Laboratory simulation of “real world” conditions has become very sophisticated, and it is easy to replicate and control tests over cable connections, bypassing antennas. But with MIMO devices, throughput depends so critically on antenna performance in realistic propagation environments, and these cannot be simulated accurately with such conducted testing.
Single-Input Single-Output (SISO) allows for greater separation of baseband and antenna testing because modem performance is not coupled to antenna performance – other than differences in receiver input levels due to antenna performance. Conversely, in MIMO the two are co-dependent: antenna performance not only affects the signal levels presented to the receivers, but also the correlation between the two (or more) receivers. MIMO performance is a function of Signal-to-Interference-plus-Noise-Ratio (SINR) and antenna correlation. How the device sees the wireless environment through the antennas can greatly affect modem performance – environmental conditions will make it harder or easier to decode the different data streams.
MIMO testing over-the-air creates a unique spatial signature that replicates realistic propagation scenarios but in a controlled, repeatable environment.
Making it realistic
MIMO performance is a function of the wireless channel and the antennas, so testing must combine such characteristics as:
- Antenna gain or efficiency
- Branch imbalance
- Dual polarized channel conditions to replicate the handset environment
- Correlation between antennas
Thanks to the highly controlled environment specified by the CTIA, MIMO OTA radiated antenna testing is currently the best approximation to real world conditions. It includes the use of an anechoic chamber, enabling spatial channels with the correct field structure so the device under test can observe the realistic expected channel during the test.
The correct field structure is replicated in the anechoic chamber using a special channel emulator and by mapping the signal levels across the probes in the chamber.