Based on orthogonal frequency-division multiple-access (OFDMA) and multiple-input multiple-output (MIMO) technologies, WiMAX systems require specialised test equipment from design to deployment stage
SHWETA DHADIWAL BAID
Worldwide interoperability for Microwave Access (WiMAX) is synonymous to wireless broadband for high-speed Internet. It provides high-speed data transmission on wireless networks in modes like point-to-multipoint (fixed WiMAX) and fully-mobile Internet access (mobile WiMAX). To achieve high-speed data transmission, it is essential that the system meets the specified standards for quality service.
WiMAX Forum has specified certain standards and certifications for WiMAX devices like customer premise equipment, mobile stations and base stations. Although all the devices comply with these standards at the time of designing, errors may creep in during manufacturing. Therefore to enhance the quality of service and efficiency, WiMAX devices and protocols must undergo rigorous tests for certifications before these are deployed in the field.
WiMAX is the first standard that adopted the next-generation OFDMA technology. This modulation scheme is entirely different from the existing ones in the cellular segment. It transmits much more data on multiple carriers than single-carrier transmissions.
Need for specialised equipment
Designing a system with multiple carriers and testing it is the biggest challenge for the design engineers. The increased complexity in the signals makes high-speed measurements very critical. You need some unconventional equipment to generate and analyse these complex signals.
OFDMA combines frequency-division duplex and time-division duplex to provide varying bandwidth and transmission time to every user. This helps in efficient resource management, but at the same time adds to the complexity of the physical layer. The receiver has to decompose the mixed signal into independent signals. This imposes a big challenge for test and measurement (T&M) at the physical layer level.
The move from single-carrier to multiple-carrier (from single-input single-output to multiple-input multiple-output) technology complicates the system further as this requires multiple antennae for transmission and reception. To test a MIMO system, multiple signal generators and an integrated signal analyser are needed to measure, display and analyse.
One of the greatest challenges before test engineers is synchronisation. Multiple signals require proper synchronisation of time and phase to get the correct information. Moreover, signal generators should offer high precision in order to make accurate and repeatable measurements.
One of the important considerations is the bandwidth of the test equipment. Various channel bandwidths defined in mobile WiMAX standards are 1.25, 5, 10 and 20 MHz. So the equipment should test at least 20 MHz of the wide-band signal. The numbers of fast Fourier transform (FFT) sub-carriers allowed for signalling are 128, 512, 1024 and 2048. That means the T&M equipment should also be capable of supporting many sub-carriers in the system.
WiMAX test equipment from lab to deployment
WiMAX is perhaps the most enigmatic wireless system deployed in the field. It takes the advantage of higher-order modulation scheme provided by ODFMA and optimised performance provided by MIMO technology. Consequently, a complete set of T&M solutions capable of handling radio frequency, multiple channels and high-speed digital signal processing is required.
WiMAX systems are built around a WiMAX base station and WiMAX mobile stations (receivers). WiMAX Forum has released RF and protocol conformance standards for base stations and mobile stations. Any device carrying WiMAX Forum emblem has to undergo conformance tests using conformance test equipment. “Once the design is verified, every individual module has to undergo tests during the manufacturing stage to meet the standards. Manufacturing tests are a small subset of conformance tests,” says Sonali Sarpotdar, application engineer, Agilent Technologies.
Design and simulation
As a device designer, you need test and measurement equipment right from the R&D stage to the deployment stage. Most of the development work for different sub-systems is done in parallel. Once the system is integrated, you need some other tests to check the workability of the entire system. Some of the common T&M instruments that are needed in the entire design flow are oscilloscopes, logic analysers, vector signal generators, spectrum analysers for swept measurements and vector signal analysers for modulation analysis.
Ideally, a base station design implementing WiMAX is expected to support air interfaces like WCDMA, CDMA2000 and GSM. It should be capable of handling single channel as well as multiple channels. The design of base station and simulation for above capabilities is critical and time-consuming.
