LTE is the next generation of all-IP mobile communication system designed and developed by 3GPP. It offers unprecedented data transmission speed and low latency to support a variety of applications and services. However, compared to wireline networks, efficient QoS provisioning for diversified applications in wireless access networks such as LTE is challenging due to unreliable and resource-constrained radio interface. In this paper, we investigate an important problem of downlink resource allocation in recently enhanced LTE-Advanced systems where a newly added feature carrier aggregation provides more flexibility in radio resource management in addition to the existing resource block level packet scheduling. The resource allocation problem can be formulated as a complex combinatorial problem with multiple constraints and is solved every time slot. We decompose this highly complex optimization problem and construct a two-tier resource allocation framework which incorporates dynamic component carrier assignment and backlog based scheduling schemes with intelligent link adaptation. An efficient algorithm is developed to dynamically allocate component carriers to users to achieve load balancing. We also present novel backlog based scheduling policies and weighted-CQI based link adaptation scheme to obtain significantly better throughput and delay fairness. Performance of the proposed schemes is evaluated against the static round-robin component carrier assignment, the well-known proportional fairness scheduling rule and the existing link adaptation scheme. Extensive simulation results demonstrate that our schemes offer both better throughput and delay performance as well as user fairness.