First of all, let's get acronyms out of the way.
OFDM - Orthogonal Frequency Division Multiplexing
OFDMA - Orthogonal Frequency Division Multiple Access
OFDM/OFDMA are the methods that we use in the modern Wi-Fi network like how FHSS/DSSS were used in prior Wi-Fi standards. The main purpose of using OFDM/OFDMA is to increase efficiency and reduce susceptibility of narrowband interference.
OFDM
OFDM is a multiplexing technology that was first introduced with 802.11a and then introduced in 802.11n and subsequent standards. Multiplexing technology is used to allow multiple signals to share a single wireless medium and OFDM is a part of a very efficient way to divide each channel into smaller frequencies called sub-carriers. OFDM stacks signals in a way that cancels out interference from various smaller adjacent frequencies.
Subcarriers
OFDM divides a channel into 64 smaller frequencies known as subcarriers. Not all subcarriers are used for data, as you need 4 pilot subcarriers for signal calibration, and some reserved for guard bands to ensure we have no interference between two channels. OFDM is even more efficient when you bond multiple channels together. Here is the breakdown -
- 4 Pilot subcarriers
- 48 Data subcarriers (802.11g/a), 52 subcarriers with (802.11n+)
OFDMA
OFDMA is an evolution of OFDM that was introduced with Wi-Fi 6 (802.11ax). This change was one of the biggest improvements with Wi-Fi 6 over the previous gen of Wi-Fi.
In the modern network, we are seeing a substantial increase in the number of devices connected to the network, especially with the increase in IoT devices. in order to create better efficiency to allow more devices to communicate at the same time, sacrificing a little bit of performance, OFDMA was introduced. OFDMA divides the channel further by introducing 256 subcarriers. Of course, we cannot use all 256 subcarriers for data as you still need guard bands to prevent interference between adjacent channels.
Resource Units (RUs)
One station typically use all of the available subcarriers with OFDM. With OFDMA, each of the stations can communicate on different subcarriers at the same time. Resource Unit is a collection of subcarriers per channel that can be assigned to different client devices in a BSS. They are also dynamic in nature; 1 RU can be assigned to a station while another station can be assigned to 3 RUs.
In summary, it is important to understand OFDM and OFDMA in relation to modern Wi-Fi since they are fundamental to understand the concept of multiplexing, how OFDM/OFDMA divides a channel into smaller frequencies, and how Resource Units can be grouped together to allow multiple clients/STAs to communicate at the same time in a BSS to make wireless technology more efficient to handle modern environment.