Drive test adapts to VoIP, moves into lab
Rick Nelson, Editor in Chief -- Test & Measurement World, 6/1/2009 2:00:00 AM
“Can you hear me now?” So asks the advertising character who personifies the complex drive testing that wireless carriers perform to test the reliability of their networks, monitoring parameters like BER (bit-error rate), FER (frame-error rate), and DCR (dropped-call rate), which in turn affect customer satisfaction (Ref. 1). Drive testing is becoming increasingly important as cellular technology evolves into wideband implementations that carry data as well as voice. And as bandwidths expand, factors such as multipath fading become critical to network performance.
 Agilent Technologies has added VoIP capabilities to its drive-test platform; the upgrade addresses carriers’ migration to LTE and WiMAX technology. Courtesty of Agilent Technologies. Useful links: Mishra, Ajay R., Fundamentals of Cellular Network Planning and Optimisation: 2G/2.5G/3G...Evolution to 4G, Wiley, 2004. Reed, Doug, “Correlation-Based Spatial Channel Modeling—White Paper #100,” Spirent Communications. Reed, Doug, “Fading Basics: Narrow Band, Wide Band, and Spatial Channels—White Paper #101,” Spirent Communications. Reed, Doug, “Path-Based Spatial Channel Modeling: SCM/SCME—White Paper #102,” Spirent Communications. |
To address evolving needs, Agilent Technologies at CTIA Wireless 2009 (Las Vegas, NV, April 1–3) introduced VoIP (voice over Internet Protocol) capabilities for its drive-test system. The system works by generating RTP (real-time transport protocol) streams that mimic VoIP traffic. The software takes time-stamped measurements as the RTP traffic flows between two endpoints in the data network. The measurements include R-Factor MOS (mean opinion score), packet loss, delay, and jitter, and can be automated within the same drive test software that simultaneously performs RF measurements. The VoIP tool works for established mobile data services such as WiMAX, UMTS, and EVDO and will work for LTE when a compliant phone is developed, Agilent reports.
Drive testing is not without its own problems. Although it remains indispensable, real-world drive-test results are not repeatable, which can complicate efforts to troubleshoot dropped-call problems experienced during an actual drive test.
To address that issue, Spirent Communications at CTIA Wireless 2009 announced that it had added virtual-drive-test-capability and fading functionality to its SR5500 wireless channel emulator. Nigel Wright, VP of product marketing, explained that the goal is to let users “capture data in the field that upsets their device and bring it back to the lab,” where the emulator enables a single scenario to be played out across multiple test runs within a consistent RF environment.
A key component of Spirent’s virtual-drive-test system is the Fading Lab engine, which can simulate channel scenarios for testing MIMO (multiple input, multiple output) and other multiple-antenna systems. In a series of white papers (Refs. 2–4), Doug Reed, solutions architect at Spirent, describes why fading must be considered in modern radio-propagation environments, noting that as bandwidths increase, so does a receiver’s ability to resolve multipath signals. He provides details on flat fading, frequency-selective fading, and shadow fading and notes that multiple antennas used in the transmitter, receiver, or both can provide significant performance improvements.
Reed also introduces the SCM (Special Channel Model) developed by the 3GPP (3rd Generation Partnership Project) to emulate real-world multipath conditions in suburban and urban areas, and he describes correlation-based and path-based spatial-channel modeling techniques that can help evaluate multiple-antenna systems’ performance. He also covers a proposal in the European WINNER (Wireless World Initiative New Radio) project to extend channel bandwidths from 5 MHz to 100 MHz; the resulting channel model is called SCME (Special Channel Model Extended).
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