July, 19982 - 1RF100 (c) 1998 Scott Baxter Wireless Systems: Modulation Schemes and Bandwidth...
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Transcript of July, 19982 - 1RF100 (c) 1998 Scott Baxter Wireless Systems: Modulation Schemes and Bandwidth...
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
July, 1998 2 - 2RF100 (c) 1998 Scott Baxter
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
July, 1998 2 - 3RF100 (c) 1998 Scott Baxter
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
July, 1998 2 - 4RF100 (c) 1998 Scott Baxter
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
July, 1998 2 - 5RF100 (c) 1998 Scott Baxter
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
July, 1998 2 - 6RF100 (c) 1998 Scott Baxter
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
FREQUENCY-DOMAIN VIEW
Vol
tage
Frequency0 fc
SFM(t)
UPPERSIDEBANDS
LOWERSIDEBANDS
July, 1998 2 - 7RF100 (c) 1998 Scott Baxter
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
July, 1998 2 - 8RF100 (c) 1998 Scott Baxter
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
July, 1998 2 - 9RF100 (c) 1998 Scott Baxter
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
FREQUENCY-DOMAIN VIEW
Vol
tage
Frequency0 fc
SFM(t)
UPPERSIDEBANDS
LOWERSIDEBANDS
information
Phase-modulated signal
July, 1998 2 - 10RF100 (c) 1998 Scott Baxter
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
July, 1998 2 - 11RF100 (c) 1998 Scott Baxter
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
July, 1998 2 - 12RF100 (c) 1998 Scott Baxter
Digital Sampling and VocodingDigital Sampling and Vocoding
July, 1998 2 - 13RF100 (c) 1998 Scott Baxter
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
July, 1998 2 - 14RF100 (c) 1998 Scott Baxter
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)
July, 1998 2 - 15RF100 (c) 1998 Scott Baxter
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)
July, 1998 2 - 16RF100 (c) 1998 Scott Baxter
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
July, 1998 2 - 17RF100 (c) 1998 Scott Baxter
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
July, 1998 2 - 18RF100 (c) 1998 Scott Baxter
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:
July, 1998 2 - 19RF100 (c) 1998 Scott Baxter
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
July, 1998 2 - 21RF100 (c) 1998 Scott Baxter
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
July, 1998 2 - 22RF100 (c) 1998 Scott Baxter
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
July, 1998 2 - 23RF100 (c) 1998 Scott Baxter
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
July, 1998 2 - 24RF100 (c) 1998 Scott Baxter
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