Améliorations de l'accès paquet en sens montant du WCDMA (2003)

Third generation (3G) cellular systems implement a new multi-access technique, the Code Division Multiple Access (CDMA). In Europe, the 3rd generation system under deployment is the Universal Mobile Telecommunications System (UMTS). It utilizes in its radio access part, the wideband CDMA standard. The major evolution of 3G systems is the provision of high data rate services. Moreover, 3G networks offer the possibility to their users to have more than one simultaneous service. A packet-switched mode of data transport has been included in 3G systems from the beginning of their conception. It is anticipated that internet-related applications along with voice, are going to be the main sources of traffic in these systems. Consequently, the packet-switched mode coexists with the circuit-switched one, the latter being present already in 2G cellular networks. Considering that the most restricting part of a wireless network is the radio access part, the provision of the requested Quality of Service (QoS) to a high number of users increases the requirement for an efficient utilization of radio resources, both by the network and by users. In this thesis, we mainly investigate the performance of packet data services in the radio access network of UMTS. We try to enhance a number of radio resource management (RRM) mechanisms that have an impact on the performance of packet data services. The proposed mechanisms aim at optimizing the utilization of resources, which is expected to increase system capacity and consequently to improve the offered QoS to packet data users. In UMTS however, packet data services are expected to coexist with voice, as mentioned above. Therefore a part of the thesis deals with the performance of both real time traffic and packet data in case of mixed traffic scenarios. The work focuses on the uplink direction of transmission, hence the transmission from the User Equipment (UE) to the network. A mechanism for which an enhancement attempt is performed is the radio link adaptation. In particular, the case of multi-service transmission is studied. The UE schedules its total allocated data rate to its activated services that are circulating in its radio bearers. At each transmission time interval (TTI), the UE selects a part of its total data rate for each radio bearer: this is performed through selection of a "transport format" to be applied during the TTI. This procedure is performed in the MAC (Medium Access Control) layer; its output is a combination of transport formats (Transport Format Combination, TFC) used by the physical layer. The procedure, named TFC selection, permits to adapt the transmission of the various services to the changing radio propagation conditions: it determines heavily the performance of the service performance. The guidelines of the TFC selection algorithm in uplink are presented in 3G standards. The principle of the algorithm is that it favors the transmission of real time traffic over packet data. However, the real time service may be degraded due to packet data under certain conditions, in particular for UEs far from the Node B. We propose a TFC selection algorithm that minimizes this degradation and offers larger coverage area for the real time services. A second mechanism under study is the packet scheduling among users. It is a procedure that is controlled by the network. Its impact on the system performance is investigated and quantified through system level simulations. We study the potential of certain features to increase the efficiency of the packet scheduling. A first feature is named fast Variable Spreading Factor (VSF): UEs far from the Node B, change rapidly their spreading factor in order to maintain their transmission power constant, which stabilizes the other cell interference. A second feature is a decentralized uplink packet data access using feedback information on the total cell interference in the cell. A third feature, called "fast packet scheduling", reduces the packet scheduling cycle. Results have shown that in cases of medium and low load in the cell the decentralized access mode reduces the delay per packet up to 25%. Fast scheduling increases the system capacity up to 10%. Moreover, it improves the QoS experienced by users in terms of throughput per user and delay per transmitted packet.