SignalMeister™ v3.0 is Keithley Instruments’ software complement to the company’s line of vector signal analyzers (VSAs) and vector signal generators (VSGs). It’s a comprehensive application that addresses generation and analysis of single-input single-output (SISO) and multiple-input multiple-output (MIMO) communications signals within a user-friendly drag-and-drop programming environment. In addition to interfacing with real test instruments, simulated signals may be generated and analyzed with the results displayed in a number of formats.
V3.0 includes signal analysis which v2.0 did not. In addition, the types of signal-generation formats have been extended, more signal operations are supported, and MIMO capabilities for both generation and analysis have been enhanced to 8×8.
SignalMeister v3.0 is available either as the Model 290101 SignalMeister RF Communications Test Toolkit CD or via a download from the company website. Either method is free. In addition, you need a key to access the licensed analysis features. This is a physical USB dongle, and your local sales representative can provide one at no charge for a 30-day evaluation. The application must recognize the key before allowing use of the analysis functions.
Top-Level Functionality
The toolbox comprises signal-generation and analysis capabilities segmented by technology, generic operations, interfaces to physical VSGs and VSAs, templates, and file-handling utilities. Technologies include WiMAX, WLAN, 3GPP (WCDMA) and 3GPP2 (cdma2000), and digital video. TD-SCDMA signals are handled by v1.2.1, also on the 290101 disk or separately downloadable.
Keithley’s Model KI 2910 and KI 2920 VSGs are supported in all their capabilities from basic single-instrument SISO through eight-instrument MIMO configurations. Similarly, the company’s Model KI 2810 and KI 2820 VSAs with up to eight-instrument MIMO systems also are included. Signal generation is handled via specific icons selected within a given technology. A 1x VSA simulator is a generic icon within the signal analyzer toolbox section and used in combination with a technology-specific or general-purpose analysis icon.
A simple group of three icons is all that’s needed to become familiar with the software’s capabilities without involving actual instruments. The first icon on the left in Figure 1 simulates the signal, the second icon represents the VSA, and the third interprets and displays the analysis results relative to the specific standard.
A general-purpose simulated analysis icon can be used in place of the third icon but only FFT, waveform amplitude, waveform I/Q, and spectrum emissions mask graphs are available, and the spectrum emissions mask isn’t plotted unless a real VSA is used. With a technology-specific analysis icon, the list of available graphs also includes constellation, error vector magnitude (EVM), ramp on/off transients, and packet power and frequency variations.
In addition to basic graphs, you can display the I/Q time-domain waveform and separate I and Q eye diagrams. Strip-chart selections vary by technology but generally record statistics per sweep. For example, WiMAX parameters include carrier leakage and channel power on a per-channel basis and a number of relative constellation error (RCE) plots for either data or pilot subcarriers specified according to power, symbol number, or subcarrier number.
A Simple SISO Example
Creating an example project was fast. First, I selected the WLAN section of the toolbox, then dragged and dropped the first and third icons. Once I had positioned them, I found the simulated VSA in the toolbox analyzer section and placed it between the others.
The easiest way to connect the relative ports is to first click the connect elements item on the toolbar and then click the icons to be connected. An arrow is drawn between them. Clicking on an arrow changes the color at the ends to red indicating that the icons have been connected.
I had some trouble with the connection arrows because I assumed that you dragged the mouse between the ports to create a connection. Once I found how SignalMeister connections worked, I had no problems. On the other hand, the arrows are somewhat independent to the extent that, when an icon is removed, the arrow(s) remain attached to the surrounding icons. This means that you need to take some care to ensure correct connections when a new icon is substituted for the one removed.
If a port is not connected, the tip of the arrow is green. If you allow sufficient space around a new icon, the remaining arrows can be repositioned after selection with the cursor, turning the ports red when the new connection is made. Alternatively, delete the old arrows when you remove an icon and start over using the connect elements tool.
The next step is to run the conflict analyzer utility. Although you may have correctly connected compatible icons, modern communications signals are so complex that it’s very easy to configure an impossible combination of lower-level parameters. You shouldn’t have any problems when working with default values, but if you incorrectly alter them, error messages provide good clues to what is wrong.
Double-clicking on an icon exposes the fields you can change and also may present a few grayed-out values that you can’t. Initially, all settings are default values specified by the relevant communications standard.
Figure 2 shows the selections available for the 802.11b signal-generation icon. Only four data rates are defined by the specification so these are in a list rather than allowing user data entry. Similarly, the preamble can only be long or short, and only a predetermined selection of filter shapes is supported. On the other hand, you have complete freedom to define the cutoff frequency and related filter width.
I found that it was very easy to specify a filter that was too narrow for the 11-Mb/s chip rate. When that happened, an error message “Start of Packet not Detected” was displayed, and no analysis graphs could be generated. Unlike 802.11a,g in which the signal bandwidth determines the filter bandwidth, 802.11b changes modulation methods with the data rate but maintains a constant 11-Mb/s chip rate.
Once the conflict analyzer confirms that all’s well, you need to build the configuration. For a simple system, this takes a few seconds. A complicated MIMO project may need two or three minutes. Now, you have a working simulation that will generate and analyze communications signals. Each time you change something, such as the MAC details accessible from the Configure PSDU (physical layer service data unit) button in Figure 2 or the amount of additive white Gaussian noise (AWGN) added to the signal, you must rebuild the system.
