CPICH

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CPICH stands for Common Pilot CHannel in UMTS and some other CDMA communications systems.

In UTRA FDD CPICH is a downlink channel broadcasted by Node-Bs with constant power. Its power usually is between 5% and 15% of the total Node-B transmit power of 43 dBm. Ie, it typically varies from 30 dBm to 34.8 dBm, although 10% or 33 dBm is most commonly used.

The pilot channel is used by the UEs to identify the scrambling code transmitted by the Node-B. There is one primary CPICH (P-CPICH), which is transmitted on spreading code 0, Spreading Factor (SF) 256 (written as C_ch,256,0), and optionally one or more secondary CPICH (S-CPICH, which are transmitted on an arbitrary chosen SF 256 codes.

A UE scanning for the presence of WCDMA signals searches through all the scrambling codes and for each code attempts to descramble the pilot. Each pilot transmits the same sequence of bits (sequence of 20 zeros in each slot for the first antenna, and an alternating sequence of 00111100001111000011 and its inverse between even and odd slots for the second antenna), therefore, the UE identifies the correct scrambling code once the descambled bits matches the known sequence.

Once the pilot is descrambled it can be used for measurements of signal quality, usually RSCP and E_c/I₀, and for timing reference when decoding other channels from the same Node-B.

Pilot signals are not a requirement of CDMA, however, they do make the UE's receiver simpler and improve the reliability of the system.

[edit] References and further reading

• A definition for CPICH in UMTS can be found in 3GPP 25.211 Physical channels and mapping of transport channels onto physical channels (FDD) (This is not an easy to read document as it is intended for an audience with substantial and detailed knowledge of the subject. Like many other standards and specifications, this text merely defines what has been agreed to be a common standard without actually explaining the used terms and categories -- it expects the reader to be familiar with the terminology and the technology in general and in specifics too. There, you have been warned... Apart from the above, this document is perhaps the most definite source of information on channels, CPICH included, in UMTS.)

• A definition for Ec/Io applicable to CPICH can be found in 3GPP 25.133 Requirements for support of radio resource management (FDD)

• Some information on the power of CPICH is given in 3GPP 25.104 Base Station (BS) radio transmission and reception (FDD)

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