Analysis of fast alpha switching for closed loop mode 1 transmit diversity with high speed downlink packet access

We present an analysis of the fast alpha switching (FAS) technique in WCDMA using closed loop (CL) mode 1 transmit diversity and utilizing the high speed physical downlink shared channel (HS-PDSCH) for data service. When applied on the HS-PDSCH channel, CL mode 1 utilizes the antenna weights used on the associated dedicated physical channel (DPCH). Therefore, when the terminal is in soft handover (SHO) in the DPCH, i.e., connected to more than one sector, the antenna weight, which is optimized for all sectors in the SHO, is hardly optimal for the HS-PDSCH, which is always received from a single sector. In the FAS technique, antenna weight calculation is based on the received pilot from the HS-PDSCH sector when the HS-PDSCH is in use and otherwise calculation is based on the pilots of all sectors in SHO. At first sight, this seems a desirable feature, but several more factors have to be considered before drawing any conclusions. The FAS technique is affected by the weight feedback delay, thus the applied weight is not always optimal for the HS-PDSCH. Moreover, the SHO probability is important for the analysis, since the gain can be achieved only when the terminals are in SHO. Therefore, dynamic system simulations are necessary to study what are the expected gains from the FAS technique in realistic scenarios.