Quick review t324_block1

Quick Review of T324 Block 1 | (Wireless Technologies) Page 1 of 8

Quick Review of T324 Block 1 | (Wireless Technologies)
• Radio waves are a particular form of electromagnetic radiation.
• Radio wave are electromagnetic waves with wavelength above 1 mm (corresponding to a frequency
of 300 GHz)
• Electromagnetic term is used because all waves involve the physics of electricity and magnetism.
• Other forms include light, X-rays and gamma rays.
• How an electromagnetic radio wave is produced? (Basis of all forms of radio communication).
1. By an k electric current in the transmitting antenna (aerial).
2. It spreads away and when part of this wave reaches a receiving antenna, a small electric signal is
set up in that antenna
3. The signal can then be amplified and processed so that any information it contains cab be
extracted.
• Period: the time between successive oscillations (cycles) is period of wave. And it is measured in
seconds.
• Frequency: the number of complete cycles which take place in seconds. And it is measured in hertz
(1Hz = 1 cycle per second).
• Wavelength: the distance between cycles. It is measured in meters (m) or fractions of a meter (m/s)
• The wave propagates away from the transmitter at a certain speed (c) {no vacuum or other matter}
and has value 299 792 458 (m/s). Inside any other form of matter the speed of propagation is less
than this (depends on matter and frequency wave).
• For radio waves in the atmosphere, the speed is only a little less than the figure for vacuum.
• Propagation speed through the atmosphere and c are often both taken to be equal to 300 000 000
m/s = 3 x 108 m/s.
• Relation between speed and wavelength and frequency:
• How do you know very important characteristics of the signal to understand it?
By knowing the scale of objects compared to the wavelength.
• How you can distinguish the different forms of electromagnetic wave?
By their frequency or wavelength. But wavelength is more commonly for electromagnetic waves
other than radio.
• Shorter wavelength are infrared radiation (1 mm – 10 µm), visible light (1 µm), ultraviolet, X-rays (10
nm – 10 pm) and gamma rays (100 pm – 1 fm).
• Radio waves are known as non-ionising radiation
• Gamma rays are produced by radioactive decay and by extreme events such as black hole collapse
and supernovae explosions.
• In the radio spectrum, in order to select the signal of interest, a receiver contains a filter which lets
through a narrow band of frequencies and attenuates all others.
• FM stands for frequency modulation, a technique for adding data to a wave.
• If two radio stations are transmitting on the same frequency or on two frequencies that are very
close then their signals get mixed up- there is interference between them which can be heard as
whistles, distortion or mixed-up sounds.
• FAT stands for frequency allocation table, is a very substantial (significant) document that defines
the allowed uses and licensing requirements of every part of the radio spectrum. It gives an

Disclaimer: this paper(s) is prepared by a AOU student of KSA, (Yaseen Alhashim). Moreover, I am not responsible of any mistake (if any). So, kindly
consider it as un-official paper & refer to the book for more information. You might notify me for any mistake by e-mail: yalhashim@gmail.com ,
http://alhashem.blogSpot.com | May, 2010


interesting picture of the full range of wireless applications that have to be planned for and
governed.

Bandwidth & Channels

• Radio waves are modulated by some form of data which could be digital or analogue.
• Radio signal is a modulated wave
• Bandwidth, the important characteristic of radio signals is that they spread over a range of radio
frequencies called bandwidth of the signal.
• What are the techniques ways to share parts of the radio spectrum?
1. Frequency Allocating Table (FAT)
2. Spread Spectrum
Instead of using individual channels, the separate signals all share the whole of the allocated
band of frequencies.
2.1. frequency-hopping spread spectrum (FHSS)
a. Each signal rapidly ‘hops’ between a large number of narrow channels in a
predetermined and unique sequence.
b. Each signal is sharing the same channel but is hopping in different sequence.
c. As long as the receiver knows this sequence, then each signal can be recovered
individually. E.g. Bluetooth
2.2. direct sequence spread spectrum (DSSS)
a. Each data bit in a signal is modulated by a unique and very fast binary code, which
causes every signal to occupy a wide frequency bandwidth.
b. If the receiver knows the code that has used to spread a signal, then this signal can be
picked out from all the others.

The propagation of radio signals

• Isotropic antennas: the transmitting and receiving antennas transmit or receive equally in all
directions called Isotropic antennas
• Decibel is a way to express a ratio of power such as received power/ transmitted power
• A key reason for using decibels is that when you have to combine two or more losses which are each
expressed in decibel, the total loss is simply the sum of the individual losses.
• An attenuation value per unit distance is often called an attenuation coefficient. To find the total
loss of a path in dB, you simply multiply this attenuation coefficient in dB/km by the path length in
km.
• Why the line of sight is often limited?
1. Curvature of the earth. The only direction of propagation that important is the one linking the
transmitter and receiver. It is represented by a the ray (by analogy with a ray of light)
2. Limiting line of sight. Is the presence of obstacles in the path such as mountains, forests,
buildings and vehicles.

