802.11ax – OFDMA

OFDMA is currently gathering a lot of attention in the WiFi community. This is because of the demand for ever-increasing growth of mobile devices, internet, multimedia, and various other services. OFDMA based systems are capable of providing high data rates, efficient sharing of resources, and take advantage of multipath. Already proven its importance in other verticals, OFDMA does look promising for various WLAN demands. Task Group 11ax does consider the existing framework and has gone a step ahead to design a much more flexible framework that can be used for WLAN communication. A few important OFDMA features include:

  • Channel Resource Allocation over Time and Frequency
  • OFDMA Deterministic and Random Access
  • Support for UL and DL MU MIMO (Uplink and Downlink Multi-User Multiple Input Multiple Output)

Channel Resource Allocation:

In 802.11ax channel resources are allocated over time and frequency, however, OFDMA transmission is organized on a per-frame basis which helps to simplify the process of resource management. This means that the stations participating in OFDMA can carry information from and to multiple OFDMA Capable stations. Various tones (Resource Units) are assigned to various Stations but the duration of all the stations is the same. This is a step ahead to achieve high efficiency when compared to legacy OFDM stations where one station uses all the resources. In OFDMA, multiple stations with different resource unites can transmit/receive data at the same time.

Similar to OFDM, there are three types of Sub-carriers:

  • Data: These are used to carry data traffic. Used for transmitting data. Modulation and coding schemes are applied to Data Subcarriers.
  • Pilot: These are used for Synchronisation between transmitter and receiver. They do not carry modulated signals.
  • Guard: These subcarriers or remaining subcarriers are used as a guard carrier or null carriers to protect against Adjacent channel interference OR for OFDMA, sub-channel interference.

OFDMA subdivides the channels into multiple smaller frequencies or Sub-channels and hence helps an AP (Access Point) to synchronize uplink and downlink communication with multiple clients simultaneously. This can help in the transmission of data from multiple users at the same time. Based on the need for the transmitting station or client, the AP can allocate a whole channel or a group of sub-carriers to that particular WLAN client. As a result of longer symbol time in OFDMA to 12.8 microseconds, the sub-carrier size has been reduced to 78.125KHz and thus giving 256 sub-carriers for a 20MHz channel.

RU TypeCBW 20CBW 40CBW 80CBW80+80 and CBW 160
26-tone RU9183774
52-tone RU481632
106-tone RU24816
242-tone RU1248
484-tone RUN/A124
996-tone RUN/AN/A12
2*996-tone RUN/AN/AN/A1

Table 1 – Maximum number of RUs for each Channel Width

Table 1 shows the spread of RUs for all types of channel bandwidths. Each RU contains a minimum of 26 tones i.e. 26 sub-carriers. These RUs are predefined by the 802.11ax tax group as columnized in RU Type in the above table. Higher the channel bandwidth, more number of sub-carriers can be allocated in a single RU. However, the allocation of these RUs is completely decided by the AP in OFDMA functionality. There are various factors considered before assigning RUs like the amount of data to be transferred, if Quality of Service implemented for this data, etc.

Table 2 – Data and Pilot Subcarrier frequencies for RUs in a 20MHz HE PPDU

Table 2 above shows how the frequencies are assigned for each of the RU types. Considering the channel Bandwidths these data changes accordingly. For each of the Channel Bandwidth, the frequencies are already defined in the 802.11ax draft. For the sake of simplicity, I have listed only 20MHz frequencies that would be used when WLAN communication happens in OFDMA. Each RU can be assigned to a single client. Thanks to MU-MIMO, around 9 users can simultaneously communicate with AP for Uplink or Downlink data traffic considering the fact that each of them needs approximately 26-tones for their transmission. 

OFDMA Deterministic and Random Access:

As the name suggests, the OFDMA designed for WLAN communication under 802.11ax can be used for deterministic as well as Random Access of transmission or reception. With OFDMA in place, most of the communication is now handled at the AP level. This results in AP taking control of the client’s transmission and reception. All this communication is based on trigger frames that AP sends in order to regulate the transmission and reception of OFDMA capable Stations. 

With the support of Multi-User MIMO for Uplink and downlink OFDMA does provides a highly efficient way of communication for WLAN Stations.

Deterministic Access:

  • Access Point identifies Stations that are ready to send data. It sends a Trigger Frame which synchronizes with the transmitting stations. It also has RUs allocated for these stations.
  • Stations use Assigned RUs and transmit data.
  • A multi-station Ack (MSBA) is sent back to all the participating stations. All the communications between stations and AP are separated by SIFS timer.
  • This mechanism can be systematically used for Uplink and Downlink communication. Various details are gathered for Downlink and Uplink Communication like the power, SNR, MCS, etc and the decision is taken for uplink Multi-User communication.

Random Access:

  • Random Access is optionally enabled. If the AP does support it, the same will be broadcasted in the beacon by AP for OFDMA Stations to be aware of the same.
  • A backoff timer called OFDMA Back Off (OBO) is used for communication of OFDMA capable clients to communicate Randomly. This timer is very similar to Random Backoff timer except that here the timer is decided based on the OFDMA Contention Window (OCW) and NOT using NAV (Virtual Carrier Sensing). The OBO counter is set to a random value.
  • AP sends a Trigger Frame (TF) with RUs allocated for Random Access
  • If the OBO counter in Station is less than the number of RUs allocated, the Station uses one of the Random RUs and transmits data.
  • If the OBO counter is greater than RUs allocated, the Station reduces the counter to that particular value as in TF Frame and waits for another Trigger Frame.

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