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Ch 5. Wireless Network Principles Myungchul Kim [email protected]

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Page 1: s5 Principles

Ch 5. Wireless Network Principles

Myungchul Kim

[email protected]

Page 2: s5 Principles

SESSION: Wireless Communication Principles

•Wireless Network Classification•Transmitters/Receivers •Antennas•Frequency Allocation•Propagation Modes •Noise Characteristics•Signal Encoding•Error Detection and Correction

Page 3: s5 Principles

Wireless Networks

SatelliteSystems

CellularNetworks

Wireless LANs

Example1: GSM, 9.6 Kbps, wide coverage

Example2: 3G, 2 Mbps, wide coverage

Example1:802.11b 11 Mbps, 100 Meters

Other examples:802.11g, HiperLAN2

Wireless WANs

PersonalArea Networks

BusinessLANs

Example1:Bluetooth1 Mbps, 10 Meters

Other examples:wireless sensor networks, UWB

Example1: Motorola Iridiumup to 64 Mbps globally

Example 2:Deep space communication

WirelessLocal Loops(Fixed Wireless)

Wireless MANs

Example1: LMDS37 Mbps, 2-4 Km

Example2:FSO 1.25 Gbps 1-2 KM

Paging Networks

Example1:FLEX, 1.2 Kbps

Example2:ReFLEX, 6.4Kbps

Page 4: s5 Principles

Factors in Designing Wireless Networks

Factor Why Important

Frequency Allocations Need to know what frequency range to operate. May need licenses tooperate in several ranges

Location services Essential because the users of mobile services change their location (e.g., acellular phone).

Also important for 911 calls.

Multiple Access mechanisms How users can share the sme medium without interfering with ach other

Antennas and Propagation Help in understanding the type of errors and how to deal with them

Signal encoding and ErrorCorrection

Better encoding can improve data rate and correction can save valueabletime.

Page 5: s5 Principles

Wireless Frequency Allocation• Radio frequencies range from 9KHz to 400GHZ (ITU)• Microwave frequency range

– 1 GHz to 40 GHz– Directional beams possible– Suitable for point-to-point transmission– Used for satellite communications

• Radio frequency range– 30 MHz to 1 GHz – Suitable for omnidirectional applications

• Infrared frequency range– Roughly, 3x1011 to 2x1014 Hz– Useful in local point-to-point multipoint applications within confined areas

Page 6: s5 Principles

Wireless Radio Spectrum; Frequency Allocation

100 Km 3KHzVLF - very low frequency

10 Km 30KhzLF - low frequency

1 Km 300KHzMF - medium frequency

100 m 3MHzHF - high frequency

10 m 30 MHzVHF - very high frequency

1m 300 MHzUHF - ultra high frequency

100 mm 3GHz

10 mm 30GHz

SHF - super high frequency

1 mm 300 GHz

EHF - extra high frequency

0.1 m 3000 GHz

THF - terribly high frequency

Source: Bekkers, R. and Smits, J., “Mobile Telecommunications”, Artech, 2000.

FrequencyWavelength

Radio Waves

Micro Waves

Infrared

X-rays

Gamma-rays

Page 7: s5 Principles

Frequency Regulations• Frequencies from 9KHz to 300 MHZ in high demand (especially VHF: 30-300MHZ)• In wireless, lower frequencies (omnidirectional)• Two unlicensed bands in the US (counterparts elsewhere)

– Industrial, Science, and Medicine (ISM): 2.4 GHz– Unlicensed National Information Infrastructure (UNII): 5.2 GHz

• Regional, national, and international issues• Procedures for military, emergency, air traffic control, etc • Different agencies license and regulate

– www.fcc.gov - US – www.open.gov.uk/radiocom -- for UK – Others (e.g., ETSI, five agencies in Japan)

• Interferences across national borders handled through Radio Communications Bureaus

Page 8: s5 Principles

ITU (International Telecom Union)

• Headquartered in Geneva (next to UN)

• Several sectors:– ITU-R(radiocommunications)- several study

groups and World Radio Conferences (WCRs)– ITU-T (standards) - subsummed formerly

CCITT– ITU-D (development) - developing countries

Page 9: s5 Principles

National Telecommunications and Information Administration (NTIA - www.ntia.gov)

• NTIA is part of The United States Commerce Department • Maintain a spectrum chart, dated March 1996, that depicts the

radio frequency spectrum allocations to radio services operated within the United States.