• What are the effects which make radio communication (VHF, UHF, SHF and EHF) possible around
the globe, which can smooth out the effect of shadows?
1. Reflection 2. Scattering
2. Refraction 3. Diffraction



• Reflection is much less effective if the reflecting object is small than a wavelength
• Reflection from buildings and vehicles are much more important for the higher frequency bands
than for the lower ones.
• When the object is much smaller than a wavelength, then the wave is scattered.
• Along with reflection, diffraction is one of the main ways in which mobile phone signals propagate in
built up areas.
• If the transmitting antenna is below roof level, then diffraction occurs around the vertical corners of
buildings.
• If the antenna is mounted high up, then diffraction down from the edges of roofs to ground level
can be important.
• In an urban environment there may be no line of sight to the transmitter, but instead waves are
only received by reflection from buildings and from the ground, scattered from road signs or
growing leaves and diffracted around corners.
• The variation of signal power in time or in space is termed fading.
• Sky wave is a very important factor for communications in the MF and HF bands, and allows these
waves to travel right round the globe in a series of hops.
• You can receive far more stations at night than during the day, because the sky waves (ionosphere)
are dominant at night.
• Interference at night from distant stations is one of the main problems with medium wave
broadcasting.
• Surface wave which essentially propagates close to the surface of the ground. Because it follows the
curvature of the earth, it allows some radio signals to be propagated beyond the line of sight
horizon.
• Together the line of sight wave, the surface wave and any reflections from the ground are called
the ground wave.
• The international consortium called Digital Radio Mondiale (DRM), which is already broadcasting
using digital modulation techniques in the LF, MF and HF bands to minimize interference and loss
of service and increase the information capacity of channel.
• Transmitting Antenna: Radio waves are produced by an oscillating electric current in the
transmitting antenna.
• Receiving Antenna: Radio waves then go on to generate a small electric current in the receiving
antenna.
• How we can make the bandwidth of an antenna wider or narrower than that of a simple dipole?
By using more complicated designs and by connecting it to suitable electronic circuitry.

• Does the omni-directional radiation pattern antenna radiate equally in all three dimensions?
No, it is not isotropic antenna and does not radiate equally in all three dimensions because there is
very little signal radiated in the up and down directions i.e. length of the rod.

• The direction along which the electric field is oriented is called the polarisation of the wave.
• For radio wave propagating parallel to the ground, the case when the electric field is also parallel to
the ground is called horizontal polarisation.
• If the electric field is perpendicular to the ground then it is vertical polarisation.



• In a vertically polarized wave, the electric field is perpendicular to the ground and so the magnetic
field which is perpendicular to it is parallel to the ground.
• Polarisation diversity is another method to increase the data that can be carried b a given radio
channel.
• Mobile phone signals are usually transmitted from the base station in mixed polarisation. Can you
think of a reason why this enhances the quality of service for mobile phones?
The mixed polarisation ensures that reception is good at all angles.
People like to hold mobile phones at whatever angle is convenient for them. To have to hold a phone,
say, vertically for it work properly would be very unpopular.

• What are the four main problems being addressed by designers of wireless systems at the present
time?
1. Strong pressures in many systems to raise the data rate
2. Fading and other multi-path effects
3. Interference from other users
4. An increasing number of users competing for limited available bandwidth

• Methods which exploit the difference between signals sent or received by different antennas are
generally called diversity techniques.
• By adjusting the relative amplitudes and phases of transmitted signals in two or more antennas it is
possible to ensure that they constructively interfere at the receiver rather than cancel, this is called
beam steering.
• More benefits can be obtained by combining multiple transmitting and receiving antennas, these
are called MIMO technologies (multiple input-multiple output).
• A key aspect of MIMO is to transmit different data from each transmitting antenna – either by
splitting a data stream between them or by using data from independent sources. So each antenna
is transmitting different data, but on the same frequency as the other antennas.

Modulation, multiplexing and multiple access

All radio transmission uses electromagnetic radiation which carries energy outwards from a transmitter and
some of the energy is captured by the receiver.

PROBLEMS

• How to get the electromagnetic radiation to carry the data that needs to transmit?
• How to prevent each transmission from interfering with all the others that might be using the same part
of the electromagnetic spectrum at the same time?

Modulation provides a solution to both problems.

