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Chapter2
Wireless Propagation Channel Models
2.1 Propagation Characteristics of Mobile Radio Channels
In an ideal radio channel, the received signal would consist of only a
single direct path signal, which would be a perfect reconstruction of the transmitted
signal. However in a real channel, the signal is modified during transmission in the
channel. The received signal consists of a combination of attenuated, reflected,
refracted, and diffracted replicas of the transmitted signal. On top of all this, the
channel adds noise to the signal and can cause a shift in the carrier frequency if the
transmitter or receiver is moving (Doppler effect). Understanding of these effects on
the signal is important because the performance of a radio system is dependent on the
radio channel characteristics.
2.1.1Attenuation
Attenuation is a general term that refers to any reduction in the strength
of a signal. Attenuation occurs with any type of signal whether digital or analog.
Sometimes called loss, attenuation is a natural consequence of signal transmission
over long distances. The extent of attenuation is usually expressed in units called
decibels (dBs).
If Ps is the signal power at the transmitting end (source) of a
communications circuit and Pd is the signal power at the receiving end (destination),
then Ps > Pd.
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The power attenuation Ap in decibels is given by the formula:
Ap = 10 log10 (Ps/Pd)
Attenuation can also be expressed in terms of voltage. If Av is the
voltage attenuation in decibels, Vs is the source signal voltage, and Vd is the
destination signal voltage, then:
Av = 20 log10 (Vs/Vd)
Attenuation is the drop in the signal power when transmitting from one
point to another. It can be caused by the transmission path length, obstructions in the
signal path, and multipath effects. Figure 2.1 shows some of the radio propagation
effects that cause attenuation. Any objects that obstruct the line of sight signal from
the transmitter to the receiver can cause attenuation.
Figure 2.1: Radio Propagation Effects
Shadowing of the signal can occur whenever there is an obstruction
between the transmitter and receiver. It is generally caused by buildings and hills, and
is the most important environmental attenuation factor.
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Shadowing is most severe in heavily built up areas, due to the
shadowing from buildings. However, hills can cause a large problem due to the large
shadow they produce. Radio signals diffract off the boundaries of obstructions, thus
preventing total shadowing of the signals behind hills and buildings. However, theamount of diffraction is dependent on the radio frequency used, with low frequencies
diffracting more then high frequency signals. Thus high frequency signals, especially,
Ultra High Frequencies (UHF), and microwave signals require line of sight for
adequate signal strength. To overcome the problem of shadowing, transmitters are
usually elevated as high as possible to minimize the number of obstructions.
Shadowed areas tend to be large, resulting in the rate of change of the signal power
being slow. Typical amounts of variation in attenuation due to shadowing are shown
in Table 2.1.
Table 2.1: Typical Shadowing in a Radio Channel [14]
Description Typical Attenuation due to Shadowing
Heavily built-up urban centre 20dB variation from street to street
Sub-urban area (fewer large
buildings)
10dB greater signal power then built-up
urban center
Open rural area 20dB greater signal power then sub-
urban areas
Terrain irregularities and treefoliage 3-12dB signal power variation
.
2.1.2Multipath Effects
There are obstacles and reflectors in the wireless propagation channel, the
transmitted signal arrivals at the receiver from various directions over a multiplicity of
paths. Such a phenomenon is called multipath. It is an unpredictable set of reflections
and/or direct waves each with its own degree of attenuation and delay.Multipath is usually described by
Line-of-sight (LOS): The direct connection between the transmitter (TX)and the receiver (RX).
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Non-line-of-sight (NLOS): The path arriving (to the receiver) afterreflection from reflectors.
The illustration of LOS and NLOS is shown in Figure 2.2.
Figure 2.2: Effect of Multipath on a Mobile Station
In a radio link, the RF signal from the transmitter may be reflected from
objects such as hills, buildings, or vehicles. This gives rise to multiple transmission
paths at the receiver. Figure 2.3 show some of the possible ways in which multipath
signals can occur.
