With the evolution of wireless standards comes a corresponding evolution in test equipment required to design, debug, and certify end-user products. The latest step forward, LTE-A (LTE-Advanced), brings higher peak data rates of up to 1 Gbit/s for downloads and 500 Mbits/s for uploads.
In the current Release 10 version that is in the works, LTE-A offers transmit bandwidths of up to 100 MHz. This is achieved through support for aggregation of up to five carriers as well as with multiple-input multiple-output (MIMO) technology, which provides spatial multiplexing. These carriers do not have to be contiguous but can be spread out in the frequency spectrum.
LTE-A is specified for MIMO configurations of up to 8 by 8 for downloads and 4 by 4 for uploads. Clustering of single carrier frequency domain multiple access (SC-FDMA) carriers enhances uplink capabilities and provides more flexibility within a frequency band even as it increases the data rate.
The timeline for LTE-A seems pointed toward early deployment in 2012. This may or may not reflect reality; network operators are still trying to get LTE on its feet. Meanwhile, the functional specification for LTE-A release 10 remains unfinished. This has not deterred design cycles and the supply chain from getting into gear. All major chipset, handset, and network equipment vendors have active LTE and LTE-A projects in the works.
For test engineers, LTE-A brings some significant test challenges over and above those already imposed by LTE. Adding more signaling capabilities, more carriers, and more flexibility means more complexity at the physical layer. For example, the carrier aggregation technique will mean the possibility of interference issues both in handsets and in basestations. Clustered SC-FDMA carriers significantly add to the challenges of amplifier design and test by creating opportunities for the generation of spurs, both in-channel and in adjacent channels.
The complexities of MIMO speak for themselves. How do you achieve differentiation between signals arriving at multiple antennas within a handset? Then there’s the matter of coexistence with older 2G and 3G cellular systems, which brings in the notion of making one transceiver chain work with multiple technologies.
All of the above signifies a shift in what is required for LTE-Advanced test regimens as opposed to earlier versions of the LTE specification. One track that design teams can take is attempting to design things properly to begin with, thus reducing the debug burden later. To that end, Agilent Technologies has launched an LTE-A baseband verification library for its EEsof Division’s SystemVue system-level design platform. The library is now part of SystemVue 2011.03, providing support from system-level architecture down through algorithm development for LTE-A baseband design.
Within the reference library are more than 170 high- and low-level models for signal processing blocks for LTE and LTE-A covering versions 8 through 10 of the standard. These verification models can be exported out of SystemVue as a compiled and licensed dynamic link library (DLL) for use in other programs and methodologies. Further, the models are made even more useful by SystemVue’s ability to link to Agilent’s measurement hardware. With the models, users can provide test vectors in any format, or bring live waveforms into SystemVue for use with the system processing building blocks.
Coupled with the models for SystemVue must be a means of stimulating the device under test (DUT). Agilent provides this through its Signal Studio application and 89600B vector signal analysis software, which can generate aggregated carriers to inject into the DUT or to use in simulation runs (see the figure). Both Signal Studio and the 89600B software support the features that have been added to LTE-A, including its enhanced uplink capabilities.