import math


def power_sum( pdb_lst ):
    """ take a list of powers in dBm, add them
        in the linear domain and return the sum
        in log
    """
    sum_lin = 0
    for pdb in pdb_lst:
        sum_lin += 10**(float(pdb)/10)*1e-3
    return 10*math.log10(sum_lin/1e-3)