Wideband Complex Modulation Analysis Using a Real-Time Digital Demodulator
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The document discusses wideband complex modulation analysis using real-time digital demodulation techniques, covering key concepts like modulation basics, I/Q modulation, and OFDM. It outlines measurement challenges, the capabilities of the RTO-K11 I/Q software interface, as well as solutions for various modulated signals, specifically addressing bandwidth and signal quality. The document concludes with an overview of Rohde & Schwarz as a key player in the electronics and communications field.
![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
17](https://image.slidesharecdn.com/widebanddemod-121015152337-phpapp01/85/Wideband-Complex-Modulation-Analysis-Using-a-Real-Time-Digital-Demodulator-17-320.jpg)
Wideband Complex Modulation Analysis Using a Real-Time Digital Demodulator
- 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 2
- 3. Modulation Modify a Signal „Modulate“ Detect the Modifications „Demodulate“ Any reliably detectable change in signal characteristics can carry information 3
- 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. 5
- 7. Measuring Complex Modulation Quadrature Actual value Error vector Q Ideal value I Inphase Error vector magnitude (EVM) 7
- 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 8
- 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. 9
- 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. 10
- 11. Complex Modulation – Offset Frequency Positive rotation Negative rotation 11
- 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. 12
- 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 13
- 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 14
- 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 15
- 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 16
- 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 17
- 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 18
- 19. How does RTO-K11 work? Complex I/Q baseband signals No downconversion required. Signals can directly be low-pass filtered 19
- 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 20
- 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 21
- 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 22
- 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 23
- 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 24
- 25. Contact Us About Rohde & Schwarz Rohde & Schwarz is an independent group of companies specializing in electronics. It is a leading supplier of solutions in the fields of test and measurement, broadcasting, radiomonitoring and radiolocation, as well as secure communications. Established more than 75 years ago, Rohde & Schwarz has a global presence and a dedicated service network in over 70 countries. Company headquarters are in Munich, Germany. Europe, Africa, Middle East +49 89 4129 12345 customersupport@rohde-schwarz.com North America 1-888-TEST-RSA (1-888-837-8772) customer.support@rsa.rohde-schwarz.com Latin America +1-410-910-7988 customersupport.la@rohde-schwarz.com Asia/Pacific +65 65 13 04 88 customersupport.asia@rohde-schwarz.com www.rohde-schwarz-scopes.com 25
Editor's Notes
- #4: To transmit a signal over the air, there are three main steps: 1. A pure carrier is generated at the transmitter. 2. The carrier is modulated with the information to be transmitted. Any reliably detectable change in signal characteristics can carry information. 3. At the receiver the signal modifications or changes are detected and demodulated. Die Modulation und Demodulation dienen also zur Aufbereitung von Informationen in eine Signalform, die die Übertragung der Informationen über eine grösstmögliche Entfernung oder beliebige, vorgegebene Entfernungen unter Wahrung des erforderlichen Störabstands gewährleistet. Dabei sind die Randbedingungen bezüglich der Kanalkapazität und die spezifischen Eigenschaften des Übertragungskanals zu berücksichtigen (frequenzabhängige Dämpfung und Phasenmass, zeit- und frequenzselektive Kanäle). Unter Modulation versteht man die Veränderung eines oder mehrerer Signalparameter (Amplitude, Frequenz oder Phase) eines Trägers in Abhängigkeit der Information. Dadurch wird dem Trägersignal die Information aufgeprägt. Nach der Modulation erscheint die Information in einer anderen Form, meistens in einem höheren Frequenzbereich ( Radio Frequency, RF ). Als Trägersignal kommt prinzipiell jede Signalart in Frage, auch Rauschen. Aber technisch haben sich nur zwei Signalformen durchgesetzt:
- #5: There are only three characteristics of a signal that can be changed over time: amplitude, phase or frequency In AM, the amplitude of a high-frequency carrier signal is varied in proportion to the instantaneous amplitude of the modulating message signal. Frequency Modulation (FM) is the most popular analog modulation technique used in mobile communications systems. In FM, the amplitude of the modulating carrier is kept constant while its frequency is varied by the modulating message signal. Amplitude and phase can be modulated simultaneously and separately, but this is difficult to generate, and especially difficult to detect. Instead, in practical systems the signal is separated into another set of independent components: I (In-phase) and Q (Quadrature). These components are orthogonal and do not interfere with each other. Signals that modulate both amplitude and phase at the same time are also called vector modulated signals because the signals can instantaneously be defined as a vector of certain amplitude and phase in a polar display. Modern communications systems demand more information capacity, higher signal quality, greater security and digital data compatibility. AM and FM, while valuable modulation methods, have proven inadequate to match today’s needs for high-volume traffic. With millions of cell phone subscribers gobbling up more voice bandwidth, we need a modulation method that can efficiently transfer information in a reliable manner.
- #6: Every signal could be instantaneously be defined as a vector which is the description of the demodulated signal. A simple way to view amplitude and phase is with the polar diagram. The carrier becomes a frequency and phase reference and the signal is interpreted relative to the carrier. The signal can be expressed in polar form as a magnitude and a phase. The phase is relative to a reference signal, the carrier in most communication systems. The magnitude is either an absolute or relative value. Both are used in digital communication systems. Polar diagrams are the basis of many displays used in digital communications, although i t is common to describe the signal vector by its rectangular coordinates of I (In-phase) and Q (Quadrature). In digital communications, modulation is often expressed in terms of I and Q . This is a rectangular representation of the polar diagram. On a polar diagram, the I axis lies on the zero degree phase reference, and the Q axis is rotated by 90 degrees. The signal vector’s projection onto the I axis is its “ I” component and the projection onto the Q axis is its “Q” component. Das Signal wird zu einem bestimmten Zeitpunkt als Zeiger dargestellt. Die Zeigerlänge entspricht der Amplitude ûc und der Winkel θder momentanen Phasenlage.