How 14.4 Mbps derived in HSDPA?

In Rel 5, HSDPA theoretical data rate is 14.4 Mbps.

lets have a look at how this data rate is achieved.

In W-CDMA, the chip rate that is being used is 3.84Mcps. That is equal to 384000 Chips per second. For HSDPA the default spreading factor, SF=16. So for sending one bit, 16 bits will be send. This is done inorder to spread the signla in 5Mhz bandwidth.

Assume that the codecs are using 16QAM modulation scheme, so whle sending one symbol, we effectively sending 4 bits. Now lets have a look at the below shown equation.

HSDPA Data rate = ((No: of chips transmitted in one sec / Spreading factor) * No of bits transmitted in one symbol)) * Number of Scrambling codes that can be decoded by the mobile (UE)
Here No: of chips transmitted in one sec = 384000
Spreading factor = 16
No of bits transmitted in one symbol = 4 (if 16 QAM is being used.)
Number of Scrambling codes that can be decoded by the mobile =15.


Now just perform the simple math, andHSDPA data rate = (( 384000/ 16 )*4)15 = 1440000 i.e is 14.4Mbps.

Assume that QPSK modulation scheme is used in the codecs, so no of bits per symbol will become 2, and the HSDPA data rate will be 7.2 Mbps.
No of Scrambling codes that can be decoded by the UE depends on the UE category.
A Cat 15 UE can decodes upto 15 Scrambling codes, and there by it can attain a max of 14.4 Mbps.

Additional Information:
In Rel 7 onwards 64 QAM is also implemented. Then 6 bits can be transmitted in each symbol. Thereby attaining a data rate of 21.6 Mbps [ (( 384000/ 16 )*6)15 ]
For more info regarding UMTS Ch9ip rates please refer 3Gpp spec - 25.211.