n meeting the LTE requirements, it comes with its own set of challenges. In particular, the antenna has to satisfy stringent linearity requirements and introduces added cost. Different choices of active elements are possible. Tuneable digital capacitor arrays can be used to realise 'variable capacitors' with ranges between fractions of a picofarad to tens of picofarads. Alternatively, conventional high-linearity RF switches can be used to create switchable antenna functionality. The active elements can be used in the matching circuit, which case the function partly overlaps with that of the adaptive matching circuits for the PA discussed below or can be part of the antenna, in which case the active element can be used to change the effective electrical length of the antenna. In any case, the physical dimension of the antenna cannot be reduced below a certain point even using a tuneable antenna: - The instantaneous bandwidth of the antenna must be well above that of the channel (20 MHz for bands above 1 GHz). - If the bandwidth is narrow, a sophisticated feedback loop must be implemented to make sure that the antenna remains tuned correctly in all possible conditions where an external object (head, hand could shift the resonant frequency. The feedback loop is typically required if capacitor arrays are used, as the fabrication tolerance can be quite high. The above feedback loop is easier to implement in the uplink (UE TX) than the downlink (UE RX), however, the antenna must have enough instantaneous bandwidth SO that if it is optimally tuned in the uplink it is automatically also tuned in the paired downlink frequency. As the bandwidth of the antenna is mostly dictated by the size of the device through some fun- damental physical relation, it is unlikely that innovations in materials can bring groundbreaking advancements in miniaturisation. 5.6.2 MIMO The baseline UE capability as specified in present 3GPP standards is two receive antennas and one transmit antenna. In the downlink, the UE must support receive dive