Benchmarking LTE-A Carrier Aggregation data throughput – More pipes, More challenges

We very recently concluded a study with the Signals Research group that for the very first time, benchmarked data throughput of carrier aggregation devices in the lab. By definition, Signals Ahead “Chips and Salsa” studies entail data throughput testing of technologies that are still in their infancy. Designed using Spirent’s industry leading expertise, the tests aim to measure data throughput in a variety of channel conditions ranging from the easy to the really adverse. It was indeed a very exciting prospect to not only see Carrier Aggregation technology in action but also see carrier aggregation devices from all over the world perform. Involving aggregation of 2 component carriers, the test setup was brewed using the Spirent 8100 recipe that calls out the now-familiar ingredients –network emulator supporting carrier aggregation, Landslide - the Core Network emulator needed for testing Application Layer throughput using a tethered data connection. As is the case with any new technology, it came as no surprise that this study brought along its own challenges.

Does the phone really support Carrier Aggregation out of the box “as advertised”?

While a lot of device vendors are beginning to advertise Carrier Aggregation and may well have the capability to support it, actual usability of the feature is really dependent on the carrier and/or the market that the device operates in.

If the operator does have the carrier aggregation feature turned on, (like some of the carriers in South Korea), we learned that the chances of the device supporting the feature is relatively high. If the operator does not have the feature turned on, chances of the carrier aggregation feature working on the device right out of the box (a rather slick one at that) are pretty slim. This usually needs specific firmware from the vendor which has the capability enabled, even though the radios and the hardware may be available on the phone.

In addition, operators may be looking to support more than one band combination for carrier aggregation and the antenna combination for each set of bands can be different. It is thus extremely important to understand which band combinations are supported by the device for carrier aggregation – again, this is influenced by the bands that the operator is looking to deploy Carrier Aggregation in.

One sure way of knowing the band combinations that the device supports for Carrier Aggregation, is by looking for the Release 10 parameters section under the “UECapabilityInformation” message sent by the device (and not the reviews on the different consumer electronics’ websites as we learned on more than one occasion!).

The bottom line is that prior knowledge of supported bands for Carrier Aggregation is essential. Although devices may advertise Carrier Aggregation support and may well have the hardware for it, the chances of carrier aggregation working depend on the market that they are sold into.

Getting to the right antennas – a game of eeny, meeny, miny, moe

Carrier Aggregation (used with any transmission mode that is not Transmission Mode 1) necessitates using at least two antennas on the device; one being the main antenna for the Primary Component Carrier and the other being for the Secondary Component Carrier.

Naturally, testing 2-component carrier aggregation on commercial, off-the-shelf devices necessitates having to understand the antenna configurations of the device – as in, identifying which of the multiple antennas (ranging from 2 to 4 as we observed) is the primary antenna and which one is the secondary, and for which band combination as described earlier.

Another dimension that we added to our test plan is that of MIMO or Transmission Mode 3 – Open Loop Spatial Multiplexing. Depending on vendor implementation, the device could require using just the two antennas for both carriers (one RF combined output for primary and another for secondary), or could require usage of all the 4 antennas (2 for each carrier).

Without direct support from the device manufacturer, all this can very quickly turn into a game of “eeny, meeny, miny, moe” to figure out the connections between the network emulator to the device.

Long story short, we’ve had our share of “fun” going through all sorts of connector combinations (All 4 antennas simultaneously, or only two at a time) to figure out the right one.

An additional challenge, is physically locating the right port – sometimes the right RF ports that are needed for Carrier Aggregation are hidden in obscure locations behind the back panel of the phone. The ones that are clearly visible, may not be the right LTE ports that are needed for testing.

On one occasion, we had to request the engineering team from a device vendor to expose the right port. The team very kindly obliged and returned the device to us after carrying out what looked to be some serious “surgery” on the unit.

It may still be early days of Carrier Aggregation deployment, but operators all over the world are making rapid progress in deployment. Apart from some of the engineering issues discussed above, there are capacity challenges that the entire industry faces when it comes to setting up Carrier Aggregation test capabilities.

More than 40 LTE bands are available of which there are currently close to 50 band combinations that support Carrier Aggregation. As more and more band combinations are added by the 3GPP, this number is going to increase rapidly over the next few years. While this study tested 10 + 10 MHz carrier aggregation, regional bandwidth combinations can differ greatly, based on spectrum owned by different carriers.

As a consequence of this, the volume of testing is expected to increase three fold which poses significant capacity challenges.

Dedicated test systems that are scalable become very important to ensure preparedness for this challenging and extremely critical test theme – our recently launched 8100 X-202 platform is positioned to help our customers do exactly that.

Learn more about testing for Carrier Aggregation download our white paper titled “LTE Advanced - Carrier Aggregation: Introduction and Implications for Mobile Device Testing.”

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