•  
• EDN Europe MAGAZINE
•  
• Free SUBSCRIPTION
• Subscription Renewal
• View Media Kit

• EDN
• EDN Australia
• EDN Asia
• EDN China
• EDN Japan

Spectrograms reveal mobile-WiMAX-zone structure

THE MOBILE WiMAX PHYSICAL-LAYER SIGNAL CONTAINS FAR MORE INFORMATION AND IS CONSIDERABLY MORE COMPLEX THAN TRADITIONAL SINGLE-CARRIER MODULATION SCHEMES, MAKING IT HARD FOR CONVENTIONAL SPECTRUM ANALYSIS TO REVEAL THE UNIQUE CHARACTERISTICS OF THIS DYNAMIC WAVEFORM: HOWEVER, THE SPECTROGRAM PROVIDES A MEANS OF INTERPRETING THE SIGNAL STRUCTURE.

The new mobile-WiMAX wireless-networking standard employs a complex multiple-access scheme to serve more users with increased flexibility by mapping data from multiple users to simultaneously transmitted OFDM (orthogonalfrequency- division-multiplexed) subchannels. Fortunately, one type of spectral display, the spectrogram, can present hundreds of contiguous or overlapping spectra at once, matching the large amount of information contained in a wideband OFDM signal, such as a WiMAX subframe.

While demodulation measurements are essential for WiMAX design and verification, spectral measurements such as spectrograms have an important function: They provide insight into the unique characteristics and structures that are present in this complex waveform, plus a variety of useful measurements that are independent of successful demodulation. Indeed, the use of spectrograms to verify signal characteristics and identify anomalies by examining an entire subframe at once can be very useful in setting up successful demodulation.

MOBILE-WiMAX SIGNAL

For most current implementations of WiMAX, base-station-to-subscriber communications occur using TDMA (time-division-multiple-access), where the downlink and uplink transmissions occupy separate slots in the time domain as RF bursts or sub-frames. Data is modulated onto OFDM subcarriers that are equally spaced in frequency—at approximately 11-kHz intervals—over the total channel bandwidth. Fixed WiMAX systems typically assign all of the OFDM subcarriers to a single subscriber at any one time. For multiplesubscriber access in the fixed system, the only channel-sharing mechanism is time-based, using a TDMA scheme. In the mobile-WiMAX system, multiple subscribers can share the frequency channel at the same time during an uplink or downlink sub-frame. This technique is called OFDMA (orthogonal frequency division multiple access), and the system assigns to each subscriber a subset of the available frequency subcarriers for a time period of one or more symbols, allowing the system to transmit several subscribers’ data simultaneously. This additional flexibility in subcarrier assignment significantly increases the complexity of the signal, as it dynamically maps data into both the frequency and time portions of the mobile-WiMAX sub-frame.

SUBCARRIERS, SUBCHANNELS

To see how the channel resources of time and frequency are dynamically allocated to multiple users, it is important to understand the relationship between subcarriers and subchannels in the mobile-WiMAX framework. Groups of physical OFDM subcarriers in the frequency domain form logical subchannels in combinations that the WiMAX specification defines. One could say the WiMAX system “thinks” in terms of logical subchannels but “acts” by transmitting physical subcarriers during the RF burst. The system assigns one or more logical subchannels to different users for some portion of the subframe (RF burst) based on their QoS (Quality of Service) needs. For example, a streaming-video application may require a larger number of subchannels (and/or a larger portion of the symbols in a subframe) than a standard websurfing application.

The group of logical subchannels and symbols assigned to a single subscriber unit is called a data burst; a data burst represents the amount of data associated with a subscriber during a subframe. The standard provides different subchannelto- subcarrier mappings to match system needs with wireless conditions. For example, the subcarriers for a logical subchannel may be adjacent to each other (the adjacent subcarrier permutation) or, more typically, the system will distribute them pseudo-randomly across the frequency channel (the distributed subcarrier permutation). Pseudo-random distributions provide the greatest amount of frequency diversity, while adjacent distribution of subcarriers is a useful technique when implementing adaptive antenna techniques such as beamforming.

