ort channels noted above are mapped on to the physical channels. These physical channels are a lay- ered structure of radio frames and time slots carrying information related to the physical layers. The physical channels are identified by a specific carrier frequency, codes (chan- nelisation/scrambling), timings, etc. There are two dedicated channels and five common (see Table 7.3). The physical channels noted in this table are the ones on which the transport channels are mapped; there are in fact a few more common channels, such as synchronisation channels (SCH), common pilot channel (CPICH), paging indication channel (PICH), etc. Table 7.3 Physical channels in a WCDMA radio network Channel Abbreviation Function/application Dedicated physical control DPCCH Dedicated higher link information channel such as user data and signalling (UL/DL) is carried on this layer Dedicated physcial data DPDCH Transmits dedicated physical-layer channel control information (UL/DL) Physcial random-access PRACH Transmits the data part of RACH channel and layer 1 control information Physical common packet PCPCH Carries CPCH transport channel channel Physical downlink shared PDSCH Carries DSCH transport channel channel Primary common control PCCPCH Carries BCH transport channel and physical channel contains only data Secondary common control SCCPCH Carries FACH and PCH transport physical channel channels 7.5.3 Uplink and Downlink Modulation Modulation characteristics are different in the uplink and downlink directions. Both direc- tions have a chip rate of 3.84 Mcps and use QPSK modulation. In the uplink direction, dedicated channels are multiplexed using complex coding schemes (also known as I/Q 3G RADIO NETWORK PLANNING AND OPTIMISATION coding), while in the downlink direction they are multiplexed with respect to time. If time- based multiplexing is used in the uplink, then during the DTX, interference that occurs will be audible, which is not the case in the downlink because transmission is continuous with common channels. 7.5.4 Upl