We derive exact BER formula for DS PPM UWB multiple access (MA) systems using the Characteristic Function (CF) method. All previous exact BER analysis were carried out in AWGN channels. Here we adopt the lognormal multipath fading channel model, which is more practical for real UWB applications. We derive the exact probability distribution function (PDF) of total noise using characteristic function instead of making any approximations on the PDF of interference. Numerical results clearly show that the BER obtained by CF method is more accurate than the GA method in large SNR range. We also plot the characteristic functions of the statistic decision variables to provide more insight into the numerical results.