QAM

I am investigating some of the proposals for 802.11n for a part time
University course and wondered if someone could help me please?

At the moment I am trying to see how certain modulation techniques increase
the data throughput at the physical layer and how these are calculated.

For example, (from the WWiSE proposal) a 64 QAM, with a code rate of 7/8,
gives 63, 126, 189, 252 Mbps using 20 MHz on 1,2,3,4 spatial streams. What
is a spatial stream and is their sufficient information on the above data to
calculate the data rate? If so, what is the formulae to calculate the data
rate please?

Thanks,

Troy