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 every slot (DL or UL).
- 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
- Target BLE (t-BLER) and the current average BLER
Re-transmissions are prioritized over first transmissions.
NetSim uses these inputs and rules to determine the schedule. The following scheduling algorithms are supported:
- 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 (active) 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.
The scheduling algorithm can be selected from gNB Properties > Interface 5G RAN > Datalink Layer
The scheduler's output is UE-wise PRB allocation in the UL and DL, at every slot.
Special cases
- Carrier aggregation: the scheduler runs on a per carrier basis.
- NSA mode: Traffic is split between 4G and 5G (eNB and gNB) above the MAC. The scheduler runs independently on the eNB and gNB.
- Application priorities and heterogenous traffic: In 5G, the types of QoS are:
- GBR, which is transmitted in RLC UM mode. In NetSim, Applications which have UGS priority set are transmitted in UM mode.
- Non-GBR, which is transmitted in RLC AM mode. In NetSim, Applications which do not have UGS priority set are transmitted in AM mode.
- The MAC scheduler allocates resources on a combined (UM plus AM) RLC requirement. Once UE wise allocation is complete, RLC would first transmit the UM mode traffic followed by the AM mode traffic, to that UE. And so on for all UEs.
- 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. Analyse throughput and fairness of 5G scheduling algorithms in a complex network scenario: https://tetcos.com/5g-scheduling-throughput-and-fairness.html
2. NetSim 5G documentation (v14.1): https://tetcos.com/downloads/v14.1/5G-NR.pdf (Sections: 3.8.2 and 4.4)