ns-3 5g-lena QoS scheduler Analysis

Original Paper: Enabling QoS Provisioning Support for Delay-Critical Traffic and Multi-Flow Handling in ns-3 5G-LENA by Katerina et al.

QoS MAC Scheduler

QoS MAC scheduler is capable of meeting the low latency requirements of delay-critical traffic, by combining the resource type, the Priority Level and the PDB of each flow, with the HOL delay and the PF metric

In this way, flows with tighter latency requirements are promoted and packet discards are avoided for these flows

Scheduling Metric

An active UE with the larger weight $W$ is scheduled first and $W$ is calculated as:

W = \sum_{n=1}^{N}w

$N$ : the number of active logical channels for a given user

$w$ is calculated for each active logical channel (flow) as:

w=\begin{cases} (100-P)\times\cfrac{r^{\gamma}}{\max(1e-9, R(\tau))}+F &\text{for non-GBR and GBR} \\ (100-P)\times\cfrac{r^{\gamma}}{\max(1e-9, R(\tau))}\times{D}+F &\text{for DC GBR } \end{cases}\\

$P$ : the default priority level, where a lower value indicates a higher priority for scheduling
$D$ : the delay budget factor, that is delay-aware weight related to the HOL packet delay and the PDB
$R(\tau)$ : the past average data rate (the average throughput within the updated window size $\tau$ )
$r$ $r$ : the instantaneous achievable data rate calculated by the spectrum efficiency and the channel bandwidth
- The potential throughput achievable with the resources intended to be scheduled by the scheduler
$F$ : the additional factor, the default value is $10$ , and the value is set to $100$ when the DRB has retransmission data

$D$ is calculated as:

D=\begin{cases} {\text{PDB}}/{0.1} &\text{if } \text{HOL} \ge\text{PDB} \\ \cfrac{\text{PDB}}{\text{PDB}-\text{HOL}} &\text{otherwise} \end{cases}

$\text{HOL}$ : head-of-line delay (HOL) that the interval between the time first packet to be transmitted pending at packet transmission queue and the time it is received by UE
$\text{PDB}$ : Packet Delay Budget

$R(\tau)$ is calculated as:

R(\tau) = (1-\alpha)R(\tau-1)+\alpha{A(\tau)}

$A(\tau)$ : the current data rate over the updated window size $\tau$

QoS LC Assignment

Allocates resources for users with multiple flows by determining how much resource to allocate to each flow

prioritizing important GBR and DC-GBR flows
ensuring fair distribution using the round-robin method.

It considers the guaranteed bit rate requirements of each flow to maximize QoS satisfaction.

How the Algorithm Works

Identify all active GBR (Guaranteed Bit Rate) and DC-GBR (Delay-Critical Guaranteed Bit Rate) flows
Compare the total demand of multiple flows with the allocated Bytes (Transport Block Size, TBS)
Distribute the allocated bytes equally among the GBR/DC-GBR flows using a round-robin (RR) method if the total demand exceeds the allocated bytes
Assign enough bytes to each flow to meet the requirements if the total demand is less than the allocated bytes
Distribute any remaining bytes among the rest of the logical channels (LCs) using the RR method

NS-3 Implementation

NrMacSchedulerUeInfoQos, inheriting from NrMacSchedulerUeInfo
NrMacSchedulerTdmaQoS, inheriting from NrMacSchedulerTdmaRR
NrMacSchedulerOfdmaQoS, inheriting from NrMacSchedulerOfdmaRR
NrMacSchedulerLcAlgorithm, inheriting from NrMacSchedulerNs3
- NrMacSchedulerLcRR and NrMacSchedulerLcQoS, inheriting from NrMacSchedulerLcAlgorithm

Evaluation

Consider a single cell topology with two users

UE1: 1 flow
- DL non-GBR traffic flow (5QI 80)
UE2: 2 flows
- DL non-GBR traffic flow(5QI 80)
- DL DC GBR traffic flow (5QI 87)

QoS requirements of 5QI 80 and 87

5QI	Resource Type	Default Priority Level	Packet Delay Budget	Packet Error Rate	Example Services
80	non-GBR	$68$	$10$ $\text{ms}$	$10^{-6}$	Low Latency eMBB applications Augmented Reality
87	Delay Critical GBR	25	5 ms	$10^{-4}$	Interactive Service - Motion tracking data, (see TS 22.261)

Evaluation Scenario

2 Traffic Scenario: saturation and non-saturation

non-saturation: bandwidth of 50 MHz (sufficient resources)
saturation: bandwidth of 5 MHz and 10 MHz

Bandwidth of 5 MHz means higher saturation

Evaluation Metrics

end-to-end delay
end-to-end throughput

Simulation Results Analysis

There are 2 checkpoints:

1️⃣ Tradeoff in QoS Scheduler

In a saturation situation, the average delay results show that DC GBR traffic meets the PDB, whereas Non-GBR traffic does not.

Due to the scheduler assigning weights with a focus on delay-critical traffic
The 5QI 80 traffic is sensitive to latency

2️⃣ Difference between UEs with the same flow

Despite UE1 and UE2 having the same 5QI 80 traffic, it can be observed that in a saturation situation, only UE1 has a very high average delay

Because DC-GBR traffic is also considered in the scheduling weight calculation for UE2

Conclusion

While satisfying the low latency of DC GBR is important, it is also necessary to consider the PDB elements of each QoS flow
Consideration of comparing weights based on flows rather than UEs