Wireless & Mobile Communications
Chapter 2: Wireless Transmission
Frequencies
 Signals
 Antennas
 Signal propagation
 Multiplexing
 Spread spectrum
 Modulation
 Cellular systems

Spectrum Allocation
twisted
pair
coax cable
1 Mm
300 Hz
10 km
30 kHz
VLF
LF
optical transmission
100 m
3 MHz
MF
HF
1m
300 MHz
VHF
VLF = Very Low Frequency
LF = Low Frequency
MF = Medium Frequency
HF = High Frequency
VHF = Very High Frequency
UHF
10 mm
30 GHz
SHF
100 m
3 THz
EHF
infrared
1 m
300 THz
visible light UV
UHF = Ultra High Frequency
SHF = Super High Frequency
EHF = Extra High Frequency
UV = Ultraviolet Light
Relationship between frequency ‘f’ and wave length ‘’ :
 = c/f
where c is the speed of light  3x108m/s
ICS 243E - Ch.2 Wireless Transmission
Spring 2003
2.2
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.
ICS 243E - Ch.2 Wireless Transmission
Spring 2003
2.3
Allocated Frequencies

ITU-R holds auctions for new frequencies, manages
frequency bands worldwide for harmonious usage (WRC World Radio Conferences)
Mobile
phones
Cordless
telephones
Wireless
LANs
Europe
USA
Japan
NMT 453-457MHz,
463-467КMHz;
GSM 890-915КMHz,
935-960КMHz;
1710-1785КMHz,
1805-1880КMHz
CT1+ 885-887КMHz,
930-932КMHz;
CT2
864-868КMHz
DECT
1880-1900КMHz
IEEE 802.11
2400-2483КMHz
HIPERLAN 1
5176-5270КMHz
AMPS, TDMA, CDMA
824-849КMHz,
869-894КMHz;
TDMA, CDMA, GSM
1850-1910КMHz,
1930-1990КMHz;
PACS 1850-1910КMHz,
1930-1990КMHz
PACS-UB 1910-1930КMHz
PDC
810-826КMHz,
940-956КMHz;
1429-1465КMHz,
1477-1513КMHz
IEEE 802.11
2400-2483КMHz
IEEE 802.11
2471-2497КMHz
ICS 243E - Ch.2 Wireless Transmission
Spring 2003
PHS
1895-1918КMHz
JCT
254-380КMHz
2.4
Signals I




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)
ICS 243E - Ch.2 Wireless Transmission
Spring 2003
2.5
Fourier Representation of Periodic Signals


1
g (t )  c   an sin( 2nft)   bn cos( 2nft)
2
n 1
n 1
1
1
0
0
t
ideal periodic signal
ICS 243E - Ch.2 Wireless Transmission
t
real composition
(based on harmonics)
Spring 2003
2.6
Signals II

Different representations of signals

amplitude (amplitude domain)
 frequency spectrum (frequency domain)
 phase state diagram (amplitude M and phase  in polar
coordinates)
Q = M sin 
A [V]
A [V]
t[s]

I= M cos 



f [Hz]
Composite signals mapped into frequency domain using
Fourier transformation
Digital signals need


infinite frequencies for perfect representation
modulation with a carrier frequency for transmission (->analog
signal!)
ICS 243E - Ch.2 Wireless Transmission
Spring 2003
2.7
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 (e.g., coaxial cable)
Antennas consist of one or several radiating elements
through which an electric current circulates
Types of antennas:






omnidirectional
directional
phased arrays
adaptive
optimal
Principal characteristics used to characterize an antenna are:

radiation pattern
 directivity
 gain
 efficiency
ICS 243E - Ch.2 Wireless Transmission
Spring 2003
2.8
Isotropic Antennas



Isotropic radiator: equal radiation in all directions (three
dimensional) - 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
x
ideal
isotropic
radiator
ICS 243E - Ch.2 Wireless Transmission
Spring 2003
2.9
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: Radiation pattern of a simple Hertzian dipole
y
y
x
side view (xy-plane)

z
z
side view (yz-plane)
x
simple
dipole
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)
ICS 243E - Ch.2 Wireless Transmission
Spring 2003
2.10
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
x
z
side view (xy-plane)
x
side view (yz-plane)
directed
antenna
top view (xz-plane)
z
z
x
x
top view, 3 sector
ICS 243E - Ch.2 Wireless Transmission
sectorized
antenna
top view, 6 sector
Spring 2003
2.11
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
/4
/2
/4
+
/2
/2
/2
+
ground plane
ICS 243E - Ch.2 Wireless Transmission
Spring 2003
2.12
Signal Propagation Ranges

