Quantization techniques for accurate soft message combining

Wireless communication is prone to errors due to always changing channel conditions. Multiple transmissions over independent channels can be leveraged by combining erroneous packets. To be able to combine, reliability information for bits, so-called soft-bits, of the previous messages has to be saved. Especially for systems with limited storage capacity, such as sensor nodes, this poses the question of how detailed the stored reliability information has to be in order to achieve a desired performance. We compare different reliability information approximations, introduce two quantization options specifically tailored for combining, and compare their Bit Error Rate (BER) performance by simulation. We conclude that our quantization with logarithmic spacing using a recursive Log Likelihood Ratio (LLR) approximation shows the most promise, because it has the best BER results and the lowest implementation complexity.