Control Plane

Control Plane (PCFICH & PDCCH)

Unlike the PDSCH data region, the control region prioritizes ultimate reliability over throughput.

PCFICH Decoding

The PCFICH dictates the size of the control region (CFI = 1, 2, or 3).

1. We extract 16 REs (4 REGs) from Symbol 0 based on the Cell ID.
2. We equalize them using CRS pilots.
3. We perform a maximum likelihood correlation against the 3GPP (32, 2) block code dictionaries.

PDCCH Transmit Diversity (SFBC)

PDCCH is strictly QPSK and utilizes Alamouti Space-Frequency Block Coding (SFBC). In src/mmse_equalizer_cpu.cpp (demap_pdcch_transmit_diversity_scalar), we bypass the expensive MMSE matrix inversion entirely.

Instead, we pair adjacent REs ($k$ and $k+1$) and apply Maximum Ratio Combining (MRC): $$Z_0 = H_{00}^* Y_0(k) + H_{01} Y_0^(k+1) + H_{10}^ Y_1(k) + H_{11} Y_1^*(k+1)$$ By scaling the output with the noise variance $\sigma^2$, we achieve MMSE-equivalent performance with zero division and zero matrix inversion.