Figure 2.3: Multipath Signals
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2.1.3.Delay Spread
The received radio signal from a transmitter consists of typically a
direct signal, plus reflections off objects such as buildings, mountings, and other
structures. The reflected signals arrive at a later time then the direct signal because of
the extra path length, giving rise to a slightly different arrival times, spreading the
received energy in time. Delay spread is the time spread between the arrival of the
first and last significant multipath signal seen by the receiver.
In a digital system, the delay spread can lead to inter-symbol
interference (ISI). This is due to the delayed multipath signal overlapping with the
following symbols. This can cause significant errors in high bit rate systems,
especially when using time division multiplexing (TDMA). Figure 2.4 shows the
effect of inter-symbol interference due to delay spread on the received signal. As the
transmitted bit rate is increased the amount of inter-symbol interference also
increases. The effect starts to become very significant when the delay spread is greater
then ~50% of the bit time.
Figure 2.4: Multipath Delay Spread
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Table 2.2 shows the typical delay spread for various environments. The
maximum delay spread in an outdoor environment is approximately 20 s, thus
significant inter-symbol interference can occur at bit rates as low as 25 kbps.
Table 2.2: Typical Delay Spread
Environment or cause Delay Spread Maximum Path Length
Difference
Indoor (room) 40 n sec - 200 n sec 12 m - 60 m
Outdoor 1 sec - 20 sec 300 m - 6 km
Inter-symbol interference can be minimized in several ways. One
method is to reduce the symbol rate by reducing the data rate for each channel (i.e.
split the bandwidth into more channels using frequency division multiplexing, or
OFDM). Another is to use a coding scheme that is tolerant to inter-symbol
interference such as CDMA.
2.1.4Doppler Shift
When a wave source and a receiver are moving relative to one another
the frequency of the received signal will not be the same as the source. When they are
moving toward each other the frequency of the received signal is higher then the
source, and when they move away from the each other the frequency decreases. Thisis called the Doppler effect. This effect becomes important when developing mobile
radio systems.
The amount the frequency changes due to the Doppler effect depends
on the relative motion between the source and receiver and on the speed of
propagation of the wave. The Doppler shift in frequency can be written:
coscos mD f
V
f
The received signal frequency
cosmcr fff
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When = 0o (mobile moving away from the transmitter)mcr
fff
When = 90o (i.e. mobile circling around)cr ff
When = 180o (mobile moving towards the transmitter)mcr fff
Where,
m = v/=maximum value of Doppler frequency
2.2 Multiple Access Techniques
Multiple access schemes are used to allow many simultaneous users to
use the same fixed bandwidth radio spectrum. In any radio system, the bandwidth that
is allocated to it is always limited. For mobile phone systems the total bandwidth is
typically 50 MHz, which is split in half to provide the forward and reverse links of the
system. Sharing of the spectrum is required in order increase the user capacity of any
wireless network. FDMA, TDMA and CDMA are the three major methods of sharing
the available bandwidth to multiple users in wireless system. There are many
extensions, and hybrid techniques for these methods, such as OFDM, and hybrid
TDMA and FDMA systems. However, an understanding of the three major methods
is required for understanding of any extensions to these methods.
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2.2.1 Frequency Division Multiple Accesses
In an FDMA system, each user has its own frequency channel. This
implies that relatively narrow filters are needed in each receiver and transmitter. Most
duplex FDMA systems must transmit and receive simultaneously. (Frequency
Division Duplex, FDD).Each user is allocated a unique frequency band in which to
transmit and receive on. During a call, no other user can use the same frequency band.
Each user is allocated a forward link channel (from the base station to the mobile
phone) and a reverse channel (back to the base station), each being a single way link.
The transmitted signal on each of the channels is continuous allowing analog
transmissions. The channel bandwidth used in most FDMA systems is typically low
(30 kHz) as each channel only needs to support a single user. FDMA is used as the
primary subdivision of large allocated frequency bands and is used as part of most
multi-channel systems.