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
ICS 243E - Ch.2 Wireless Transmission
sender
transmission
distance
detection
interference
Spring 2003
2.13
Signal Propagation I

Radio wave propagation is affected by the following
mechanisms:

reflection at large obstacles
 scattering at small obstacles
 diffraction at edges
reflection
ICS 243E - Ch.2 Wireless Transmission
diffraction
scattering
Spring 2003
2.14
Signal Propagation II

The signal is also subject to degradation resulting from
propagation in the mobile radio environment. The principal
phenomena are:



pathloss due to distance covered by radio signal (frequency
dependent, less at low frequencies)
fading (frequency dependent, related to multipath propagation)
shadowing induced by obstacles in the path between the
transmitted and the receiver
shadowing
ICS 243E - Ch.2 Wireless Transmission
Spring 2003
2.15
Signal Propagation III

Interference from other sources and noise will also impact signal
behavior:

co-channel (mobile users in adjacent cells using same frequency) and
adjacent (mobile users using frequencies adjacent to
transmission/reception frequency) channel interference
 ambient noise from the radio transmitter components or other
electronic devices,

Propagation characteristics differ with the environment through
and over which radio waves travel. Several types of environments
can be identified (dense urban, urban, suburban and rural) and are
classified according to the following parameters:





terrain morphology
vegetation density
buildings: density and height
open areas
water surfaces
ICS 243E - Ch.2 Wireless Transmission
Spring 2003
2.16
Pathloss I

Free-space pathloss:
To define free-space propagation, consider an isotropic source
consisting of a transmitter with a power Pt W. At a distance ‘d’
from this source, the power transmitted is spread uniformly on
the surface of a sphere of radius ‘d’. The power density at the
distance ‘d’ is then as follows:
Sr = Pt/4d2

The power received by an antenna at a distance ‘d’ from the
transmitter is then equal to:
Pr = PtAe/4d2
where A is the effective area of the antenna.
e
ICS 243E - Ch.2 Wireless Transmission
Spring 2003
2.17
Pathloss II

Noting that Ae = Gr/(4/2)
where Gr is the gain of the receiver

And if we replace the isotropic source by a transmitting
antenna with a gain Gt the power received at a distance ‘d’ of
the transmitter by a receiving antenna of gain Gr becomes:
Pr = PtGrGt/[4(d/)]2

In decibels the propagation pathloss (PL) is given by:
PL(db) = -10log10(Pr/Pt) = -10log10(GrGt/[4(d/)]2)

This is for the ideal case and can only be applied sensibly to
satellite systems and short range LOS propagation.
ICS 243E - Ch.2 Wireless Transmission
Spring 2003
2.18
Multipath Propagation I

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
ICS 243E - Ch.2 Wireless Transmission
Spring 2003
2.19
Multipath Propagation II

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, this is referred
to as Inter Symbol Interference (ISI)
multipath spread (in secs) = (longest1 – shortest2)/c
For a 5s symbol duration a 1s 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.
ICS 243E - Ch.2 Wireless Transmission
Spring 2003
2.20
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)

Additional changes in


power
distance to sender
obstacles further away
long term
fading
 slow changes in the average power
received (long term fading)
short term fading
ICS 243E - Ch.2 Wireless Transmission
Spring 2003
t
2.21
Multiplexing Techniques

Multiplexing techniques are used to allow many users to
share a common transmission resource. In our case 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 in wireline systems, it is desirable to allow the
simultaneous transmission of information between two
users engaged in a connection. This is called duplexing.

Two types of duplexing exist:

Frequency division duplexing (FDD), whereby two frequency
channels are assigned to a connection, one channel for each
direction of transmission.

Time division duplexing (TDD), whereby two time slots (closely
placed in time for duplex effect) are assigned to a connection,
one slot for each direction of transmission.
ICS 243E - Ch.2 Wireless Transmission
Spring 2003
2.22
Multiplexing

Multiplexing in 3 dimensions

time (t) (TDM)
 frequency (f) (FDM)
 code (c) (CDM)

Goal: multiple use
of a shared medium
channels ki
k1
k2
k3
k4
k5
k6
c
t
c
t
s1
f
s2
f
c
t
s3
ICS 243E - Ch.2 Wireless Transmission
Spring 2003
f
2.23
Narrowband versus Wideband

These multiple access schemes can be grouped into two
categories:

Narrowband systems - the total spectrum is divided into a
large number of narrow radio bands that are shared.

