ario with FTP traffic using up to 80 MHz transmission bandwidth 17,23]. As mentioned earlier, LBT is a mechanism used by devices operating in the unlicensed bands to determine the presence of other signals in the channel prior to transmission and to avoid collisions with other transmissions. This protocol allows several users and different technologies to use the same channel without pre-coordination. In LAA, the LBT procedure is initialized when the SCell has data to transmit. Prior to that, the cell is completely turned off to all transmissions, including cell reference signals. Then, the LAA cell will initiate CCA to determine if the channel is idle. If there are no signals detected in the channel, then the transmission can proceed. This procedure is illustrated in Fig. 8.18. If the channel is not idle, the device performs a slotted random backoff procedure, in which a random number of slots are withdrawn from the contention window. The contention window is increased expo- nentially with the occurrence of collisions and is reset to the minimum value once the trans- mission succeeds. Given the random nature of the backoff procedure, different devices will have different backoff intervals, improving channel adaptation [14,20]. 8.1.4.2 LTE-Wi-Fi Aggregation An alternative approach to deploying LTE in the unlicensed spectrum, which was more acceptable to the Wi-Fi industry, was through the aggregation of LTE and Wi-Fi carriers. This solution was meant to enhance LTE performance by partly offloading traffic to an available Wi-Fi link. In this scheme, an LTE payload is split at the PDCP level and some traffic is tunneled over the Wi-Fi link while the remaining traffic is sent over the native LTE connection. The LWA approach uses Wi-Fi access points to augment LTE RAN by encapsulating LTE data in IEEE 802.11 MAC frames, such that it appears as a Wi-Fi frame 878 Chapter 8 Frame-based equipment (FBE) LBT Fixed frame period Fixed frame period Fixed frame period CCA failure CCA success CCA failure CCA success CCA period 