Wireless communication

Electromagnetic waves are formed when an electric field (shown as blue arrows) couples
with a magnetic field (shown as red arrows). The magnetic and electric fields of an
electromagnetic wave are perpendicular to each other and to the direction of the wave.
Electromagnetic waves travel VERY FAST – around 300,000 kilometres per second (the
speed of light).At this speed they can go around the world 8 times in one
second..
Radio transmission can take place using many different frequency bands, with each band
having certain advantages and disadvantages.
Frequency and wave length are directly coupled
λ= c/f
where wave length λ, speed of light c ≅3x108m/s, and frequency f

• Electromagnetic Spectrum—name for the range of
electromagnetic waves when placed in order of
increasing frequency
Each of these colors actually corresponds to a different wavelength of light.
Waves in the electromagnetic spectrum vary in size from very long radio waves
the size of buildings, to very short gamma-rays smaller than the size of the
nucleus of an atom.

Electromagnetic Spectrum
Radio waves are basically frequencies that travel at the speed of light and deliver
information to transmitters and receivers. Radio waves can easily suffer from interference
due to natural causes such as stars and gases that emit radio waves. They can propagate from
millimeters to thousands of miles, also the frequency ranges from 3KHz to 300GHz.

All electromagnetic waves travel at the same speed. (300,000,000 meters/second) in
a vacuum.
They all have different wavelengths and different frequencies.
Long wavelength-lowest frequency
Short wavelength highest frequency
The higher the frequency the higher the energy.

Things to remember
The higher the frequency, the more energy the wave has.
EM waves do not require media in which to travel or move.
EM waves are considered to be transverse waves because they are made of
vibrating electric and magnetic fields at right angles to each other, and to the
direction the waves are traveling.
Inverse relationship between wave size and frequency: as wavelengths get
smaller, frequencies get higher.

Frequencies and Regulations
 Radio frequencies are scarce resources.
 The International Telecommunications Union (ITU) located in Geneva is
responsible for worldwide coordination of telecommunication activities. {ITU-R
handles standardization in wireless sector.
 The ITU-R has split the world into three regions:
 Region-1 covers Europe, the Middle East, countries of soviet union and Africa
 Region-2 includes Greenland, North and South America
 Region-3 the Far East, Australia and New Zealand
 The ITU-R holds, the World Radio Conference (WRC), to periodically discuss and
decide frequency allocations for all three regions.

Signals
 Physical representation of data
 Function of time and location
 Signal parameters: parameters representing the value of data
Classification
 continuous time/discrete time
 continuous values/discrete values
 analog signal = continuous time and continuous values
 digital signal = discrete time and discrete values
Signal parameters of periodic signals:
period T, frequency f=1/T, amplitude A, phase shift ϕ
Sine wave is used as a special periodic signal for a carrier. The general function of
a sine wave is

Fourier representation of signals
Sine waves are of special interest because it is possible to
construct every periodic signal g by using only sine and cosine
functions according to a fundamental equation of Fourier
The c determines the Direct Current (DC) component of the
signal, the coefficient an and bn are the amplitudes of the nth
sine and cosine function.

Signal Representation
Different representations of signals
 amplitude (amplitude domain)
 frequency spectrum (frequency domain)
 constellation diagram (amplitude M and phase ϕ in polar coordinates)
A tool to display frequencies is a spectrum analyzer. Fourier transformations are
a mathematical tool for translating from the time domain into the frequency
domain and vice versa (using the inverse Fourier transformation).
 The x-axis represents a phase of 0 and is also called In-Phase (I).
 A phase shift of 90° or π/2 would be a point on the y-axis, called Quadrature
(Q).

Signal Propagation
Transmission range
• communication possible low
error rate
Detection range
• detection of the signal possible
• no communication possible,
high error rate
Interference range
• signal may not be detected
• signal adds to the background
noise
• Propagation in free space always like light (straight line)
• Receiving power proportional to 1/d² where, d = distance between
sender and receiver – Inverse Square Law.
• Free Space Loss
• Path Loss or Attenuation

Signal Propagation
Radio wave propagation is affected by the following
mechanisms:
 fading (frequency dependent)
 shadowing
 reflection at large obstacles
 refraction depending on the density of a medium
 scattering at small obstacles
 diffraction at edges

Multipath propagation
Signal can take many different paths between sender and receiver due to
reflection, scattering, diffraction.

