CHAPTER four

Wireless Networks Principles

1 Wireless Network Principles

Chapter Contents
 Wireless networks basics,
 Wireless communications,
 Applications,
 Simple reference model,
 Wireless transmissions,
 Frequencies
 Signal,
 Signal Propagation,
 Antennas,
 Multiplexing,
 Modulation
 Media Access Control
2 Wireless Network Principles
Wireless Network Basics
 WLAN uses radio waves to connect devices to the
Internet
 A wired network uses cables to connect devices to the
Internet
 The most common wired networks use cables
connected to Ethernet ports on network router on one
end and to a computer or other device on the cable's
opposite end.

3 Wireless Network Principles

 Wireless Communications
 Wireless communication is the transfer of information between
two or more points that are not connected by an electrical
conductor.
 Wireless communication generally works through
electromagnetic signals that are broadcast by an enabled device
within the air, physical environment or atmosphere.
 The way of accessing a network or other communication
partners i.e. without a wire .
 The wire is replaced by the transmission of electromagnetic
waves through ‘the air’

4 Wireless Network Principles

What Is a Wireless Network?: The Benefits
 Convenience:
Access network resources from any location
 Productivity:

Get the job done and encourages collaboration.

 Easy setup:

No cables, so installation can be quick and cost effective.

 Expandable:

Easily expand wireless networks with existing equipment

 Security:

Provide robust security protections.

 Cost:

Eliminate or reduce wiring costs, less cost to operate

5 Wireless Network Principles

Applications
 Vehicles
 Emergencies
 Business
 Replacement of wired networks
 Infotainment
 Mobile and Wireless devices
 Sensor (Sensing the Door)
 Embedded controllers (keyboard, washing machines)
 Pager (Display short message)
 Mobile Phones (Migrate, Color graphic display touch screen)
 Personal Digital assistant (Accompany calendar, notepad)
 Pocket computer
 Notebook / Laptop
6 Wireless Network Principles
Simple Reference Model

Application Application

Transport Transport

Network Network Network Network

Data Link Data Link Data Link Data Link

Physical Physical Physical Physical

Radio Medium

7 Wireless Network Principles

Wireless Transmission
 The frequencies used for transmission are all regulated.
 Multiplexing is used because the medium is always shared.
 Multiplexing schemes have to ensure low interference
between different sender.
 Modulation is needed to transmit digital data via certain
frequencies.

8 Wireless Network Principles

 Frequencies - Spectrum Allocation
twisted coax cable
pair optical transmission

1 Mm 10 km 100 m 1m 10 mm 100 1 m
300 30 kHz 3 MHz 300 MHz 30 GHz m 300
Hz 3 THz THz

VLF LF MF HF VHF UHF SHF EHF infrared visible UV

light
VLF = Very Low Frequency UHF = Ultra High Frequency
LF = Low Frequency SHF = Super High Frequency
MF = Medium Frequency EHF = Extra High Frequency
HF = High Frequency UV = Ultraviolet Light
VHF = Very High Frequency

Relationship between frequency ‘f’ and wave length ‘’ :

 = c/f
where c is the speed of light  3x108m/s

9 Wireless Network Principles

Frequencies Allocated for Mobile Communication
 VHF & UHF ranges for mobile radio
 allows for simple, small antennas for cars
 deterministic propagation characteristics
 less subject to weather conditions –> more reliable connections
 SHF and higher for directed radio links, satellite
communication
 small antennas with directed transmission
 large bandwidths available
 Wireless LANs use frequencies in UHF to SHF spectrum
 some systems planned up to EHF
 limitations due to absorption by water and oxygen molecules
 weather dependent fading, signal loss caused by heavy rainfall, etc.

10 Wireless Network Principles

 Band chart
MF (Medium Frequency) 300-3000 kilohertz
HF (High Frequency) 3 - 30 megahertz
VHF (Very High Frequency) 30-300 megahertz
UHF (Ultra High Frequency) 300-3000 megahertz
SHF (Super High Frequency) 3-30 gigahertz
EHF (Extremely High Frequency) Anything above 30 gigahertz
 Common amateur activity falls into three major
bands: HF, VHF, and UHF bands. 