Electronic design automation (EDA) tools used for designing a device now offer a mobile WiMAX library to help wireless systems designers and verification engineers to speed up the development of WiMAX devices. “The library provides signal processing models, preconfigured simulation set-up, signal sources and receivers to facilitate simulation of fully-coded systems,” shares Sarpotdar. “Moreover, the library can provide transmitter measurements (such as error vector magnitude and complementary cumulative distribution function) and receiver measurements (such as receiver sensitivity and bit error rate) that directly help the designers,” she adds.
Expressing his views on EDA tools, Mahesh Basavaraju, assistant manager, product support and applications, Rohde & Schwarz, says, “Using specialised EDA tools, engineers can see the interaction between architecture, circuit devices, parameters and RF impairments.”
RF signal generation and verification
WiMAX operates in microwave radio frequencies. “The base and mobile stations have to be tested for their RF performance in order to maintain the best possible quality of radio link, using the least amount of carrier power,” says Sarpotdar. For troubleshooting of the design and prototypes of transmitter and receiver, T&M equipment should be capable of working at the baseband as well as in the RF domain.
It is difficult to generate OFDM signals for test performance and even more difficult to decompose them. Many new vector signal generators directly help test engineers by generating the OFDM signals. However, you need vector signal analyses as you deal with multiple-carrier and OFDM signals, where accurate phase measurement is important. “Signal generators need to be flexible to create signals that designers wish to test their system with. These should also comply with the set standards so that designers can trust the test results,” says Sarpotdar.
While dealing with multiple signals, it is important that phase distortions are corrected and the signal is recovered in time. The signal generators used must ensure fast set-up time and rapid switch-in frequencies and amplitudes.
A conventional signal generator is capable of generating one signal. You cannot be carrying multiple signal generators to test a MIMO system. “There equipment combine two independent vector signal generators in one box for 2×2 MIMO testing. You can simulate MIMO for conditions like fading and phase coherence that are very useful for various test cases,” explains Basavaraju.
Signal decomposing and analysis
The complexity of implementing the MIMO capability in WiMAX presents unique measurement challenges. Test receivers should be capable of emulating multiple channels for MIMO test. Equipment are available with specialised and critical instrumentation for testing multiple antennae systems.
2. High-speed capture and analysis
3. Vector signal analysis (due to different modulation schemes)
4. Multiple-channel testing
5. ODFM testing
6. Repeatable and reliable results
7. Low phase noise
Signal analyser software today have a unique ability to acquire the signal, analyse it and demodulate it before computing and displaying a wide range of useful parameters. Some equipment designed using state-of-the-art RF and digital signal processing (DSP) technologies are capable of performing such a task.
“The real-time spectrum analyser series, along with signal analysis software, provides in-depth measurement for troubleshooting of the WiMAX device at the design stage,” says Antonio Policek, marketing manager, Asia Pacific, Tektronix. “Some high-performance signal analysers also go up to 20 GHz of frequencies, capturing and sampling frequencies up to 110 MHz,” he adds.
According to Policek, there are some portable/handheld RF spectrum analysers that are used by regulatory bodies and administrative bodies to monitor the frequency usage.
There are a number of software tools that can generate OFDM waveforms. T&M manufacturers have made these tools easy to use by providing drop-down menus. You can configure the number of frames to be generated, zones, individual bursts, modulation format, etc.
Conformance tests and certification
The entire WiMAX deployment is a combined effort of WiMAX equipment manufacturers, WiMAX solution providers and operators providing WiMAX services. To avail the best quality of service and avoid issues like ‘dropped call’ and ‘network busy,’ all the devices must undergo tests and certifications as per the standards of WiMAX Forum. “The forum precisely defines mandatory and optional test cases that every device has to face,” shares Basavaraju. Conformance tests are done to verify the interoperability of devices and network at various interfaces.