Multipath propagation
Due to the finite speed of light, signals travelling along different paths with
different lengths arrive at the receiver at different times. This effect
(caused by multi-path propagation) is called delay spread: the original
signal is spread due to different delays of parts of the signal.
The energy intended for one symbol now spills over to the adjacent
symbol, an effect which is called intersymbol interference (ISI). The higher
the symbol rate to be transmitted, the worse the effects of ISI will be, as
the original symbols are moved closer and closer to each other.
In case of mobile senders and receivers, the paths the signal can travel
varies, the power of the received signal changes considerably over time.
These quick changes in the received power are also called short-term
fading. Depending on the different paths the signals take, these signals
may have a different phase and cancel each other. An additional effect is
the long-term fading of the received signal. This long-term fading is caused
by varying distance to the sender or more remote obstacles.

Multiplexing
Multiplexing means combining multiple streams of information for transmission over
a shared medium. Demultiplexing performs the reverse function, split a combined
stream arriving from a shared medium into the original information streams. For
wireless communication, multiplexing can be carried out in four dimensions: space,
time, frequency, and code.

Frequency division multiplexing (FDM) describes schemes to subdivide the frequency
dimension into several non- overlapping frequency bands. Each channel is now
allotted its own frequency band as indicated. Senders using a certain frequency band
can use this band continuously.
Time division multiplexing (TDM) is a more flexible scheme where a channel is given
the whole bandwidth for a certain amount of time, i.e., all senders use the same
frequency but at different points in time.
Code division multiplexing (CDM) is a relatively new scheme in commercial
communication systems. All channels use the same frequency at the same time for
transmission. Separation is now achieved by assigning each channel its own ‘code’.

Modulation
The two types of signals used in a telecommunication network are-
1. Analog 2. Digital
Analog signals take an infinite number of values in a range.
Digital signals take a limited number of values in a range.
(a) Analog signal (b) Digital signal
Modulation is the process of varying one or more properties
of a high frequency periodic waveform, called the carrier
signal, with respect to a modulating signal.
In modulation, a message signal, which contains the information is
used to control the parameters of a carrier signal, so as to impress
the information onto the carrier.

Types of Modulation
Digital data, digital signals: simplest form of digital encoding of digital data
Digital data, analog signal: A modem converts digital data to an analog signal so
that it can be transmitted over an analog
Analog data, digital signals: Analog data, such as voice and video, are often
digitized to be able to use digital transmission facilities
Analog data, analog signals: Analog data are modulated by a carrier frequency to
produce an analog signal in a different frequency band, which can be utilized on
an analog transmission system
There are two basic types:- Analog modulation and Digital modulation
The main difference between analog modulation and digital modulation is in the
manner that they transmit data. With analog modulation, the input needs to be in
the analog format, while digital modulation needs the data in a digital format.

Analog Modulation
Analog modulations techniques are as follows
AM: The carrier amplitude is changed according to the information signal.
FM: The carrier frequency is changed according to the information signal.
PM: The carrier phase is changed according to the information signal.
Applications: Radio and television broadcast stations use AM and FM form of
modulation.

Digital Modulation
In digital modulation, an analog carrier signal is modulated by a digital bit
stream. Digital modulation methods can be considered as digital-to-analog
conversion, and the corresponding demodulation or detection as analog-to-
digital conversion.
Digital modulation modulates three parameters of sinusoidal signal. The three
parameters are Amplitude A, phase θ, and frequency f.
s(t) = A·cos( 2π·fc·t + θk )
There are three type digital modulation.
ASK: Amplitude Shift Keying
FSK: Frequency Shift Keying
PSK: Phase Shift Keying

Radio Transmitter & Receiver
Apart from the translation of digital data into analog signals, wireless
transmission requires an additional modulation, an analog modulation
that shifts the center frequency of the baseband signal generated by the
digital modulation up to the radio carrier. The main reasons are length of
the antennas, Frequency division multiplexing and the medium
characteristics.
The Digital Modulation schemes discussed vary in spectral efficiency,
power efficiency and robustness.

