Testing TD-LTE with real world test technologies: Page 5 of 7

July 24, 2012 // By Erik Org, Azimuth Systems
Many wireless data networks were initially designed to offer symmetrical data capacities as the original killer wireless application, voice, required equal capacity for both uplink and downlink. Spectrum blocks were licensed and auctioned as paired spectrum suitable for Frequency Division Duplexing (FDD) protocols, which served carriers and subscribers well when voice was the primary application.
more than -85 dBm. Noise floor is often expressed by test equipment vendors as a noise power spectral density. Assuming 25º C and a 10 MHz wide signal as would be typical for 3GPP LTE, the noise power spectral density of the test equipment would need to be less than -155 dBm/Hz to insure that the signal fidelity was maintained even during fading conditions. If the noise floor was greater than this, it is possible that as the emulator provides the fading channel, the emulator could also introduce a noise level that will cause demodulation errors in the receiver as a direct result of the channel emulator noise floor and not of the device under test.

Implications of beamforming

In the TD-LTE environment, many service providers and equipment vendors have been considering beamforming in deployments. By focusing the transmission energy in beams:
• Higher range can be reached;
• Less energy can be used for the same range;
• Interference can be mitigated;
• Network capacity can be increased;
• Overall improvements in system performance.

Beamforming algorithms are distinguished on the basis of the algorithm used to select the “beamformer.” With regards to TD-LTE, one must consider that “air” is reciprocal in nature; i.e. the downlink wireless path looks exactly like the wireless path in the uplink. There may also be multipath reflections, wireless channel variations, phase changes etc. as the signal traverses the wireless environment. In general, beamforming algorithms utilize the characteristics of the air interface such as channel variations and reciprocity.

Channel emulation provides a methodology to reproduce over-the-air conditions in the lab for testing and benchmarking different devices. Channel emulation can be used to validate improvements and performance gains due to beamforming algorithms. However, robust test of a beamforming device in the lab will require the use of a channel emulator that is bidirectional and has reciprocal and balanced paths in a cabled lab environment (figure 2).

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