Conquering the WiMAX Test and Measurement Challenge

Only a few years ago, people were asking whether or not mobile WiMAX would become a reality. In fact as recently as February of this year, The Economist magazine headlined an article “WhyMAX?” questioning how WiMAX would fare against entrenched wireless networks. Although the question still remains, since WiMAX is already being deployed throughout the world and even by the U.S. Department of Defense. Nearly 80% of telecommunications carriers have indicated they are considering WiMAX, and more than 200 carriers throughout the world are either conducting WiMAX trials or are making preparations to deploy it. So the question is no longer if WiMAX will be deployed but when, at what levels, and for what applications. From the perspective of wireless component, subsystem, and system manufacturers, there is little risk, since WiMAX has tremendous momentum, is strongly backed by some the of the world’s largest electronics manufacturers, and is being deftly administrated by the WiMAX Forum, which has more than 470 members from all segments of the WiMAX “ecosystem”.

In what applications WiMAX will be best received remains in question, but as a complement to or replacement for microwave and T1 cellular backhaul its future is virtually assured. In addition, WiMAX appears to have a future in countries with limited communications infrastructure, and to provide communications service in rural areas of any country. How well it fares when pitted against next-generation cellular systems will determine its ultimate success, since the performance of both the upcoming Long-Term Evolution (LTE) enhancement to UMTS and the Ultra Mobile Broadband (UMB) enhancement to 1xEV-DO claim to deliver performance similar to that of WiMAX. As a competitor to DSL and cable for high-speed delivery of voice, video, and data, it has much to offer but faces entrenched services that already deliver high quality of service.

The Test and Measurement Challenge

At a superficial level, “WiMAX measurements” are no different than those for any other wireless network, since they are based on fully-documented standards. However, scrape the surface just a little and differences appear, because by any standard of measure, WiMAX is at the very least equal in complexity to and perhaps more complex than any wireless system ever deployed. While cellular services took decades to reach their current level of performance, WiMAX “comes out of the box” as a high-speed digital telecommunications network with advanced features. It is an all-IP-core network that incorporates not just one higher-order modulation scheme but several, all bundled under the umbrella of Orthogonal Frequency Division Multiplexing (OFDM) and Orthogonal Frequency Division Multiple Access (OFDMA).

In addition, WiMAX takes full advantage of Multiple Input Multiple Output (MIMO) technology to optimize performance, which requires an unprecedented collaboration of RF and high-speed digital signal processing technologies, and complex software. Consequently, characterizing the performance of a WiMAX system is a tall order, requiring a complete set of standard-based measurements and performance requirements that all WiMAX equipment must satisfy to ensure that user equipment and infrastructure not only performs acceptably, but is fully interoperable as well.

The Rohde & Schwarz Advantage

Continuity is the key to effective characterization of WiMAX systems, from R&D through production and conformance testing. Without it, the test process becomes a patchwork of incompatible instruments and software cobbled together that takes too much time, too much money, and provides too little confidence in both the repeatability and reliability of the measurements. Rohde & Schwarz test solutions encompass all stages of WiMAX system design, from R&D through precompliance, conformance, and production. All are supported by software dedicated to WiMAX testing that results in a highly-integrated environment.

This white paper looks at the measurement challenges presented by WiMAX, provides insight into the measurements and their intent, and offers tips on how to implement test processes in the most cost-effective manner. It also includes extensive information about the measurement requirements posed by MIMO as well as a thorough discussion of MIMO itself.

Broadband Signal Challenges

Any signal with a broad bandwidth is susceptible to the potentially destructive effects of fading, a problem which WiMAX effectively deals with through use of OFDM. However, these signals have a very high crest factor, which means that power amplifiers must be extremely linear under every anticipated operating condition, a challenge faced by both amplifiers and the RF power transistors that empower them. OFDMA also has a complex physical layer, which makes interoperability tests a challenge.

The large bandwidth of WiMAX signals also requires power amplifiers and I/Q modulators to have flat frequency response. Transmitting signals that employ higher-order modulation schemes requires very good modulation accuracy along with a receiver that can distinguish between constellation points even when noise, fading, and interference are present. So in addition to generating multiple signals, receiver tests for MIMO systems require multiple fading channels to be simulated. The end result is that testing WiMAX equipment requires high-performance instruments that can generate and analyze standards-based signals and simulate interference effects, while making the process as simple as possible for the user.

The wide bandwidth of WiMAX also requires special testing of transceiver components. In a transmitter, a low-frequency or I/Q signal from the baseband chip modulates an RF carrier, which is then amplified. To optimize the performance of the transmitter, it is necessary to distinguish the performance of the baseband chip, I/Q modulator, and amplifier. This means that high-performance signal generators and signal analyzers be used to generate, demodulate, and analyze broadband RF signals, as well as their corresponding baseband signals.

