Transcript Cellular Radio and Personal Communication Lecture 3

Cellular Mobile Communication
Systems
Lecture 3
Engr. Shahryar Saleem
Assistant Professor
Department of Telecom Engineering
University of Engineering and Technology
Taxila
TI - 1011
TI - 1011
1
Communication Issues and Radio
Propagation
TI - 1011
2
Small scale/Fast fading
• Multipath = several delayed replicas of the
signal arriving at the receiver
• Fading = constructive and destructive adding
of the signals
• Rapid fluctuation in amplitude with time
• Results in poor signal quality
• Digital communications
– High bit error rates
TI - 1011
3
Constructive and Destructive
Fading
TI - 1011
4
Modeling of Multipath Fading
• Fluctuations of the signal amplitude results due to the
addition of signals arriving with different phases
• Phase difference because the signals have travelled
different distance by different paths
• Phase difference causes rapid fluctuations in the
received signal amplitude
TI - 1011
5
Modeling of Multipath Fading
• Fluctuations are modeled as a random variable with a
particular distribution
• Generate a histogram of the received signal strength
• The density function corresponds to the distribution of
the fluctuating values of the received signal strength
• Commonly used distribution for multipath fading is
Rayleigh distribution, whose PDF is given by
• Where r is the random variable corresponding to
signal amplitude
TI - 1011
6
Modeling of Multipath Fading
• When a strong LOS signal component also exists
• The distribution is found to be Ricean
• The PDF of such a distribution is
• K determines how strong the LOS component is with respect to the
rest of the multipath
• Small scale fading results in very high bit errors
• Techniques used to mitigate the effects of small scale fading
– Error Control coding with interleaving
– Diversity schemes
– Directional antennas
TI - 1011
7
Small Scale Fading Multipath Delay
Spread
• Small scale fading is divided into fading based on
– Multipath time delay spread
– Doppler spread
• Multipath Delay Spread
– In Time Domain
• a transmitted narrow pulse arrives as multiple paths with
different strengths and delay
– In Frequency Domain
• the response is not flat (suffers from frequency selective
fading)
• Inter Symbol Interference (ISI) caused due to multipath
delay spread > the symbol Period
TI - 1011
8
Small Scale Fading Multipath Delay
Spread
•
•
ISI results in irreducible errors that are caused in the detected signal
The effect of ISI can be modeled using a Wideband multipath channel,
whose impulse response is:
•
Where αi = Rayleigh distributed amplitude of multipath
E{αi}= 2σi => mean local strength,
Ti = multipath arrival time
•
Two types of fading based on multipath delay spread
•
•
Flat Fading: where the multipath delay spread < symbol period
Frequency Selective Fading: where Delay spread > symbol Period
TI - 1011
9
Small Scale Fading/ Time
Dispersion Parameters
• In order to compare different multipath channels and to develop
general guidelines for wireless systems parameter which quantify
the multipath channel are used.
• Mean Excess delay, Rms delay spread and excess delay spread
defines the parameters of multipath channel that can be determined
form the power delay profile
• Mean excess delay is;
• RMS multipath delay spread is the measure of the data rate that can
be supported and is given as;
TI - 1011
10
Small Scale Fading/ Time
Dispersion Parameters (cont)
• Where
• Coherence bandwidth is the range of
frequencies over which the channel can be
considered “flat” i.e. two frequencies of the
same signal experience equal amplitude fading.
