Published on: Dec 2, 2022
So far, many of our blogs have talked about how mmWave enables new and exciting applications for 5G and the future 6G networks. Besides these cellular networks, there is another wireless network technology that is indispensable in our daily lives – the WiFi network. There has also been significant research in mmWave WiFi, culminating in two industrial standards: IEEE 802.11ad [1] and its successor IEEE 802.11ay [2]. In this blog, we will introduce IEEE 802.11ad and its significance for the future of WiFi.
IEEE 802.11ad overview and features
In 2012, IEEE 802.11ad expanded the WiFi family to include mmWave operation in the 60 GHz band. It represented a significant achievement as it introduced, for the first time, specifications for Medium Access Control (MAC) and Physical (PHY) layer WiFi operation in the mmWave frequency bands. Most crucially, this included procedures for directional communication, which is necessary because of the increased attenuation in the 60 GHz band. The most important features of the IEEE 802.11ad standard are introduced below.
Physical (PHY) layer characteristics: IEEE 802.11ad defines three PHY layers: the control PHY, the Single Carrier (SC) PHY, and the Orthogonal Frequency-Division Multiplexing (OFDM) PHY. The control PHY is designed for robust communication at low Signal-to-Noise Ratio (SNR) before beamforming training and is used for control messages. The SC and OFDM PHYs are designed for data transmissions. SC PHY provides power-efficient, low-complexity communication and a throughput of up to 4.62 Gbps. On the other hand, the OFDM PHY is designed for high performance and can support peak data rates of 6.75 Gbps. IEEE 802.11ad’s ability to support multi-Gbps throughput is enabled by a channel bandwidth of 2.16 GHz, which is much wider than the channel bandwidth at sub-6 GHz frequencies. With this, IEEE 802.11ad can support emerging throughput-demanding applications, such as wireless backhauling, virtual/artificial reality and multimedia streaming over IP.
Fig. 1 Beacon Interval structure introduced in in IEEE 802.11ad
Redesign of the Beacon Interval (BI): The BI is the basic organizational unit for channel access in WiFi. Usually, it starts with a beacon sent by the Access Point (AP) and lasts for approximately 100 ms. The beacon advertises the WiFi network to nearby users that might want to join and is therefore crucial for association. IEEE 802.11ad significantly modified the BI, dividing it into two main periods, as seen in Figure 1. The first period is reserved for management information before data exchange can take place in the second period. It was necessary to expand the single beacon transmission from previous WiFi standards into a larger period due to the need for directional transmission and the unknown location of unassociated users. This includes sending multiple beacons with different directional beampatterns to reach users all around the AP, assigning a separate period for initial beamforming training of new users and for exchanging management information with beamtrained stations.
Medium Access: IEEE 802.11ad offers two options for data exchange – in periods with contention-based channel access that is typical for WiFi, or with a new scheduled approach. In the first case, users access the channel on demand without a fixed schedule and compete to transmit. Before a user transmits, they listen to the channel or “carrier sense” and based on the received energy determine if it is already occupied. However, in mmWave the carrier sensing is very unreliable because of the use of directional antennas and the low transmission range, making it difficult to accurately sense whether someone else is transmitting. Therefore, IEEE 802.11ad introduced scheduled access where the AP creates a transmission schedule for users. This ensures that no collisions will happen, but is also inefficient for bursty traffic (like web traffic) and thus, is not always appropriate. Lastly, IEEE 802.11ad defines a third option where polling is used to dynamically allocate channel resources to users.
Beamforming Training: As explained in previous blogs (Where to point the beam? and Have you ever heard (of) Beamforming?), beamforming training is crucial for mmWave, since it allows devices to align their beams to take advantage of the increased gains of directional beampatterns. IEEE 802.11ad has two beamforming training mechanisms – the Sector Level Sweep (SLS) and the Beam Refinement Protocol (BRP). SLS trains the initial beampatterns for communication and is mandatory to implement. It is composed of a series of packets, each one transmitted or received with a different beampattern. The optimal beampattern is chosen by measuring the signal quality of each received packet. SLS is simple, reliable and robust, however, it has high overhead and scales badly. The optional BRP phase is used for further beam refinement to get maximal gain. It reduces the overhead of SLS as multiple beampatterns can be trained in a single packet, however it is more complex to implement since it requires fast switching between beampatterns.
Fast Session Transfer: mmWave networks have limited range and are very vulnerable to blockage and obstacles, making them unreliable. To overcome this, IEEE 802.11ad offers Fast Session Transfer (FST) which switches from 60 GHz to the more stable 2.5/5 GHz band when the connection degrades. In this way, the device does not experience communication outages, offering a better user experience.
Relay Operation: Another mechanism to improve mmWave reliability and extend range is relay mode, where two devices communicate through a third, relay device. There are two operation modes for relays – link switching, where either the direct or the relay link is active at one time, and link cooperating, where both links are simultaneously used to improve the signal quality.
The approval of IEEE 802.11ad allowed vendors to develop commercial products that support mmWave WiFi, resulting in a wide range of devices available today. TP-LINK Talon AD7200 [6] was the first router to support IEEE 802.11ad. MicroTik has since launched a whole range of products at 60 GHz [7]. Another notable router is the Netgear Nighthawk X10 [8], as shown in Figure 2 below. There has also been development on user devices – including the Dell Wireless Docking Station WLD15 [9], and recently, ASUS ROG Phone II [10] became the first smartphone to support 60 GHz WiFi.
Fig 2: The Netgear Nighthawk X10
Codifying mmWave into a WiFi standard was an incredibly important step towards the widespread use of mmWave band. IEEE 802.11ad redesigned WiFi to deal with the challenges of the 60 GHz band, allowing for a proliferation of commercial devices that offer multi-Gbps throughput. However, it was not able to fully reach the potential of mmWave technology, leading to the development of its successor, IEEE 802.11ay. In the next editions of our blog we will introduce you to the newest member of the mmWave WiFi standard, IEEE 802.11ay and the exciting opportunities it presents.
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References
[1] IEEE, “Wireless LAN(MAC and PHY Specifications Amendment 3: Enhancements for Very High Throughput in the 60 GHz Band,” IEEE Std 802.11ad, 2012.
[2] IEEE, “Wireless LAN MAC and PHY Specifications Amendment 2: Enhanced Throughput for Operation in License-Exempt Bands Above 45 GHz,” IEEE Std 802.11ay, 2021.
[3] https://b5g-mints.eu/blog9/
[4] https://b5g-mints.eu/blog14/
[6] https://www.tp-link.com/us/home-networking/wifi-router/ad7200/#overview
[7] https://mikrotik.com/products/group/60-ghz-products
[8] https://www.netgear.com/home/wifi/routers/ad7200-fastest-router/
[10] https://rog.asus.com/phones/rog-phone-ii-model/
