Compression and Decompression

A preprocessing block-based bit shift block generates the optimum bit-shifts for a resource block of 12 resource elements (REs). The block reduces the quantization noise, especially for low-amplitude samples. Hence, it reduces the error vector magnitude (EVM) that compression introduces. The compression algorithm is almost independent of the power value. Assuming the complex input samples is x = x1 + jxQ, the maximum absolute value of the real and imaginary components for the resource block is:

${maxI}_n= max\left( \left(x{I}_{12\left(n-1\right)+1}\right) ,\left({ xI}_{12\left(n-1\right)+2}\right), \dots , \left({ xI}_{12n} \right)\right)$

${maxQ}_n= max\left( \left(x{Q}_{12\left(n-1\right)+1}\right) ,\left({ xQ}_{12\left(n-1\right)+2}\right), \dots , \left({ xQ}_{12n}\right) \right)$

The maximum value of the resource block n is:

${maxVal}_n=max\left( {maxI}_n ,{ maxQ}_n \right)$

Having the maximum absolute value for the resource block, the following equation determines the left shift value assigned to that resource block:

${lshift}_n=\left(\begin{array}{c}bitWidth- \left({\log }_2\left({maxVal}_n\right)\right)-1 if {maxVal}_n<2^{bitWidth-1}\\ 0 else\end{array}\right)$

Where bitWidth is the input bit width.

The IP supports compression ratios of 8, 9, 10, 11, 12, 13, 14, 15, 16.

For a compression ratio of 16 (udCompParam field = 0, according to the O-RAN WG specification), the IP does not perform compression or decompression.

Mu-Law Compression and Decompression

The algorithm uses Mu-law companding technique, which speech compression widely uses. This technique passes the input uncompressed signal, x, through a compressor with function, f(x), before rounding and bit-truncation. The technique sends compressed data, y, over the interface. The received data passes through an expanding function (which is the inverse of the compressor, F^-1(y). The technique reproduces the uncompressed data with minimal quantization error.

The Mu-law IQ compression algorithm follows the O-RAN specification.