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Optimal allocation of resources in an asynchronous CDMA channel with identical SINR requirements for all users

: Boche, H.; Stanczak, S.

IEICE Transactions. E, English transactions. B, Communications E86 (2003), No.1, pp.397-405
ISSN: 0916-8516
ISSN: 1745-1345
Journal Article
Fraunhofer HHI ()

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.