10ec81- wireless communication

10EC81- Wireless Communication

Department of ECE, Canara Engineering College

UNIT- 7

Radio wave propagation and propagation models

EM waves below 2Mhz tend to travel as ground waves, These wave tend to follow the

curvature of the earth and lose strength rapidly as they travel away from the antenna. They do

not penetrate through ionospheric layers.

Frequencies between 2 to 30Mhz propagate as skywaves, Bouncing off of ionosheric layers,

these EM waves may propagate completely around the earth through multiple reflections or

hops between the ground and the ionosphere.

Frequencies above 30Mhz tend to travel in straight lines they are limited in their

propagation by the curvature of the earth.

Wave propagation effects at UHF and above

Reflection – Reflection occurs for EM waves incident upon some types of large

surface. For smooth surface the EM waves undergo a specular reflection, means that

the angle of incidence equals to the angle of reflection.

Scattering – scattering occurs when the signal is incident upon a rough surface or

obstacles smaller than the wavelength. This case produces diffuse reflection.

Diffraction – Diffraction causes EM waves to appear to bend around corners. EM

wave incident upon a sharp corner causes the generation of a weak point source that

can illuminate a shadow or area behind the object.



Path loss models for various coverage areas

Free space propagation model

PR = PTGTGR ( λ / 4Πd ) 2

GT GR Are the transmitting and receiving antenna gain

d is the distance between transmitting and receiving antenna

PR = Po / d2

Po received signal strength at a distance of 1 meter

Other path loss models



Two-ray model – This model assumes a direct LOS signal between the transmitter and the

receiver and another signal path that consists of a reflected signal of a flat surface of the

earth.

The composite signal received at the M.S will consists of EM waves that add either

constructively or destructively.

PR = PTGTGR ( hT2hR

2 / d

2 )

Path loss = 40 log d – (10 logGR + 10 log GT + 20 log hT + 20 log hR )

Multipath and Doppler effects

The models considered previously do not address real time fluctuation of the received signal

strength.

Multipath delay spreading leads to both time dispersion of the received signal and frequency

selective fading.

Error detection and correction coding

Error control coding is the term used to denote a technique that codes the transmitted bits in a

way that attempts to control the overall bit rate



Block codes may be used to determine whether an error has occurred during data

transmission. Schemes that use block codes to correct errors that might have occurred during

data transmission are known as forward error correction.

Convolutional and turbo encoders A continuous stream of bits is mapped into output

stream that possesses redundancy. The redundancy introduced depends upon incoming bits

and several of preceding bits. Turbo encoders are modified form of combined convolutional

encoders.

Block interleaving

It is a technique used by mobile wireless systems to combat the effect of bit errors introduced

during transmission of errors.

Examples of coding and interleaving

The error control code used by the system may be able to correct one bit error out of a block

of 8 bits, however it is not able to correct a burst of errors. If the bits of the block can be

interleaved with bits from the other blocks then in theory burst of errors can be spread out

over other six blocks



Digital Modulation Techniques

MSK- Continuous phase FSK similar to OQPSK. MSK is encoded with bits alternating

between quarternary components with Q component delayed by half symbol period.

BPSK encodes 0’s and 1’s as transmitter output signals with either 180 or 0 deg output

phases.

QPSK encodes information both in phase and amplitude of the transmitted signal.

OQPSK uses 4 different values of the phase to transmit.. In QPSK the phase of the signal

can jump as much 180 at a time , when the signal is LPF these phase shift results in large

amplitude fluctuations. By offsetting the timing of the odd and even bits by one bit period

or half symbol period, the Inphase and quadrature components will never change at the

same time. i.e it limits the phase shift by no more than 90 deg at a time. This yields much

lower amplitude fluctuations than non offset QPSK.

Π/4 QPSK uses 2 identical constellations which are rotated by 45 deg with respect to one

another. Usually either the even or odd symbols are used to select points from one of the



constellations and other symbols select points from other constellation . These also reduces

the phase shift from a maximum of 180 deg but only to a maximum of 135 deg.

OFDM

OFDM was chosen as modulation scheme supports high speed packet data transfer. It is the

modulation scheme for the IEEe 802.11a wireless standard

It is a form of multi carrier, multi symbol, multirate FDM in which user gets to use all the

FDM channels together.

It possess the property of orthogonality.

Spread Spectrum Modulation Techniques

Frequency hopping spread spectrum : FHSS consists of a system that changes the centre

frequency of transmission on a periodic basis in a pseudorandom sequence.

There are usually limited number of different carrier frequencies to hop to and the hopping

sequence is designed in such a fashion as to keep the occurrence of various hopping

frequencies statistically independent from one another.

