ical Layer Enhancement Besides the dual connectivity, the main areas impacting the physical layer are the use of 256 Quadrature Amplitude Modulation (256QAM), small cell on/off procedure and over the air synchronization between eNodeBs, which are elaborated further on the following sections. 4.6.1 256QAM for LTE Downlink One of the new LTE features introduced in Rel-12 is the support for higher order modulation, that is, 256QAM, for downlink data transmission. The main motivation for introduction of 256QAM is the increase in peak data rates, achievable mainly in small cells. While 256QAM is, in principle, applicable also in macro cells, the required very high SINR of more than 20 dB is not often achieved in macro cell environments. Moreover, achieving high SINR also imposes stringent RF requirement for the eNodeB transmitter, which are easier to meet at lower output power levels, that is, in small cells. Similarly, for the UE, support for 256QAM requires better receivers from RF requirements point of view. From the physical layer point of view, 256QAM is a relatively simple feature. Changes in the specifications relate to extensions in signalling of modulation and coding scheme, transport block sizes as well as to channel quality indicator definition. In terms of peak data rates, the largest transport block size with 256QAM is about 30% larger than that of 64QAM. Correspondingly, new UE categories Cat11-Cat15 have been introduced to accommodate support for 256QAM with increased peak data rates as presented in Chapter 3. A UE with 256QAM support can achieve roughly 200 Mbps per 20 MHz downlink carrier peak data rate. 4.6.2 Small Cell ON/OFF Switching and Enhanced Discovery Switching cells on and off is possible already based on earlier LTE releases and the SON-based schemes standardized in Release 9 have been successfully used especially when the network load is very low. With these schemes, cells in OFF (i.e. dormant) state do not transmit any signals and consequently UEs are not able to detect those cells. To re