11 Wireless Network Principles

Signal
 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 
 sine wave as special periodic signal for a carrier:

s(t) = At sin(2  ft t + t)

12 Wireless Network Principles

 Signal propagation
 Radio wave propagation (Receiving power) is influenced by
 Shadowing induced by obstacles in path between the transmitted & receiver
 reflection at large obstacles
 refraction depending on the density of a medium
 scattering at small obstacles
 diffraction at edges
 pathloss due to distance covered by radio signal (frequency dependent, less
at low frequencies)
 fading (frequency dependent, related to multipath propagation)

shadowing reflection refraction scattering diffraction

Signal propagation ranges
 Transmission range
 communication possible
 low error rate
 Detection range
 detection of the signal sender

possible
transmission
 no communication
possible distance
detection
 Interference range
interference
 signal may not be
detected
 signal adds to the
background noise

14 Wireless Network Principles

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

signal at sender
signal at receiver

 Positive effects of multipath:

 enables communication even when transmitter and receiver are not in LOS
conditions - allows radio waves effectively to go through obstacles by getting
around them thereby increasing the radio coverage area

15 Wireless Network Principles

 Cont’d
 Negative effects of multipath:
 Time dispersion or delay spread: signal is dispersed over time due
signals coming over different paths of different lengths
 Causes interference with “neighboring” symbols, Inter Symbol
Interference (ISI)

multipath spread (in secs) = (longest1 – shortest2)/c

For a 5ms symbol duration a 1ms delay spread means about a 20%
intersymbol overlap.
 The signal reaches a receiver directly and phase shifted (due to
reflections)
 Distorted signal depending on the phases of the different parts,
this is referred to as Rayleigh fading, due to the distribution of the
fades. It creates fast fluctuations of the received signal (fast fading).
 Random frequency modulation due to Doppler shifts on the different
paths. Doppler shift is caused by the relative velocity of the receiver to
the transmitter, leads to a frequency variation of the received signal.
16 Wireless Network Principles
 Effects of Mobility
 Channel characteristics change over time and location
 signal paths change
 different delay variations of different signal parts
 different phases of signal parts

 quick changes in the power received (short term fading)

long term
power
fading

Additional changes in
 distance to sender
 obstacles further away
t
short term fading
 slow changes in the average power
received (long term fading)

17 Wireless Network Principles

 Antennas
 Antennas are used to radiate and receive EM waves (energy)
 Antennas link this energy between the ether and a device
such as a transmission line
 Antennas consist of one or several radiating elements
through which an electric current circulates
 Electromagnetic waves propagate along transmission lines
and through space.
 The antenna is the interface between these two media and
is a very important part of the communication path.
 First, antennas are passive devices.
 Therefore, the power radiated by a transmitting antenna
cannot be greater than the power entering from the
transmitter.
18 Wireless Network Principles
 Cont’d
 In fact, it is always less because of losses.
 We will speak of antenna gain, but remember that gain in
one direction results from a concentration of power and is
accompanied by a loss in other directions.
 Antennas achieve gain the same way a flashlight reflector
increases the brightness of the bulb: by concentrating
energy.
 The second concept to keep in mind is that antennas are
reciprocal devices;
 that is, the same design works equally well as a transmitting
or a receiving antenna and in fact has the same gain.
 In wireless communication, often the same antenna is used
for both transmission and reception.
19 Wireless Network Principles
Cont’d

 Essentially, the task of a transmitting antenna is to

convert the electrical energy travelling along a
transmission line into electromagnetic waves in space.
 At the receiving antenna, the electric and magnetic
fields in space cause current to flow in the
conductors that make up the antenna.

20 Wireless Network Principles

Types of Antennas
 Types of antennas:
 Omnidirectional
 Directional
 Phased arrays
 Adaptive
 Optimal
 Principal characteristics used to characterize an
antenna are:
 Radiation pattern
 Directivity
 Gain
 Efficiency

21 Wireless Network Principles

Omnidirectional Antennas: simple dipoles
 Real antennas are not isotropic radiators but, e.g., dipoles with
lengths /4, or Hertzian dipole: /2 (2 dipoles)
 shape/size of antenna proportional to wavelength

/4
/2

 Example:
y
Radiation pattern
y
of a simple Hertzian
z
dipole

simple
x z x dipole
side view (xy-plane) side view (yz-plane) top view (xz-plane)

 Gain: ratio of the maximum power in the direction of the main lobe
to the power of an isotropic radiator (with the same average power)

22 Wireless Network Principles

Cont’d

 Simple Antennas
 The simplest antenna, in terms of its radiation
pattern, is the isotropic radiator
 it has zero size, is perfectly efficient, and radiates
power equally in all directions.
 the isotropic radiator makes a good reference with
which to compare the gain and directionality of other
antennas. That is because, even though this antenna
cannot be built and tested, its characteristics are
simple and easy to derive.

