Real-Time Spectrum Analysis for Troubleshooting n/ac WLAN Devices

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1 Real-Time Spectrum Analysis for Troubleshooting n/ac WLAN Devices Application Brief WLAN devices operate in the license-exempt 2.4 GHz ISM and 5 GHz UNII bands, where they must share spectrum with many other wireless devices that can cause interference. Verifying the performance of WLAN devices in the presence of interfering signals that may be time-varying is an important part of design verification. Troubleshooting performance problems in real-world environments is another challenge, and being able to capture interfering signals that may be present and impacting performance, even if they are transient and very short in duration, can provide important insight. This application note will explain how real-time spectrum analysis can help address these test challenges. Overview The IEEE n standard, ratified in 2009, added advanced signal processing and modulation techniques and multipleinput multiple-output (MIMO) 40 MHz channels at the physical (PHY) layer. At the Media Access Control (MAC) layer, protocol extensions make more efficient use of available bandwidth. Together, these High Throughput (HT) enhancements can boost data rates up to 600 Mbps more than a ten-fold improvement over 54 Mbps of a/g. IEEE ac is the next-generation WLAN standard, extending on n and improving data rates, network robustness, reliability, and RF bandwidth utilization efficiency ac is a new standard for Very High Throughput (VHT) which can provide data rate over 1 Gbps and only operates in the 5 GHz bands. As an extension of n, ac uses similar strategies of higher order modulation types (up to 256 QAM), wider bandwidth (up to 160 MHz), and more spatial streams to achieve more than three times the performance n, with speeds up to 1.35 Gbps. The 3X speed improvement means that a single-antenna ac device can provide up to 450 Mbps throughput. From a network architecture or operation perspective, these two standards complement each n is still needed to support the 2.4 GHz band for 2.4 GHz b/g legacy devices, but ac can be inserted by replacing existing Access Points (APs) in 5 GHz band with dual-radio (802.11n and ac) APs, or installed as a network overlay.

2 Problem Various types of wireless LAN systems have become very popular and are required applications for mobile devices, tablets, and laptop and desktop computers. The n and b/g systems, however, share the 2.4 GHz frequency band with industrial, scientific, and medical (ISM) equipment, and hence, the electromagnetic noise emitted from ISM equipment may cause interference in wireless links. In the 2.4 GHz band, there are several sources of interfering signals, including microwave ovens, cordless phones, Bluetooth -enabled devices, and neighboring wireless LANs. A microwave oven operating within ten feet (less than 3 meters) of an access point may also cause WLAN performance to drop. In the real world, the oven must be operating for the interference to occur, which may not happen very often depending on the usage. Bluetooth-enabled devices, such as mobile phones and headsets, will cause performance degradations if operating in close proximity to stations. Even in the less crowded 5 GHz band that can be used by n/ac, cordless telephones and nearby WLAN devices can cause interference, and WLAN transmitters must also avoid channels that are used by government, military, or weather radar systems. Devices operating in the 5.25 to 5.35 GHz bands or 5.47 to GHz bands are required to meet requirements for Dynamic Frequency Selection (DFS) to avoid interference with radar signals. Figure 1 is an illustration of the frequency allocation for ac in the United States. Figure 1. IEEE ac frequency allocation in the United States Solution In the real world, the interference environment of a WLAN device is unpredictable and can have widely varying characteristics. A real-time spectrum analyzer allows engineers to capture and understand the interfering signals that may be impacting the performance of the WLAN system, even when they are intermittent or short in duration, making it a valuable tool for troubleshooting real-world performance problems. Interference testing with known signals is an important part of design verification, and a real-time spectrum analyzer can be used to verify the characteristics of complex test signals such as frequency-hopping Bluetooth, multi-channel WLAN, or radar signals, as well as to analyze the response of the device under test. Real-time spectrum analysis (RTSA) is an optional capability for Agilent PXA and MXA signal analyzers that can capture and analyze the most elusive signals. RTSA is designed to address the measurement challenges associated with dynamic RF signals such as the burst packet transmissions of WLAN. PXA and MXA with RTSA can: Make gap-free analysis within up to 160 MHz bandwidth; Provide 100 percent POI (Probability of Intercept) for signals with durations as short as 3.57 µs; Use FMT (Frequency Mask Trigger) to detect and capture intermittent RF signals. Combining the Agilent N9077A WLAN measurement application and RTSA capability with the PXA or MXA allows the signal analyzer to pinpoint interference caused by many signals in the ISM and UNII (2.4 GHz or 5 GHz) bands. Off-air antenna ISM band demonstration In order to identify the signals in the ISM band over-the-air, we can connect a type-n to-bnc adapter and paper clip to a PXA or a MXA. Because a ¼ wave at 2.5 GHz is approximately 3 cm, you can use a simple bent paper clip and type-n to-bnc adapter (DO NOT insert the paper clip in the analyzer s front panel connector! Plug the paper clip into the BNC adapter instead.). Set the PXA to measure the 2.45 GHz ISM band with preamp on. Refer to Figure 2, the 2.5 GHz ISM (industrial/ scientific/medical) band spans 100 MHz from 2.4 to 2.5 GHz. 2

