Join us for a LIVE WEBINAR on this topic! Wednesday, November 14, 2:00pm ET http://bit.ly/XPgjO7 Wide bandwidth modulation is becoming more common in communications. The emergence of the 802.11ac wireless Ethernet standard has extended the modulation bandwidth to 160 MHz which requires very wide band measurement equipment to measure. This presentation illustrates the details of a measurement method that uses a real time digital down converter and post processing software that measures the performance of this signal.

1. Wideband Complex Modulation
Analysis Using a Real-Time
Digital Demodulator

2. Agenda
l Modulation basics
l I and Q modulation
l OFDM
l Complex frequency offset
l Measuring complex modulatioon
l Error vector magnitude
l Real time digital down conversion and demdulation
l Measurement example: 802.11ac
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3. Modulation
Modify a
Signal
„Modulate“
Detect the Modifications
„Demodulate“
Any reliably detectable change in signal
characteristics can carry information
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4. Different Modulation Schemes
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5. I/Q vector display
In the baseband the modulating signal can be represented as a vector
l of certain magnitude and phase or
l with certain inphase (I) and quadrature (Q) component
Quadrature
Q
ag
M
Phase
I Inphase
l I and Q carry the information to be transmitted and need to be
analyzed in order to extract that information.
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6. Constellation Diagram
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7. Measuring Complex Modulation
Quadrature Actual value
Error vector
Q
Ideal value
I Inphase
Error vector magnitude (EVM)
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8. OFDM
l Orthogonal Frequency Division Multiplex (OFDM) is a multi-
carrier transmission technique, which divides the available
spectrum into many subcarriers, each one being modulated
by a low data rate stream,
Single Carrier
Transmission
(e.g. WCDMA)
5 MHz
Typically several 100 sub-carriers with spacing of x kHz
(Orthogonal )
Frequency Division
Multiplexing ((O)FDM)
e.g. 5 MHz
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9. OFDM signal generation chain
l OFDM signal generation is based on Inverse Fast Fourier Transform
(IFFT) operation on transmitter side:
Data QAM N Useful
1:N OFDM Cyclic prefix
source Modulator symbol IFFT N:1 OFDM
symbols insertion
streams symbols
Frequency Domain Time Domain
l On receiver side, an FFT operation will be used.
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10. OFDM Summary
Advantages and disadvantages
Advantages
l Very sensitive to frequency
l High spectral efficiency due to
synchronization,
efficient use of available
l Phase noise, frequency and clock offset,
bandwidth,
l Scalable bandwidths and data rates, l Sensitive to Doppler shift,
l Robust against narrow-band co- l Guard interval required to minimize
channel interference, effects of ISI and ICI,
Intersymbol Interference (ISI) l High peak-to-average power ratio
and fading caused by multipath (PAPR), due to the independent
propagation, phases of the sub-carriers mean that
l Can easily adapt to severe they will often combine constructively,
channel conditions without l High-resolution DAC and ADC required,
complex equalization l Requiring linear transmitter circuitry, which
l 1-tap equalization in frequency suffers from poor power efficiency,
- Any non-linearity will cause intermodulation
domain, distortion raising phase noise, causing Inter-
l Low sensitivity to time Carrier Interference (ICI) and out-of-band
synchronization errors, spurious radiation.
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11. Complex Modulation – Offset Frequency
Positive rotation Negative rotation
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12. Complex Signal Analyzer
BW < 2*fs
A/D
RF Down
preselector
conversion
A/D
Complex
Detector
Application
software
l Down converter translates RF to IF
l Complex detector translates signal to complex baseband
l Complex spectrum centered at DC
l A/D converters digitize I and Q signals at > 2x the modulation
bandwidth
l Application software measures EVM, constellation, etc.
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13. Measurement Challenge for Wideband
Signals
l A/D converter typically samples at hundreds of MHz
l High resolution 12 to 14 bit ADC
l Limited bandwidth (160 MHz)
l Wideband signals can have spectra > 160 MHz
l 802.11ac is at 160 MHz today
l Use an oscilloscope to acquire the RF or IF signal
l Wide frequency range (many GHz)
l Relatively low resolution: less than 6 effective bits
l Deep memory requirements (100 ps sample interval = 10
Msamples/ms)
l High processor load (down conversion and detection)
l Improved oscilloscope solution using ASIC
l ASIC performs down conversion and detection in real time
l Low memory requirement (signal at information rate)
l Higher resolution: 7 effective bits
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14. RTO-K11 I/Q Software Interface
Acquires modulated signals and outputs the corresponding
I/Q data for further analysis
l Does a hardware-based
downconversion of the input
signals to I/Q
l Resamples the I/Q to a required
sample rate
l Supports RF, I/Q and low-IF
signals
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15. RTO-K11 I/Q Software Interface
Following input signal formats are supported:
l Real RF signals
 Downconversion  Filtering  Resampling
 One input channel needed per signal  up to 4
signals can be recorded in parallel
l Complex I/Q baseband signals
 Filtering  Resampling
 Two input channels needed per signal (one for I,
one for Q)  up to 2 signals can be recorded in
parallel
l Complex modulated signals in low-IF range
 Downconversion  Filtering  Resampling
 Two input channels needed per signal (one for I,
one for Q)  up to 2 signals can be recorded in
parallel
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16. How does RTO-K11 work?