• Graphically partitions the radio frequency spectrum, extending from 9 kHz to 300 GHz, into over 450 frequency bands

• Copies of this chart can be viewed on line at http://www.ntia.doc.gov/osmhome/allochrt.html; and printed copies of this chart are available from the U.S. Government Printing Office (ph: 202 512 1800; stock #: 003-000-00652-2 cost is: $6.00

Page 10: s5 Principles

Location Based Services (LBSs)

P u b l i c S w i t c h e dT e l e p h o n eN e t w o r k( P S T N )

M o b i l eS w i t c h i n gC e n t e r( M S C )

B a s e T r a n s c e i v e r S t a t i o n ( B T S ) M o b i l e U s e r

C e l l 1

C e l l 2

C o r d l e s s c o n n e c t i o n

W i r e d c o n n e c t i o n

H L R V L R

H L R = H o m e L o c a t i o n R e g i s t e r

V L R = V i s i t o r L o c a t i o n R e g i s t e r Techniques: •Cell-id based•GPS assisted•Angle of arrival•Others

Page 11: s5 Principles

• Table 5-4

Page 12: s5 Principles

Wireless Transmission

• Wireless Communication systems consist of:– transmitters

– Antennas: radiates electromagnetic energy into air

– Receivers

• In some cases, transmitters and receivers are on same device, called transceivers (e.g., cellular phones)

Transmitter Receiver

AntennaAntenna

Page 13: s5 Principles

Transmitters Amplifier

Oscilator

Mixer Filter Amplifier

Antenna

Transmitter

Suppose you want to generate a signal that is sent at 900 MHz and the original source generates a signal at 300 MHZ. • Amplifier - strengthens the initial signal• Oscilator - creates a carrier wave of 600 MHz• Mixer - combines original signal with oscilator and produces 900 MHz (does modulation, etc)• Filter - selects correct frequency (required by FCC)• Amplifier - Strengthens the signal before sending it (higher f in some cases)

Receivers perform similar operations but in reverse direction

Page 14: s5 Principles

Antennas• An antenna is an electrical conductor or system of

conductors to send/receive RF signals– Transmission - radiates electromagnetic energy into

space– Reception - collects electromagnetic energy from space

• In two-way communication, the same antenna can be used for transmission and reception

OmnidirectionalAntenna (lower frequency)

DirectionalAntenna (higherfrequency)

Page 15: s5 Principles

Radiation Patterns• Radiation pattern

– Graphical representation of radiation properties of an antenna

– Depicted as two-dimensional cross section

• Reception pattern– Receiving antenna’s equivalent to radiation pattern

Antenna Types

• Isotropic antenna (idealized)– Radiates power equally in all directions

• Dipole antennas– Half-wave dipole antenna (or Hertz antenna)

– Quarter-wave vertical antenna (or Marconi antenna)

• Parabolic Reflective Antenna (highly focussed, directional)

Page 16: s5 Principles

Smart Antennas• Basic idea: propagate signals to follow objects as they

move around and minimize noise. • Mixture of:

– Switched beam systems: a number of fixed beams at an antenna site – the beam with least interference and best signal strength is chosen.

