Let’s take some time to discuss both OFDM and OFDMA technologies used for Wi-Fi communications. 802.11a/g/n/ac radios currently use Orthogonal Frequency Division Multiplexing (OFDM) for single-user transmissions on an 802.11 frequency. 802.11ax radios can utilize Orthogonal frequency-division multiple access (OFDMA) which is a multi-user version of the OFDM digital-modulation technology. OFDMA subdivides a channel into smaller frequency allocations, called resource units (RUs).
Both OFDM and OFDMA divided a channel into subcarriers through a mathematical function known as an inverse fast Fourier transform (IFFT). The spacing of the subcarriers is orthogonal, so they will not interfere with one another despite the lack of guard bands between them. This creates signal nulls in the adjacent subcarrier frequencies, thus preventing inter-carrier interference (ICI).
What are some of the key differences between OFDM and OFDMA? As shown in Figure 1, a 20 MHz 802.11n/ac channel consists of 64 subcarriers. Fifty-two of the subcarriers are used to carry modulated data; four of the subcarriers function as pilot carriers; and eight of the subcarriers serve as guard bands. OFDM are sometimes also referred to as OFDM tones. In this blog series, we will use both terms interchangeably. Each OFDM subcarrier is 312.5 KHz.
Figure 1: 802.11n/ac 20 MHz channel—OFDM subcarriers
802.11ax introduces a longer OFDM symbol time of 12.8 μs, which is four times longer than the legacy symbol time of 3.2 μs. Subcarrier spacing is equal to the reciprocal of the symbol time. As a result of the longer symbol time, the subcarrier size and spacing decreases from 312.5 KHz to 78.125 KHz. The narrow subcarrier spacing allows better equalization and therefore enhanced channel robustness. Because of the 78.125 KHz spacing, an OFDMA 20 MHz channel consists of a total of 256 subcarriers (tones) as depicted in Figure 2.
Figure 2: Subcarrier spacing
Just like with OFDM, there are three types of subcarriers for OFDMA, as follows:
Data Subcarriers: These subcarriers will use the same modulation and coding schemes (MCSs) as 802.11ac and two new MCSs with the addition of 1024-QAM.
Pilot Subcarriers: The pilot subcarriers do not carry modulated data; however, they are used for synchronization purposes between the receiver and transmitter.
Unused Subcarriers: The remaining unused subcarriers are mainly used as guard carriers or null subcarriers against interference from adjacent channels or sub-channels.
These tones are grouped into smaller sub-channels, known as resource units (RUs). By subdividing the channel, parallel transmissions of small frames to multiple users can happen simultaneously. The data and pilot subcarriers within each resource unit are both adjacent and contiguous within an OFDMA channel.