rf100

Wireless CDMA RF

Engineering: Week 1

Wireless CDMA RF

Engineering: Week 1

Integrated RF/CDMA/Performance Training

•Wireless Industry Intro. •Modulation Techniques •Mult. Access Methods •Wireless system Architectures •RF Propagation •Physics •Mechanisms •Models •Link Budgets •Margins •Pred. Tools •Meas. Tools •Wireless Antennas •Intro: Principles •Families/Types •Choosing the right antenna •Selecting ants. •Other devices •Tests/Problems •Traffic Engineering •Units, principles •Traffic tables •Wireless appls. •Introduction to CDMA •Spread Sp. Principles •CDMA’s Codes •Fwd & Rev Channels •System Architecture •Power Control •Phone Architecture •Handoff Process •Ec/Io, Eb/No •phone’s limitations •Call Processing •CDMA Messages

•CDMA Flow Examples •Critical CDMA Issues

•Interference control •Managing Soft HO% •Capacity constraints •Forward big picture •Reverse big picture •Sys Architecture details

•Lucent •Nortel •Motorola •System Growth Mgt. •Stopgap measures •Longterm strategies •Multiple carriers •Intercarrier Handoff •Intro to Optimization •Perspectives •Bottom-up: mobile •Top-down: OMs •Survey of Tools •Performance Goals •Design Implications

Monday Tuesday Wednesday Thursday Friday

Course RF100: RF Introduction, CDMA Principles, Understanding System Design & Performance Issues

Course RF200: Optimization Principles, Tools, Techniques, and Real-Life Examples/Exercises •Optimization Overview

•RF100 Fast Review •General Q&A •Meet the CDMA performance indicators •Signatures of CDMA transmission problems •The classic CDMA death scenario •Introduction to Performance Data •System-side tools and their implications

•Intro to Mobile Tools •Collection Tools •Grayson, LCC, HP •PN Scanners •HP, Grayson, Berkeley •Post-processing •Analyzer, DeskCat •Drive-test Demo files

•Grayson •LCC

•Intro to Post-Processing •Analyzer, DeskCat

•Handsets as test tools •Drive-Test Demo Lab

•RSAT/Collect 2000! •Grayson Inspector •Data Analysis and Post-Processing

•Analyzer, DeskCat •what events did you see? •Identifying root causes •Parameter & configuration changes •Operators’ Corporate RF Benchmarking Overview •PN Scanner Lab •HP, Grayson, Berkeley •Gathering data, interpreting problems •Applied Optimization •common scenarios

Wireless Systems:

How did we get here? What’s it all about?

Wireless Systems:

How did we get here? What’s it all about?

RF100 Chapter 1

MTS, IMTS

Radio Hasn’t Been Around Long!

Days before radio...

• 1680 Newton first suggested

concept of spectrum, but for visible light only

• 1831 Faraday demonstrated that

light, electricity, and magnetism are related

• 1864 Maxwell’s Equations:

spectrum includes more than light • 1890’s First successful demos of

radio transmission

U

N S

First Wired Communication: Telegraphy

„ Samuel F.B. Morse had the idea of the telegraph on a sea cruise in the 1833. He studied physics for two years, and In 1835 demonstrated a working prototype, which he patented in 1837.

„ Derivatives of Morse’ binary code are still in use today „ The US Congress funded a demonstration line from

Washington to Baltimore, completed in 1844.

„ 1844: the first commercial telegraph circuits were coming into use. The railroads soon were using them for train dispatching, and the Western Union company resold idle time on railroad circuits for public telegrams, nationwide „ 1857: first trans-Atlantic submarine cable was installed

Samuel F. B. Morse

at the peak of his career

Field Telegraphy

during the US Civil War, 1860’s

Submarine Cable Installation

Wired Communication for Everyone:

Telephony

„ By the 1870’s, the telegraph was in use all over the world and largely taken for granted by the public, government, and business.

„ In 1876, Alexander Graham Bell patented his telephone, a device for carrying actual voices over wires.

„ Initial telephone demonstrations sparked intense public interest and by the late 1890’s, telephone service was available in most towns and cities across the USA

Telephone Line Installation Crew Alexander Graham Bell and his phone

Radio Milestones

„ 1888: Heinrich Hertz, German physicist, gives lab demo of existance of electromagnetic waves at radio frequencies „ 1895: Guglielmo Marconi demonstrates a wireless radio

telegraph over a 3-km path near his home it Italy

„ 1897: the British fund Marconi’s development of reliable radio telegraphy over ranges of 100 kM

„ 1902: Marconi’s successful trans-Atlantic demonstration „ 1902: Nathan Stubblefield demonstrates voice over radio „ 1906: Lee De Forest invents “audion”, triode vacuum tube

• feasible now to make steady carriers, and to amplify signals „ 1914: Radio became valuable military tool in World War I „ 1920s: Radio used for commercial broadcasting

„ 1940s: first application of RADAR - English detection of incoming German planes during WW II

„ 1950s: first public marriage of radio and telephony

-MTS, Mobile Telephone System

„ 1961: transistor developed: portable radio now practical „ 1961: IMTS - Improved Mobile Telephone Service

„ 1970s: Integrated circuit progress: MSI, LSI, VLSI, ASICs „ 1979, 1983: AMPS cellular demo, commercial systems

Guglielmo Marconi

radio pioneer, 1895

Lee De Forest

vacuum tube inventor

MTS, IMTS

Overview of the Radio Spectrum

Frequencies Used by Wireless Systems

3 4 5 6 7 8 9 10 12 14 16 18 20 22 24 26 28 30 GHz

30,000,000,000 i.e., 3x1010_Hz

Broadcasting Land-Mobile Aeronautical Mobile Telephony

Terrestrial Microwave Satellite

0.3 0.4 0.5 0/6 0.7 0.8 0.9 1.0 1.2 1.4 1.6 1.8 2.0 2.4 3.0 GHz 3,000,000,000 i.e., 3x109 _Hz UHF TV 14-69 UHF GPS DCS, PCS Cellular 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.2 1.4 1.6 1.8 2.0 2.4 3.0 MHz 3,000,000 i.e., 3x106 _Hz AM LORAN Marine 3 4 5 6 7 8 9 10 12 14 16 18 20 22 24 26 28 30 MHz 30,000,000 i.e., 3x107_Hz

Short Wave -- International Broadcast -- Amateur CB

30 40 50 60 70 80 90 100 120 140 160 180 200 240 300 MHz

300,000,000 i.e., 3x108_Hz

FM VHF TV 7-13

„ In the late 1970’s, the FCC (USA Federal Communications Commission) allocated 40 MHz. of spectrum in the 800 MHz. range for public mobile telephony.

„ FCC adopted Bell Lab’s AMPS (Advanced

Mobile Phone System) standard, creating cellular as we know it today

• The USA was divided into 333 MSAs (Metropolitan Service Areas) and over 300 RSAs (Rural Service Areas)

„ In 1987, FCC allocated an additional 10 MHz. of “expanded spectrum” „ By 1990, all MSAs and RSAs had competing licenses granted and at

least one system operating.