23 Wireless Network Principles

Half-wave dipole antenna
 on the other hand, is a simple, practical antenna
which is in common use.
 An understanding of the half-wave dipole is
important both in its own right and as a basis for the
study of more complex antennas.

24 Wireless Network Principles

Cont’d
 The word dipole simply means it has two parts, as
shown.
 A dipole antenna does not have to be one-half
wavelength in length like the one shown in the figure,
but this length is handy for impedance matching, as
we shall see.
 A half-wave dipole is sometimes called a Hertz
antenna, though strictly speaking the term Hertzian
dipole refers to a dipole of infinitesimal length.

25 Wireless Network Principles

Cont’d

 Typically the length of a half-wave dipole, assuming

that the conductor diameter is much less than the
length of the antenna, is 95% of one-half the
wavelength measured in free space.
 the free- space wavelength is given by
λ =c/ƒ
where
λ = free-space wavelength in meters
c = 300 × 10^6 m/s
ƒ = frequency in hertz

26 Wireless Network Principles

Directional Antennas
 Often used for microwave connections (directed point to point
transmission) or base stations for mobile phones (e.g., radio
coverage of a valley or sectors for frequency reuse)
y y z
directed
x z x antenna

side view (xy-plane) side view (yz-plane) top view (xz-plane)

z
z

x
sectorized
x antenna

top view, 3 sector top view, 6 sector

27 Wireless Network Principles

Isotropic Antennas
 Isotropic radiator: equal radiation in all directions
(3D) - only a theoretical reference antenna
 Real antennas always have directive effects (vertical
and/or horizontal)
 Radiation pattern: measurement of radiation around
an antenna

y z
ideal
isotropic
x radiator

28 Wireless Network Principles

Array Antennas
 Grouping of 2 or more antennas to obtain radiating
characteristics that cannot be obtained from a single
element
 Antenna diversity
 switched diversity, selection diversity
 receiver chooses antenna with largest output
 diversity combining
 combine output power to produce gain
 cophasing needed to avoid cancellation
/2 /2
/4 /2 /4 /2

+
+
ground plane

29 Wireless Network Principles

Mobile & Portable Antennas
 The portable and mobile antennas used with cellular
systems have to be omnidirectional and small,
especially in the case of portable phones.
 Easier to achieve at 1900 MHz than at 800 MHz.
 Many PCS phones must double as 800-MHz cell
phones, however, so they need an antenna that works
well at 800 MHz.

30 Wireless Network Principles

Cont’d
 The simplest suitable antenna is a quarter-wave
monopole, and these are the usual antennas supplied
with portable phones.
 For mobile phones, where compact size is not quite as
important, a very common configuration consists of a
quarter-wave antenna with a half-wave antenna
mounted collinearly above it.

31 Wireless Network Principles

 Multiplexing
 Whenever the bandwidth of a medium linking two devices is
greater than the bandwidth needs of the devices, the link can
be shared.
 Multiplexing is the set of techniques that allows users
simultaneous transmission of multiple signals across a
single data link. The users are mobile and the transmission
resource is the radio spectrum. Sharing a common
resource requires an access mechanism that will control
the multiplexing mechanism.
 As data and telecommunications use increases, so does traffic.
 A Multiplexer (MUX) is a device that combines several signals
into a single signal.
 Demultiplexer (DEMUX) is a device that performs the inverse
operation.
32 Wireless Network Principles
Types Multiplexing
 Multiplexing can be carried out in 4 dimensions
 space (si) – Space Division Multiplexing (SDM)
 time (t) – Time Division Multiplexing (TDM)
 frequency (f) – Frequency Division Multiplexing(FDM)
 code (c) – Code Division Multiplexing (CDM)

