Thursday, 20 September 2018

OFDM & OFDMA in context of WiFi 802.11ax technology.

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). By subdividing the channel, parallel transmissions of small frames to multiple users can happen simultaneously.
Think of OFDMA as a technology that partitions a channel into smaller sub-channels so that simultaneous multiple-user transmissions can occur. For example, a traditional 20 MHz channel might be partitioned into as many as nine smaller sub-channels. Using OFDMA, an 802.11ax AP could simultaneously transmit small frames to nine 802.11ax clients. OFDMA is a much more efficient use of the medium for smaller frames. The simultaneous transmission cuts down on excessive overhead at the MAC sublayer as well as medium contention overhead. The goal of OFDMA is better use of the available frequency space. OFDMA technology has been time-tested with other RF communications. For example, OFDMA is used for downlink LTE cellular radio communication.
To illustrate the difference between OFDM and OFDMA, please reference both Figures 1 and 2. When an 802.11n/ac AP transmits downlink to 802.11n/ac clients on an OFDM channel, the entire frequency space of the channel is used for each independent downlink transmission. In the example shown in Figure 1, the AP transmits to six clients independently over time. All 64 subcarriers are used when an OFDM radio transmits on a 20 MHz channel. In other words, the entire 20 MHz channel is needed for the downlink communication between the AP and a single OFDM client. The same holds true for any uplink transmission from a single 802.11n/ac client to the 802.11n/ac AP. The entire 20 MHz OFDM channel is needed for the client transmission to the AP.
Figure 1- OFDM transmissions over time
 As shown in Figure 2, an 802.11ax AP can partition a 20 MHz OFDMA channel into smaller sub-channels for multiple clients on a continuous basis for simultaneous downlink transmissions. In a future blog, you will learn that an 802.11ax AP can also synchronize 802.11ax clients for simultaneous uplink transmissions. It should be noted that the rules of medium contention still apply. The AP still has to compete against legacy 802.11 stations for a transmission opportunity (TXOP). Once the AP has a TXOP, the AP is then in control of up to nine 802.11ax client stations for either downlink or uplink transmissions. The number of resource units (RUs used can vary on a per TXOP basis.
Figure 2- OFDMA transmissions over time

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