Introduction

OFDMA as technology has existed for quite some time and has been successfully used in Cellular Communication systems notably LTE. The introduction of OFDMA in Wi-Fi as part of 802.11ax extension (also called Wi-Fi 6) was mainly done with a goal to improve the higher density performance, mainly

Higher Aggregate Throughput
Lower Latency

A Wi-Fi deployment is considered high density if there are about 30 clients in a small area (~1000 square feet)

In the first part of our series, we would do tests with VoIP calls between Wi-Fi 6 clients connected to a Wi-Fi6 AP with OFDMA enabled. We would also compare with a case where the OFDMA feature has been disabled on the AP.

Test Setup

To test the impact of OFDMA, we have used the test setup which contains 2 LinA boxes which have 12 intel AX200 cards each. So we have a total of 24 clients with 1 AP in a single RF Chamber. We would use SIP calls for generating VoIP traffic. At the time of the test, support for Buffer Status Report is not present for Intel AX200 and hence OFDMA allocation doesn’t happen on UL for these clients. However, BAs from the clients are using OFDMA

The test equipment gives detailed information on traffic (latency, jitter, loss, throughput) as well as a sniffer output with analysis of types of frames seen on air.

Configuration

Mode	802.11ax
Channel	44
Operating Bandwidth	80 MHZ
Number of Spatial Streams	2
Total Number of Clients	24

Test Results with VoIP Calls

As a first step, we have decided to do VOIP calls with a SIP client (PJSIP) and an asterisk server at the back end. The test AP is an enterprise grade AP.

Tests were done in groups of 2,4,8,12 i.e 2 clients of LinA1 box would call 2 clients of LinA2 box(there was both DL and UL traffic in this case). The results for the LinA1 box are tabulated below.

OFDMA OFF

SIP Calls	Latency (ms)	Rx Jitter (ms)	Loss. (%)
12	85.93	33.64	1.48

OFDMA ON

SIP Calls	Latency (ms)	Rx Jitter (ms)	Loss (%)
2	4.211	2.087	0
4	7.172	4.854	0
8	15.921	13.672	0
12	23.947	19.164	0.14

The system also calculates the Total % of DL Data with MU Frames from the sniffer logs. The data is shown in the chart below along with latency.

Client wise distribution of DL MU Frame allocation is as below

2 Clients 4 Clients 8 Clients 12 Clients

Inferences

There is indeed improvement in latency with OFDMA for the number of clients. they are considered (4-24). Compared to the case when OFDMA is turned off, the latency is reduced by a quarter (1/4th the value).
Latency seems to increase linearly with an increase in the number of clients even with OFDMA (double the number of clients is leading to double the latency).
OFDMA DL allocation, measured by the total percentage of data in MU Frames, goes up beyond 80% in the test for 16 – 24 clients but not so good for lower numbers of clients (4-8). This is counterintuitive(More users are preferred OFDMA generally ?)
OFDMA DL allocation between the clients participating in the test is fair as seen in the pie chart for the clients participating in the SIP Calls.