The system designer would create a covariance matrix that needs inversion to be used in
the MMSE weight calculation.
Generally speaking, the MIMO technique can be applied across any radio access technology
(TDMA, CDMA, OFDMA). With respect to UMTS, MIMO is being used for HSDPA (specifically
HS-DSCH physical channel) in order to increase the peak throughput data rates. Here data rates on
the order of 20 Mbps can be expected when MIMO is used with HOM. In this application the
architecture is a 2 2 MIMO, there can be a maximum of two spatial data streams, that is, primary
and secondary. The NodeB HS-DSCH transmission builds upon the Closed Loop Transmit Diversity
(CLTD) technique used in the previous releases (see Fig. 9.24).
The first two weights (e.g., w1 and w2) are actually signaled back to the NodeB whereas the last
two weights can be derived from the first two. This reduces the uplink signaling required for the UE
to adjust the NodeB transmit antenna weights. The two data streams will most likely have different
modulation schemes operating on them along with differing FEC code rates. It is then expected that
the UE would maintain the CQI for each of the data streams and communicate CQI values to the
NodeB so relative adjustments can be made.
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