WiMAX in the Last Several Miles

802.16 2004 OFDM Signal Complexities
The basic idea of OFDM is shown by the nine frequency-domain signals in Figure 1. They are harmonics displayed on a linear frequency axis: the center frequencies are 2�, 3�, 4ׅ9� the lowest frequency signal, which is the basis for orthogonality.

Figure 1. Overlapping Orthogonal Subcarriers

The product of two sinusoids is equal to the sum of two cosine waves at the sum and difference of the two original frequencies. That is, sin(n) � sin(m) = �cos(m – n) + �cos(m + n). If the sine waves are harmonically related, m is an integer multiple of n. The integral of a sine or cosine wave over a single cycle is zero. Integrating either cosine term over one cycle of the fundamental frequency n is equal to zero. This shows that, ideally, there is no interference between overlapping harmonically related subcarriers.

If m were not an integer multiple of n, integrating for one period of frequency n would not always correspond to an integer number of cycles at frequency m – n or m + n. This is what happens in conventional frequency division multiplexing (FDM) in which the frequencies are not harmonically related. The subcarriers must have a guard band between them to reduce the interference that otherwise would occur. The available spectrum is not used as efficiently as in OFDM where the subcarriers actually overlap each other.

Multipath signals arrive at the receiver at slightly different times, causing a delay spread. To avoid the delayed end of one symbol interfering with the start of the next, a cyclic prefix is added to each symbol.

Typically, this is accomplished by adding the last several microseconds of a symbol to its beginning. This small amount of time acts as a guard band in the time domain to avoid intersymbol interference (ISI). It is easily accomplished and avoids the cost and complexity of equalization to remove symbol distortion.

In contrast to the 64 subcarriers used in 802.11a and g WiFi OFDM systems, fixed WiMAX uses 256 subcarriers, of which 192 are modulated with data and eight are used as pilot signals. The remaining 56 carry no data and form guard bands between the broad group of 200 subcarriers and activity at higher and lower frequencies that possibly could interfere.

The greater range that WiMAX provides compared to that of WiFi relates directly to the greater number of subcarriers. �A receiver using 256 ODFM can tolerate delay spreads [in the multipath data] up to 10� greater than systems using 64 OFDM.�¹

Because the wideband channel does not have uniform characteristics across its entire width, the eight pilot subcarriers are used to estimate channel behavior. The pilot frequencies are distributed throughout the band. Data is transmitted in bursts preceded by a preamble, and during the preamble, known symbols are transmitted on the pilot subcarriers. From the receiver response to the pilot signals, an estimate of channel characteristics is made.

This approach allows the modulation format to be decided based on the quality of the channel. The modulation choices are binary phase-shift keying (BPSK), quadrature phase-shift keying (QPSK), 16-state quadrature amplitude modulation (16QAM), and 64QAM. The required quality of the channel increases from BPSK with 64QAM requiring the best channel performance. Because of the dynamic nature of NLOS operation, the type of modulation is allowed to change for successive bursts.

Figure 2 shows the arrangement of downlink (DL) and uplink (UL) subframes within a frame as well as the major parts within each subframe. Transmission is in bursts corresponding to separate user information. Depending on whether the frequency band being used is licensed or not, frequency-division duplexing (FDD), time-division duplexing (TDD), or a combination is used for UL and DL.

Figure 2. 802.16 2004 OFDM Frame Structure
Courtesy of Rohde & Schwarz
Please click here to see larger image

Ideally, OFDM subcarriers don�t interfere with each other, but channel and hardware impairments will degrade performance. Testing is intended to reveal degradation caused by the nonideal nature of a real channel and the actual system implementation. Deleterious effects include but are not limited to I/Q gain imbalance, I/Q quadrature skew, phase noise, frequency error, nonlinear distortion, and the addition of spurious signals.

Further Complexities of 802.16e
Mobile WiMAX is addressed by specification 802.16e, which adds scalability to 802.16 2004 systems. This means that instead of a constant 256 subcarriers, as many as 2,048 or as few as 128 may be used with a corresponding range of bandwidths from 20 MHz to 1.25 MHz. Further, the frame size runs from 2 ms to 20 ms.

