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Wi-Fi After 802.11n: It’s 802.11ac

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It has been relatively silent for a while on how Wi-Fi is going to develop now that the 802.11n standard has become widely accepted. But behind the scenes, companies in the IEEE are silently working on the next generation of the standard with first and interesting results.

Perhaps they have run out of single letters as the next version of the Wi-Fi specification will be called 802.11ac. Wikipedia contains an interesting entry on the enhancements and links to a current draft specification of the IEEE. The link is pretty interesting as it is the first time I see the IEEE publish drafts in public. Previously, things were kept inside the IEEE community until things were finished. A new openness?

Anyway, so here’s the features currently under development:

Wider channel bandwidths

The initial 802.11b, a and g standards were defined for channel bandwidths of 20 MHz. 802.11n then introduced channel bundling to 40 MHz. While this in theory doubles the available data rate, the issue especially in the 2.4 GHz band is that foremost in cities there many access points are on air and an enlargement to 40 MHz makes it even less likely that one can find an unused spot in the band.

As a consequence several access points use the same channel and hence capacity of a network becomes dependent on how much data is transferred in other networks on the same channel. If the channel used by other networks is fully overlapping, interference is limited but data can still not be transferred, while a packet is sent or received by another access point. The collision avoidance scheme makes sure that packets are seen by other access points and clients so lower speeds are not a consequence of interference but of the transmitters waiting to catch an opportunity to send their own packets to their own access points on the channel. There is also the scenario in which channels are only partly overlapping. In this case only a part of the channel used by an access point is used and the packets are therefore not detected.

This results in a reduction of throughput due to perceived interference as due to the only partial channel overlap the packets of the other network can not be correctly received and thus, collision avoidance does not work.

With 802.11ac, bandwidth aggregations of up to 80 MHz and 160 MHz are defined, making the issues described above even worse. In 2.4 GHz it's not even possible to aggregate 160 MHz as the overall channel is smaller than that. There is more spectrum available in the 5 GHz band but even here, 80 MHz or 160 MHz channel aggregations will be a stretch. Also, networks using the 5 GHz band have a more limited coverage area than on 2.4 GHz, as signal absorption through walls and by obstacles is much higher than on 2.4 GHz.

More MIMO Streams and Multi-User MIMO

802.11n introduced Multiple Input Multiple Output (MIMO) transmission, i.e. transmitting several independent spatial streams over the same channel simultaneously. 2, 3 and 4 antennas for the same number of spatial streams are defines so far and 802.11ac increases the number to up to 8 spatial streams. Enjoy 8 antennas on your access point and mobile devices.

Mobile devices can have fewer antennas an will consequently only be able to use the number of spatial streams in line with their number of antennas. 802.11ac, however, specifies Multi User MIMO, i.e. if the access point can handle more data streams than mobile devices, several devices can send their data simultaneously. In the other direction, the access point can send several MIMO streams to different devices simultaneously.

Even Higher Modulation

The current state of the art Wi-Fi uses up to 64-QAM modulation if access point and mobile device are close to each other. 64-QAM encodes 6 data bits per transmission step. 802.11ac takes this one step further to 256-QAM, i.e. transmitting 8 bits per transmission step. The coding rate for such transmissions, which is a measure for the number of user data bits to error detection and correction bits is 3/4 and 5/6.

Backwards compatibility

And, of course, a most important requirement is that 802.11ac devices and access points must be able to co-exist with their older 802.11 devices.

An Interesting Future

A marvelous challenge to put all of this in a specification. And even more of a challenge, I am sure, to put this into real devices and make it work in a backward compatible way. Things remain interesting!

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转载请注明:自由的风 » Wi-Fi After 802.11n: It’s 802.11ac

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