OFDMA

WiFi-6 or 802.11ax is the next generation of WiFi Technology. It is expected to make a huge difference in the way people and more importantly IoT devices access the network.  One of the major differences between the current generation and the next generation is the use of OFDMA. 802.11a/g/n/ac uses orthogonal frequency division multiplexing while WiFi6 (802.11ax) uses Orthogonal frequency division multiple access (OFDMA). So what makes it unique? 

Orthogonal frequency division multiple access is a multi-user version of the popular orthogonal frequency division multiplexing(OFDM) digital modulation scheme. Multiple access is achieved in OFDMA by assigning subsets of subcarriers to individual users. It allows the simultaneous low-data-rate transmission to several users. OFMDA helps WiFi6 to operate efficiently in high-density environments, serving several users simultaneously.

OFDM vs OFDMA

Let’s consider an example of 3 people traveling in a car. So the car is a medium here. As in  the figure below, people use the car in two ways. They can travel one by one in the same car over a different period, (OR) all 3 people travel in the same car at the same time. The second method is a more efficient utilization of the space in the car. This is an idealized example of the difference in operation between OFDM and OFDMA.

To understand technically, consider a  transmission in 20 MHz channel to six different clients using OFDM and OFDMA. The 20MHz channel is internally divided into 64 sub-carriers in OFDM and 256 sub-carriers in OFDMA. Figure (1) depicts the OFDM transmission between AP and clients where the entire frequency space (all 64 sub-carriers)  is used for one client. That is the entire 20MHz channel is occupied for transmission irrespective of the packet size. OFDM makes it inefficient when you have shorter data to send, which may be the case very often in real-time.

(Figure 1)

Figure (2) shows how the OFDMA  divided channel and allocated the subcarriers to different clients for simultaneous transmission. OFDMA allocates the unused channel to another client. It’s up to AP to allocate the entire channel or just a few subcarriers based on need. Thus makes effective utilization of bandwidth and so OFDMA does better in a high-density environment.

(Figure 2)

Flexibility in Bandwidth Utilization

When an OFDM uses the entire channel, some sub-carriers may be wasted. They may be left idle since there is not enough data to be transmitted. However, OFDMA can allocate the available sub-carriers to different users and achieves effective bandwidth utilization. There are 256 subcarriers (tones) in a 20 MHz channel, and these group of subcarriers are called as resource units (RUs). The standardized resource units in a 20 MHz channel are 26, 52, 104 and 242. The minimum bandwidth allocation is 26 tone resource unit (RU) – approximately 2MHz.

How does OFDMA give us better number?

CSMA wait time, PHY headers and transmission to multiple users are some of the common factors which make OFDM inefficient. Each OFDM transmission has its own CSMA wait time and PHY header. It becomes worse with shorter packets to send when overhead size is more than the useful information being transmitted. However, OFDMA addresses this issue with common overheads and parallel transmission to multiple users. Hence it improves the numbers in throughput.

Following case study shows the result with simultaneous TCP DL and UDP UL traffic. You can see that OFDMA does better by being able to get TCP ACKs without contention using the trigger frame. The trigger frame allocates the bandwidth for UL transmission for sending ACKs without doing CSMA/CA.

Considering the growth projection in the number of WiFi devices, OFDMA is going to be the key in WiFi6. Despite being the most prominent feature of 802.11ax, has its complexities to come out in real-time. However, OFDMA a good step forward in evolving WiFi.

Look out for our upcoming blogs on UL and DL OFDMA and differences between them.