Wireless Transmission Fundamentals
(Dayem’s book, Chapter 4)
(Nico’s book, Chapter 2)
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Electromagnetic Spectrum
Wireless Propagation Models
Digital Modulation Techniques
Multiple Access
Performance Issues
Cellular and Ad Hoc Concepts
Link Budget Analysis
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Electromagnetic Waves

predicted by British physicist James Maxwell in
1865, and observed by German physicist Heinrich
Hertz in 1887
 These waves are created by the movement of
electrons and have the ability to propagate through
space.
 using appropriate antennas, transmission and reception
of electromagnetic waves through space becomes
feasible.
 the speed of electron vibration determines the wave’s
frequency.

Hertz: how many times the wave is repeated in 1
sec. (to honor Heinrich Hertz)
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Wavelength and Amplitude

l = wavelength, f = frequency, c = speed of
light
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Electromagnetic Spectrum

spectrum: range of electromagnetic
radiation
 band: spectrum parts
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Radio Waves

HF band enables worldwide transmission:
 HF signals are reflected off the ionosphere and thus
can travel very large distances
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Microwaves

small wavelengths compared to radio waves
 easily attenuated by objects
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Infrared

emitted by very hot objects
 such as human body (night vision applications)
 frequency depends on the temperature of the
emitting body

line-of-sight, point-to-point
 of no use outdoors (interfered by heat of sun)

short-rang: 10 meters
 IrDA: Infrared Data Association
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Microwave and Infrared Bands

Most wireless networking traffic is in the
microwave frequency bands.
 some licensed, some unlicensed

Infrared:
 for short-range wireless communication
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Spectrum Regulation

ITU = Int’l Telecommunications Union
 a worldwide spectrum regulation org.
 the world is split into 3 parts:
American continent
Europe, Africa, and former Soviet union
rest of Asia and Oceania

Rules of assigning spectrum
 lottery
 auction
 comparative bidding
such as pricing, technology, etc.
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Licensed Microwave Band

Examples: cellular, paging, PCS
 Use of a license is typically in an order of
10 years.
 A company can’t have the license and not use
it.
 Bandwidth is regarded as a resource that the
public wants and needs.
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Unlicensed Microwave Band

Also on the same microwave band, but no
license required.
 To avoid interfering primary (licensed) users,
spreading spectrum is required.
 Two types:
FHSS: Frequency-hopping spread spectrum
DSSS: Direct sequence spread spectrum

Also known as ISM band.
 industrial, scientific, and medical
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Model of Wireless Propagation
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Free space path loss
Doppler shift
Slow/fast fading
Error modeling
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Shannon’s Formula

an upper bound on the bit rate W of any channel of
bandwidth H Hz:
W = H log2(1 + S/N)
S/N = signal to thermal noise ratio
 However, in real world, the upper bound is
difficult to achieve due to:
 free space path loss
 proportional to r-2, where r is the distance between transmitter
and receiver (sometimes at higher exponent)
 Doppler shift
 a signal transmitter and receiver are moving relative to one
another
 slow/fast fading
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Slow Fading
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Definitions

Reflection:
 when an electromagnetic wave falls on an
object with dimension very large compared to
the wave’s wavelength

Scattering:
 when obstructed by objects with dimensions in
the order of the wavelength

Diffraction (or shadowing):
 when the wave falls on an impenetrable object
 in which case, the secondary waves are formed
behind the obstructing body
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Fast Fading: Multipath Effect

waves traveling along different paths may
be completely out of phase when they reach
the antenna (thereby canceling each other)
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
Multipath propagation delay can degrade
performance in indoor/outdoor environment.
 When the path length differences are short, the
effect is smaller.

multipath fading is also referred as fast
fading
 When LOS (line of sight) exists, this kind of
fading is known as Ricean Fading
 When LOS does not exist, this kind of fading
is known as Rayleigh Fading
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Propagation Models


We say that the relative strength
of signal x, P(x), to that of
signal y, P(y), is D dB, if
 D = 10 log10(P(x)/P(y))
In free space, the average path
loss (PL) at a distance of r is (in
dB):
 PL(r) = PL(r0) +
10n log(r/r0)
 r0 = reference distance
(typically 1 Km for
macrocells; and 100 m for
microcells)
 n = environmental factor
(typically >= 2)

