July, 19982 - 1RF100 (c) 1998 Scott Baxter Wireless Systems: Modulation Schemes and Bandwidth Wireless Systems: Modulation Schemes and Bandwidth RF100.

July, 1998 2 - 1RF100 (c) 1998 Scott Baxter

Wireless Systems:Modulation Schemes and Bandwidth

Wireless Systems:Modulation Schemes and Bandwidth

RF100 Chapter 2

fc

fc

Upper Sideband

Lower Sideband

fc

fc

I axis

Q axis

a

b

c

QPSK

I axis

Q axis

c

a

b

p

r

v/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 [ c t + ]

The complete, time-varying radio signal

Amplitude (strength) of the signal

Natural Frequency of the signal

Phase of 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 communicationsSSB

LSB USB


Amplitude Modulation (“AM”) DetailsTIME-DOMAIN VIEWof AM MODULATOR

x(t) = [1 + amn(t)]cos c twhere:

a = modulation index (0 < a <= 1)

mn(t) = modulating waveform

c = 2 fc, the radian carrier freq.

a

1

+

+

x(t)

cos c

mn(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

Vol

tage

Frequency0 fc

mn(t)BASEBAND

x(t)

UPPERSIDEBAND

LOWERSIDEBAND

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 c

mn(t)

[1 + amn(t)]

Sat.

Lin.

TIME-DOMAIN VIEW:AM DETECTOR(non-coherent)

x(t)

mn(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

sFM(t) =A cos [c t + mm(x)dx+ ]t

t0

where:A = signal amplitude (constant)

c = radian carrier frequency

mfrequency deviation indexm(x) = modulating signal

= initial phase


Vol

tage

Frequency0 fc

SFM(t)

UPPERSIDEBANDS

LOWERSIDEBANDS


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

sFM(t)m(x) ~VCO

x

LO

HPA

TIME-DOMAIN VIEW:FM DETECTOR

x

LO

LNA PLLsFM(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 Armstrong1890 - 1954


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

sPM(t) =A cos [c t + mm(x) + ]

where:A = signal amplitude (constant)

c = radian carrier frequency

mphase deviation indexm(x) = modulating signal

= initial phase


Vol

tage

Frequency0 fc

SFM(t)

UPPERSIDEBANDS

LOWERSIDEBANDS

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-emphasis are often used in FM systems

TIME-DOMAIN VIEW:FM DETECTOR FOR PM

x

LO

LNA PLLsFM(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

sFM(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

Frequency0

fc

fc

Upper Sideband

Lower Sideband

fc

fc


Digital Sampling and VocodingDigital 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


transmission



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 fM , its highest frequency. 2 x fM is called the Nyquist Rate.

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

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* fM

• 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 fM Hz., it is completely described by specifying its samples taken at instants of time spaced 1/2 fM s.•The signal can be completely recovered from its samples taken at the rate of 2 fM samples per second or higher.

m(t)

Sampling

Recoverym(t)

p(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

– -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 10000Frequency, Hz

C-Message Weighting

t

0

1

23456879101112131415

16

4

16

1

3

15

8

34

8

A-LAWysgn(x) A|x|

ln(1A) for 0x1A

(where A87.6)

ysgn(x) ln(1A|x)|ln(1A) for 1

A x1

µ-Lawy sgn(x)

ln(1 |x|)ln(1 )

(where255)

Companding

Band-Limiting

x = analog audio voltagey = 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 HomomorphicLPC (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 A few different strategies and algorithms used in voice compression:


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) CDMAPan-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


Digital ModulationDigital 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 KeyingPSK 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

Time1 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 • FHSS Frequency-Hopping Spread Spectrum


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: QPSKQ Axis

I Axis

ShortPN Q

cos t

sin t

User’schips

ShortPN I

Mobiles: OQPSKQ Axis

I Axis

ShortPN Q

cos t

sin t

User’schips

1/2 chip

ShortPN 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


End of Section

July, 19982 - 1RF100 (c) 1998 Scott Baxter Wireless Systems: Modulation Schemes and Bandwidth Wireless Systems: Modulation Schemes and Bandwidth RF100.

Documents