73Mbps LTE for new iPad?

I was watching the Apple presentation last week and I found it a bit odd that Philip Schiller quoted an LTE maximum speed of 73Mbps for the new iPad. It is common knowledge that Qualcomm produce category 3 LTE chipsets at the moment which support a theoretical maximum speed of 100Mbps, so why 73Mbps?

Well it turns out it is related to the messy spectrum situation in the U.S. and the fact that the two biggest carriers that will launch the new iPad (Verizon and AT&T), only have 10MHz blocks for LTE at the moment.

LTE allows for flexible carrier allocation (up to 20MHz) and the number of Resource Blocks is tied to the amount of spectrum. With a 10MHz block, 50 Resource Blocks are available.

Looking at 3GPP spec 36.213, we find a table that indicates the maximum transport block size for a given number of resource blocks per 1ms TTI.

If we look at the column for 50 Resource Blocks we find that the maximum TB is 36696 bits.

So 36696 x 1000 = 36696000 bits per second. As LTE allows for 2x2 MIMO, this is doubled to 73392000 bits per second or 73Mbps, which is the figure Apple quoted.

Of course this is a theoretical maximum speed, which requires perfect RF conditions, and the user using all the resources in the cell. Both of course are quite unlikely (unless in a lab) so real speeds can vary.