Modulation is supplies techniques for grafting data onto electromagnetic waves

• The key feature of modulated signal is that their spectrum is an entirely different frequency range from
that of the baseband signal. This is important for a variety of different specifications.



• Wireless communication required the signal to be located very precisely (exactly) within an allocated
frequency band i.e. radio broadcasting, tune the receiver.
• An important application of modulation is in modems used to transmit digital data over analogue
telephone lines. These modems have to fit the digital signal within the bandwidth of analogue
telephony (300 Hz to 3.4 kHz).
• In PSK (phase shift keying) are both segments a sinusoid of the same frequency and differ only in their
phase. They are invariably separated by 180°.°
• Because we shift the wave to the left or right by half a cycle of baseband, this is why the difference
between these two waves is described as 180°.
• What advantages would FSK and PSK have over the two amplitude modulation methods, namely OOK
and ASK? Activity 8, b1, p75
OOK and ASK modulation methods results in a change of amplitude. FSK and PSK do not change the
amplitude.

• Oscillator is an electronic circuit which provides a continuous sinusoidal output at a given frequency.
• ASK/OOK, PSK and FSK are therefore special cases (digital versions) of AM, PM and FM, respectively.
• We use the word ‘keying’ to emphasize that they are digital signals.
• Differential encoding is merged with binary PSK and the combination is described as differential phase
shift keying, DPSK. It can be described by the rule: if the data is a 1, change the phase by 180. If the data
is a 0, don’t change the phase.
• In DPSK, the receiver provides a 1 as the output if the phase changes between successive symbols in
the received waveform, and a 0 if the phase is the same between successive symbols.
• DPSK is used in the 802.11b Wi-Fi standard at the lower data rates, in Bluetooth system for connecting
peripherals wirelessly to computer or mobile phones.
• BPSK is the basic type of phase shift keying, which uses only two symbols (hence binary)
• What is the different/advantages between BPSK and quadrature QPSK ?
BPSK supplies only 1 bit per symbol and gives lower data rate. QPSK supplies 2 bits per symbol, and
tends to be associated with higher data rate.

• In FSK, a technique called ‘continuous phase’ can be used to smooth the transitions of frequency.
Frequency shift keying that uses this technique is called continuous-phase frequency shift keying
(CPFSK).
• A modulation scheme with parameters CPFSK with frequency spacing of 0.5S is called minimum shift
keying (MSK). Where S, is signaling rate
• Sidebands are the lobes on either side of the carrier frequency. Upper
sideband above the carrier frequency and a lower sideband below it.
• When power is distributed continuously across a band of frequencies, it
is not meaningful to speak of the power transmitted at any particular
frequency. It could 1 MHz, 1.01 MHz, there is infinite number of frequencies, so the power is not spread
equally, and this is why the vertical axis is labeled power density.
• The concept of cutoff frequency allows a more precise definition of the bandwidth of a signal or
channel.



• The modulation techniques that are widely used for wireless digital data transmission are quadrature
phase shift keying (QPSK) and quadrature amplitude modulation (QAM)

• Sinusoidal waves (of some frequencies) with a phase difference of quarter (or three-quarters) of a cycle
are said to be in a quadrature phase relationship.
• I wave, or in-phase wave, and the other Q wave, or quadrature wave
• QPSK can achieve twice the data rate of a comparable BPSK scheme for a given bandwidth.
• Sinusoids that have a quadrature phase relationship are said to be orthogonal.
• Spectral efficiency is a performance measure of how well a data transmission system uses the frequency
spectrum. It is measured in bit per second per hertz.

• Data rates and service ‘based on block1, pp101-102’
Service Data Rate
3G phone system 1 Mbit/s
HSDPA ‘high speed downlink packet access’ 10 Mbit/s
802.11b/g Wi-Fi 11 Mbit/s and 54 Mbit/s
802.11n 30 Mbit/s
WiMAX ‘Worldwide Interoperability for Microwave 70 Mbit/s
Access’ and Ultra wideband “for short distance”

