HY539: Spring 2005 Wireless networks and mobile computing

Lecture2: Radio Channel Issues

Prof. Maria PapadopouliAssistant Professor

Department of Computer ScienceUniversity of North Carolina at Chapel Hill

Review of Last Lecture

Heterogeneous networks of devices with different capabilities

Pervasive computing spaces

Mobile Computing Challenges Battery capacity, Energy and Bandwidth constraints Mobility

Intermittent connectivity Delays Packet losses

Wireless networks more vulnerable than the wired ones Wireless infrastructures cannot support QoS for applications with

real-time constraints

Support Intelligent Mobile Clients

Efficient position sensing mechanisms Adaptive systems Monitor the environment and adapt based

on their resources (battery, application requirements, channel capacity, throughput) in an energy-efficient manner

Intelligent and robust wireless infrastructures

Roadmap

High-level introduction to

Mobile data access Physical layer

Mobile Data Access

Infrastructure Client-Server paradigm

Ad Hoc (without infrastructure) Peer-to-Peer paradigm

Fundamentals

Wireless channel model Antenna Impairments Radio Propagation Digital modulation and detection

techniques Error control techniques

Digital Radio Communications

Baseband Modulation

CarrierRadio

Channel

Transmitter

DataIn

Carrier Bit &FrameSync

Detection

Receiver

DecisionDataOut

Conversion of a stream of bits into signal

Conversion of the signal to a stream of bits

Adds redundancy to protect the digital information from noise and interference

Bits mapped to signal (analog signal waveform)

e.g., GFSK e.g., TDMA, CDMA

Transmitter & Radio Channel

TransmitterReceiver

Transmitter Multi-path Fading +

Noise

Receiver

Antenna Made of conducting material Radio waves hitting an antenna cause electrons to

flow in the conductor and create current Likewise, applying a current to an antenna creates

an electric field around the antenna. As the current of the antenna changes, so does the

electric field. A changing electric field causes a magnetic field,

and the wave is off …

Antenna (cont’d)

The gain is the extent to which it enhances the signal in its preferred direction

Measured in dBi, decibels relative to an isotropic radiator

Isotropic antenna: radiates power with unit gain uniformly in all directions

Assignment (for your log)

Different types of antennas Multiple & directional antennas

State of art Cost

Channel Coding

Often used to protect the digital information from noise and interference and reduce the number of bit errors

Accomplished by selectively introducing redundant bits into the transmitted information stream

These additional bits allow detection and correction of bit errors in the received data stream

Types of Impairments Noise (thermal, human) Radio frequency signal path loss Fading at low rates Inter-Symbol interference (ISI) Shadow fading Co-channel interference Adjacent channel interference

Impairments

Impacts radio system design Impact on indoor and outdoor

communications Difficult to control

Adjacent Channel Interference

Interference from signals adjacent in frequency to the desired signal

Results from imperfect receiver filters which allow nearby frequencies to leak into the passband

Prevented by keeping the frequency separation between each channel in a given cell as large as possible

Inter-Symbol Interference (ISI)

Overflowing symbols

ISI (cont’d) Waves that take different paths from the transmitter to

the receiver will travel different distances and be delayed with respect to each other

Waves are combined by superposition, but the effect is that the total waveform is garbled

Delay spread: time between the arrival of the first wavefront and the last multipath echo

Longer delay spreads require more conservative coding 802.11b networks can handle delay spreads of up to

500 ns, but performance is much better when the delay spread is lower

When delay spread is large, many cards reduce transmission rate

Limits of wireless channel How many bits of information can be transmitted

without error per sec over a channel with a bandwidth B, when the average signal power is limited to P watt, and the signal is exposed to an additive, white (uncorrelated) noise of power N with Gaussian probability distribution

Shannon (1916-2001) Norbert Wiener (1894-1964)

Shannon’s limit For a channel without shadowing, fading, or

ISI, the maximum possible data rate on a given channel of bandwidth B is

R=Blog2(1+SNR) bps, where SNR is the received signal to noise ratio

Shannon’s is a theoretical limit that cannot be achieved in practice but design techniques improve data rates to approach this bound

Signal-to-noise ratio (SNR) The ratio between the magnitude of

background noise and the magnitude of un-distorted signal (meaningful information) on a channel

Higher SNR is better (i.e., cleaner) It determines how much information each

symbol can represent

Propagation models One of the most difficult part of the radio channel

design Done in statistical fashion based on measurements

made specifically for an intended communication system or spectrum allocation

Predicting the average signal strength at a given distance from the transmitter

Large-scale propagation model: signal strength over large T-R separation distances

Small-scale or fading model: rapid fluctuations of the received signal strength over very short travel distances or short time durations (order of seconds)