Amplitude Shift Keying (ASK)
Amplitude-shift keying (ASK) is a form of amplitude modulation that
represents digital data as variations in the amplitude of a carrier wave.
Like AM, ASK is also linear and sensitive to atmospheric noise, distortions,
propagation conditions on different routes in PSTN, etc. Both ASK modulation
and demodulation processes are relatively inexpensive. The ASK technique is
also commonly used to transmit digital data over optical fiber.

Frequency Shift Keying (FSK)
The simplest form of FSK, also called binary FSK (BFSK),
assigns one frequency f1 to the binary 1 and another frequency f2
to the binary 0.
To avoid sudden changes in phase, special frequency modulators with
continuous phase modulation, (CPM) can be used. Sudden changes in phase
cause high frequencies, which is an undesired side-effect.
FSK needs a larger bandwidth compared to ASK but is much less susceptible
to errors.

Phase Shift Keying (PSK)
Phase-shift keying (PSK) is a digital modulation scheme that conveys data by
changing, or modulating, the phase of a reference signal (the carrier wave).
PSK uses a finite number of phases; each assigned a unique pattern of binary
digits.
To receive the signal correctly, the receiver must synchronize in frequency
and phase with the transmitter. This can be done using a phase lock loop
(PLL). Compared to FSK, PSK is more resistant to interference, but receiver
and transmitter are also more complex

Advanced FSK- Minimum Shift Keying
• Bandwidth needed for FSK depends on the distance between the
carrier frequencies.
• Special pre-computation avoids sudden phase shifts

 MSK is basically BFSK without abrupt phase changes, i.e., it belongs
to CPM schemes.
 Data bits are separated into even and odd bits, the duration of each bit
being doubled.
 Depending on the bit values (even, odd) the higher or lower frequency,
original or inverted is chosen
 The scheme also uses two frequencies: f1, the lower frequency, and f2,
the higher frequency, with f2 = 2f1, i.e. the frequency of one carrier is
twice the frequency of the other

if the even and the odd bit are both 0, then the higher frequency f2 is inverted (i.e.,
f2 is used with a phase shift of 180°)
if the even bit is 1, the odd bit 0, then the lower frequency f1 is inverted. This is the
case, e.g., in the fifth to seventh columns of figure.
if the even bit is 0 and the odd bit is 1, as in columns 1 to 3, f1 is taken without
changing the phase,
 if both bits are 1 then the original f2 is taken.
A high frequency is always chosen if even and odd bits are equal. The signal is inverted if
the odd bit equals 0.
Adding a so-called Gaussian low pass filter to the MSK scheme results in Gaussian
MSK (GMSK), which is the digital modulation scheme used for many European
wireless standards like GSM, DECT etc.
GMSK is a spectrally efficient modulation scheme and is particularly useful in
mobile radio systems. It has a constant envelope, spectral efficiency, good BER
performance.

Advanced Phase Shift Keying
In M-ary or multiple phase-shift keying (MPSK), there are more than two phases, usually
four (0, +90, -90, and 180 degrees) or eight (0, +45, -45, +90, -90, +135, -135, and 180
degrees). If there are four phases (m = 4), the MPSK mode is called quadrature phase-
shift keying or quaternary phase-shift keying (QPSK), and each phase shift represents
two signal elements.
If there are eight phases (m = 8), the MPSK mode is known as octal phase-shift keying
(OPSK), and each phase shift represents three signal elements. In MPSK, data can be
transmitted at a faster rate, relative to the number of phase changes per unit time, than
is the case in BPSK.

Quadrature Amplitude Modulation
 Quadrature Amplitude Modulation (QAM): combines amplitude
and phase modulation
 it is possible to code n bits using one symbol
 2n discrete levels, n=2 identical to QPSK
 Bit error rate increases with n, but less errors compared to
comparable PSK schemes

16-QAM 64-QAM
QAM is used in many radio communications and data delivery applications.
For domestic broadcast applications for example, 64 QAM and 256 QAM are often used in
digital cable television and cable modem applications. In the UK, 16 QAM and 64 QAM are
currently used for digital terrestrial television using DVB - Digital Video Broadcasting. In the US,
64 QAM and 256 QAM are the mandated modulation schemes for digital cable as standardized
by the SCTE in the standard ANSI/SCTE 07 2000.
In addition to this, variants of QAM are also used for many wireless and cellular technology
applications.