Downconversion of real RF signals
The digitized data from the ADC is downconverted to the baseband
l Carrier frequency range:
1 Hz to 5 GHz
l Frequency position of the RF spectrum:
Normal Inverse
x(t)e-j2πfct x(t)ej2πfct
- 2fc - fc fc - fc fc 2fc
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17. How does RTO-K11 work?
Downconversion of complex modulated signals in low-IF range
The digitized data from the ADC is downconverted to the baseband
l Carrier frequency range:
1 Hz to 5 GHz
l Frequency position of the RF spectrum:
Upper sideband & normal position Lower sideband & inverse position
x(t)e-j2πfct ej2πfct
fc - fc
Upper sideband & inverse position Lower sideband & normal position
[x(t)e-j2πfct]* [x(t)ej2πfct]*
fc - fc
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18. Complex low-IF signals
Example:
l Low-IF receiver:
 A modulated RF signal is mixed down to a non-zero low intermediate frequency
(typ. a few MHz).
 Purpose is to avoid DC offset and 1/f noise problems of subsequent components,
like A/D converters
 Nowadays e.g. widely used in the tiny FM receivers incorporated into MP3 players
and mobile phones; is becoming commonplace in both analog and digital TV
receiver designs.
B
RTO A
cos(2πfIFt)
exp(j2πfot) fc
A X ADC
x(t)
X LPF B
X ADC fIF
-sin(2πfIFt) C C
DC
offset
ADC
analog frontend digital backend
fIF
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19. How does RTO-K11 work?
Complex I/Q baseband signals
No downconversion required.
Signals can directly be low-pass filtered
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20. How does RTO-K11 work?
Low-pass filtering and resampling
l Sample rate range:
freely selectable between 1 kSa/s and 10 GSa/s
l Filter bandwidth = Relative bandwidth x Sample rate
Relative bandwidth: 4 % … 80 %
Within the filter BW the filter has a flat frequency response (no 3 dB bandwidth)
Nyquist!!!
Filter BW Sample Rate
Transfer to aquisition memoy
l Record Length: freely selectable between
1 kSa and 10 MSa (6 MSa for more than 2 channels)
l Acquisition time = Record length / Sample rate
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21. How to deal with carrier frequencies > 4 GHz?
Carrier frequencies > 4 GHz
require external downconversion
I/Q or
RF > 4 GHz external RF < 4 GHz
down RTO
DUT conversion
IF = 500 MHz
RF > 4 GHz
DUT
LAN
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22. What makes the RTO-K11 so interesting?
l RTO with K11 extends the available I/Q analysis bandwidth:
 Maximum I/Q analysis bandwidth of R&S Spectrum Analysers is 160 MHz for the
FSW
 For analysis bandwidth > 160 MHz use the RTO (allows for bandwidths up to 4 GHz)
⇒ Wideband applications, like e.g.
 Wideband Radar and Pulsed RF signals
 High data rate satellite links
 Frequency hopping communications
l The RTO offers 4 parallel inputs
 1 RF input on a Spectrum Analyzer
⇒ MIMO applications
 analyzing up to 4 Tx antennas with just one RTO  e.g. 4x4 MIMO LTE
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23. How to analyze the data RTO-K11 provides?
l RTO-K11 provides different data formats (e.g. csv) that can easily be
imported into generic customer tools, like for example Matlab
l RTO-K11 is a generic interface for signal analysis options from 1ES
running on an external PC*
 FS-K96 OFDM Vector Signal Analysis
 FS-K112 NFC Analysis Software
 FS-K10xPC LTE Analysis Software
* roadmaps to be defined
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24. What I/Q signal quality does RTO-K11 provide?
RTO versus Spectrum Analyzer
l Advantage RTO:
 I/Q analysis bandwidth: SpecAn ≤ 160 MHz versus RTO < 4 GHz
 Spectrum flatness: FSW: ± 0.3 dB @ 80 MHz I/Q bandwidth, fcenter ≤ 8 GHz
RTO1044: ± 0.1 dB @ 100 MHz I/Q bandwidth, fcenter ≤ 3 GHz
l Advantage Spectrum Analysis:
 Carrier frequencies >> 4 GHz
 ADC resolution: SpecAn 12 to 16 bit versus RTO 8 bit
 Frontend: Less noise and non-linearities in the SpecAn
⇒ Spectrum Analyzer will provide better I/Q analysis results, e.g. EVM
Nevertheless, I/Q performance of RTO is quite good:
l low-noise frontend, full BW even at 1 mV/div, single core ADC (> 7 ENOB)…
l e.g. 802.11a signal: EVM with RTO < -40 dB
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25. Contact Us
About Rohde & Schwarz
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the fields of test and measurement, broadcasting, radiomonitoring and radiolocation, as well as secure communications.
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