„ In the 1990’s, additional technologies were developed for cellular • TDMA (IS-54,55,56, IS-136) (also, GSM in Europe/worldwide) • CDMA (IS-95)

„ US Operators did not pay for their spectrum, although processing fees (typically $10,000’s) were charged to cover license administrative cost

Development of North American Cellular

333 MSAs

300+ RSAs

North American Cellular Spectrum

„ In each MSA and RSA, eligibility for ownership was restricted

• “A” licenses awarded to non-telephone-company applicants only • “B” licenses awareded to existing telephone companies only

• subsequent sales are unrestricted after system in actual operation

Downlink Frequencies (“Forward Path”) Uplink Frequencies (“Reverse Path”) 824 835 845 870 880 894 869 849 846.5 825 890 891.5 Frequency, MHz

Paging, ESMR, etc.

A B A B

Ownership and

Licensing

Frequencies used by “A” Cellular Operator

Initial ownership by Non-Wireline companies

Frequencies used by “B” Cellular Operator

Development of North America PCS

51 MTAs

493 BTAs

A D B E F C unlic._data unlic._voice A D B E F C

1850 1910 1930 1990

15 5 15 5 5 15 15 5 15 5 5 15

PCS SPECTRUM ALLOCATIONS IN NORTH AMERICA

„ By 1994, US cellular systems were seriously overloaded and looking for capacity relief

• The FCC allocated 120 MHz. of spectrum around 1900 MHz. for new wireless telephony known as PCS (Personal Communications Systems), and 20 MHz. for unlicensed services

• allocation was divided into 6 blocks; 10-year licenses were auctioned to highest bidders „ PCS Licensing and Auction Details

• A & B spectrum blocks licensed in 51 MTAs (Major Trading Areas )

• Revenue from auction: $7.2 billion (1995)

• C, D, E, F blocks were licensed in 493 BTAs (Basic Trading Areas)

• C-block auction revenue: $10.2 B, D-E-F block auction: $2+ B (1996)

• Auction winners are free to choose any desired technology

• About half the C-block winners were unable to pay for their licenses. They wrangled in and out of court, with final disposition in 2005.

Major PCS Auction Winners

The Largest Players, Areas, and Technologies

„ Sprint PCS

• Began as partnership of Sprint, TCI, Cox Cable

• Bid & won in 2/3 of US markets A or B blocks

• Sprint won D and/or E blocks in remaining markets

• CDMA: Mix of Nortel, Lucent, Motorola

„ AT&T Wireless Systems

• Bid & won a majority of markets in A&B Blocks

• will combine and integrate service between its new PCS 1900 systems and its former McCaw cellular 800 MHz. properties

• IS-136: mix of Lucent and Ericsson equipment

„ Other CDMA Operators

• Primeco: partnership of various operators

• GTE, others

„ GSM Operators

• Western Wireless, OmniPoint, BellSouth, GTE, Powertel, Pacific Bell

• Mix of Ericsson, Nokia, and Nortel networks „ For auction details, check www.fcc.gov

Sprint PCS

CDMA

AT&T Wireless

IS-136

Primeco

CDMA

Western Wireless Pacific Bell Powertel BellSouth OmniPoint Aerial

GSM

Progress in Radio Technology Development

Systems, Signals, & Devices

RADAR Spark Vacuum Tubes Discrete Transistors MSI LSI VLSI, ASICS Devices Modulation CW AM FSK FM PM PSK QAM DQPSK GMSK

Radio Communication Systems

Mobile Telephony_30-50MHz 150MHz 450MHz 800MHz 1900MHz AM Bcst_1MHz FM Bcst_100MHz VHF-TV Bcst UHF-TV Bcst HF_Amateur Marine Military VHF_{Land Mobile} Microwave Point-to-Point Microwave Satellite 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 Time

Evolution of Wireless Telephony

Standards, Technologies, & Capacity

1960 1990

Standards Evolution

MTS_150MHz IMTS_150MHz 450MHz AMPS_800MHz N_AMPS D-AMPS CDMA PCS_1900MHz GSM CDMA AMPS, etc ESMR_800MHz

System Capacity Evolution - Users

Dozens Hundreds 100,000’s 1,000,000’s

Technology Evolution

Analog _{AM, FM} Digital Modulation DQPSK GMSK Access Strategies FDMA TDMA CDMA

Vacuum Tubes Discrete Transistors MSI LSI VLSI, ASICs

AMPS = Advanced Mobile Phone System N_AMPS = Narrowband AMPS (Motorola)

D-AMPS = Digital AMPS (IS-54 TDMA)

PCS-1900 = Personal Communication Systems FDMA = Frequency Division Multiple Access TDMA = Time Division Multiple Access

Trends in Radio Communications

Time

Cost per Subscriber

System Complexity System Capacity

Radio Frequencies Used

Analog Digital

Centralized Distributed

Technology:

System

Organization:

Wireless Systems:

Modulation Schemes and Bandwidth

Wireless Systems:

Modulation Schemes and Bandwidth

RF100 Chapter 2

f_c f_c Upper Sideband Lower Sideband f_c I axis Q axis a b φ c QPSK I axis Q axis c a φ b p r v π/4 shifted DQPSK

1 0 1 0

1 0 1 0

1 0 1 0

Characteristics of a Radio Signal

„ The purpose of telecommunications is to send information from one place to another „ Our civilization exploits the transmissible

nature of radio signals, using them in a sense as our “carrier pigeons”

„ To convey information, some characteristic of the radio signal must be altered (I.e., ‘modulated’) to represent the information „ The sender and receiver must have a

consistent understanding of what the variations mean to each other

• “one if by land, two if by sea” „ Three commonly-used RF signal

characteristics which can be varied for information transmission:

•

Amplitude

•

Frequency

•

Phase

SIGNAL CHARACTERISTICS

S

(t) = A cos

[ ω

_c t +

ϕ

]

The complete, time-varying radio signal

Amplitude(strength) of the signal

Natural Frequency

of the signal

Phaseof the signal

Compare these Signals:

Different Amplitudes Different Frequencies Different Phases

AM: Our First “Toehold” for Transmission

„ The early radio pioneers could only turn their crude transmitters on and off. They could form the dots and dashes of Morse code. The first successful radio experiments happened during the mid-1890’s by experimenters in Italy, England, Kentucky, and elsewhere.

„ By 1910, vacuum tubes gave experimenters better control over RF power generation. RF power could now be linearly modulated in step with sound vibrations. Voices and music could now be transmitted!! Still, nobody anticipated FM, PM, or digital signals. „ Commercial public AM broadcasting began in the early 1920’s. „ Despite its disadvantages and antiquity, AM is still alive:

• AM broadcasting continues today in 540-1600 KHz.

• AM modulation remains the international civil aviation standard, used by all commercial aircraft (108-132 MHz. band).

• AM modulation is used for the visual portion of commercial television signals (sound portion carried by FM modulation)

• Citizens Band (“CB”) radios use AM modulation

• Special variations of AM featuring single or independent

sidebands, with carrier suppressed or attenuated, are used for marine, commercial, military, and amateur communications

SSB

Amplitude Modulation (“AM”) Details

TIME-DOMAIN VIEW of AM MODULATOR x(t) = [1 + am_n(t)]cos ω_c t where: a = modulation index (0 < a <= 1) m_n(t) = modulating waveform ω_c= 2π f_c, the radian carrier freq.

