EEE440 Modern Communication Systems

Download Report

Transcript EEE440 Modern Communication Systems

EEE440 Modern
Communication Systems
Wireless and Mobile
• What is wireless communication?
• What is mobile communication?
• Different types of mobile wireless systems
– Cellular
– Wireless LAN
– Wireless MAN
– Wireless PAN
– Mobile Ad-hoc network (MANET)
Characteristics of Mobile Wireless
radio propagation
6 common effects
• Propagation loss
• Fading
– Large Scale (Shadow fading)
– Small scale (multipath fading)
• Doppler shift
• Frequency-selective fading
• Time-selective fading
Characteristics of Mobile Wireless
radio propagation
• In free space propagation, the power incident on
a receiving antenna is given by the free space
received power equation
  
 4d 
• In wireless environment where obstacles exist,
the average power decrease with distance at a
rate greater than d2 (usually 3 or 4)
• This is commonly known as the propagation loss
Characteristics of Mobile Wireless
radio propagation
• The actual power received over a relatively long
distance will vary randomly about the average
– A good approximation reveals that the power
measured in decibel follows a gaussian or normal
distribution centred about its average value with some
standard deviation ranging typically from 6 to 10dB.
– The power probability distribution is commonly called
a log-normal distribution
– This is commonly referred to as shadow fading
Characteristics of Mobile Wireless
radio propagation
• The actual power received over a much
smaller distance also vary considerably
due to the destructive/constructive
interference of multiple signals that follow
multiple paths
– This is commonly referred to as multipath
Characteristics of Mobile Wireless
radio propagation
• The three effects can be modelled by the
following equation
  21010 g
d PT GT GR
Propagation loss
Characteristics of Mobile Wireless
radio propagation
• Terminal mobility with respective to the
incoming wave introduces a frequency
shift called Doppler shift
• Signal fades due to the movement of the
Characteristics of Mobile Wireless
radio propagation
• The effect of multipath fading depends of the
signal bandwidth
• For a relatively large bandwidth, different
frequency components of the signal being
handled differently over the propagation path
leading to signal distortion called frequency
selective fading
• This is manifested in inter-symbol interference
(ISI) due to successive digital symbols overlap
into adjacent symbol intervals
• For narrower signal bandwidth, non-selective of
flat fading occur
Characteristics of Mobile Wireless
radio propagation
• Time selective fading occurs when the
channel changes its characteristics during
the transmission of the signal
• The change in the channel characteristics
is proportional to the receiver mobility
Propagation loss
• The average power measured at the
receiver at a distance d from the
transmitter is given by
PR  g d PT GT GR
• g(d) represents the path loss with the
general expression
g d   kd n
Propagation loss
• There are many models for the path loss
• A common two-ray model is most often used
g d   kd 4
g d   kd 4
Propagation loss: Two-ray model
• The two-ray model is the simplest representation
that models the effect on the average received
power of multiple rays due to reflection,
diffraction and scattering
• It treats the case of a single reflected ray
• Provides reasonably accurate results in
macrocellular environment with relatively high
BS antenna and/or L.O.S conditions
• Assumes that the signal arrives directly through
a L.O.S path and indirectly through perfect
relfection from a flat ground surface
Propagation loss: Two-ray model
Propagation loss: Two-ray model
• The reflected signal shows up with a delay relative to the direct path
signal and as a consequence, may add constructively (in phase) or
destructively (out of phase)
• Propagation starts out with an R2 falloff rate and then transitions to a
R4 falloff rate at greater ranges.
• The "point" where this transition occurs is often called the Fresnel
• The nulls are representative of points where direct and reflected
signals cancel while the humps show points where signals add.
• In practice, ground reflections are usually somewhat diffuse (rough
mirror instead of polished) and so the sharp nulls get filled in.
• In macrocellular communications systems, operating distances are
usually large enough so that signal strength can be thought of as
falling off at an R4 rate.
Propagation loss: Other models
• Various measurement based empirical laws
have been developed to estimate median path
loss for moderate to large macrocells based on
frequency, building environment, and antenna
heights; most notably the Okumura & Hata
• Generally describes median path loss as
Median Path Loss (dB) = A + B log10( R )
A is median path loss in dB at 1km
B is the rate at which median signal strength falls off (10
times )
R is range in km
Propagation loss: Other models
• Free space A&B values at 1900 MHz are
A=98 dB and B=20 consistent with an R2
signal strength fall-off rate
• In dense residential areas with a 100'
basestation antenna height, model
indicates A&B values of A=132 dB and
B=38 for an R3.8 signal strength fall-off
Propagation loss: Microcells
• For microcells, some investigators use
d 
g (d )  d  n1 1  
 db 
Where n1 , n2 are two separate integers
db is a measured breakpoint
Propagation loss: Okumura model
• One of the most common model used for signal
prediction in large urban macrocells
• Applicable over distance of 1-100km and frequency
ranges of 150-1500 MHz
• Measurement made in Tokyo using base station heights