Wideband systems - the total spectrum is used by each mobile
unit for both directions of transmission. Only applicable for
TDM and CDM.
ICS 243E - Ch.2 Wireless Transmission
Spring 2003
2.24
Frequency Division Multiplexing (FDM)

Separation of the whole spectrum into smaller frequency bands
 A channel gets a certain band of the spectrum for the whole time –
orthogonal system
 Advantages:

no dynamic coordination
necessary, i.e., sync. and
framing
 works also for analog signals
 low bit rates – cheaper,
delay spread

k1
k2
k3
k4
k5
k6
c
f
Disadvantages:

waste of bandwidth
if the traffic is
distributed unevenly
 inflexible
 guard bands t
 narrow filters
ICS 243E - Ch.2 Wireless Transmission
Spring 2003
2.25
Time Division Multiplexing (TDM)


A channel gets the whole spectrum for a certain amount of
time – orthogonal system
Advantages:

only one carrier in the
medium at any time
 throughput high - supports bursts
k1
 flexible – multiple slots
 no guard bands ?!

Disadvantages:
k2
k3
k4
k5
k6
c
f

Framing and precise
synchronization
necessary
 high bit rates
at each
t
Tx/Rx
ICS 243E - Ch.2 Wireless Transmission
Spring 2003
2.26
Hybrid TDM/FDM




Combination of both methods
A channel gets a certain frequency band for a certain
amount of time (slot).
Example: GSM, hops from one band to another each time
slot
Advantages:
k1

better protection against
tapping (hopping among
frequencies)
 protection against frequency
selective interference

k2
k3
k4
k5
k6
c
f
Disadvantages:

Framing and
sync. required
t
ICS 243E - Ch.2 Wireless Transmission
Spring 2003
2.27
Code Division Multiplexing (CDM)



Each channel has a unique code
k1
(not necessarily orthogonal)
All channels use the same spectrum
at the same time
Advantages:
k2
k3
k4
k5
k6
c

bandwidth efficient
 no coordination and synchronization
necessary
 good protection against interference
and tapping

f
Disadvantages:

lower user data rates due to high
gains required to reduce
interference
 more complex signal regeneration
ICS 243E - Ch.2 Wireless Transmission
t
Spring 2003
2.19.1
2.28
Issues with CDM

CDM has a soft capacity. The more users the more codes that are
used. However as more codes are used the signal to interference
(S/I) ratio will drop and the bit error rate (BER) will go up for all
users.

CDM requires tight power control as it suffers from far-near effect.
In other words, a user close to the base station transmitting with
the same power as a user farther away will drown the latter’s
signal. All signals must have more or less equal power at the
receiver.

Rake receivers can be used to improve signal reception. Time
delayed versions (a chip or more delayed) of the signal (multipath
signals) can be collected and used to make bit level decisions.

Soft handoffs can be used. Mobiles can switch base stations
without switching carriers. Two base stations receive the mobile
signal and the mobile is receiving from two base stations (one of
the rake receivers is used to listen to other signals).

Burst transmission - reduces interference
ICS 243E - Ch.2 Wireless Transmission
Spring 2003
2.29
Types of CDM I

Two types exist:

Direct Sequence CDM (DS-CDM)
spreads the narrowband user signal (Rbps) over the full spectrum by
multiplying it by a very wide bandwidth signal (W). This is done by
taking every bit in the user stream and replacing it with a pseudonoise
(PN) code (a long bit sequence called the chip rate). The codes are
orthogonal (or approx.. orthogonal).
This results in a processing gain G = W/R (chips/bit). The higher G the
better the system performance as the lower the interference. G2
indicates the number of possible codes. Not all of the codes are
orthogonal.
Frequency
Code
CDMA
Time
ICS 243E - Ch.2 Wireless Transmission
Spring 2003
2.30
Types of CDM II

Frequency hopping CDM (FH-CDM)
FH-CDM is based on a narrowband FDM system in which an individual
user’s transmission is spread out over a number of channels over time
(the channel choice is varied in a pseudorandom fashion). If the carrier
is changed every symbol then it is referred to as a fast FH system, if it
is changed every few symbols it is a slow FH system.
A
B
B
B
A
A
B
A
A
A
ICS 243E - Ch.2 Wireless Transmission
A
A
B
B
B
A
B
B
Spring 2003
2.31
Orthogonality and Codes

An m-bit PN generator generates N=2m - 1 different codes.

Out of these codes only ‘m’ codes are orthogonal -> zero
cross correlation.

For example a 3 bit shift register circuit shown below
generates N=7 codes.
Mod2 Adder (1+0=1, 0+1=1, 0+0=0, 1+1=0)
+
1
Initial State:
2
1
0
1
0
0
1
1
ICS 243E - Ch.2 Wireless Transmission
3
1
1
0
1
0
0
1
Spring 2003
1
1
1
0
1
0
0
2.32
Orthogonal Codes

A pair of codes is said to be orthogonal if the cross correlation is
zero: Rxy(0) = 0 .

