Wireless Transmission Fundamentals (Dayem’s book, Chapter ...

tseng:1

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

tseng:2國立交通大學國立交通大學國立交通大學國立交通大學資訊工程系資訊工程系資訊工程系資訊工程系曾煜棋曾煜棋曾煜棋曾煜棋教授教授教授教授

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)


Wavelength and Amplitude

� l = wavelength, f = frequency, c = speed of

light


Electromagnetic Spectrum

� spectrum: range of electromagnetic

radiation

� band: spectrum parts


Radio Waves

� HF band enables worldwide transmission:

� HF signals are reflected off the ionosphere and thus can travel very large distances


Microwaves

� small wavelengths compared to radio waves

� easily attenuated by objects


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


Microwave and Infrared Bands

� Most wireless networking traffic is in the

microwave frequency bands.

� some licensed, some unlicensed

� Infrared:

� for short-range wireless communication

國立交通大學國立交通大學國立交通大學國立交通大學資訊工程系資訊工程系資訊工程系資訊工程系曾煜棋曾煜棋曾煜棋曾煜棋教授教授教授教授


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.


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.


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

tseng:12

Model of Wireless Propagation

� Free space path loss

� Doppler shift

� Slow/fast fading

� Error modeling 國立交通大學國立交通大學國立交通大學國立交通大學資訊工程系資訊工程系資訊工程系資訊工程系曾煜棋曾煜棋曾煜棋曾煜棋教授教授教授教授


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


Slow Fading


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


Fast Fading: Multipath Effect

� waves traveling along different paths may

be completely out of phase when they reach

the antenna (thereby canceling each other)



� 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


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) + Xδ

� Xδ

= zero-mean Gaussian

random variable with

standard deviation δ


tseng:19

Digital Modulation Techniques

� Binary Modulation

� Phase Shift Keying

� Minimum Shift Keying

� π/4-Shifted QPSK 國立交通大學國立交通大學國立交通大學國立交通大學資訊工程系資訊工程系資訊工程系資訊工程系曾煜棋曾煜棋曾煜棋曾煜棋教授教授教授教授


Basics

� Convert digital stream into the analog signal A(t)cos(wt + θ), where w = 2πf.

� 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


� Most systems modulate the information

onto a carrier centered in a (small) allocated

spectrum.




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


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


Quarternary Phase Shift Keying

� QPSK:

�multi-level modulation: 2 bits per symbol

�more spectrally efficient, more complex

receiver



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


ππππ/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 π/4

� example:

� 10 10 01 (Fig. 2.27)

(i.e., -π/4, -π/4, +5π/4)



Hybrid of PSK + ASK

� QAM = Quadrate Amplitude Modulation

�mixture of PSK and ASK

� 3 bits at a time

tseng:29

Multiple Access

� defining how nodes in a wireless

network to share a common

medium



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


FDMA

� frequency division multiple access

** NMT = nordic Mobile Telephony


TDMA

� time division multiple access


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


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


CSMA

� Before sending, sense the carrier.


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.


Hidden-Node Problem

� CSMA has the following problem:

�when two nodes are too far away, carrier

sensing is difficult


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?



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


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

tseng:42

Spread Spectrum

� FHSS

� DSSS



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:


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.



� 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.


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


primary

user

primary

user


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



� The frequency spectrum is spread out and

the spectral energy is x times lower.

� It’s so low that primary users are not interfered.



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




tseng:53

Link Budget Analysis

� “Tutorial on Basic Link Budget

Analysis” Application Note, June

1998, AN9804.1, Intersil Co.



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)


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


� 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)


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


� 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

tseng:59

Performance Increasing Techniques

for Wireless Networks

� antenna diversity

� coding

� power control



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


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


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



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



Coding

� Parity check

� Hamming code

� Cyclic redundancy check (CRC)


Power Control

� properly tuning the transmission power to

reduce coverage and interference


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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