To take into account of the
shadowing effect
 PL(r) = PL(r0) +
10n log(r/r0) + Xd
 Xd = zero-mean Gaussian
random variable with
standard deviation d
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Digital Modulation Techniques
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Binary Modulation
Phase Shift Keying
Minimum Shift Keying
p/4-Shifted QPSK
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Basics

Convert digital stream into the analog signal
A(t)cos(wt + ), where w = 2pf.
 The characteristics in this formulation that
may be changed are:
 amplitude
 frequency
 phase

Ex: ASK = amplitude shift keying; FSK =
frequency shift keying; PSK = phase shift
keying
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
Most systems modulate the information
onto a carrier centered in a (small) allocated
spectrum.
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Binary Modulation Scheme

Amplitude Shift Keying (ASK):
 using ON/OFF to represent 1/0
 “keying”: like a telegraph key

Frequency Shift Keying (FSK):
 1/0 represented by two different frequencies
separated by some distance
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Binary Phase Shift Keying

Binary Phase Shift Keying (BPSK)
 use alternative sine wave phases to encode bits
 simple to implement
 very robust, used extensively in satellite
communications
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Quarternary Phase Shift Keying

QPSK:
 multi-level modulation: 2 bits per symbol
 more spectrally efficient, more complex
receiver
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Differential PSK (DPSK)

1 = changing the phase relative to the
previous symbol by some amount
 0 = having the same phase as the previous
symbol
 adv: self-clocked
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p/4-Shifted QPSK
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
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coding by bit pairs
varying the phase of the
current bit pair to the
phase of the previous bit
pair by a multiple of p/4
example:
 10 10 01 (Fig. 2.27)
(i.e., -p/4, -p/4, +5p/4)
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Hybrid of PSK + ASK

QAM = Quadrate Amplitude Modulation
 mixture of PSK and ASK
 3 bits at a time
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Multiple Access

defining how nodes in a wireless
network to share a common
medium
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Objectives

MAC layer is to define how a user access a
channel when he needs one.
 Random access: ALOHA and CSMA
 Ordered access: Token bus and Token Ring
 Deterministic access: FDMA, TDMA, and
CDMA
 Combinations: TDMA-over-FDMA, TDDCDMA, and TDMA/CSMA
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FDMA

frequency division multiple access
** NMT = nordic Mobile Telephony
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TDMA

time division multiple access
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CDMA

code division multiple access
 each station has a “station code”
 each bit is encoded by station code
code 1 is mapped to 1
code 0 is mapped to -1
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ALOHA

A type of packet-radio network.
 The first well-known wireless network as
well as network system.
 Very simple, but not efficient!

Variations:
 pure-ALOHA: whenever desired, send the
packet
 slotted-ALOHA: further divide time axis into
slots
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CSMA
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Before sending, sense the carrier.
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Persistent and Non-persistent CSMA
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Persistent CSMA:
 when the medium is busy, a station can
persistently wait for the medium to become
idle, and then transmit with a probability p
 This is called 1-persistent or p-persistent
CSMA.

Non-persistent CSMA:
 A station can stop monitoring the wireless
medium, and listen to the medium again at
predefined time.
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Hidden-Node Problem
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CSMA has the following problem:
 when two nodes are too far away, carrier
sensing is difficult
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CSMA/CA

CA = collision avoidance
 sender first does carrier sense
 sender broadcasts RTS (request to send) to
receiver
 receiver broadcasts CTS (clear to send) to
sender
 then send data packet

Q: Is CSMA/CD possible in wireless
network?
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Ordered MAC Techniques

Can a token-ring or token-bus protocol be
applied to a wireless network?

Problems:
 mobility (nodes joining or leaving the ring)
 token loss
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Comparison and Summary
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Random access: CSMA
 under light load: fast response time
 under heavy load: throughput declines
 simplicity

Deterministic protocols: TDMA, FDMA
 guaranteed bandwidth
 larger average delay
 small delay variance

Hybrid: CSMA/TDMA
 adaptive, higher overhead
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Spread Spectrum


FHSS
DSSS
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Spread Spectrum Technology

Spread spectrum must be used in ISM band.
 Two major technologies:
 Frequency Hopping SS (FHSS)
 Direct Sequence SS (DSSS)

Located at the PHY of the network stack:
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FHSS

Most Wireless LANs
use the ISM bands as
secondary users.