Σ

a 1 + + x(t) cos ω_c m_n(t)

„ AM is “linear modulation” -- the

spectrum of the baseband signal translates directly into sidebands on both sides of the carrier frequency

„ Despite its simplicity, AM has definite

drawbacks which complicate its use for wireless systems:

• Only part of an AM signal’s energy actually carries information

(sidebands); the rest is the carrier

• The two identical sidebands waste bandwidth

• AM signals can be faithfully

amplified only by linear amplifiers

• AM is highly vulnerable to external noise during transmission

• AM requires a very high C/I (~30 to 40 dB); otherwise, interference is objectionable FREQUENCY-DOMAIN VIEW Voltage Frequency 0 f_c m_n(t) BASEBAND x(t) UPPER SIDEBAND LOWER SIDEBAND CARRIER

Circuits to Generate & Detect AM Signals

„ AM modulation can be simply accomplished in a saturated amplifier

• superimpose the modulating waveform on the supply

voltage of the saturated amplifier

„ AM de-modulation (detection) can be easily performed using a

simple envelope detector

• example: half-wave rectifier • this “non-coherent” detection

works well if S/N >10 dB. „ AM demodulation can also be

performed by coherent detectors • incoming signal is mixed

(multiplied) with a locally generated carrier

• enhances performance when S/N ratio is poor (<10 dB.) TIME-DOMAIN VIEW: AM MODULATOR x(t)

∼

∼

cos ω_c m_n(t) [1 + am_n(t)] Sat. Lin. TIME-DOMAIN VIEW: AM DETECTOR (non-coherent) x(t)

∼

m_n(t) information RF carrier Modulated signal

Better Quality: Frequency Modulation (“FM”)

„ Frequency Modulation (FM) is a type of angle modulation

• in FM, the instantaneous frequency of the signal is varied by the

modulating waveform

„ Advantages of FM

• the amplitude is constant

– simple saturated amplifiers can

be used

– the signal is relatively immune

to external noise

– the signal is relatively robust;

required C/I values are typically 17-18 dB. in wireless

applications

„ Disadvantages of FM

• relatively complex detectors are required

• a large number of sidebands are produced, requiring even larger bandwidth than AM TIME-DOMAIN VIEW

s

_FM(t) =A cos

[

ω_c t + m_ω m(x)dx+ϕ₀

]

t t₀ where:

A = signal amplitude (constant)

ω_c = radian carrier frequency

m_ω = frequency deviation index

m(x) = modulating signal ϕ₀ = initial phase FREQUENCY-DOMAIN VIEW Voltage Frequency 0 f_c S_FM(t) UPPER SIDEBANDS LOWER SIDEBANDS

Circuits to Generate and Detect FM Signals

„ One way to build an FM signal is a

voltage-controlled oscillator

• the modulating signal varies a reactance (varactor, etc.) or otherwise changes the

frequency of the oscillator

• the modulation may be

performed at a low intermediate frequency, then heterodyned to a desired communications

frequency

„ FM de-modulation (detection) can

be performed by any of several types of detectors

• Phase-locked loop (PLL)

• Pulse shaper and integrator

• Ratio Detector TIME-DOMAIN VIEW: FM MODULATOR

s

FM(t) m(x)

_~

VCO

x

LO HPA TIME-DOMAIN VIEW: FM DETECTOR

x

LO LNA PLL

s

FM(t) m(x) information FM modulated signal

The Inventor of FM

Major Edwin H. Armstrong was one of the most famous

inventors in the early history of radio. In 1918, he invented the superheterodyne circuit -- and implemented the basic mixing principle of heterodyne frequency conversion used in virtually all modern radio receivers. Others got the credit. In 1933, he invented wide-band frequency modulation.

Armstrong’s primary motivation was to improve the audio quality of broadcast transmission, which had suffered from noise and static because it used AM modulation.

Promotion and commercial development of FM placed Armstrong in competition with David Sarnoff and Radio Corporation of America. Sarnoff and RCA were promoting television, and worried Armstrong’s FM would compete with TV and slow its public acceptance.

Mainly due to RCA influence, the US FCC decided to change the frequencies allocated for FM broadcasting, obsoleting hundreds of FM transmitters and 500,000+ home receivers Armstrong had helped finance as an FM demonstration. In 1954, despondent over these setbacks, Armstrong took his

life. But today, the technology he started is used not only in broadcasting and the sound portion of TV, but also in land mobile and first-generation analog cellular systems.

Edwin Howard

Armstrong

Sister of FM: Phase Modulation (“PM”)

„ Phase Modulation (PM) is a type of angle

modulation, closely related to FM

• the instantaneous phase of the signal is varied according to the modulating waveform

„ Advantages of PM: very similar to FM

• the amplitude is constant

– simple saturated amplifiers can

be used

– the signal is relatively immune

to external noise

– the signal is relatively robust;

required C/I values are typically 17-18 dB. in wireless

applications

„ Disadvantages of PM

• relatively complex detectors are required, just like FM

• a large number of sidebands are produced, just like FM, requiring even larger bandwidth than AM

TIME-DOMAIN VIEW

s_PM(t) =A cos

[

ω_c t + m_ω m(x) +ϕ₀

]

where:

A = signal amplitude (constant)

ω_c = radian carrier frequency

m_ω = phase deviation index

m(x) = modulating signal ϕ₀ = initial phase FREQUENCY-DOMAIN VIEW Voltage Frequency 0 f_c S_FM(t) UPPER SIDEBANDS LOWER SIDEBANDS information Phase-modulated signal

Circuits to Generate and Detect PM Signals

„ PM and FM signals are identical with only one exception: in FM, the analog modulating signal is inherently de-emphasized by 1/F

„ Consequences of this realization:

• the same types of circuitry can be used to generate and detect both analog PM or FM, determined by filtering the modulating signal at baseband

• FM has poorer signal-to-noise ratio than PM at high modulating frequencies. Therefore,

pre-emphasis and de-pre-emphasis are often used in FM systems

TIME-DOMAIN VIEW: FM DETECTOR FOR PM

x

LO LNA PLL

s

FM(t) m(x)

The phase of an FM signal is proportional to the integral of the amplitude of the modulating signal. The phase of a PM signal is proportional

to the amplitude of the modulating signal. TIME-DOMAIN VIEW: PHASE MODULATOR

s

FM(t) m(x)

~

Phase Shifter

x

LO HPA information Phase-modulated signal

How Much Bandwidth do Signals Occupy?

„ The bandwidth occupied by a signal

depends on:

• input information bandwidth

• modulation method

„ Information to be transmitted, called “input” or “baseband”

• bandwidth usually is small, much lower than frequency of carrier

„ Unmodulated carrier

• the carrier itself has Zero bandwidth!!