• Bc= 1/5 σt
TI - 1011
11
Example
TI - 1011
12
Small Scale Fading Doppler
Spectrum
• Delay spread and coherence bandwidth describe the time dispersive
nature of the channel
• These parameters do not give any information about the time
varying nature of the channel caused by either relative motion
between the mobile and base station, or movements of objects in
the channel
• Doppler spread and coherence time are the parameters which
describe the time varying nature of the channel
• Doppler spread BD, is the measure of the spectral broadening
caused by the time rate of change of mobile radio channel
• Defined as the range of frequencies over which the received
Doppler spectrum is non-zero
TI - 1011
13
Small Scale Fading Doppler
Spectrum
• When a sinusoidal tone of frequency fc is transmitted
• The received signal spectrum also known as the Doppler spectrum
will have components in the range of fc-fd to fc+fd
• fd is the Doppler shift
• The amount of spectral broadening depends on fd which is the
function of the relative velocity of the mobile, and the angle θ
between the direction of motion of the mobile and the direction of
arrival of the scattered waves
• If the based band signal bandwidth is much greater than BD, the
effect of the Doppler spread are negligible at the receiver
• This type of channel is a slow fading channel
TI - 1011
14
Fade Duration & Fade Rate
• Fade Duration: For what time a signal strength will be below a
particular value
• Fade Rate: how often it crosses a threshold value (frequency of
transitions or fading rate)
• Important in designing coding schemes and interleaving sizes for
efficient performance
• Doppler spectrum is the spectrum of fluctuations of the received
signal strengths
• Where fm is the maximum Doppler frequency and is
given as
fm = vm /λ
TI - 1011
15
Coherence Time
• It is the average time for which the channel can be assumed to be
constant
• A good approximation for the coherence time is
• If the symbol duration > Tc, symbol is distorted (Fast Fading)
• If the symbol duration < Tc, symbol is not distorted (Slow Fading)
TI - 1011
16
Radio Channel Characteristics and
Mitigation Techniques
Issue
Performance Affected
Mitigation Technique
Shadow Fading
RSS
Fade Margin- Increase
TX power or decrease
cell size
Fast Fading
BER
Packet Error Rate
Error Control Coding
Interleaving
Frequency Hopping
Diversity
Multipath Delay
spread
ISI & irreducible error
rates
Equalization
DS- spread spectrum
OFDM
Directional Antennas 17
TI - 1011
Emerging Channel Models
• Position location important for emergency and locationaware applications
• Models developed for communication systems are no
longer sufficient to address the performance of
Geolocation schemes
• Smart antennas and adaptive antennas needs to know
the Angle of Arrival (AOA) of the multipath components
TI - 1011
18
Wideband Channel Models for
Geolocation
• Position location applications
• Civilian
–
–
–
–
Intelligent Transportation System
Public safety (Enhanced 911 or E-911)
Cargo Tracking
Accident reporting
• Military application
– Small Unit Operations (SUO) in restrictive RF environment e.g.
buildings, tunnels etc
• Traditional GPS system fails due to lack of sufficient
power indoors and the harsh multipath environment
TI - 1011
19
Requirements for Position Based
Systems
• For PBS applications detecting the Direct Line of Sight (DLOS) path
between TX and RX is extremely important
• DLOS is the straight line connecting the TX and RX even if there are
obstacles in between them
• Detecting DLOS is important because the Time of Arrival (TOA) or
AOA of the DLOS path corresponds to the distance between the TX
and RX
• For telecom application bit error rate is important
• For PBS apps, error in detecting the right multipath component can
result in different distance measurements
• Thus, PBS using AOA require wide bandwidth to resolve multipath
component and detect the arrival of first path
TI - 1011
20
Classification of channel Profiles
for Geolocation Apps
•
•
•
Channel profiles for Geolocation Apps are divided into three
categories
DDP (Dominant Direct Path)
– In this case traditional GPS receiver (designed for outdoor
applications) where multipath components are weaker detects the
DLOS path which is the strongest in channel profile (thus
accurately detecting the TOA)
NDDP (Nondominant Direct Path)
– GPS receiver detects a DLOS path but it is not the Dominant path
– For these profiles traditional GPS receivers which is expected to
lock to the strongest path
– Detects a TOA that leads to error in position estimation
– The amount of error is the distance associated with the difference
between the TOA of the strongest path and the TOA of the DLOS
path
TI - 1011
21
Classification of channel Profiles
for Geolocation Apps
• Rake Receivers can be used to detect the TOA of the
DLOS path for NDDP
• UDP (Undetected Direct Path)
– The GPS system cannot detect the DLOS path
• If dynamic range of receiver is power of strongest
signal/power of the weakest detectable path
• UDP can be neglected
TI - 1011
22
Rake Receivers
• Radio receiver designed to counter the effects of
multipath
• Uses several “sub- receivers” each delay slightly to tune
in to the individual multipath components
• Each component is decoded independently
• Later recombined to make the most of the different
transmission x-tics of each transmission path
• Results in higher signal - to - noise ratio in multipath
environment than in clean environment
TI - 1011
23
SIMO and MIMO Channel Models
• Spatial wide band channels
– Provide the delay-power spectrum as in multipath delay spread
– AOA of the multipath components
• SIMO (single input multiple output)
– Typical cellular environment
• Mobile transmitters are simple
• Base Station
– Complex receiver with smart antennas with M antenna elements
– L multipath components arrive at the BS from different MT (l)
with different
•
•
•
•
Amplitudes (α)
Phases (φ)
Delay (T)
Directions(θ)
TI - 1011
24
MIMO
•
•
•
•
•
•
•
•
Stands for Multiple Input Multiple Output
N mobile antennas
M base station antennas
Spectral efficiency improves
Example
MIMO channel
Experimental results with a 4X4 antenna array system
For MIMO, spectral efficiency of 27.9 b/s/Hz as
compared with spectral efficiencies of 2 b/s/Hz in
traditional radio system
TI - 1011
25