- In this paper, we investigate a new approach for the spectrum allocation in UWB systems. This approach consists in a cross-layer scheme that takes into consideration the different users channel quality and quality of service (QoS) requirements. The new scheme is based on the WiMedia solution proposed for multiband OFDM UWB systems. The main objective is to propose a lowcomplexity solution for the spectrum allocation that can manage all the users constraints. Thus, we study the optimal solution of the spectrum allocation and formulate it as a convex optimization problem. Then, we show that our proposed scheme reduces significantly the complexity of the optimal solution. Moreover, we show through simulations, that the new approach and the optimal solution have close performance in term of error rate and they outperform WiMedia solution proposed for UWB systems. Keywords - OFDM, spectrum allocation, QoS, UWB.