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By Damon Watson | 08 August 2022 | 0 Comments

What is OFDMA? Difference between OFDMA and MU-MIMO?

The key features of WiFi 6 (802.11ax) are MU-MIMO and OFDMA. We've introduced MU-MIMO and explored the difference between MIMO and MU-MIMO in the previous article. Now it's OFDMA's turn. What advantages can OFDMA bring to the Internet and how does it differ from MU-MIMO? Read on and you will know.
 

What is OFDMA?


OFDMA refers to Orthogonal Frequency Division Multiple Access. It is a multi-user version of OFDM that is single-user. The data transmission adopts OFDM before the occurrence of WiFi 6, which identifies users in terms of time segment. Only one user occupies all the subcarriers on a channel and sends a complete packet within each time slot. The throughput of OFDMA is three times higher than that of OFDM for short data packets or multiple endpoints. OFDMA combines transmissions and sends the frame to multiple endpoints. Therefore, the transmission latency is lower and efficiency is higher, which makes OFDMA fit for the application of IoT devices, videos, online games, etc.
 

How does OFDMA Work?


With the aid of OFDMA, multiple clients' data can be sent to or received from the AP in a single transmission. The access point uses groups of sub-carriers within the RF channel simultaneously for the downlink, whereas previously it would have sent the data in a sequential series of packets, consuming the entire RF channel bandwidth. Multiple clients transmit at once in the uplink direction, with their signals arriving at the access point on various sub-carriers to enable parallel reception. Reduced header and contention overhead improve efficiency, and small-packet throughput is noticeably higher, but what makes OFDMA awesome is the powerful control over individual transmissions.

 

Wi-Fi transmission is a complex interaction of numerous parameters such as transmit bit rate, transmit power, receive sensitivity and noise levels, channel bandwidth, and others. For maximum effectiveness and performance, OFDMA enables to optimize these parameters for each transmission and each client and then group them in various combinations.
 

What are the Advantages and Disadvantages of OFDMA?


Advantages:

 

Higher frequency variety and efficiency

 

OFDMA improves the communication efficiency of multi-client concurrent scenarios especially for high-density deployment by reducing the cost of time of Preamble, SIFS, and Contention. 

 

QoS 

With the utilization of OFDMA,there is not only the ability of more granular downlink resource allocation in the time domain and frequency domain but also the ability to explicitly allocate resource units in the uplink for the first time in Wi-Fi communication. This bidirectional resource element allocation capability is similar to LTE resource blocks (RBs) and is analogous to virtual resources or "slices" in 5G. As you can imagine, this 802.11ax slice can have various properties, such as bandwidth, latency, and jitter, enabling finer QoS than was possible before with 802.11ac.

 

Higher User Bandwidth

The OFDMA technology used in 802.11ax divides the 20/40/80/160 MHz channel into multiple resource units (RUs). For a 20MHz channel, an RU may consist of 26, 52, 106 or 242 sub-carriers corresponding to RUs of approximately 2 MHz, 4 MHz, 8 MHz and 20 MHz bandwidth. The data of the client is carried on each RU, so from the perspective of the total time-frequency resources, in each time slice, data can be sent to multiple clients at the same time.

 

More flexible in that it can open and close channel and subchannel
Better network coverage

 

Disadvantages:

 

The frequency diversity depends on how the subcarriers are assigned to the users so it will become complicated.
Extra power is required because it is always on and ready to send transmissions.
There is higher sensitivity than other channel types.
 

OFDM vs MU-MIMO, What's the Difference?


MU-MIMO and OFDMA are complementary.

MU-MIMO improves space utilization and effective bandwidth and reduces latency at the same time. But MU-MIMO is not stable enough, making it get affected by STA. It allows multi-user to visit by using different spatial streams.

OFDMA supports multi-channel transmission in the range of frequency, which is a great choice for low-bandwidth and small-packet applications. It improves the utilization of spatial streams and transmission efficiency and reduces the procedure latency. The technology is stable and not easy to be influenced by STA.

OFDMA fits low-broadband applications while MU-MIMO suits high-broadband applications. With a great combination of two items, WiFi 6 performs better.

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