Network Performance of Transmit and Receive Diversity in HSDPA under Different Packet Scheduling Strategies

The target of this work is to assess the performance of WCDMA FDD networks implementing the High Speed Downlink Packet Access (HSDPA) concept of UMTS when transmit and receive diversity techniques are used in combination with different packet scheduling strategies. In HSDPA, the data traffic is transmitted on a time-shared channel with constant power and fixed spreading factor. The transmission is adapted to the state of the radio channel by varying the modulation and coding scheme (MCS) and the number of used multi-codes accordingly to the channel quality estimates fed back from the UE. This adaptive variation of the transmission parameters is referred to as link adaptation (LA). Since the feedback information from the UE is subject to delays, the performance of the LA is affected by the UE speed. The aforementioned use of different MCSs implies that there is a non-linear relationship between the achieved throughput and the received signal-to-noise ratio (SNR) at the UE. Moreover, there is a fast physical layer retransmission scheme with chase combining and incremental redundancy (Hybrid Automatic Repeat Request, H-ARQ), which provides time diversity. In addition, HSDPA facilitates fast packet scheduling at the Node B (Base Transceiver Station), which enables the possibility to consider the fast variations of the radio channel in the scheduling decisions. The goal is to evaluate the network capacity gain of transmit and receive diversity techniques in HSDPA, conditioned on the selected packet scheduling algorithm. Based on the specifications on this system, a multi-cell dynamic network simulator has been built. This simulator includes the basic propagation mechanisms, models for all the considered transmit/receive diversity schemes, support for different modulation and coding schemes, a power controlled associated channel for each packet user, LA with delays and measurement errors, retransmissions in the physical layer with soft combining of the retransmissions, models for the soft combining loss, stop and wait functionality to manage the retransmission scheme, simple traffic models and different packet scheduling algorithms. Results have been obtained for the HSDPA cell throughput in a Pedestrian A environment at 3 km/h for three different schedulers and three transmit and receive diversity techniques when half of the power and code resources are reserved for HSDPA; for the transmit and receive diversity techniques we included Space-Time Transmit Diversity (STTD), Closed Loop Transmit Diversity (CLTD), and dual-antenna Rake (2Rake). The combination of CLTD and 2Rake gives the highest throughput irrespective of the chosen scheduler. In general, the overall highest value is achieved with proportional fair scheduling. (Juan Ramiro Moreno, Preben E. Mogensen; Klaus I. Pedersen, Nokia)