A look at how the new 802.11ad WiFi standard compares with its predecessors, 802.11ac and 802.11ax.
The IEEE has ratified a standard called 802.11ad, also known as WiGig. It's the next frontier for WiFi after 802.11ax debuts in the market this year. WiFi progression for the past 10 years has been evolutionary. WiGig, however, has some fundamental differences from its predecessors, as I outline in this article.
1. Range and coverage
While WiFi currently works on 2.4 GHz and 5 GHz frequencies, WiGig is designed for 60 GHz frequency. Since radio-signal attenuation is proportional to the square of frequency, the WiGig signal attenuates 100x more than the current WiFi signal at a given distance. WiGig fights this attenuation using narrow signal beams on transmit and receive sides. Even then, WiGig's range is only about 30 feet. Further, since the 60 GHz signal does not penetrate obstacles such as walls and human bodies, the coverage of one WiGig cell is limited to a single room, with clear line of sight between the transmitter and the receiver.
2. Multiple-antenna transmit/receive
WiFi currently uses a multiple-antenna technique called MIMO for “spatial multiplexing.” The objective is to increase throughput over the wireless link. Radio signals travel from transmitter to receiver over multiple paths due to reflections from the surrounding objects; this is called “multipath.” MIMO leverages multipath by using signal processing to create multiple transmission streams over wireless between the transmitter and the receiver, called spatial streams (SS). Different information streams are transmitted concurrently over different SS. Today, most clients have one or two antennas, so 1-SS and 2-SS transmissions are most common.
WiGig uses multiple antennas in a different way, called “beamforming," which is required to fight signal attenuation at 60 GHz. WiGig beamforming uses phased antenna array, in which relative phase shifts are introduced in signals at different antennas in the array to create a flashlight-like transmit/receive beam in the desired direction. This provides a signal power boost in that direction, which is called “array gain.” A WiGig access point can have up to 64 antennas to create as many as 128 beams.
Note that WiFi currently supports beamforming, but implements it differently. It combines antenna signals to not only attain a signal power boost with array gain, but also to harness multipath for diversity gain. This requires more complex signal processing based on detailed channel feedback. Beamforming is currently optional in WiFi and practical with small number of antennas, such as up to four.
Elaborate procedures are required in WiGig for signal beam alignment between the transmitter and the receiver, and are provided in the 802.11ad MAC protocol. For beam alignment, an AP transmits beacons successively on different beams to allow a client to register the best-heard beam from the AP. This is called Sector Level Sweep (SLS). Then the client performs SLS by transmitting on its beams to help the AP register the best-heard beam from the client. Each side provides feedback to the other side about the best-heard beams, which are then used for communication between them. There are also provisions for fine-tuning of beam alignment with Beam Refinement Protocol (BRP) and continuous alignment with beam tracking. Beam alignment is also done to support newly introduced direct client-to-client transfer mode.
3. Multiple access
A multiple access scheme called CSMA is dominant in today’s WiFi. In CSMA, a device that is ready to transmit first performs channel sensing and backs off if the channel is found to be busy. CSMA distributes control of channel access among devices, while avoiding collisions. A collision involves two devices transmitting simultaneously to the same receiver. In WiGig however, due to the use of directional transmit and receive beams, CSMA suffers from a “deafness” problem. For example, if device A is transmitting to device B using beamforming, device C cannot detect this ongoing transmission unless C’s receive beam is aligned with the transmit beam of A. Deafness results in a higher rate of collisions.
So, a new channel access mode called Service Period (SP) is added in WiGig, in which transmission schedules are allocated by APs to clients. Overall time on the channel is organized into intervals called Beacon Intervals (BI). Each BI starts with SLS beacons from an AP as described above, followed by client SLS frames, then by SP schedule distribution to clients. The remaining time of BI is partitioned into CSMA and SP access windows. SP access is expected to be the preferred channel access in WiGig.
4. Basic service set types
Today’s WiFi supports two modes of operation: infrastructure BSS and ad hoc BSS. In infrastructure BSS, all clients talk through the AP. In ad hoc BSS, clients talk directly to one another.
WiGig introduces a mixed mode called PBSS in which there is a central coordinator, such as an AP, but clients do not have to talk through the AP. Clients can also talk to one another directly via CSMA or SP schedule distributed by the AP. This is designed for applications such as a tablet that's streaming HD video to a display so it doesn't have to send video traffic through the AP, but can still connect elsewhere in the network through the AP. Interestingly, with SP access described above, it is now possible to schedule one client to talk to the AP and two other clients to talk directly to one another at the same time, assuming their signal beams aren’t aligned.
It may be tempting to say that WiGig is the fastest WiFi. However, the maximum possible data rate supported in 11ax is 9.6 Gbps, while that in WiGig is 7 Gbps. Data rates realized in practice are often order of magnitude smaller than the maximum limit.
More importantly, WiGig may encounter a cleaner RF environment at 60 GHz. WiGig won’t cause interference between regions separated by walls. Thus, WiGig may not suffer from the interference problem that we see today in crowded 2.4 GHz and 5 GHz WiFi. That said, the 2 GHz-wide channels of WiGig have further potential for data-rate improvement, which 802.11ad's successors -- such as 802.11ay -- will tap into.
Future WiFi devices are expected to be multi-band supporting 2.4 GHz, 5 GHz, and 60 GHz radios. There will be intelligence built into devices to use the most appropriate band to support user activity. Connection transfers between bands will also be supported, for example, using the WiFi Agile Multiband scheme from the WiFi Alliance. Thus, WiGig is expected to seamlessly add extra capacity for WiFi applications of the future.