Wireless Data Networks
Lecture 2 – Wireless Communication Basics cont.
cont
Dr Chandimal Jayawardena

In mobile radio systems, unlike wired
networks, electromagnetic signals are
transmitted in free space.
The propagation of electromagnetic
waves in free space is extremely
complex. Depending on the frequency
and the corresponding wavelength,
electromagnetic waves propagate as
ground waves, surface waves, space
waves or direct waves
waves.
The type of propagation is correlated
with the range, or distance, at which a
signal can be received.
The general rule is that at a given
transmit power the higher the frequency
of the wave to be transmitted, the
shorter the range reached.

Free Space Propagation
Another factor that determines the range of
electromagnetic waves is their power.
The field strength of an electromagnetic wave
in free space decreases in inverse proportion
to the distance to the transmitter, and the
receiver input power therefore fades with the
square of the distance
distance.

Free Space Loss
FSL is given by the following equation,
where f is frequency in MHz and d is
q y
distance in km.

Two-Path Propagation over
Flat Terrain
Free-space propagation is of
little practical importance in
mobile communications,
communications
because in reality obstacles
and reflective surfaces will
always appear in the
propagation path.
Along with attenuation
caused by distance, a
radiated wave also loses
energy through reflection,
transmission and diffraction
due to obstacles.

Attenuation
Weather conditions cause changes to
the atmosphere, which in turn affect
the propagation conditions of waves.
Attenuation is frequency-dependent and
has a considerable effect on some
frequencies,
frequencies and a lesser one on others
others.

Fading
Fading refers to fluctuations in
the amplitude of a received
signal that occur owing to
propagation related
interference.
Multipath propagation caused
by reflection and the scattering
y g
of radio waves lead to a
situation in which transmitted
signals arrive phase-shifted over
paths of different lengths at the
receiver and are superimposed
there.
This interference can
strengthen, distort or even
eliminate the received signal.

Shadowing
Obstacles in the Line-of-Sight (LOS) path
between transmitter and receiver outdoors
(mountains and buildings) or inside buildings
(walls) hinder direct wave propagation and
therefore prevent th use of th shortest and
th f t the f the h t t d
in most cases least interfered (strongest)
path between transmitter and receiver and
receiver,
cause additional attenuation to the signal
level, which is called shadowing.
, g

Refraction

Reflection

Diffraction and Scattering
The spreading out of a wave around an obstacle is
called diffraction
When light hit
Wh li ht hits small particles a phenomenon called
ll ti l h ll d
scattering is possible

Signal-to-Interference Ratio
(SIR)
If the power of a received signal exceeds PminR then
the receiver can decode it and retrieve information.
However PminR is not the only requirement a wireless
However,
signal needs to meet for successful reception.
As the wireless medium is shared, other simultaneous
radio transmissions may occur. They have impact on
di t ii Th h i t
the possibility of successful radio reception.
To successfully receive a radio transmission, the
y ,
power level of the wanted signal must be several
degrees higher than any other interfering signal
arriving at the receiver at the same time.
g

Noise
Assuming a single transmission at a time, SIR
would always be infinite. However, in real
wireless system noise must be taken into
account.
It is constantly present. Noise does not
depend on the existence of other
simultaneous transmissions
transmissions.
Since it is unwanted power at the receiver it
can be treated as interference
interference.

Digital Modulation
In digital wireless communication
systems a discrete clock is used to
exchange symbols of fixed duration.
The modulation of a carrier signal in
one or more domains such as
frequency,
frequency phase and amplitude
provides the ability to carry multiple bits
within such a symbol.
symbol

Modulation Techniques

Carrier frequency
A carrier frequency is an electromagnetic
wave that is combined with the
information signal and carries it across
the communications channel.

Modulation Aspects and
Techniques
There are three aspects of the basic carrier wave that
can be modulated:
Amplitude
Frequency
Phase or angle
The three corresponding techniques are as follows:
Amplitude modulation (AM)
p ()
Frequency modulation (FM)
Phase modulation (PM)
Other modulation techniques
Amplitude shift keying (ASK) — Turning the amplitude all the way off
Frequency shift keying (FSK) — Hopping to an extreme frequency
Phase shift keying (PSK) — Shifting the phase 180 degrees

Amplitude Shift Keying (ASK)

Frequency Shift Keying (FSK)

Phase Shift Keying (PSK)

PSK Systems

DQPSK

Quadrature Amplitude
Modulation

Duplexing Schemes
In general, duplexing schemes provide
a separation of the send and receive
signals of a terminal or Subscriber
Station (SS).
Partitioning of the wireless medium by
duplex separation prohibits the self
self-
interference of a station so that it does
not receive its own transmitted signal
signal.

Time-Division Duplexing
The alternate transmitting
and receiving of data on a
single frequency channel is
referred t as Ti
f d to Time Di i i
Division
Duplexing (TDD).
In TDD, the times for
transmitting and receiving
are periodically alternating
as depicted in the figure.
Thus the uplink (UL) and
p ()
downlink (DL) directions are
separated in the time
domain.

Frequency-Division Duplexing
In the case of a
Frequency Division
Duplex (FDD) scheme,
scheme
the UL and DL of a
wireless network are
separated in the
frequency domain.
The stations transmit
and receive in different
frequency bands as
illustrated in the figure.

Multiplexing
Multiplexing serves to share the radio channel capacity between
competing stations.
A wireless medium seen as a transmission resource can be divided into
multiple dimensions, namely frequency, time, code and space.
Access to a multiplexed resource is by means of a multiple access rule
so that the multiplexing scheme and the multiple access rule are
strongly related. In addition, the duplexing scheme needs to be
specified to fully characterize a transmission system.
p y y
Today’s wireless communication protocols combine several multiple
access rules and duplexing schemes.
For instance the current enhancements of IEEE 802.16 that combine
TDD, TDMA with SDMA.
,
GSM can be regarded as an FDMA/TDMA/FDD system while one
version of UMTS combines for instance FDMA/CDMA/TDMA and TDD.

Multiplexing Schemes