Mobile WiMAX PHY: Modulation and Frequency Tests

by Peter Cain, Agilent Technologies , TechOnline India - October 15, 2008

Wish you knew more about WiMAX testing needs? Check out Agilent's take on how to test for WiMAX.

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. .

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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Fig 25. The constellation of the CDMA ranging burst is indicated by the row of red dots in the center. The circles indicate the target EVM. The diameter is user-definable.


Fig 26. VSA modulation properties dialog box showing settings for pilot tracking and equalizer training.


Modulation coding (CTC, CC)
Convolution coding and convolutional turbo code are FEC codes mandated by the WiMAX Forum for use in mobile WiMAX to ensure robust data transmission. Convolutional turbo coding (CTC) improves data throughput under suitable channel conditions. The downlink interval usage code (DIUC) is the indicator of a burst modulation profile, which can be set in the VSA software under the 802.16 Demodulation Properties tab. (Selecting the incorrect profile will prevent the recovery of the UL MAP information from the DL signal.)

Special functions. Testing initial and periodic ranging, HARQ
The standard requires the MS to be able to adjust its transmission according to ranging information sent by the BS. The ranging information includes power adjustment, frequency adjustment, and timing adjustment. This initial ranging procedure enables the MS to obtain frequency synchronization within a specified tolerance with the BS. The MS will not attempt uplink transmission until it obtains synchronization. During normal operation, the MS keeps track of frequency changes in the BS through periodic ranging and it will defer transmission when synchronization is lost. The E6651A provides the mechanisms for testing ranging operation, with logging of associated messages such as RNG-REQ and RNG-RSP.

Hybrid automatic repeat request (HARQ) functionality, in which the receiver asks the transmitter for retransmission of the lost or incorrectly received packet of the mobile, can also be tested with the E6651A. Figure 28 shows an example of how the E6651A test set provides this emulation.


Frequency and timing tests: Power versus time (PvT)
This measurement requires the instrument to have an instantaneous (not swept) bandwidth at least as wide as the signal, and examines the time domain burst shape of the signal and rise and fall edges of the power envelope.

Although the 802.16e-2005 standard does not define any conformance test for the power versus time mask, a pass/fail limit mask test is commonly used to make sure DSP and RF switching occur at the right time. With wideband signals, the impact of amplitude switching transients on the spectrum may be less noticeable than on narrow band signals such as GSM. This is because the signal bandwidths are comparable to the inverse of the rise and fall times. However, it is still important to make sure PvT is within reasonable limits to avoid unexpected interoperability problems with receivers from different suppliers.

The RTG/TTG measurement indirectly tests the signal, but the PvT measurement helps identify other issues. As the arrows in Figure 29 indicate, the PvT measurement shows the multiple stages during the start of a burst and a short period after turn-on, when there is no data transmission. In this example, there are two steps in the turn on process, with a clear amplitude overshoot at the start of the unmodulated second step. When superimposed on the first transmitted symbol, it will give an amplitude tracking problem to the BS receiver.


{pagebreak}The MXA has useful features in the PvT measurement. The burst detection parameters will help automatically search for the bursted signal. If the signal needs to be verified against an external timing source, the burst auto detection function can be disabled in the MXA. In addition, the burst detection algorithm searches a burst from the beginning of the captured data and picks up the first one that satisfies the algorithm. For example, if there are two valid bursts in the captured data, the second burst will be disregarded. As for WiMAX OFDMA, the captured TDD signal could contain both downlink and uplink bursts. In this case, use an external trigger with an appropriate trigger delay so that the downlink or uplink burst comes to the beginning of the captured data selectively.

Spectrogram testing
A spectrogram shows the way spectrum is used throughout the WiMAX frame. WiMAX signals have distinct spectral patterns dependant on the zone type and burst allocations. Spectrograms allow pattern matching for rapid recognition or assessment of incorrectly con¿gured signals.


A spectrogram displays spectral energy (horizontal) versus time (vertical). Using a signal that has been captured (recorded) it shows spectrum use and the presence of any outside interference in the band with red being the highest power and blue being the lowest. The FFT Overlap ratio, set in percentages in , determines the total time for the vertical scale. Increasing the overlap "slows down" the speed the signal is replayed.


