From The Editor | May 15, 2012

Radio Channel Emulation Scaling Up For LTE-Advanced

PaulK

By Paul Kruczkowski, Editor

With wireless standards becoming increasingly complex and network environments continuously changing, field testing of chipsets, devices, base stations, and other mobile broadband products has become unfeasible. As a result, radio channel emulators have become essential tools in enabling carriers and device manufacturers provide better products and service to the end-user. Emulators replicate the real-world wireless channel environment, simulating fading, shadowing, noise, and spatial channel conditions in a controlled laboratory setting, so performance tests can be both realistic and repeatable.  No one has benefitted more from emulator technology than today’s 4G LTE OEMs and operators, and two important emulator capabilities helping them move testing from the field to the anechoic chamber are the virtual drive test and over-the-air MIMO test.

The virtual drive test (VDT) essentially replaces repetitive, time-consuming, and costly drive tests with fully automated lab analysis. The challenge is to emulate a real-word radio channel that includes multiple base stations (eNodeB) and terminal devices (UE) and accounts for the eNodeBs, UEs, antennas, interferences, and propagation, as well as link and network algorithms.  Geometric-based stochastic channel models (CSCMs) in the channel emulator are enhanced by using VTD software to generate a measured characterization of the radio environment obtained from a channel sounder on a single drive test.  Introducing measured parameters such as data throughput in the serving cell, reference signal receive power, carrier-to-interference and noise power delay profiles of all cells, and the corresponding GPS coordinates of the measurements to the model results in virtual drive test results that are repeatable and correlate to field tests. Stability and performance testing, throughput measurements, handover and call success rate testing, and interference mitigation are some of the analyses that can be completed using this technique.

Another advanced feature of radio channel emulators that is critical to LTE device testing today and will continue to grow in importance is over-the-air (OTA) MIMO testing. OEMs need to verify the quality of MIMO-capable user equipment (UE), and operators must determine if the UE is suitable for use on their LTE mobile broadband network.  The OTA MIMO test environment consists of a communications tester or base station, a channel emulator, and an anechoic chamber that houses both the device under test (DUT) — fixed or mounted on a turntable — and multiple MIMO antennas.  The channel emulator recreates path loss, shadowing, multipath fading, delay spread, Doppler spread, angle spread, and polarization effects, as well as adding noise and interference. Depending on their test objectives, engineers can choose from single cluster channel models or more complex multi-cluster channel models that are based on field measurements. OTA MIMO tests evaluate the entire mobile device and permit end-to-end system performance analysis while the DUT operates. Engineers can also measure quality of service (QoS) parameters, bit error rate (BER), and frame error rate (FER).

Radio channel emulator tests like VDT and OTA MIMO will be relied on to simulate and troubleshoot the next wave of broadband wireless challenges.  For example, broadband wireless providers are constantly implementing new MIMO techniques as part of their effort to squeeze capacity and efficiency out of their existing spectrum until new spectrum allocations are available. In addition, ongoing development related to the LTE-Advanced standard (3GPP Release 10 and 11) will require emulator technology that supports wider bandwidths, multiple bands, and higher-order MIMO technology.

There are several radio channel emulators on the market that support VDT and OTA MIMO testing. However, one instrument that recently caught my attention was Elektrobit’s (EB’s) newly released EB Prosim F32, a scalable radio channel emulator that can address the current and future challenges of LTE testing. The Prosim F32 has 32 RF channels and 128 fading channels in a single unit, which is 4 times the capacity of its predecessor (the EB Prosim F8). When I spoke to Janne Kolu, VP of test tools at EB, he told me that the Prosim F32 supports emulation of the LTE-Advanced (3GPP Release 10 and 11) standard, addressing bi-directional 8x8 MIMO — including testing requirements of spatial multiplexing, beamforming, and spatial diversity. It also supports multiple RF bands, carrier aggregation up to 320 MHz, and relaying and coordinated multipoint transmission CoMP testing.

Next-generation radio channel emulators, of which the Prosim F32 is a prime example, will prove to be powerful tools in the laboratories of LTE-Advanced chipset and infrastructure developers. Expanded channel capacities will allow today’s critical tests — VDT and OTA MIMO — to be performed on tomorrow’s technology. Emulator scalability will also allow mobile operators to perform complex system modeling to maximize current network capacity while providing a path for expansion for the virtual field trials of the future.

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