Added relative dB lookup table (for SNR)

ft8/decode.c

@@ -9,6 +9,21 @@
 // #define LOG_LEVEL LOG_DEBUG
 // #include "debug.h"
 
+// Lookup table for y = 10*log10(1 + 10^(x/10)), where
+//   y - increase in signal level dB when adding a weaker independent signal
+//   x - specific relative strength of the weaker signal in dB
+// Table index corresponds to x in dB (index 0: 0 dB, index 1: -1 dB etc)
+const float db_power_sum[] = {
+    3.01029995663981f, 2.53901891043867f, 2.1244260279434f, 1.76434862436485f, 1.45540463109294f,
+    1.19331048066095f, 0.973227937086954f, 0.790097496525665f, 0.638920341433796f, 0.514969420252302f,
+    0.413926851582251f, 0.331956199884278f, 0.265723755961025f, 0.212384019142551f, 0.16954289279533f,
+    0.135209221080382f, 0.10774225511957f, 0.085799992300358f, 0.06829128312453f, 0.054333142200458f,
+    0.043213737826426f, 0.034360947517284f, 0.027316043349389f, 0.021711921641451f, 0.017255250287928f,
+    0.013711928326833f, 0.010895305999614f, 0.008656680827934f, 0.006877654943187f, 0.005464004928574f,
+    0.004340774793186f, 0.003448354310253f, 0.002739348814965f, 0.002176083232619f, 0.001728613409904f,
+    0.001373142636584f, 0.001090761428665f, 0.000866444976964f, 0.000688255828734f, 0.000546709946839f
+};
+
 /// Compute log likelihood log(p(1) / p(0)) of 174 message bits for later use in soft-decision LDPC decoding
 /// @param[in] wf Waterfall data collected during message slot
 /// @param[in] cand Candidate to extract the message from