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Okey Ugweje, PhD Page 1

Radio System Design

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Noise in Communications Systems

Interference Issues in Radio Communications

Eb/N0 vs SNR; BER vs. Noise

Spectral Efficiency and System Limitation Bandwidth Limitations

Inter-modulation Distortion

Bandwidth Limitations

RF System Design Considerations

Characteristics of Receiver Design Noise Figure

Receiver Sensitivity

Dynamic Range

Power Output

Day 4: Radio System Design

RF/Microwave Systems : PDDProgram Schedule

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Origination of Noise in communication systems

External to the system, e.g., atmospheric, solar,cosmic man-made, etc

Internal to the system, e.g., thermal noise, shot

noise

Effect of external noise depends on system location &

configuration, while effect of internal noise is

independent of location & configuration

Noise can be classified in two broad categoriesA) Additive noise

Any noise that remains in the system when the

input signal disappears, e.g., thermal noise,

crosstalk, ISI, etc

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B) Multiplicative noise

Noise caused by inherent randomness within the signal

itself

Produced in the system only when the signal is present.

Eg. Shot noise, phase noise, multipath fading, etc.

Noise arises in various forms

Data m(t) is corrupted in the Tx by thermal noisedue to thepresence of electronic devices (e.g., Audio Amplifier)

Carrier c(t) is not a pure sine wave - in fact, it containsharmonic distortions

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Modulated signal s(t) experiences multiplicative noise intransmission from Tx thru the channel due to turbulence in

the air and propagation mechanisms

s(t) also suffers from additive noise during transmission

automobiles, static electricity, lightning, power lines, etc

Thermal and short noise at the receiver is also a factor

All forms of noise degrade system performance

In comparison, additive noise is the most annoying

usually contains most power and is of most interest in many

applications

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Noise Modeling

In the channel, the signal experience attenuation, time delay(precisely known) and additive noise

Most disturbances, interference, attenuation, etc., are usually

classified as noise

The most important type of noise that occur in communication

system is said to be white noise, n(t)

Usually n(t) is assumed to be additive, white and Gaussiannoise (AWGN) with power spectral density G

n

(f)

Transmitter Channel Receiver

+ r(t)s(t)

n(t)

(noise)

(modulated signal ) (received signal )

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White Noise is a random process having a flat (constant)power spectral density Gn(f), over the entire frequency range

white because it is analogous to white light

assumed to be a Gaussian random process

usually additive in nature

Hence, this type of noise is commonly called Additive, White

and Gaussian (AWGN) with power spectral density No/2

0( )2

n

NG f

White Noise and Filtered Noise

(f)Gn

f

2-sided power spectral density of noise

0

0

2

N

2 0( )2

NVar df n t

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9/29 Okey Ugweje, PhD Page 9

Since white noise has infinite bandwidth, it cannot be used in

system design

wideband and cannot be expressed in terms of quardrature

components

However, in most commun systems operating at fc, the channelbandwidth B (or W), is small compared to fc

narrowband systems

Hence, it is mathematically convenient to represent the white

noise process in terms of the quadrature components

But it must be filtered

Accomplished by passing the signal plus noise through an

ideal BPF having a passband as(f)G

n

f

fc-fc

0

2

N

0

B B

noise is said to be bandlimitted

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Signal-to-Noise Ratio

Signal-to-noise ratio (SNR) is the figure of meritfor evaluatingthe performance of analog communication systems

A certain signal m(t) (orx(t)) is transmitted with powerPT s(t) is corrupted by additive noise n(t) during transmission

Channel may also attenuate (and/or distort) the signal

At Rx, we have a signal mixed with noise

Signal and noise power at the receiver input are Siand NiRx processes the signal (filters, demodulation, etc.) to yield

the desired signal powerSo, plus noise powerNo

Transmitter Channel Receiver

y0(t)

n(t)

yi(t)

Si, N

i

x(t)

m(t)

s(t)

S0, N

0

input output P

T

0 0

( ) ( ) ( )oy t s t n t

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Assume that:

Noise n(t) is zero-mean Gaussian with Gn(f) = N

0/2 or/2

Noise is uncorrelated with s(t)

