802.11n documentation

doc/app_notes/ieee80211n.md

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+## IEEE 802.11n (Wi-Fi 4)
+
+The 4th generation of Wi-Fi (i.e. 802.11n-2009) is a generation leap over its predecessor Wi-Fi 3 (i.e. 802.11g-2003). It was coined as high throughput (HT) since it improves both the physical layer and the MAC layer.
+
+The physical layer performance boosted from 54Mbps -> 600Mbps and a total of 5 major improvements were amended on top of Wi-Fi 3.
+
+### PHY layer improvments
+
+#### More subcarriers
+Wi-Fi 3 utilizes 48 OFDM data subcarriers and Wi-Fi 4 increased this number to 52, thereby increasing the throughput to 52/48 * 54Mbps = 58.5Mbps.
+
+![](./subcarriers.png)
+
+#### Forward error correction
+The most efficient coding rate used in Wi-Fi 3 was 3/4 but Wi-Fi increased this value to 5/6 by squeezing more bits. This increased the throughput by (5/6)/(3/4) * 58.5Mbps = 65Mbps.
+
+#### Guard interval
+As a measure to combat inter-symbol interference (ISA), Wi-Fi 3 utilizes 800nsec of guard interval between consecutive OFDM symbols. Wi-Fi 4 shortens this value to 400nsec and this resulted in a throughput change of 4usec/3.6usec * 65Mbps = 72.2Mbps.
+
+![](./guard-interval.png)
+
+#### MIMO
+Wi-Fi 4 was the first to introduce MIMO and standardized 4x4 spatial streams. This quadrupled the throughput to 4*72.2Mbps = 288.9Mbps.
+
+![](./mimo.png)
+
+#### 40MHz bandwidth
+The last thing Wi-Fi 4 introduced to the physical layer is a 40MHz bandwidth utilizing 108 OFDM data subcarriers. This increased the throughput to 108/52 * 288.8Mbps = 600 Mbps.
+
+![](./40mhz.png)
+
+### MAC layer improvements
+
+On top of the PHY layer improvements, Wi-Fi 4 also introduced frame aggregation at the MAC layer to ease the medium access contention. Two types of frame aggregation are standardized in Wi-Fi 4; A-MPDU and A-MSDU. While A-MSDU is efficient in medium occupation, a single packet error will make the whole frame unusable and require a complete retransmission. However, A-MPDU aggregates multiple MPDUs by adding headers to each packet and a single packet error only requires a single packet retranmission. As such, A-MPDU gained traction.
+
+![](./mpdu-aggr.png)
+
+
+## Supported 802.11n ammendments
+
+- 52 subcarriers
+- 5/6 code rates
+- 400nsec short guard interval.
+
+Current theoretical throughput = 72.2Mbps.
+
+## Unsupported 802.11n ammendments
+
+- MIMO
+- 40MHz bandwidth
+- Frame aggegation
+