Analysis of WLAN digital modulations by a RF Vector Spectrum Analyzer

Author(s):
M. Bertocco - UNIVERSITY OF PADOVA - DEPARTMENT OF INFORMATION ENGINEERING
C. Narduzzi - UNIVERSITY OF PADOVA - DEPARTMENT OF INFORMATION ENGINEERING
D. Fortin - UNIVERSITY OF PADOVA - DEPARTMENT OF INFORMATION ENGINEERING
A. Sona - UNIVERSITY OF PADOVA - DEPARTMENT OF INFORMATION ENGINEERING

Industry:
University/Education

Products:
RF Measurements, LabVIEW, PXI/CompactPCI, Spectral Measurements Toolkit

The Challenge:
The realization of a test bench for the analysis of digital signals for wireless communications (WLAN) at physical layer, at both the frequency domain and the I/Q domain.

The Solution:
The use of the 5660 RF Vector Signal Analyzer allowed us to acquire the wideband digital signal from a commercial WLAN Acces Point, to demodulate it and to analyse it.

The purpose of this work is the assessment of the performance of short range wireless communication systems in presence of controlled in-channel interference. In particular WLAN IEEE 802.11b networks are investigated by the analysis of the digital signal at physical layer. To this aim a suitable test bench for the measurement of physical layer parameters like channel power, and the estimate of modulation quality indices like EVM (Error Vector Magnitude) and MER (Modulation Error Ratio), is required.
Generally for power measurements of wideband RF signals a superetherodyne spectrum analyzer together with a software implementing the channel power method can be used. Furthermore “modulation domain” analysis requires the signal to be sampled and demodulated. The most important problems encountered in this kind of testing are due to the characteristics of the digital signal defined by the IEEE 802.11b standard, like high frequency carrier (2.400 - 2.485 GHz ISM band), large bandwidth (20 MHz), burst packet transmission. In particular, burst transmission represents a difficulty for measurements at the physical layer. In fact when the estimate of a parameter is realized by averaging over a number of acquisitions measurement accuracy can be strongly affected, since untriggered, continuous acquisition would mean that only a few burst is acquired, while for the majority just background noise is present.
The NI 5660 RF Vector Signal Analyzer allows to overcome some of these difficulties thanks to the frequency range from 9 kHz to 2.7 GHz and the 20 MHz real-time bandwidth provided by the downconverter. Furthermore the 14-bit 64 MSample/s digitizer allows to perform both frequency domain and I/Q domain measurements. The external trigger input of the digitizer module allowed us to solve the burst transmission problem by a suitably designed trigger circuit.

Measurement test bench
The transmitter utilized for this work is a D-Link 524 Access Point (AP), controlled by a Toshiba notebook. The D-Link software tools are used to define the AP set-up parameters, like the transmission standard (802.11b, 802.11g), the data rate, the channel frequency, etc. For our analysis the Access Point is configured with 1 Mb data rate trasmission. The RF signal from the transmitter’s output connector is sent through a directional coupler to the NI 5660 RF Vector Signal Anlyzer, by the main line, and to the trigger circuit, by the coupled port. The trigger circuit built for our test bench is designed to detect the AP’s output signal burst at a frequency in the range 2.4 - 2.485 GHz and generate a TTL compatible signal, sending it to the digitizer’s external digital trigger input.
The measurement of the signal channel power is performed by the use of the RFSA Analyzer tool of the Spectral Measurement Toolkit. The RFSA front panel enables the selection of the external trigger source, and this allows to accomplish the acquisition just of the signal burst, thus leading to an accurate evaluation of the effective average signal power. A specific LabVIEW VI called WLAN_Measurement, whose front panel is reported in Fig. 1, has also been developed for the demodulation of the 802.11b signal, which is BPSK modulated for 1 - 2 Mbps data rates. This VI provides the visualization of the constellation graph for each acquired burst, which is a helpful first indication of the signal modulation quality and its evolution over time. An accurate modulation quality estimate is obtained through EVM and MER measurements, whose results are displayed by the WLAN_Measurement VI. Furthermore it provides a display of the eye-diagram, which is very useful for application with a multipath affected signal described hereinafter.
The first test has been realized for the evaluation of the effect of the signal attenuation on the estimate of the Modulation Error Ratio at the receiver input connector. In order to avoid the influence of interference the signal is sent through a cable directly to the RF Analyzer. The signal power is progressively reduced by two HP variable attenuators, 0 - 70 dB with 10 dB step, and 1 - 10 dB with 1 dB step. A second test, represented in Fig.2, has been defined to evaluate the performance of 802.11b in the presence of multipath. A multipath affected channel has been emulated in the following way: the Access Point output signal is split and sent via cables through two different paths, the main path and the interfering one. The first consists of a 1 m long cable and a 30 dB attenuator, the interfering path consists of a sequence of cables with two HP variable attenuators. The overall length of the second path varies from 10 m to 30 m, in order to cause different delays. The signal from the two paths are then recombined, sent to the RF Vector Signal Analyzer and analyzed by the WLAN_Measurement VI. EVM measurements upon the varying of the attenuation of the interfering signal for four different delays (63 ns, 74 ns 106 ns, 150 ns) have been thus performed. The experimental results obtained are reported in Fig. 3.

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