„ AM-modulated carrier

• Notice the upper & lower sidebands • total bandwidth = 2 x baseband

„ FM-modulated carrier

• Many sidebands! bandwidth is a complex Bessel function

• Carson’s Rule approximation 2(F+D)

„ PM-modulated carrier

• Many sidebands! bandwidth is a complex Bessel function

Voltage Time Time-Domain (as viewed on an Oscilloscope) Frequency-Domain (as viewed on a Spectrum Analyzer) Voltage Frequency 0 f_c f_c Upper Sideband Lower Sideband f_c

Claude Shannon:

The Einstein of Information Theory

„ The core idea that makes CDMA possible was first explained by Claude Shannon, a Bell Labs research mathematician

„ Shannon's work relates amount of information carried, channel bandwidth, signal-to-noise-ratio, and detection error probability

• It shows the theoretical upper limit attainable

In 1948 Claude Shannon published his landmark paper on information theory, A Mathematical

Theory of Communication. He observed that

"the fundamental problem of communication is that of reproducing at one point either exactly or approximately a message selected at another point." His paper so clearly established the foundations of information theory that his

framework and terminology are standard today. Shannon died Feb. 24, 2001, at age 84.

SHANNON’S

CAPACITY EQUATION

C

=

B

_ω log₂

[

1 + S

]

N

B_ω = bandwidth in Hertz

C = channel capacity in bits/second S = signal power

Digital Sampling and Vocoding

Introduction to Digital Modulation

„ The modulating signals shown in previous slides were all analog. It is also possible to quantize modulating signals, restricting them to discrete values, and use such signals to perform digital modulation. Digital

modulation has several advantages over analog modulation:

„ Digital signals can be more easily regenerated than analog

• in analog systems, the effects of noise and distortion are cumulative: each demodulation and remodulation

introduces new noise and distortion, added to the noise and distortion from previous demodulations/remodulations.

• in digital systems, each demodulation and remodulation produces a clean output signal free of past noise and distortion

„ Digital bit streams are ideally suited to multiplexing - carrying multiple streams of information intermixed using time-sharing

transmission demodulation-remodulation transmission demodulation-remodulation transmission demodulation-remodulation

Theory of Digital Modulation: Sampling

„ Voice and other analog signals first must be converted to digital form (“sampled”) before they can be transmitted digitally „ The sampling theorem gives the

requirements for successful sampling

• The signal must be sampled at least twice during each cycle of f_M , its highest frequency. 2 x f_M is called the Nyquist Rate.

• to prevent “aliasing”, the analog signal is low-pass filtered so it contains no frequencies above f_M

„ Required Bandwidth for Samples, p(t)

• If each sample p(t) is expressed as an n-bit binary number, the

bandwidth required to convey p(t) as a digital signal is at least N*2* f_M

• this follows Shannon’s Theorem: at least one Hertz of bandwidth is

required to convey one bit per second of data

• Notice: lots of bandwidth required!

The Sampling Theorem: Two Parts

•If the signal contains no frequency higher than f_M Hz., it is completely described by specifying its samples taken at instants of time spaced 1/2 f_M s.

•The signal can be completely recovered from its samples taken at the rate of 2 f_M samples per second or higher.

m(t)

Sampling

Recoverym(t)

The Mother of All Telephone Signals: DS-0

„ Telephony has adopted a world-wide PCM standard digital signal, using a 64 kb/s stream derived from sampled voice data

„ Voice waveforms are band-limited (see curve)

• upper cutoff beyond 3500-4000 Hz. to avoid aliasing

• rolloff below 300 Hz. For less sensitivity to “hum” picked up from AC power mains „ Voice waveforms sampled 8000 times/second

• A>D conversion has 1 byte (8 bit)

resolution; thus 256 voltage levels possible

• 8000 samples x 1 byte = 64,000 bits/second

• Levels are defined logarithmically rather than linearly, to handle a wider range of audio levels with minimum distortion

– µ-law companding is used in North America & Japan

– A-law companding is used in most other countries -10dB -20dB -30dB -40dB 0 dB 100 300 1000 3000 10000 Frequency, Hz C-Message Weighting t 0 1 2 3 4 5 6 8 7 9 10 11 12 13 14 15 16 4 16 1 3 15 8 3 4 8 A-LAW y= sgn(x) A|x|_{ln(1+ A) for 0 ≤ x≤} 1 A (where A= 87.6) y= sgn(x)ln(1+ A|x)| ln(1+ A) for 1 A< x ≤1 µ-Law y= sgn(x)ln(1+ µ| x|) ln(1+ µ) (whereµ= 255) Companding Band-Limiting

x = analog audio voltage y = quantized level (digital)

Was Digital Supposed to Give More Capacity!?

„ A DS-0 telephone signal, carrying one person talking, is a 64,000

bits/second data stream.

„ Shannon’s theorem tells us we’ll need at least 64,000 Hz. of

bandwidth to carry this signal, even with the most advanced

modulation techniques (QPSK, etc.)

„ But regular analog cellular signals are only 30,000 Hz. wide! So

does a digital signal require more bandwidth than analog?!!

„ YES -- unless we do something fancy, like compression.

„ We DO use compression, to reduce the number of bits being

transmitted, thereby keeping the bandwidth as small as we can

„ The compressing device is called a Vocoder (voice coder). It both

compresses the signal being sent, and expands the signal being

received

„ Every digital mobile phone technology uses some type of Vocoder

•

There are many types, with many different characteristics

Vocoders: Compression vs. Distortion

„ Objective: to significantly reduce the number of bits which must be transmitted, but without creating objectionable levels of distortion

„ We are concerned mainly with telephone applications, with voice signal already band-limited to 4 kHz. max. and sampled at 8 kHz.

„ The objective is toll-quality voice reproduction „ General Categories of Speech Coders

• Waveform Coders

– attempt to re-create the input waveform

– good speech quality but at relatively high bit rates • Vocoders

– attempt to re-create the sound as perceived by humans – quantize and mimic speech-parameter-defined properties – lower bit rates but at some penalty in speech quality

• Hybrid Coders

– mixed approach, using elements of Waveform Coders & Vocoders

– use vector quantization against a codebook reference – low bit rates and good quality speech

Meet some Families of Speech Coders

Waveform Coders

PCM (pulse-code modulation), APCM (adaptive PCM) DPCM (differential PCM), ADPCM (adaptive DPCM) DM (delta modulation), ADM (adaptive DM)

CVSD (continuously variable-slope DM) APC (adaptive predictive coding)

RELP (residual-excited linear prediction) SBC (subband coding)

ATC (adaptive transform coding)

Hybrid Coders

MPLP (multipulse-excited linear prediction) RPE (regular pulse-excited linear prediction) VSELP (vector-sum excited linear prediction) CELP (code-excited linear prediction)

Vocoders

Channel, Formant, Phase, Cepstral, or Homomorphic LPC (linear predictive coding)

STC (sinusoidal transform coding)

MBE (multiband excitation), IMBE (improved MBE)

„ Objective: to significantly reduce the number of bits which must be transmitted, but without creating objectionable levels of distortion

„ We are concerned mainly with telephone applications, with voice signal already band-limited to 4 kHz. max. and sampled at 8 kHz.