• Transmitters on 2.4 GHz band (Wi-Fi & Bluetooth) are required not to exceed maximum allowed
transmission power.
• In Europe the maximum allowed power is 100 mW, and in USA the maximum power output is 1 W.
• Power density is a measure of the concentration of power in relation to frequency.
• Overlapping channels mean that service is likely to be impaired for users of the overlapping channels.
• Shannon, there is three-way relationship between channel capacity, bandwidth and signal-to-noise
ratio.
• Frequency-Hopped spread spectrum works by changing the carrier frequency of the modulated signal-
hopping from one frequency to another.
• Slow frequency hopping the signal modulates a carrier (e.g. OOK or PSK) at one frequency for several
symbols, then the carrier frequency changes and for the next few symbols the signal modulates the new
carrier.
• Benefit of frequency hopping
I. When the channel characteristics are better for some frequencies than others, for whatever
reason.
II. Frequency hopping signals are difficult to disrupt i.e. to ‘jam’ in military applications.
III. Offers some resistance to eavesdropping (listen in) because to follow the signal you have to
know the hopping sequence.
• Interval between frequencies hops is longer than the symbol period. That means the several symbols
are spread over a single frequency.
• Fast frequency hopping the carrier frequency changes at a rate faster than the date rate.
• Frequency hopping is a way of avoiding interference in Wi-Fi & Bluetooth frequency 2.4 GHz.
• CDMA code division multiple access, is widely used in some mobile telephony techniques.
• Multiple access, is the sharing of communication channel among many users.



• OFDM orthogonal frequency division multiplexing.
• Frequency division multiplexing (FDM) is generally described as a way of allowing several
communication channels to coexist within a particular section of frequency spectrum.
• OFDM uses many carriers called subcarriers. Each subcarrier is associated with its own stream of data.
• Benefit of OFDM is that it allows a high data rate to be achieved by increasing the bandwidth.
• Electromagnetic compatibility (EMC) is a standard for electrical goods, so they do not affect and are not
affected by other devices.
• Aspect to EMC
I. Emissions – there are limits to the amount of power a device is allowed to radiate at different
frequencies
II. Immunity – the device must function normally in the presence of radio waves up to a certain
power at different frequencies.

• Radio signal must be strong enough to compete with:
I. Unwanted transmissions from transmitters using the same frequency or band of frequency
II. Natural noise
III. Artificial noise from unrelated equipment

• List some of services where regulation has to be done at world level, rather than exclusively by a
national regulator like Ofcom.
Generally, any services which affect more than one country are likely to be subject to international
written agreement, for example:
I. Satellite services
II. Broadcasting
III. Radio astronomy
IV. Radio amateurs
V. Maritime and aviation bands
• List some of the factors that determine the level of emissions from the license holder.
I. Transmitter power
II. Number of transmitters
III. Transmitter location
IV. Where the transmitters are operated continuously or frequently
V. Propagation conditions

• Why 5 GHz band is being actively considered for radio local area network (RLAN)?
It is required large bandwidth and need to operate at gigahertz frequencies. They operate with low
power over a short range.

• Effective radiated power (ERP) is a measurement of transmitter power that is often used for setting
maximum limits, as it takes into account the directional properties of the antenna.
• Adjacent channel rejection is the ability of a receiver to filter out signals in the next frequency channel
above or below the channel it is tuned to receive.
• Ad hoc network are isolated groups of mobile stations that communicate with each other, and cannot
communicate with stations outside their radio range.



• Infrastructure networks allow a station to communicate with other stations outside its radio range. E.g.
Wi-Fi devices as Access points.
• What is the protocol in the context of a communication network?
Protocols are rules governing (control) the transmission and reception of data.
• TCP/IP layers
o Application layer (FTP, SMTP, HTTP)
Provide general services that are used by some users-to-user applications
o Transport layer (TCP connection-oriented, UDP connectionless)
Provides services related to end-to-end connection and that are independent of the types of network
o Internet layer (IP connectionless)
Managing the routing and forwarding of data across network
o Network access layer (PPP)
Provides a means of communicating with devices directly connected to a network

• The network access layers in the routers and bridge are split into two halves to indicate that different
protocols may be involved in the different links

• Bridge filters frames by reading the destination address in the frame. The bridge will only forward a
frame onto a connected collision domain if the frame is addressed to a computer on the opposite of the
bridge.
• Router uses information about the structure of the network as a whole, and it can be used to send frame
from one computer to any other on the LAN.
• What is the two main aspect of network access control in Warless network?
1. Sending data over a specific physical link
2. Providing a range of services to higher layers that control the way data is sent over a transmission
channel. This is called medium access control (MAC) sub-layer.

• What are the two complementary approaches in Wi-Fi networks?
• Carrier sense multiple access with collision avoidance (CSMA/CA)
• Connection-free period

• Network Security
» Availability of service (unavailable because of heavy traffic, hardware/software failures..)
» Confidentiality (user may which to keep the content of a message secret)
» Data integrity (protect data from transmission errors)
» Authentication (user may want to be sure that a received message was sent by the user whom concerned
» Non-repudiation (a user may wish to prevent another user denying sending a message that they
received)
• Main user requirement under threat for each of the four types of attack
A fabrication attack threatens the authentication requirement
An interception attack threatens the confidentiality requirement
An interruption attack threatens the availability requirement
An modification attack threatens the data integrity requirement


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