33 Wireless Network Principles

 Space Division Multiplexing(SDM)
channels ki
 SDM, (3D) space si is represented
via circles indicating the interference range k1 k2 k3 k4 k5 k6
 Goal: multiple use of a shared medium
c
 The channels k1 to k3 can be mapped onto
t c
three ‘spaces’ s1 to s3 which clearly
t
separate channels and prevent
s1
interference ranges from overlapping. f
 The space between the interference s2
f
ranges is called guard space. c
 Three spaces are required for remaining t
channels k4 to k6
 Important: guard spaces needed! s3
 Example: FM Radios f
 Disadvantages: if channels established within same
space(radio stations in the same city)
34 Wireless Network Principles
 Frequency Division Multiplexing(FDM)
 FDM is an analog multiplexing technique that combines analog signals.
 Separation of the whole spectrum into smaller frequency bands
 A channel gets a certain band of the spectrum for the whole time
 Advantages:
 no dynamic coordination k1 k2 k3 k4 k5 k6
necessary
c
 works also for analog signals
f
 Low bit rates – cheaper,

delay spread

 Disadvantages:
 waste of bandwidth
if the traffic is
distributed unevenly
 inflexible t

 guard spaces

35 Wireless Network Principles

 Time Division Multiplexing(TDM)
 TDM is a digital multiplexing technique for combining several low-
rate digital channels into one high-rate one.
 A channel gets the whole spectrum for a certain amount of time
 Advantages:
 only one carrier in the
medium at any time k1 k2 k3 k4 k5 k6

 throughput high - supports bursts

 flexible – multiple slots c
 no guard bands f

 Disadvantages:
 Framing and precise
synchronization
necessary
 high bit rates t

36 Wireless Network Principles

 Hybrid - Time and Frequency Multiplexing
 Combination of both methods (TDM and FDM)
 A channel gets a certain frequency band for a certain amount of time
 Example: GSM
 Advantages:
 better protection against
tapping k1 k2 k3 k4 k5 k6
 protection against frequency
c
selective interference
 higher data rates compared to f
code multiplex
 but: precise coordination
required
t

37 Wireless Network Principles

 Code Division Multiplexing
 Each channel has a unique code
k1 k2 k3 k4 k5 k6
 All channels use the same spectrum
at the same time
c
 Advantages:
 bandwidth efficient
 no coordination and synchronization
necessary
 good protection against interference
f
and tapping
 Disadvantages:
 lower user data rates
 more complex signal regeneration
t
 Implemented using spread spectrum
technology

38 Wireless Network Principles

 Modulation
 Digital modulation
 digital data is translated into an analog signal (baseband)
 Analog modulation
 shifts center frequency of baseband signal up to the radio carrier

 Modulation and Demodulation

analog
baseband
digital
signal
data digital analog
101101001 modulation modulation radio transmitter

radio
carrier

analog
baseband
digital
signal
analog synchronization data
Wireless Network Principles
demodulation decision 101101001 radio receiver

radio
39 carrier
 Areas of research in wireless & mobile communication
 Wireless Communication
 transmission quality (bandwidth, error rate, delay)
 modulation, coding, interference
 media access, regulations
 ...
 Mobility
 location dependent services
 location transparency
 quality of service support (delay, jitter, security)
 ad-Hoc & sensor
 Portability
 power consumption
 limited computing power, sizes of display, ...
 usability
40 Wireless Network Principles
Media Access Control
 MAC stands for Media Access Control.
 A MAC layer protocol is the protocol that controls access to the
physical transmission medium on a LAN.
 It tries to ensure that no two nodes are interfering with each
other’s transmissions, and deals with the situation when they do.

41 Wireless Network Principles

Motivation
 Can we apply media access methods from fixed networks?