Direct sequence spread spectrum

In DSSS a spreading code is applied to the baseband data stream at the transmitter and the

same spreading code is applied to the received signal to perform demodulation.

Diversity Techniques

Most effective technique that can be used to mitigate the effect of multipath fading is to

employ some form of time space or frequency diversity either or both the transmission and

reception of the desired signal.



The basic idea behind these solution s is that fading will not remain same over different

signal paths or over different frequencies over the same path.

RAKE receiver

To combat fading several innovative receiver implementations have been created. Multipath

signals will arrive at the receiver over the mobile radio channel , these receivers exploit that

fact by isolating the signal paths at the receiver.

If the fading of multipath signals is different then the isolation process yield the diverse

signals needed to improve receiver performance.

Rake receiver is used for the equalization of multipath



Smart antennas

Typical GSM System Hardware

Base station controller



Specific BSC parts

Group switch, sub-rate switch exchange/interface circuits, transcoder rate adaptation

unit, system control, power supply, and environmental conditioning unit

The exchange/ interface unit is basically mux/demux provide interconnections to the

MSC, PDN or RBS’s

The connection from the MSC to the BSC provides 64kbps PCM voice signals.

Once voice signals from the PSTN have been transcoded they are multiplexed

together and forwarded to the proper RBS over T1/E1/J1 at a much lower data rate.

Vocoded speech from the RBS must be transcoded to PCM and multiplexed before

being sent to the PSTN.

Radio base station

RBS is typically self contained unit that contains several subsystems that perform the

necessary operations to provide a radio link for the mobile subscriber.

Communication link exists between the RBS and BSC to provide transfer of user data and

network signaling functions.

Radio base station subsystems



- Distribution switch unit, timing and control, transmitter/receiver units, and combining and

distribution units

RBS transceiver unit

Signal processing and control subsystem, transmitter units, and receiver units



Answers to Problems and Questions

Chapter 8

Section 8.1

1. The function/purpose of a transmission line is to direct or guide a signal from one point to

another in the most efficient manner.

2. The most common techniques used to compensate for noise problems encountered when

transmitting digital information over conductor-based transmission lines is to use various

encoding schemes and transmission protocols that mitigate these effects.

3. Fiber-optic cables have several important advantages over conductor-based transmission

lines. The potential for almost unlimited bandwidth is one advantage and

another advantage is the lack of susceptibility to electromagnetic noise.

Section 8.2

4. To convert the range of frequencies from ELF to EHF (3 kHz to 300 GHz) to a

corresponding range of wavelengths, one needs to use the following equation:

cf

For 3 kHz,

8

5

3

3 101 10 100 km

3 10c

f

For 300 GHz,

8

3

9

3 101 10 1 mm

300 10c

f

5. The three EM wave propagation effects most likely to effect cellular wireless operation

are: reflection, scattering, and diffraction.

6. The EM wave propagation effect that can illuminate a shadow behind an object is known

as diffraction.



7. In the context of EM wave propagation, the term “multipath” refers to the fact that

typically for cellular frequencies many signals usually arrive at the receiver simultaneously

due to reflections from the environment.

8. Due to the many signals that arrive at the cellular receiver simultaneously due to various

propagation effects, use of the free space path loss model is inappropriate for wireless cellular

operation.

9. Using the free space path loss model, if the transmitted power is 600 mW at a frequency of

850 MHz, the received signal power at a distance of 5,000 meters

is given by:

First determine for 850 MHz, 6

6300 10 .353

850 10m

Then, assuming transmitting and receiving antenna gains of 1.0,

2 2

22

6

4

12

.353600

4 4 5000

.353600 600 5.621 10

6.28 10

.6 31.6 10 18.96

R T T RP P G G mWd

mW mW

W pW

This yields a received signal power in dBm of:

10

129

10 103

10 log1

18.96 1010log 10log 18.96 10

1 10

77.22

PSignal Power in dBm

mW

dBm

.

Finally, the path loss is given by:

T RPath Loss P P

Converting 600 mW into dBm yields,



10

3

10 103

10log1

600 1010log 10log 600 27.78

1 10

PSignal Power in dBm

mW

dBm

and, 27.78 77.22 105 T RPath Loss P P dB

10. The basic difference between the two-ray path loss model and the free-space path loss

model is that the two-ray model assumes an additional signal path for a reflected signal that

also reaches the receiver. This yields a different equation (see equation 8.4) for the

received signal power.

11. Using the distance-power gradient model with 4 the same conditions as

problem #9 yields:

First,0P , must be determined.

