To determine the # of TRUs you need, first to have to know the number of the subscriber the cell will serve (by mean of statistic data or forecast, for example 1000 subscribers), and the average traffic per subscriber (for example 20mErl/sub).
Hence you will calculate the total traffic that the cell must offer:
1000sub * 20mE/sub = 20 Erlang
Suppose that your cell will work at blocking probability GOS=1%.
From here you will have the total traffic channel the cell must provide:
TCHs = 30
You know that 1TRU = 8TCHs, so you will need 4 TRUs for this cell.
After having the # of TRUs, you have to calculate the # of SDCCHs, and so on.
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Brgds