HSPA+ R7 supports 2x2 downlink MIMO that uses two transmit
antennas at the Node B to transmit orthogonal (parallel) data streams to
the two receive antennas at the device. Using two antennas and
additional signal processing at the receiver and the transmitter, MIMO
can increase the system capacity and double user data rates without
using additional Node B power or bandwidth. Additionally, MIMO
beamforming provides gains for cell edge users where parallel MIMO
streams may not be possible.
To be most effective, parallel MIMO streams need a high signal-to-noise
ratio (SNR) at the device and a rich scattering environment. High SNR
ensures that the device will be able to decode the signal successfully
and a rich scattering environment ensures that the two data streams
remain orthogonal. The MIMO benefit is therefore maximized in a dense
urban (city) environment, as there is enough scattering and cell sizes are
small (potentially high SNR at the device). In rural environments with
large cell sizes and less scattering, the MIMO gains will be smaller.
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