between 30-100m
• The empirical propagation loss formula at distance d
parameterised by the carrier frequency fc is given by
PL (d ) dB  L( f c , d )  A ( f c , d )  G(ht )  G(hr )  GAREA
Propagation loss: Hata model
Propagation loss: Hata model
Shadow fading
• Long term shadow fading due to variations in
radio signal power due to encounters with terrain
obstructions such as hills or manmade
structures such as buildings
• The measured signal power differ substantially
at different locations even though at the same
radial distance from a transmitter
• Represents the medium scale fluctuations of the
radio signal strength over distances from tens to
hundreds of meters
Shadow Fading
• Many empirical studies demonstrate that
the received mean power fluctuates about
the average power with a log-normal
• Can be well-approximated by a gaussian
random variable with standard deviation, δ
Shadow fading
Consider the signal power equation in dB.
PR,dB  10log10  2  x  10log10 g (d )  PT ,dB  10log10 GT G R
The shadow-fading random variable x, measured
in dB is taken to be a zero-mean gaussian
random variable with variance δ2
f ( x) 
2 2
2 2
Shadow fading
PR,dB  10log10  2  x  10log10 g (d )  PT ,dB  10log10 GT G R
PR,dB  10log10  2  pdB
• Ignoring the multipath effect, α
P R,dB  10log10 g (d )  PT ,dB  10log10 GT G R
• The term pdB is the local-mean power
modelled as a gaussian random variable
with average value P R,dB
• The pdf for pdB is
 pdB P
R , dB
f ( pdB ) 
2 2
2 2
Shadow fading
• Typical value of δ range from 6 to 10dB
Multipath fading
• The actual power received over a much smaller distance
vary considerably due to the destructive/constructive
interference of multiple signals that follow multiple paths
to the receiver
• The direct ray is actually made up of many rays due to
scattering multiple times by obstructions along its path,
all travelling about the same distance
• Each of these rays appearing at the receiver will differ
randomly in amplitude and phase due to the scattering
Multipath fading
Multipath fading
• It is found that the multipath can be
modelled by using the Rayleigh/Ricean
• With Rayleigh statistics, the pdf of the
random variable α is given by
f ( )  2 e
2 r
Multipath fading
Multipath fading
• Rayleigh fading is viewed as a reasonable model for
urban environments where there are many objects in the
environment that scatter the radio signal before it arrives
at the receiver
• there is no dominant propagation along a LOS between
the transmitter and receiver.
• The central limit theorem holds that, if there is sufficiently
much scatter, the channel impulse response will be wellmodelled as a Gaussian process irrespective of the
distribution of the individual components
• such a process will have zero mean and phase evenly
distributted between 0 and 2π radians.
• The envelope of the channel response will therefore be
Rayleigh distributed
Multipath fading
Multipath fading
• If the environment is such that, in addition
to the scattering, there is a strongly
dominant signal seen at the receiver,
usually caused by a LOS, then the mean
of the random process will no longer be
zero, varying instead around the powerlevel of the dominant path.
• Such a situation may be better modelled
as Rician fading.
Multipath fading
Doppler shift
• How rapidly the channel fades will be
affected by how fast the receiver and/or
transmitter are moving
• Motion causes Doppler shift in the
received signal components
• the change in frequency of a wave for a
receiver moving relative to the transmitter
Doppler shift
• Say a mobile phone moving at velocity v km/hr in the x direction and
the radio wave impinging on the receiver at an angle βk
• The motion introduces a Doppler frequency shift,
fk = vcos βk/λ
• If the ray is directed opposite to the mobile’s motion (β=0), then
• The frequency of the signal has increased by the Doppler spread, fk
Frequency selective fading
• The effect of multipath fading on the reception of signals
depends on the signal bandwidth
• For relatively large bandwidth, different parts of the
transmitted signal spectrum are attenuated differently,
• This is manifested in the inter-symbol interference (ISI)
• For narrower bandwidth signals, non-selective of flat
fading occur
• The delay spread, is the variation in the propagation
delays of multiple scattered rays
• Digital symbol intervals, Ts smaller than 5 or 6 times the
delay spread,ds give rise to frequency selective fading
(Ts <2πds)
• Typical values of delay spread are 0.2µs (rural area),
0.5µs (suburban area), 3-8µs (urban area), <2 µs (urban
microcell) and 50-300ns (indoor picocell)
Frequency selective fading
• Slow or fast fading depends on the coherence time, Tc
• Coherence time is the measure of period over which the
fading process is correllated
• Tc is related to the delayspread, Tc=1/ds
• The fading is said to be slow if the symbol duration, Ts is
smaller than the coherence time.
• Frequency selective fading occur when the signal
bandwidth exceeds the coherence bandwidth, B=1/2πds
Time selective fading
• Occurs when the channel changes its characteristics
during signal transmission
• Directly proportional to receiver mobility
• Receiver mobility causes the signal to change rapidly
enough in comparison with the coherence time,
• The Doppler effect leads to time selective fading
• However, if the signal itself changes rapidly enough with
respect to the reciprocal of the Doppler frequency
spread, fk , distortion will not happen
• There is a minimum bandwidth beyond which the time
selective fading can be eliminated
• Ts > Tc.9/(16πfk)