17b8dedc6Smmehari 27b8dedc6Smmehari## IEEE 802.11n (Wi-Fi 4) 37b8dedc6Smmehari 47b8dedc6SmmehariThe 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. 57b8dedc6Smmehari 6*e8b22efaSMichael Mehari### PHY layer improvements 7*e8b22efaSMichael MehariAt the physical layer, 5 major improvements were amended on top of Wi-Fi 3 and this has increased the throughput from 54Mbps to 600Mbps. 87b8dedc6Smmehari 97b8dedc6Smmehari#### More subcarriers 107b8dedc6SmmehariWi-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. 117b8dedc6Smmehari 127b8dedc6Smmehari 137b8dedc6Smmehari 147b8dedc6Smmehari#### Forward error correction 15*e8b22efaSMichael MehariThe 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 has increased the throughput to (5/6)/(3/4) * 58.5Mbps = 65Mbps. 167b8dedc6Smmehari 177b8dedc6Smmehari#### Guard interval 18*e8b22efaSMichael MehariAs 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 has increased the throughput to 4usec/3.6usec * 65Mbps = 72.2Mbps. 197b8dedc6Smmehari 207b8dedc6Smmehari 217b8dedc6Smmehari 227b8dedc6Smmehari#### MIMO 23*e8b22efaSMichael MehariWi-Fi 4 was the first to introduce MIMO and standardized 4x4 spatial streams. This has quadrupled the throughput to 4*72.2Mbps = 288.9Mbps. 247b8dedc6Smmehari 257b8dedc6Smmehari 267b8dedc6Smmehari 277b8dedc6Smmehari#### 40MHz bandwidth 28*e8b22efaSMichael MehariThe last thing Wi-Fi 4 introduced to the physical layer is a 40MHz bandwidth utilizing 108 OFDM data subcarriers. This has increased the throughput to 108/52 * 288.8Mbps = 600 Mbps. 297b8dedc6Smmehari 307b8dedc6Smmehari 317b8dedc6Smmehari 327b8dedc6Smmehari### MAC layer improvements 337b8dedc6Smmehari 34*e8b22efaSMichael MehariOn 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 used 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 complete retransmission. However, A-MPDU aggregates multiple MPDUs by adding headers to each packet and a single packet error only requires single packet retransmission. As such, A-MPDU gained traction. 357b8dedc6Smmehari 367b8dedc6Smmehari 377b8dedc6Smmehari 387b8dedc6Smmehari 39*e8b22efaSMichael Mehari## Supported openwifi 802.11n amendments 407b8dedc6Smmehari 417b8dedc6Smmehari- 52 subcarriers 427b8dedc6Smmehari- 5/6 code rates 437b8dedc6Smmehari- 400nsec short guard interval. 447b8dedc6Smmehari 457b8dedc6SmmehariCurrent theoretical throughput = 72.2Mbps. 467b8dedc6Smmehari 47*e8b22efaSMichael Mehari## To be supported openwifi 802.11n amendments 48*e8b22efaSMichael Mehari- Frame aggregation 49*e8b22efaSMichael Mehari 50*e8b22efaSMichael Mehari## Not supported openwifi 802.11n amendments 517b8dedc6Smmehari 527b8dedc6Smmehari- MIMO 537b8dedc6Smmehari- 40MHz bandwidth 54
