# Digital modulations

Document Sample

```					          WCA102
Fundamentals of Digital
Modulation

Digital Modulation – Introduction
Digital Modulation in Wireless Communications

1
Agenda

 Introductions
 Who Cares?
 What is Modulation
 IQ Modulation Types
 Filters and How Things Go Wrong
 Measurements

 Spectral efficiency – use of a narrow bandwidth to
send a large amount of data
Effective use of limited frequency resources
 Good privacy and security features
Digital encryption techniques may be employed
 Lower power consumption
 Repeatable, more easily produced
 Reduced device size
Modulation for Wireless

 Media
Carrier
V(t) = A cos(2πfc t + Φ)

 The 3 essential parameters
Amplitude value    A(t)   ― Amplitude Modulation
Frequency value    f(t)   ― Frequency Modulation
Phase value       φ(t)    ― Phase Modulation
Analog Modulation

 Amplitude Modulation
 Frequency Modulation
 Phase Modulation
TV color image signal (including Amplitude Modulation)
Transmission of a Digital Message

 Basically, it’s the same as Analog Modulation
Methods

V(t) = A(t) cos(2πfc t + Φ)
FSK:     Frequency shift keying
V(t) = A(t) cos(2πf(t) t + Φ)
PSK:     Phase shift keying
V(t) = A(t) cos(2πf(t) t + Φ(t))
 Digital modulation: Amplitude, frequency and/or
Phase are used to represent a digital state

 Amplitude shift keying
1’s or 0’s represented by different amplitudes
Could be accomplished with an AM system

+

=

I: In phase component
I
Q
0
I
(0)     (1)
Q

Amplitude variation on I axis   0
FSK

 Frequency shift keying
Select frequency based on each bit, 0 or 1
Could be done with simple FM system

+
=
FSK in IQ
 Frequency change causes
constant-rate phase change versus
the reference carrier
 Amplitude remains constant on the
IQ circle
Phase
1 Symbol only turns π(ex)

Q
π

Time
I         1Sp        2Sp     3Sp    4Sp     5Sp

 If the phase change is 90 degrees
－π
in one symbol period, the
modulation type is called Minimum
Pos offset    Neg. Offset    Pos. Offset
Shift Keying (remember this one)
PSK

 Phase shift keying
At the bit transitions invert the phase by 180°

=
+
Representation of PSK in IQ

 PSK
Specifically, BPSK(Binary Phase Shift Keying)
I

Q
0
(1)            I
(0)

Q

0
Change Phase to 180°
Relative to reference
Digital Modulation in
Modern Wireless Systems

13
Digital Modulation Block Diagram

Raw Data       Compression,       011010100101                                 01 10 10 10 01 01
Convert to
Error Correction,                                   Symbols
110101           Encryption

01       00
I - Signal
Low Pass Filter
To IQ
Q - Signal                                  Modulator
10        11                    Low Pass Filter

Modulation Mapping

I - Signal
RF
Modulation,
Q - Signal                                              Amplifier
Upconversion
Raw Data Conversion

Raw Data    Compression,
011010100101   Convert to
Error Correction,
Interleaving,                      Symbols
110101        Encryption

 Raw data comes from the user
Digitized voice, keystrokes, jpegs…
 Compression is employed for efficiency
 Error correction is applied for transmission quality
 Interleaving creates signal-dropout resistance
 Encryption is applied for security
Data Bits, to Symbols

Raw Data    Compression,       011010100101                01 10 10 10 01 01
Convert to
Error Correction,                   Symbols
110101        Encryption

 Symbols are represented by the possible states of
digital modulation
 Higher order modulation allows more bits per
symbol
 What in the world does that mean?
