Impact of MAC Scheduling algorithms on throughput in a Multi UE scenario

In this example we understand how the scheduling algorithm affects the UDP download throughput of a multi-user (UE) system where the UEs are at different distances from the gNB. Open NetSim, Select Examples ->5G NR ->Scheduling then click on the tile in the middle panel to load the example as shown in below screenshot

Figure 4‑31: List of scenarios for the example of Scheduling

The following network diagram illustrates, what the NetSim UI displays when you open the example configuration file.

Figure 4‑32: Network set up for studying the Scheduling

UEs at different distances and channel is not time varying#

Configuring the scheduling algorithm, and parameter settings in example config files

  1. Set grid length as 5000m from Environment setting.

  2. Set distance as follows.

    1. gNB_7 to UE_8 = 500m

    2. gNB_7 to UE_9 = 1000m, and

    3. gNB_7 to UE_10 = 1500m

  3. Go to Wired link properties and set the following properties as shown below Table 4‑21.

Wired Link Properties
Uplink Speed 5000 Mbps
Downlink Speed 5000 Mbps
Uplink and downlink BER 0.0000001

Table 4‑21: Wired Link Properties

  1. Go to gNB properties à Interface (5G_RAN), set the following properties as shown below Table 4‑22. In the first sample the scheduling type is set to Round Robin, in the second to Proportional fair, and in the third to Max throughput.
Properties
Data Link Layer Properties
Scheduling Type Varies: Proportional Fair, Max throughput, Round Robin
Physical Layer Properties
CA Type SINGLE_BAND
CA Configuration n78
CA1
Numerology 1
Channel Bandwidth 100 MHz
Outdoor_Scenario URBAN_MACRO
LOS NLOS Selection USER_DEFINED
LOS Probabillity 1
Pathloss Model 3GPPTR38.901-7.4.1
Shadow Fading Model None
Fading and Beamforming NO_FADING_MIMO_UNIT_GAIN
O2I Building Penetration Model Low Loss Model

Table 4‑22: gNB >Interface (5G_RAN) >Data Link layer properties

  1. Set Tx_Antenna_Count as 2 and Rx_Antenna_Count as 1 in gNB properties.

  2. Set Tx_Antenna_Count as 1 and Rx_Antenna_Count as 2 in all the UEs.

  3. Go to Application properties and set the following properties as shown below Table 4‑23.

Application Properties
Application 1 Application 2 Application 3
Application Type CBR CBR CBR
Source ID 12 12 12
Destination ID 8 9 10
QoS UGS UGS UGS
Transport Protocol UDP UDP UDP
Packet Size 1460Bytes 1460Bytes 1460Bytes
Inter-arrival time 10μs 10μs 10μs
Start Time 1s 1s 1s

Table 4‑23: Application properties

  1. Plots are enabled in NetSim GUI.

  2. Run Simulation for 1.5s and note down throughput value in the results window in each sample. Recall that each sample has a different scheduling algorithm configured.

Results and discussions

The results with all the three UEs simultaneously downloading data is as given below.

Throughput (Mbps)
Scheduling Application 1 Application 2 Application 3 Aggregate
Round Robin 170.55 88.81 42.46 301.82
Proportional Fair 170.55 88.81 42.46 301.82
Max Throughput 510.36 0 0 510.36

Table 4‑24: UDP download throughputs for different scheduling algorithms when all three 3 UEs simultaneously downloading data

Next, consider a scenario with only one of the UEs seeing DL traffic (we don’t provide inbuilt configuration file for this, and since it is a simple exercise for a user) First, run for the UE at 1000m, then for UE at 1500m and finally for UE at 2000m. This gives the maximum achievable throughput per node since the gNB resources (bandwidth) is not shared between 3 UEs and is fully dedicated to just one UE. The results are below.

Distance from gNB (m) Application ID Throughput (Mbps) Remarks
500 1 510.36 UE 1 alone has full buffer DL traffic
1000 2 266.77 UE 2 alone has full buffer DL traffic
1500 3 127.56 UE 3 alone has full buffer DL traffic

Table 4‑25: UE throughputs if they were run standalone (without the other UEs downloading data)

The PHY rate is decided per the received SNR. Therefore, a UE closer to the gNB will get a higher date rate than a UE further away. In this example the distances from the gNB are such that UE10_Distance > UE9_Distance > UE8_Distance.

In Round Robin PRBs are allocated equally among all three nodes. However, throughputs are in the order UE8 > UE9 > UE10 because of their distances from the gNB. The individual throughputs seen by each of the UEs is exactly $\frac{1}{3}$ of the throughput as shown in Table 4‑25.The PF scheduler results will match that of the RR scheduler since the channel is not time varying. In Max throughput scheduling the PRBs are allocated such that the system gets the maximum download throughput. The nearest UE will get all the resources and its throughput will be 3 times the throughput of the UE which got the max throughout in RR.

UEs equidistant with time varying channel. RR vs. PF scheduling#

A difference in the performance of the RR and PF schedulers can be seen when the channel is time varying (of the order of the coherence time which is 10ms). We consider the following case: all UEs are initially at a distance d= 2000 m from the gNB. Then the UEs move away from the gNB at the same speed of 0.1m every 10ms (or 0.01s ), which is a speed of 10m/s. The simulation is run for 10s, and the UEs end up at a distance of 2000 + 10 × 10 = 2100m from the gNB. Note that UEs are at all times equidistant from the gNB and hence pathloss is the same (at all times) for all UEs. To induce time varying randomness in the channel we enable log normal shadow fading. Thus, every time the UE moves, NetSim draws a normally distributed random variable from N  ∼ (0, 4) dB, as the additional loss. Under these conditions, the RR scheduler would allot resources to the UEs in a round robin fashion, whereas the PF scheduler would give preference to the UE which sees the best channel. The channel quality is dependent on the draw from N ∼ (0, 4) since all UEs pathlosses are equal. The results are shown in Table 4‑27.

Configuring the scheduling algorithm, and parameter settings in example config files

  1. Set grid length as 8000m from Environment setting

  2. Distance between gNB and UEs is 2000m.

  3. File based mobility is set in all UEs to move away from gNB at the > speed of 0.1m every 10ms(or 0.01s ).

  4. Go to gNB properties à Interface (5G_RAN), set the following > properties as shown below table. In the first sample the > scheduling type is set to Proportional fair, in the second to > Round Robin.

Properties
Data Link Layer Properties
Scheduling Type Varies: Proportional Fair, Round Robin
Physical Layer Properties
CA Type SINGLE_BAND
CA Configuration n78
CA1
Numerology 0
Channel Bandwidth 10 MHz
Outdoor_Scenario URBAN_MACRO
LOS NLOS Selection USER_DEFINED
LOS Probabillity 1
Pathloss Model 3GPPTR38.901-7.4.1
Shadow Fading Model LOG NORMAL
Fading and Beamforming NO_FADING_MIMO_UNIT_GAIN
O2I Building Penetration Model Low Loss Model

Table 4‑26: Data Link Layer Properties

  1. Go to Application properties and set the properties as shown Table 4‑23 with Inter arrival time of 389 μs.

  2. Run Simulation for 11s and note down throughput value in the results window in each sample.

Results

UE ID

RR Throughput

(Mbps)

PF Throughput

(Mbps)

UE1 12.33 13.99
UE2 12.32 14.13
UE3 12.15 13.89

Table 4‑27: Comparison of PF vs. RR throughput in a case involving time varying channels