Amplifier Testing

The large WiMAX bandwidth signals are also a challenge for designers of power amplifiers and I/Q modulators, which are typically characterized by providing an extremely pure signal at their input and analyzing the resulting output signal to see if (or by how much) it has deteriorated. In-band deterioration is measured in terms of spectral flatness and flatness difference, which is the difference in level between adjacent subcarriers. The modulated OFDM subcarriers add up to produce an RF signal with high dynamic range, typically quantified as the difference between peak and average power (crest factor). WiMAX signals have high crest factors of about 12 dB. To avoid modulation errors, the power amplifier must remain linear under these conditions, which requires simulation of two correlated fading channels and radio frequency signals.

Test parameters are those recommended by amplifier manufacturers and are evaluated by changing different input parameters such as frequency, power level etc. The test parameters include output burst power, frequency error, symbol clock error, crest factor, Error Vector Modulation (EVM), Adjacent Channel Power Ratio (ACPR), spectrum flatness, spectrum difference, and spectrum mask.

Output Burst Power

The measurement of output power versus input power for an amplifier is one of evaluating linearity. Output power of an amplifier is typically linear until reaching a point at which it begins to cause compression. The region in which output power increases linearly with increases in input power is regarded as the “linear amplification region”, and is where the amplifier will perform at its best. Power measurements allow the 1 dB compression point with reference to the linear region to be determined, after which further increases of the input power will cause the amplifier to work in the saturation region.

For WiMAX measurements, the signal is not continuous and has a burst structure. Using the R&S®FSQ, R&S®FSL and R&S®FSP spectrum analyzers as an example – along with Option R&S®FSx-K93 – burst power can be measured directly. The burst power measurements are obtained in terms of minimum RMS burst power, average RMS burst power and maximum RMS burst power.

Frequency Error

It is essential in a WiMAX system that the receiver accurately track the transmitter’s frequency. Frequency error measurement is the carrier frequency error relative to the spectrum analyzer’s center frequency. A frequency error between the transmitter and the receiver will cause shifts in the spectrum of each subcarrier relative to the FFT receiver frequencies to the point that the spectral nulls are no longer aligned with the FFT frequencies. This results in intercarrier interference (ICI). This measurement allows the frequency error to be determined when the guard period is changed.

Symbol Clock Error

Symbol clock error is the difference between measured and reference symbol clock relative to the system sampling rate. A symbol clock that is lower than the reference symbol clock will make the OFDM signal longer than required and cause the subcarrier spacing to decrease. A symbol clock that is greater than the reference clock will make the OFDM signal shorter and cause the subcarrier spacing to increase. This will create intercarrier interference and is detrimental to the signal’s EVM performance.

EVM

EVM is one of the most important test parameters for ensuring that an amplifier can produce more power and yet maintain signal quality. EVM is a measurement of the quality of the modulated signal and measurement results can be used to ensure that the receiver signal-to-noise ratio (SNR) does not degrade more than a specified minimum value because of the transmitter’s SNR. An amplifier may distort the input signal and therefore worsen the EVM performance because of compression effects or nonlinearities. Measurement of EVM allows amplifier quality to be evaluated with different modulation schemes.

ACPR

ACPR is used to characterize the distortion of amplifiers causing interference to adjacent channels. It is specified as the ratio of power measured in the adjacent channel to the amount of power in the main channel. The R&S® FSQ spectrum analyzer allows the user to measure this test parameter easily. The measurement results include adjacent, alternate, and second alternate channels and are taken from the lower and upper frequency bands. The plot makes it possible to analyze variations in ACPR performance when the input power is varied.

Spectrum flatness and spectrum difference

Spectral flatness is a measurement of power variations in subcarriers of a WiMAX signal and of deviation in average power in each subcarrier from the average power over all subcarriers. Spectrum difference shows the adjacent subcarrier power difference in the preamble part of the burst. The spectrum mask evaluates the spectral profile of the transmitter to ensure that no excessive power is transmitted outside the main channel. The analyzer shows a limit line representing the spectrum mask for the selected frequency band.

OFDMA testing

OFDMA combines frequency division duplex (FDD) and time division duplex (TDD), which means that a specific amount of spectrum and time are allocated to each subscriber. Varying bandwidths and transmission times are allocated to users based on their needs and level of service. OFDMA offers considerable flexibility in the efficient use of resources, but accomplishes it with a complex physical layer. From the test and measurement perspective, this complexity poses a challenge for the user interface of test equipment because many parameters must be set and the impact of the parameter must be displayed both graphically and in text form. It also poses the risk that transceiver manufacturers will have configuration problems. Fortunately, emulating transmitters and receivers with signal generators and signal analyzers that have flexible parameters helps solve these configuration problems.