Hence output power is

The output signal-to-noise ratio (SNR) is

For a baseband system

2 2 2

0 0 0 0 0( ) ( ) ( )E y t E s t E n t S N

2

0

0 2

0 0

( )

( )

S E s tSoSNRN N E n t

o

Mean Signal Power

Noise Power

Part 4: Digital Baseband Communications

0

iSS

SNRN N Bb b

used as a standard formaking comparisons of

the various analog

modulation schemes

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Unipolar

(orthogonal)

Bipolar (antipodal)

P QE

Nb

b

o

HG

2P QE

Nb

b

o

HG

Bipolar signals require a

factor of 2 increase in energy

compared to Unipolar

Since 10log102 = 3 dB, wesay that bipolar signaling

offers a 3 dB better

performance than Unipolar0 2 4 6 8 10 12 14 16 18 20

10-10

10-8

10-6

10-4

10-2

10

0

Eb/No (dB)

Probability

ofBitError

Othogonal

Antipodal

QE

N

b

oHG

QE

N

b

o

2

HG3-dB

BER vs. Noise

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Comparing BER Performance

0 2 4 6 8 10 12 14 16 18 2010

-10

10-8

10-6

10-4

10-2

100

Eb/No (dB)

Probab

ility

ofBitError

OthogonalAntipodal

7 8 10 4.

9 2 102

.

ForEb/No = 10 dB Pb,orthogonal = 9.2x10

-2

Pb,antipodal = 7.8x10-4

For the same received signal to noise ratio, antipodal

provides lower bit error rate than orthogonal

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Probability of Error Performance

Coherent

Noncoherent

Coherent orthogonal BFSK performance is identical to coherentASK

Eb/N0 penalty of noncoh. detection is only about 1 dB lower

Note:noncoherent FSK performance is not nearly as bad as

ASK

P QE

Nb

b

o

HG

PE

Nb

b

o

H

1

2 2exp

Part 5: Digital Bandpass Communication

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BER Performance for DPSK

0

1exp

2b

B

EP

N

Part 5: Digital Bandpass Communication

Theoretical performance forCPSK and DPSK is shown

here for an AWGN channel

BER for CPSK is exactlythe same as that derived for

bipolar baseband

transmission

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16/29 Okey Ugweje, PhD Page 16Part 5: Digital Bandpass Communication

MASK

MPSK

MFSK

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whereP is spectral power density in W/Hz

T is absolute Kelvin temperature (293 = room temp, 20

Celsius)

k is Boltzmanns constant 1.38x10-23 Ws/deg. h is Plancks constant 6.6x10-34 Ws2.

For frequencies below 1011 Hz, we can treat the spectral

power density as a uniform value kT

Quantum Limited Spectral Noise Power

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Bandwidth may be derived from half-power points or other

criteria

Ideal Uniform Spectrum Noise

System Names BW (kHz) Noise Power (W) Noise Power (dBm)

TACS, SMR 25 1x10-19 -129.8

AMPS, TDMA 30 1.2x10-16 -129.05

GSM,

DCS1900

200 8.3x10-16 -120.8

CDMA 1000 4.2x10-15 -113.8

CDMA figure is broadband for entire composite signal without

despreading gainFor individual user including effects of despreading, the

equivalent B is taken as the bit rate of the vocoder (14,400 b/s

in IS-95 applications)

On this basis, noise power is -132.25 dBm for an individual user

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Shot noise = flow of electrons or current at weak signal levels;

random impacts of individual electrons in active devices (diodes,

transistors, etc.)

In2 = qIGf,

where

In

is the standard deviation of the shot noise current,

q = 1.6x10-19As, the charge of the electron,

I is the dc signal current through an active junction, and

G is a factor dependent on geometry of the structure

f is bandwidthNote that In is notrelated to temperature.

