LTE eNodeB Scheduler and Different Scheduler Type

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What is a LTE Scheduler ?

Scheduling is a process through which eNodeB decides which UEs should be given resources (RBs), how much resource (RBs) should be given to send or receive data .In LTE, scheduling is done at per subframe basis i.e. every 1 mili second. The entity which is govern this is know as scheduler.

A scheduler takes input from OAM as system configuration e.g. which scheduling algorithm is to be enable (round robin, Max C/I, Proportional Fair, QoS aware etc), consider QoS information (Which QCI, GBR/N-GBR etc.) and channel quality information (CQI, Rank, SINR etc) to make the decisions.

A LTE scheduler performs following function for efficient scheduling:

  • Link Adaptation: It selects the optimal combination of parameters such as modulation, channel Coding & transmit schemes i.e. Transmission Mode (TM1/TM2/TM3/TM4) as a function of the RF conditions.
  • Rate Control: It is in charge of resource allocation among radio bearers of the same UE which are available at the eNB for DL and at the UE for UL.
  • Packet Scheduler: It arbitrates access to air interface resources on 1ms-TTI basis amongst all active
    Users (Users in RRC Connected State).
  • Resource Assignment: It allocates air interface resources to selected active users on per TTI basis.
  • Power Control: Provides the desired SINR level for achieving the desired data rate, but also controls
    the interference to the neighbouring cells.
  • HARQ (ARQ + FEC): It allows recovering from residual errors by link adaptation.
LTE Services and Scheduling Mechanism:

The Services/ Applications are broadly classified into two categories as Real time services and Non-Real time services. Real time services includes Conversational Voice, Video Phony [Conversational Video], MPEG Video [Non Conversational Video], Real-time gaming etc. Non-Real time services include Voice Messaging, Buffered Streaming, ftp, www, email, Interactive gaming etc.The data transmission characteristics of these services are:

  • Delay tolerance
  • Data Packet Size [Fixed or Variable]
  • Periodic or Aperiodic data transmission
  • Packet error loss rate, etc.

Some or all of these characteristics determine what kind of Packet schedulers are required at the LTE MAC to adhere to the required QoS requirements of the relevant applications. LTE MAC supports the following three types of Scheduling:

  • Dynamic Scheduling
  • Persistent Scheduling
  • Semi-Persistent Scheduling
  1. Dynamic Scheduling: Every TTI, MAC checks for the UEs to be scheduled, the Data Availability for each UE to be scheduled and the feedback from the UE on the Channel conditions. Based on these data, it can schedule the resources for the UE through the PDCCH. If data is not available, UE will not get scheduled. All Services can be scheduled using Dynamic Scheduling, but at the expense of the Control signalling [PDCCH Usage – a scarce resource].
  2. Persistent Scheduling: In this case, Packets are scheduled on a fixed basis, similar to the Circuit Switched fashion. Here, it does not depend on the Channel Condition. The Resource allocation remains constant for the period of the call.
  3. Semi-Persistent Scheduling: It is a Hybrid way of scheduling, which tries to overcome the drawbacks of the Dynamic Scheduling and the Persistent Scheduling.

Semi-Persistent Scheduling

The figure above show conversational voice is considered for persistent scheduling. It is clear that, because of the fixed resource allocation, UE will end up in under utilizing the allocated resources, because of non-availability of the sufficient data during the persistently scheduled TTIs. Here, because the Speaker sometimes speaks and sometimes user may give pauses during the conversation, Voice activity period and Voice Inactivity period exist in the Speech data. If the Speech Codec without VAD is involved, then it sends the Voice Payload at the end of every voice frame length of 20ms during the Voice activity period and sends nothing during the Voice Inactivity period. So, the fixed resource allocations will get utilized during the Voice activity periods to transmit the received voice payloads over the air interface and it gets unused during the Voice inactivity periods, which is a critical drawback. Semi persistent scheduling addresses this drawback in a unique way.

As shown above, whenever the Voice Payloads arrive at the L2, the MAC Scheduler will activate the SPS resources and whenever there are no transmissions for few of the transmission opportunities, the SPS resources were implicitly released. Again, it gets activated, when the voice payloads arrives at the next Voice activity period. During the Voice Inactivity period and after the implicit release of the SPS resources, these radio resources will be allocated for different UEs, which are in need of it. It is clear that, only those services, which are real time in nature, with fixed packet size payloads, and fixed periodicity of the payload arrival, can effectively and efficiently utilize. The Semi-Persistent Scheduling. Such services or applications, which fulfils these characteristics are conversational voice, conversational video [only Conversational voice part of the video], and any other real time applications [Only Conversational Voice part]

Different Types of Schedulers:
  • Round Robin: The RR scheduler selects and schedules UEs in a round robin manner, thereby creating an equal resource share. The disadvantage of this approach is that UEs with sub-optimalCQIs may be allocated Physical Radio Resources (PRBs), thus reducing the overall cell throughput.
  • Max CQI : The max-CQI scheduler selects the schedulable UEs based on the experienced CQI. The UEs with the highest CQI therefore become candidates for scheduling thereby increasing the overall cell throughput. The disadvantage of this approach is that UEs with lower CQI are denied scheduling instances, thus being starved for throughput and leading to degraded user experience.
  • Proportional Fair: The PFS is expected to strike a balance between the traditional Round Robin (RR) scheduler and the max Throughput Scheduler (also known max-CQI (Channel Quality Indicator) scheduler). The PFS scheduler performs in such a manner that it considers resource fairness as well as maximizing cell throughput (in addition to other possible
    performance metrics).

For a Max C/I scheduler, the Sector throughput improves while cell edge throughput drops compared to a PF scheduler where sector throughput may not be as good as Max C/I but cell edge throughput thoroughly improves.

Scheduler Type Max C/I Round Robin Proportional Fair (PF)
How it works Allocates resources to the
user with the instantaneous best RF conditions. UE with the best channel conditions is always prioritized		 Resources are shared
across users over time
regardless of the RF conditions.		 Sharing the cell throughput but as a function of RF conditions and bearer
priorities
Pros Very Good
Throughput		 Resources shared in an
equal manner		 Trade-off between
fairness and cell throughput
Cons Cell Edge UEs starved of
scheduling instances leading to degraded user
experience.		 UEs with sub optimal
CQI conditions will reduce
the cell throughput		 Implementation complexity
and overall cell throughput will not be the
highest

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