Test equipment must not only be fast but allow changes to be easily made to WiMAX parameters. This flexibility in changing WiMAX parameters is extremely useful during interoperability testing with a signal analyzer between subscriber and base station equipment. For example, the contents of the DL-Map and UL-Map can automatically be demodulated down to the bit level, which allows the bits demodulated by the signal analyzer to be compared with those demodulated by the mobile station. During roubleshooting of an interoperability test, this is one of the first required steps because the subscriber station must demodulate the DL-Map in order to know what subcarriers to demodulate and when.

Once the DL-Map is demodulated correctly, the next step is to verify the contents of the UL-Map, which tells each subscriber station how to configure its transmitter in terms of which sub-carriers to use and when to use them. The base station can only demodulate the signal correctly if the mobile station transmits its information at the requested subcarriers with the correct timing. This highlights that timing is a parameter that may cause interoperability problems. Signal analyzers can increase the speed of the troubleshooting process because they can record and display the signal in both link directions.

MIMO Measurement Challenges

Signal generators provide standard-specific baseband or RF signals for use in MIMO receiver tests and allow flexible configuration and parameterization of the signals. In addition to setting the usual RF parameters such as frequency, bandwidth, and power of the test signal, it is necessary to select different data and control channel combinations, modulation/coding schemes, and data sources, as well as adding impairments to the signal. Additive White Gaussian Noise (AWGN) and simulation of different propagation channels are important to stress receiver algorithms under more realistic conditions.

Interferer scenarios provide another challenge, requiring the coexistence of the test stimulus with other technologies. The signal analyzers required for transmitter tests and for verification of RF components analyze signals in terms of power, spectrum characteristics, and modulation quality. In the case of MIMO, it includes analysis of signals from more than one transmit antenna.

While many RF tests can be performed with a single instrument, scenarios and RF performance testing require RF test systems that combine signal generators and signal analyzers. These complex test scenarios include receiver tests with specific interferer scenarios such as multiple interferers on different technologies. The instruments allow tests to be automated, which is imperative in the case of WiMAX and MIMO measurements that have very long test times. It also is useful in R&D for pre-compliance tests and to verify the RF performance of prototypes. Both non-signaling and signaling test systems are required depending on test requirements, and the RF test systems employed in R&D can be upgraded to become complete conformance test systems for use later in the development process (during the validation and certification, for example).

For testing the signaling aspects of MIMO, standard-compliant protocol testers are used that act as network simulators for terminal testing. To enable MIMO operation, signaling messages (to set up bearers on the air interface, for example) must include MIMO-specific information elements. The combination of MIMO with other procedures like adaptive modulation and coding, hybrid ARQ (Automatic Repeat Request) retransmission protocols, and scheduling is another important test element. All of these tests rely on fast feedback from the terminal to the network to adapt to the conditions of the radio channel, so the latency between uplink and downlink interaction is crucial. Since one of the primary benefits of MIMO is its ability to increase throughput, verification of the end-to-end connection between terminal and network is necessary to ensure that all protocol layers can handle the high data rates required for high quality of service.

Both downlink and uplink MIMO testing is important and of the two, downlink MIMO testing is the most challenging. Downlink MIMO testing refers to verification of MIMO functionality and performance in base station transmitters and mobile station receivers. For terminal receiver testing, a base station MIMO transmitter must be provided as the stimulus for different test scenarios. For a 2x2 MIMO system, two baseband signals and two RF signals must be generated for two different transmit antennas. For higher-order scenarios like 4x4, even more signals are needed. As should be obvious, measurement solutions for MIMO testing can easily become quite complex, so it is essential to limit the testing efforts whenever possible.

MIMO receiver tests and fading channel simulation

Receiver performance tests require a fading channel simulation in order to reflect realistic radio channel conditions. WiMAX conformance tests specify the propagation conditions for different test scenarios. Fading simulation is especially important for MIMO, because its performance is significantly influenced by propagation conditions. Fortunately, complex models are able to simulate realistic radio channel conditions in different environments by incorporating stochastic modeling of parameters. However, they require long simulation or test times and are not necessarily the best choice for mobile radio R&D and conformance testing. Less complex models may still provide sufficiently good performance with reproducible results in these environments.

