Digital Signal Analysis Digital Signal Analysis X-Series Spectrum - - PowerPoint PPT Presentation

Digital Signal Analysis Digital Signal Analysis

Page 1 X-Series Spectrum Analyzers Seminar 2010

Objectives

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

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Di it l M d l ti O i Digital Modulation Overview

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Vector Signal Analysis on one page

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Transmitter Overview

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Receiver Overview

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Signal Characteristics to Modify

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Polar Display / IQ Relationship

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Quadrature Amplitude Modulation

QPSK 2 bits /symbol Symbol Rate = 1/2 bit 64 QAM Symbol Rate 1/2 bit rate 64 QAM 6 bits /symbol Symbol Rate = 1/6 bit rate 16 QAM 4 bits /symbol Symbol Rate = 1/4 bit rate 32 QAM 5 bits /symbol 128 QAM 7 bits /symbol Symbol Rate = 1/7 bit Symbol Rate = 1/5 bit rate Symbol Rate = 1/7 bit rate Page 9 X-Series Spectrum Analyzers Seminar 2010

Comparing OFDM and SC-FDMA QPSK example using N=4 subcarriers QPSK example using N=4 subcarriers

The following graphs show how this sequence of QPSK symbols is

1, 1

• 1,-1
• 1, 1

1, -1 1, 1

• 1,-1
• 1, 1

1, -1

this sequence of QPSK symbols is represented in frequency and time

V

CP

V

CP

F 60 kHz Frequency

CP

15 kHz Frequency

fc

OFDMA

Data symbols occupy 15 kHz for

• ne OFDMA symbol period

SC-FDMA Data symbols occupy N*15 kHz for 1/N SC FDMA b l i d

60 kHz Frequency

fc

• ne OFDMA symbol period

1/N SC-FDMA symbol periods

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IQ Modulator

1 0,0 0,1 1,0 1,1

symbol

1

symbol

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1

X-Series Spectrum Analyzers Seminar 2010

IQ Demodulation

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Error Vector Concept

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How EVM is calculated

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Digital Radio Impairments and Digital Radio Impairments and Measurements

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IQ Imbalance

Common Radio Impairments

IF Filter 0 d I/Q Modulator

I

Transmitter

Compression IQ Imbalance

PA

+

0 deg 90 deg IF LO

Symbol Encoder

DAC BB Filter

Q

RF LO

Q

Phase Noise

DAC/DSP Error Spurious

I/Q Demodulator

I

Receiver

Incorrect Coefficients

Spurious

RF Filter IF Amp LNA IF Filter 0 deg 90 d IF LO ADC BB Filter

Symbol Decoder

I

I t f

RF LO 90 deg

Q

Interferers Ripple & Tilt Thermal Noise

RF LO

Ripple & Tilt

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Wrong filter coefficients

IF Filter 0 deg I/Q Modulator

Symbol

I

PA

+

90 deg IF LO

Symbol Encoder

DAC BB Filter

Q

RF LO

Baseband Filter Impulse Response

Overshoot from incorrect alpha

Baseband Filter Freq. Response

For this example, the alphas for the Nyquist filter were 0.2 in the transmitter and 0.35 in the receiver.

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Amplifier Nonlinearity

IF Filter I/Q Modulator

I

Transmitter

Compression

IF Filter PA

+

0 deg 90 deg IF LO

Symbol Encoder

DAC BB Filter

Q

RF LO

Q

Traditional Two-Tone Test Testing a Digitally Modulated Signal

IMD

With compression

f1 f2

Without compression

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f1 f2

Without compression

X-Series Spectrum Analyzers Seminar 2010

Amplifier Nonlinearity - CCDF

Wh t th th b bl ff t f C i ? What are the other observable effects of Compression? CCDF Measurements

AWGN Si l AWGN Signal as a reference

y probability

Compressed QPSK Signal Non-compressed

p

QPSK Signal dB above the average

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g

X-Series Spectrum Analyzers Seminar 2010

Amplifier Nonlinearity - Demodulation

What are the other observable effects of Compression? Vector (IQ) Diagram ( ) g

Without compression Increased EVM With compression

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I/Q Impairments

I/Q Modulator

I

Imbalances

IF Filter PA

+

0 deg 90 deg IF LO

Symbol Encoder

DAC BB Filter RF LO

Q

• Gain Imbalance
• Quadrature Error

Quadrature Error

• DC Offset

P th Diff D l

• Path Difference - Delay

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I/Q Impairments – Gain Imbalance

IQ Constellation QPSK Summary Table QPSK Summary Table

Gain Imb. = 1.02 dB (Ideally 0 dB) Ideal Measured ( y ) Ideal (square) (rectangle)

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I/Q Impairments – Quadrature Error

IQ Constellation QPSK Summary Table QPSK Summary Table

• Quad. Error = 5.9 deg.

(Ideally 0 deg*) Ideal M d ( y g ) Ideal (square) Measured (parallelogram)

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

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What is causing this problem?

RF Filter IF Amp IF Filter 0 deg I/Q Demodulator

S b l

I

Receiver

LNA 90 deg IF LO ADC BB Filter

Symbol Decoder

Q

Interferers

RF LO

Spectrum A

Spectrum B No Spurious

Spectrum A

Spectrum B Signals ACP Looks Good

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Signal A: Demodulation is Good

S A

Demod

Points randomly distributed

Spectrum A

EVM Low (0.4%) ( )

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Signal B: Demodulation

S B

Demod

Points are not randomly distributed

Spectrum B

EVM High (3.6%) g ( ) Mag & Phase Errors High and comparable

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and comparable

X-Series Spectrum Analyzers Seminar 2010

Signal B: EVM Spectrum Shows Spur

S i Si l

EVM Spectrum

Spurious Signal

• 36dBc spur was

buried under the modulated carrier

Spectrum B

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Infamous V-shape

RF Filter IF Amp IF Filter 0 deg I/Q Demodulator

S b l

I

Receiver

LNA 90 deg IF LO ADC BB Filter

Symbol Decoder

Q

RF LO

Symbol Rate

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Digital Modulation Measurements Constellation Errors Constellation Errors

64 QAM C t ll ti

Ideal Symbol Point

64 QAM Constellation

Q Random Noise Ideal Symbol Point Phase Noise I AM Distortion PM Distortion D l Di i /ISI Delay Distortion/ISI Interference

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Demonstration

X-Series Spectrum Analyzers Seminar 2010 Page 30

THANK YOU! THANK YOU!

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