A Long-Term Evolution (LTE) system offers high peak data rates, low latency, improved system capacity, and low operating cost resulting from simple network architecture. An LTE system also provides seamless integration to older wireless network, such as GSM, CDMA and Universal Mobile Telecommunication System (UMTS). In LTE systems, an evolved universal terrestrial radio access network (E-UTRAN) includes a plurality of evolved Node-Bs (eNBs) communicating with a plurality of mobile stations, referred as user equipments (UEs).
Enhancements to LTE systems are considered so that they can meet or exceed International Mobile Telecommunications Advanced (IMT-Advanced) fourth generation (4G) standard. One of the key enhancements is to support bandwidth up to 100 MHz and be backwards compatible with the existing wireless network system. Carrier aggregation (CA) is introduced to improve the system throughput. With carrier aggregation, the LTE-Advance system can support peak target data rates in excess of 1 Gbps in the downlink (DL) and 500 Mbps in the uplink (UL). Such technology is attractive because it allows operators to aggregate several smaller contiguous or non-continuous component carriers (CC) to provide a larger system bandwidth, and provides backward compatibility by allowing legacy users to access the system by using one of the component carriers.
In a mobile network, the bandwidth requirement of each UE changes with the amount of data the UE is transmitting and receiving. In LTE systems, an eNB may dynamically allocate resources among UEs. Carrier aggregation allows the mobile network to use the bandwidth more efficiently. However, it also increases the complexity of resource management. A light-weighted component-carrier management scheme is thus desirable. One of the issues is how to efficiently activate or deactivate one or more component carriers on a UE. One scenario is upon receiving CC activation/deactivation messages from an eNB, a UE needs to efficiently analyze the messages and make decisions to trigger actions. In other scenarios, the UE can decide to activate a plurality of CCs or deactivate a plurality of CCs based on an internal timer, or a combination of internal CC states and a received CC activation/deactivation message from an eNB.
To make the system more efficient, carrier aggregation also requires changes in scheduling mechanisms, including Buffer Status Report (BSR) and Power Headroom Report (PHR). The BSR procedure provides the serving eNB with information about the amount of data available for transmission in the UL buffers of the UE. It is performed in the medium access control (MAC) layer between the UE and the eNB. When the BSR is triggered at a transmission time interval (TTI), a BSR control element is included in a medium access control-packet data unit (MAC-PDU), or a transport block (TB), to be delivered to the eNB. There are several BSR triggers. A set of events can trigger a regular BSR. When BSR timer expires, it triggers a periodic BSR. A padding BSR is triggered only when there is enough padding bit. CA imposes a question of how to prepare the BSR when all UE grants from all CCs in one TTI are considered. Another scheduling mechanism is PHR. A UE uses the PHR procedure to provide the serving eNB with a power offset between a maximum transmitting power of the UE and a current transmitting power of the UE. With multiple CCs configured and dynamic activation and deactivation for each CC, it is important to consider the impact for the PHR during these CA procedures.
Carrier aggregation also provides new ways to link DL and UL. Without CA configured, each DL CC broadcast a UL CC in System Information Block 2 (SIB2). This is a cell specific UL CC where a UE finds its Physical Radio Access Channel (PRACH) resource and transmits Packet Uplink Control Channel (PUCCH) and Physical Uplink Shared Channel (PUSCH). The network cells all possess this cell specific linking. Since SIB2 linking almost never changes, it can be called static linking. With CA, multiple UL CCs can be configured to a UE. Therefore, besides the static SIB2 linking, new types of DL-UL linking are created.
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