Subcarriers are grouped to form subchannels. Depending on whether all the subchannels that are supported by a particular number of subcarriers are used, allocation of subcarriers may be different. These permutations are referred to as fully used subchannelization (FUSC) or partially used subchannelization (PUSC).

Many additional considerations contribute to the exact configuration of both UL and DL OFDMA frames. Also, different forms of diversity are possible such as multiple-antenna transmission supported through an advanced antenna systems (AAS) option.

Testing WiMAX
Given the complicated nature of a WiMAX signal, thorough testing demands specialized equipment. Equally important, testing is required on many levels. At one end of the range are operational profiles with physical-layer signal impairments at the other.

Profiles
To ensure that devices interoperate correctly, the WiMAX Forum has developed performance profiles with which each device must comply to be certified. It also is important that the profiles guide market development by encouraging manufacturers to design within established subsets of possible WiMAX system parameters. As of May 2006, 14 fixed WiMAX products had been certified. The 802.16e specification was ratified only late in 2006.

Table 2 lists five test setups used at the most recent PlugFest to determine that communications could be correctly established between a single BS and from one to three mobile stations (MSs). Because this was the first PlugFest to be held following ratification of the 802.16e mobile WiMAX specification, the testing concentrated on mobile operation.

Table 2. PlugFest Test SetupsCourtesy of the WiMAX Forum

Different system profiles exist for fixed and mobile operation. Fixed WiMAX has five certification profiles that define interoperating products according to frequency band, channelization, and duplexing mode. These profiles cover the 3.5-GHz band with both TDD and FDD and the 5.8-GHz band with TDD. For example, profile 3.5F1 applies to a 3.5-MHz bandwidth on a 3.5-GHz carrier operating in the FDD mode. Table 1 shows the range of parameters applying to different mobile WiMAX bands.

Release 1 of the profiles emphasizes mandatory features. A second release will address quality of service (QoS), advanced encryption standard (AES), and automatic repeat request (ARQ) optional modules. As more products enter the market, the number of test cases run on each profile also will increase from the initial 75.

Physical-Layer Analysis
The WiMAX Forum 2006 report �State of the Art WiMAX Test Equipment� lists the Agilent Technologies type 89600 Vector Signal Analyzer (VSA) with the B7Y option (OFDMA modulation analysis), Anritsu�s Signature Model MS2781B VSA with options 22 (30-MHz Demodulation Bandwidth and IQ Baseband Input) and 41 (WiMAX Modulation Analysis), and the Rohde & Schwarz Model FSQ VSA with options K92 (OFDM) and K93 (OFDMA) as suitable instruments for making WiMAX physical-layer measurements. A large, comprehensive table in this report correlates product capabilities and WiMAX specification requirements for each instrument.

Commenting on the large number of choices possible within WiMAX, Eric Hakanson, product marketing manager, Microwave Measurements Division at Anritsu, said, �802.16 2004 is not a standard but a toolbox that offers suggestions about how to develop profiles, so it allows engineers a lot of flexibility in testing WiMAX products. Anritsu has developed flexible solutions that can measure everything within the toolbox. For example, WiMAX accommodates a wide range of channel bandwidths and FFT sizes.

�Anritsu�s Signature integrates a full suite of physical-layer measurements of both fixed and mobile WiMAX signals. Its open Windows environment makes it easy to integrate popular simulation and analysis tools. For example, the Windows environment provides a seamless interface with MATLAB�,� he continued. �This permits designers of WiMAX products to develop their own measurements and then view live measurement results, post-processed by MATLAB, directly on Signature�s display. The open platform architecture also allows us to easily evolve test capability to satisfy any changes that may come from future WiMAX requirements.�

Agilent�s 89600 Series VSAs are factory preconfigured VXI systems containing one or two baseband, IF, or RF channels; time capture memory; and software with several options. For WiMAX, the Model 89641 DC to 6-GHz RF instrument is suitable. The 89600 VSA software can be used separately and complements an Agilent spectrum analyzer such as the E4440 PSA or N9020 MXA but also can be used with Infiniium Scopes and 16900 Series Logic Analyzers. It can be installed in the MXA Analyzer as a software option.

