We design new receiver structures suited for the ultra wide band (UWB) standard IEEE 802.15.4a. The design objective is a receiver robust to multiuser access interference (MAI), which is a particular severe impairment in simple wireless personal area networks (W-PAN). Unlike most approaches of the literature that focus on the optimisation of decoding, we jointly optimize both demodulation and decoding, deriving new maximum likelihood (ML) schemes with enhanced capabilities in MAI rejection. In particular, a generalized Gaussian mixture model is assumed for the description of MAI, which includes as sub-cases the Gaussian and Laplacian models but has the potential to describe much more general scenarios. Simulations carried out on the IEEE 802.15.4a standard and including channel estimation show that the proposed approach significantly outperforms existing solutions in various channel and interference scenarios.