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