FA-72237 / Error-correcting codes / Member archive

LLR quantizer saturates to the unsigned range · case 02

Large samples produce LLRs outside the decoder word size.

Member previewVariant 2 · 3 implementations · 8 checks per implementation

Case contract

Quantise soft demodulator samples to signed integer LLRs for a hard/soft decoder. Each sample y (integer) is scaled (v = y * scale, in hundredths), rounded to the nearest integer with halves rounded away from zero, and saturated symmetrically to +-(2^(bits-1) - 1). Positive LLR means bit 0, negative bit 1, zero is an erasure (None). Return [llrs, hard decisions].

Why this case matters

Fixed-point receivers quantise LLRs before feeding Viterbi or LDPC decoders; bias at this stage costs coding gain.

One recorded failure

Sample boundary fixture

This sample comes from the broken implementation of a controlled reproducer.

Boundary fixture	Actual	Expected	Outcome
regression [[136, 302, -270, -121, 100, 337, -271], 2, 3]	`[[3, 6, -5, -2, 2, 7, -5], [0, 0, 1, 1, 0, 0, 1]]`	`[[3, 3, -3, -2, 2, 3, -3], [0, 0, 1, 1, 0, 0, 1]]`	Failed

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