With increasing pressure to lower test costs, many RF test engineers face the challenge of reducing measurement time. As you might expect, wireless LAN (WLAN) device testing is no exception. Whether you are creating an automated test system for design validation or final production test, optimizing a test system for measurement speed has become increasingly important. Thus, consider the following tips to reduce the measurement time of WLAN devices:
- Measure error vector magnitude (EVM) over only part of a burst
- Use composite measurements
- Measure only the necessary span
- Use the fastest CPU available
In this discussion, all WLAN measurements are configured using the NI WLAN Measurement Suite, which contains both generation and analysis toolkits for NI LabVIEW and LabWindows™/CVI software. For more information on how to configure PXI WLAN test systems, read Configuring Software-Defined WLAN Test Systems.
1. Tip 1: EVM Over Full Versus Partial Burst
To perform faster EVM measurements, calculate the measurement over a partial burst rather than over an entire burst. By default, the NI WLAN Analysis Toolkit performs 802.11a/g (OFDM) EVM measurement as the root-mean-square (RMS) of all subcarriers over each symbol in the entire burst. Similarly, the toolkit reports 802.11b (DSSS) EVM measurements as the RMS of all chips in the burst. However, in some cases, you can reduce measurement time by specifying that an EVM measurement be performed over only a portion of the burst. Observe this trade-off in Figure 1, which illustrates the relationship between the numbers of symbols used to compute the measurement and the measurement time for an 802.11a/g BPSK (6 Mbit/s) burst.
Figure 1. This shows the relationship between standard deviation and symbols measured for BPSK burst.
As you can see in Figure 1, you can significantly reduce measurement time simply by analyzing a portion of the burst instead of every symbol. However, analyzing only a portion of the burst produces a slight trade-off in measurement repeatability. In this case, the standard deviation (in dB over 100 measurements) increases slightly.
2. Tip 2: Use Composite I/Q Measurements
To reduce WLAN measurement time, perform composite measurements instead of configuring each measurement separately. With the NI WLAN Analysis Toolkit, you can obtain all time domain measurements (power versus time, EVM, and frequency offset) with a single composite measurement. Because composite measurements calculate multiple measurement results on single burst, it is more efficient than performing each measurement sequentially. For 802.11b signals, you can use composite measurements to measure EVM, power, power ramp-up time, and power ramp-down time. In Table 1, you can compare the time required to measure each of these individually instead of in parallel.
Table 1. Compare the measurement time of composite versus single measurements for 802.11b.
From Table 1, you can calculate that for an 11 CCK burst, an EVM, TXP, and a ramp-up/ramp-down measurement would take 126 ms when performed sequentially but only 64 ms when performed in parallel.
3. Tip 3: Reduce the Span of Spectral Mask Measurements
To shorten WLAN signal measurement time, reduce the span of the spectral mask measurement. While the IEEE 802.11 standard defines a 60 MHz mask for 802.11a/g signals and a 66 MHz mask for 802.11b signals, you can occasionally use a narrower span in production test to save on measurement time. For example, an engineer performing production test might use only a 44 MHz span for an 802.11b signal to save on measurement time. Because the NI PXIe-5663 6.6 GHz RF vector signal analyzer (VSA) offers an instantaneous bandwidth of 50 MHz, you can perform spectrum measurements that are less than 50 MHz in span without retuning the front end of the instrument. Figure 2 illustrates this concept.
Figure 2. This chart shows a WLAN 802.11a/g mask versus span for an NI PXIe-8106 controller (NI-RFSA 2.2 or later).
As Figure 2 illustrates, measurement spans between 50 MHz and 100 MHz require the NI PXIe-5663 VSA to retune once at the cost of 6 ms longer measurement times. Thus, you can significantly reduce measurement time when configuring a span such that the RF analyzer front end is not required to retune.
4. Tip 4: Use the Fastest CPU Available
A final method to reduce the measurement time of WLAN signals is to use the fastest CPU available. The CPU is one of the most important components of a software-defined PXI measurement system. Especially for RF measurements, CPU performance is often the single most significant factor gating faster measurement performance. Fortunately, you can use modern multicore CPUs with the highly parallel measurement algorithms of the WLAN Analysis Toolkit to deliver extremely fast measurement results.
While actual system performance can depend on a variety of factors such as memory availability and other applications running in the background, a strong correlation exists between the CPU performance and measurement time for automated test systems.
Table 2. Key specifications of various PXI Express controllers can affect overall measurement speed.
Several CPU characteristics can affect overall measurement speed. Some of the most significant of these include number of processing cores, CPU clock speed, front side bus, L2 cache size, and system memory. Observe these effects by benchmarking 802.11a/g measurement times in Figure 3. As this figure illustrates, high-performance multicore controllers such as the NI PXIe-8106 perform EVM measurements faster across all data rates and modulation schemes.
Figure 3. Measurement time improves with faster CPUs.
Note that even though the NI PXIe-8130 has a faster clock rate than the NI PXIe-8106, the results suggest that other factors such as an L2 cache size also influence measurement speed. Despite this, you can clearly determine that CPU has a significant effect on measurement time. Thus, using the fastest CPU available is a simple way to reduce measurement time without compromising measurement quality.
A variety of factors can affect the overall measurement time of wireless LAN signals. While you can adjust many measurement settings to improve measurement time, these settings can occasionally require you to make repeatability, accuracy, or completeness trade-offs. Thus, the easiest way to improve test throughput without sacrificing measurement quality is to always use the fastest CPU available.
For more information on reducing measurement time, see Optimizing WLAN Test Systems for Measurement Speed.
David Hall is an RF product manager for National Instruments. He holds a bachelor's degree in computer engineering from Penn State University.
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