6.8 Design and Verification Challenges of MIMO
This section describes the design and verification challenges associated with spatial multiplexing and diversity-enabled radios for LTE. The impact of this technology on transmitter and receiver hardware and software will be considered as well as the effects of different antenna arrangements.
It has been said that MIMO is a baseband (digital coding) challenge. There is some truth to this belief, but not all situations are so straightforward. When designs become more compact, the isolation and performance of RF and analog circuits are not ideal. RF currents are notorious for leaking onto non-RF circuits, and the effects can be difficult to filter out. As a result, MIMO performance degrades. The transmit and receive antennas are key components in the signal chain. Their performance is not easy to model, and in the most complex beamforming cases it is necessary to use real-time calibration mechanisms to maintain performance. The receivers in the UE have a particularly demanding task, since they have to recover the MIMO signal using algorithms that balance performance with space in the baseband IC (BBIC). Also, the UE has to report accurate real-time channel state information to the eNB. There are other important factors that impact MIMO performance indirectly, such as battery power consumption in the UE. These factors are dealt with in Section 6.13 on battery drain testing.
To work in practice, spatial multiplexing ...