Figure2.5. Time and bandwidth occupancy of three user signals with FDMA
2.2.1.1 Advantages Very Simple to design Narrowband (no ISI) Synchronization is easy No interference among users in a cell
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2.2.1.2 Disadvantages
Narrowband interference Static spectrum allocation Freq. reuse is a problem High analog filters large guard band required
2.2.2 Time Division Multiple Access
In TDMA, a set ofNusers share the same radio channel, but each user
only uses the channel during predetermined slots. A frame consists ofNslots, one for
each user. Frames are repeated continuously.
The transmit bandwidth isNtimes the bandwidth that would be needed
to accommodate a single user. Thus the receiver can be built with broader filters,
which are less expensive and smaller than those required for FDMA operation.
Mostly, TDMA is combined with Time Division Duplex (TDD), in which
transmission and reception do not occur simultaneously, but during different slots.
This obviates the need for costly duplex filters. In a downlink (base to mobile),
TDMA is simple to implement: it is just a matter of multiplexing Nuser signals. In
the uplink (mobile to base), TDMA is more difficult: the signals from all users have to
be aligned in time. Often this is achieved through a feedback loop with timing
information. Relatively fast power-up and power-off times are needed to avoid that
signals from users interfere with signals in other slots.
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Figure2.6: Time and Bandwidth Occupancy of Three User Signals in TDMA
System, Each user has its Own Time Slot
2.2.2.1 Advantages
Better suited for digital Often gets higher capacity ( 3 times higher here) Relaxes need for high quality filters
2.2.2.2 Disadvantages
Strict synchronization and guard time needed Still susceptible to jamming, other-cell interference Often requires equalizer
TDMA is normally used in conjunction with FDMA to subdivide the
total available bandwidth into several channels. This is done to reduce the number of
users per channel allowing a lower data rate to be used. This helps reduce the effect of
delay spread on the transmission. Figure 2.7 shows the use of TDMA with FDMA.
Each channel based on FDMA, is further subdivided using TDMA, so that several
users can transmit of the one channel. This type of transmission technique is used by
most digital second generation mobile phone systems. For GSM, the total allocated
bandwidth of 25MHz is divided into 125, 200 kHz channels using FDMA. These
channels are then subdivided further by using TDMA so that each 200 kHz channel
allows 8-16 users.
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Figure 2.7: TDMA / FDMA hybrid, showing that the bandwidth is split into
frequency channels and time slots
2.2.3 Code Division Multiple Access
Code Division Multiple Access (CDMA) is a spread spectrum
technique that uses neither frequency channels nor time slots. With CDMA, the
narrow band message (typically digitized voice data) is multiplied by a large
bandwidth signal that is a pseudo random noise code (PN code). All users in a CDMA
system use the same frequency band and transmit simultaneously. The transmitted
signal is recovered by correlating the received signal with the PN code used by the
transmitter. Figure 2.8 shows the general use of the spectrum using CDMA.
Figure 2.8. Code division multiple access (CDMA)
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CDMA technology was originally developed by the military during
World War II. Researchers were spurred into looking at ways of communicating that
would be secure and work in the presence of jamming. [17]
Figure 2.9 shows the process of a CDMA transmission. The data to be
transmitted (a) is spread before transmission by modulating the data using a PN code.
This broadens the spectrum as shown in (b). In this example the process gain is 125 as
the spread spectrum bandwidth is 125 times greater the data bandwidth. Part (c)
shows the received signal. This consists of the required signal, plus background noise,
and any interference from other CDMA users or radio sources. The received signal is
recovered by multiplying the signal by the original spreading code. This process
causes the wanted received signal to be despread back to the original transmitted data.
However, all other signals that are uncorrelated to the PN spreading code become
more spread. The wanted signal in (d) is then filtered removing the wide spread
interference and noise signals.
Figure 2.9: Basic CDMA transmissions
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2.2.3.1 Advantages
Signal hiding and non-interference with existing systems. Anti-jam and interference rejection Information security Accurate Ranging Multiple User Access Multipath tolerance
2.2.3.2 Disadvantages
The near-far problem occurs at a CDMA receiver problem. Users nearthe base station are received with high power. Users far from the base
station are received with low power.
