wireless propagation

8/7/2019 Wireless Propagation

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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


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


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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The path losses predicted Walfisch-Ikegami model is greater than Hatas

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.

wireless propagation

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