3 We see two or more different signals in this band. Some wider signals are WLAN beacons or data traffic. Some narrower signals are likely from Bluetooth and/or cordless phones. Other signals (impulsive, broadband, frequency stepping or sweeping) may also be present briefly. It is very easy switching to the RTSA mode if it is installed in the PXA or MXA. With the same analyzer setting, the maximum 160 MHz of real-time bandwidth and overlap processing, the PXA or MXA can clearly show what is happening across the full ISM band and nearby frequencies. Figure 3 is a real-time spectrum analysis display of the 2.4 GHz band, which is a demanding analysis situation due to the wide bandwidth, short bursts (especially Bluetooth), and relatively long time interval between bursts of some signals. Figure 2. A general spectrum trace with Max Hold display for ISM band signals With real-time spectrum analysis, we can see some narrowband signals among the wider bandwidth WLAN beacons and data traffic, and some swept or stepped signals through the ISM band, some of which are very small signals. The PXA and MXA provide 75 or 72 db of spurious-free dynamic range respectively, across the full 160 MHz analysis bandwidth. Figure 3. A real time analysis of 2.4 GHz ISM band 3

4 WLAN n multi-burst signal with Bluetooth Without an actual antenna, we can simulate a multi-burst WLAN n signal with Bluetooth using Agilent N7620B Signal Studio for Pulse Building software with customized WLAN I/Q waveform and Agilent N7606B Signal Studio for Bluetooth software. In an actual WLAN network, there will be signals in different channels. Using a traditional spectrum analyzer, the WLAN spectrum cannot be fully displayed and recognized. The RTSA function in the PXA or MXA provides users a method to display the different WLAN signals in real time. The WLAN signals can be a combination of different frequencies, formats, bandwidths, and powers. All of the information can be accurately measured by a PXA or MXA with realtime spectrum analysis. Figure 4. Signal Studio for Pulse Building software user interface for multiple WLAN burst signals Figure 4 shows the user interface of the N7620B Signal Studio for Pulse Building software for generating signals with multiple WLAN bursts. Combined with N7607B Signal Studio for Bluetooth software with hopping setting, the information can then be downloaded to an Agilent MXG signal generator for demonstration purposes. This mixed signal can now be measured with the PXA in traditional spectrum analyzer mode. To change the trace display to Max Hold, change Detector to Peak. You can see the WLAN and Bluetooth combined spectrum trace in Figure 5, but not the details of the combined WLAN signals. Figure 5. A combined trace display of WLAN and Bluetooth signal in swept spectrum analyzer mode 4

5 To view other details of the WLAN signal such as power versus time and spectrum versus time, switch to real-time spectrum analyzer mode. We can now see the spectrum with power density display in Figure 6. It is easy to recognize the WLAN signals at each channel and the hopping Bluetooth signal at some of the 79 channels from GHz to 2.48 GHz. You can also change to the Power vs. Time and Spectrogram view (refer to Figure 7). Now all of information is updated in real-time, which can tell you the signal changes in spectrum domain, time domain and spectrogram view. In this example, the 80 MHz real-time bandwidth is equal to the selected frequency span, thus capturing in one acquisition multiple WLAN transmissions, so you can see more accurately the changes in power. But in a general-purpose spectrum analyzer, as it uses sweep mode, it cannot observe all signal changes such as power levels unless it is sweeping that specific signal at the time. A PXA or MXA with RTSA has no problem capturing such signal dynamics. Figure 6. WLAN multi-burst with Bluetooth signals showing on RTSA Figure 7. Power versus time, real-time spectrum and spectrogram 5

6 WLAN multi-burst signals with pulse interference As previously mentioned, there will be signals from different channels together with many types of interference signals, such weather radar operating in the same 5 GHz band as a WLAN signal. Using a traditional spectrum analyzer, the WLAN spectrum and pulse interference cannot be accurately displayed and recognized. Using RTSA, it is very easy to identify a very short duration radar signal. Figure 8 shows a combination of three WLAN signals with an interfering radar signal. A generalpurpose spectrum analyzer cannot capture this type of random signal, even when using Max Hold (as in figure 9). However, the RTSA with persistence spectrum display clearly illustrates how the signals change over time. The RTSA function in the PXA or MXA provides users a method to view the WLAN signal and intermittent radar signal in the same band. It is important to monitor the entire band of the WLAN signal while also monitoring the radar signal for the dynamic frequency selection test. Figure 8. A simulated radar signal interfering with WLAN signals in the 5 GHz band Figure 9. Swept spectrum analyzer mode for the signal shown on figure 8 6

7 Summary of Results IEEE n/ac WLAN technology currently requires the maximum bandwidth up to 80 or 160 MHz in the ISM 2.4 GHz and UNII 5 GHz bands, respectively, where it is share with other un-licensed equipment and devices. This application brief provided some examples of how to identify RF interference and WLAN signals. Supporting adequate bandwidth and sufficient dynamic range for WLAN signals, the signal analyzer is an important analysis tool for making standards-based measurements of WLAN signals. PXA and MXA signal analyzers also offer real-time spectrum analysis capabilities as a cost-effective solution to capture and discern interference versus actual WLAN signals. Related Products N9030A PXA and N9020A MXA signal analyzers with real-time spectrum analysis capabilities: Related Literature X-Series Measurement Applications, brochure, EN Real-Time Spectrum Analyzer (RTSA), X-Series Signal Analyzers N9030A/ N9020A-RT1 & -RT2, technical overview, EN WLAN a/b/g/n/ac X-Series Measurement Application N9077A & W9077A, technical overview, EN Signal Studio for Pulse Building N7620A, technical overview, EN 7

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