The 89600 VSA software must run on a separate PC linked to the scope or an E4440 Spectrum Analyzer with the appropriate options. The software with the B7Y option provides simultaneous and correlated WiMAX time-, frequency-, and modulation-domain analysis capabilities. A large number of display formats and user-selectable scaling are supported. By being PC-based, the program�s responsiveness increases with computer performance.

Rohde & Schwarz addresses WiMAX measurements with the FSQ Signal Analyzer for R&D applications and the FSL Spectrum Analyzer for production test applications. Justin Stallings, product manager at the company, explained, �Because the two instruments use a common remote-control command structure, the K92/K93 OFDM/OFDMA firmware options for the FSQ are easily adapted to the FSL, providing a streamlined transition from development to production.

�In R&D applications, the FSQ offers both RF and baseband interfaces, providing test flexibility at all levels of component and system test,� he continued. �For future requirements, architectural options accommodate 120-MHz IQ demodulation bandwidth for analysis.�

A point stressed in Rohde & Schwarz WiMAX white papers is the need for sufficient resolution bandwidth to handle the maximum 20-MHz wideband signal. The FSQ also offers features such as display of error vector magnitude (EVM) vs. time, facilitating transient event analysis. This is in addition to the usual constellation diagram, frequency response, group delay, and similar presentations that have become well-established tools for analysis of digitally modulated signals.

Tektronix introduced WiMAX support software for the 8-GHz RSA 3408A Real-Time Spectrum Analyzer in November 2006 so this instrument was not included in the WiMAX Forum report. With the RSA-IQWIMAX Analysis software package, all aspects of both 802.16 2004 and 802.16e can be addressed.

For example, you can analyze frequency settling time and phase errors that occur during a burst transmission. In addition to a comprehensive feature set, the RSA3408A captures intermittent or random events, enabling design errors to be detected, diagnosed, and resolved quickly. The RSA-IQWIMAX software runs on a separate PC communicating via Ethernet or GPIB and provides one-button measurements with automatic detection and auto-configure.

Together, Tektronix and Litepoint have developed the IQmax one-box tester for WiMAX production test. It is a lower-cost solution capable of testing all critical WiMAX physical-layer parameters. IQmax offers the functionality and look and feel of the RSA-IQMAX Software used with the RSA3408A.

In addition to these instruments, lower-frequency VSAs such as the Keithley Instruments Model 2810 2.5 GHz VSA are suitable for 2.4-GHz mobile WiMAX measurements. Walt Strickler, the company�s wireless test marketing director, said, �The Model 2810 with its 30-MHz bandwidth, when connected to a signal demodulation tool such as MATLAB, can track the evolving WiMAX standard and be easily reconfigured for multiple test cases as testing requirements are modified.�

Physical-Layer Signal Sources
Vector signal generators (VSG) create WiMAX signals for development or test purposes. In addition to the obvious RF capabilities required, the WiMAX Forum test-equipment report lists further necessary characteristics.

A VSG must generate a signal with controllable noise and selectable impairments so that a receiver�s capability to demodulate the signal in the presence of these imperfections can be tested. In addition, it is desirable that the generator simulate channel fading so the receiver can be tested under this condition.

Because a representative WiMAX signal can be very complex, the Agilent, Rohde & Schwarz, and Anritsu products listed by the forum all can generate signals based on stored modulation files.

Complex signal description requirements are handled by Agilent�s Signal Studio software: type N7613 for fixed WiMAX and type N7615 for mobile. The software is used in conjunction with the E4438C ESG or N5182A MSG VSG. The MSG features fast waveform switching and very good adjacent channel power ratio (ACPR) performance.

With either generator, the N5115B Baseband Studio software adds real-time channel-fading capabilities to the Signal Studio waveforms. Different types of fading can be selected as represented by several Stanford University Interim (SUI) and International Telecommunication Union (ITU) channel fading models. The fading also can be applied to signals from a real device.

The Rohde & Schwarz SMU200A VSG with option SMU-K49, digital standard IEEE 802.16, produces 802.16 2004-compatible signals. Additional white Gaussian noise (AWGN) can be added with option SMU-K62. A fading simulator (SMU-B14) also is available as well as a second RF channel (SMU-B203). A single-channel instrument is suitable for receiver sensitivity testing while a second channel can be used to provide an OFDM-modulated interference signal.

