Digital Modulation Test Tools: Instruments With Attitude

Diverse well describes the myriad digital modulation schemes used in modern voice and data communications. The alphabet soup of terminology extends well beyond the familiar QAM and GMSK fundamentals to specific communications protocols such as CDMA, Bluetooth, and WLAN specifications 802.11a and b. Channel widths range from 200 kHz for GSM systems to 22 MHz for 802.11b.

Digital modulation encodes two or more data bits into a single symbol. For this reason, the symbol rate, which is related to the minimum required channel bandwidth, can be much lower than the actual data rate. However, problems of frequency reuse, number of simultaneous users, and noise immunity are amenable to wide bandwidth solutions such as DSSS CDMA systems.

Generally, simple modulation schemes have greater transmission range, lower error rates, and lower speeds associated with them. For example, GSM channels use GMSK with a 200-kHz bandwidth to achieve a 9.6-kHz data rate. The ratio of data rate to bandwidth is not as bad as it seems when you consider that a GSM channel only transmits during one of eight time slots.

GMSK represents the state of each 2-data-bit symbol as either a 45°-, 135°-, 225°- or 315°-phase shift as does MSK modulation. But for GMSK, a Gaussian filter smoothes the phase transitions and reduces the required bandwidth. The RF envelope of GMSK-modulated signals has a constant power because only the phase is affected.

In contrast to the fundamentally TDMA characteristics of GSM transmissions, CDMA systems transmit and receive on multiple channels simultaneously in the same frequency range. CDMA uses DSSS techniques with orthogonal codes to distinguish among users.

In contrast to DSSS systems, Bluetooth uses a frequency-hopping form of spread-spectrum technology. All Bluetooth users reside within the same 83.5-MHz wide channel in the 2,400-MHz to 2,483.5-MHz ISM band. Separate 1,600 hops/s frequency vs. time hopping sequences distinguish among users.

Figure 1 is an overview of typical activity within the ISM band, produced by the Celerity Model CS20310C Wireless LAN/Bluetooth Test Station. All baseband modulation is captured at a 268-MS/s rate. For monitoring purposes, FFTs are performed every 100 ms and the results assembled to form the display shown. The vertical scale runs from 2.400 GHz to 2.500 GHz, with time along the horizontal axis. No attempt is made in the monitor mode to provide fine time resolution.

The display includes 802.11b transmissions in successive 22-MHz high bands along the lower third of the screen and a few near the middle. These are, respectively, channels 1 and 6 of the 11 defined 802.11b channels—two of only three that don’t overlap each other. The individual bright dots are a number of Bluetooth channels hopping throughout the entire ISM band. Finally, the horizontal activity at 2,494 MHz represents a prototype of a DSSS device no longer in production.

When the complex nature of air-interface signals is considered, it is clear why testing and relevant test equipment are divided in line with the OSI seven-layer model. Test sets are capable of emulating an element of the communications system and operate on all seven layers. More general-purpose test equipment can quantify physical-layer and data-link performance but cannot determine the meaning of received data, for example.

Physical-Layer Test Tools

Any RF signal can be expressed as the sum of an in-phase and a quadrature component. For example, if

z(t) = A(t) • cos[wt + f(t)]

substituting a trigonometric identity for the cosine of the sum of two angles gives

z(t) = I(t) • cos(wt) – Q(t) • sin(wt)

where: 
I(t) = A(t) • cos(ft)
Q(t) = A(t) • sin(ft)

The baseband in-phase I(t) and quadrature Q(t) terms contain all the information needed to define the modulation signal s(t), which can be written as a complex quantity

s(t) = I(t) + jQ(t)=A(t) • ejf(t)

This format is the basis of vector signal generators that develop a succession of I/Q data pairs. The resultant signal corresponding to each pair of values can be represented as a vector of length A(t) rotated by the angle f(t).1

Vector Signal Generators

The separation of I/Q pair generation and RF modulation is shown in Figure 2. In the Anritsu MG3681A Digital Modulation Signal Generator, 14-b I/Q data drives two identical DACs to generate analog I/Q modulation. These internal signals or selectable external I/Q signals are used to drive the vector modulator.

Accurately generating the baseband I/Q signals isn’t easy. In the MG3681A, separate expansion modules and associated software provide TDMA or CDMA baseband modulation. Besides the complexity of the modulation formats, it is important that the I and Q data words change simultaneously even at the generator’s top data rate.