PERMUTATION ZONES

The portion of the subframe or RF burst that uses a common subchannel- to-subcarrier mapping is called a permutation zone or simply a zone. Thus, changing zone definitions allow important radio characteristics to be changed one or more times during an RF burst. A commonly used zone is DL-PUSC (downlink partial usage of subchannels), which maps a single subchannel to 24 subcarriers over time periods in increments of two symbol times. The DL-PUSC zone is always the first zone of every DL subframe. The WiMAX specifications define seven different types of zones, and a single RF burst can contain multiple different zones.

Figure 1 shows the symbol time and logical subchannel assignments for a downlink zone with eight different data bursts. This graphic representation is often called a zone map or zone definition grid. In this figure, data burst #1 has an assignment of seven subchannels (168 subcarriers) over two symbols in time. Data burst #2 has an assignment of nine subchannels over eight symbols. Burst #3 uses the same subchannels as burst #1 but follows burst #1 in time. In this example, there are five additional data bursts in this zone of the subframe. There are also portions at the end of this zone that contain no user data; only pilot subcarriers are actually transmitted during these subchannels. Also visible at the beginning of the subframe in Figure 1 is the medium-accesscontrol (MAC) layer information that consists of the downlink-MAP (DLMAP) the uplink-MAP (UL-MAP) and the frame-control header (FCH). Additional information concerning the subframe components and definitions can be found in other Agilent application notes (references 1 and 2). The zone-definition grid shown in Figure 1 was taken from an actual demodulation measurement using an Agilent 89600 series VSA (vectorsignal analyzer) configured with autodetection of the OFDMA subframe structure.

SUBFRAME CHARACTERISTICS

Spectrograms are made up of a sequence of ordinary spectrum measurements, where each spectrum measurement is compressed to a height of 1 pixel row on the display and the amplitude values of the spectra are encoded as color. This produces a display of spectrum vs time, containing hundreds or even thousands of spectrum measurements, and an entire WiMAX UL/DL frame is visible. The spectrogram preserves all of the individual spectrum data, and, using a spectrogram marker, the user can select individual spectrum measurements from the spectrogram for more detailed analysis.

The spectrogram allows easy visual recognition of the major signal characteristics, including preamble and symbol—and often zone—transitions, changes in subchannel assignment and some indication of subcarrier allocation. These measurements all occur without the need for digital demodulation of the WiMAX waveform.

Figure 2 compares a spectrogram of a DL-PUSC zone with its corresponding zone map, that of Figure 1 (now rotated to align the time axes. As with the map, the Agilent 89600 series VSA produced the spectrogram, enabling a direct comparison of the logical and physical signal structures of this WiMAX signal.

The spectrogram displays many important features of the zone, including the preamble—where the transmitter sends only every third subcarrier—the OFDM symbol transitions, the noise-like regions of data transmission, the changes in occupied bandwidth, and even the two symbols at the end of the zone where no data bursts are defined and the system only transmits regularly-spaced pilots. It is also possible to see fine detail in the pilots-only symbols and observe the alternating location of the pilots from symbol to symbol.

Depending on the needs of the measurement, the user could modify this spectrogram for greater time or frequency resolution, or expand it to more easily resolve and measure individual subcarriers in the preamble or in the portions of the zone where some subcarriers are untransmitted.

Note in particular the spectrum changes in the second half of the zone, where data bursts do not require the use of all logical subchannels. Beginning with symbol 7, one subchannel is unused, resulting in 24—out of 720— untransmitted data carriers. Because of the pseudo-random distribution of the subcarriers in this subchannel, and because the system is still transmitting all pilot subcarriers, there is no visible effect on the spectrum or spectrogram. However, beginning with symbol 9, there are 12 logical subchannels with no data, for a duration of two symbols, resulting in 288 untransmitted data subcarriers and a distinct change in the spectrogram and spectrum of the signal.

