The document discusses various channel impairments that can affect communication signals as they propagate through a channel. It describes additive noise such as thermal noise generated by electronic components. It also discusses phenomena such as fading caused by multipath propagation, as well as other distortion effects like attenuation and interference that can corrupt signals. Intersymbol interference occurs when multipath causes signals to spread and overlap adjacent symbols. The document notes several types of noise and interference sources and explains how they degrade the signal-to-noise ratio.

1. Channel Impairments
BY
Muhammad Uzair Rasheed
2009-CPE-03
UCE&T BZU MULTAN

2. Performance Criterion
How a “good” communication system can be differentiated
from a “sloppy” one?
For analog communications
ˆ
– How close is m(t ) to m(t ) Fidelity!
– SNR is typically used as a performance metric
For digital communications
– Data rate and probability of error
– No channel impairments, no error
– With noise, error probability depends upon data rate,
signal and noise powers, modulation scheme

3. Noise
Noise is unwanted signals generated by different
atmospheric conditions or other external and internal
sources.
These signals are added or combined with the transmitted
signal. This is denoted by: r(t) = s(t) + n(t).
Noise signals are random and unpredictable in nature.
There are various types of noise signals generated from
different sources. One of the sources is thermal noise
generated by the motion of the electrons movement during
transmission. This is unavoidable noise. It is known as
Additive white Gaussian noise (AWGN).
Noise has to be eliminated at the receiver end to recover
the original signal.

5. Additive noise
Internal noise generated by electronic components
such as resistors and solid-state devices – thermal
noise
External noise: e.g. noise from another user in the
same frequency band – co-channel/multi-user
interference

6. Noise
• External Noise
• Internal Noise

7. Noise
Summary
• External Noise 8MHz-1.43GHz
• atmospheric Noise
• Solar Noise
• Cosmic Noise
• Industrial Noise (1-600
MHz)

8. Internal Noise
• It is Due to Active and Passive Devices in
receiver.
• Random noise
• Random noise power is proportional to the
bandwidth over which it is measured.

9. Channels and their characteristics
Wired and wireless channels. (freq. range,
channel capacity and other factors).
One problem in signal transmission is the
additive white Gaussian noise.
It is the noise generated by the internal
components like resistors and capacitors.
It is known as thermal noise.
External noise.
Amplitude and phase distortion and multi path
fading.

10. Modeling Transmission Channels
Channel impulse
response
Channel transfer function
Channel transfer function
/linear/nonlinear + r (t ) = s(t ) ⊗ c(t ) + n(t )
s (t ) /linear/nonlinear
c (t ) n(t ) (AWGN channel (usually transfer
channel function is linear) and n(t) is Gaussian,
white noise)
Information is always transmitted in channels as radio path (wireless
cellular channel, microwave link, satellite link) or in wireline channels as
coaxial cable, fiber optic cable or wave guide. Note that information
storage is also a transmission channel
Most common channels we discuss are linear Additive, White
Gaussian Noise (AWGN) channels or linear, fading channels
Note that the AWGN channel output is convolution of channel impulse
response c(t) and channel input signal s(t) and has the noise term n(t)
as additive component:
r (t ) = s ⊗ c (t ) + n(t ) = ∫ s(t )c (τ − t )dt + n(t )
u
10
(u: where integrand exists)

11. Signal to noise ratio (SNR)
It is defined as the ratio of signal power to noise power.
During transmission, the power of noise decreases the power of
signal.
Lower SNR means poor performance.
SNR decreases along the length of the channel.
Solution for this is to pump more power to the signal so that at the
receiving end, the signal is received with better SNR.
Increasing signal power reduces the effect of channel noise.
Larger SNR allows transmission over a longer distance.
Lower SNR means more error at the receiving end.
Certain minimum SNR in necessary for transmission.
SNR is usually given in decibel (dB): SNR(dB) = 10 log10(SNR).

