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Communication Channel. Outline. Information Transmission Attenuation: dB Equivalent Noise Temperature Communication Limits Broadband Channel BER . Frequency Response. All communication channels modify/ distort signals transmitted.

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communication channel

Communication Channel

Property of R. Struzak

  • Information Transmission
  • Attenuation: dB
  • Equivalent Noise Temperature
  • Communication Limits
  • BroadbandChannel
  • BER

Property of R. Struzak

frequency response
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)

Property of R. Struzak

frequency response measurement
Frequency Response Measurement






Spectrum Analyzer


Property of R. Struzak

time response
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

Property of R. Struzak


Time Response Measurement







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t domain f domain
T-Domain & F-Domain

Property of R. Struzak

nonlinearity bdr

Output power (dBm)

Noise Floor

Nonlinearity: BDR



MDS = Minimum

Detectable Signal

(Output Noise Floor)



power (dBm)



(Blocking Dynamic Range)

Property of R. Struzak

nonlinarity sfdr
Nonlinarity: SFDR

Extrapolated Third-Order Distortion

Extrapolated Linear Output


Output power (dBm)

Extrapolated Second-Order Distortion


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)





OIP2 = Output Referred Second-Order Intercept Point

OIP3 = Output Referred Third-Order Intercept Point

Property of R. Struzak

2 tone test
2-Tone Test


Signal power (dBm)






IIP3 = 1/2 + P

f1 ~ f2

Property of R. Struzak

communication channel1
Communication Channel





Input signal

Output signal






Propagation Channel


Signal transformationsdue to natural phenomena;external noise/signals added


signal processing


signal processing

Property of R. Struzak

main natural phenomena affecting communication
Main Natural PhenomenaAffecting Communication
  • Attenuation
  • Noise/ interference
    • Additive (thermal noise)
    • Multiplicative (fading)

Property of R. Struzak

loss db
Loss & dB
  • Abbreviation fordecibel(s). One tenth of the common logarithm of the ratio of relative powers, or power ratios, equal to 0.1 B (bel).

Property of R. Struzak

various dbs
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

Property of R. Struzak

radio transmission loss components
Radio Transmission Loss Components

ITU-R Rec.

Property of R. Struzak

sum of two signals deterministic linear system
Sum of Two Signals (Deterministic, Linear System)

Resultant signal

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uncertainty due to noise
Uncertainty due to Noise

Small uncertainty,

Signals can easily be differentiated

Large uncertainty,

Signals cannot easily be differentiated

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thermal noise
Thermal Noise

N = kTB

N – available noise power from resistor [W]

k – Boltzmann’s constant (1.37 x 10-23 [J/o])

T – temperature [oK]

B – frequency bandwidth [Hz]


Thermal Noise = fundamental limiting factor

Property of R. Struzak

equivalent noise temperature
Equivalent Noise Temperature






Identical Output







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communication channel 2
Communication Channel (2)

Original message


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)


s(t) = U(m, f)

Transport medium

x(t) = V(s, )


y(t) = W(x)


(received) message

y = W{V[,U(m,f)]}

Property of R. Struzak

shannon s law
Shannon’s 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 proportionaltoB, 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

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communication limits
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

Property of R. Struzak

transmission time speed
Transmission Time & Speed

Property of R. Struzak

data rate per hz vs snr
Data Rate per Hz vs. SNR

Property of R. Struzak

bit rate boud rate
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.

Property of R. Struzak

wideband channel

C = Blog2{1 + [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(1+S/N0B) = 1.44 loge(1+S/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.

Property of R. Struzak

wideband channel 2
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

Property of R. Struzak

data rate vs bandwidth wideband channel
Data Rate vs. Bandwidth(Wideband Channel)

C = B log [1+ S / kT]

Thermal noise asymptote: C = 1.44 S / kT

Property of R. Struzak

ber vs s n
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



Property of R. Struzak

ber vs input signal
BER vs Input Signal


Errors due to

thermal noise,


Sampling jitter

Errors due to


spurious interference (overload)

Input signal level

Property of R. Struzak

countermeasures against errors
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

Property of R. Struzak

retransmission schemes
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.

Property of R. Struzak

channel summary
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

Property of R. Struzak

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

Property of R. Struzak