The original intention of WiMAX was to replace broadband cable networks such as DSL. With the adoption of the 802.16e-2005 standard, broadband mobile radio applications can now also be implemented. This has become possible through extensions at the MAC layer, which handles call setup and data processing. There have also been significant improvements to the physical layer of the standard.
While stationary applications primarily employ the OFDM multi-carrier method, mobile applications use the expanded OFDMA method exclusively. A major expansion of OFDMA over OFDM is in the number of carriers. In OFDMA, 128, 512, 1024 or 2048 carriers can be used.
With OFDM, subscribers are served one after the other. This means all carriers are assigned to the same subscriber, and the same modulation and power are used in every timeslot.
With OFDMA, several subscribers are served simultaneously. Several physical carriers are combined into sub-channels, and each subscriber is assigned a specific number of sub-channels, depending on the bandwidth required. The channel assignment is defined in what is referred to as downlink and uplink maps. Here, channels are assigned to specific subscribers for a specific time.
Signal generators are a valuable aid in the development of WiMAX receiver systems. The task is to code the data and the modulation mode for the various subscribers in line with the standard, to distribute the information to the appropriate carriers and to generate the maps.
The development of WiMAX transmit systems, on the other hand, calls for spectrum and signal analyzers. In signal analysis, the carriers have to be reallocated to the respective subscribers in line with the map definitions, demodulated, and results represented in a clear cut manner.
The SMU200A, SMATE200A and SMJ100A signal generators from Rohde & Schwarz, with an SMx-K49 option installed, can deliver OFDM signals in accordance with the 802.16-2004 standard as well as OFDMA signals in accordance with the 802.16e-2005 standard.
The SMU200A can be equipped with two different baseband modules, fading options, additional noise sources and two RF paths to generate a wide variety of test scenarios. The latest version of the software allows for the generation of WiMAX multi-carrier scenarios with only a single baseband path and RF path in the generator.
With the FSQ-K93 option installed, the FSQ signal analyser will measure WiMAX OFDMA signals directly without requiring an external PC.
Analysing signals from basestations, for example, requires an exact definition of the downlink map. This can be implemented in two ways. The user can enter the required data using a data editor in the analyser’s GUI. Alternatively, an existing configuration can be read from the signal generator. Data to be defined includes, for example, the number of subscribers and the modulation modes, the active sub-channels and the permutation zones.
Measurements can be started when the complete set of parameters has been defined, which includes the frequency, the guard interval, the recording time, etc. The measured burst is displayed. A green bar indicates successful demodulation of the signal.
All data relevant to the analysis of the WiMAX OFDMA signal is displayed in a list. The data is listed separately for each subscriber, a feature that is particularly important in OFDMA applications. Data is displayed numerically and graphically, which is useful in developing WiMAX applications.
Dr. Wolfgang Wendler is product manager spectrum analysis at Rohde & Schwarz.
See also: Electronics Weekly's roundup of content related to WiMax