The advent of 4G/Long-Term Evolution (LTE) technology and multiple-input multiple-output (MIMO) antenna schemes in the wireless realm has brought significant changes in the way wireless devices and networks are tested.
In the past, simpler antenna configurations and lower-speed data protocols could be tested in a so-called “tabled” environment. Manufacturers simply plugged a cable into a port on the device and piped in an emulation of the wireless channel. It took the antennas completely out of the equation and went directly to the heart of the matter, which was receiver and transmitter performance as well as checking algorithms for cellular handoffs.
But alas, things are no longer so simple, and over-the-air (OTA) testing is now the order of the day. OTA test means that someone has to get into a vehicle with the prototype handset and physically evaluate it in field conditions where subscribers will actually use it. This is a rather expensive proposition to do once, never mind every time a design change is made to a handset or basestation.
This is just one of the changes in test methodologies that has been brought to bear by the move to 4G/LTE. In this article, we’ll look at this and other ways in which wireless test is changing and why.
Test Challenges Abound
One of the biggest issues in RF design for 4G/LTE is the ongoing spectrum fragmentation and the multiplicity of radios within handsets. Triband devices are required in Europe unless you’re staying put. The same goes for North America, only it’s three different bands. Asia Pacific? Can you say seven bands? This means significant design and test challenges for RF engineers in ensuring that performance is consistent across those multiple bands.
Also near the top of the list in terms of challenges is interoperability. In the United States, 4G is new enough that coverage is still a matter of “islands” centered on large metropolitan areas. Thus, handsets must be able to hand off seamlessly between existing 3G networks and the newer 4G/LTE networks. They must do so without dropping data connections or calls, and optimization is required on both the network and device sides of the link.
The issue of dealing with voice calls in smart phones also poses test challenges. The era of ubiquitous voice-over-LTE is at least a year or more out. Meanwhile, service providers are falling back on intermediate measures.
For example, Verizon uses its 1X network for voice while data is moved on the 3G and/or 4G/LTE networks. This two-radio approach can have a significant effect on battery life if design criteria aren’t exacting.
Data performance is itself a critical test area. With channel bandwidth of 20 MHz, data rates should theoretically reach 50 Mbytes/s. But what does that translate to in terms of real-world speeds? Many factors come into play here, including MIMO implementations and network issues.
Closely related is how handsets handle data retry for 4G/LTE. When the handset is trying to establish a data link to a server, does it relentlessly hammer the signaling channel until it connects, or does it bow out gracefully and retry after some interval?
The biggest change in the way mobile devices are tested is due to the arrival of MIMO technology. MIMO has brought a proliferation of test schemes that involve intensive field testing of devices.
The issue is how well you can translate conditions captured in the field to the testbench. This is critically important to network operators, infrastructure vendors such as Nokia and Siemens, and chipset makers such as Qualcomm.
So-called “drive testing” with mobiles reveals what most of us already know—depending on where you might be relative to the nearest basestation sites, calls may drop or data throughput may fall off. But recreating this environment in the test lab, replete with multipath reflections and Doppler shift, is exceedingly difficult.
For example, on a drive through midtown Manhattan, your handset might be within range of a basestation a block or two away at one moment and suddenly have that signal blocked by large buildings. If the algorithms in the handset do not embody sufficient intelligence, it can result in a dropped call. “These types of scenarios can’t be modeled with statistical channels,” says Erik Org, Azimuth’s senior marketing manager.
One solution to this conundrum is a product such as Azimuth Systems’ ACE MX MIMO channel emulator (Fig. 1). As part of Azimuth’s “Field-to-Lab” methodology, the ACE MX channel emulator can be used to prepare and replay drive-test RF data (see “MIMO Channel Emulator Selected For Testing 4G Wireless Broadband” at www.mobiledevdesign.com).