MIMO Performance and Condition Number in LTE Test

Application Note 2 As companies rush to get Long Term Evolution (LTE) products to market, engineers face tough challenges in testing these often complex devices. The mandate to include Multiple-Input Multiple-Output (MIMO) means many engineers are working to optimize their multi-antenna architectures. To achieve time-to-market goals, the MIMO solutions under development need to work correctly in real-world situations. Defining situations when MIMO transmissions will improve system performance can be complicated, but it is a critical aspect of successful implementation. Of particular interest is how to quickly determine whether a MIMO channel is capable of supporting spatial multiplexing under a given signal-to-noise ratio (SNR). Fortunately with the proper test equipment, a figure of merit called " condition number " can provide a short-term indication of the SNR required to properly recover a MIMO transmission over the selected wireless channel. This application note will review the basic concepts of MIMO with specific application to spatial multiplexing in 3rd Generation Partnership Project (3GPP) LTE. It will show measurements of channel coefficients and associated condition numbers as they relate to the SNR at the LTE receiver. How antenna and channel correlation affects system performance will also be discussed, along with recommendations for the best measurement tools to use when developing LTE products and systems. Multi-antenna wireless systems have been shown to improve data capacity through spatial multiplexing and improve system reliability through antenna diversity and spatial coding such as Space-Time Block Coding (STBC) and Space-Frequency Block Coding (SFBC). The suitability of a MIMO wireless system for spatial multiplexing is largely dependent on the characteristics of the wireless channel, the antenna configuration and the ability of the receiver to accurately recover the channel coupling matrix coefficients. The complexity of the wireless channel including channel correlation, interference and noise rate may create difficulties when measuring the operation and troubleshooting a MIMO system. Fortunately, the operation of a LTE MIMO system with application to spatial multiplexing can be quickly verified with a calculation of the channel matrix " condition number " using a vector signal analyzer (VSA) such as the Agilent 89601A. The condition number is a deterministic calculation for evaluating the performance of the wireless channel and estimating the associated increase in SNR required for successful signal demodulation in the LTE MIMO system. For example, a condition number close to the ideal value of 0 dB would imply perfect channel conditions for the application of spatial …