mirror of
https://github.com/open-sdr/openwifi.git
synced 2024-12-25 08:21:17 +00:00
58 lines
2.4 KiB
Markdown
58 lines
2.4 KiB
Markdown
<!--
|
|
SPDX-FileCopyrightText: 2019 Jiao Xianjun <putaoshu@msn.com>
|
|
SPDX-License-Identifier: AGPL-3.0-or-later
|
|
-->
|
|
|
|
## 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.
|
|
|
|
### PHY layer improvements
|
|
At the physical layer, 5 major improvements were amended on top of Wi-Fi 3 and this has increased the throughput from 54Mbps to 600Mbps.
|
|
|
|
#### 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.
|
|
|
|
|
|
|
|
#### 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 has increased the throughput to (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 has increased the throughput to 4usec/3.6usec * 65Mbps = 72.2Mbps.
|
|
|
|
|
|
|
|
#### MIMO
|
|
Wi-Fi 4 was the first to introduce MIMO and standardized 4x4 spatial streams. This has quadrupled the throughput to 4*72.2Mbps = 288.9Mbps.
|
|
|
|
|
|
|
|
#### 40MHz bandwidth
|
|
The 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.
|
|
|
|
|
|
|
|
### 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 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.
|
|
|
|
|
|
|
|
|
|
## Supported openwifi 802.11n amendments
|
|
|
|
- 52 subcarriers
|
|
- 5/6 code rates
|
|
- 400nsec short guard interval.
|
|
|
|
Current theoretical throughput = 72.2Mbps.
|
|
|
|
## To be supported openwifi 802.11n amendments
|
|
- Frame aggregation
|
|
|
|
## Not supported openwifi 802.11n amendments
|
|
|
|
- MIMO
|
|
- 40MHz bandwidth
|