1*7b8dedc6Smmehari 2*7b8dedc6Smmehari## IEEE 802.11n (Wi-Fi 4) 3*7b8dedc6Smmehari 4*7b8dedc6SmmehariThe 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. 5*7b8dedc6Smmehari 6*7b8dedc6SmmehariThe physical layer performance boosted from 54Mbps -> 600Mbps and a total of 5 major improvements were amended on top of Wi-Fi 3. 7*7b8dedc6Smmehari 8*7b8dedc6Smmehari### PHY layer improvments 9*7b8dedc6Smmehari 10*7b8dedc6Smmehari#### More subcarriers 11*7b8dedc6SmmehariWi-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. 12*7b8dedc6Smmehari 13*7b8dedc6Smmehari 14*7b8dedc6Smmehari 15*7b8dedc6Smmehari#### Forward error correction 16*7b8dedc6SmmehariThe 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. 17*7b8dedc6Smmehari 18*7b8dedc6Smmehari#### Guard interval 19*7b8dedc6SmmehariAs 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. 20*7b8dedc6Smmehari 21*7b8dedc6Smmehari 22*7b8dedc6Smmehari 23*7b8dedc6Smmehari#### MIMO 24*7b8dedc6SmmehariWi-Fi 4 was the first to introduce MIMO and standardized 4x4 spatial streams. This quadrupled the throughput to 4*72.2Mbps = 288.9Mbps. 25*7b8dedc6Smmehari 26*7b8dedc6Smmehari 27*7b8dedc6Smmehari 28*7b8dedc6Smmehari#### 40MHz bandwidth 29*7b8dedc6SmmehariThe 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. 30*7b8dedc6Smmehari 31*7b8dedc6Smmehari 32*7b8dedc6Smmehari 33*7b8dedc6Smmehari### MAC layer improvements 34*7b8dedc6Smmehari 35*7b8dedc6SmmehariOn 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. 36*7b8dedc6Smmehari 37*7b8dedc6Smmehari 38*7b8dedc6Smmehari 39*7b8dedc6Smmehari 40*7b8dedc6Smmehari## Supported 802.11n ammendments 41*7b8dedc6Smmehari 42*7b8dedc6Smmehari- 52 subcarriers 43*7b8dedc6Smmehari- 5/6 code rates 44*7b8dedc6Smmehari- 400nsec short guard interval. 45*7b8dedc6Smmehari 46*7b8dedc6SmmehariCurrent theoretical throughput = 72.2Mbps. 47*7b8dedc6Smmehari 48*7b8dedc6Smmehari## Unsupported 802.11n ammendments 49*7b8dedc6Smmehari 50*7b8dedc6Smmehari- MIMO 51*7b8dedc6Smmehari- 40MHz bandwidth 52*7b8dedc6Smmehari- Frame aggegation 53*7b8dedc6Smmehari 54
