What’s the Difference Between OFDMA and MU-MIMO in 11ax?
The IEEE's latest standard, 802.11ax, has been designed inside and out for high efficiency. Also known as Wi-Fi 6, 11ax promises real-world performance improvements that will keep today’s mobile users and IoT devices connected and happy. To achieve that, over 50 features have been proposed to be part of the 11ax standard. Once adopted into the standard, they will be rolled out to the market in phases – Wave 1 and Wave 2, just like with 802.11ac.
Multi-user Multiple-Input, Multiple-Output (MU-MIMO) and Orthogonal Frequency-Division Multiple Access (OFDMA) are two of the most significant technical enhancements in 11ax. Both MU-MIMO and OFDMA are multiuser technologies that enable simultaneous bidirectional communication between an access point (AP) and end users. So how are they different?
Let's dive deeper into each technology, and explore how they have different uses.
MU-MIMO
In essence, the MU-MIMO technology remains the same between 11ac and 11ax. When multiple clients are trying to access the medium at the same time, the AP uses RF multipath to send frames to multiple clients at the same time instance. This technology is called MU-MIMO and it utilizes diversity in space. The difference between downlink MU-MIMO in 11ac and 11ax is that in the latter, the groups of clients are now bigger (up to eight clients in a group) instead of a maximum of four clients in one group. In contrast, MU-MIMO on the uplink is a new feature in 802.11ax that will roll out in the next phase of the standard.
The use of MU-MIMO in today's Wi-Fi devices has increased multifold and it is a commonly adopted technology in the Wi-Fi space. One of the drawbacks experienced in the 11ac flavor of MU-MIMO was the difficulty in forming client groups. Because of this, even a 4×4:4 spatial stream access point would seldom reap the benefits of higher gains as compared to Single-User MIMO. Downlink MU-MIMO in 11ax allows for up to eight clients to be grouped together.
As seen in the diagram above, APs can now group clients and transmit to these client groups in a sequential manner. The diagram represents beamforming reports, but this concept applies to other packet types as well. When uplink MU-MIMO is introduced in Wave 2 of 11ax, the bottleneck caused by acknowledgement mechanisms in the uplink will be eliminated and traffic over TCP/IP protocols will benefit from better performance on the uplink. Thus, MU-MIMO is a multiuser technology that increases capacity, facilitates higher speeds and is ideal for applications that require high bandwidths.
OFDMA
Orthogonal Frequency-Division Multiple Access (OFDMA) divides available channel bandwidth into several mutual orthogonal subcarriers or resource units (RUs). Access to multiple users is granted in OFDMA by assigning subsets of these subcarriers to individual users. Downlink OFDMA is one of the complex features of 11ax that allows a single downlink transmission (from AP to clients) to be split by frequency within a channel. Uplink OFDMA is similar to downlink OFDMA but in the reverse direction, i.e. from multiple clients to the AP. Since many clients will try to transmit at the same time, they need to be coordinated. To achieve this coordination, the AP sends trigger frames to the clients to inform them which subcarriers (or RUs) they can use to send data. OFDMA is also used by LTE networks as the means of multiuser transmission.
At first, it may appear as though OFDMA offers no added benefit as compared to single-user Orthogonal Frequency Division Multiplexing (OFDM) because the speed of transmit links remains unchanged. So, when half the channel is allocated for frame transmission in OFDMA, it would take twice the time. But, in OFDM, each time a frame transmission has to occur, there is significant overhead caused by medium contention. In OFDMA though, since transmit opportunities are shared by several frames bundled together, medium contention overhead is much lesser. Efficient use of the channel therefore leads to increased overall efficiency in small-packet applications in dense environments.
Two Technologies, Designed for Different Uses
OFDMA and MU-MIMO are thus complementary technologies. While OFDMA is ideal for low-bandwidth, small-packet applications such as IoT sensors, MU-MIMO increases capacity and efficiency in high-bandwidth applications like mission-critical voice calls and video streaming.
