Channel Impairments

BY Muhammad Uzair Rasheed

2009-CPE-03UCE&T BZU MULTAN

Performance Criterion

How a “good” communication system can be differentiated from a “sloppy” one?

For analog communications– How close is to 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

)(tm )(ˆ tm

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.

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

Noise

• External Noise• Internal Noise

Noise

• External Noise• atmospheric Noise• Solar Noise• Cosmic Noise• Industrial Noise (1-600

MHz)

Summary 8MHz-1.43GHz

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.

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

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:

Channel transfer function/linear/nonlinear

Channel transfer function/linear/nonlinear +

( )n t

( ) ( ) ( ) ( )r t s t c t n t

(AWGN channel (usually transferfunction is linear) and n(t) is Gaussian,white noise)

( )s t

channel

( ) ( ) ( ) ( ) ( ) ( )u

r t s c t n t s t c t dt n t

(u: where integrand exists)

( )c t

Channel impulse response

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

SNR = Average signal power

Average noise power

SNR (dB) = 10 log10 SNR

Signal Noise Signal + noise

Signal Noise Signal + noise

HighSNR

LowSNR

t t t

t t t

Signal-to-Noise Ratio

error

No errors

SNR

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

                                                           

EE 541/451 Fall 2006

Shannon CapacityShannon 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

sbitSNRBC /)1(log2

15

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

Shannon Channel Capacity

C = BT log2 (1 + SNR) bps

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

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

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)

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.

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.

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

Eeng 360 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).

Data 1

bT 0 bT0bT bT

Data 0

bT 0 bT0bT bT

Channel Input

Pulse width TTbb

Channel Output

Pulse width TTbb

Reasons for ISIMultipath 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.

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.

Assignment