For two m-bit codes: x1,x2,x3,...,xm and y1,y2,y3,...,ym:
For example: x = 0011 and y = 0110. Replace 0 with -1, 1 stays as is.
Then:
x = -1 -1 1 1
y = -1 1 1 -1
----------------Rxy(0) = 1 -1 +1 -1 = 0
ICS 243E - Ch.2 Wireless Transmission
Spring 2003
2.33
Example of an Orthogonal Code: Walsh Codes

In 1923 J.L. Walsh introduced a complete set of orthogonal
codes. To generate a Walsh code the following two steps
must be followed:

Step 1: represent a NxN matrix as four quadrants (start off with
2x2)

Step 2: make the first, second and third quadrants indentical
and invert the fourth
b
b
1
b
=
b’
1
bb
b b’
1
bb
b b’
bb
b b’
=
0
Code 1
0
1
Code 2
or
0
2 codes: 11 and 10
bb
b b’
0
11
10
11
10
ICS 243E - Ch.2 Wireless Transmission
11
10
00
01
or
2 codes: 00 and 01
0
0
0
0
0
1
0
1
Spring 2003
00
01
11
10
Code 1
Code 2
Code 3
Code 4
2.34
Modulation

Digital modulation

digital data is translated into an analog signal (baseband)
 ASK, FSK, PSK - main focus in this chapter
 differences in spectral efficiency, power efficiency, robustness

Analog modulation


shifts center frequency of baseband signal up to the radio
carrier
Motivation
smaller antennas (e.g., /4)
 Frequency Division Multiplexing
 medium characteristics


Basic schemes



Amplitude Modulation (AM)
Frequency Modulation (FM)
Phase Modulation (PM)
ICS 243E - Ch.2 Wireless Transmission
Spring 2003
2.35
Modulation and Demodulation
digital
data
101101001
digital
modulation
analog
baseband
signal
analog
modulation
radio transmitter
radio
carrier
analog
demodulation
analog
baseband
signal
synchronization
decision
digital
data
101101001
radio receiver
radio
carrier
ICS 243E - Ch.2 Wireless Transmission
Spring 2003
2.36
Digital Modulation


Modulation of digital signals known as Shift Keying
1
0
Amplitude Shift Keying (ASK):
1

very simple
 low bandwidth requirements
 very susceptible to interference
t
1

0
1
Frequency Shift Keying (FSK):

needs larger bandwidth
t

Phase Shift Keying (PSK):
1
0
1

more complex
 robust against interference
t
ICS 243E - Ch.2 Wireless Transmission
Spring 2003
2.37
Advanced Frequency Shift Keying






bandwidth needed for FSK depends on the distance
between the carrier frequencies
special pre-computation avoids sudden phase shifts
 MSK (Minimum Shift Keying)
bit separated into even and odd bits, the duration of each
bit is doubled
depending on the bit values (even, odd) the higher or lower
frequency, original or inverted is chosen
the frequency of one carrier is twice the frequency of the
other
even higher bandwidth efficiency using a Gaussian lowpass filter  GMSK (Gaussian MSK), used in GSM
ICS 243E - Ch.2 Wireless Transmission
Spring 2003
2.38
Example of MSK
1
0
1
1
0
1
0
bit
data
even
0101
even bits
odd
0011
odd bits
signal
value
hnnh
- - ++
low
frequency
h: high frequency
n: low frequency
+: original signal
-: inverted signal
high
frequency
MSK
signal
t
No phase shifts!
ICS 243E - Ch.2 Wireless Transmission
Spring 2003
2.39
Advanced Phase Shift Keying

BPSK (Binary Phase Shift Keying):






Q
bit value 0: sine wave
bit value 1: inverted sine wave
very simple PSK
low spectral efficiency
robust, used e.g. in satellite systems
1
10
0
Q
QPSK (Quadrature Phase Shift
Keying):
2 bits coded as one symbol
 symbol determines shift of sine wave
 needs less bandwidth compared to
BPSK
A
 more complex
Often also transmission of relative,
not absolute phase shift: DQPSK Differential QPSK (IS-136, PACS,
PHS)
ICS 243E - Ch.2 Wireless Transmission
11
I


I
00
01
t
11
Spring 2003
10
00
01
2.40
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
Q

0010
0011
0001
Example: 16-QAM (4 bits = 1 symbol)
0000
I
1000

ICS 243E - Ch.2 Wireless Transmission
Symbols 0011 and 0001 have the
same phase, but different amplitude.
0000 and 1000 have different phase,
but same amplitude.
 used in standard 9600 bit/s
modems
Spring 2003
2.41
Spread spectrum technology: CDM