3 Ways 802.11ax Makes Wi-Fi Better
Introduced in 1997, the IEEE 802.11 standard, more commonly known as Wi-Fi, has continually evolved to address the need of increased speeds in enterprise Wi-Fi networks. Of late, however, data rate and throughput have become table stakes in any high-density WLAN deployment.
That's because there has been an explosion in the number of client devices per household. Offices and public spaces like malls, stadiums and concert venues also boast of highly dense client environments. The demand has shifted from “high speed Wi-Fi” to “fast and efficient Wi-Fi in extremely dense environments.”
Enter 11ax. With the introduction of 802.11ax (also known as Wi-Fi 6), the wireless industry is now delivering bandwidth and efficiency several times that of the legacy 802.11b.
A quick recap of the key features of 11ax:
- Power save mode with enhancements to ensure that Wi-Fi scanning does not drain the battery of handheld and portable devices, including IoT devices.
- Uplink and downlink OFDMA is now mandatory, and as a result of the reduced congestion and overhead, high-density environments and large public venues will see a dramatic improvement in capacity.
- Uplink and downlink MU-MIMO, which was originally introduced in the downlink in the 11ac Wave 2 standard (MU-MIMO), will be included for the uplink in Wave 2 of 11ax. It allows multiple devices to simultaneously transmit at once, and that increases efficiency of the network.
- High-order modulation with 1024 QAM will bring a 25% increase in peak data rates under high signal-to-noise conditions.
And much more!
Another Wi-Fi Revolution is Here
A revolution is in store for large enterprises, service providers and end-users. Here are three ways 802.11ax will make the Wi-Fi user experience better:
1. Improved efficiency for large public venues and support for more data. Moving forward, “overall efficiency” is the means to determine Wi-Fi performance, instead of the traditional throughput and speed measurements the industry has used for the last two decades.
An important enhancement in 11ax is a 4x longer duration of OFDM symbols as compared to 11ac. This drives increased efficiency and higher data rates. As a result, outdoor deployments such as on a large campus or in a stadium will see significantly better capacity and up to four times greater speeds for clients even at the cell boundaries.
Imagine a stadium full of happy sports fans, with phones in hand, streaming video and posting to social media over great Wi-Fi coverage. You read that right!
2. Improved battery life and power savings. 11ax enhances the power save mechanism in which the specific time to access wireless medium is negotiated between the AP and the clients. With the help of the new Target Wake Time (TWT) capability, the clients are made aware when it is their turn to transmit by a triggering mechanism that awakens the client at pre-negotiated times. With TWT, there is reduced contention for the medium between clients, and hence there is much less overlap between several users trying to transmit at the same time. The sleep time of Wi-Fi clients increases multifold.
Because of this capability, handheld or battery-powered portable devices can conserve more power than ever before. IoT devices also can make use of this power saving mechanism, and we expect to see more sensors and smart devices designed to take advantage of this capability.
Imagine spending the day at a public place such as a zoo or a shopping mall and reaching for your phone at the end of the day to see that the battery isn’t about to die. Yep!
3. Dual-band operation. The 11ac standard operates only in the 5GHz band. Unlike its predecessor, 11ax operates in both 2.4GHz and 5GHz bands. 11ax aims at improving the efficiency of both bands, while providing backward compatibility with the previous amendments of the IEEE standard.
Many wireless devices today operate solely in the 2.4 GHz band, such as fitness watches, wearables, mesh links and certain cell phone radios. The 2.4 GHz band is widely used in healthcare industry as well. Improved spectral efficiency supports the explosive growth of devices in the 2.4 GHz band, providing a better Wi-Fi experience for legacy and new Wi-Fi client devices.
Imagine the innovation as the next wave of smart connected devices are invented. And the peaceful coexistence of old and new mobile devices.
802.11ax promises real-world performance improvements that will keep today’s mobile users and IoT devices connected and happy. I’m excited to see all the new possibilities!
802.11ax white paper