The biggest problem with Antennas for mobiles and now the tablets have been how to arrange antennas for MIMO since the wavelength needs to be λ/4. The picture gives an idea how the antenna size changes with different frequencies. Higher frequencies are better for having multiple antennas as their length and the distance between then decreases.
From a presentation by Shirook M. Ali, RIM in the 4th LTE North America Conference, 8 - 9 November
2011, Dallas, Texas, USA.