In another approach to providing both I/Q baseband signal generation and RF modulation, the IFR 2029 Vector Modulator comprises a digital Arb that drives a vector modulator. Suitable baseband I/Q signals are generated from IFR-supplied files, or the user develops custom waveforms with MATLAB® from The MathWorks or a similar simulation program. The 2029 uses downloaded data to modulate the RF output from a separate external generator. A single-box solution is provided in the company’s 3410 Series Digital RF Signal Generators.

For engineers opting to develop their own waveforms, IFR outlines some of the necessary considerations in an application note.2 For example, the 2029’s front-panel software supports four types of files: an IFR proprietary format with a *.iq extension, the ESG binary format used by Agilent Technologies, the 16-b and the 32-b Little Endian formats, and the 32-b IEEE floating-point notation.

Many details also are included regarding sampling rates and signal timing relationships. The importance of one comment about filter-delay compensation should not be underestimated. Basically, you must compensate for delays introduced by filters when attempting to produce a cyclical waveform.

This point is particularly interesting because a waveform may be cyclical in the sense that it can be repeated without an amplitude vs. time discontinuity. This does not mean that the symbol transition from the end to the (repeated) beginning is valid. So, when calculating the number of samples needed to capture a complete cycle of baseband modulation, it’s also necessary to ensure that an integer number of symbols are represented.

The Agilent Technologies E4438C ESG Signal Generator takes a different approach to creating the baseband signal, splitting the generation between internal firmware and external software. The generator supports 3GPP W-CDMA, cdma2000 and IS-95A, TDMA, and GPS signals via built-in firmware personalities. You run Signal Studio software on your PC to develop suitable signals to test Bluetooth, 802.11a and b, enhanced multitone, and TD-SCDMA formats.

In terms of hardware, the generator features 16-b I/Q data and 400-MHz DACs to provide 4× oversampled, 100-MHz baseband modulation. The added 2 b of resolution more accurately represent complex phase/amplitude relationships than the 14 b standard in other products. In addition, the 100-MHz bandwidth means that multiple 22-MHz 802.11b signals can be simultaneously simulated, for example.

Oversampling 4× supports improved accuracy. Sampling at a rate significantly above Nyquist provides a larger distance between the desired signal and undesired signal images. This, in turn, allows more straightforward reconstruction filters to be used. Fewer artifacts are caused either by the filter stopband intruding into the signal bandwidth or by incomplete image rejection.

The Rohde & Schwarz Model AMIQ Dual-Channel Arb provides 14-b resolution I/Q data or 16-b optionally. Clock frequencies range up to 100 MHz. An automatic amplitude/offset alignment as well as fine adjustment of the channel-to-channel skew ensure symmetry of the two channels. Other than amplitude or phase errors that may be introduced by the RF modulator, the error vector can be minimized.

WinIQSIM™ Waveform Simulation Software is used off-line to develop suitable modulation signals. The AMIQ can be used to drive the I/Q inputs of a vector signal generator and test I/Q modulators/demodulators. Versions of the company’s SMIQ vector signal generator with option SMIQB60 have built-in functionality equivalent to the separate AMIQ Arb.

An application note examines the generation of multicarrier signals for amplifier testing.1 The authors compare the ACLR obtained from three different equipment setups. In the first case, four SMIQs were used to generate separate RF signals, which fed a four-way, 50-W power combiner. The result was a -74-dB figure.

When two channels were generated by each of two SMIQs with the outputs fed into a two-way, 50-W power combiner, the ACLR increased to -64 dB. Finally, all four channels were simulated by one vector signal generator with a -62-dB result.

For CDMA signals, the peak-to-average ratio and, consequently, the complementary cumulative distribution function depend on the selected channelization codes, the correlation of the user data, and the timing offsets between code channels. Phase randomization can be applied to a single Arb-generated multicarrier signal to approach the results obtained from using several non-phase-locked separate generators.

Promoting a Get Real theme, Celerity Systems’ Model CS20310C Wireless LAN/Bluetooth Test Station captures, analyzes, and plays back ISM-band signals. Alternatively, you can generate custom baseband signals via MATLAB and more conventional signals using the company’s Vector Signal Simulation software.

