Applicable Versions
NetSim StandardNetSim Pro


Applicable Releases
v13.2
v13.3
v14.0


In NetSim, the MAC scheduler at each base station (gNB) decides the UE-wise PRB allocation for every slot. In FDD, both PDSCH and PUSCH allocations are output per slot, while in TDD, the appropriate (PDSCH or PUSCH) allocation is output depending on the slot type (DL or UL).

The scheduler takes the following inputs for each gNB and attached UE:
  • Number of MIMO layers
  • DL - PDSCH SINR at each layer, CQI at each layer, MCS at each layer
  • UL - PDSCH SINR at each layer, CQI at each layer, MCS at each layer
  • DL Buffer status: Buffer Fill, Type of traffic (GBR/Non-GBR)
  • UL Buffer status: Buffer Fill, Type of traffic (GBR/Non-GBR)
  • DL HARQ context - RV, HARQ-ID, NDI 
  • UL HARQ context - RV, HARQ-ID, NDI

The scheduler also uses the number of PRBs available in the gNB


The scheduling mechanism can be selected from gNB Properties > Interface 5G RAN > Datalink Layer


v14


v13


NetSim supports the following scheduling algorithms:

  • Round Robin: It divides the available PRBs among the active flows, i.e., those logical channels that have a non-empty RLC queue. The MCS for each user is calculated according to the received CQIs.
  • Proportional fair: The Proportional Fair (PF) scheduler works by scheduling a user when its instantaneous channel quality is high relative to its own average channel condition over time. The PF scheme is based on the current data rate for each user and the exponentially weighted moving average (EWMA) data rate over an immediately prior predetermined interval for each user. In comparison with the round-robin (RR) scheduler in which UEs are cyclically scheduled irrespective of the channel condition, the PF scheduler maximizes the system throughput while maintaining long-term fairness in the allocation of resources between users.
  • Max throughput: It allocates PRBs to the active flow(s) to maximize the achievable rate. In other words, it selects the user that sees the highest CQI.


Re-transmissions are prioritized over first transmissions. 


The scheduler's output is UE-wise PRB allocation in the UL and DL.

Assumptions and limitations
  • If fading is enabled, the channel changes every coherence time. However, within the coherence time, for every UE, the channel is flat across the system bandwidth, i.e., the channel fading is not frequency-selective. 
  • As a result of the above assumption, the scheduler just maps PRB counts to UEs. For example, there could be 100 PRBs allocated across 3 UEs as 50, 30, and 20, but PRBs within a slot are not differentiated in frequency.
  • The scheduler allocation (PRBs to UEs) is instantaneously applied in the DL and in the UL without any delay.
  • Transmit power is equal for all users. For one user, transmit power is split equally between the MIMO layers.
  • Control packets such as the buffer status report (BSR), UL assignments, etc., are assumed to be sent out of band. The resources for transmission of these control packets are part of the overhead, which is reduced from the available resources as defined in the NetSim manual, section 3.9.20.


Useful links


1. Documentation: https://www.tetcos.com/help/v13.3/Technology-Libraries/5G-NR.html#mac_scheduler_introduction

2. Examples on MAC scheduler: https://www.tetcos.com/help/v13.3/Technology-Libraries/5G-NR.html#impact_of_mac_scheduling_algorithms_on_throughput_in_a_multi-ue_scenario