UMTS downlink cell power include: CPICH fixed power + PDSCH dynamic shared between users.
If RRU power increased from 40W to 80W, CPICH power keep as 4W. So, remaining power available of PDSCH is increased from 36W to 76W.
CPICH is always transmitted but PDSCH is transmitted based on user traffic/TBS/code used.
CPICH’s EcNo is measured at UE = CPICH RSCP / total RSSI
Because CPICH power is kept unchaged, so CPICH RSCP measured at UE is the same as before. But power available for PDSCH increased, so PDSCH power transmitted to users may increased, total RSSI measured at UE increased. That’s why CPICH EcNo decreased, but DL throughput increased 50%.
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