modulation numérique : principes et diagnostics · modulation numérique : principes et...

Modulation numérique :

Principes et diagnostics

Journée technique Grenoble

2 Octobre 2012

Frédéric BIS MB Electronqiue

Objectives

- Provide a digital modulation overview- Review common digital radio impairments

Digital Modulation Overview

3

Transmitter Overview

4

Receiver Overview

5

Signal Characteristics to Modify

6

Polar Display / IQ Relationship

7

IQ Modulator

1 0

0,00,1

8

1

0

1,01,1

symbol

IQ Demodulation

9

• BPSK encodes one binary bit per symbol by a 180o phase change of the Carrier

• The Occupied spectrum bandwidth is proportional to the symbol frequency.

• Quadrature modulation formats increases the bit rate by 2X using the same symbol clock frequency and therefore same occupied bandwidth.

• Quadrature Phase Shift Keyed modulation adds 2 BPSK carriers that are the same frequency but 90o in relative phase.

Modulation Formats

��

��

Q��

��

�

��

��

� ��

� ��

��

�

��

��

��

��

Q

�

�

�

Quadrature Amplitude Modulation

16 QAM

4 bits /symbol

Symbol Rate = 1/4 bit

rate

QPSK

2 bits /symbol


rate64 QAM

6 bits /symbol


rate

32 QAM

5 bits /symbol


rate

128 QAM

7 bits /symbol


rate

BPSK 1

Number of bits per symbol

Transmission bandwidth

Modulation format

F

I

Q

01

Q 0001

Constellation

Digital Modulation Characteristics

Symbol Rate = #symbols/sec. (Hz)

QPSK 2

16 QAM 4

F/2

F/4

I

0001

1011

I

Q 0000

Modulation examples: APSK

13

Amplitude & Phase Shift Keying

How to demodulate ? Custom APSKDemod Setup

1. Determine number of rings (max. 8)

2. For each ring, define:• number of states (evenly spaced)• relative magnitude of ring (0-1.00)• relative phase of ring (0-180 deg)

14

22.5o

0o

45o

Rel.PhaseRing 0: 2 states, mag. 0.30

Ring 1: 4 states, mag. 0.55Ring 2: 12 states, mag. 1.00

Who Needs Custom APSK?

• Example applications:• APSK constellations (Aero/Def, mostly SATCOM)• Unusual constellations (staggered APSK)• High-order PSK (e.g. 16PSK, 32PSK)

Any modulation format where the constellation can be modeled with 1-8 concentric rings of arbitrary magnitude, arbitrary phase rotation and N evenly–spaced states.

• High-order PSK (e.g. 16PSK, 32PSK)• Amplitude Shift Keying

• But not:• Where states are un-evenly spaced (empty states are OK!)• “Square” constellations with more than 3 states per side.

15

16PSK

16

DuoBinary

17

3ASK-2PSK

18

8QAM

19

SOQPSK “Shaped Offset QPSK”

• A type of Continuous Phase Modulation (CPM).

• Phase transitions are shaped by a filter with a specified time response.

• Typical symbol rates: 10’s of Ksym/sec to 10’s of Msym/sec.

• Several types of SOQPSK exist –• SOQPSK-MIL (defined by Mil STD 188-181C) •SOQPSK-TG (defined by IRIG 106-04)

20

•users: RF telemetry equipment..

SOQPSK• Choose Preset to Standard , then select “SOQPSK-TG (IRIG 106)”. There are no standard symbol rates.

• Example signal and Recall Demo provided.

21

• This is how users typ. want to see it (i.e. with I&Q artificially aligned).• This is what it actually looks like (constant mag, continuous phase).

OFDM Modulation :What is OFDM?

Orthogonal Frequency Domain Multiplexing

•OFDM is a modulation format that achieves:– high data throughput by transmitting on hundreds or

thousands of carriers simultaneously.

– high spectral efficiency by spacing the carriers very – high spectral efficiency by spacing the carriers very closely.

– high data integrity by transmitting at a relatively slow symbol rate.

22

Orthogonal Subcarriers

•Overlapping Carriers But No Inter-Carrier Interference (Ideally!)

23

Symbol #2

Symbol #3

Symbol #4

OFDM Symbols & Subcarriers Simplified view

Symbol #0

Symbol #1

Time

Freq

-5 -4 -3 -2 -1 0 +1 +2 +3 +5+4Subcarrier Number

……

24

Symbol #2

Symbol #3

Symbol #4

OFDM Symbols & SubcarriersReal world view

Preamble

Symbol #1

Time

Freq

-5 -4 -3 -2 -1 0 +1 +2 +3 +5+4Subcarrier Number

……

25

OFDM vs. Single Carrier ModulationFrequency Domain View

many carriers

BW = #carriers x spacing

OFDM

1 carrierBW =

Sym(1+α)

Single Carrier QAM

#carriers x spacing

Carrier #0 always null

Sym(1+α)

26

OFDM vs. Single Carrier ModulationTime Domain View

Single Carrier 64QAM802.11a OFDM

1 Sym = 1 Sample = .083 usec

Data rate = 54 Mbits/sData rate = 54 Mbits/s

1 Sym = 64 Samples = 4.0 usec

27

Sharing the Resource: OFD MA

+

=

User1 (low rate): 112 subcarriers

“Multi-Access”

+

=

User3 (hi-rate): 448 subcarriers

User2 (med-rate): 280 subcarriers 840 subcarrier signal

28

OFDMA Resource Map

DL B

urst 9

DL B

urst 8

DL Burst 5DL Burst 1

DL Burst 7DL Burst 4DL Burst 3

UL-M

AP

Pream

bleF

CH

DL-M

AP

Sub

carr

iers

Example:802.16e Mobile WiMAX

• Shows allocation of subcarriers by time and frequency.• Subcarriers are usually grouped into logical channels.• Each channel can have different modulation, power level, coding, etc.