You can define in detail up to 64 bursts in both the UL and DL. As the company�s Application Note 1EF57 stated, regardless of the apparent greater complexity of 802.16e OFDMA signals, these too can be generated by the SMU200A. The main difference between working with OFDM and OFDMA signals is the definition of the waveform at the signal generator and signal analyzer. Because of the complexity of the signal definition, a good editor is helpful to the user.

The note explained, �A map has to be clearly defined for DL and UL for [generating and] analyzing the signals. The used subchannels and zone types have to be defined as well. In OFDMA mode, the allocation of physical carriers to logical subchannels has to be calculated considering complex permutation algorithms, which can vary according to these different zones. Moreover, different segments can be defined for allocating subchannels to different BSs working in parallel.�

Anritsu�s Mr. Hakanson said that the company�s Model MG3700A VSG with up to 512-MS memory and modulation bandwidth in excess of 120 MHz was a good complementary instrument to the Signature analyzer. �To address WiMAX, there are a couple of options available to designers. Given that the MG3700A is Arb-based, it has the capability to transmit waveforms created in off-line applications such as MATLAB. Another way to create waveforms is via the company�s IQproducer� Software.�

IQproducer and Agilent�s Signal Studio are similar to the degree that both PC applications are used to produce complex waveforms that are downloaded to the connected VSG. IQproducer contains a menu listing several different types of capabilities such as CDMA, multicarrier, and WiMAX, from which you make a selection. Both products produce output data files compatible only with specified VSGs.

Spirent Communications doesn�t provide actual WiMAX signal generation, but the Model SR5500 Wireless Channel Emulator allows you to include NLOS fading in your test setup. The effects of 24 independent paths recombined according to preloaded SUI and ITU models together with optional AWGN ensure realistic test conditions.

Adam Rachlin, senior product manager at the company, said, �The SR5500 supports future upgrades of the technology by adding software/firmware functions that include complex correlation between signal paths, which is being adopted by the WiMAX Forum. Also, enhancements are accessible via an intuitive GUI.�

Conformance Test System
The Aeroflex MiNT T2230 and T2231 Protocol Conformance Test (PCT) Systems were developed jointly with AT4 Wireless. This equipment will be used by CETECOM (Spain), the WiMAX Forum accredited laboratory, for mobile WiMAX device certification.

�The MiNT WiMAX system consists of a control PC and a number of signaling units,� according to Paul Argent, the company�s director of infrastructure products. �Each signaling unit can emulate a single BS or a single MS. The MiNT signaling unit utilizes a software-defined radio (SDR). This provides a completely flexible solution.

�Simply by changing the firmware that is loaded into the signaling unit, either BS or MS emulation can be provided, fully scalable in bandwidth and number of subcarriers. Additionally, each signaling unit contains one or two banded radio cards. This feature supports advanced capabilities such as 2+2 MIMO [coming in a future profile].�

Summary
WiMAX is incrementally more complex than WiFi. It is based on the OFDM technology proven in WiFi but expanded to support greater distances and higher bandwidths for both fixed and mobile applications. Several types and models of test equipment address WiMAX system development and troubleshooting, many recognized as suitable by the WiMAX Forum in its recent test-equipment report.

A common theme is the importance of precise signal definition and analysis. Sometimes this can be accomplished on-board the actual test instrument as in the Rohde & Schwarz SMU200A. Many other instruments generate or analyze the required RF signals, but the WiMAX-specific detail is handled by a separate software program running on a PC.

The software is either a general-purpose application such as MATLAB or an instrument manufacturer�s proprietary program. Channel emulation capabilities also can vary. They may be provided by a specialized instrument, or alternatively, be resident within a VSG or be part of PC-based signal-definition software.

As with WiFi, PlugFests help manufacturers verify interoperability before subjecting their devices to formal certification. If you are developing WiMAX test equipment or devices, a PlugFest is an excellent opportunity to evaluate your product and obtain expert advice.

Reference
1.�WiMAX Technology and Deployment for Last-Mile Wireless Broadband and Backhaul Applications,� Fujitsu Microelectronics America, 2004.

February 2007