Analysis of IEEE 802.11n wireless network technology

IEEE 802.11n technology has greatly improved the wireless transmission rate through technical improvements in the physical layer and MAC layer, increasing the bandwidth from 54Mbps to 300Mbps. The core of 802.11n - MIMO-OFDM

OFDM modulation technology modulates a high-speed data stream into multiple sub-data streams with lower rates, and then communicates through a physical channel that has been divided into multiple sub-carriers, thereby reducing the chance of ISI (inter-symbol interference).
MIMO (multiple-input multiple-output) technology uses multiple antennas at both the transmitting and receiving ends of the link to turn multipath propagation into a favorable factor, thereby exponentially increasing the capacity and spectrum utilization of the communication system without increasing the channel bandwidth, so as to achieve an increase in the rate of the WLAN system.
Combining MIMO with OFDM technology has produced MIMO OFDM technology, which uses array antennas in the OFDM transmission system to achieve spatial diversity, improve signal quality, and increase the tolerance of multipath, so that the effective transmission rate of the wireless network has a qualitative improvement.

Dual-band (20-MHz and 40-MHz bandwidth)
IEEE 802.11n combines two adjacent 20MHz bandwidths into a 40MHz communication bandwidth, which can be used as two 20MHz bandwidths in actual operation (one is the primary bandwidth, the other is the secondary bandwidth, and can work with either 40MHz bandwidth or a single 20MHz bandwidth when sending and receiving data), which can double the rate. At the same time, for IEEE 802.11a/b/g, in order to prevent adjacent channel interference, a small portion of the bandwidth boundary is reserved on both sides of the 20MHz bandwidth channel. Through band bonding technology, these reserved bandwidths can also be used for communication, further improving throughput.

Figure 20/40-MHz bandwidth throughput

Short Guard Interval
Short GI (Guard Interval) is an improvement made by 802.11n over 802.11a/g. When the RF chip uses OFDM modulation to send data, the entire frame is divided into different data blocks for transmission. For the reliability of data transmission, there will be GI between data blocks to ensure that the receiving side can correctly parse each data block. The transmission of wireless signals in space will cause delays on the receiving side due to factors such as multipath. If the subsequent data blocks are sent too fast, they will interfere with the previous data block, and GI is used to avoid this interference. The GI duration of 11a/g is 800us, while the Short GI duration is 400us. When using Short GI, the rate can be increased by 10%. In addition, Short GI is independent of bandwidth and supports 20MHz and 40MHz bandwidths. Optimization of the data link layer
Conflicts caused by channel competition and the backoff mechanism introduced to resolve conflicts greatly reduce the system throughput. In order to solve these two problems at the MAC layer, 802.11n adopts the frame aggregation technology and the block acknowledgment mechanism.

Frame aggregation technology includes aggregation for MSDU (A-MSDU) and aggregation for MPDU (A-MPDU):

A-MSDU
A-MSDU technology refers to aggregating multiple MSDUs into a larger payload in a certain way. The MSDU here can be considered as an Ethernet message. Usually, when an AP or wireless client receives a message (MSDU) from the protocol stack, it will be marked with an Ethernet message header, which we call A-MSDU Subframe here; and before sending it out through the radio port, it needs to be converted into the 802.11 message format one by one. The A-MSDU technology aims to aggregate several A-MSDU Subframes together and encapsulate them into an 802.11 message for transmission. This reduces the overhead of the PLCP Preamble, PLCP Header and 802.11MAC header required to send each 802.11 message, while reducing the number of response frames and improving the efficiency of message transmission.

The difference between A-MPDU
and A-MSDU is that A-MPDU aggregates MPDUs encapsulated by 802.11 messages. Here, MPDU refers to data frames encapsulated by 802.11. By sending several MPDUs at a time, the PLCP Preamble and PLCP Header required to send each 802.11 message are reduced, thereby improving system throughput.

Block Acknowledgement
To ensure the reliability of data transmission, the 802.11 protocol stipulates that each unicast data frame must be responded to immediately with an ACK frame. After receiving the A-MPDU, the receiver needs to process each MPDU and send an ACK frame for each MPDU.
Block Acknowledgement uses one ACK frame to complete the response to multiple MPDUs, thereby reducing the number of ACK frames in this case.

Spatial Multiplexing Power Save
When using 802.11n services, the problem of power capacity becomes more prominent due to the installation of multiple antennas. Therefore, the 802.11n protocol has made improvements in power saving processing and adopted Spatial Multiplexing (SM) Power Save technology. Its technical principle is that when there is no data forwarding, only one antenna of the STA is in working state, and the other antennas are in dormant state, thereby achieving the purpose of saving power. SM Power Save defines two power management modes: dynamic SM Power Save and static SM Power Save.

Backward compatibility
The 802.11n protocol allows access by 802.11a/b/g users. The signals sent by 802.11n devices may not be parsed by 802.11a/b/g devices, which causes 802.11a/b/g devices to send signals directly into the air, resulting in conflicts in channel usage. To solve this problem, when 802.11n runs in mixed mode (that is, when there are 802.11a/b/g devices in the network at the same time), a preamble that can be correctly parsed by 802.11a or 802.11b/g devices will be added to the header of the sent message. This ensures that 802.11a/b/g devices can detect 802.11n device signals and enable conflict avoidance mechanisms, thereby achieving interoperability between 802.11n devices and 802.11a/b/g devices.