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.