OPTIMIZE CPICH AND CELL PARAMETERS IN 3G+ AND LTE E-UTRAN

I-             UTRAN CELL PARAMETERS OPTIMIZATION

There are numerous configurable base station parameters which influence strongly the system and determine the capacity of the network:

·         CPICH power

·         Antenna setting (tilt, azimuth, height, antenna pattern)

·         Soft handover parameters

1-  THE CPICH

1.1        RADIO NETWORK ACCESS

CPICH stands for Common Pilot Channel in 3G and some other CDMA communications systems. It provides to cell:

• Initial system synchronization

• Channel estimation for the dedicated channel

After turning on the power and while roaming in the network, a mobile phone determines its serving cell by choosing the best CPICH signal. In WCDMA FDD cellular systems, CPICH is a downlink channel broadcast by NodeB with constant power and of a known bit sequence. Its power is usually between 5% and 15% of the total NodeB transmit power. A common the CPICH power is 10% of the typical total transmits power of 43 dBm.
The Primary Common Pilot Channel is used by the UEs 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), which is transmitted using spreading code 0 with a spreading factor of 256. Optionally a NodeB may broadcast one or more secondary common pilot channels (S-CPICH), which use arbitrarily chosen 256 codes, written as Cch,256,n where 0 < n < 256.

A UE searching for a NodeB will first use the Primary and Secondary Synchronization Channels (P-SCH and S-SCH respectively) to determine the slot limits and frame timing of a candidate P-CCPCH. The code of the P-SCH is the same for all cells and all MNO. There are 512 set of 16 Scrambling code (For 1 frame  = 15 secondary codes for S-SCH + 1 primary scrambling code for P-CPICH). Searching for the primary scrambling code is reduced to the set of 8 possible Primary Scrambling Codes per group. Indeed, there are 64 groups of 15 S-SCH codes (1 frame per group of S-SCH code). From 512 choices, only 1 scrambling primary code will be chosen within the 8 possible. At this point the correct Primary Scrambling Code can be determined through the use of a matched filter, configured with the fixed channelization code, 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, and the mobile synchronized, it can receive on the P-CCPCH (BCH transport channel) system information of the serving cell. The channel can be used for measurements of signal quality, usually comprising of RSCP and Ec/Io. Timing and phase estimations can also be made, providing a reference that helps to improve reliability when decoding other channels from the same NodeB.

Thus, CPICH power determines the cell coverage capacity. Incerasing or decreasing the CPICH power will enlarge or shrink the cell coverage area. Therefore, by suitably adjusting the CPICH power of the NodeB, the number of users per cell can be balanced among neighboring cell, which reduces the inter-cell interference, stabilizes network operation and ease radio resource management. But CPICH value too high can also decrease cell coverage capacity because of interference with neighboring cell (pilot pollution), while a too low value can create “hole coverage”, area where CPICH power is weak for the mobile phone decode the signal. Mobiles in this area are so not covered.

1.2        COVERAGE OPTIMIZATION BY CPICH

CPICH value parameter optimization can provide good cell coverage, especially indoor coverage:

• Increasing CPICH power (With up to 2dB) may perform enhanced indoor coverage as long as it won’t bring congestion, and we are not in a soft handover area. A scope need to be defined when planning a network. For indoor coverage, depending on if you want to cover first wall (window) or a deep penetration, you may respectively increase your CPICH power of a scope of 10dB or up to 25-30dB.

• Closely monitor your congestion level when increasing CPICH power. If there are lots of users in the area, your cell may be congested.

• It would always better to add more sites and possibly repeater, especially if your buildings belongs to a dense area with important clients and it is in a corporate setting without affecting performance on the ground outside. No miracle can be done without sufficient NodeB. We can’t absolutely expect indoor coverage with 2-3 Km distance between 2 NodeB. This distance must be reduced to 700m, 1km max to expect indoor coverage.