etween the system throughput and fairness, but they cannot deal with the above-mentioned problem, since all of them assume that the users can be scheduled on the same number of carriers [5-33]. The base station performs admission control and carrier load balancing in order to allocate the users to different component carriers. Different methods for balancing the load across component carriers are possible and will impact on system performance [10]. Once the users are assigned to a certain component carrier(s), packet scheduling is performed where time-frequency resources are assigned to each of the allocated users on different component carriers. While independent physical layer transmission is assumed, packet scheduling can be performed either independently within each component carrier or jointly across multiple component carriers. The objective is to optimize the resource allocation process with the existence of multiple component carriers. In this chapter, we review the physical layer and MAC layer aspects of bandwidth extension and carrier aggregation schemes that have been utilized in the IEEE 802. 16m and the 3GPP LTE-Advanced [2,21-24]. It will be seen that the two technologies use very similar techniques in order to support larger bandwidths in a contiguous or non-contiguous spectrum. 11.1 PRINCIPLES OF MULTI-CARRIER OPERATION One of the distinctive features of the 4th generation of cellular systems is the ability to operate at extremely large bandwidths. The RF spectrum utilized for such large bandwidth operation may comprise contiguous or non-contiguous bands (virtual wideband operation). The component carriers corresponding to each frequency band can be assigned to unicast and/or multicast and broadcast services. The multi-carrier operation enables control and operation of a number of contiguous or non-contiguous component carriers (several physical layers) using a single MAC instantiation. In FDD mode, each downlink or uplink frequency channel is individually referred to as a component carrier and is