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Wireless Transmission Fundamentals(Dayem’s book, Chapter 4)

(Nico’s book, Chapter 2)

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 continentEurope, Africa, and former Soviet unionrest 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 spectrumDSSS: Direct sequence spread spectrum

Also known as ISM band. industrial, scientific, and medical

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Model of Wireless Propagation

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 shifta 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) + X

X = zero-mean Gaussian random variable with standard deviation

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Digital Modulation Techniques

Binary Modulation Phase Shift Keying Minimum Shift Keying /4-Shifted QPSK

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Basics Convert digital stream into the analog signal

A(t)cos(wt + ), where w = 2f. 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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/4-Shifted QPSK coding by bit pairs varying the phase of the

current bit pair to the phase of the previous bit pair by a multiple of /4

example: 10 10 01 (Fig. 2.27)

(i.e., -/4, -/4, +5/4)

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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, TDD-

CDMA, 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 1code 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

Before sending, sense the carrier.

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Persistent and Non-persistent CSMA

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

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 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. They must use SS in

order not to interfere with the primary users.

FHSS: send info in different frequencies on different time slots.

Hopping Pattern In each time slot, the

occupied frequencies are separated by some distance to avoid interference.

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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

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 spectrumsince low transmission power is sufficient for 5

meters

Orthogonal FSKsimplicity: 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

transmissionwill occupy 2 x 11 = 22 MHz of bandwidth due to

spreading

DQPSK (differential quadrature phase shift keyed) modulation

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Performance Increasing Techniquesfor Wireless Networks

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