Hi, When we compute the true loglikelihood ratios in bit interleaved co-ordinate modulation, it has a factor of the noise variance and hence the LLRs can be very large when the SNR is high(or equivalently the noise variance is low). The LLRs need to be quantized to be sent to the decoder. Can anyone explain or proivde a reference to the criteria used for LLR quantization. Should the LLRs be scaled differently for different SNR ranges? Regards ujar

# LLR quantization in BICM ..

Started by ●October 13, 2012

Reply by ●October 13, 20122012-10-13

"ujar" <suren.raju@gmail.com> wrote:>When we compute the true loglikelihood ratios in bit interleaved >co-ordinate modulation, it has a factor of the noise variance and >hence >the LLRs can be very large when the SNR is high(or equivalently the noise >variance is low). The LLRs need to be quantized >to be sent to the decoder. >Can anyone explain or proivde a reference to the criteria used for LLR >quantization. Should the LLRs be >scaled differently for different SNR >ranges?If SNR is good, everything else is good. Hence there is no practical point in the separate consideration for this case. Just clip the LLRs to maximum value. Vladimir Vassilevsky DSP and Mixed Signal Consultant www.abvolt.com

Reply by ●October 13, 20122012-10-13

On Sat, 13 Oct 2012 04:12:53 -0700 (PDT), ujar <suren.raju@gmail.com> wrote:>Hi, >When we compute the true loglikelihood ratios in bit interleaved co-ordinat= >e modulation, it has a factor of the noise variance and hence the LLRs can = >be very large when the SNR is high(or equivalently the noise variance is lo= >w). The LLRs need to be quantized to be sent to the decoder. Can anyone exp= >lain or proivde a reference to the criteria used for LLR quantization. Shou= >ld the LLRs be scaled differently for different SNR ranges? > >Regards >ujarThis is decoder-dependent to some degree and you should design to what the decoder you're using needs for the conditions in which you'll use it. In general, however, there is very little difference in performance for most decoders if you design to the expected low SNR conditions and just leave it at that. Eric Jacobsen Anchor Hill Communications www.anchorhill.com