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5G air interfaces require channel measurements

Posted: 18 Nov 2015 ?? ?Print Version ?Bookmark and Share

Keywords:5G? air interface? algorithms? channel impulse response? measurement systems?

Channel parameter estimations
Much of the research to date has focused on a single channel. MIMO (multiple input, multiple output) channels, however, introduce spatial and correlation information. The key issue with MIMO channels is how to estimate the spatial parameters. This includes parameters such as AoA (angle of arrive), AoD (angle of departure), and AS (angular spread). There are several channel parameter estimation algorithms that can be considered including beamforming based, subspace based, and ML (Maximum Likelihood) based. For consistency, coherence, and estimation performance, the ML-based estimation algorithms provide the very good performance for MIMO channel parameter estimations. Specifically, the SAGE (space-alternating generalized expectation-maximisation) algorithm (ML based) with relativity low computing, is widely accepted by the research community.

Calibration and synchronisation
Calibration and synchronisation are paramount to getting accurate, repeatable results. Synchronisation of the transmitter and receiver sub-systems can be achieved using two Rubidium clocks to provide a stable, high precision synchronised 10MHz reference clock to the transmitter and receiver as shown in figure 2. In addition, triggering must be used to synchronise the sounding stimulus signal generation and acquisition. With-in a mmWave measurement system shown in figure 2, the following calibrations need to be considered:
???System calibration, also called "back-to-back" calibration, involves physically connecting the transmitter to the receiver to align the frequency reference and system clocks. This can then be used to get accurate amplitude, phase and time of arrival estimates.
???The differential IQ outputs of a base band AWG can have timing, gain and quadrature errors, which can impact signal quality. An IQ mismatch calibration is used to address any imbalance of the in-phase and quadrature-phase signal that is output from the AWG.
???The multi-channel, wideband digitizer or oscilloscope can have inter-channel time and phase variants that will impact the measurement results. Various methods can be used to measure the cross-channel skew. One method is to measure the magnitude and phase differences across a large frequency range for each channel and apply a wideband correction filter.
???Antenna and power calibration also need to be considered. Check with the antenna manufacturer for calibration data. If not provided, antenna array phase pattern measurement could be performed in a microwave chamber and compared with the theoretical performance of the antenna array.

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Figure 2: Measurement System includes Rubidium clocks for precise Tx and Rx synchronisation and acquisition trigger to align signal generation and data capture.

Conclusion
There are many challenges in the characterisation of new 5G mmWave air interfaces. When considering time-varying channels with multi-path propagation, the measurement systems can be complex. The measurement system should support mmWave, wideband signals and multiple channels, calibration, and synchronisation. Such support can produce accurate and repeatable measurements with good channel parameter estimation algorithms for the characterisation and development of realistic and accurate channel models.

About the author
Sheri DeTomasi contributed this article.


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