ith LBT requirements. In case the channel is sensed as free according to LBT rules, the eNodeB or UE may resume transmission for, for example, 6-10ms If the channel is sensed as occupied, the eNodeB or UE will need to continue to suspend transmission until the channel is sensed as unoccupied 16 LTE-Advanced Evolution according to LBT rules. In addition to LBT, additional restrictions related to unlicensed band operation include the following: Limitations related to the occupied channel bandwidth: According to European Telecommunications Standards Institute (ETSI) regulations, the Occupied Channel Bandwidth, defined as the bandwidth containing 99% of the power of the signal, shall be between 80% and 100% of the declared nominal channel bandwidth. Limitations related to the maximum power spectral density (PSD): For most cases, the requirement is stated with a resolution bandwidth of 1 MHz. For example, the ETSI 301893 specifications require 10 dBm/MHz for 5150-5350 MHz. Similar limi- tations are involved also in the United States, governed by Federal Communications Commission (FCC). The UE's peak PSD for 5.15-5.725 MHz is 11 dBm/MHz in the United States. These regulatory requirements necessitated a few significant modifications to the LTE air interface. For the downlink, unlike in licensed band operation, all transmissions are omitted unless there is data to transmit for the UE, with the exception of Discovery Reference Signals (DRSs) facilitating, for example, cell search and synchronization. For the uplink, the resource allocation had to be modified to match the requirements related to the occupied channel bandwidth. This was achieved by introducing a block-interlaced resource allocation scheme, where uniformly spaced resources span at least 80% of a carrier. Figure 16.11 illustrates the supplemental downlink solution using the unlicensed band. The licensed band carries both uplink and downlink traffic including all voice and other traffic requiring reliable mobility and high quality of service (QoS). The unlicen