFDR
Extrapolated Third-Order Distortion
Extrapolated Linear Output
OIP2
Output power (dBm)
Extrapolated Second-Order Distortion
OIP3
IIP3 Third-Order Intercept Point IIP2
Second-Order Intercept Point MDS Minimum
Detectable Signal (Output Noise Floor) SFDR
(2/3)(IIP3 MDS) Spurious-Free Dynamic Range
Noise floor
Input power (dBm)
MDS
IIP3
IIP2
SFDR
OIP2 Output Referred Second-Order Intercept
Point
OIP3 Output Referred Third-Order Intercept Point
10
2-Tone Test
P
?
Signal power (dBm)
f1
f2
2f1-f2
2f2-f1
Frequency
IIP3 1/2? P
f1 f2
11
Communication Channel
Signal transmitted
Signal received
Input signal
Output signal
Information destination
Information source
Transmitter
Propagation Channel
Receiver
Signal transformationsdue to natural
phenomenaexternal noise/signals added
Transmitter signal processing
Receiver signal processing
12
Main Natural PhenomenaAffecting Communication
  • Attenuation
  • Noise/ interference
  • Additive (thermal noise)
  • Multiplicative (fading)

13
Loss dB
  • Abbreviation for decibel(s). One tenth of the
    common logarithm of the ratio of relative powers,
    or power ratios, equal to 0.1 B (bel).

14
Various dBs
  • dBi In the expression of antenna gain, the
    number of decibels of gain of an antenna
    referenced to the zero dB gain of a free-space
    isotropic radiator.
  • dBm dB referenced to one milliwatt. dBm is
    often used in communication work as a measure of
    absolute power values. Zero dBm means one
    milliwatt.
  • dB?V dB referenced to 1 microvolt. Used often
    for receiver sensitivity measurement.
  • dBmV dB referenced to one millivolt across 75
    ohms. This is 1.33 10-5 milliwatts.
  • dBv dB relative to 1 volt peak-to-peak. dBv is
    often used for television video signal level
    measurements.
  • dBW dB referenced to one watt. Zero dBW means
    one watt.
  • Note There are also other dBs in use!
  • Source Telecommunication Glossary 2000

15
Radio Transmission Loss Components
ITU-R Rec.
16
Sum of Two Signals (Deterministic, Linear System)
Resultant signal
17
Uncertainty due to Noise
Small uncertainty, Signals can easily be
differentiated
Large uncertainty, Signals cannot easily be
differentiated
18
Thermal Noise
  • N kTB
  • N available noise power from resistor W
  • k Boltzmanns constant (1.37 x 10-23 J/o)
  • T temperature oK
  • B frequency bandwidth Hz

1J1Ws
Thermal Noise fundamental limiting factor
19
Equivalent Noise Temperature
Actual Receiver
SN
Internal Noise
Identical Output Signal-to-Noise Ratio
SN
Noise-less Receiver
kTeB
20
Communication Channel (2)
Original message m(t)
Transmitter s(t) U(m, f)
  • m(t) message (information, data)
  • s(t) signal carrying the message
  • f f(a,b,c,, t) (carrier function)
  • a,b,c, modulation parameters
  • U, V, W operators
  • ? noise, interference, perturbations
  • x(t) perturbed signal at the receiver input
  • y(t) reproduced message
  • Task make y m (within an acceptable error)

Transport medium x(t) V(s, ?)
Receiver y(t) W(x)
Reproduced (received) message y WV?,U(m,f)
21
Shannons Law
  • The maximum rate of information transmission
    without errors through a communication channel
    equals the channel capacity
  • The channel capacity of a noisy channel is
    limited. It depends on the channel bandwidth B
    and signal-to-noise power ratio SNR it is
    proportional to B, and increases with SNR

Notes (1) Isolated system. (2) AWGN (Additive
White Gaussian Noise) only. (3) Noise-like signal
using full bandwidth. (4) No signal-noise
correlation. (5) Ideal coding, but Shannon says
nothing how to implement such a code. Special
coding required that may take very log time, but
the signal latency is ignored. (6) Claude
Shannon, 1948
22
Communication Limits
  • Claude Shannon defined the limits for
    communication channels
  • C channel capacity (max. data rate), bps
  • B frequency band, Hz
  • S/N received signal-to-noise power ratio