Mapping symbols to I and Q
IQ Mapping

 What is Mapping:              Example
 Translate a Symbol to a
point in the IQ space                Q

(01)        (11)
01     00

10      11                                  I
Modulation Mapping

(00)        (10)
Differential Modulation

 QPSK(Quadrature PSK)             DQPSK(Differential QPSK)
 Assign the value to points       The value is based on the
in IQ Space                        transitions between 2 points

Q                                    Q
(01)
(01)              (11)                                (00)

I                                   I

(11)         (10)
(00)              (10)
00= 0 01= +90
10= -90 11= +180
Higher Order Modulation

 8PSK(8-PSK)                         p/4 DQPSK
 Assign the value to points          The value is based on the
in IQ Space                          transitions between 2 points
 3 points per symbol                 Eliminates Zero Crossings
Q                                    Q
(110)                               (11)
(001)                 (011)

(01)               (00)
(111)
I                                   I
(100)
(10)
(000)
(010)
(101)                                         00= -45 01= +135
10= -135 11= +45
More Higher Order Modulation
Modulation)
 Each IQ symbol location is
represented by 4 data bits      (000100) (001100) (011100) (010100)   (110100) (111100)(101100) (100100)
Modulation)
(000101) (001101) (011101) (010101)   (110101) (111101)(101101) (100101)
 Each symbol is now worth 5 bits

Q                          (000111) (001111) (011111) (010111)   (110111) (111111)(101111) (100111)

(000110) (001110) (011110) (010110)   (110110) (111110)(101110) (100110)    I
(0010) (0110) (1110) (1010)

(000010) (001010) (011010) (010010)   (110010) (111010)(101010) (100010)
(0011) (0111) (1111) (1011)
I
(000011) (001011) (011011) (010011)   (110011) (111011)(101011) (100011)
(0001) (0101) (1101) (1001)
(000001) (001001) (011001) (010001)   (110001) (111001)(101001) (100001)

(0000) (0100) (1100) (1000)
(000000) (001000) (011000) (010000)   (110000) (111000) (101000) (100000)
Why Not Just Keep Going?
    Errors in IQ modulation create symbol errors in
transmission                                                     Q
    Vector Errors are created (what’s that?)
    Noise in the transmission channel create symbol
errors
    Inaccuracies in the receiver creates errors
    Signal-to-noise requirements increase with higher   (0010)   (0110)   (1110)   (1010)
order modulations

Q
(0011)   (0111)   (1111)   (1011)
I
(01)                        (11)
(0001)   (0101)   (1101)   (1001)

I
(0000)   (0100)   (1100)   (1000)

(00)                        (10)
The World’s Most Popular Modulation

 Gaussian Minimum Shift Keying
 Gaussian Filtered Form of FSK
 Sum of I and Q results in a constant amplitude circle
Symbol Rate and Bit Rate
 Modulation type determines number of bits per symbol
 BPSK              1 bit/symbol
 DBPSK             1 bit/symbol
 QPSK              2 bit/symbol
 p/4 DQPSK         2 bit/symbol
 DQPSK             2 bit/symbol
 8PSK              3 bit/symbol
 16QAM             4 bit/symbol
 64QAM             5 bit/symbol
 256QAM            6 bit/symbol
 For a fixed symbol rate, having more bits will provide a faster
transfer rate
 Setting up a WCA requires you to know the modulation type
and symbol rate, not the bit rate

 32QAM                    HPSK
 ADSL etc                 Hybrid Phase Shift Keying
 256QAM                      Also known as
 Microwave                 Orthogonal Complex
 Some Cable Modem          Keying (OCQPSK)
 1024QAM
 Used in CDMA2000
 Still experimental
 VSB
 OQPSK
 Vestigial Side Band
 Offset QPSK
 8VSB, 16VSB
 Used to avoid zero
crossings                US Digital Broadcast TV
 DQPSK
Filters, For Spectrum Control
01      00
I - Signal
Low Pass Filter
To IQ
Q - Signal                     Modulator
10      11                     Low Pass Filter

Modulation Mapping
Sources of Error

Raw Data       Compression,       011010100101                                 01 10 10 10 01 01
Convert to
Error Correction,                                   Symbols
110101           Encryption

01       00
I - Signal
Low Pass Filter
To IQ
Q - Signal                                  Modulator
10        11                    Low Pass Filter

Modulation Mapping

I - Signal
RF
Modulation,
Q - Signal                                              Amplifier
Upconversion
Sources of Error

cos(2πfct)
I     LPF

fc
BPF

90 90°
90

Q      LPF

sin(2πfct)
Errors Receiving the Signal

cos(2πfct)
LPF       I

fc
BPF
90 90°
90

LPF        Q
sin(2πfct)
 This could be your customers receiver, or it could be a
WCA vector spectrum analyzer
Common Measurements

29
What data was sent?

