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.
[stextbox id=”info” caption=”Parametric requirement for WiMAX T&M”]1. Wide bandwidths
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[/stextbox]
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.
[stextbox id=”info” caption=”T&M equipment for WiMAX”]1. EDA software: Agilent’s ADS
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[/stextbox]
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
