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Standards rarely explain why they chose particular values or settled on a particular design. My guess is that the slot duration was chosen as a trade-off between (a) wanting it to be long enough to be able to carry a sufficient number of data bits, and (b) short enough to minimize scheduling delays. The actual choice is pretty arbitrary, and a half-millisecond slot (or a millisecond subframe) allows the eNB to schedule transmissions at a faster interval than HSDPA allowed, and it's a nice round number to boot. Note that the number of symbols in a slot (7) is a function of the symbol duration (plus the cyclic prefix), which is itself the inverse of the subcarrier spacing (15 kHz).

Hi Don,

             Thanks for your Explanation...Yes what to you said is correct..I'm not able to find in any 3GPP Spec about this 0.5ms slot reasoning...And may be for latency improvement also they can have this 0.5ms...

One More basic..Sorry for Posting it here...

Do you know the Reason for Choosing 12 Subcarriers per RB,why can't it be more or less..

Why they settled down to 12...

Thanks in advance...

Shankar ganesh

Same sort of answer: it was chosen arbitrarily to give enough resources to carry a reasonable amount of data, while having enough resource blocks in the channel to share them among a reasonable number of users. The bandwidth of a resource block is 12 x 15 = 180 kHz, which is almost the size of a GSM radio channel, but that's probably coincidence.

The 0.5 ms duration provides low latency through small scheduling intervals while maintaining low overhead related to higher layer protocols