– Adaptive antennas: array of antennas that can adjust patterns based on noise, interference, and location of objects

• Great deal of activity • Liberti, J. and Rappaport, T., “Smart Antennas for

Wireless Communications”, Prentice Hall

Page 17: s5 Principles

Smart Antennas

a) Coverage Pattern (Top View)for a Switched Beam Antenna with 4 Elements

b) Coverage Pattern (Top View)for an Adaptive Antenna givingPreferential treatment for the User and minimizing the Interferers

user

Interferer

Interferer

Interferer

Page 18: s5 Principles

Terrestrial Microwave (1GHz to 40GHz)• Description of common microwave antenna

– Most common: Parabolic "dish", 3 m in diameter– Fixed rigidly and focuses a narrow beam– Achieves line-of-sight transmission to receiving antenna (relays used in

between)– Located at substantial heights above ground level

• Applications– Long haul telecommunications service (instead of fiber, coax) -- requires

less repeaters but line of sight– Short point-to-point links between buildings (e.g, closed circuit TV,

LANs, bypass local telephone companies)– Most common BW= 4GHZ (can give up to 200 Mbps)

• Loss proportional to log (d/w)

Page 19: s5 Principles

Satellite Microwave (1GHz to 20 GHz, typically)

• Description of communication satellite– Microwave relay station– Used to link two or more ground-based microwave

transmitter/receivers– Receives transmissions on one frequency band (uplink),

amplifies or repeats the signal, and transmits it on another frequency (downlink)

• Applications– Television distribution (e.g., PBS uses satellite exclusively)– Long-distance telephone transmission between telephone

exchange offices– Private business networks (lease channels, expensive)

Page 20: s5 Principles

Broadcast Radio (30 MHz to 1GHz)• Description of broadcast radio antennas

– Omnidirectional (main differentiator from microwave) – Antennas not required to be dish-shaped– Antennas need not be rigidly mounted to a precise alignment

• Applications– Broadcast radio

• VHF and part of the UHF band; 30 MHZ to 1GHz• Covers FM radio and UHF and VHF television

– Due to new apps, the frequency range is expanded frequently

• Infrared – does not penetrate walls– used in remote control devices

Page 21: s5 Principles

Propagation Modes

Earth

Earth

Earth

a) Ground Wave Propagation

b) Sky Wave Propagation

c) Line-of-Sight Propagation

TransmissionAntenna

ReceivingAntenna

Signal

Signal

Ionosphere

Signal

Page 22: s5 Principles

• Table 5-5

Page 23: s5 Principles

LOS Wireless Transmission Impairments

• Attenuation and attenuation distortion

• Free space loss

• Noise

• Atmospheric absorption

• Multipath

• Refraction

• Thermal noise

Page 24: s5 Principles

Attenuation• Strength of signal falls off with distance over

transmission medium• Attenuation factors for unguided media:

– Received signal must have sufficient strength so that circuitry in the receiver can interpret the signal

– Signal must maintain a level sufficiently higher than noise to be received without error

– Attenuation is greater at higher frequencies, causing distortion

• Approach: amplifiers that strengthen higher frequencies

Page 25: s5 Principles

Categories of Noise

• Thermal Noise

• Intermodulation noise

• Crosstalk

• Impulse Noise

Page 26: s5 Principles

Multipath Propagation

R

D

S

R = R e f le c t io nS = S c a t te r in gD = D if f r a c t io n

Page 27: s5 Principles

Digital versus analog communications Fig 5-13

Page 28: s5 Principles

Signal Encoding (Modulation)

• Modulation of digital signals– When only analog transmission facilities are

available, digital to analog conversion required

• Modulation of analog signals– A higher frequency may be needed for effective

transmission– Modulation permits frequency division

multiplexing– PCM and variants used frequently

Page 29: s5 Principles

Pulse Code Modulation

• Based on the sampling theorem (sample rate should be higher than twice highest frequency)

• Each analog sample is assigned a binary code– Analog samples are referred to as pulse amplitude

modulation (PAM) samples

• The digital signal consists of block of n bits, where each n-bit number is the amplitude of a PCM pulse

• 8000 samples per second, 8 bits for levels (256)

Page 30: s5 Principles

Pulse Code Modulation

Amplitude

Samples

This shows 12 samples, each sample represents the amplitude of the wave. These samples as sent as digital data and then reconstructed into the original signal on the receiving side.

