Getting the best performance out of your Mobile WiMax design - Embedded.com

Getting the best performance out of your Mobile WiMax design

The latest version of the WiMAX standardaddresses the mobile services required to free users from the tethersof a predetermined location. As with any new technology comes the needto test its hardware—in the research labs, for product qualificationand conformance testing to the standard; on the production line; andfor troubleshooting and maintenance.

Proper and accurate testing helps ensure full interoperability ofWiMAX equipment from all manufacturers. Since WiMAX is intended as asolution for metropolitan broadband wireless access applications, thereis a need to address multipath effects from buildings and otherobstructions. For this, the OFDM versions of WiMAX weredeveloped and used in fixed WiMAX.

Mobile WiMAX employs orthogonal frequency division multiple access(OFDMA), a multiple- user version of OFDM. This allows a more flexibleassignment of bandwidth to multiple users than TDMA and provides lower latency.

Mobile WiMAX and fixed WiMAX have similar channel bandwidths andthey share modulation types. A key difference between them is thatMobile WiMAX has been designed to hand over user connections andmaintain connections across different base stations and coverage areas.A comparison between fixed WiMAX and Mobile WiMAX is shown in Table 1 below.

Table1: Mobile WiMAX and fixed WiMAX have similar channel bandwidths andboth share modulation types.

In WiMAX systems with OFDM, several modulation formats are used,with the modulation adapted to specific transmission requirements.Using this adaptive modulation approach, raw transmission rates of upto 73Mbit/s are possible for a 20MHz bandwidth.

The OFDM symbols used in fixed WiMAX systems are based on an inverse256-point FFT to make the frequency-to-time conversion, while the OFMDAversion uses a variable FFT size from 128 up to 2,048 (except 256).

A single user in an OFDM WiMAX system can use all subcarriers at anygiven time. In OFDMA, subsets of subcarriers are assigned to multipleusers, allowing a number of subscribers to be served simultaneously.

Using subchannelization, specific carrier groups are used for eachsubscriber. These subcarrier assignments change dynamically to overcomethe effects of multipath interference.

WiMAX systems can be used in time-division-duplex (TDD ), frequency-division-duplex(FDD) or half-duplex FDD configurations. In a TDD approach, thebase station and the subscriber station each transmit on the samefrequency although separated in time.

The base station transmits a downlink subframe, followed by a shortgap called a transmit/ receive transition gap and then individualsubscribers transmit the uplink subframes. Subscribers are accuratelysynchronized so that their transmissions do not overlap with each otherwhen they arrive at the base station. Following all uplink subframes,another short gap called a receive/ transmit transition gap isallocated before the base station can start transmitting again.

The use of preambles at the beginning of each subscriber uplinksubframe allows the base station to synchronize on each subscriberstation.

Mobile WiMAX in South Korea is known as WiBro , or wireless broadband.WiBro is based on the same IEEE 802.16e-2005 standard as Mobile WiMAXbut it is designed to be slightly more robust in terms of thesubscriber's speed relative to the base station. WiBro uses TDD onlyand an 8.75MHz maximum channel bandwidth. WiBro, which uses the 2.3GHzband, is interoperable with WiMAX equipment and is expected to competewith cable, DSL and WLANs in South Korea.

Signal testing
For evaluating WiMAX receivers and components, known test signals froma signal generator take the place of signals from a WiMAX transmitter.To emulate signals for receiver evaluation, the generator must providethe frequency range, modulation types and modulation bandwidthnecessary to match the signal requirements of the WiMAX standard forthe equipment under test.

Because the WiMAX signal has a burst nature, with differences inamplitude level from the start (preamble) of the burst through theburst data, a signal generator for WiMAX receiver testing should alsoprovide programmable power control to mimic the dynamic powercharacteristics of WiMAX signals. It must also be able to accuratelymeasure bit error ratio(BER).

Once WiMAX test signals are generated, they must be analyzed.Testing a base station unit or portable devices for Mobile WiMAXrequires the use of a signalanalyzer with the ability to emulate the operation of an IEEE802.16e base station, since the analyzer must be able to detect andrecord the full range of frequencies and modulation formats used by aWiMAX system.