Long memory always is an advantage in Arb-based instruments, but according to Mark Pritchard, new business technologist at Celerity Systems, “The capacity of the CS20310C is about 20 s of the entire ISM band. We’ve optimized the hardware to give the highest sensitivity and most memory depth possible. With our instrument, you actually can record reality.”

Vector Signal Analyzers

The counterpart to signal generation is signal capture and analysis. Celerity’s CS20310C Test Station provides software-based, post-acquisition FFT analysis of the captured time-domain data. In addition, the monitor mode shown in Figure 1 displays signal activity prior to capture. Corresponding oscilloscope and spectrum-analyzer views of the same signals also are available.

The Agilent 89600 Series Vector Signal Analyzers feature a large number of analysis and display modes together with extensive analysis routines. These facilities are particularly useful when determining which of many possible sources actually is causing a problem.

Maximum modulation bandwidth is 40 MHz for the baseband Model 89610A, 36 MHz for the DC to 2.7-GHz Model 89640A, and 36 MHz for the 6.0-GHz Model 89641. The Model 89611A works with your own 70-MHz IF tuner and has a 36-MHz demodulation capability. With both time and frequency domain compatibility, the analyzers can display the detailed behavior of complex signals in a variety of formats.

Lee Meyer, the company’s spectrum analyzer product manager, explained that the 89600 Vector Signal Analyzers are PC-based. The VXI hardware comprises RF downconversion followed by a wideband ADC with analysis and control accommodated by software in a PC.

In addition to the option of increasing throughput by buying a more powerful PC, this functional division means the analysis software can be applied to digitized signals from other sources such as the company’s PSA and ESA spectrum analyzers or Infiniium DSOs. Combining a DSO with the 89600 software, for example, enables analysis of modulation bandwidths of 1 GHz and beyond.

Tektronix recently introduced the WCA200A Series Wireless Communications Analyzers. Specifically designed to address the needs of 2.5G and 3G developers, the product combines digital demodulation and midrange spectrum-analysis features. A new display mode called a codogram presents CDMA channel activity as a function of both time and power (lower right-hand quadrant of Figure 3).

Chris Larsen, the company’s product marketing manager for the wireless product line, said, “Many analyzer functions are available as one-button setups for common standards such as GSM, W-CDMA, and EDGE. However, the instrument also has general-purpose demodulation capabilities a user can program to suit his needs.”

A very specific capability, but an important one, is the analysis of so-called compressed-mode W-CDMA signals. During an interfrequency handoff, time is required to make measurements on the two different W-CDMA frequencies. The time is provided by compressing transmission frames. Power is increased during these frames to maintain signal quality. Because of the variety of ways in which frames can be shortened, making sense of operation during the compressed mode can be difficult without the WCA200A.

A more general-purpose capability of this FFT-based analyzer is triggering in the frequency domain. There’s also an optional 256-MB memory that supports capture of up to 10 s of  W-CDMA signals. Maximum modulation bandwidth is 15 MHz.

References

  1. Generating 3GPP Multi Carrier Signals for Amplifier Tests with R&S SMIQ03HD and WinIQSIM™, Rohde & Schwarz, June 2002.
  2. Creating IQ Data Files for the 2029 Vector Modulator, IFR, 2000.

FOR MORE INFORMATION:

Glossary

3GPP 
ACLR 
ADC 
Arb 
CDMA 
DAC 
DSO 
DSSS 
EDGE 
FFT 
GMSK 
GPS 
GSM 
ISM 
MSK 
OSI  
QAM 
TDMA 
TD-SCDMA 

W-CDMA 
WLAN 3rd generation partnership project
adjacent channel leakage ratio
analog to digital converter
arbitrary waveform generator
code division multiple access
digital-to-analog converter
digital storage oscilloscope
direct sequence spread spectrum
enhanced data rates for global evolution
fast Fourier transform
Gaussian minimum shift keying
global positioning system
global system for mobile communications
industrial, scientific, and medical
minimum shift keying
open systems interconnection
quadrature amplitude modulation
time division multiple access
time-division synchronous code division multiple access
wideband CDMA
wireless local area network

Return to EE Home Page

Published by EE-Evaluation Engineering
All contents © 2003 Nelson Publishing Inc.
No reprint, distribution, or reuse in any medium is permitted
without the express written consent of the publisher.

February 2003


Sponsored Recommendations

Comments

To join the conversation, and become an exclusive member of Electronic Design, create an account today!