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802.11ax: Optimistic CSMA for Efficient Channel Reuse in WiFi

by Hemant Chaskar on Jul 9, 2016

802.11ax is the new 802.11 standard currently in the making. Unlike earlier 802.11 standards that mainly focused on increasing raw link speeds, the design objective now is to increase airtime efficiency. One feature it introduces is OFDMA (Orthogonal Frequency-Division Multiple Access) to address the airtime inefficiency caused by short WiFi frames. The other is dynamic sensitivity control, which modifies traditional CSMA (Carrier Sense Multiple Access) to address airtime inefficiency caused by co-channel interference during channel reuse.

What Table Manners Teach about Co-Channel Interference

Imagine that you are with a group in a crowded restaurant. There are other groups of people at other tables. You listen when someone at your table is speaking. You also overhear people at other tables. If the person at your table is loud enough, you can make out what is being said even in the noise. When you speak, your volume depends on the volume of people at other tables. In general, if you can maintain a comfortable margin over the ambient noise, the people at your table can hear you.

Co-Channel Interference

The crowded restaurant analogy is a way of thinking about “co-channel interference” in WiFi. A WiFi cell consists of an access point (AP) and its connected clients, and is called a Basic Service Set (BSS). Different BSS’s operate on different channels, but since the total number of channels is limited, the channels need to be reused over some distance. If the reuse distance is small, the BSS’s operating on the same channel cause mutual interference. This co-channel interference is severe in dense WiFi deployments.

The 802.11 protocol tackles co-channel interference with a technique called carrier sense. A client or an AP (called a station) listens to the channel before transmitting. If it can hear the preamble of another WiFi frame on that channel, it defers its own transmission. The standard dictates that WiFi radios must be able to hear preambles received at -82 dBm or higher signal strengths. This is called the “carrier sense threshold.” In practice, radios can hear preambles received at even lower signal strengths. These low and static carrier sense thresholds often cause BSS’s to take turns at transmission when they can hear one another, even when co-channel interference between them isn’t strong. In the restaurant analogy, this would be like waiting to speak even when you can barely hear some conversation at another table. The co-channel interference effectively splits the channel’s total capacity among the co-channel BSS’s that are near one another, instead of the channel capacity being spatially replicated.

BSS Coloring

802.11ax sets out to solve the problem by including a new field called “BSS color” in the preambles of WiFi frames. It is an identifier of the BSS in which the corresponding frame is transmitted. All stations within hearing distance of the frame preamble read BSS color during the carrier sense process. They can thus identify the BSS of the frame transmission as well as determine its received signal strength. The decision to transmit or defer is now governed by the new logic as follows.

Parallel Transmissions in Nearby BSS’s

If the BSS color indicates that the detected transmission is in the station’s own BSS, then the station defers its transmission. However, if the BSS color indicates that the detected transmission is in a different BSS, then the station need not defer its transmission (need not set the clear channel assessment or CCA state to BUSY), as long as the received signal strength of the detected preamble is below a threshold.

Additional aspects of the above process include:

  1. configuration of a threshold for co-channel interference from another BSS to be used in the defer/transmit decision process,
  2. using just enough transmit power above the co-channel interference to limit interference to nearby BSS’s (called “transmit power control” or TPC), and
  3. use of the RTS/CTS mechanism, to ensure that the receiver can actually decode the transmission over the co-channel interference.

Some of the above parameters can be communicated to the clients from the AP and optimized on an ongoing basis.

Comparison with Proprietary Techniques

The above is conceptually similar to the proprietary sensitivity adjustment techniques that some APs provide today to permit parallel transmissions in the wake of co-channel interference. However, the proprietary techniques are applicable only to AP transmitters, obviously because AP radio implementations are in the AP vendors’ control. The 802.11ax technique will work on both APs and clients as it is standards based. Also, proprietary techniques did not have the BSS color identifier in the preamble, so decisions were based only on signal strength thresholds only. Finally, a standards-based technique provides a more objective approach compared to a lot of heuristics embedded in the proprietary techniques.

All things said, we will have to wait for ratification and penetration of 802.11ax before real world benefits can be quantified. Fingers crossed!

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Topics: 802.11ax, Dynamic Sensitivity Control, BSS coloring, air time efficiency, carrier sense, co-channel interference, CSMA