2. Simulation software: AWR’s Visual System Simulator (VSS)
3. Vector signal generators: Agilent’s E4438C (ESG), N5182A (MXG) and PSG, R&S SMU200A
4. Signal analysers and software tools: R&S FSQ, Agilent’s N5106A, Tektronix RSA 6000, RSA-IQWiMAX, N7613A and N7615A Signal Studio, Tektronix G35
5. Conformance testers: R&S TS8970, N6434A, E6651A, N6430A
6. One-box solutions: R&S CMW270, Agilent’s N8300A
On the device side. WiMAX Forum has associated with AT4 Wireless, a designated certification laboratory and the WiMAX Forum lead lab. All the required testing services for obtaining WiMAX certification for both the base stations and mobile stations are currently available at AT4 Wireless’ WiMAX Laboratory.
AT4 Wireless has developed MiNT (mobile communications integrated tester)—a family of test platforms for mobile communications. MiNT T2110 platform covers the official test cases for certification of base stations and subscriber stations according to the WiMAX Mobile radio test specification and supports the initial profiles by the WiMAX Forum.
The network conformance test (NCT) system provides a validated test regime for ‘testing and test control notation version-3’ (TTCN-3) network conformance testing. Network conformance and protocol conformance test (PCT) systems can be combined together as these use the same test infrastructure—WiMAX certification test program.
The radiated performance test (RPT) is undergoing evaluations and will be a part of certification testing for Mobile WiMAX. At the RPT testing facility installed by AT4 Wireless, one can perform both 2-D (polar) and 3-D (spherical) antenna pattern measurements of transmit power and receiver sensitivity for notebooks, PCMCIA cards and other terminals with WiMAX. This will help to ensure the quality of the product.
On the software side. WiMAX systems are built around an IP core network consisting of access service network and connectivity service network. It runs on WiMAX protocols, which are tested using a protocol analyser. “The aim of the WiMAX network is to accommodate the maximum number of users and every user expects the best quality of service,” says Sarpotdar. The final end-user experience depends on all the elements of the network performing to their specifications.
For protocol conformance test, TTCN-3 provides the scripting language. The tester should provide protocol designers with a TTCN-3 scripting interface that supports script development for design needs, as well as the ability to run the European Telecommunications Standards Institute (ETSI) validated protocol test cases in accordance with the WiMAX Forum PCT certification requirements.
Talking about the security aspect, Policek says, “Another critical area where testing is needed is security and its implementation within the IEEE 802.16e framework. The need for security is increasing, as is subscribers’ demand for it. Security solutions emerging among new cellular standards are increasingly complex.” He adds, “WiMAX technology needs to inter-work flawlessly with other technologies including 3G and its evolutions. This adds yet another dimension to the development test puzzle.”
One-box test systems
Many T&M manufacturers believe in providing an integrated test solution as it is easier to use for engineers and designers. Typically, a one-box solution for the WiMAX system has a signal waveform generator, a signal analyser and some software tools that provide measurement and analysis of the system.
Agilent’s N8300A tester combines the capabilities of both a signal generator and a signal analyser in one box. It incorporates software customisations to test and calibrate specific WiMAX chipsets from different manufacturers. The N8300A is optimised for testing WiMAX radios during manufacturing.
CMW270, a complete test product from Rohde & Schwarz, was chosen as the ‘Best in Test’ in 2008 by the readers of Test and Measurement World, a trade publication and website. It is an all-in-one solution for testing mobile WiMAX equipment accurately and fast. It allows users to select from a non-signaling to the signaling mode to test a base station emulator in simulated network condition.
User experience matters
“The end-user experience is important. Thus it is important to perform quality-of-service tests to check for throughput, packet errors and latency. A complete network check is necessary to ensure that there are no dark spots and that all the areas have adequate signal strength,” says Sarpotdar.
The network frequently needs to grant high data rates to subscribers attempting to run throughput-intensive applications. Developers must have a means to guarantee data capture and data processing performance up to the maximum WiMAX base station throughput.
Every manufacturer has new software and instrumentation that increase the bandwidth to process wider-bandwidth signals for WiMAX. New T&M equipment and software dedicated to RF and WiMAX testing greatly reduce the development and deployment time. These reduce the manufacturing and maintenance costs as well and help improve the user experience.
The author is a senior technology journalist at EFY