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Friday, December 16, 2011
Thursday, December 15, 2011
Wednesday, December 14, 2011
Short summary of the functions and features of MIMO
Multiple Input Multiple Output (MIMO) may
in a most generalized way be viewed as the use of pre-coding with multiple
antennas at both the transmitter and the receiver arranged to operate in one of
the 2 following ways:
- Single stream: both diversity and array (beamforming) gain can
be accomplished for increasing the carrier-to-interference-ratio which is
typically used in bad channel conditions or at the cell edge borders for
increased coverage. The pre-coding weights are selected such that the data
streams from the two antennas can be combined coherently into a single
stream. A new and modified type of CQI is used that consists of 5 bits
carrying the Pre Coding Information (PCI) and the CQI itself.
- Dual stream: Transmission in multiple layers or streams for
increasing the maximum achievable data rate. This is also known as spatial
multiplexing and requires high carrier-to-interference-ratio and is therefore
typically only used near the nodeB and in good channel conditions.
Pre-coding is used in this case to create two orthogonal data streams that
can carry separate flows of information. By choosing the weights for the
second stream as the orthogonal eigenvectors of the covariance matrix at
the receiver, the two streams will not interfere with each other and the
bit rate may be doubled in this way. The physical layer HARQ processing
for each stream is identical to the single stream case meaning that one
ack/nack is transmitted for each stream. The CQI in this case has been
extended to 8 bits and contains separate information for each flow.
All devices supporting MIMO has to be capable
of receiving 15 channelization codes. The system has been extended in later/coming releases for higher number of layers/ranks each requiring an additional pair of transmit/receive antennas and allowing for a doubling in the bitrate for each doubling in the layers/rank.
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