Basics of scheduling in LTE

Scheduling is the process through which eNB decides which UEs should be given resources to send or receive data . In LTE, scheduling is done per subframe level (i.e. each 1 ms TTI)

Before getting into basics of scheduling, it is important to understand following key terms:

CQI:

CQI (Channel quality indicator) is a four digit value sent to eNB by UE as a feedback for downlink channel.CQI informs eNB about the channel quality in downlink. This helps eNB to allocate proper MCS (Modulation and coding scheme) and RB (Resource block) for UE

BSR:

BSR (Buffer Status Report) is a UE way of informing network that it has certain data in its buffer and it requires grants to send this data

QoS:

QoS (Quality of Service) defines how a particular user data should be treated in the network. QoS is implemented between UE and PDN Gateway and is applied to a set of bearers.  e.g. VoIP packets are prioritized by network compared to web browser traffic.


Now let's see how the scheduling works
  • UE computes the CQI value from downlink channel and sends it to the eNB
  • UE sends BSR reports to eNodeB
  • Based on BSR, CQI and UE QoS, eNodeB computes MCS value and PRB mapping information and send it to the UE in downlink

Factors that affects scheduling: 

  • Traffic Volume: Schedules those UEs with bearers waiting data in buffer
  • QoS Requirement: Schedules and allocates resources to UE to meet its QoS requirement
  • Radio Conditions: Schedules resources for UE that best suits its radio environment



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