802.11ac

What Is 802.11ac?

The 802.11ac standard, also referred to as Wi-Fi 5, marks the fifth generation of the 802.11 protocol. With significant technological advancements, 802.11ac represents a substantial improvement in wireless transmission speeds, escalating from 600 Mbit/s in 802.11n to 6.93 Gbit/s. This enhancement opens up vast opportunities for a multitude of applications that rely on high-volume wireless data transfer. Currently, 802.11ac is extensively employed in enterprise and residential wireless communications, profoundly reshaping our work and lifestyle dynamics.

How Does 802.11ac Emerge?

Since the IEEE introduced the first-generation 802.11 standard in 1997, wireless access has progressively become the dominant method for Internet connectivity, with the 802.11 standards evolving accordingly. This evolution is closely linked to the advent of smart home devices, the widespread adoption of wireless networks in offices and campuses, and the use of wireless terminals in environments such as shopping malls, supermarkets, manufacturing facilities, and warehouses. To meet new service demands and bridge the gap with wired network bandwidth, the IEEE officially launched the 802.11ac standard in 2013. Building on the 802.11n architecture, 802.11ac enhances channel bandwidth, multiple-input multiple-output (MIMO) capabilities, and modulation techniques, while maintaining compatibility with 802.11n. 802.11ac significantly boosts throughput from 600 Mbit/s to 6.93 Gbit/s, unlocking vast potential for applications requiring high-traffic wireless communication.

What Are 802.11ac Wave 1 and 802.11ac Wave 2?

To swiftly address the growing traffic demands, the Wi-Fi Alliance (WFA) introduced 802.11ac in two phases: Wave 1 in 2013 and Wave 2 in 2015. The figure below illustrates the differences between these two 802.11ac generations as defined by the WFA and the IEEE 802.11ac standard.

What Is the Maximum Transmission Rate of 802.11ac?

Throughput is commonly used to measure the data transmission performance of wireless networks. The maximum throughput in 802.11ac reaches 6.93 Gbit/s, almost ten times higher than the 600 Mbit/s available in 802.11n. Many wireless devices on the market today have single-spatial-stream antennas, making the transmission rate of a single spatial stream another important performance metric. Compared to 802.11n, 802.11ac offers a significant boost in transmission rate. In 802.11ac, a single spatial stream can achieve a maximum transmission rate of 433 Mbit/s at 80 MHz bandwidth, while eight spatial streams can reach up to 6.93 Gbit/s at 160 MHz bandwidth.

802.11ac vs. 802.11n

802.11ac builds upon the previous 802.11n standard, achieving a higher transmission rate through several enhancements:

• Introducing new technologies and extending existing ones to increase maximum throughput and the number of access users, such as 256-QAM and MU-MIMO.

• Optimizing protocols to reduce complexity, for instance, by removing implicit transmit beamforming (TxBF) and providing only one channel sounding mode and one feedback mode.

• Maintaining compatibility with earlier 802.11 protocols by improving the physical layer (PHY) frame structure and channel management across different bandwidths.

Key Features of 802.11ac

• Higher Bandwidth

802.11ac operates on the 5 GHz frequency band, which offers more spectrum resources for communication. Building on the 20 MHz and 40 MHz bandwidths, 802.11ac enables channel bonding into 80 MHz, 80+80 MHz (non-contiguous), and 160 MHz channels, significantly enhancing throughput and user experience.

• Dynamic Channel Management

802.11ac establishes a dynamic channel management mechanism to handle the coexistence of various channel bandwidths. This mechanism permits data transmission on other available sub-channels when 80 MHz and 160 MHz sub-channels are occupied, ensuring full utilization of channel resources.

802.11ac defines an enhanced Request to Send/Clear to Send (RTS/CTS) mechanism to determine available channels.

Dynamic bandwidth management is intended for spectrum multiplexing, improving channel use efficiency and reducing interference between channels. This mechanism allows two APs to operate on the same channel.

• MU-MIMO (Multi-User Multiple Input Multiple Output)

Multiple spatial streams can greatly enhance single-user service throughput. However, most devices on the current network, particularly mobile smart devices, only support a single spatial stream. Compared to devices with multiple spatial streams, single-spatial-stream devices take up air interface resources for a longer time to transmit the same amount of data. As the number of users continues to grow, these single-spatial-stream devices can become a bottleneck. MU-MIMO addresses this issue effectively. It allows an AP to simultaneously transmit different data to up to four users without changing the user bandwidth or frequency band.

• MCS (Modulation and Coding Scheme)

To enhance throughput, 802.11ac employs higher-order modulation, specifically 256-QAM, which supports 3/4 and 5/6 coding rates and boosts modulation efficiency. This results in up to 10 MCS modes available in 802.11ac.

A higher MCS value signifies greater maximum throughput, as different modulation modes utilize varying numbers of bits per subcarrier: 2 bits in BPSK mode, 4 bits in 16-QAM mode, 6 bits in 64-QAM mode, and 8 bits in 256-QAM mode. Higher-order modulation modes offer increased modulation efficiency, although the efficiency gains diminish for higher-order modes.

While 256-QAM enhances modulation efficiency, it demands a more favorable radio environment and a higher signal-to-noise ratio (SNR) compared to 64-QAM. Thus, MCS8 and MCS9 are typically used in scenarios where STAs (stations) are near APs (access points), where the useful signal strength is high and interference is low, making it easier to meet the SNR requirement (SNR = useful signal strength/interference signal strength).

• Frame Aggregation

On a Wi-Fi network, frames are transmitted on the air interface using CSMA/CA mode. When many frames are transmitted, collisions can decrease air interface efficiency. To address this, 802.11n introduced frame aggregation technology at the MAC layer, allowing for the aggregation and encapsulation of MAC service data units (MSDUs) and MAC protocol data units (MPDUs). This method encapsulates multiple frames with a single PHY header, enhancing encapsulation efficiency and reducing air interface occupation and preemption. To further boost efficiency and reliability, 802.11ac increases the sizes of MPDUs and A-MPDU frames and exclusively uses A-MPDU.