HSPA Data Rates Calculation

In UMTS, in each slot the maximum number of bits transmitted is 2560. The correct term to use is chips rather than bits. If you want to know where this 2560 comes from or why chips then please refer 3GPP TS 25.211
There are 15 slots per 10ms frame so since the TTI for HSDPA is 2ms, there will be 3 slots. So there will be a total of 7680 chips.
QPSK has 2 bits per symbol = 7680 * 2 chips for 2ms = 15360 chips/2ms = 15360 * 1000 /2 chips per second
Now the SF is fixed at 16
= (15360 * 1000) / (2 * 16)
= 480 Kbps
Terminal that uses 15 QPSK codes will get 480 * 15 = 7.2Mbps
On other hand 16 QAM will have 4 bits per symbol so the rate would be 7.2 * 2 = 14.4Mbps

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