Same Channel, No Interference

Spatial Frequency Reuse (SFR) technology ensures that neighboring WiFi stations are allowed to transmit on the same radio channel at the same time. Normally, they would interfere with each other. However, in some situations the signal strength and signal-to-noise ratio in the individual cells are large or good enough to allow transmissions to take place.

This feature improves channel capacity by allowing access points to make smarter decisions about when to transmit data. In general, it can be assumed that cell sizes will have to be reduced in future WiFi plans to benefit from the higher data rates offered by the new QAM-1024 modulation method.

According to the 802.11 standard, the service set refers to all devices on a WiFi network. A basic service set (BSS) is created by synchronizing the basic parameters of several devices that can be addressed by a service set identifier (SSID). The BSS coloring method was introduced with the 802.11ah standard and is used to assign a different "color" to each BSS. To increase capacity in dense WiFi environments, the ability to reuse frequencies between BSSs has to increase. However, with the media access rules used in the past, the WiFi terminals were moved from one BSS to another co-channel BSS without increasing network capacity (Figure 2).

Figure 2: BSS coloring allows the use of additional channels.

The BSS coloring method available in WiFi 6 optimizes the competition overhead of the devices because of overlapping transmission channels. By doing so, it enables spatial reuse of these channels. WiFi 6 devices can distinguish between different BSSs by adding a number (color) to the physical layer (PHY) header. The new channel access behavior is based on this assigned color. For example, the same color bit indicates intra-BSS communication (communication on the same WiFi network), whereas different color bits indicate inter-BSS communication (communication between different WiFi networks). Inter-BSS detection means that a WiFi receiver has to view the medium as busy and postpone its transmission until the channel is free.

An adaptive implementation can increase the signal detection (SD) threshold for inter-BSS frames while maintaining a lower threshold for intra-BSS traffic. BSS coloring thus reduces channel conflicts that result from the previously valid 4dB signal detection thresholds. The aim of BSS coloring is therefore to enable the reuse of transmission channels without causing significant interference. The crucial point here is that the medium is only considered occupied if a color is unambiguously identified by a transmitter.

Improved Battery Life

Each new WiFi standard extends battery life, because the radio range is usually increased and data is transmitted faster. One previously unchanged burden on the battery life of clients was always having to check whether the access point was available and ready for data transmission. WiFi 6 therefore introduces a new feature known as target wake time (TWT) scheduling.

TWT allows access points to tell clients when they can go to sleep and provides a schedule for when the device should wake up. These sleep times can be very short periods of time (literally milliseconds between sleeping and waking). TWT nevertheless increases the amount of time the device is idle, therefore improving battery life. For WiFi 6, the TWT mechanism enshrined in the 802.11ah standard has been modified to support WiFi participants who have not negotiated a hibernation agreement with the AP.

TWT is very useful for both mobile and Internet of Things (IoT) devices. TWT uses fixed policies on the basis of expected traffic activity between 802.11ax clients and an 802.11ax AP to set a scheduled wake time for each client, which means that WiFi 6 IoT clients can sleep for hours or even days.

With the 802.11ac standard, the TWT function is only supported by 80MHz-capable WiFi clients, whereas the new WiFi 6 specification also supports 20MHz clients. Therefore, TWT is available for low-power, low-complexity devices (e.g., portable devices, sensors, IoT devices).

Precise Device Synchronization

Thanks to WiFi technology, almost all devices can now be connected wirelessly. In the past, this technology still suffered from minor timing difficulties, like where synchronicity was important for video and audio streaming. Now the Wi-Fi Alliance has found a way to overcome this shortcoming. The Wi-Fi TimeSync certification industry group has published a specification for precise time synchronization for WiFi devices [2].

When WiFi technology came onto the market more than 20 years ago, only data that was not time-critical had to be transmitted without errors between computers. Clock synchronization was not an issue. Because of the deep penetration of WiFi technology in the fields of home entertainment and media production, the timing question has become significant.

For example, stereo sound can be produced over loudspeakers located on opposite sides of a room. If WiFi is used to transmit the information, the connecting cables between the transmitter and the loudspeakers are no longer necessary. However, as long as these devices do not output the signals synchronously, the stereo effect is lost. Outputting the right sound in a home theater becomes even more difficult when the TV controls a subwoofer and several speakers wirelessly.

Wi-Fi Certified TimeSync was developed for exactly these kinds of applications. It ensures that the clocks on the various devices are synchronized to less than a microsecond, which allows each component to perform its task at the right time.

TimeSync not only synchronizes the clocks on each certified device automatically, it can also be used to synchronize entire groups of devices. For example, the systems of several manufacturers can work together in a time-dependent application. (See the "Applications for Wi-Fi TimeSync" box.)