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Editor's Note: This is a multi-part excerpt from Agilent Technologies' Application Note: Mobile WiMAX PHY Layer (RF) Operation and Measurement.
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.
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If abnormalities are seen in the transmit spectrum, it makes sense to check if the modulator is working as reflected by the error vector magnitude (EVM) measurements. To verify the modulation performance of the mobile, use a basestation (BS) emulator and establish connection with the mobile station (MS), or command the MS to transmit a specific burst. A vector signal analyzer (VSA) is needed to perform modulation measurements.
Configuring the demodulator for uplink (UL) measurement
In normal operation, the BS sends the preamble, FCH, DL-MAP, and UL-MAP to instruct the MS how to receive and transmit bursts. The demodulation measurement also needs to be told, or find out, what the UL transmission looks like before it can make the demodulation measurements. Burst information can come from several sources, including a Signal Studio .scp setup file, the MAP file obtained from the downlink (DL) signal, or using a manually-entered configuration. The Agilent VSA is able to auto detect the UL signal if it is a single burst. When measuring noisy signals, the auto-detection algorithm may not be able to determine the correct data burst modulation type. In such situations, configuring the analyzer to use manually-specified data tone modulation formats, which can be done using the burst profiles in the VSA's demodulation properties, extends the measurement dynamic range.
Burst allocation effects
A new feature associated with the flexibility of OFDMA is that frequently not all the subcarriers are used during the UL transmission from an individual MS. This has a novel impact on the way distortion affects in-band performance. In-band distortion products cause the expected EVM degradation on subcarriers that form part of the wanted signal. Elsewhere, instead of there being no signal power, the distortion acts as interference to other MSs expecting to occupy the first MS's unused subcarrier.
To understand distortion of an OFDMA signal, a comparison can be drawn with a multi-tone signal of uncorrelated tones. If there are no vacant tones, distortion creates inter-modulation products, which are most noticeable as spectral re-growth-- affecting the SEM measurement. If some tones are not used, intermodulation distortion will also put spectral energy at the frequency of the unused tones. With the OFDMA signal, variations in the payload data make this interference look like noise.
At a constant transmit power, the level of distortion associated with used and unused subcarriers varies markedly, as shown in Figure 23. The EVM of used subcarriers increases with the sub-channel allocation because progressively more of the distortion components lie on wanted subcarriers. For the unused subcarriers, the reverse is true. When nearly all subcarriers are allocated to the wanted signal, only a small proportion of distortion spectral components fall in unused subcarriers, so the total power drops. When only a few sub-channels are used for the wanted signal, most of the distortion now occupies the unused subcarrier spectrum, resulting in an increase in unused subcarrier EVM (RCE).
Figure 23 shows the variation in results for the same transmit power and demonstrates how important it is to know the design of the burst (sub-channels allocation) to achieve repeatable measurements. This explains why the WiMAX Forum is specific about the burst type for performing most tests.
Modulation accuracy RCE (EVM)
The modulation accuracy is the deviation of error vector from a recovered target value; hence it is a relative measurement. As noted above, the measurement is carried out on both modulated and un-modulated carriers to ensure that the MS does not degrade the link for itself or other users. The performance limit is the same for a given output power and modulation. The Mobile radio conformance tests (MRCT) document specifies the value for relative constellation error (RCE) for different burst profiles. The measurement is done with tracking enabled for amplitude, frequency, and timing errors. The equalizer is set to operate on both pilots and data, and remove amplitude, phase, and timing errors, thereby matching the expected capability of a BS receiver.
Fig 24. Constellation of an UL signal. The red dot in the center refers to the RCE on the un-modulated carriers. The BPSK pilots are shown in black and the QAM data bursts in green and red.
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