CPICH
Encyclopedia
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 UE
s to first complete identification of the Primary Scrambling Code used for scrambling Primary Common Control Physical Channel (P-CCPCH
) transmissions from the Node B. Later CPICH channels provide 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 is transmitted using spreading code
0 with a spreading factor of 256, notationally written as Cch,256,0. Optionally a Node B may broadcast one or more secondary common pilot channels (S-CPICH), which use arbitrarily chosen 256 codes, written as Cch,256,n where
.
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 for transmissions on the Node B's second antenna. The first antenna of a base station always transmits all zeros for CPICH.
A UE searching for a WCDMA Node B will first use the primary and secondary synchronisation channels (P-SCH and S-SCH respectively) to determine the slot and frame timing of a candidate P-CCPCH, whether STTD is in use, as well as identifying which one of 64 code groups is being used by the cell. Crucially this allows to UE to reduce the set of possible Primary Scrambling Codes being used for P-CPICH to only 8 from 512 choices. At this point the correct PSC can be determined through the use of a matched filter
, configured with the fixed channelisation code Cch,256,0, looking for the known CPICH bit sequence, while trying each of the possible 8 PSCs in turn. The results of each run of the matched filter can be compared, the correct PSC being identified by the greatest correlation result.
Once the scrambling code for a CPICH is known, the channel can be used for measurements of signal quality, usually with RSCP
and Ec/N0. Timing and phase
estimations can also be made, providing a reference that helps to improve reliability 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.
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
DBm
dBm is an abbreviation for the power ratio in decibels of the measured power referenced to one milliwatt . It is used in radio, microwave and fiber optic networks as a convenient measure of absolute power because of its capability to express both very large and very small values in a short form...
.
The Primary Common Pilot Channel is used by the UE
UE
Ue may refer to:* Ue , a digraph used in some writing systems. Its variant Ü is a letter representing a vowel in the Estonian, German, Hungarian, and Turkish alphabets, and in pinyin* Ue , letter in many Asian languages...
s to first complete identification of the Primary Scrambling Code used for scrambling Primary Common Control Physical Channel (P-CCPCH
CCPCH
CCPCH stands for Common Control Physical CHannel in UMTS and some other CDMA communications systems. It is a broadcast radio channel by which a mobile phone or User equipment can decode and determine important system parameters before establishing a dedicated communications link.In a FDD UMTS...
) transmissions from the Node B. Later CPICH channels provide 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 is transmitted using spreading code
Spread spectrum
Spread-spectrum techniques are methods by which a signal generated in a particular bandwidth is deliberately spread in the frequency domain, resulting in a signal with a wider bandwidth...
0 with a spreading factor of 256, notationally written as Cch,256,0. Optionally a Node B may broadcast one or more secondary common pilot channels (S-CPICH), which use arbitrarily chosen 256 codes, written as Cch,256,n where
.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 for transmissions on the Node B's second antenna. The first antenna of a base station always transmits all zeros for CPICH.
A UE searching for a WCDMA Node B will first use the primary and secondary synchronisation channels (P-SCH and S-SCH respectively) to determine the slot and frame timing of a candidate P-CCPCH, whether STTD is in use, as well as identifying which one of 64 code groups is being used by the cell. Crucially this allows to UE to reduce the set of possible Primary Scrambling Codes being used for P-CPICH to only 8 from 512 choices. At this point the correct PSC can be determined through the use of a matched filter
Matched filter
In telecommunications, a matched filter is obtained by correlating a known signal, or template, with an unknown signal to detect the presence of the template in the unknown signal. This is equivalent to convolving the unknown signal with a conjugated time-reversed version of the template...
, configured with the fixed channelisation code Cch,256,0, looking for the known CPICH bit sequence, while trying each of the possible 8 PSCs in turn. The results of each run of the matched filter can be compared, the correct PSC being identified by the greatest correlation result.
Once the scrambling code for a CPICH is known, the channel can be used for measurements of signal quality, usually with RSCP
RSCP
In the UMTS cellular communication system, received signal code power denotes the power measured by a receiver on a particular physical communication channel. It is used as an indication of signal strength, as a handover criterion, in downlink power control, and to calculate path loss...
and Ec/N0. Timing and phase
Phase (waves)
Phase in waves is the fraction of a wave cycle which has elapsed relative to an arbitrary point.-Formula:The phase of an oscillation or wave refers to a sinusoidal function such as the following:...
estimations can also be made, providing a reference that helps to improve reliability 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.
Further reading
- The authoritative definition for CPICH in UMTS is given by ITUItuItu is an old and historic municipality in the state of São Paulo in Brazil. The population in 2009 was 157,384 and the area is 641.68 km². The elevation is 583 m. This place name comes from the Tupi language, meaning big waterfall. Itu is linked with the highway numbered the SP-75 and are flowed...
and ETSI project 3GPP3GPPThe 3rd Generation Partnership Project is a collaboration between groups of telecommunications associations, known as the Organizational Partners...
25.211 Physical channels and mapping of transport channels onto physical channels (FDD)
- 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)