For manufacturers of components and equipment, the Third-Generation Partnership Project (3GPP) Long Term Evolution (LTE) specifications present some new challenges. For example, LTE has been widely heralded for its use of multipleinput multiple-output (MIMO) antenna techniques. In addition, it uses multiple channel bandwidths, different transmission schemes for the uplink and downlink, and both frequency- and time-division-multiplexing (FDD and TDD) transmission modes. Interference issues also may arise, as LTE will co-exist with current second- and third-generation (2G and 3G) cellular systems.
To explain how engineers can ensure that LTE-system components meet their high performance targets, Agilent Technologies created the application note, "Stimulus-Response Testing for LTE Components." This 38-page document covers devices like power amplifiers (PAs) and radio-frequency integrated circuits (RF ICs) for transmitters, receivers, or transceivers. It does not include the testing of basestation or user-equipment transmitters or receivers. Specifically, it examines stimulus-response testing using LTE signals and measurements of transmitted signals from RF ICs.
The note begins by providing a brief overview of the physical layer (PHY) in LTE standards. That PHY uses orthogonal frequency division multiple access (OFDMA) for the downlink and singlecarrier frequency division multiple access (SC-FDMA) for the uplink. With these multiple-access technologies, multiple users can transmit at the same time. In addition, six different channel bandwidths may be used ranging from 1.4 to 20.0 MHz. Fixed subcarrier spacing is 15 or 7.5 kHz for multimedia broadcast multicast service (MBMS). The number of OFDM carriers changes for different channel bandwidths.
The breadth of the application note, which focuses on testing, examines the requirements for LTE-system components. It begins with LTE conformance testing and proceeds to the implications of testing components rather than full systems. Although the complete characterization of these components may demand additional measurements, such as noise figure or power consumption, those tests are not covered in this document.
In the amplifier section, for example, the document explains that the LTE signal's high crest factor requires a large dynamic range for the PA in the transmitter. Although SC-FDMA is used in the uplink to reduce the crest factor, amplifiers for user equipment must still be capable of wide dynamic range. If the amplifier is operated in its nonlinear region, gain compression or clipping will resultthus causing distortion. By operating the amplifier backed-off from its compression point, it can remain in its linear region. Distortion is therefore minimizedbut at the cost of reduced efficiency.
The designer must grasp an amplifier's performance under different conditionsfor example, with input signals that have different peak-to-average power ratios (PAPRs). It is then possible to determine the best tradeoff within the constraints. Among the tests that are typically used are power measurements, modulation-quality measurements like error vector magnitude (EVM), and distortion measurements, such as adjacent-channel leakage ratio and spectral regrowth. Although this is just a brief example of this application note's content, it shows how the document explains the unique features of the LTE specifications and how they must be handled in terms of testing.
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