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Wideband Complex Modulation Analysis Using a Real-Time Digital Demodulator
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Agenda
l Modulation basicsl I and Q modulationl OFDMl Complex frequency offset
l Measuring complex modulatioonl Error vector magnitude
l Real time digital down conversion and demdulationl Measurement example: 802.11ac
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Modulation
Detect the Modifications„Demodulate“
Any reliably detectable change in signal characteristics can carry information
Modify a Signal
„Modulate“
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Different Modulation Schemes
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I/Q vector displayIn the baseband the modulating signal can be represented as a vector l of certain magnitude and phase orl with certain inphase (I) and quadrature (Q) component
Inphase
PhaseM
ag
Quadrature
I
Q
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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Constellation Diagram
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Measuring Complex Modulation
Inphase
Quadrature
I
Q
Actual value
Ideal value
Error vector
Error vector magnitude (EVM)
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OFDM
5 MHz
Single Carrier Transmission (e.g. WCDMA)
e.g. 5 MHz
(Orthogonal )Frequency Division
Multiplexing ((O)FDM)
Typically several 100 sub-carriers with spacing of x kHz
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,
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Frequency Domain Time Domain
OFDM signal generation chain
l OFDM signal generation is based on Inverse Fast Fourier Transform (IFFT) operation on transmitter side:
Data source
QAM Modulator
1:NN
symbol streams
IFFT OFDM
symbolsN:1 Cyclic prefix
insertion
Useful OFDM symbols
l On receiver side, an FFT operation will be used.
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OFDM SummaryAdvantagesOFDM SummaryAdvantages and disadvantages
l High spectral efficiency due to efficient use of available bandwidth,l Scalable bandwidths and data rates,
l Robust against narrow-band co-channel interference, Intersymbol Interference (ISI) and fading caused by multipath propagation,
l Can easily adapt to severe channel conditions without complex equalizationl 1-tap equalization in frequency
domain, l Low sensitivity to time
synchronization errors,
l Very sensitive to frequency synchronization,l Phase noise, frequency and clock offset,
l Sensitive to Doppler shift,l Guard interval required to minimize
effects of ISI and ICI,l High peak-to-average power ratio
(PAPR), due to the independent phases of the sub-carriers mean that they will often combine constructively,l High-resolution DAC and ADC required,l Requiring linear transmitter circuitry, which
suffers from poor power efficiency, - Any non-linearity will cause intermodulation
distortion raising phase noise, causing Inter-Carrier Interference (ICI) and out-of-band spurious radiation.
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Complex Modulation – Offset Frequency
Positive rotation Negative rotation
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Complex Signal Analyzer
l Down converter translates RF to IFl Complex detector translates signal to complex basebandl 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.
preselectorDown conversion
A/D
BW < 2*fs
Application software
A/D
ComplexDetector
RF
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Measurement Challenge for Wideband Signalsl A/D converter typically samples at hundreds of MHzl High resolution 12 to 14 bit ADCl Limited bandwidth (160 MHz)
l Wideband signals can have spectra > 160 MHzl 802.11ac is at 160 MHz today
l Use an oscilloscope to acquire the RF or IF signall Wide frequency range (many GHz)l Relatively low resolution: less than 6 effective bitsl Deep memory requirements (100 ps sample interval = 10
Msamples/ms)l High processor load (down conversion and detection)
l Improved oscilloscope solution using ASICl ASIC performs down conversion and detection in real timel Low memory requirement (signal at information rate)l Higher resolution: 7 effective bits
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RTO-K11 I/Q Software Interface
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
Acquires modulated signals and outputs the corresponding I/Q data for further analysis
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RTO-K11 I/Q Software InterfaceFollowing 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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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
fc- fc fc- fc
x(t)e-j2πfct
- 2fc
x(t)ej2πfct
2fc
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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
fc
x(t)e-j2πfct
- fc
ej2πfct
Upper sideband & inverse position Lower sideband & normal position
fc
[x(t)e-j2πfct]*
- fc
[x(t)ej2πfct]*
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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.
-sin(2πfIFt)
X
cos(2πfIFt)
X
ADC
ADC
exp(j2πfot)
X LPFx(t)
analog frontend digital backend
RTO
fIF
fIF
DC offset ADC
A
B
C
Afc
B
C
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How does RTO-K11 work?
Complex I/Q baseband signals
No downconversion required.Signals can directly be low-pass filtered
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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 rateRelative bandwidth: 4 % … 80 %Within the filter BW the filter has a flat frequency response (no 3 dB bandwidth)
Filter BW Sample Rate
Nyquist!!!
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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How to deal with carrier frequencies > 4 GHz?Carrier frequencies > 4 GHz require external downconversion
DUTRF > 4 GHz external
downconversion
I/Q orRF < 4 GHz
RTO
DUT
LAN
IF = 500 MHz
RF > 4 GHz
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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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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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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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