„ The objective is toll-quality voice reproduction

Speech Coders Used Mobile Technologies:

„ Vocoders are usually described by their output rate (8 kilobits/sec, etc.) and the type of algorithm they use. Here’s a list of the vocoders used in currently popular wireless technologies:

bits/sec 64k 32k 32k 16k 13/7/4/2 v 13k 9.6k 8k 6.7k 6.4k 8/4/2/1 v 8/4/2/1 v 4.8k 2.4k Algorithm log PCM ADPCM LD-CELP APC QCELP RPE-LTP MPLP VSELP VSELP IMBE QCELP QCELP CELP LPC-10 Standard (Year) CCITT G.711 (1972) CCITT G.721 (1984) CCITT G.728 (1992) Inmarsat-B (1985) CTIA, IS-54/J-Std008 (1995) CDMA Pan-European DMR, GSM (1991) BTI Skyphone (1990)

CTIA IS-54 (1993) TDMA

Japanese DMR (1993) Inmarsat-M (1993)

Enhanced Vocoder, 1997 CDMA

CTIA, IS-95 (1993) CDMA

US, FS-1016 (1991) US, FS-1015 (1977) MOS 4.3 4.1 4.0 n/avail n/avail 3.5 3.4 3.5 3.4 3.4 n/avail 3.4 3.2 2.3 8k EFRC IS-136 (1997) TDMA enhanced n/avail

The Latest Vocoder Technology, 2005

„ CDMA Family:

•

SMV (selective multirate vocoder)

„ ETSI GSM/WCDMA Family

Digital Modulation

Modulation by Digital Inputs

„ For example, modulate a signal with this digital waveform. No more continuous analog variations, now we’re “shifting”

between discrete levels. We call this “shift keying”.

• The user gets to decide what levels mean “0” and “1” -- there are no inherent values

„ Steady Carrier without modulation

„ Amplitude Shift Keying

ASK applications: digital microwave

„ Frequency Shift Keying

FSK applications: control messages in

AMPS cellular; TDMA cellular

„ Phase Shift Keying

PSK applications: TDMA cellular,

GSM & PCS-1900

Our previous modulation examples used continuously-variable

analog inputs. If we quantize the inputs, restricting them to

digital values, we will produce digital modulation.

Voltage

Time

1 0 1 0

1 0 1 0

1 0 1 0

1 0 1 0

Digital Modulation Schemes

„ There are many different schemes for digital modulation, each a

compromise between complexity, immunity to errors in transmission,

required channel bandwidth, and possible requirement for linear amplifiers „ Linear Modulation Techniques

• BPSK Binary Phase Shift Keying

• DPSK Differential Phase Shift Keying

• QPSK Quadrature Phase Shift Keying IS-95 CDMA forward link

– Offset QPSK IS-95 CDMA reverse link

– Pi/4 DQPSK IS-54, IS-136 control and traffic channels

„ Constant Envelope Modulation Schemes

• BFSK Binary Frequency Shift Keying AMPS control channels

• MSK Minimum Shift Keying

• GMSK Gaussian Minimum Shift Keying GSM systems, CDPD

„ Hybrid Combinations of Linear and Constant Envelope Modulation • MPSK M-ary Phase Shift Keying

• QAM M-ary Quadrature Amplitude Modulation

• MFSK M-ary Frequency Shift Keying FLEX paging protocol

„ Spread Spectrum Multiple Access Techniques

• DSSS Direct-Sequence Spread Spectrum IS-95 CDMA

Modulation used in CDMA Systems

„ CDMA mobiles use offset QPSK modulation

• the Q-sequence is delayed half a chip, so that I and Q never

change simultaneously and the mobile TX never passes through (0,0)

„ CDMA base stations use QPSK modulation

• every signal (voice, pilot, sync, paging) has its own amplitude, so the

transmitter is unavoidably going through (0,0)

sometimes; no reason to include 1/2 chip delay

Base Stations: QPSK

Q Axis I Axis Short PN Q Σ cosωt sin ωt User’s chips Short PN I

Mobiles: OQPSK

Q Axis I Axis Short PN Q Σ cosωt sin ωt User’s chips 1/2 chip Short PN I

CDMA Base Station Modulation Views

„ The view at top right shows the

actual measured QPSK phase

constellation of a CDMA base

station in normal service

„ The view at bottom right shows

the measured power in the code

domain for each walsh code on a

CDMA BTS in actual service

•

Notice that not all walsh codes

are active

•

Pilot, Sync, Paging, and

certain traffic channels are in

use

Wireless Systems:

Multiple Access Technologies & Standards

Wireless Systems:

Multiple Access Technologies & Standards

Multiple Access Technologies

„ FDMA (example: AMPS)

Frequency Division Multiple Access

•

each user has a private frequency (at

least in their own neighborhood)

„ TDMA (examples: IS-54/136, GSM)

Time Division Multiple Access

•

each user has a private time on a private

frequency (at least in their own

neighborhood)

„ CDMA (examples: IS-95, J-Std. 008)

Code Division Multiple Access

•

users co-mingle in time and frequency

but each user has a private code (at

least in their own neighborhood)

Freq uenc y Tim_e Power Freq uenc y Tim_e Power Freq uenc y Tim_e Power

FDMA

TDMA

CDMA

Conventional Technologies:

Recovering the Signal / Avoiding Interference

„ In ordinary radio technologies, the desired signal must be stronger than all interference by at least a certain margin called C/I (carrier-to-interference ratio)

• the type of signal modulation determines the amount of interference which can be tolerated, and thus the required C/I

„ In conventional systems, the C/I is controlled mainly by the distance between co-channel cells

• frequency usage is planned so that co-channel users don’t have interference worse than C/I

• any undesired interference we face is coming from the nearest co-channel cells, far away

• if the signal is delicate, then we need a big C/I and the co-channel cells must be very far away

• if the signal is more rugged, we can tolerate more interference (smaller C/I) allowing the co-channel cells a bit closer without bad effects

2 3 4 5 6 7 4 6 4 7 2 7 2 5 3 5 3 6 1 1 1 1 1 1 1

AMPS-TDMA-GSM

Figure of Merit: C/I

(carrier/interference ratio

)

AMPS: +17 dB

TDMA: +14 to 17 dB

Handoffs and C/I

„ One purpose of handoff is to keep the call from dropping as the mobile

moves out of range of individual cells „ Another purpose of handoff is to

ensure the mobile is using the cell with the best signal strength and best C/I at all times

„ Notice in the signal graphs at lower right how the mobile’s C/I is

maintained at a usable level as it goes from cell to cell

A B RSSI, dBm -120 -50 C D Sites C/I

A

B

C

D

AMPS Tech-nology NAMPS TDMA GSM CDMA Analog FM Modulation Type Analog FM DPQSK GMSK QPSK/OQPSK 30 kHz. Channel Bandwidth 10 kHz. 30 kHz. 200 kHz. 1,250 kHz. C/I ≅ 17 dB Quality Indicator C/I ≅ 17 dB C/I ≅ 17 dB C/I ≅ 17 dB E_b/N_o ≅ 6dB

CDMA: Using A New Dimension

„ All CDMA users occupy the same frequency

at the same time! Frequency and time are

not used as discriminators

„ CDMA operates by using a new dimension,

CODING, to discriminate between users

•

In CDMA, we do not try to immediately

recover the pulses of energy from each

user. Instead, we watch long groups of

the totals of everybody’s pulses, and

detect little patterns which are the

“signature” of the user we wish to

decode

„ In CDMA, the interference originates mainly

from nearby users in the same general area

„ Each user is a small voice in a roaring

crowd -- but with a uniquely recoverable

code

CDMA

Figure of Merit: C/I

AMPS: +17 dB

TDMA: +14 to +17 dB

GSM: +7 to 9 dB.

CDMA: -10 to -17 dB.