2 2

0

4

.353600

4 4 1

.6 7.896 10 .4738

T T RP P G G mWd

mW

Then

4

0 12

18

.4738.4738 5000

625 10

.758 10 151.2

R

mWP P d mW

W or dBm

And, the path loss is given as,

27.78 151.2 178.98 T RPath Loss P P dB

12. Using the distance-power gradient model with 3 the same conditions as

problem #9 yields:

First,0P , must be determined.

2 2

0

4

.353600

4 4 1

.6 7.896 10 .4738

T T RP P G G mWd

mW

Then



3

0 9

15

.4738.4738 5000

125 10

3.79 10 114.2

R

mWP P d mW

W or dBm

And, the path loss is given as,

27.78 114.2 141.98 T RPath Loss P P dB

Section 8.3

13. The basic operation of an ARQ scheme is as follows: the transmission protocol will

cause the system to call for a retransmission of a data packet when necessary (i.e. the bit error

rate is too high).

14. The basic purpose of a block code is to determine whether or not an error has occurred

during transmission.

15. When a 256-bit digital word is applied to a convolutional encoder with R = 1/3, a total of

3 256 768 output bits are produced.

16. The basic process involved in the block interleaving of data bits before transmission is as

follows: the bits of one data block are interleaved with the bits of another data block. In

theory, this process will provide an important advantage in the transmission of digital data. If

a noise burst causes several bit errors to occur, the bits effected will not all belong to the

same data block. Hence the errors are spread out and thus easier to correct.

Section 8.4

17. The basic advantage that digital modulation offers is bandwidth efficiency. Digital

modulation techniques make it easier to transmit more bits per Hz of bandwidth.

18. 8-PSK modulation is a process by which three bits are encoded per transmitted symbol.

Each symbol has a different phase angle from a reference phase angle.



19. An OFDM modulation system uses several to many carriers that are all transmitted

simultaneously. Each carrier transmits a sub-symbol that may encode one to many bits of

data. The entire transmitted symbol consists of the sum of all the sub-symbols.

20. The overall data rate for an OFDM system that transmits 32 kbps over each carrier and

uses 16 carriers is given by:

32 16 512 kbps kbps

Section 8.5

21. FHSS operation is where the carrier frequency is changed on a periodic basis in a

pseudorandom sequence.

22. DSSS operation is where a spreading code is applied to the baseband data stream at the

transmitter and the same spreading code is applied at the receiver to perform demodulation.

Section 8.6

23. Ultra-wideband radio technology uses extremely short or narrow pulses (sub-nanosecond)

with a correspondingly ultra-wideband frequency spectra to transmit information (data) over

relatively short distances (meters).

24. The typical type of radio pulse used by UWB is a short sub-nanosecond pulse.

Section 8.7

25. The basic theory behind the use of diversity in a wireless system is as follows: any fading

of the transmitted signal that occurs will not remain the same over time nor will it be the

same over different signal paths or be the same for different frequencies. Therefore, if some

form of time, space, or frequency diversity is used, the effects of signal fading can be

mitigated.



26. The basic theory behind the operation of a RAKE receiver is as follows: since there are

going to be multiple signals arriving at a receiver due to multipath propagation, this fact can

be exploited by isolating the signal paths at the receiver. Once the signals have been isolated

(they become the diverse signals), they may be used to improve the performance of the

receiver.

27. The usual implementation of space diversity for a wireless system is to use multiple

transmitting and/or receiving antennas. These antennas must be physically some distance

separate or apart from each other.

28. Polarization diversity is implemented for a cellular wireless system by using several

antennas with different polarizations.

Section 8.8

29. An RF combining unit performs the function of combining several RF signals. Typically,

several transmitter outputs may be applied to an RF combiner and the signal sources do not

interact with each other. A typical application is to use a combiner to allow several

transmitters to share the same antenna.

30. The distribution switch unit in a GSM radio base station serves the function of providing

correct timing to the system. User data being carried on a T-1 data link from the BSC needs

to be connected to the correct RBS transceiver and the correct timeslot for

transmission.

31. The purpose of a cellular duplex filter is to allow for the sharing of an antenna for both

transmitting and receiving.

32. Maintenance is usually performed on a modern cellular radio base station through the use

of an operations and maintenance terminal (OMT). Through a craft interface port connection,

the technician may access the RBS managed objects using OMT software. The OMT

software usually allows the technician to isolate the problem to a specific field replaceable



unit (FRU). Furthermore, the OMT software may be used to upgrade or re-provision the

RBS.

33. OMT software is the typical designation of software that can be used to troubleshoot

problems with a RBS system. This software can also be used to update the system

configuration.

10ec81- wireless communication

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