In Long-Term Evolution (LTE), time-frequency resources can be divided into 1 millisecond (ms) subframes, where each 1 ms subframe includes two 0.5 ms slots, and each slot (with normal cyclic prefix (CP) durations) includes seven single carrier frequency-division multiplexing (SC-FDMA) symbols in the time domain for uplink (UL) communication and seven orthogonal frequency-division multiplexing (OFDM) symbols in the time domain for downlink (DL) communication. In the frequency domain, resources within a slot are divided into physical resource blocks (PRBs), where each resource block spans 12 contiguous subcarriers.
In current LTE systems, resources are usually assigned using a 1 ms minimum transmission time interval (TTI) when data is available for transmission between an eNodeB (eNB) and user equipment (UE). This is referred to as dynamic scheduling. Within each scheduled TTI for UL communication, a UE transmits data over a physical uplink shared channel (PUSCH) in PRB-pairs indicated by an uplink grant to the UE from an eNB that schedules the data transmission. For DL communication, the eNB transmits data over a physical downlink shared channel (PDSCH) in PRB-pairs preceded by a DL assignment from the eNB. The DL assignment information is provided to the UE in a control channel, which is generally referred to as a physical downlink control channel (PDCCH).
Discontinuous reception (DRX) functionality is used in LTE to allow UEs to conserve power. Without DRX, a UE is always awake and continuously monitors all subframes of a PDCCH for DL assignments. With DRX, the UE powers down a portion of its circuitry during a DRX sleep mode when there are no expected data packets to be received. The eNB configures DRX with a set of DRX parameters shared with the UE. These DRX parameters can be application dependent such that power and resource savings are maximized. The eNB then schedules DL assignments during periods when the UE is actively monitoring for them.
While DRX operation results in power savings, it comes at the expense of latency. On average, a UE using DRX receives transmissions later as compared to when DRX is not used. This delay can be problematic when the transmissions are associated with a time-sensitive application executing on the UE.
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