Symbol #

DL Burst 2DL Burst 6

DL B

urst 9

DL B

urst 8

29

Who Uses OFDM?

WPA

N Bluetooth 1.0WiMedia

802.11b

802.16

WLA

NW

MA

N

802.11a/g 802.11n

2.0 3.0

802.11ad802.11ac

802.16d 802.16e 802.16m

Mob

ileB

road

cast

Analog

1GAnalog

2GGSM, CDMAPDC, PHS

3GWCDMACDMA2K

3.9GFDD-LTETDD-LTE

4GLTE-Adv.

ATSC

DVBISDB

DVB-TISDB-T

DVB-HDAB

30

Digital Radio Impairments and Measurements

31

IF Filter

PA

RF LO

+

0 deg

90 deg

IF LO

I/Q Modulator

Symbol

EncoderDACBB Filter

I

Q

Transmitter CompressionIQ Imbalance

Phase Noise

Incorrect Coefficients

DAC/DSP ErrorSpurious

Common Radio Impairments

RF Filter IF Amp

RF LO

LNA

IF Filter0 deg

90 deg

IF LO

I/Q Demodulator

ADC BB FilterSymbol

Decoder

I

Q

Receiver

Interferers

Ripple & Tilt

Incorrect CoefficientsSpurious

Thermal Noise

32

Vector Signal Analysis on one page

33

Error Vector Concept

34

How EVM is calculated

35

Wrong filter coefficients

IF Filter

PA

RF LO

+

0 deg

90 deg

IF LO

I/Q Modulator

Symbol

EncoderDACBB Filter

I

Q

Baseband Filter Impulse Response

36

Baseband Filter Freq. Response

Overshoot from incorrect alpha

For this example, the alphas for the Nyquist filter were

0.2 in the transmitter and 0.35 in the receiver.

Amplifier Nonlinearity

IF Filter

PA

RF LO

+

0 deg

90 deg

IF LO

I/Q Modulator

Symbol

EncoderDACBB Filter

I

Q

Transmitter Compression

37

Traditional Two-Tone Test

f1 f2

IMD

Testing a Digitally Modulated Signal

With compression

Without compression

Amplifier Nonlinearity - CCDFWhat are the other observable effects of Compression?

CCDF Measurements

AWGN Signal as

a referenceprobability

38

Compressed

QPSK SignalNon-compressed

QPSK Signal

dB above the average

probability

Amplifier Nonlinearity - Demodulation

What are the other observable effects of Compression?

Vector (IQ) Diagram

Without compression

Increased EVM

39

With compression

I/Q Impairments

IF Filter

PA

RF LO

+

0 deg

90 deg

IF LO

I/Q Modulator

Symbol

EncoderDACBB Filter

I

Q

Imbalances

• Gain Imbalance• Quadrature Error• DC Offset• Path Difference - Delay

40

I/Q Impairments – Gain Imbalance

IQ Constellation

QPSK Summary Table

41

Ideal

(square)

Measured

(rectangle)

Gain Imb. = 1.02 dB

(Ideally 0 dB)

I/Q Impairments – Quadrature Error

IQ Constellation

QPSK Summary Table

42

Ideal

(square)

Measured

(parallelogram)

Quad. Error = 5.9 deg.

(Ideally 0 deg*)

* meaning that I and Q are ideally 90 deg. apart

RF Filter IF Amp

RF LO

LNA

IF Filter0 deg

90 deg

IF LO

I/Q Demodulator

ADC BB FilterSymbol

Decoder

I

Q

Receiver

Interferers

What is causing this problem?

Spectrum A Spectrum B

43

ACP Looks Good

No Spurious

Signals

Spectrum A Spectrum B

Signal A: Demodulation is Good

DemodPoints randomly

distributed

Spectrum A

44

EVM Low (0.4%)

Signal B: Demodulation

DemodPoints are not

randomly distributed

Spectrum B

45

EVM High (3.6%)

Mag & Phase Errors High

and comparable

Signal B: EVM Spectrum Shows Spur

Spurious Signal

EVM Spectrum

-36dBc spur was

buried under the

modulated carrier

46

Spectrum B

RF Filter IF Amp

RF LO

LNA

IF Filter0 deg

90 deg

IF LO

I/Q Demodulator

ADC BB FilterSymbol

Decoder

I

Q

Receiver

Symbol Rate

Infamous V-shape

47

Digital Modulation MeasurementsConstellation Errors

64 QAM ConstellationQ

Random Noise

Phase Noise

Ideal Symbol Point

48

IAM Distortion

PM Distortion

Delay Distortion/ISI

Interference

Demonstration

THANK YOU!THANK YOU!

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