rithm 2 applies to the message part, then we use initially the inner loop power control algorithm 1 in the power control pream- ble, with a step size of 2dB. In either one of the cases, the power control algorithm and step size revert to ones used for the message part as soon as the sign of the TPC commands reverses for the first time [11]. 4.3.7 Downlink Physical Channels 4.3.7.1 Downlink Transmit Diversity Table 4.16 outlines possible applications of open and closed loop transmit diversity modes on different types of downlink physical channel. Simultaneous use of STTD and closed loop modes on the same physical channel is not possible. Furthermore, when Tx diversity applies to any of the downlink physical channels, it also apply to the P-CCPCH and SCH. In addition, the PDSCH and the DPCH associated with this PDSCH shall use the same transmit diversity mode. A transmit diversity mode (open loop or closed loop) on the associated DPCH may not change while the duration of the PDSCH frame, and within the slot prior to the PDSCH frame. Nevertheless, changing from closed loop mode 1 to mode 2 or vice versa, is possible. Table 4.16 Application of Tx Diversity Modes on Downlink Physical Channel Types Physical channel type Open loop mode Closed loop P-CCPCH S-CCPCH PDSCH CSICH V, may apply; X, does not apply. 4.3.7.2 Open Loop Transmit Diversity Space Time Block Coding Based Transmit Antenna Diversity (STTD) employs a space time block coding based transmit diversity. It is optional in UTRAN but mandatory at the UE. STTD encoding works on blocks of four consecutive channel bits. Figure 4.15 illustrates a block diagram of a generic STTD encoder for channel bits bo, b1, b2, b3. Channel coding, rate matching and interleaving occurs as in the non-diversity mode. The bit bi has real valued {0} for DTX bits and {1, -1} for all other channel bits. The UTRA Physical Layer Design Time Switched Transmit Diversity for SCH (TSTD) can apply to the SCH; like STTD, it is optional in UTRAN but mandatory in the UE. 4.3.7.3 Closed Loop Tran