Our measurements at UNC

Free-space propagation model

Used to predict received signal strength when the transmitter and receiver have a clear, unobstructed line-of-sight path between them

Examples: satellite, and microwave line-of-sight radio links

Derived from first principles - power flux density computation Any radiating structure produces electric and magnetic fields:

its current flows through such antenna and launches electric and magnetic fields

The electrostatic and inductive fields decay much faster with distance than the radiation field

At regions far way from the transmitter, the electrostatic and inductive fields become negligible and only the radiated field components need be considered

Free Space Model Pr(d)=PtGtGr2/[(4)2d2L]

Pt,Pr: transmitter/receiver power

Gt, Gr: transmitter/receiver antenna gain

G = 4Ae/2

L: system loss factor (L=1 no loss) Ae: related to the physical size of the antenna: wavelength in meters, f carrier frequency,

c :speed of light = c/f

Path Loss

Difference (in dB) between the effective transmitted power and the received power

Radio wave propagation

Electromagnetic wave propagation mechanisms are diverse

Due to reflection, diffraction, scattering

Propagation Mechanisms (cont’d)

Reflection: when a propagating electromagnetic wave impinges upon an object which has very large dimensions when compared to the wavelength of the propagating wave

Reflections occur from the surface of the earth, buildings, and walls

Diffraction: when the radio path between transmitter and receiver is obstructed by a surface that has sharp irregularities (edges)

Secondary wavelets into a shadowed region Scattering: when the medium through which the wave travels

consists of objects with dimensions that are small compared to the wavelength and where the number of obstacles per unit volume is large (e.g., street signs, lamp posts)

Reflected energy is spread out/diffused in all directions

Multipath Propagation

Wall

Wall

Transmitter Cabinet

Reflection

Diffraction (Shadow Fading)

Scattering

Receiver

Mobile radio channel A single direct path between the base station and the

mobile is seldom the only physical means for propagation Hence, the free space propagation model is inaccurate

when used alone

Two-ray ground reflection model considers both the direct path and a ground reflected propagation path between transmitter and receiver

Reasonably accurate for predicting the large-scale signal strength over distances of several km for mobile radio systems that use tall tower (heights which exceed 40m) or for line-of-sight micro-cell channels in urban environment

Two-ray ground reflection model

d

T (transmitter)

R (receiver)ht

hr

Pr(d)=PtGtGrhr2ht2/d4

Wave combination by superposition

When multiple waves converge on a point, the total wave is simply the sum of any component waves

Modulation

The process of taking information from a message source (baseband) in a suitable manner for transmission

It involves translating the baseband signal onto a radio carrier at frequencies that are very high compared to the baseband frequency

Demodulation

The process of extracting the baseband from the carrier so that it may be processed and interpreted by the receiver (e.g., symbols detected and extracted)

Why not modulate the baseband

We must consider the fact that for effective signal radiation the length of the antenna must be proportional to the transmitted wave length For example, voice range 300-3300Hz At 3kHz at 3kbps would imply an antenna of 100Km! By modulating the baseband on a 3GHz carrier the

antenna would be 10cm

To ensure the orderly coexistence of multiple signals in a given spectral band

To help reduce interference among users For regulatory reasons

Modulations Techniques

Carrier wave s : s(t)=A(t)*cos[(t)] A(t) time varying amplitude Time varying angle (t)(t)= + (t) phase (t), : radian frequency

Ideal Digital Modulation Provides low bit error rates at low received

signal-to-noise ratio Performs well in multi-path and fading

conditions Occupies a minimum of bandwidth Is easy and cost-effective to implement

Existing modulation schemes do not simultaneously satisfy all of these requirements

Performance of Modulation Schemes

Tradeoff between fidelity and signal power To increase noise immunity, it is necessary to

increase the signal power

Power efficiency: the amount by which the signal power should be increased to obtain a certain level of fidelity (ie acceptable bit error probability) depends on the particular type of modulation

Bandwidth efficiency: the ability to accommodate data within a limited bandwidth

Modulation Examples: Frequency Hopping

Time slot

Frequencyslot

Timing the hops accurately is the challenge

0

1

2

3

4

5User A

User B

Reading material on 802.11 802.11 Wireless Networks, The definitive guide.

Matthew S. Gast, O'Reilly, 2002, ISBN 0-596-00183-5. http://www.csd.uoc.gr/~maria/802.11.book.pdf

Papers: http://sss-mag.com/pdf/802_11tut.pdf

http://sss-mag.com/pdf/80211p.pdf Theoretical paper on its performance:

http://www.ece.utexas.edu/~jandrews/ee381k/EE381KTA/802.11_throughput.pdf