Although the CDMA C/I is negative, the decoding

process recovers the user’s energy while discarding others’ energy. The final net result is Eb/No, typically about +6 db. We’ll study this in detail later.

The CDMA Migration Path to 3G

1xEV-DO Rev. A IS-856 1250 kHz. 59 active users Higher data rates on data-only CDMA carrier 3.1 Mb/s DL 1.8 Mb/s UL RL FL Spectrum 1xEV-DO Rev. 0 IS-856 1250 kHz. 59 active users High data rates on data-only CDMA carrier 2.4 Mb/s DL 153 Kb/s UL

CDMAone CDMA2000 / IS-2000

Technology Generation Signal Bandwidth, #Users Features: Incremental Progress 1G AMPS Data Capabilities 30 kHz. 1 First System, Capacity & Handoffs None, 2.4K by modem 2G IS-95A/ J-Std008 1250 kHz. 20-35 First CDMA, Capacity, Quality 14.4K 2G IS-95B 1250 kHz. 25-40 •Improve d Access •Smarter Handoffs 64K 2.5G? 3G IS-2000: 1xRTT 1250 kHz. 50-80 voice and data •Enhanced Access •Channel Structure 153K 307K 230K 3G 1xEV-DV 1xTreme 1250 kHz. Many packet users High data rates on Data-Voice shared CDMA carrier 5 Mb/s 3G IS-2000: 3xRTT F: 3x 1250k R: 3687k 120-210 per 3 carriers Faster data rates on shared 3-carrier bundle 1.0 Mb/s RL FL RL FL RL FL RL FL RL FL RL FL RL FL

Modulation Techniques of 1xEV Technologies

„ 1xEV, “1x Evolution”, is a family of alternative fast-data schemes that can be implemented on a 1x CDMA carrier.

„ 1xEV DO means “1x Evolution, Data Only”, originally proposed by Qualcomm as “High Data Rates” (HDR).

• Up to 2.4576 Mbps forward, 153.6 kbps

reverse

• A 1xEV DO carrier holds only packet data, and does not support circuit-switched voice

• Commercially available in 2003

„ 1xEV DV means “1x Evolution, Data and Voice”.

• Max throughput of 5 Mbps forward, 307.2k reverse

• Backward compatible with IS-95/1xRTT voice calls on the same carrier as the data

• Not yet commercially available; work continues

„ All versions of 1xEV use advanced modulation techniques to achieve high throughputs.

QPSK

CDMA IS-95, IS-2000 1xRTT, and lower rates of 1xEV-DO, DV

16QAM

1xEV-DO at highest rates

64QAM

1xEV-DV at highest rates

GSM Technology Migration Path to 3G

Integrated voice/data (Future rates to 12 MBPS using adv. modulation?) Technology Generation Signal Bandwidth, #Users Features: Incremental Progress 1G various analog Data Capabilities various various various 2G GSM 200 kHz. 7.5 avg. Europe’s first Digital wireless none 2.5G or 3? GPRS 200 kHz. Many Pkt. users •Packet IP access •Multiple attached users 9-160 Kb/s (conditions determine) 3G EDGE 200 kHz. fast data many users 8PSK for 3x Faster data rates than GPRS 384 Kb/s mobile user 3G UMTS UTRA WCDMA 3.84 MHz. up to 200+ voice users and data 2Mb/s static user

TDMA IS-136 Technology Migration Path to 3G

2G CDPD 30 kHz. Many Pkt Usrs 19.2 kbps US Packet Data Svc. Technology Generation Signal Bandwidth, #Users Features: Incremental Progress Data Capabilities 2G TDMA IS-54 IS-136 30 kHz. 3 users USA’s first Digital wireless none 2.5G or 3? GPRS 200 kHz. Many Pkt. users •Packet IP access •Multiple attached users 9-160 Kb/s (conditions determine) 3G EDGE 200 kHz. fast data many users 8PSK for 3x Faster data rates than GPRS 384 Kb/s mobile user 3G UMTS UTRA WCDMA 3.84 MHz. up to 200+ voice users and data Integrated voice/data (Future rates to 12 MBPS using adv. modulation?) 1G AMPS 30 kHz. 1 First System, Capacity & Handoffs None, 2.4K by modem 2Mb/s static user 2G GSM 200 kHz. 7.5 avg. Europe’s first Digital wireless none

Not BWA; for comparison only 802.16 BPSK to 256QAM OFDM 54 Mb/s TDD, FDD various 2-11 GHz 10-66 GHz 802.20 Mobile BWA

4G: Broadband Wireless Access Technologies

Technology Modulation Type Max Raw Data Rate Access Method Frequency Band Infrared IRDA various 4 Mb/s

Single User per Optical Carrier Optical 802.11b CCK 11 Mb/s DSSS 2.4 GHz 802.11a BPSK, QPSK, 16QAM, or 64QAM 54 Mb/s DSSS 5 GHz HIPERLAN Type 1 FSK or GMSK 23.5 Mb/s OFDM 5 GHz HIPERLAN Type 2 BPSK, QPSK, 16QAM, or 64QAM 54 Mb/s various. 5 GHz Bluetooth GFSK FH 1 Mb/s various 2.4 GHz BLUETOOTH 802.11A, B, WIFI, WILAN Infrared IRDA High Hopes!

High-Tier $$$ Low-Tier $ 1G: AMPS

4G – Evolution or Revolution?

„ There’s a revolution going on!

• New 2.5G services arriving now, new 3G arriving 2002 through 2005 • A groundswell of commercial (and even free!) WILAN deployment „ 3G networks and 4G networks have their own unique advantages „ Ultimately 3G and 4G will be integrated by wireless operators!

Technology Environment _{Infrastructure Owner}Service Provider/

PSTN IP/VPNs

2G: TDMA, GSM, IS95 CDMA, IDEN

2.5G: GPRS, EDGE

3G: IS2000 1xRTT, 1xEV DO, 1xEV DV UMTS WCDMA 4G: Wireless LAN 802.11b “Wi-Fi” 802.11a, g HIPERLAN Type 1 HIPERLAN Type 2 Bluetooth Infrared freenetworks.org

Near-Universal Macro-Coverage

Hotspots

Global and US Wireless Snapshot 4Q 2003

„ Total Worldwide Wireless customers surpassed total worldwide landline customers at year-end 2002, with 1,00,080,000 of each.

„ 2/3 of worldwide wireless customers use the GSM technology „ CDMA is second-most-prevalent with 17.0%

„ In the US, CDMA is the most prevalent technology at 45.7% „ Both CDMA and GSM are growing in the US

• most IS-136 TDMA systems are converting to GSM + GPRS + EDGE

Worldwide USA

Total Wireless Users GSM users CDMA users TDMA users IDEN users Analog users 1,320,000,000 100% 870,000,000 65.9% 224,000,000 17.0% 124,000,000 9.4% 68,000,000 5.2% 34,000,000 2.6% 141,000,000 100% 33,732,506 23.9% 64,503,287 45.7% 26,375,232 18.6% 11,978,382 8.5% 4,510,594 3.2%

Other Misc.

A Quick Survey of Wireless Data Technologies

„ This summary is a work-in-progress, tracking latest experiences and reports from all the high-tier (provider-network-oriented) 2G and 3G wireless data technologies

„ Have actual experiences to share, latest announced details, or corrections to the above? Email to [email protected]. Thanks for your comments!

