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Tutorial Topic Sections
Intended to be read in sequence

1 - Establishment of the 802.11ac and 802.11ad Standards 6 - QAM Modulation and OFDM Symbols
2 - Transmit Output Power 7 - Comparing 802.11ac and 802.11ad QAM and OFDM Implementation
3 - Oxygen Absorption of RF at 60 GHz 8 - Real-World Expectations for 802.11ac and 802.11ad
4 - Channel Width and Guard Interval 9 - Antenna Differences: Beamsteeering, Gain and Range
5 - MIMO and Implementation of Multiple Spatial Streams 10 - Overall Perspective and Conclusions

Channel Width and Guard Interval

The massive channel width in 802.11ad (2.16 GigaHertz wide!) is the primary reason why 11ad is expected to provide greater throughput than 802.11ac. Let’s see what the details are behind 802.11ac offering up to 6.93 Gbps and 802.11ad offering up to 7 Gbps. At first glance the roughly 7 Gbps maximum throughput offered by both 802.11ac and 802.11ad appear to be the same – they’re not. Both 11ac and 11ad maximum throughput specifications assume the best case scenario for all aspects of the transmission. Best case scenario for transmission includes minimum noise, maximum channel width, maximum modulation rate and minimum inter-symbol spacing. Maximum connection rate also assumes the maximum number of MIMO spatial streams will be available – they never are. In the real-world you will never achieve the best case scenario for transmission with any wireless technology.

The first thing we’ll consider is the intersymbol Guard Interval which can be either 400 ns (nanoseconds) or 800 ns in 802.11ac (and 802.11n). The normal setting for Guard Interval is “Automatic” – the access point picks the GI. The Guard Interval allows time for all significant reflected signals to be gone before the start of the next symbol transmission. If there aren’t many undesirable reflections (long path; long time delay) then the 400 ns GI can be used. Of course, some reflections are required to use Multiple-Input/Multiple-Output (MIMO) but too many long path reflections are bad. When there are too many long delay paths then the previous symbols reflections will “walk on” the current symbol. Long path reflections almost always occur with high transmit power and high gain antennas in large, enclosed areas. Even in a carpeted office with a large cubicle area it’s not uncommon to discover that the Guard Interval is adjusted to 800 ns to mitigate long reflections. In practice it’s almost always the case that the 800 ns Guard Interval is selected by a typical commercial-grade access point. There are very few environments that can support a 400 ns Guard Interval consistently. With the 800 ns Guard Interval in use you achieve roughly 10% less throughput than would be the case with a 400 ns Guard Interval. Whatever theoretical 11ac connection is determined based on channel width and modulation type, knock it down by 10% for the 800 ns Guard Interval.