Page 31: s5 Principles

Delta Modulation• Analog input is approximated by staircase

function– Moves up or down by one quantization level

() at each sampling interval

• The bit stream approximates derivative of analog signal (rather than amplitude)– 1 is generated if function goes up– 0 otherwise

Page 32: s5 Principles

Delta Modulation

Signal

Amplitude

Time

Analog Signal

Staircase Function

Page 33: s5 Principles

Multiple Access Techniques

Time

Frequency Session1

Session2

Session3

Session4

Frequency Division Multiple Access (FDMA)

TimeF

requencyTime Division Multiple Access (TDMA)

PCM, PSK (6 ms frames)Used by AT&T wireless,Bellsouth, Ericsson

Session2

Session3

Session1

Session4

TimeF

requency

Code Division Multiple Access (CDMA)

All sessionsbased on a code

Rarely usedat present

Spread spectrum, DirectUsed by Sprint PCS, 3G systems

Page 34: s5 Principles

FDMA and TDMA• FDMA:

– FM radio divides the spectrum into 30 Khz channels.

– FDMA divides 30 Khz channels into 3 (10 KHz each)

– Base station cost is high and very limited capacity

• TDMA: – available since 1992

– each subscriber transmits at different times

– 6 millisecond frames, each divided into 1 ms time slots

– each time slot has a header and data

– errors may corrupt headers and cause time slots and in some cases the whole frame is lost

– TIA standard IS-54 defines the TDMA interface between a mobile station and cell-site radio (uses PCM for speech encoding, DQPSK for modulation)

– Call quality is similar to FDMA but can handle more calls (AT&T)

– Several extensions of TDMA (can support 15 users per voice channel)

Page 35: s5 Principles

CDMA • Based on spread spectrum - direct sequencing is more prevalent (TIA IS-95)• Groups of bits from digitized speech are tagged with a unique code that is

associated with a cellular call.• Several cellular calls are combined and transmitted over 1.25MHz and then

reassembled on the receiver side

• Receiver detects a signal by tuning to correct phase position between incoming and

locally generated signals from code • Speech coder operates at a variable rate (fully when user is talking) • Adjusts for near-far power adjustments (nearer stations generate less powerful

signals)• When powered on, the mobile system knows the CDMA frequency, so it tunes to

that frequency and searches for a pilot signal (pilot signals represent base stations) • Mobile station will pick the strongest pilot and register• When moving from cell to cell, new pilot is picked up

Page 36: s5 Principles

TDMA versus CDMA Controversy

• TDMA and CDMA are accepted TIA (telecom Industry Association) standards (IS-54, IS-95)

• Hardware vendors are lobbying hard • Many, many variants in industry• Performance reports are conflicting and confusing in terms of:

– Call clarity: CDMA appears to be better but questioned

– Network capacity: CDMA may be more efficient than TDMA

– Privacy: CDMA codes provide more privacy

– Economy: TDMA allows same equipment for multiple users

– Maturity: TDMA is very mature (in use since 1992)

– More features; TDMA offers more but CDMA can do it also

Page 37: s5 Principles

Spread Spectrum

Page 38: s5 Principles

Spread Spectrum• Input is fed into a channel encoder

– Produces analog signal with narrow bandwidth

• Signal is further modulated using sequence of digits – Spreading code or spreading sequence

– Generated by pseudonoise, or pseudo-random number generator

• Effect of modulation is to increase bandwidth of signal to be transmitted

• On receiving end, digit sequence is used to demodulate the spread spectrum signal

• Signal is fed into a channel decoder to recover data

Page 39: s5 Principles

Frequency Hopping Spread Spectrum (FHSS)

• Signal is broadcast over seemingly random series of radio frequencies

• Signal hops from frequency to frequency at fixed intervals

• Channel sequence dictated by spreading code

• Receiver, hopping between frequencies in synchronization with transmitter, picks up message