Problem of radio transmission: frequency dependent fading
can wipe out narrow band signals for duration of the
interference
Solution: spread the narrow band signal into a broad band
signal using a special code

power
protection against narrow band interference
interference
spread
signal
power
signal
spread
interference
detection at
receiver
f
protection against narrowband interference

f
Side effects:

coexistence of several signals without dynamic coordination
 tap-proof

Alternatives: Direct Sequence, Frequency Hopping
ICS 243E - Ch.2 Wireless Transmission
Spring 2003
2.42
Effects of spreading and interference
P
P
i)
user signal
broadband interference
narrowband interference
ii)
f
f
sender
P
P
P
iii)
iv)
f
v)
f
f
receiver
ICS 243E - Ch.2 Wireless Transmission
Spring 2003
2.28.1
2.43
Spreading and frequency selective fading
channel
quality
1
2
5
3
6
narrowband channels
4
frequency
narrow band
signal
guard space
channel
quality
1
2
2
2
2
2
spread
spectrum
ICS 243E - Ch.2 Wireless Transmission
spread spectrum channels
frequency
Spring 2003
2.29.1
2.44
DSSS (Direct Sequence Spread Spectrum) I

XOR of the signal with pseudo-random number (chipping
sequence)

many chips per bit (e.g., 128) result in higher bandwidth of the
signal
t
b

Advantages
user data

reduces frequency selective
fading
 in cellular networks
0
chipping
sequence
01101010110101
01101011001010
precise power control necessary
ICS 243E - Ch.2 Wireless Transmission
=
resulting
signal
Disadvantages

XOR
tc
base stations can use the
same frequency range
several base stations can
detect and recover the signal
soft handover

1
tb: bit period
tc: chip period
Spring 2003
2.30.1
2.45
DSSS (Direct Sequence Spread Spectrum) II
spread
spectrum
signal
user data
X
transmit
signal
modulator
chipping
sequence
radio
carrier
transmitter
correlator
lowpass
filtered
signal
received
signal
demodulator
radio
carrier
products
sampled
sums
data
X
integrator
decision
chipping
sequence
receiver
ICS 243E - Ch.2 Wireless Transmission
Spring 2003
2.31.1
2.46
FHSS (Frequency Hopping Spread Spectrum) I

Discrete changes of carrier frequency


sequence of frequency changes determined via pseudo
random number sequence
Two versions

Fast Hopping:
several frequencies per user bit
 Slow Hopping:
several user bits per frequency

Advantages

frequency selective fading and interference limited to short
period
 simple implementation
 uses only small portion of spectrum at any time

Disadvantages


not as robust as DSSS
simpler to detect
ICS 243E - Ch.2 Wireless Transmission
Spring 2003
2.32.1
2.47
FHSS (Frequency Hopping Spread Spectrum) II
tb
user data
0
1
f
0
1
1
t
td
f3
slow
hopping
(3 bits/hop)
f2
f1
f
t
td
f3
fast
hopping
(3 hops/bit)
f2
f1
t
tb: bit period
ICS 243E - Ch.2 Wireless Transmission
td: dwell time
Spring 2003
2.33.1
2.48
FHSS (Frequency Hopping Spread Spectrum) III
user data
modulator
modulator
frequency
synthesizer
transmitter
hopping
sequence
narrowband
signal
received
signal
data
demodulator
hopping
sequence
spread
transmit
signal
narrowband
signal
demodulator
frequency
synthesizer
ICS 243E - Ch.2 Wireless Transmission
receiver
Spring 2003
2.34.1
2.49
Concept of Cellular Communications

In the late 60’s it was proposed to alleviate the problem of
spectrum congestion by restructuring the coverage area of mobile
radio systems.

The cellular concept does not use broadcasting over large areas.
Instead smaller areas called cells are handled by less powerful
base stations that use less power for transmission. Now the
available spectrum can be re-used from one cell to another
thereby increasing the capacity of the system.

However this did give rise to a new problem, as a mobile unit
moved it could potentially leave the coverage area (cell) of a base
station in which it established the call. This required complex
controls that enabled the handing over of a connection (called
handoff) to the new cell that the mobile unit moved into.