PAGING ETSI / GSM US CDMA IS-136 ANALOG AMPS Cellular 9.6 – 4.8 kb/s w/modem IS-136 TDMA 19.2 – 9.6 kb/s GSM CSD 9.6 – 4.8 kb/s GSM HSCSD 32 – 19.2 kb/s IDEN 19.2 – 19.2 kb/s IS-95 14.4 – 9.6 kb/s IS-95B 64 -32 kb/s CDPD 19.2 – 4.8 kb/s discontinued GPRS 40 – 30 kb/s DL 15 kb/s UL EDGE 200 - 90 kb/s DL 45 kb/s UL 1xRTT RC3 153.6 – 80 kb/s 1xRTT RC4 307.2 – 160 kb/s 1xEV-DO 2400 – 600 DL 153.6 – 76 UL 1xEV-DO A 3100 – 800 DL 1800 – 600 UL 1xEV-DV 5000 - 1200 DL 307 - 153 UL WCDMA 0 384 – 250 kb/s WCDMA 1 2000 - 800 kb/s WCDMA HSPDA 12000 – 6000 kb/s Flarion OFDM_{1500 – 900 kb/s} TD-SCDMA In Development Mobitex 9.6 – 4.8 kb/s obsolete

Wireless System Capacity

Each wireless technology (AMPS, NAMPS, D-AMPS, GSM, CDMA) uses a specific modulation type with its own unique signal

characteristics

„ Signal Bandwidth determines how many RF signals will “fit” in the operator’s licensed spectrum

„ Robustness of RF signal

determines tolerable level of interference and necessary

physical separation of cochannel cells

„ Number of users per RF signal directly affects capacity

„ In the following page, we will

develop the number of users and traffic in erlangs per site for each of the popular wireless

technologies

AMPS, D-AMPS, N-AMPS

CDMA 30 30 10 kHz Bandwidth 200 kHz 1250 kHz 1 3 1 Users 8 Users 22 Users 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 2 3 4 4 3 2 5 6 1 7

Typical Frequency Reuse N=7

Typical Frequency Reuse N=4

Typical Frequency Reuse N=1

Vulnerability: C/I ≅ 17 dB Vulnerability: C/I ≅ 6.5-9 dB Vulnerability: E_bN_o≅ 6 dB

Comparison of Wireless System Capacities

Fwd/Rev Spectrum kHz. 12,50012,500 12,500 15,00015,000 15,000 5,000 5,000 5,000

Technology AMPS TDMA CDMA TDMA GSM CDMA TDMA GSM CDMA

Req'd C/I or Eb/No, db 17 17 6 17 12 6 17 12 6

Freq Reuse Factor, N 7 7 1 7 4 1 7 4 1

RF Signal BW, kHz 30 30 1250 30 200 1250 30 200 1250

Total # RF Carriers 416 416 9 500 75 11 166 25 3

RF Sigs. per cell @N 59 59 9 71 18 11 23 6 3

# Sectors per cell 3 3 3 3 3 3 3 3 3

#CCH per sector 1 1 0 1 0 0 1 0 0

RF Signals per sector 18 18 9 22 6 11 6 2 3

Voicepaths/RF signal 1 3 22 3 8 22 3 8 22

SH average links used 1 1 1.66 1 1 1.66 1 1 1.66

Unique Voicepaths/carrier 1 3 13.253 3 8 13.253 3 8 13.253

Voicepaths/Sector 18 54 198 66 48 242 18 16 66

Unique Voicepaths/Sector 18 54 119 66 48 145 18 16 39

P.02 Erlangs per sector 11.5 44 105.5 55.3 38.4 130.9 11.5 9.83 30.1 P.02 Erlangs per site 34.5 132 316.5 165.9 115.2 392.7 34.5 29.49 90.3

Capacity vs. AMPS800 1 3.8 9.2 4.8 3.3 11.4 1.0 0.9 2.6

Capacity of Multicarrier CDMA Systems

Fwd/Rev Spectrum kHz. 12,500 1,800 3,050 4,300 5,550 6,800 8,050 9,300 10,550 11,800 13,050 14,300 Technology AMPS CDMA CDMA CDMA CDMA CDMA CDMA CDMA CDMA CDMA CDMA CDMA

Req'd C/I or Eb/No, db 17 6 6 6 6 6 6 6 6 6 6 6

Freq Reuse Factor, N 7 1 1 1 1 1 1 1 1 1 1 1

RF Signal BW, kHz 30 1250 1250 1250 1250 1250 1250 1250 1250 1250 1250 1250

Total # RF Carriers 416 1 2 3 4 5 6 7 8 9 10 11

RF Sigs. per cell @N 59 1 2 3 4 5 6 7 8 9 10 11

# Sectors per cell 3 3 3 3 3 3 3 3 3 3 3 3

#CCH per sector 1 0 0 0 0 0 0 0 0 0 0 0

RF Signals per sector 18 1 2 3 4 5 6 7 8 9 10 11

Voicepaths/RF signal 1 22 22 22 22 22 22 22 22 22 22 22

SH average links used 1 1.66 1.66 1.66 1.66 1.66 1.66 1.66 1.66 1.66 1.66 1.66 Unique Voicepaths/carrier 1 13.3 13.3 13.3 13.3 13.3 13.3 13.3 13.3 13.3 13.3 13.3

Voicepaths/Sector 18 22 44 66 88 110 132 154 176 198 220 242

Unique Voicepaths/Sector 18 13 26 39 53 66 79 92 106 119 132 145

P.02 Erlangs per sector 11.5 7.4 18.4 30.1 43.1 55.3 67.7 80.2 93.8 105.5 119.1 130.9 P.02 Erlangs per site 34.5 22.2 55.2 90.3 129.3 165.9 203.1 240.6 281.4 316.5 357.3 392.7 Capacity vs. AMPS800 1 0.64 1.60 2.6 3.7 4.8 5.9 7.0 8.2 9.2 10.4 11.4

f

1 2 3 4 5 6 7 8 9 1011

Physical Principles of

Propagation

Physical Principles of

Propagation

Introduction to Propagation

„ Propagation is the heart of every radio link. During propagation, many processes act on the radio signal.

• attenuation

– the signal amplitude is reduced by various natural mechanisms. If there is too much attenuation, the signal will fall below the reliable detection threshold at the receiver. Attenuation is the most important single factor in propagation.

