Low-loss compression of CPRI baseband data

Richard Maiden (Altera)
EDN (September 17, 2014)

Modern wireless infrastructure systems use fibers running a CPRI (Common Public Radio Interface) protocol to communicate frequency, phase, complex data, and control information. The demand for wireless data has, and is set to continue to increase exponentially. Both operators and equipment providers are seeking ways in which to minimize the capital investment and operational costs of running multiple high data rate fibers between baseband units and radio units.

This paper describes a method of using Mu-Law compression for Gaussian-like waveforms – for example, baseband IQ data, as used in CPRI interfaces. Mu-law compression is common in audio applications and is efficient to implement, but it has an excessive loss in fidelity for baseband signals. This paper describes offset-power-of-two schemes with a reduced number of segments to provide for an efficiently implemented lower Mu value more suited to baseband signals. This flexible compression scheme has 2:1 compression ratio for less than 1% EVM (Error Vector Magnitude) degradation on a standard LTE (Long Term Evolution) test waveform.

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