4G LTE: MIMO and MRC

MIMO and MRC:-

    o LTE system uses MIMO (multiple input multiple output) as advanced multiple antenna technique in order to achieve the ambitious requirements for throughput and spectral efficiency.

    o maximal ratio combining (MRC) : is used to enhance link reliability in challenging propagating conditions when signal strength is  low and multipath conditions are challenging. 

 

 Downlink MIMO:-

    o a 2x2 configuration for MIMO i.e. 2 transmit antennas at the base station and 2 receive antennas at the terminal side. 

    o Configurations with 4 antennas are also being considered.

    o In downlink MIMO is used in two modes 

        - Spatial multiplexing: allows to transmit different streams of data simultaneously on the same downlink resource block(s) >>>> increase the data rate

        -  Transmit diveristy : Here the receiver will has many copies of the transmitted signal 

        -   In case the channel conditions do not allow spatial multiplexing, a transmit diversity
scheme will be used instead, so switching between these two MIMO modes is possible
depending on channel conditions. 

  Uplink MIMO:-

    o Uplink MIMO schemes for LTE will differ from downlink MIMO schemes to take into account
terminal complexity issues.

   o For the uplink, MU-MIMO can be used.

   o Multiple user terminals may transmit simultaneously on the same resource block. 

   o This is also referred to as spatial domain multiple access (SDMA).