The number of channels available on the VSA simulation is selectable for testing MIMO configurations, and you can add AWGN into the simulator for presentation of more realistic data. Actual performance metrics and graphical output are obtained from the technology-specific analysis icon.
Figure 3 shows the constellation diagram available as an analysis graph for the 802.11b 11-Mb/s data rate. The black dots indicate the four nominal quadrature phase shift keying (QPSK) symbol positions resulting from progressive 90-degree phase shifts. The dark blue and red triangles represent the header and preamble symbols, respectively. The aqua triangles are the data symbols.
Because the preamble and header always are sent at a 1-Mb/s rate using binary phase shift keying (BPSK), the dark blue and red triangles occupy only two areas opposite each other. At 11 Mb/s and 5.5 Mb/s, the constellation has a QPSK characteristic similar to that for the 2-Mb/s rate with basic QPSK modulation. Although the plots look the same, a lot more is going on at the higher rates.
At 11 Mb/s, six of the eight bits that compose a symbol are used to select one of 64 eight-bit complex Walsh/Hadamard complementary codes—the two Cs in complementary code keying (CCK). Each bit in the selected code can have one of four values, +1, -1, +j, or -j, and indicates a change in position rather than absolute position. This is the meaning of differential QPSK (DQPSK), where a constellation position represents the change between the present value and the previous value. The other two bits of the original eight data bits determine a further QPSK rotation to the CCK modulation.
This approach maintains a constant 11-Mb/s chip rate for all four data rates. On the other hand, modulation for the two higher speeds is developed without involving the Baker spreading codes used for the lower rates. Modulation/demodulation of 11 Mb/s and 5.5 Mb/s is done with lookup tables in which the CCK sequence references the original data word. With Baker codes, the original data bits are two’s complement added to the codes both for spreading and despreading.
Transmit filtering restricts signal frequencies to the intended channel, and the panel shown in Figure 2 allows selection of Gaussian (default), Blackman, Bartlett, Hamming, spectrally raised cosine (SRC), spectrally root raised cosine (SRRC), and Hann shapes. You can set cutoff frequency and filter width, the width being the propagation time through the filter and proportional to the number of taps. A longer time corresponds to sharper filtering and less adjacent channel interference.
For Figure 3, the filter parameters were left at the default Gaussian, 7.8-MHz, 400-ns settings, but -20 dBc of AWGN was added to the signal in the simulated VSA icon. This makes the positions of the symbols easier to explain than in a constellation with perfectly aligned ideal and actual markers. Although real signals suffer both phase and amplitude impairments that result in this type of diagram, the level of noise is much more than ordinarily would be encountered. As an example, an EVM value of -20 dB is equivalent to an EVM of 0.1 or 10%. A good 1% EVM corresponds to -40 dB.
In Figure 4, EVM is shown as a function of time with color-coding the same as in Figure 3. So, the preamble is transmitted first for about 140 µs, the header for a further 50 µs, and then the data.
A Four-Channel 3GPP WCDMA Example
All signals are created at baseband around zero frequency. When a real signal generator is used, this isn’t a problem because the signal is generated around the carrier center frequency. For example, the 802.11b spectrum extends approximately 11 MHz either side of the carrier for a total 22-MHz bandwidth. However, to develop a signal with frequency-shifted components without an actual generator, the baseband waveform first has to be resampled.
In the example shown in Figure 5, the original signal was created with 4x oversampling so it has a 20-MHz bandwidth rather than WCDMA’s nominal 5 MHz. To satisfy the Nyquist criteria, resampling is done at 50 MHz. WCDMA uses 5-MHz channels and CDMA based on direct sequence spread spectrum (DSSS) with a 3.84-Mc/s chip rate.
The 4×4 channel block can be used either as a combiner or splitter and configured to provide equal power in either case. The effect of the splitter is to generate four identical signals that then are frequency shifted. The second 4×4 channel block combines the four signals. Finally, -45dBc AWGN is added in the 1x VSA simulator, and the general-purpose analysis block provides graphical output. Figure 6 shows the FFT of the final four-channel signal.
Summary
If you already have a Keithley VSA or VSG, SignalMeister is an obvious addition that will greatly enhance the usefulness of these instruments. Even without actual test instruments, the software provides an easy way to investigate various wireless communications protocols.
Alternatively, you could model a particular protocol in a general-purpose simulation tool. This would take a reasonable amount of time after you became very familiar with the specifications for a certain technology. Then, should you wish to compare one technology with another or at a different data rate, you could repeat the exercise several times.
Or, you could just use SignalMeister. Many of the capabilities have been demonstrated in the examples, but general-purpose operations such as I/Q gain imbalance, invert Q, I/Q offset, time delay, and a separate AWGN source also are available.
In addition to the toolbox, the program includes a comprehensive help section that provides detailed background information for each of the technologies and related topics. As should be evident from the details of the examples, wireless protocols are complicated. And, they are constantly changing. A tool like SignalMeister results from the combined experience of many industry experts and is kept up to date with the latest revisions. Why not take advantage of it?
April 2009