• multipath and group delay distortions

– the signal diffracts and reflects off irregularly shaped objects, producing a host of components which arrive in random timings and random RF

phases at the receiver. This blurs pulses and also produces intermittent signal cancellation and reinforcement. These effects are overcome

through a variety of special techniques

• time variability - signal strength and quality varies with time, often dramatically

• space variability - signal strength and quality varies with location and distance

• frequency variability - signal strength and quality differs on different frequencies

„ To master propagation and effectively design wireless systems, you must know:

• Physics: understand the basic propagation processes

• Measurement: obtain data on propagation behavior in area of interest

• Statistics: analyze known data, extrapolate to predict the unknown

Frequency and Wavelength: Implications

„ Radio signals in the atmosphere

propagate at almost speed of light

λ = wavelength

C = distance propagated in 1 second F = frequency, Hertz

„ The wavelength of a radio signal

determines many of its propagation

characteristics

• Antenna elements size are

typically in the order of 1/4 to 1/2 wavelength

• Objects bigger than a wavelength can reflect or obstruct RF energy • RF energy can penetrate into a

building or vehicle if they have apertures a wavelength in size, or larger λ/2

λ

= C / F

for AMPS: F= 870 MHz

λ

= 0.345 m = 13.6 inches for PCS-1900: F = 1960 MHz

λ

= 0.153 m = 6.0 inches

Propagation Effects of Earth’s Atmosphere

„ Earth’s unique atmosphere supports life (ours

included) and also introduces many propagation

effects -- some useful, some troublesome

„ Skywave Propagation: reflection from Ionized

Layers

•

LF and HF frequencies (below roughly 50

MHz.) are routinely reflected off layers of the

upper atmosphere which become ionized by

the sun

•

this phenomena produces intermittent

world-wide propagation and occasional total outages

•

this phenomena is strongly correlated with

frequency, day/night cycles, variations in

earth’s magnetic field, 11-year sunspot cycle

•

these effects are negligible for wireless

More Atmospheric Propagation Effects

„ Attenuation at Microwave Frequencies

• rain droplets can substantially attenuate RF

signals whose wavelengths are comparable to, or smaller than, droplet size

• rain attenuations of 20 dB. or more per km. are possible

• troublesome mainly above 10 GHz., and in tropical areas

• must be considered in reliability calculations during path design

• not major factor in wireless systems propagation „ Diffraction, Wave Bending, Ducting

• signals 50-2000 MHz. can be bent or reflected at boundaries of different air density or humidity

• phenomena: very sporadic unexpected long-distance propagation beyond the horizon. May last minutes or hours

• can occur in wireless systems

Refraction by air layers Ducting by air layers >100 mi. “Rain Fades” on MIcrowave Links

Dominant Mechanisms of Mobile Propagation

Most propagation in the mobile

environment is dominated by these three mechanisms:

„ Free space

• No reflections, no obstructions – first Fresnel Zone clear

• Signal spreading is only mechanism • Signal decays 20 dB/decade

„ Reflection

• Reflected wave 180°out of phase • Reflected wave not attenuated much • Signal decays 30-40 dB/decade

„ Knife-edge diffraction

• Direct path is blocked by obstruction • Additional loss is introduced

• Formulae available for simple cases „ We’ll explore each of these further...

Knife-edge

Diffraction

Reflection

with partial cancellation

B A

d

D

Free-Space Propagation

„ The simplest propagation mode

• Antenna radiates energy which spreads in space • Path Loss, db (between two isotropic antennas)

= 36.58 +20*Log₁₀(F_MHZ)+20Log₁₀(Dist_MILES )

• Path Loss, db (between two dipole antennas)

= 32.26 +20*Log₁₀(F_MHZ)+20Log₁₀(Dist_MILES )

• Notice the rate of signal decay:

• 6 db per octave of distance change, which is 20 db per decade of distance change

„ Free-Space propagation is applicable if:

• there is only one signal path (no reflections) • the path is unobstructed (i.e., first Fresnel zone

is not penetrated by obstacles)

First Fresnel Zone =

{Points P where AP + PB - AB <

λ/2

}

Fresnel Zone radius d = 1/2 (λD)^(1/2)

1st Fresnel Zone

B A d D Free Space “Spreading” Loss energy intercepted by receiving antenna is proportional to 1/r2 r

Reflection With Partial Cancellation

„ Mobile environment characteristics:

• Small angles of incidence and reflection

• Reflection is unattenuated (reflection coefficient =1)

• Reflection causes phase shift of 180 degrees „ Analysis

• Physics of the reflection cancellation predicts signal decay of 40 dB per decade of distance

Heights Exaggerated for Clarity

HT_FT

HTFT

D_MILES

Comparison of Free-Space and Reflection Propagation Modes

Assumptions: Flat earth, TX ERP = 50 dBm, @ 1950 MHz. Base Ht = 200 ft, Mobile Ht = 5 ft.

Received Signal in Free Space, DBM Received Signal in Reflection Mode Distance_MILES -52.4 -69.0 1 -58.4 -79.2 2 -64.4 -89.5 4 -67.9 -95.4 6 -70.4 -99.7 8 -72.4 -103.0 10 -75.9 -109.0 15 -78.4 -113.2 20 Path Loss [dB ]= 172 + 34 x Log (D_Miles )

- 20 x Log (Base Ant. Ht_Feet) - 10 x Log (Mobile Ant. Ht_Feet)

Signal Decay Rates in Various Environments

We’ve seen how the signal decays

with distance in two basic modes

of propagation:

„ Free-Space

•

20 dB per decade of distance

•

6 db per octave of distance

„ Reflection Cancellation

• 40 dB per decade of distance

• 12 db per octave of distance

„ Real-life wireless propagation

decay rates are typically

somewhere between 30 and 40

dB per decade of distance

Signal Level vs. Distance

-40 -30 -20 -10 0 Distance, Miles 1 2 3.16 5 6 7 8 10 One Octave of distance (2x) One Decade of distance (10x)

Knife-Edge Diffraction

„ Sometimes a single well-defined

obstruction blocks the path, introducing

additional loss. This calculation is fairly

easy and can be used as a manual tool

to estimate the effects of individual

obstructions.

„ First calculate the diffraction parameter

ν from the geometry of the path

„ Next consult the table to obtain the

obstruction loss in db

„ Add this loss to the

otherwise-determined path loss to obtain the total

path loss.

„ Other losses such as free space and

reflection cancellation still apply, but

computed independently for the path as

if the obstruction did not exist

H

R

₁

R

₂

ν

atten dB 0 -5 -10 -15 -20 -25 -4 -3 -2 -1 0 1 2 3 -5

(

+

)

ν = -H 2 λ 1 1 R₁ R₂

Local Variability: Multipath Effects

„ The free-space, reflection, and diffraction mechanisms described earlier explain signal level variations on a large scale, but other mechanisms introduce small-scale local fading

„ Slow Fading occurs as the user moves over hundreds of wavelengths due to shadowing by local obstructions

„ Rapid Fading occurs as signals received from many paths drift into and out of phase

• the fades are roughly λ/2 apart in space:

7 inches apart at 800 MHz., 3 inches apart at 1900 MHz

• fades also appear in the frequency domain and time domain

• fades are typically 10-15 db deep, occasionally deeper

• Rayleigh distribution is a good model

for these fades

„ these fades are often called “Rayleigh fades” Multi-path Propagation A d 10-15 dB λ/2 Rayleigh Fading

Combating Rayleigh Fading: Space Diversity

„ Fortunately, Rayleigh fades are

very short and last a small

percentage of the time

„ Two antennas separated by

several wavelengths will not

generally experience fades at the

same time

„ “Space Diversity” can be

obtained by using two receiving

antennas and switching

instant-by-instant to whichever is best

„ Required separation D for good

decorrelation is 10-20

λ

•

12-24 ft. @ 800 MHz.

•

5-10 ft. @ 1900 MHz.

Signal received by Antenna 1 Signal received by Antenna 2 Combined Signal D