Hopping Pattern
 In each time slot, the
occupied frequencies
 They must use SS in
are separated by some
order not to interfere
distance to avoid
with the primary users.
interference.
 FHSS: send info in
different frequencies
on different time slots.
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
FHSS is different from FDM (frequency
division multiplexing).
 Example: (Fig. 4.7)
 In the 2.4 GHz band of ISM, we have a space
of 80 MHz. (2400~2483MHz)
 A typical bandwidth of the information signal
is 1 MHz.
Maximum occupancy is 1MHz regulated by FCC.
 One time slot = 0.1 sec.
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Primary vs. Secondary Users

In FHSS, a typical power limit is 1 watt.
 For primary users, the power limit is much larger.
 So the interference from FHSS will not be noticeable
primary users.

For FHSS secondary user,
 when there is 1 primary user  there will be a
throughput loss of 1/80 = 1.25%;
 when there are 2 primary users  there will be a
throughput loss of 2/80 = 2.5%.
 fig 4.8
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primary
user
primary
user
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DSSS

The input data stream is transferred to a
chip stream that is x times higher by XOR.
 a chip is 0 or 1, but is called so to distinguish
from a bit
 example: x = 11, 13, 15, 16 chips/bit
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
The frequency spectrum is spread out and
the spectral energy is x times lower.
 It’s so low that primary users are not interfered.
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Comparison of Interference

Degradation due to existence of interference:
 FHSS: linear to the level of interference
 DSSS:
degraded by half after a certain point (since it
typically occupies 50% of the bandwidth)
won’t work after a certain level
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Link Budget Analysis

“Tutorial on Basic Link Budget
Analysis” Application Note, June
1998, AN9804.1, Intersil Co.
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Communication Basics

When evaluating a wireless link, there are 3
most important questions to be answered:
 How much radio frequency (RF) power is
available?
 How much bandwidth is available?
 What is the required reliability?
evaluated by BER (bit error rate)
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Link Budget Example 1
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Wireless Link to Modem
 required rate: 40 Kbps (28.8 Kbps plus
framing, overhead, checksum)
 range: 5 meters
 BER: 10-6
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
Choices of Technology:
 900 MHz
2.4GHz and 5GHz are not selected since the
required rate is low.
 no spread spectrum
since low transmission power is sufficient for 5
meters
 Orthogonal FSK
simplicity: two separated frequencies (one for “1”
and the other for “0”)
separated by 40 kHz (called “orthogonal” since
frequency-separation/bit-rate = 1)
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Link Budget Example 2

Wireless USB
 required data rate = 2 Mbps (1.408 Mbps plus
framing, overhead, and checksum)
 range = 30 meters
 BER = 10-6
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
Selection of Technologies:
 ISM band in 2.4 GHz (with 83MHz of band to
use)
 DSSS spreading to support long distance
transmission
will occupy 2 x 11 = 22 MHz of bandwidth due to
spreading
 DQPSK (differential quadrature phase shift
keyed) modulation
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Performance Increasing Techniques
for Wireless Networks
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antenna diversity
coding
power control
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Diversity

definition:
 to send multiple copies of the same
information signal through several channels

goal:
 to combat fading in wireless channels

example:
 time, frequency, antenna
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Antenna Diversity

also known as space diversity
 method
 a set of array elements (also referred to as
branches), spaced sufficiently apart from each
other
 usually 2 elements

can combat multipath fading
 because multipath fading is usually
independent at distances in the order of
channel’s wavelength
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Example

a 2-branch diversity system
 a number of algorithms have been proposed to
reconstruct the original transmission
 ex: pick the strongest signal from one of the
antennas
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Smart Antennas

multi-antennas that change in order to adapt
to the conditions of wireless channels
 can focus toward the receivers
 can focus to the transmitters
 also known as beamforming

Already available for
several years
 not widely used due to costs
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Coding

Parity check
 Hamming code
 Cyclic redundancy check (CRC)
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Power Control

properly tuning the transmission power to
reduce coverage and interference
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Summary

What have we discussed?








Electromagnetic Spectrum
Wireless Propagation Models
Digital Modulation Techniques
Multiple Access
Performance Issues
Cellular and Ad Hoc Concepts
Link Budget Analysis
Performance Improvement Techniques
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