MIMO in LTE (2) - Rx Diversity receiver combining techniques (1)

As mentioned in an earlier post about MIMO, in Rx diveristy, the receiver needs to combine multiple streams from different antenna into a single stream. The challenge here is how to use "effectively" the information from allmthe antennas. In fact, it is a just a matter of choose the appropriate weight for each received signals (see figure below). There are multiple ways:

Selective Combining (SC):
The receiver selects the antenna with the highest received signal power and ignore observations from the other antennas. w=[1,0,0,0...].

On flat-fading Rayleigh channel, if the channels from each receiving antenna is independent, then:
- with two receive antenna the effective big energy to noise ratio is 1.5 times Eb/N0
- with three receive antennas the effective bit energy to noise ratio is 1.833 times Eb/N0

- with four receive antennas, the effective bit energy to noise ratio is 2 times Eb/N0

MIMO in LTE (1)

MIMO is vital for LTE system's performance. From communication channel point of view, below transmission schemes can be used:

1) SISO - single input single output, in other word, non-MIMO

2) SIMO - A single data stream is input into the channel, multiple outputs from the channel, i.e. Rx Diversity. This is to mitigate the effect of multi-path fading by transmitting same information via multiple antennas. The multiple antennas created sufficiently de-correlated channels, therefore signals can be combined at the receiver end. MRC (Maximum Ratio Combining), SC(Selective Combining), EGC(Equal Gain Combining) or IRC(interference rejection combining) can be used at the receiver.

3) MISO - Multiple layer of data stream is input into the channel, single outputs from the channel, i.e. Tx diversity. Depends on the number of Tx antenna ports, SFBC(Space-Frequency Block Coded) or SFBC+FSTD(Frequency Switch Transmit Diversity) can be used for 2x or 4x Tx diversity respectively.

4) MIMO - Multiple-Input, Multiple-Output channel. At Tx side, serial to parallel mapping maps the data stream to multi-parallel streams, and at the Rx side, parallel to serial mapping combine the received data streams back into a single stream.