• Multi-carrier modulation (MCM) is a method of
transmitting data by splitting it into several
components, and sending each of these
components over separate carrier signals. The
individual carriers have narrow bandwidth , but
the composite signal can have broad bandwidth.
• The advantages of MCM include relative immunity
to fading caused by transmission over more than
one path at a time (multipath fading), less
susceptibility than single-carrier systems to
interference caused by impulse noise, and
enhanced immunity to inter-symbol interference.
Limitations include difficulty in synchronizing the
carriers under marginal conditions, and a
relatively strict requirement that amplification be
linear.
• MCM was first used in analog military
communications in the 1950s. Recently, MCM has
attracted attention as a means of enhancing the
bandwidth of digital communications over media
with physical limitations. The scheme is used in
some audio broadcast services.

In telecommunication and radio communication, spread-spectrum techniques
are methods by which a signal (e.g. an electrical, electromagnetic, or acoustic
signal) generated with a particular bandwidth is deliberately spread in the
frequency domain, resulting in a signal with a wider bandwidth.
• Spread spectrum is an technique used in radio transmission based on the
concept that the narrowband signal is manipulated (scrambled) prior to
transmission in such a way that the signal occupies a much larger part of the
RF spectrum than strictly needed. This makes the signal more robust against
interference and jamming.
• The manipulation requires a pseudo random noise code which, in the
original concept, was only known to the parties at each end of the radio
connection. Spread spectrum technology was invented in the 1940s, and has
been used extensively since then for military and other applications that
require robustness and resistance to jamming or eavesdropping.

 Because Spread Spectrum signals are noise-like, they are hard to detect.
 Spread Spectrum signals are harder to jam (interfere with) than
narrowband signals
 Spread Spectrum transmitters use similar transmit power levels to narrow
band transmitters.
 The Ability to Selectively Address. If we are clever about how we
spread the signal, and use the proper encoding method, then the signal can
only be decoded by a receiver which knows the transmitter's code.
 Bandwidth Sharing. If we are clever about selecting our modulation
codes, it is entirely feasible to have multiple pairs of receivers and
transmitters occupying the same bandwidth.
 Security from Eavesdropping. If an eavesdropper does not know the
modulation code of a spread spectrum transmission, all the eavesdropper
will see is random electrical noise rather than something to eavesdrop

(i) Original signal to be transmitted.
(ii) The sender spreads the signal and converts the narrow-band
signal to broadband (Power level can be much lower without
losing data)
(iii) During transmission, narrow and broadband noise gets added.
(iv) The receiver despreads the given signal, narrow band
interference is spread, leaving the broadband as it is.
(v) Receiver applies a band pass filter cutting off left & right of
narrow band signal.

Spreading the spectrum can be achieved in two different ways
Direct Sequence Spread Spectrum (DSSS)
 Data signal is multiplied by a spreading code, and resulting signal
occupies a much higher frequency band
 Spreading code is a pseudo-random sequence
Frequency Hopping Spread Spectrum (FHSS)
 Data signal is modulated with a narrowband signal that hops from
frequency band to frequency band, over time
 The transmission frequencies are determined by a spreading, or
hopping code (a pseudo-random sequence)

 DS modulation is achieved by modulating the carrier wave with a
digital code sequence which has a bit rate much higher than that
of the message to be sent. This code sequence is typically a
pseudorandom binary code (often termed "pseudo-noise" or PN),
specifically chosen for desirable statistical properties.
 The time period of a single bit in the PN code is termed a chip,
and the bit rate of the PN code is termed the chip rate.
tc = Chip Period
tb = Bit Period
Spreading factor S = tb/tc
S*original bandwidth is the new bandwidth.
It determines the BW of the resulting signal

XOR of the signal with pseudo-random number (chipping
sequence)
many chips per bit (e.g., 128) result in higher bandwidth of the signal
• Most civil applications need a
spreading factor of 10 to 100.
• Military applications use a
speeding factor of around
10,000.
• Barker codes exhibit a good
robustness against interference
and insensitivity to multipath
propagation. 10110111000
(802.11 wireless LANS), 11, 110,
1110, 11101, 1110010,
1111100110101