12. Signal-to-Noise Ratio
Signal Noise Signal + noise
High
SNR
t t t
No errors
Signal Noise Signal + noise
Low
SNR
t t t
Average signal power error
SNR =
Average noise power
12
SNR (dB) = 10 log10 SNR

13. SNR
The ratio of a signal power to the noise power
corrupting the signal.

14. Shannon Capacity
 Shannon Theory
– It establishes that given a noisy channel with information capacity C and
information transmitted at a rate R, then if R<C, there exists a coding
technique which allows the probability of error at the receiver to be made
arbitrarily small. This means that theoretically, it is possible to transmit
information without error up to a limit, C.
– The converse is also important. If R>C, the probability of error at the
receiver increases without bound as the rate is increased. So no useful
information can be transmitted beyond the channel capacity. The theorem
does not address the rare situation in which rate and capacity are equal.
 Shannon Capacity
C = B log 2 (1 + SNR ) bit / s
EE 541/451 Fall 2006

15. Shannon Channel Capacity
C = BT log2 (1 + SNR) bps
Arbitrarily reliable communications is possible if the transmission
rate R < C.
If R > C, then arbitrarily reliable communications is not possible.
“Arbitrarily reliable” means the BER can be made arbitrarily small
through sufficiently complex coding.
C can be used as a measure of how close a system design is to the
best achievable performance.
Bandwidth BT & SNR determine C
15

16. Example
Find the Shannon channel capacity for a telephone
channel with BT = 3400 Hz and SNR = 10000
C = 3400 log2 (1 + 10000)
= 3400 log10 (10001)/log102 = 45200 bps
Note that SNR = 10000 corresponds to
SNR (dB) = 10 log10(10000) = 40 dB
16

17. Attenuation
Attenuation (in some contexts also called extinction) is the
gradual loss in intensity of any kind of flux through a
medium. For instance, sunlight is attenuated by dark
glasses, and X-rays are attenuated by lead.
Attenuation affects the propagation of waves and signals in
electrical circuits

18. Signal attenuation
Large scale – path loss, shadowing
Small scale – fading
Amplitude and phase distortion
Multipath – Inter-symbol interference (ISI)
(multipath is also the cause for fading)

19. Fading
Fading is the distortion that a carrier-modulated telecommunication
signal experiences over certain propagation media. A fading channel is
a communication channel that experiences fading. In wireless systems,
fading is due to multipath propagation and is sometimes referred to as
multipath induced fading.

20. Distortion
A distortion is the alteration of the original shape (or other
characteristic) of an object, image, sound, waveform or
other form of information or representation. Distortion is
usually unwanted.

21. Interference
Interference is anything which alters, modifies, or disrupts
a signal as it travels along a channel between a source and
a receiver.
The term typically refers to the addition of unwanted
signals to a useful signal.
Interference types
•Constructive Interference.
•Destructive Interference.
Examples

22. Intersymbol Interference
• Intersymbol interference (ISI) occurs when a pulse spreads out in such a way
that it interferes with adjacent pulses at the sample instant.
• Example: assume polar NRZ line code. The channel outputs are shown as
spreaded (width Tb becomes 2Tb) pulses shown (Spreading due to band limited
channel characteristics).
Channel Input Channel Output
Pulse width Tb Pulse width Tb
Data 1
− Tb 0 Tb − Tb 0 Tb
Data 0
− Tb 0 Tb − Tb 0 Tb
Eeng 360 22

23. Reasons for ISI
Multipath propagation
One of the causes of intersymbol interference is what is known as multipath
propagation in which a wireless signal from a transmitter reaches the receiver via
many different paths. The causes of this include reflection (for instance, the signal
may bounce off buildings), refraction (such as through the foliage of a tree) and
atmospheric effects such as atmospheric ducting and ionospheric reflection. Since all
of these paths are different lengths - plus some of these effects will also slow the
signal down - this results in the different versions of the signal arriving at different
times. This delay means that part or all of a given symbol will be spread into the
subsequent symbols, thereby interfering with the correct detection of those symbols.
Additionally, the various paths often distort the amplitude and/or phase of the signal
thereby causing further interference with the received signal.

24. Bandlimited channels
Another cause of intersymbol interference is the transmission of a signal
through a bandlimited channel, i.e., one where the frequency response is
zero above a certain frequency (the cutoff frequency). Passing a signal
through such a channel results in the removal of frequency components
above this cutoff frequency; in addition, the amplitude of the frequency
components below the cutoff frequency may also be attenuated by the
channel.
This filtering of the transmitted signal affects the shape of the pulse that
arrives at the receiver. The effects of filtering a rectangular pulse; not only
change the shape of the pulse within the first symbol period, but it is also
spread out over the subsequent symbol periods. When a message is
transmitted through such a channel, the spread pulse of each individual
symbol will interfere with following symbols.

25. Assignment