CPICH

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CPICH stands for Common Pilot Indicator CHannel in UMTS and some other CDMA communications systems. In WCDMA FDD cellular systems, CPICH is a downlink channel broadcast by Node Bs with constant power and of a known bit sequence. Its power is usually between 5% and 15% of the total Node B transmit power. Commonly, the CPICH power is 10% of the typical total transmit power of 43 dBm. The Primary Common Pilot Channel is used by the UEs to first complete identification of the Primary Scrambling Code
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  CPICH stands for Common Pilot Indicator CHannel in UMTS and some other CDMAcommunications systems.In WCDMA FDD cellular systems, CPICH is a downlink channel broadcast by NodeBs with constant power and of a known bit sequence. Its power is usually between 5%and 15% of the total Node B transmit power. Commonly, the CPICH power is 10% of the typical total transmit power of 43 dBm.The Primary Common Pilot Channel is used by the UEs to first complete identificationof the Primary Scrambling Code used for scrambling Primary Common ControlPhysical Channel (P-CCPCH) transmissions from the Node B. Later CPICH channelsprovide allow phase and power estimations to be made, as well as aiding discovery of other radio paths. There is one primary CPICH (P-CPICH) for each Cell, which istransmitted using spreading code 0 with a spreading factor of 256, notationally writtenas Cch,256,0[1]. Optionally a Node B may broadcast one or more secondary commonpilot channels (S-CPICH), which use arbitrarily chosen 256 codes, written as Cch,256,nwhere 0 < n < 256.The CPICH contains 20 bits of data, which are either all zeros, or in the case that Space-Time Transmit Diversity (STTD) is employed, is a pattern of alternating 1's and 0's fortransmissions on the Node B's second antenna[2]. The first antenna of a base stationalways transmits all zeros for CPICH.A UE searching for a WCDMA Node B will first use the primary and secondarysynchronisation channels (P-SCH and S-SCH respectively) to determine the slot andframe timing of a candidate P-CCPCH, whether STTD is in use, as well as identifyingwhich one of 64 code groups is being used by the cell. Crucially this allows to UE toreduce the set of possible Primary Scrambling Codes being used for P-CPICH to only 8from 512 choices. At this point the correct PSC can be determined through the use of amatched filter, configured with the fixed channelisation code Cch,256,0, looking for theknown CPICH bit sequence, while trying each of the possible 8 PSCs in turn. Theresults of each run of the matched filter can be compared, the correct PSC beingidentified by the greatest correlation result.  Once the scrambling code for a CPICH is known, the channel can be used formeasurements of signal quality, usually with RSCP and Ec/N0. Timing and phaseestimations can also be made, providing a reference that helps to improve reliabilitywhen decoding other channels from the same Node B.Pilot signals are not a requirement of CDMA, however, they do make the UE's receiversimpler and improve the reliability of the system.
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