UMTS Closed Loop Mode Transmit Diversity Used in DPCH and HS-PDSCH.
The closed-loop-mode transmit diversity can only be applied to the UMTS downlink channel if there is an associated UMTS uplink channel. Thus this mode can only be used with UMTS dedicated channels. The chief operating principle of the closed loop mode is that the UMTS UE can control the transmit diversity in the base station by sending adjustment commands in FBI bits on the UMTS uplink DPCCH. The UE uses the UMTS base station’s common pilot channels to estimate the channels separately. Based on this estimation, it generates the adjustment information and sends it to the UMTS UTRAN to maximize the UE’s received power.
There are actually two modes in the UMTS closed-loop Mode Transmit Diversity method. In mode 1 only the phase can be adjusted and in mode 2 the amplitude is adjustable as well as the phase. Each uplink UMTS time slot has one FBI bit for closed-loop-diversity control. In mode 1 each bit forms a separate adjustment command, but in mode 2 four bits are needed to compose a command.