The Long Term Evolution Advanced standard, also known as LTE Advanced, LTE-Advanced or LTE-A, is a mobile communication standard adopted by the 3rd Generation Partnership Project (3GPP) as a major enhancement of the LTE standard. The LTE-Advanced standard requires downstream transmission rates up to 1 Gbps and upstream transmission rates be up to 500 Mbps. However, a system bandwidth of 20 MHz as defined in the Release 8 standard is insufficient to meet the requirement of high transmission rates. Therefore, the LTE-Advanced system bandwidth is extended to 100 MHz in a time-division duplexing (TDD) model and to 40 MHz in a frequency-division duplexing (FDD) model to support the requirement of high transmission rates. Accordingly, carrier aggregation technology is proposed in the LTE-Advanced standard to support aggregation of multiple continuous or non-continuous frequency bands to achieve a resultant bandwidth of up to 100 MHz in the TDD model or 40 MHz in the FDD model for the entire system.
Under the carrier aggregation technology, each constituent carrier of the system bandwidth is called a component carrier (CC). In communication systems prior to the LTE-Advanced standard, multiple users within a distinct cell share one component carrier and the component carrier scheduling is based on one component carrier. However, with the carrier aggregation technology implemented in an LTE-Advanced system, carrier aggregation scheduling based on multiple component carriers, whether inter-band continuous or non-continuous, is required. The basic function of carrier aggregation is about whether and how to schedule a component carrier for a user. According to an established standard of 3GPP specification R10, currently there are two main types of component carrier scheduling: independent scheduling and joint scheduling.
With independent scheduling (also referred to as individual scheduling), a given user equipment attached to aggregated multiple component carriers can be scheduled by multiple schedulers. A typical scenario is that oncoming traffic, e.g., a queue of data packets awaiting for transmission, of a carrier aggregation user is loading to a component carrier randomly. There is not necessarily a component carrier selection process. When a given component carrier is selected for the traffic, resource blocks on a physical downlink shared channel (PDSCH) can be scheduled by using any scheduling policies such as, for example, maximum carrier-to-interface (Max C/I), proportionality fair (PF), or round robin (RR). If there is a data packet scheduler that is responsible for mapping the data packet(s) awaiting transmission to a corresponding component carrier, the scheduling can be independently conducted between multiple schedulers. Otherwise, one user queue is shared by each scheduler. Distributed control is adopted to decide which of the data packets can be transmitted on its own component carrier.
With joint scheduling, in contrast to independent scheduling, the schedulers implement centralized management of all the user queues and resources. The scheduler determines which component carrier, among multiple component carriers already attached to a given carrier aggregation user, is best-suited for an oncoming traffic according to information exchanged between component carrier schedulers. However, more signaling overhead is involved. This centralized management can make better use of multiple users' diversity gain and a frequency selective gain. Theoretically, joint scheduling tends to have better system performance than independent scheduling yet with greater complexity.
Thus, a fundamental difference between independent scheduling and joint scheduling is whether a component carrier selection process is used. Although independent scheduling tends to be simpler in terms of implementation, resource utilization in a system based on independent scheduling may be lower relative to joint scheduling. Further, although joint scheduling may result in relatively higher resource utilization, implementation of joint scheduling tends to be more complex.
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