icensed SCells adheres to the principle of fair channel utilization while identifying improved transmission opportunities across available spectrum. Rel-13 LAA supports two methods for identifying and utilizing secondary chan- nels: (1) prior to transmission, a single random backoff is completed on any carrier along with CCA on other channels; (2) multiple SCells must individually perform random backoff before transmitting simultaneously. The single and multi-carrier LBT schemes are illustrated and compared in Fig. 8.16 for a scenario in which three LAA SCells are assigned a common random backoff counter. The performance of multi-carrier LBT over 80 MHz has been eval- uated and shown in Fig. 8.17. The overall system performance in terms of mean user throughput as a function of served traffic per access point per operator show that, from the coexistence point of view and the impact on the non-replaced Wi-Fi network, both classes of multi-channel LAA LBT schemes are viable and can increase the performance of a multi-carrier Wi-Fi network compared to the case when it is coexisting with another Wi-Fi network [17,23]. Backoff complete, quick End of burst check on other SCells Post-TX backoff SCell 1 SCell 2 Interferer SCell 3 Interferer End of burst Backoff complete, wait until wait Ilimit Post-TX backoff Wait limit SCell 1 SCell 2 Interferer SCell 3 Interferer Figure 8.16 Comparison of LAA multi-carrier LBT access schemes with single and multiple random backoff channels (17,23,31,32). Operation in Unlicensed and Shared Spectrum 877 Downlink throughtput Uplink throughtput Operator Wi-Fi uplink Operator B Wi-Fi uplink (LAA Alt 1) Operator B Wi-Fi uplink (LAA Alt 2) Operator Wi-Fi downlink Operator Wi-Fi downlink Operator LAA Alt Operator Wi-Fi downlink Operator LAA Alt 2 Operator B Wi-Fi downlink Total served traffic per operator per AP (Mbps) Total served traffic per operator per AP (Mbps) Figure 8.17 Mean user throughput versus served traffic per access point per operator for the indoor multi- carrier deployment scen