In summary, the essential elements of a cellular system are:

Low power transmitter and small coverage areas called cells

Spectrum (frequency) re-use

Handoff
ICS 243E - Ch.2 Wireless Transmission
Spring 2003
2.50
Cell structure

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
ICS 243E - Ch.2 Wireless Transmission
Spring 2003
2.35.1
2.51
Cellular Network
Other MSCs
F1,F2,..,F6
(IS 41)
F7,F8,..,F12
PSTN
F7,F8,..,F12
MSC
F1,F2,..,F6
Base
Station
Handoff
Cell
MSC: Mobile Switching Center
PSTN: Public Switched Telephone Network
(Theoretical)
Practical Cell - coverage depends on antenna location and
height, transmitter power, terrain, foliage, buildings, etc.
ICS 243E - Ch.2 Wireless Transmission
Spring 2003
2.52
Some Definitions

Forward path or down link - from base station down to the mobile

Reverse path or up link - from the mobile up to the base station

The mobile unit - a portable voice and/or data comm. transceiver.
It has a 10 digit telephone number that is represented by a 34 bit
mobile identification number -> (215) 684-3201 is divided into two
parts: MIN1: 215 translated into 10bits and MIN2: 684-3201
translated into 24bits. In addition each mobile unit is also
permanently programmed at the factory with a 32 bit electronic
serial number (ESN) which guards against tampering.

The cell - a geographical area covered by Radio Frequency (RF)
signals. It is essentially a radio communication center comprising
radios, antennas and supporting equipment to enable mobile to
land and land to mobile communication. Its shape and size
depend on the location, height , gain and directivity of the
antenna, the power of the transmitter, the terrain, obstacles such
as foliage, buildings, propagation paths, etc. It is a highly irregular
shape, its boundaries defined by received signal strength! But for
traffic engineering purposes and system planning and design a
hexagonal shape is used.
ICS 243E - Ch.2 Wireless Transmission
Spring 2003
2.53
More definitions


The base station (BS) - a transmitter and receiver that
relays signals (control and information (voice or data)) from
the mobile unit to the MSC and vice versa.
The mobile switching center (MSC) - a switching center that
controls a cluster of cells. Base stations are connected to
the MSC via wireline links. The MSC is directly connected to
the PSTN and is responsible for all calls related to mobiles
located within its domain. MSCs intercommunicate using a
link protocol specified by IS (International Standard) 41.
This enables roaming of mobile units (i.e. obtaining service
outside of the home base). The MSC is also responsible for
billing, it keeps track of air time, errors, delays, blocking,
call dropping (due to handoff failure), etc. It is also
responsible for the handoff process, it keeps track of signal
strengths and will initiate a handoff when deemed
necessary (note to handoff or not to handoff is not a trivial
issue!)
ICS 243E - Ch.2 Wireless Transmission
Spring 2003
2.54
The Basic Cellular Communication Protocol I



Every mobile unit whether at home or roaming, has to
register with the MSC controlling the area it is in. If it does
not register then the MSC does not know of its existence
and will not be able to process any of its calls.
The home location register (HLR) is used to keep
information regarding a mobile unit/user, it is a database for
storing and managing subscriber information. When
roaming, a mobile unit registers with a foreign MSC and
data from its HRL is relayed to the visitor location register
(VLR). The VLR is a dynamic database used to store
roaming mobile subscriber information. The HLR and VLR
communicate via the MSCs using IS 41.
The cellular system uses out of band signalling. Most of the
control information is sent over different channels from the
user information (voice or data) channels. Inband signalling
is used for control during the connection (disconnect,
handoff, etc.)
ICS 243E - Ch.2 Wireless Transmission
Spring 2003
2.55
The Basic Cellular Communication Protocol II

A mobile unit when enabled (power on) scans the control
channels and tunes to the one with the strongest signal. The
control channels are known and carry signals pertaining to the
cell sites, e.g. transmission power to be used by the mobile unit in
a particular cell. This process is called initialization.

If the mobile wants to initiate a call, it sends in a service request
on the reverse path control link. The service request contains the
destination phone number and identification information (MIN1,
MIN2, and ESN) of the source mobile unit to verify the originator.

When the base station receives the request, it relays it to the MSC.
The MSC then checks to see it is it a number of another mobile or
of a fixed user. If the latter the call is forwarded to the PSTN. If the
former, it checks to see if the destination mobile unit is a
subscriber (local or visitor/roamer). If not it relays the call to the
PSTN to forward to the appropriate MSC.
ICS 243E - Ch.2 Wireless Transmission
Spring 2003
2.56
The Basic Cellular Communication Protocol III

If the destination is within its cluster it sends out a paging
message to all the base stations. Every base station then
relays this message by broadcasting it on its control
channel. If the destination mobile unit is enabled (power on)
it will detect this message and respond to the base station.

The base station relays this response to the MSC. The MSC
then allocates channels to both the source mobile unit and
the destination mobile unit. The corresponding base
stations pass this information on to the respective mobile
units. The mobile units then tune to the correct channels
and the communication link is established.
ICS 243E - Ch.2 Wireless Transmission
Spring 2003
2.57
Spectrum and Capacity Issues

Spectrum is limited
Allocated Spectrum
F1 F2 F3 F4 F5 F6 F7 F8 F9
FDM
F1,F2,...F9: frequency channels
ICS 243E - Ch.2 Wireless Transmission
Spring 2003
2.58
Frequency Re-use I

To be able to increase the capacity of the system, frequencies must
be re-used in the cellular layout (unless we are using spread
spectrum techniques).

