MIMO Operation s marred by Inter Channel Interference and Inter Antenna Synchronisation, because it involves simultaneous transmission/reception on Multiple Antennas. The transmission is pre-coded according to the PMI (Precoding Matrix Indicator) which is used at the receiver to identify the transmission from each of the antennas.
The ICI mitigation and synchronisation requirement make the receptor design too complicated and also limit the gain to 1.5X rather than the theoretical 2X for a 2X2 MIMO.
An improvisation in the Modulation used for MIMO operation is proposed in a IEEE paper which will not require simultaneous transmission/reception. Only one antenna will transmit and one antenna will receive the signal. This will remove the ICI and the requirement for antenna synchronisation, while still giving increased throughput with the use of MIMO (the increase will be less than the theoretical conventional MIMO, but will match the practical gain).
The new modulation technique uses the antenna as a modulation unit (called Space Modulation).
The Space Modulated Symbol is formed by the combination of the conventional modulation symbol (QPSK or 16-QAM) and the index of the antenna used for transmission.
Example for No of Tx Antenna = 4
With 4 antennas the transmission can be from antenna 1,2,3 or 4. The symbols therefore are:
With QPSK used for Modulating the bit stream, the no of bits in the Space Modulated Symbol = 2(QPSK)X2=4. This is same as 16-QAM. Therefore using Space Modulation on top of QPSK will result in 2X data speed, same as in 16-QAM.
(QPSK Symbol)
The increase in the data rate is logarithmic with the number of Tx Antennas in use.As compared to MIMO, where the increase in data rate is linear with no of antennas.
MIMO
New Data Rate = (No of Antenna)x(Original Data Rate)
MIMO with Space Modulation
New Data Rate = log2(No of Antenna)x(Original Data Rate)
For DL only MIMO operation, the design of the UE receiver can be greatly simplified. Multiple Rx antennas will provide diversity gain, and a single Rx antenna will also work.
The ICI mitigation and synchronisation requirement make the receptor design too complicated and also limit the gain to 1.5X rather than the theoretical 2X for a 2X2 MIMO.
An improvisation in the Modulation used for MIMO operation is proposed in a IEEE paper which will not require simultaneous transmission/reception. Only one antenna will transmit and one antenna will receive the signal. This will remove the ICI and the requirement for antenna synchronisation, while still giving increased throughput with the use of MIMO (the increase will be less than the theoretical conventional MIMO, but will match the practical gain).
The new modulation technique uses the antenna as a modulation unit (called Space Modulation).
The Space Modulated Symbol is formed by the combination of the conventional modulation symbol (QPSK or 16-QAM) and the index of the antenna used for transmission.
Example for No of Tx Antenna = 4
With 4 antennas the transmission can be from antenna 1,2,3 or 4. The symbols therefore are:
With QPSK used for Modulating the bit stream, the no of bits in the Space Modulated Symbol = 2(QPSK)X2=4. This is same as 16-QAM. Therefore using Space Modulation on top of QPSK will result in 2X data speed, same as in 16-QAM.
(QPSK Symbol)
 |
| 4 Tx Antenna adding 2 bits to the Space Modulated Symbol |
The increase in the data rate is logarithmic with the number of Tx Antennas in use.As compared to MIMO, where the increase in data rate is linear with no of antennas.
MIMO
New Data Rate = (No of Antenna)x(Original Data Rate)
MIMO with Space Modulation
New Data Rate = log2(No of Antenna)x(Original Data Rate)
For DL only MIMO operation, the design of the UE receiver can be greatly simplified. Multiple Rx antennas will provide diversity gain, and a single Rx antenna will also work.