Frequency errors
As mentioned earlier, the MS needs to adjust its transmission as per the ranging code from the BS. The transmit center frequency should be within ±2 percent of the subcarrier spacing compared to the BS center frequency. With a subcarrier spacing of the order of 10 kHz, this means transmit frequency errors of less than 200 Hz. Figure 32 shows an occupied bandwidth measurement on the MXA for a 10 MHz signal that includes a transmit frequency error reading. The measurement in this mode is not demodulating the signal. It is intended as a simple way to indicate if there is any mistuning, or if the signal bandwidth is not what was expected. The measurement requires the burst to occupy all sub-channels over a 12-symbol period or longer. The sweep time should be made long enough to reduce the result variation to an acceptable level.


Fig 32. Frequency error reported along with occupied bandwidth measurement on MXA.

A more precise frequency error measurement is available using the OFDMA demodulator. It is available in both the MXA OFDMA and 89600 VSA applications. As shown in Figure 33, in the latter, it is reported along with the modulation measurements.


Fig 33. Frequency error reported along with modulation measurements.

Transmitter reference timing
The MS and serving BS need to be synchronized in time and frequency so that the signals from all the MSs in the network arrive at the same time at the BS antenna port to avoid intersymbol interference. The transmitter needs to adjust its timing as well, as commanded by the BS, which provides target timing uncertainty as stated in the MRCT document. To verify this synchronization performance, a timing test can be performed using the Agilent E6651A test set.

MS receive to transmit transition gap (MSRTG) and MS transmit to receive transition gap (MSTTG)
In TDD operation, the downlink and uplink transmissions share the same frequency, but are separated by a gap that is an integral multiple of physical slots. These gaps allow both the BS and MS to switch from receive/transmit mode to transmit/receive mode. The transitions are important to check for problem areas that affect normal operation such as amplitude and phase transients in RF control circuits and RF/BB timing alignment. The MS certi¿cation test is designed to ensure the last symbol of the DL is received by the MS and the ¿rst symbol and last transmitted signals from the MS have an acceptable modulation error. Symbol-by-symbol results are measured in both MXA and 89600 VSA.

The MSRTG and MSTTG measurements test both the receiver and transmitter in the MS. The test is carried out using a BS emulator to instruct the MS to send and receive data at fixed positions in the frame, over a duration of 100 bursts. RCE measurements are made on the first and last symbols of the UL transmission. It is recommended that a normal data burst is used to test the first symbol using eight or more sub-channels to reduce measurement uncertainty. A PER measurement is made using the MS's receiver to ensure it successfully recovers both the earliest and latest symbol data in the DL signal.

Click here to download the entire app note.

References
[1] Institute of Electrical and Electronics Engineers, Inc., IEEE Std 802.16-2004; IEEE Standard for Local and metropolitan area networks; Part 16: Air Interface for Fixed Broadband Wireless Access, Oct. 1, 2004. [2] Institute of Electrical and Electronics Engineers, Inc., IEEE Std 802.16e-2005; IEEE Standard for Local and metropolitan area networks; Part 16: Air Interface for Fixed Broadband Wireless Access Systems, Amendment 2: Physical and Medium Access Control Layers for Combined Fixed and Mobile Operation in Licensed Bands and Corrigendum 1, Feb. 28, 2006. [3] Agilent Technologies, WiMAX Concepts and RF Measurements, IEEE 802-16-2004 WiMAX PHY layer operation and measurements, Application Note, literature number 5989-2027EN, Jan. 5, 2005. [4] Pandharipande, A.; "Principles of OFDM", IEEE Potentials, Volume 21, Issue 2, Apr-May 2002. pg 16 " 19. [5] Eklund, Carl, et.al. WirelessMAN, Inside the IEEE802.16 Standard for Wireless Metropolitan Networks, IEEE Press, 2006.

About the Author
Peter Cain was awarded a 1st Class degree in Electronic Engineering from Southampton University in the United Kingdom and has worked in the Test & Measurement industry for 20 years, 16 of those being at Agilent/Hewlett-Packard. Peter has worked in engineering, project & program management and more recently technical solution planning. A lapsed scuba diver, Pete continues to like "building" things, and has a young son to provide the excuses.

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