Frequencies cannot be re-used in adjacent cells because of cochannel interference. The cells using the same frequencies must be
dispersed across the cellular layout. The closer the spacing the
more efficient the scheme!
Fx:subset of
frequencies
used in a
cell
Cochannel
Interference
F2
F1
F1
F2
Minimum
Re-use distance
ICS 243E - Ch.2 Wireless Transmission
Spring 2003
2.59
Frequency Re-use II

For an omni-directional antenna, with constant signal
power, each cell site coverage area would be circular
(barring any terrain irregularities or obstacles).

To achieve full coverage without dead spots, a series of
regular polygons for cell sites are required.

The hexagonal was chosen as it comes the closest to the
shape of a circle, and a hexagonal layout requires fewer
cells (when compared to triangles or rectangles, it has the
largest surface area given the same radius R) -> less cells.

Goal is to find the minimum distance between cells using
same frequencies.
ICS 243E - Ch.2 Wireless Transmission
Spring 2003
2.60
Frequency re-use distance I
i,j - integers -> intercell distance
along cell centers
i
A
60%
i,j: multiples of 31/2R
j
D
A
R
D - min. dist.
D=31/2R[i2+j 2+ij]1/2
R = radius of hexagonal
R: cell radius
i,j are integers
v
R
(u,v)
3
1
31/2R
31/2R
R
u
D
2
300
1
(0,0)
u2-u1=3 1/2Ri
v2-v1=31/2 Rj
ICS 243E - Ch.2 Wireless Transmission
Spring 2003
2.61
Frequency re-use distance II

For two adjacent cells: D=31/2R

The closest we can place the same frequencies is called the
first tier around the center cell (minimal re-use distance ->
lower -> more capacity!).

For simplicity we only take the first tier of cells into account
for co-channel interference (i.e., we ignore 2nd, 3rd, etc.
tiers, cause much less interference, negligible!).
Original cell
Cluster of “N” cells with
different frequencies
First tier of interferers
Second tier of interferers
They are all equidistant
away from each other (D)
Each cell has exactly six equidistant interfering cells
ICS 243E - Ch.2 Wireless Transmission
Spring 2003
2.62
Frequency re-use distance III
Radius = D
D
First Tier
(all use same
frequencies as
center cell)
Radius
R
Cluster of “N” cells with
frequencies different
from center cell
(large hexagon)
ICS 243E - Ch.2 Wireless Transmission
Spring 2003
2.63
Frequency re-use distance III

Radius = dist. between two co-channel cells =
(3R2[i2+j2+ij])1/2 = D

Since the area of a hexagon is proportional to the square of
the distance between its center and a vertex (i.e., its radius),
the area of the large hexagon is:
Alarge = k[Radius]2 = k[3R2[i2+j2+ij]]
where k is a constant.

Similarly the area of each cell (i.e., small hexagon) is:
Asmall = k[R2]

Comparing these expressions we find that:
Alarge/Asmall = 3[i2+j2+ij] = D2/R2
ICS 243E - Ch.2 Wireless Transmission
Spring 2003
2.64
Frequency re-use distance IV

From symmetry we can see that the large hexagon
encloses the center cluster of N cells plus 1/3 the number of
the cells associated with 6 other peripheral hexagons. Thus
the total number of cells enclosed by the first tier is:
N+6(1/3N) = 3N

Since the area of a hexagon is proportional to the number
of cells contained within it:
Alarge/Asmall = 3N/1 = 3N

Substituting we get:
3N = 3[i2+j2+ij] = D2/R2

Or:
D/R = q =(3N)1/2
 “q”
is referred to as the reuse ratio!
ICS 243E - Ch.2 Wireless Transmission
Spring 2003
2.65
Co-channel Interference I

The co-channel interference ratio S/I is given as:
S
S
--- = -----------------------N
I
i
  I k
k=1
S = desired signal power in a cell (note that many texts use “C”
instead of S), Ik = interference signal power from the kth cell, Ni =
number of interfering cells.

If we only assume the first tier of interfering cells, then Ni=6,and
all cells interfere equally (they are all equidistant!).

The signal power at any point is inversely proportional to the
inverse of the distance from the source raised to the g power.
(2<g<5)
ICS 243E - Ch.2 Wireless Transmission
Spring 2003
2.66
Co-channel Interference II

Ik is proportional to Dg , and S is proportional to Rg , where g
is the propagation path loss and is dependent upon terrain
environment. For cellular systems it is often taken as = 4.

