Optimal Allocation of Resources in an Asynchronous CDMA Channel with Identical SINR Requirements for All Users
Holger Boche, Sławomir Stańczak · Publikationsdatenbank der Fraunhofer-Gesellschaft (Fraunhofer-Gesellschaft) · 2003
A great deal of effort has been spent to de-velop strategies for allocation of resources in DS-CDMA sys-tems in order to mitigate effects of interference between users. Here, the choice of spreading sequences and appropriate power allocation play a crucial role. When developing such strategies, CDMA system designers need to ensure that each user meets its quality-of-service requirement expressed in terms of the signal-to- interference+noise ratio. We say that a set of users is ad-missible in a CDMA system if one can assign sequences to the users and control their power so that all users meet their quality-of- service requirements. In [1], the problem of admissibility in a synchronous CDMA channel was solved. However, since the simplistic setting of perfect symbol synchronism rarely holds in practice, there is a strong need for investigating asynchronous CDMA channels. In this paper, we consider a K-user asyn-chronous CDMA channel with processing gain N and identical performance requirements for all users assuming chip synchro-nism. We solve the problem of admissibility of the users in such a channel if N · K, and identify optimal sequences. We also show that constant power allocation is optimal. Results obtained in this paper give valuable insights into the limits of asynchronous CDMA systems.