Let the code to be transmitted be 110.
Let the Chip Barker Code be
10110111000
Hence the transmitted code is:
11111111111 11111111111 00000000000 XOR
10110111000 10110111000 10110111000
- 01001000111 01001000111 10110111000
At the Receiver : The transmitted signal is XORed with the same chip
sequence.
01001000111 01001000111 10110111000
XOR 10110111000 10110111000 10110111000
Resulting in :
11111111111 11111111111 00000000000
This is the original signal 110

 The DSSS receiver is more complex than the transmitter. Noise and multi-path
propagation require additional mechanisms to reconstruct the original data
along with demodulation.
 The receiver has to know the original chipping sequence, i.e., the receiver
basically generates the same pseudo random sequence as the transmitter.
During a bit period, which also has to be derived via synchronization, an
integrator adds all these products.
 Calculating the products of chips and signal, and adding the products in an
integrator is also called correlation, the device a correlator. Finally, in each bit
period a decision unit samples the sums generated by the integrator and
decides if this sum represents a binary 1 or a 0.
 A rake receiver uses n correlators for the n strongest paths. Each correlator is
synchronized to the transmitter plus the delay on that specific path. As soon as
the receiver detects a new path which is stronger than the currently weakest
path, it assigns this new path to the correlator with the weakest path. The
output of the correlators are then combined and fed into the decision unit

 Total available BW is split into many channels of smaller BWs
 Transmitter & Receiver stay in one of these channels and hop to
another channel
 The implementation is a combination of FDM & TDM
 Hopping sequence defines the transition sequence amongst channels
 Dwell Time : Time spent on a channel with certain frequency
 The bandwidth of a frequency hopping signal is simply w times the
number of frequency slots available, where w is the bandwidth of each
hop channel.
 In Slow Hopping FHSS, one frequency is used for several bit periods
 In Fast Hopping FHSS, transmitter changes frequency several times
during single bit transmission.
 Slow Hopping FHSS are not as immune to narrow band interference
as the fast hopping FHSS

 Spreading is simpler in FHSS
 FHSS uses only a portion of the Bandwidth at any given time.
 DSSS are more resistant to fading and multi-path effects.
 DSSS signals are had to detect in the absence of the
knowledge of spreading code.

Implements space division multiplex: base station covers a certain
transmission area (cell)
Mobile stations communicate only via the base station
Advantages of cell structures:
 higher capacity, higher number of users
 less transmission power needed
 more robust, decentralized
 base station deals with interference, transmission area etc.
locally
Problems:
• fixed network needed for the base stations
• handover (changing from one cell to another) necessary
• interference with other cells
Cell sizes from some 100 m in cities to, e.g., 35 km on the country side
(GSM) - even less for higher frequencies

4 cell cluster with 3 sector antennas
• To avoid interference, different
transmitters within each other’s
interference range use FDM.
• Cells are combined in clusters – on the
left side three cells form a cluster, on the
right side seven cells form a cluster. All
cells within a cluster use disjointed sets of
frequencies.
• To reduce interference even further,
sectorized antennas can be used. The
figure shows the use of three sectors per
cell in a cluster with four cells.

• Fixed Channel Allocation (FCA) means fixed assignment of
frequencies to cell clusters and cells respectively which is not very
efficient if traffic load varies. It is used in GSM and requires careful
traffic analysis
• Fixed allocation of frequency channels do not optimize channel
usage, if the traffic pattern is varying.
• Cells with higher traffic pattern can borrow frequencies from its
neighbors (which do not carry heavy traffic) and this scheme is
termed BCA (Borrowing Channel Allocation)
• In DCA (Dynamic Channel Allocation), channels (frequencies) will
be allocated dynamically. Frequencies can only be borrowed. The
borrowed frequency can be blocked for the surrounding cells, to
reduce interference.

• CDM cells are commonly said to ‘breathe’.
• While a cell can cover a larger area under a light load, it
shrinks if the load increases. The reason for this is the growing
noise level if more users are in a cell.
• CDM systems: cell size depends on current load
• Additional traffic appears as noise to other users
• If the noise level is too high users drop out of cells

Wireless communication

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