Therefore:
–g
g
S
R
1
q
--- = ----------------- = ---------------- = ----I
–g
–g
6
6D
6 q

The relationship between SNR (signal to noise ratio - Eb/No)
and S/I for cellular systems with Rayleigh fading channels:
SNR = S/I(db) – 9db.
ICS 243E - Ch.2 Wireless Transmission
Spring 2003
2.67
For a given S/I how to get N

Recall that: D/R = q =(3N)1/2

An S/I = 18db (decibels=10logS/I) = 63.1, gives an
acceptable voice quality.

Therefore q = [6x63.1]1/4 = 4.41 when g = 4

Substituting for N we get N = (4.41)2/3 equals approx. 7

This means that if we have 49 frequency channels available,
each cell gets 49/7 = 7 frequency channels.

If we have 82 available then 82/7 = 11.714 -> which means
that 5 cells will have 12 and 2 cells will have 11!

How does that translate to “i and j” for a cell layout?
N = [i2+j2+ij], find i,j that satisfy the equation!
ICS 243E - Ch.2 Wireless Transmission
Spring 2003
2.68
Calculating i, j, and D from N
7
7
2
1
7
j
i
2
6
1
1
D
5
3
4
N=7 -> i=2, j=1
f2
f4
f7
f2
f6
f1
f3
f5
f4
f7
f1
f3
f2
ICS 243E - Ch.2 Wireless Transmission
D = 4.41R
f3
f5
f6
2
f5
f2
Spring 2003
2.69
Frequency planning


Frequency reuse only with a certain distance between the
base stations
Standard model using 7 frequencies:
f3
f5
f4
f2
f6
f1
f3
f5
f4
f7
f1
f2

Fixed frequency assignment:

certain frequencies are assigned to a certain cell
 problem: different traffic load in different cells

Dynamic frequency assignment:

base station chooses frequencies depending on the
frequencies already used in neighbor cells
 more capacity in cells with more traffic
 assignment can also be based on interference measurements
ICS 243E - Ch.2 Wireless Transmission
Spring 2003
2.36.1
2.70
Increasing Capacity

We can see that by reducing the area of a cell we can
increase capacity as we will have more cells each with its
own set of frequencies.

What is drawback of shrinking the size of the cells (cell
splitting)? Increase in the number of handoffs -> increased
load on the system! Also need more infrastrucutre -> base
stations (each cell needs a BS).

An easier solution exists, sectorization. It does not reduce
handoffs, its advantage: it does not require more
infrastructure.
ICS 243E - Ch.2 Wireless Transmission
Spring 2003
2.71
Sectorization I

We can also increase the capacity by using sectors in cells.

Directional antennas instead of being omnidirectional, will
only beam over a certain angle.
F1+F2+F3=Fa 120%
F1
F3
F2
f3
f2
f1
f1
f2
f3
f1
f3
f2
f2
f1
f1
F6
F2
60%
F3
F4
F5
6 sectors
f3
f2
f3
F1
Fa: A cell’s set of frequencies
3 sectors
f3
F1+F2+F3+F4+F5+F6=Fa
f3
f2
f2
f2
f1 f
f1 f
f1 f
h
h
3
3
3
h1 2
h1 2
g2 h3 g2 h3
g2
g1
g1
g1
g3
g3
g3
f3
3 cell cluster
ICS 243E - Ch.2 Wireless Transmission
3 cell cluster with 3 sectors
Spring 2003
2.72
Sectorization II

What does that mean?

We can now assign frequency sets to sectors and decrease
the re-use distance or improve S/I ratio (i.e. signal quality).

Question: By how much? Depends on number of sectors
(i.e., 60% or 120%).
“A”: set of frequencies in a sector
A”
A
First Tier
(all use same
frequencies in
sectors as
A” center cell)
A:Do not interfere with
“A”sector of center cell
A”A’
A
A’
A’:Cause Cell site to
mobile interference
ICS 243E - Ch.2 Wireless Transmission
A’
A”:Cause Mobile to cell
site interference
Spring 2003
2.73
Other Capacity or Signal Improvement Tech.

Dynamic channel allocation (DCA): allows cells to borrow
frequencies from other cells within the cluster if not used
by them. Can be used to alleviate hotspots. Another
implementation basically has all channels available to all
cells, they get allocated based upon demand.

Power control: by reducing the transmitted power, the
battery life of a mobile can be extended. It also helps in
reducing -channel and adjacent channel interference.
ICS 243E - Ch.2 Wireless Transmission
Spring 2003
2.74