OFDMA and single carrier FDMA have been selected as the downlink and uplink multiple access schemes for the E-UTRA air interface currently being studied in 3GPP (which is a standard based collaboration looking at the Long Term Evolution (LTE) of mobile telecommunication systems). In the LTE system, a base station (eNB) which communicates with a number of user devices (UEs) allocates the total amount of time/frequency resource (depending on bandwidth) among as many simultaneous users as possible, in order to enable efficient and fast link adaptation and to attain maximum multi-user diversity gain.
The LTE system provides for both real time (RT) services, such as VoIP, and non-real time (NRT) services, such as web browsing. The time/frequency resources that allow UEs to use these services may be allocated dynamically (ie on demand at the time that they are needed) or semi-persistently depending on the service used. For example, for real time services such as VoIP, where the amount of resources needed is known in advance, the UEs can be allocated the resources in advance (semi-persistently allocated), for example at the time of the call setup. For more “bursty” traffic, such as web browsing traffic, the resources would be allocated dynamically based on the amount of data to be transmitted between the UE and the eNB at the time.
For the transmission of downlink dynamically allocated data, a scheduler in the eNB has the flexibility to dynamically send data on a subframe-by-subframe basis, to pick the preferred transmission parameters (e.g. resource allocation, Modulation and Coding Scheme (MCS), Hybrid Acknowledge Repeat Request (HARQ), Power, etc.) to adapt to the changing link conditions between the eNB and the particular UE and to prioritize among users when scheduling on the current subframe.
In the LTE system, the physical downlink control channel (PDCCH) carries the scheduling assignments and other control information. A PDCCH consists of an aggregation of one or more consecutive control channel elements (CCEs), where a CCE occupies a fraction of the eNB's available physical radio resources. The total number of CCEs that are available depends on the system bandwidth of the eNB and the number of OFDM symbols reserved for PDCCH transmission in a given sub-frame.
In LTE Rel-12, 3GPP are considering introducing a new scheduling approach—multi-subframe scheduling; in which the eNB sends a scheduling assignment that persists for several subframes (ie. resources are assigned to a UE for multiple subframes with one scheduling assignment). The eNB may use multi-subframe scheduling for some UEs and traditional single-subframe scheduling for other UEs at the same time.