 Data Display
 Time vs.
Amplitude
 Error Summary
Error Summary

 Error Vector
Magnitude
 Magnitude
and Phase
Error
 Freq. Error
 IQ offset
Modulation Errors vs. Time

 Amplitude errors
correlated to EVM
 WCA is especially
good at this
What can a WCA do?
Tektronix
 Modulation types                        Wireless
Communication
Analyzer
WCA230A /

 Symbol rates        Modulation types
WCA280A
BPSK, QPSK, Pai/4
Shift DQPSK,
8PSK, 16QAM,
64QAM, 256QAM,
 Filter types                            GMSK, GFSK
Symbol rates        to 12.8 Msps
Filter types        Measurement Filter:

 One button setups
Root Cosine
Reference Filter:
Cosine, Gauss
Filter Parameter:
α/BT： 0.0001 to 1,

 Standards                               0.0001 step

One button setups   PDC, PHS, NADC,
TETRA, GSM,
CDPD, Bluetooth

Standards           W-CDMA
W-CDMA
GSM/EDGE(opt.24)
Summary

 Digital modulation is cheaper, faster, more accurate,
more efficient, more secure
 Higher order modulation is used for greater
transmission rates in the same spectrum occupancy
 Higher order modulation is more susceptible to
noise
 Baseband filters are used to control spectrum
 Wireless Communications Analyzers are used to
evaluate modulation quality
WCA is particularly good at connecting effects in
multiple domains
Product Line Contact Information

 Dedicated Regional Contacts        Worldwide Factory Contacts

 Kurt Krukenberg                   Dave McDonald
phone: +1 503-627-5039             phone: +1 503-627-1279
Regional Product Manager           TSC Primary Contact
Americas                           WCA200A and WCA300

phone: +44 1344-392249             phone: +81 3-3448-3272
Regional Product Manager           Product Manager
EMEA                               WCA200A & Wireless Apps

 Charles Wu                        Jerry Harris
phone: +852 258-56774              phone: +1 503-627-4827
Product Line Representative        Product Manager
Asia/PacRim                        WCA300 & Non-Wireless Apps
Slide Archive (extras)

36
Filters Alter The Signal

01      00
I - Signal
Low Pass Filter
To IQ
Q - Signal                     Modulator
10      11                     Low Pass Filter

Modulation Mapping
Common Filter Types

 Gaussian

 Raised Cosine

 Root Raised Cosine

 Setting up the WCA requires
knowledge of what filter is
used
Effect of Roll-off
 Usually α is between 0.2 - 0.5   α=0.0   α=1.0
 α determines the bandwidth
BW = (1+α) *Symbol rate
Put IQ on the Carrier Wave

cos(2πfct)
I     LPF

fc
BPF

90 90°
90

Q      LPF

sin(2πfct)
Structure of Tx/Rx

Encode

Data                  Error correction
Scrambling                          Interleaving 1st   Interleaving 2nd
stream                    encoding

IQ mapping               IQ modulation

IQ demodulation              IQ de-mapping

De-interleaving   De-interleaving     Error correction                     Data
Scrambling
2nd               1st              decoding                         stream

```
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