4x4 MIMO - The almost forgotten LTE feature?
When LTE was standardized by 3GPP in Rel. 8 (in 2009), 4x4 MIMO was part of the feature set. It did not play a role at all at that time since LTE rollout in a basic configuration with two transmitters at the base station and two receivers in the UE (LTE USB sticks in the early days, later smartphones) was already sufficiently complex. The realization of four receivers in a smartphone was a technological blocking point for the ecosystem in these early LTE days. Nowadays, the pressure for higher capacity is increasing and smartphone designs (and chipsets) have improved significantly, so that commercial 4x4 MIMO capable smartphones are entering the market. Mobile network operators can now exploit the higher spectral efficiency and higher network capacity of 4x4 MIMO by also investing on the base station side (third and fourth transmitter path).