23
Transmission Time Speed
24
Information Transmission
The Shannons channel can transmit the same
amount of information using various combinations
of Time, Bandwidth, and Signal/ Noise
SNR
T
SNR
2T
T
2B
B
25
Data Rate per Hz vs. SNR
26
Bit Rate Boud Rate
  • The bit rate defines the rate at which
    information is passed
  • The boud (or signalling) rate (Bd) is a unit of
    modulation rate and defines the number of symbols
    per second.
  • Each symbol represents n bits, and has M signal
    states, where M 2n. This is called M-ary
    signalling.

27
Wideband Channel
C B log21 S/(NoB)
Noise density, W/Hz (const)
Received signal power, W
Bandwidth, Hz
Capacity (data rate), bit/s
With signal power S and noise power density N0
constant, enlargement of the bandwidth increases
also noise. For B ? ?, (S/N0B) ? 0 and
log2(1S/N0B) 1.44 loge(1S/N0B) ? 1.44S/N0B,
or R ? 1.44S/N0. With thermal noise only, C ?
1.44S/kT. R does not become greater with any
further increase of B. In these conditions,
S?0.693kTR.
28
Wideband Channel 2
  • With large bandwidth involved, the assumption of
    flat channel frequency response and/or white
    noise is likely not to be valid. In such a case,
    the following equation is frequently used

Delogne P, Bellanger M, The impact of Signal
Processing on an Efficient Use of the Spectrum,
Radio Science Bulletin No 289, june 1999, 23-28
29
Data Rate vs. Bandwidth(Wideband Channel)
C B log 1 S / kT
Thermal noise asymptote C 1.44 S / kT
30
BER vs. S/N
  • BER or bit error ratio The number of erroneous
    bits divided by the total number of bits
    transmitted, received, or processed over some
    stipulated period.
  • It is usually expressed as a coefficient and a
    power of 10 e.g. 2.5 erroneous bits out of
    100,000 bits transmitted would be 2.5 10-5.
  • Acceptable BER 10-3 for a voice link, 10-9 for a
    data link
  • BER decreases with S/N to a degree that depends
    on the signal processing applied

BER
S/N
31
BER vs Input Signal
BER
Errors due to thermal noise, Quantization,
Sampling jitter
Errors due to self-induced spurious
interference (overload)
Input signal level
32
Countermeasures Against Errors
  • Repeating transmission/ Error control
  • Increase S/N (filtration/ frequency, time,
    direction selection)
  • Noise-resistant Modulation/ Demodulation /
    Encoding/ Decoding
  • Spreading/De-spreading signals
  • Applied during signal generation, transmission,
    reception in digital/ analogue technology

33
Retransmission Schemes
  • Stop and Wait
  • Only one packet at a time can be transmitted. The
    tranbsmitter waits for an acknowledgment (ACK),
    positive or negative, from the receiver. If no
    ACK is received after a fixed amount of time
    (timeout) the packet is retransmitted
  • Go-Back-N
  • Extension of Stop and Wait. Transmitter sends up
    to N packets without reception of corresponding
    ACK. On reception of negative ACK or when the
    timeout expires, the packets are retransmitted.
  • Selective Repeat
  • Extension of Go-Back-N. Only the packet in error
    is retransmitted. Requires packet buffering and
    reordering at the receiver end.

34
Channel Summary
  • Information is carried by signals that are
    limited in time, frequency, and energy
  • Signal travel distance with limited speed
    require time to travel at a distance
  • During transmission, signal suffer attenuation
    and is affected by noise, etc.
  • The channel capacity is limited

35
References
  • Many good books, e.g.
  • Pierce JR, An Introduction to Information Theory,
    Dover Publ.
  • Dunlop J, Smith DG, Telecommunications
    Engineering, Chapmann Hall
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