Fading channel models for Single Input Single Output (SISO) systems have been in use for many years, and reflect the propagation conditions in different environments by modeling the positions of base station and terminal as well as the expected impact of the environment on the propagation. These channel models can also be used to simulate Multiple Input Single Output (MISO) and Single Input Multiple Output (SIMO) setups for transmit and receiver diversity. With the setup of a signal generator for a transmit diversity receiver test, the signal generator provides two baseband signals corresponding to two different transmit antennas. The baseband signal can be selected according to a certain standard and can be parameterized flexibly in terms of bandwidth, power, resource allocations, and data sources. In order to reflect spacetime coding for transmit diversity, a different coding type can be selected for each of the two antennas. Two baseband fading simulators make it possible to add propagation effects to the transmit signals of each antenna. Different propagation models can be selected.

For SIMO or receiver diversity testing, a set-up like that shown in Figure 6 would be employed. The signal generator provides one baseband signal according to one transmit antenna. The transmit signal is input into two fading simulators with correlated or uncorrelated fading. Afterward the signals are converted to RF and provided to two RF outputs to connect to the dual antenna terminal. The channel models for the SIMO and MISO case must be extended for MIMO systems in order to reflect the spatial dimension. The broader bandwidths of WiMAX must also be considered.

In a MIMO system, the channels between each of the transmit antennas and each of the receive antennas must be modeled separately. In a 2x2 system, four channels can be modeled independently. Assuming uncorrelated fading processes on the different channels is likely to be too optimistic, so correlation parameters must be reflected as well.

Extension of the ITU models employed for traditional wireless networks is one approach for a WiMAX MIMO channel model that has a reasonable level of complexity and good performance. However, for WiMAX, pedestrian and vehicular channel models are extended to incorporate spatial correlation matrices for each multipath component.

Here the signal generator provides the baseband signals for two transmit antennas. Besides space time coding for the two antennas, selection of precoding matrices may be desired to create typical MIMO signals. Four baseband fading simulators provide the fading characteristics for the channels between each transmit and each receive antenna and correlation properties can be individually set. For full flexibility, it is possible to specify the full complex correlation matrix for each multipath component, but it is also possible to use a simplified model and specify only complex correlation coefficients between the transmit and receive antennas. The faded signals are then summed before RF conversion and provided to the two RF outputs that can be connected to the dual antenna terminal.

MIMO transmitter tests

Signal analyzers are employed for MIMO base station transmitters in the downlink case. The typical transmitter measurements are performed to ensure that the transmit chain fulfills the requirements of output power, frequency error, RF spectrum emissions, and modulation accuracy. These measurements can be made separately with the signal analyzer for each antenna. Each MIMO antenna port usually transmits a different pilot pattern so that the receiver is able to distinguish the antenna signals and do the channel estimation.

MIMO conformance tests

MIMO is also important in conformance testing for both RF and signaling parameters. The WiMAX Forum evaluates a number of MIMO-specific mobile radio conformance tests for terminals and base stations in the context of its Wave 2 radio certification tests. The tests address pure RF requirements as well as verification of MIMO operation in combination with other procedures. Both receiver and transmitter tests for base stations and terminals are included.

For mobile station receivers, MIMO may be tested based on the demodulation and decoding performance for Matrix A and Matrix B, with block error rate (BLER) as the performance measure. The test includes different modulation and coding schemes and various fading channel conditions. The feedback mechanism from the terminal to the base station must also be verified to determine if the mobile is recommending the correct MIMO mode Matrix A or B based on a certain pre-defined channel condition. On the base station side, transmit MIMO processing must be verified, including pilot formatting for Matrix A and Matrix B and evaluation of modulation quality (error vector magnitude, EVM) for each transmit chain.

Beamforming

WiMAX systems can employ transmit and receive beamforming, which adds to the measurement challenge. Transmit beamforming, for example, requires verification of whether the mobile receiver can handle the reception of dedicated pilots. Receiver sensitivity for different modulation and coding schemes and test channel conditions including AWGN and fading must also be tested. There should not be any degradation in receiver performance caused by the higher power generated by beamforming. The base station performance for transmit beamforming must also be tested, and correct signaling of dedicated pilots verified.

Test scenarios for uplink MIMO are also important to see if the mobile can use the correct uplink subchannels and uplink pilot patterns. This is important in the WiMAX uplink, because different terminals may simultaneously access the same radio resource (transmit collaborative MIMO) and correct operation of the scheme is essential to avoid uplink interference. Verification of power boosting on the uplink pilots is also desirable.

On the base station side, verification of uplink MIMO is strongly recommended. The base station receiver must support the collaborative MIMO features. That is, it must be able to correctly demodulate and decode Matrix B MIMO transmissions from two terminals. Base station receiver sensitivity must be good enough to cope with mixed modulation and coding schemes and different transmit power values from the two different terminals.