For production environments aiming to save the space of a separateanalyzer and signal generator, there are measurement tools thatfunction as bothanalyzer and generator.

Transmitter measurements
What types of tests are needed for Mobile WiMAX transmitter testing?Transmitter tests as defined by WiMAX standards include maximum outputpower, transmitter spectral flatness, transmitter relativeconstellation error (RCE) and error vector magnitude (EVM), transmitterpower level control, transmit spectral mask (for unlicensed- bandoperation), adjacentchannel power ratio, spurious levels and harmoniclevels.

Licensing authorities generally establish spectral masks withspecific requirements for their areas. Armed with a wideband signalanalyzer, all these WiMAX RF transmitter measurements can be madeincluding tests of frequency, power level, interference and modulationquality.

Since WiMAX systems are so dependent on accurate digital modulationand demodulation functions, many of the measurements that characterizea Mobile WiMAX device's transmitter relate to modulation quality.

To avoid interference, transmitted channels must remain within theirspecified limits and at specified power levels. Thus, two of the morebasic WiMAX transmitter measurements have to do with characterizing thefrequency and power of a WiMAX device's transmitted signal.

Because WiMAX signals are transmitted in bursts, transmitter testsfor frequency accuracy require a signal analyzer with enoughinstantaneous bandwidth to capture the full signal of interest. Thechannel bandwidths for various WiMAX operating frequency ranges andstandards are shown in Table 2 below.

Table2: To avoid interference, transmitted channels must remain within theirspecified limits and at specified power levels.

Measurements of frequency accuracy require that a WiMAX signal bedemodulated before measurement. The WiMAX specifications call for aWiMAX transmitter that is within 2ppm of its set frequency, which isequal to 7kHz for a 3.5GHz WiMAX device.

The WiMAX standards also refer to a “mesh-capable” device havinglooser frequency requirements, with a specification of 20ppm of the setfrequency.

In terms of power, note that WiMAX devices send signals in frames inwhich the amplitude varies from one end of the frame to another. Forexample, the power at the beginning of the frame (preamble) isgenerally at least 3dB higher than the power level of the data part ofthe frame.

The WiMAX specifications require relative amplitude accuracy within±0.5dB for basic WiMAX power measurements.

In a WiMAX burst signal, received- signal-strength indication (RSSI)is used, but only in the preamble part of the burst. Because of thecomplexity of the WiMAX burst signal, more than just a basic powermeter must be used for accurate power measurements.

Due to the wide modulation bandwidth of WiMAX signals, it is notpossible to evaluate their modulation quality with a conventionalspectrum analyzer. Instead, accurate WiMAX transmitter measurementsrequire programmable signal analyzer and application software.

More advanced fixed and Mobile WiMAX transmitter measurements areoften presented as a function of time due to the burst nature of theWiMAX signals. Transmitter-quality measurements include received timesignals, EVM/RCE, spectral flatness, EVM vs. time, adjacent channelpower ratio, adjacent channel spectral flatness, power spectral densityand cumulative complementary distribution functions.

For WiMAX receiver testing, a methodology similar to transmittertesting is used, but a high-performance signal generator is used inplace of a WiMAX system's normal transmitter.

Typical WiMAX receiver measurements include testing for sensitivity,maximum input level, adjacent-channel and alternate channel rejection,reference timing accuracy, BER and subscriber station uplink transmittime tracking accuracy, which is similar to reference timing accuracyin that the subscriber station is instructed to change timing.

The best measurement capability is meaningless without the abilityto interpret the test results. Due to the complexity of IEEE 802.16e-2005 OFDMA signalsand the need for accurate digital demodulation to perform many of theMobile WiMAX transmitter measurements, it is easy to miss a problem ormisinterpret test results. Incorrectly defined data bursts, forexample, can cause demodulation during the test process to fail.Attention to detail and sound measurement practices should always beexercised for maximum accuracy in WiMAX measurements.

Eric Hakanson is Product MarketingManager in the Microwave Measurements Division at Anritsu Co.

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