 Example CSMA/CD
 Carrier Sense Multiple Access with Collision Detection
 send as soon as the medium is free, listen into the medium if a
collision occurs (original method in IEEE 802.3)
 Problems in wireless networks
 signal strength decreases proportional to the square of the
distance
 the sender would apply CS and CD, but the collisions happen at
the receiver
 it might be the case that a sender cannot “hear” the collision,
i.e., CD does not work
 furthermore, CS might not work if, e.g., a terminal is “hidden”

42 Wireless Network Principles

 Motivation - hidden and exposed terminals
 Hidden terminals
 A sends to B, C cannot receive A
 C wants to send to B, C senses a “free” medium (CS fails)
 collision at B, A cannot receive the collision (CD fails)
 A is “hidden” for C

A B C

 Exposed terminals
 B sends to A, C wants to send to another terminal (not A or B)
 C has to wait, CS signals a medium in use
 but A is outside the radio range of C, therefore waiting is not necessary
 C is “exposed” to B

43 Wireless Network Principles

 Motivation - near and far terminals
 Terminals A and B send, C receives
 signal strength decreases proportional to the square of the
distance
 the signal of terminal B therefore drowns out A’s signal
 C cannot receive A

A B C

 If C for example was an arbiter for sending rights, terminal B would

drown out terminal A already on the physical layer
 Also severe problem for CDMA-networks - precise power control
needed!

44 Wireless Network Principles

Access methods SDMA/FDMA/TDMA
 SDMA (Space Division Multiple Access)
 segment space into sectors, use directed antennas
 cell structure
 FDMA (Frequency Division Multiple Access)
 assign a certain frequency to a transmission channel between
a sender and a receiver
 permanent (e.g., radio broadcast), slow hopping (e.g., GSM),
fast hopping (FHSS, Frequency Hopping Spread Spectrum)
 TDMA (Time Division Multiple Access)
 assign the fixed sending frequency to a transmission channel
between a sender and a receiver for a certain amount of time
 The multiplexing schemes are used to control medium
access!
45 Wireless Network Principles
 Access method CDMA
CDMA (Code Division Multiple Access)
 all terminals send on the same frequency probably at the same time and
can use the whole bandwidth of the transmission channel
 each sender has a unique random number, the sender XORs the signal with
this random number
 the receiver can “tune” into this signal if it knows the pseudo random
number, tuning is done via a correlation function
Disadvantages:
 higher complexity of a receiver (receiver cannot just listen into the
medium and start receiving if there is a signal)
 all signals should have the same strength at a receiver
Advantages:
 all terminals can use the same frequency, no planning needed
 huge code space (e.g. 232) compared to frequency space
 interferences (e.g. white noise) is not coded
 forward error correction and encryption can be easily integrated

46 Wireless Network Principles

Comparison SDMA/TDMA/FDMA/CDMA

47 Wireless Network Principles

CSMA/CD MAC

 MAC schemes from wired networks, CSMA/CD as used in original

specification of IEEE 802.3 (aka Ethernet).
 CSMA/CD architecture used in Ethernet is a common MAC layer
standard.
 It acts as an interface between the Logical Link Control sublayer and
the network's Physical layer.
 A sender senses the medium (a wire or coaxial cable) to see if it is
free. If the medium is busy, the sender waits until it is free. If the
medium is free, the sender starts transmitting data and continues to
listen into the medium. If the sender detects a collision while
sending, it stops at once and sends a jamming signal.
 Contention-based protocols
 CSMA — Carrier Sense Multiple Access
 Ethernet (CSMA/CD) is not enough for wireless (collision at receiver cannot
detect at sender). Hence hidden terminal problem.
48 Wireless Network Principles
 MACA - Collision Avoidance
 MACA (Multiple Access with Collision Avoidance) uses short signaling
packets for collision avoidance
 RTS (request to send): a sender request the right to send from a
receiver with a short RTS packet before it sends a data packet
 CTS (clear to send): the receiver grants the right to send as soon as
it is ready to receive
 Signaling packets contain
 sender address, receiver address, packet size
 Variants of this method can be found in IEEE802.11 as DFWMAC
(Distributed Foundation Wireless MAC)
 MACA Protocol solved hidden and exposed terminal problems:
 Sender broadcasts a Request-to-Send (RTS) and the intended receiver
sends a Clear-to-Send (CTS).
 Upon receipt of a CTS, the sender begins transmission of the frame.
 RTS, CTS helps determine who else is in range or busy (Collision Avoidance).
49 Wireless Network Principles
MACA examples
 MACA avoids the problem of hidden terminals
 A and C want to
send to B
 A sends RTS first
RTS
 C waits after receiving
CTS from B CTS CTS
A B C

 MACA avoids the problem of exposed terminals

 B wants to send to A, C
to another terminal
 now C does not have RTS RTS
to wait for it cannot
CTS
receive CTS from A
A B C
50 Wireless Network Principles