Quasi-orthogonal codes cause self-interference, which dominates theperformance in most CDMA systems.
2.3 Path loss propagation model
Path loss models describe the signal attenuation between a transmit and
a receive antenna as a function of the propagation distance and other parameters.
Some models include many details of the terrain profile to estimate the signal
attenuation, whereas others just consider carrier frequency and distance. Antenna
heights are other critical parameters.
2.3.1 Free-Space path loss
We consider the system show-n in Figure 2.10, where a cell-site
transmitter is transmitting at an average power level of PT. We want to find the
received power level, PR, at the receiving antenna (MS) located at a distance, d, from
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the transmitter.[3]
For an isotropic antenna, in free space:
PR=2
4 d
PT
................................................................................................ (2.1)
where:
PT=average power level of transmitter
D=distance between transmitter & receiver
PR=power density at the receiver
For an antenna radiating uniformly in all directions (spherical pattern. the power
density, PR at the receiver is given by Eq. (2.1)
Figure 2.10: A Simple Model for Path Loss in Free Space
When a directional transmitting antenna with a power gain factor GT, is used, the
power density at the receiver site is GT times Eq (2.1)
Transmitting system
PT,Gain GT
Receiving System PR,Gain GR
d
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The amount of power captured by the receiver is PR times the aperture area, AR, of the
receiving antenna. The aperture area is related to the gain of the receiving antenna by
GR= 24 RA ........................................................................ (2.2)
Where:
=f
c
f=the transmission frequency in Hz.
C=8
103 m./s is the free-space speed of propagation for electromagnetic waves
AR is the effective area, which is less than the physical area by efficiency factor PR
Typical values for R range from 60% to 80%. The total received power , PR is:
PR=ARR........................................................................(2.3)
Substituting the values ofR & AR from Eq (2.1)&(2.2) into Eq(2.3) together with the
transmitting antenna gain GT we get
PR= RTT GGPd
2
4
................................................................. (2.4a)
Eq(2.4a) includes only the power loss from the spreading of the transmitted wave. If
other losses such as atmospheric absorption or ohmic losses of the waveguides
leading to the antenna, are also present, Eq ( 2.4a) is modified as
0LL
GG
P
P
p
RT
T
R ........................................................................................................... (2.4b)
Where
2
4
d
LP
denotes the loss associated with propagation of electromagnetic waves
from the transmitter to the receiver
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Lp depends on the carrier frequency and separation distance, d. This loss is always
present. L0= loss factor for additional losses. When we express Eq. (2.4a) in terms of
decibels, we get
04
log20 LGGPd
P RTTR
................................................................... (2.5)
The product PTGTis called the Equivalent Isotropic Radiated Power (EIRP) and term
d
4log20 is referred to as free-space loss (L p)in dB.
2.3.2 Hata-Okumura Model
Most of the propagation tools use a variation of Hatas model. Hatas
model is an empirical relation derived from the technical report made by Okumura so
that the results could be used in computational tools. Okumuras report consists of a
series of charts that have been used in radio communication modeling. The following
are the expressions used in Hatas model to determine the mean loss L50. Hatas
model is applicable to urban, suburban, and open environment. [3]
Urban Area
L50 = 69.55 + 26.l6log fc 13.82loghb a(hm) + (44.9 6.551oghb)logR dB
where
fc = frequency (MHz)
L50 = mean path loss (dB)
h b= BS antenna height (m)
a(hm) = correction factor formobile antenna height dB
R = distance front BS km;.