• Advantages– Eavesdroppers hear only unintelligible blips– Attempts to jam signal on one frequency succeed only at knocking

out a few bits

Page 40: s5 Principles

Frequency Hopping Spread Spectrum (FHSS)

f1 f2 f3 f4 f5 f6 f7 f8

Frequency

Energy4 7 5 1 6 8 3 2

DataBits

Page 41: s5 Principles

Direct Sequence Spread Spectrum (DSSS)

• Each bit in original signal is represented by multiple bits in the transmitted signal

• Spreading code spreads signal across a wider frequency band – Spread is in direct proportion to number of bits used

• One technique combines digital information stream with the spreading code bit stream using exclusive-OR

Page 42: s5 Principles

Code-Division Multiple Access (CDMA)

• Basic Principles of CDMA– D = rate of data signal– Break each bit into k chips

• Chips are a user-specific fixed pattern

– Chip data rate of new channel = kD

Page 43: s5 Principles

CDMA Example

• If k=6 and code is a sequence of 1s and -1s– For a ‘1’ bit, A sends code as chip pattern

• <c1, c2, c3, c4, c5, c6>

– For a ‘0’ bit, A sends complement of code• <-c1, -c2, -c3, -c4, -c5, -c6>

• Receiver knows sender’s code and performs electronic decode function

• <d1, d2, d3, d4, d5, d6> = received chip pattern• <c1, c2, c3, c4, c5, c6> = sender’s code

665544332211 cdcdcdcdcdcddSu

Page 44: s5 Principles

CDMA Example• User A code = <1, –1, –1, 1, –1, 1>

– To send a 1 bit = <1, –1, –1, 1, –1, 1>– To send a 0 bit = <–1, 1, 1, –1, 1, –1>

• User B code = <1, 1, –1, – 1, 1, 1>– To send a 1 bit = <1, 1, –1, –1, 1, 1>

• Receiver receiving with A’s code– (A’s code) x (received chip pattern)

• User A ‘1’ bit: 6 -> 1• User A ‘0’ bit: -6 -> 0• User B ‘1’ bit: 0 -> unwanted signal ignored

Page 45: s5 Principles

Coding and Error Control

Page 46: s5 Principles

Coping with Data Transmission Errors

• Error detection codes– Detects the presence of an error

• Automatic repeat request (ARQ) protocols– Block of data with error is discarded– Transmitter retransmits that block of data

• Error correction codes, or forward correction codes (FEC)– Designed to detect and correct errors

Page 47: s5 Principles

Error Detection Process

• Transmitter– For a given frame, an error-detecting code (check bits)

is calculated from data bits– Check bits are appended to data bits

• Receiver– Separates incoming frame into data bits and check bits– Calculates check bits from received data bits– Compares calculated check bits against received check

bits– Detected error occurs if mismatch

Page 48: s5 Principles

Wireless Transmission Errors• Error detection requires retransmission

• Detection inadequate for wireless applications– Error rate on wireless link can be high, results in a

large number of retransmissions– Long propagation delay compared to transmission

time

• Best to correct errors by using – Block Error Correction– Turbo Codes

Page 49: s5 Principles

Block Code (Error Correction) • Hamming distance – for 2 n-bit binary sequences, the number of different bits

– E.g., v1=011011; v2=110001; d(v1, v2)=3

• For each data block, create a codeword

• Send the codeword

• If the code is invalid, look for data with shortest hamming distance (possibly correct code)

Datablock (k=2) Codeword (n=5)

00 00000

01 00111

10 11001

11 11110

Suppose you receive codeword 00100 (error)

Closest is 00000 (only one bit different)

Page 50: s5 Principles

Turbo Codes• Good block error correction requires large codewords • Large codewords are complex to process and waste

bandwidth• Turbo codes break the codewords into two

– Two encoders on transmitters– Two decoders on receivers

• Shown to be very efficient • Main limitation: decoding is complex and introduces delays• Many applications in deep space communications • Very active area of work

Page 51: s5 Principles

Summary

• Wireless Network Classification• Transmitters/Receivers • Antennas• Frequency Allocation• Propagation Modes • Noise characteristics• Signal Encoding• Error Detection and Correction