great to hear that you were able to get 256 QAM running.
The [k,d] value-pair corresponds to slope (k) and intercept (d) of a linear approximation of the BICM curves around the SNR regime relevant for a given CQI.
Basically, you should take a look at the following two papers to understand what I mean:
– Fig. 1 of “Low Complexity Approximate Maximum Throughput Scheduling for LTE” shows the linear approximation of the BICM curves over CQI that is employed for calculation of [k,d].
– Actually, the BICM curves are a function of SNR rather than CQI as used in Fig. 1. above. However, there is an approximate linear relationship btw. SNR and CQI according to Fig. 4 of “System level simulation of LTE networks”.
–> To calculate the [k,d] values for 256 QAM, you first of all have to determine the appropriate extended SNR to CQI mapping (Fig. 4). Provided this is still linear for the CQIs corresponding to 256 QAM, you can then determine the [k,d] values by linearly approximating the 256 QAM BICM curve similar to Fig. 1.
Hope that helps.