MIMO (Multiple-Input Multiple-Output) Solution

MIMO Basics

MIMO systems use multiple input and multiple output antennas operating on a single channel (frequency). At the transmitter side, the signal is space–time encoded and transmitted from N_T antennas. At the receiving side, the signals are received from N_R antennas (see Figure 1). The space–time decoder combines the signal received by the N_R antennas and transmitted from the N_T antennas after having estimated the channel matrix (N_T×N_R).

Figure 1: Generic MIMO block diagram for the downlink

The objective of the MIMO solution is to exploit the space and time diversity of the channels on the different radio paths between each combination of transmit/receive antennas to improve the reception sensitivity and/or to improve the channel capacity. There are several families of MIMO solutions. The two extreme ones are the spatial diversity MIMO schemes and the spatial multiplexing MIMO schemes.

In addition, several MIMO schemes exist that are a mix between SM and spatial diversity schemes. The diversity order and capacity increase depends on the space–time code and number of antennas.

More recently, MIMO schemes using pre-coding have been defined. In these cases, the space–time code depends on a feedback from the receiver on the channel states. Indeed, this solution requires a closed-loop operation and additional signalling between the receiver and the transmitter.

Finally, MIMO can also be generated from signals transmitted from different BSs (virtual MIMO). This requires time synchronisation of the BS but also a synchronisation of the scheduler of the BS involved in the transmission. 

System Design Aspects of BS and MS

As indicated in Figure 1, MIMO operation has a significant impact on the design of the BS and the MS. Indeed, in addition to the MIMO codes, the algorithms for encoding and decoding MIMO signals, there is the requirement to implement several transmitter and several receivers both at the BS and MS sides. This may be critical for the MS side. Indeed, implementing several receiver chains implies several antennas on a small device and additional hardware and processing power. Moreover, several transmitters at the MS side also mean significant additional power consumption. Efficient implementation of MIMO at the MS side has then some technological challenges to be solved.

The preferred antenna configuration for MIMO is when the multiple antennas are perfectly uncorrelated. In that case, the performances of MIMO are optimal. Good correlation is obtained if at the BS side the antennas are separated by 10 to 20 λ and at the MS side by at least λ. The latter requirement makes solutions with more than two antennas at the MS impractical.

In some cases, the additional effort needed for the space–time decoder is significant. This also imposes some limits on the number of transmit antennas at the BS side.