Hi,
That's exactly the question, why is this trade-off?
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Not sure I understand the question you are asking. It all depends on energy per bit. (not chip) Energy being power over time period. CDMA allows a given amount of bit energy to be spread over any amount of spectrum. Even though the energy per Hz goes down for wider spectrum, the energy per re-correlated bit remains nearly the same. Raise the data rate and you must raise the power to maintain same energy per bit.