REAL-TIME MEASUREMENTS AND SPECTROGRAMS

To show the WiMAX frame/subframe structure, the analyser must construct the spectrogram while operating in a realtime mode. That is, the spectra for the measurement should be produced from time records where there is no data missing or omitted from the spectrum calculations, and the time records for each spectrum overlap. A typical overlap fi gure for these measurements is 75 to 95%. To carry out these heavily overlapped real-time measurements of broadband signals such as WiMAX, it is necessary to use time capture, a feature of most vector signal analyzers. During time capture, the measured data is streamed directly to memory, without gaps and without performing analysis (which would slow capture and prevent gap-free recording). Then spectrum/spectrogram measurements are made during playback of the recording, where the overlap percentage is varied to adjust the effective “speed” of the playback and the time increment represented by each new spectrum in the display. The same time capture recordings can also be used for demodulation. For troubleshooting purposes, it can be very benefi cial to compare both spectrum and demodulation measurements of the same WiMAX frames or RF bursts.

 

At the left and right edges of the spectrogram in Figure 2, the OFDM symbol transitions, which would ordinarily be a subtle spectral feature, are now obvious. This temporary spectral spreading might not be noticed in a single spectral display, but the spreading and its periodic nature are obvious in the spectrogram. Also, it is easy to see that the preamble occupies a wider bandwidth than the data carriers of the PUSC zone.

VIEW COMPLETE SUBFRAMES

The spectrogram can also allow the user to examine multi-zone subframes and even complete DL-UL frames in their entirety. Figure 3 shows a complete downlink subframe containing three zones. This article already discussed the spectrogram characteristics of the first zone, and Figure 2 showed them with more detail. As Figure 3 illustrates, zone 2 shows a similar spectral pattern as that of zone 1, particularly near the end of the zone, where a large number of subchannels are inactive. Based on the spectrogram display, it is clear that zone 2 uses a distributed allocation of subcarriers similar to that of zone 1.

A very different spectral pattern is obvious in the middle of zone 3. This zone is an AMC (adaptive modulation and coding) zone of nine symbols in length, and a single distinct region of lower spectral energy is visible during symbols 4 to 6 of this zone. This region of lower energy is much broader during the last three symbols in this zone. The zone maps arranged to the right of this spectrogram show the source of the change in spectrum and power: During symbols 4 to 6 of this zone, a single logical subchannel is untransmitted, resulting in 16 untransmitted subcarriers out of 768 total data subcarriers. Though this is a significantly smaller proportion of untransmitted subcarriers than the similar region described in zone 1 above (24 out of 720 in that zone), the spectral minimum it creates is much more obvious because the missing subcarriers are adjacent in this zone.

The spectrogram display of the end of this zone shows the significant spectrum and power effects of nine unused logical subchannels (144 adjacent subcarriers) during symbols 7 to 9 of this zone. In this portion of the channel and during these symbols, the system transmits only pilot subcarriers. Note also that the spectrogram confirms that the slot length—the smallest time increment for a data burst—is three symbols in this zone and two symbols in the two previous zones.

OVERLAP DEFINES INTERVAL

For the measurement in Figure 3, the VSA is configured with a lower percentage of overlap processing than in Figure 2, in order to display a longer portion of the waveform in time. Increasing the overlap produces measurement displays with greater detail—in time—while reducing the overlap provides a measurement of a longer time period in a single display.

The VSA software includes a large number of signal recordings—multiple WiMAX signals and dozens of others— including the one used for these measurements. The software includes a demonstration-license mode in which you can measure and demodulate included recordings, plus complete measurement tutorials and online help. A demonstration version of the software is available from Agilent without cost or obligation, in either disk or download form (Reference 3).

REFERENCES

Agilent Technologies, WiMAX Concepts and RF Measurements, IEEE 802-16-2004 WiMAX PHY layer operation and measurements, Application Note, Literature No. 5989-2027EN, Jan 5, 2005. Agilent Technologies, IEEE802.16e WiMAX OFDMA Signal Measurements and Troubleshooting, Application Note 1578, Literature No. 5989-2382EN, June 6, 2006. www.aglient.com/find/89600.

 

AUTHOR’S B IOGRAPHY

Ben Zarlingo is product manager for communications test in Agilent Technologies’ signal-analysis division in Everett, Washington. His primary focus for the past dozen years has been emerging communications technologies and measurements using spectrum and vector-signal analyzers.