Mobile WiMAX PHY: RF Test Suite and Transmitter Measurements

Part 1 of this article introduces the WiMAX standard and provides in-depth explanation of the IEEE 802.16 profiles.
Part 2 profiles control mechanisms and the mobile station.
Part 3 covers the RF Test Suite and transmitter measurement set up.
Part 4 covers transmitter power, including spectral flatness, spurious emissions, and adjacent channel power. .
Part 5 covers modulation and frequency tests.

This Part covers the RF Test Suite and Transmitter Measurement set up.

PHY Layer (RF) Test Suite
It is useful to have an understanding of concepts such as FFT, OFDMA, uplink, downlink, zones, slot, and symbol as we prepare to discuss measurements on the PHY layer.

There are two main sources of measurement definitions on mobile WiMAX: those from the IEEE standard (802.16e), and others created by the WiMAX Forum. IEEE 802.16-2004 and 2005 merged Sections 8.4.10/11/12/13 detail the measurements that concern the performance of the PHY layer and the conditions under which they are performed. The mobile Radio Conformance Test (MRCT) document from the WiMAX Forum specifies the test requirements for radio conformance.

Both the signals from the BS, which affect MS receiver results, and the mobile transmitter can vary symbol by symbol. Defining the test signal and the measurement gating interval being used (intentionally or not) is vital to achieving traceable and repeatable measurements.

Note: To understand transmitter performance in a BS, refer to Agilent's IEEE 802.16e WiMAX OFDMA Signal Measurements and Troubleshooting, application note 1578, literature number 5989-2382EN.

Transmitter Measurements
The transmitter path in the MS consists of baseband processing, IQ modulation, filtering, frequency up conversion to RF, and power amplification. To confirm all these stages are function properly, it is recommended to start with the basics of getting frequency, power, and timing correct, before moving on to modulation measurements frequency (bandwidth). This approach means important signal problems will be identified fast.

The danger of missing fundamental problems is particularly acute in applications such as IEEE 802.16's OFDMA due to the signal's complexity and the possibility that demodulation will fail due to configuration discrepancies such as preamble ID, PRBS setting, or incorrect definition of data bursts. This is why it is important to isolate problems as early as possible in the measurement and troubleshooting process and eliminate the maximum number of simple signal problems before attempting digital demodulation. This is especially true when working on baseband digital signal processor (DSP) operations for creating such things as signals with correct modulation and pilot configuration. Experience has shown there are numerous possibilities for demodulation measurement problems in the initial turn on.

Given the burst nature of the transmitted signal and the different signal formats, this section discusses the conditions under which a test is performed, the measurements set up, aspects of device control, and specifics of the tests.

Test conditions and measurement setup
Measurements on the transmitted signal from the MS require set up information for the burst, including bandwidth, frequency, UL/DL ratio or sub-frame duration, FFT size, zone type, and MAP (sub-channel and slot allocation). This information allows the test equipment to perform the appropriate frequency, power, time, and modulation measurements. As discussed later, using a vector signal analyzer with an auto zone detect feature, such as the Agilent 89600S Series VSA, simplifies demodulation measurements because the VSA detects the bursts without having to know the MAP. Basic frequency and time measurements do not require burst information, as these measurements are done prior to doing demodulation. Time gated measurements are often essential, and generally offer greater measurement clarity.

Differences between a DL and UL
The UL signal has no preamble, but it has more sub-channels than the DL (35 instead of 30 for a 10 MHz PUSC), different zone specięcations, and data bursts that are "wrapped" back to the initial symbol they are using if more slots are required. Currently, there is only one transmitter in the MS, whereas the BS can have two or more transmitters. Live UL signals will also contain ranging bursts, sounding bursts, and channel quality indicator (CQI) bursts.

Figures 7 to 9 show a WiMAX TDD frame and the various parts of the frame for a 10 MHz signal created in the Agilent N7615B Signal Studio software for mobile WiMAX. Burst 1 in Figure 9 shows the burst wrapping in the UL signal.

Figure 10 shows measurements made on this signal using the Agilent VSA 89601A software for mobile WiMAX, highlighting differences in the DL and UL. The upper left trace shows the power versus time waveform of the burst. The upper right trace illustrates the time waveform showing the nine symbols in UL. The center left graph shows the frequency domain representation of the DL preamble, and the lower left shows a data burst in a PUSC zone. The center right trace shows the sparse use of subcarriers in a ranging burst, while the lower right graph shows the spectrum of a data burst symbol in the UL.

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