Network Level Performance of Uplink Synchronous WCDMA

Orthogonal codes are a powerful method to reduce the multiple access interference (MAI) and thereby increase the network capacity in WCDMA systems. However, synchronisation is not easy to maintain in the uplink since the transmission is started from the users, and so far orthogonal channelisation codes were only considered for the downlink in UMTS. With the purpose of facilitating uplink synchronism, USTS (Uplink Synchronous Transmission Scheme) is proposed within the 3GPP standardisation body, and it will likely become a work item for release 6. USTS is applicable for low mobility and low dispersive environments such as indoor and pedestrian that allows reducing the uplink intra-cell interference by means of making the own cell MAI orthogonal. Synchronisation is reached in USTS by means of a conventional tracking loop procedure, which is suitable for low mobility terminals. In our investigations, the performance of USTS has been assessed at network level. The main problem with this technique is that the number of orthogonal channelisation codes is limited. A solution to overcome this problem is to use a new scrambling code and thus reuse the whole set of channelisation codes. The problem is that the signals associated to different scrambling codes are not orthogonal. This code restriction mitigates practically all the benefit of USTS, even in environments where orthogonality can be kept almost perfect. Nevertheless, USTS could give high performance if it is combined together with other techniques that make better use of the number of available channelisation codes. Assuming up to 50 users per scrambling code USTS can increase the number of users by approximately 20 % for the case with soft handover and 50 % user activity; with two-antenna receive diversity the increase is about half of that. Receive diversity reduces the other cell interference, and therefore increases the performance of USTS, but on the other hand, the gain from diversity reception allows a higher number of users in the system in general and therefore conflicts with the limitation on the number of available codes. (José Outes, Preben E. Mogensen; Klaus I. Pedersen, Nokia)