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LTE: Up for a test

Posted: 24 Feb 2009 ?? ?Print Version ?Bookmark and Share

Keywords:LTE? WiMAX? broadband? wireless mobile? device test?

Similarities and differences
"One similarity, for example, is that both WiMAX and LTE have configurable OFDM FFT sizes [number of subcarriers] ranging from 128 to 2,048 subcarriers," Hall said. "In addition, both support the use of multiple modulation schemes, ranging from QPSK to 64-QAM on each subcarrier. As a result, the specific measurements required for each standard are also quite similar.

"For example, engineers testing either standard will need to evaluate the same common performance metrics, such as EVM [error vector magnitude], EVM vs. subcarrier, subcarrier flatness and adjacent channel power/spectrum mask."

Differences between the standards at the physical layer, however, prevent engineers from using common measurement algorithms for both. For example, OFDM subcarriers in WiMAX are spaced by 10.94kHz, and OFDM signals in LTE are spaced by 15kHz. "Because of differences such as this one, engineers can't really use a WiMAX test instrument to perform an EVM measurement on an LTE device, and vice versa," said Hall. Thus, while the same performance metrics are used to characterize the RF performance of WiMAX and LTE devices, different measurement algorithms are required for each.

Aggressive timeline (Click to view full image.)

LTE's momentum continues to grow, according to market watcher Maravedis Inc. The firm claims that more than 100 operators, including many from the CDMA camp, have committed to LTE. By comparison, nearly 400 operators have the more mature WiMAX technology in deployment or in trials.

Jan Whitacre, LTE program manager at Agilent Technologies, noted that "technical specifications for LTE have yet to be completed, and commercial deployment is not expected before 2010." That gives test equipment companies such as Agilent, Aricent, Azimuth, Ixia and Spirent, among others, some legroom to provide test solutions for LTE equipment conformance and interoperability.

There are plenty of engineering challenges, according to Whitacre. For one, "WiMAX and LTE utilize nontraditional frequency reuse schemes, which can create intercell interference, especially at the edges of the cell--just when a critical handoff needs to occur."

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