Bandwidth Allocation and Traffic Handling
The scheduler resides in the eNodeB to dynamically allocate uplink and downlink resources over the uplink and downlink shard channel U-SCH and D-SCH, respectively. Uplink scheduling is performed per SC-FDMA while downlink is performed for OFDMA. The eNodeB calculates the time-frequency resources given the traffic volume and the QoS requirements of each radio bearer. However, the resources are allocated per UE and not per radio bearer.
The uplink and downlink schedulers are invoked to allocate resources every TTI. The minimum TTI duration is of one subframe length; that is, 1 ms. However, the LTE specification allows adaptive downlink TTI duration where multiple subframes can be concatenated to produce a longer TTI duration. This concatenation reduces the overhead for higher layers. The TTI length can be set dynamically by the eNodeB through defining the modulation and coding scheme used and the size of the resource blocks. Otherwise, it can be set semi-statically through higher layer signaling. Adaptive TTI length can be used to improve the Hybrid Automatic Repeat Request (HARQ) performance or the support of lower data rates and quality of service. In the following two sections we summarize the operation of the downlink scheduler and uplink scheduler.
The unicast downlink transmission is carried over the shared downlink channel (D-SCH) and the operation takes place at the MAC layer of eNodeB. At each TTI, ...