Efficient selection of OVSF codes for the downlink DCH in WCDMA

We examine the problem of unmovable dedicated channels (DCH) at the forward link in 3G mobile communications systems employing WCDMA. An unmovable DCH is defined as one which cannot be reallocated to another OVSF (orthogonal variable spreading factor) code because it is related to some service with very strict delay requirements. The relatively high total signaling delay for a single OVSF code reassignment makes the reallocation of such channels impractical. Therefore, a reassignment procedure that involves unmovable DCH channels cannot be performed. This is a very significant problem, since the complete elimination of code blocking is accomplished only if a code reassignment procedure can always be performed. We introduce a code selection scheme, named selective fewer codes blocked (S-FCB), which takes into account the different delay requirements of incoming calls during the assignment process and avoids the scattering of unmovable DCHs across the OVSF code tree. Thus the effectiveness of the code reassignment procedure is increased and code blocking is significantly decreased.