instance artificially introducing a coupling line between the two antennas that partially cancel in phase the natural coupling of the two antennas. As at least two antennas have to be embedded, the MIMO requirement has a significant impact on the dimensions and mechanical design of the user terminals. We have seen above that one cannot expect a reduction in the size of the antennas even if they are made tuneable or switchable: so, the mechanical designers have to allocate enough space for two antennas of reasonable size (for instance 40 mm (W) X 8 mm (L) X 6 mm (H) if a full ground plane is placed under the antennas), while taking into account all the constraints required to make the MIMO antenna system work properly. In particular, the placement must ensure that the isolation between antennas is sufficient and pattern correlation is low; moreover, the antennas must not be covered or otherwise affected by the user hand and head in typical use conditions, and comply with specific absorption rate (SAR), and hearing-aid compatibility (HAC) and other regulations. It is worth noting that performance in the presence of the hand(s) but not the head will be of greatest importance in high-speed data mode (e.g. streaming videos); it is not sufficient if just one antenna is not affected by the hand(s), as the 2 X 2 MIMO system will effectively collapse into a lower-rate 2 X 1 MISO channel. All this means that it will be even more important than before that the antenna designers are involved from the concept stage of any new terminal design. To summarise, the decision to deploy LTE in the lower frequency (<1 GHz) bands has created enormous pressure on the antenna design for the user terminals, and set off a race between designers to produce effective solutions, although most of these solutions have yet to be proved in real devices and real networks. Hopefully, the high data rate will demand large displays, and SO the typical device size is bound to grow, mitigating some of the problems and challenges. 27 26 At higher frequen