The range of the parameters for which the Hata model is valid is:
150 fc 1500 MHz
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30 hb200m
1hm10 m
1R 20 m
a(hm) is computed as:
Small or medium sized city:
a(hm)= (1.1log fc-.7)hm - (1.56log fc-0.8) dB
Large city
dBhha mm 1.154.1log29.8)(2 , fc200MHz
or,
a(hm)=3.2(log11.75hm)2-4.97 dB, fc400dB
Suburban area
dBf
urbanLL c
4.5
28log2
2
5050
Open area
L50=L50(urban)-4.78(log fc)2+18.33log fc - 40.94 dB
Hatas model does not account for any of the path-specific correction
used in Okumuras model.Okumuras model tends to average over some of the
extreme situations and does not respond sufficiently quickly to rapid changes in the
radio path profile. The distance-dependent behavior of Okumuras model is in
agreement with the measured values. Okumuras measurements are valid only for the
building types found in Tokyo.
Okumuras model requires that considerable engineering judgment be
used, particularly in the selection of the appropriate environmental factors. Data is
needed in order to be able to predict the environmental factors from the physical
properties of the buildings surrounding a mobile receiver, In addition to the
appropriate environmental factors, path-specific corrections are required to convert
Okumuras mean path-loss predictions to the predictions that apply to the specific
path under study. Okumuras techniques for correction of irregular terrain and other
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path-specific features require engineering interpretations and are thus not readily
adaptable for computer use.
MATLAB Program is given in APPENDIX-B
2.3.3 Walfisch--Ikegami Model
This model (also known as the European committee of Scientific and
Technology COST 231 model) is used to estimate the path loss in an urban
environment for cellular communication (Figure 2.11) The model is a combination of
the empirical and deterministic models for estimating the path loss in an urban
environment over the frequency range of -2000 MHz. This model is used primarily in
Europe for GSM systems and in me propagation models in the United States. The
model contains three elements: free-space loss, roof-to-street diffraction and scatter
1055, and multiscreen diffraction and scatter loss from other structures loss. The
expressions used this model are
L50=Lf+Lrts+Lms
Or,
L50=Lf whenLrts+Lms 0
Figure 2.11: The Walfisch-Ikegami Propagation Model [3]
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Where,
Lf= free-path loss
L rts = rooftop-to-street diffraction & scatter loss
L ms = multiscreen loss
Free-space loss isgiven as:
L f= 32.4 + 20IogR +20logf dB
The rooftop-to-street diffraction and scatter lois is given as:
L rts=-16.9-10logW+ l0log fc + 20log hm + l0 dB
where:
W= street width (m), and
hm=hr- hm(m)
L0=-9.646 dB 0 35 degree
L0 = 2.5 + 0.075(- 35) dB 3555 degree
L0=4 - 0.l14 (-55) dB 5590 degree
Where
=incident angle relative to the street.
The multiscreen lossis given as:
Lms =Lbsh+ka+kdlogR+kflogfc-9logb
Where
b=distance between buildings along the radio path (m)
Lbsh=-18log11+b, hb>hr
Lbsh=0, hbhr
ka=54-0.8hb, R500m, hbhr
ka=54-1.6hb R , R
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model. Hatas model ignores effects from street width, street diffraction, & scatter
losses
which the Walfisch-ikegami model includes.
MATLAB Program is given in APPENDIX-C
2.4 Conclusion
Mobile cellular wireless systems operate under harsh and challenging
channel conditions. The wireless channel is distinct and much more unpredictable
than the wireline channel because of factors such as multipath and shadow fading,
Doppler shift, and time dispersion or delay spread. These factors are all related to
variability that is introduced by the mobility of the user and the wide range of
environments that may be encountered as a result. Bandwidth of the signal could
increase or decrease leading to poor and/or missed reception. If maximum Doppler
shift is less than the data rate, there is slow fading channel. If maximum Doppler
shift is larger than the data rate, there is fast fading channel. In outer space, the path
between two antennas has no obstructions and no objects where reflections can occur.
Thus the received signal is composed of only one component. When the two antennas
are on the earth, however, there are multiple paths from the transmitter to the receiver.
The effect of the multiple paths is to change the path loss between two points. Several
empirical models have been suggested and used to predict propagation path losses.
We have discussed two widely used modelsthe Hata-Okumura model and the
Walfisch-Ikegami Model have also presented the models suggested for use in IMT-
2000 specifications.