• Home
  • /
  • IOT
  • /
  • WiFi 6 enhancements explained: TWT and 1024-QAM

WiFi 6 enhancements explained: TWT and 1024-QAM

Digging into two further WiFi 6 enhancements – target wake time and 1024 quadrature amplitude modulation.
Andre Kannemeyer
By Andre Kannemeyer, National chief technical officer (CTO) at specialist distributor Duxbury Networking.
Johannesburg, 11 May 2020

Dense device environment deployment will be made possible with WiFi 6. This will result in the support of higher service-level agreements to more concurrently connected users and devices with more diverse usage profiles.

In my previous Industry Insight, I discussed basic service set colouring and in this article I will focus on two further WiFi 6 enhancements – target wake time (TWT) and 1024 quadrature amplitude modulation (1024-QAM).

TWT is a WiFi 6 enhanced power-saving mechanism which conserves battery life by enabling devices to determine when and how frequently they will wake up to send or receive data.

A TWT is a negotiated agreement, based on expected traffic activity between the access point (AP) and clients, to specify a scheduled target wake-up time for WiFi 6 clients in powersave (PS) mode. The negotiated TWTs allow an AP to manage client activity by scheduling client stations to operate at different times, in order to minimise contention between the clients. A TWT reduces the required amount of time that a client station in PS mode needs to be awake. This allows the client to ‘sleep’ longer and reduce energy consumption.

As opposed to legacy client power-saving mechanisms such as delivery traffic indication map, which require sleeping client devices to wake up in microsecond intervals, TWT could theoretically allow client devices to sleep for hours.

Although the primary goal of WiFi 6 is increased efficiency, more speed is not a bad thing.

TWT is thus an ideal power-saving method for mobile devices and Internet of things (IOT) devices that need to conserve battery life.

As depicted in Figure 1, a TWT frame exchange is used between the AP and the clients to negotiate a scheduled TWT. For each WiFi 6 client there can be as many as eight separate negotiated scheduled wake-up agreements for different types of application traffic. Once the negotiation is complete, the clients sleep and then awaken at the targeted intervals.

Figure 1: Target wake time
Figure 1: Target wake time

Most IOT devices with a WiFi radio currently transmit in the 2.4GHz frequency band. The ideal would be for IOT device manufacturers to take advantage of WiFi 6 radios in their IOT devices as opposed to other communication technologies such as Bluetooth Low Energy, Sigfox and Zigbee.

Although the primary goal of WiFi 6 is increased efficiency, more speed is not a bad thing. Elevated efficiency and more speed are not mutually exclusive goals. Quadrature amplitude modulation (QAM) uses both the phase and amplitude of a radio frequency signal to represent data bits. WiFi 6 will support 1024-QAM and new modulation and coding schemes that define higher data rates.

For comparison, 256-QAM (introduced in 802.11ac) modulates eight bits per symbol, whereas 1024-QAM modulates 10 bits per symbol − a potential 25% increase in data throughput.

Much like 256-QAM, very high signal-to-noise ratio thresholds (35 decibels or more) will be needed in order for WiFi 6 radios to use 1024-QAM modulation. Pristine radio frequency environments with a low noise floor and close proximity between a WiFi 6 client and a WiFi 6 AP will most likely be needed.

A constellation diagram, also known as a constellation map, is a two-dimensional diagram often used to represent QAM modulation. A constellation diagram is divided into four quadrants, and different locations in each quadrant can be used to represent data bits. Areas on the quadrant relative to the horizontal axis can be used to represent various phase shifts. Areas relative to the vertical axis are used to represent amplitude shifts.

The number of points in the modulation constellation chart determines the number of bits conveyed with each symbol. Figure 2 shows a comparison of constellation charts between 256-QAM and 1024-QAM modulation. As you can see, 1024-QAM has many more constellation points.

Error vector magnitude (EVM) is a measure used to quantify the performance of a radio receiver or transmitter in regard to modulation accuracy. With QAM modulation, EVM is a measure of how far a received signal is from a constellation point. Any WiFi 6 radios that use 1024-QAM modulation will need strong EVM and receive sensitivity capabilities.

It is expected there will be over 30 billion connected things this year, so the higher wireless throughput represented by 1024-QAM will play a large role in ensuring quality-of-service in high-density locations such as transportation hubs, sports stadiums, convention centres and auditoriums.

Figure 2: 256- and 1024-QAM
Figure 2: 256- and 1024-QAM