wideband complex modulation analysis using a real-time digital demodulator
DESCRIPTION
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.TRANSCRIPT
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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