Shot noise is a problem for the circuit designer, not the system

designer

Its effect is included in the Noise Figure

Shot Noise

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Example:

A 30 kHz bandwidth Rx rated at 7 dB NF has equivalent

input noise level of -129 + 7 = -122 dBm

Minimum analog received signal must be -122+18=-104

dBm for good noise-limited reception

Noise Figure of Receiver

The composite effect of all noise generated in the receiver is

expressed by a figure of merit called Noise Figure (NF)

NF of an amplifier, or the entire front end of a Rx, is the ratio

in dB of Signal/Noise at the output divided by S/N at the

input

The input to a Rx is the antenna, and the assumed noise

source there is the kT thermal noise of space

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Inter-modulation (IM or Intermod) is an effect arising from very

strong signals

It relates to the upper end of the dynamic range of signal

power

IM produces small signals at various frequencies which add to

other sources of system noise and reduce the sensitivity of

receivers

It relates to the lower end of the dynamic range of signal

power as well

Inter-modulation Involves

Mixing of the signals

Power amplifier transfer characteristics of active and passive

devices

In short, IM is just unwanted modulation

Inter-modulation

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Active Inter-modulation

produced in transmitters andreceivers

Passive Inter-modulation

Produced in antenna

Also produced in other pointsof rectification

Inter-modulation Issues

Finding Inter-modulation

Eliminating Inter-modulation

Available Inter-modulation

prediction software

Sources of Inter-modulation

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Try to prevent or reduce the amplitude of strong RF signals

reaching receivers in wireless systems

Reduce or eliminate at the source, if feasible (spurious

emissions from electric lamps, signs, elevator motors, etc.)

Shielding, enclosure, modification of antenna directionality to

reduce the penetration of electromagnetic waves

Identify and eliminate secondary non-linear radiators: parallel

metal joints with conductive connections, ground all parts of

metal fences, rain gutters, etc., (also improves lightning

protection)

Conducted RF from wires, etc. entering receiver can bereduced via low pass or band pass filters, ferrite beads, etc.

Use Notch filters to remove source RF, or specific harmonics or

products

What to do about IM?

N li Eff t & I t d l ti

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Almost everything is slightly (or extremely) non-linear

Only free space is theoretically a true linear medium

Particularly non-linear are:

all active semiconductor devices

corroded electrical connections, etc.

Non-linear Effects & Inter-modulation

When high RF current levels are present in non-linear devices,waveform distortion occurs

A distorted (clipped, peaked, etc.) non-sinusoidal waveform is

equivalent to a sum of sine waves of several different

frequencies (Fourier series)Product waveforms can also occur when 2 freqs are mixed

due to the non-linearity

If nonlinear device characteristics are known (intercept point,

etc.), IM amplitudes can be accurately computed

M d l ti I t d l ti

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When 2 signals are intentionally combined in a non-linear

device we call the effect modulation

Amplitude modulator, or quad phase modulator

Mixer, down or up converter in superheterodyne receivers

When 2 (or more) signals are unintentionally combined in a

non-linear device, the effect is known as inter-modulation (a

pejorative term)

An analogy:

Botanists use soil to grow plants. But on your living room

carpet, soil is just dirt

We use modulation to transmit signal, however, when it

happens without our direction, we dont want it

IM signals increase system noise, or cause distinctiverecognizable interference signals

Modulation vs. Inter-modulation

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Complete link: Uplink + Downlink

Fixed services analysis and transparent repeaterAnalysis of uplink is done the same way as the downlink

Design Parameters:

Uplink:Transmission power of earth station

Antenna characteristics of earth station

Receiver characteristics and satellite antennas

Downlink:

Transmission power of Transponder

Antenna transmission characteristics

Antenna characteristics and Earth station receiver

Radio System Design Above 10 GHz

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Global System BER:

Eb/No of uplink and downlink Signal to Noise Ratio of receiver (satellite or Earth

station) is defined by 4 ways:

1. Eb/No (dB)

2. C/N (dB): Carrier-to-noise

3. C/No (dB)4. C/T (dBW/K):

the ratio of carrier power with the equivalent

noise temperature


R S G

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Relation between the Signal-to-noise ratio

C/N= (Eb/No) log2M; and in dB : C/N= Eb/No(dB)+10 (log10(M))

No= KtWatt/Hz and N = NoBWatts in the bandwidth of B Hz

K = 1.38x10-23 Joule/Kelvin (Boltzmann Constant) T = Equivalent noise temperature in Kelvin

M = Signal number in M-PSK constellation


R di S t D i

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Radio System DesignRadio System Design Above 10 GHz

Budget link:

To guarantee a ratio Eb/No (or C/N) sufficiently large

to attempt a given BER

The link Budget is divided in two parts:

The power budget and the noise budget

Power Link Budget for downlink

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