WLAN 802.11ac (Wi-Fi 5) Tutorial

IEEE 802.11ac is a Wi-Fi standard introduced as part of the IEEE 802.11 family, commonly known as Wi-Fi 5. It was developed to meet the growing demand for higher throughput and better performance in wireless local area networks (WLANs), especially for HD video streaming, high-density environments, and faster internet speeds. It operates exclusively in the 5 GHz band and supports much higher data rates than its predecessor, 802.11n, making it ideal for applications that require high bandwidth and low latency.

In previous versions of the 802.11 series of standards, designs were primarily for single-user operation. 802.11ac introduced the multi-user concept with the Multi-User MIMO feature. Beamforming was added to cover more area, focusing beams on specific regions to improve signal quality. This WLAN 802.11ac tutorial covers 802.11ac features, frame structure, physical layer, MAC layer, frequency, radio network planning, and more.

802.11ac Features

As mentioned, 802.11ac products are available in the 5 GHz frequency band only and not available for the 2.4 GHz frequency band, unlike 802.11n.

Following are key features of 802.11ac:

• Frequency Range: 5 GHz
• Modulation Scheme: OFDM and DSSS/CCK
• Backward Compatibility: Supports legacy 802.11n systems
• Data Modulation: Supports BPSK, QPSK, 16QAM, 64QAM, and 256QAM, designated by MCS0 to MCS9
• Channel Bandwidth: 20MHz, 40MHz, 80MHz, 160MHz
• OFDM data subcarriers/pilots: 52/4, 108/6, 234/8, 468/16
• OFDM coding rate: 5/6
• Short guard interval: 400 ns
• Max. Spatial streams Supported: 8
• FFT sizes: 64, 128, 256, 512
• Beamforming/Spatial streams: Introduces standardized beamforming, which focuses the signal towards specific devices, improving range and reliability. Supports up to 8 spatial streams, improving data throughput.
• Max. Data rate: 6.93Gbps using 160MHz bandwidth, 8 spatial streams, MCS9, 256QAM, with a short guard interval
• Distance: About 80 m with 3 antennas (about 10 meters more than 802.11n)
• MIMO configuration: 4 X 4
• **Supports Only NDP(Null Data Packet) explicit beamforming
• Supports single-user transmission as well as multi-user transmission
• PHY layer frame(VHT mode) consists of L-STF, L-LTF, L-SIG, VHT-SIG-A, VHT-STF, VHT-LTF, VHT-SIG-B and Data part.

WLAN 802.11ac Frame Structure

802.11ac uses the same basic frame structure as previous 802.11 standards but with enhancements for higher throughput and efficiency. Key changes include the following:

• MAC Frame: The frame consists of management, control, and data frames. Improvements for more efficient aggregation of frames, like A-MPDU (Aggregated MAC Protocol Data Unit), reduce overhead.
• PHY Frame: The physical frame includes a PLCP (Physical Layer Convergence Protocol) preamble and data fields. Enhanced support for 80 MHz and 160 MHz channels leads to better data transfer performance.

Figure-1 depicts WLAN 802.11ac frame Structure. As shown it consists of L-STF, L-LTF, L-SIG, VHT-SIG-A, VHT-STF, VHT-LTF, VHT-SIG-B & Data part.

WLAN 802.11ac PHY Layer

The 802.11ac physical layer is designed to deliver high throughput and efficiency through various improvements:

• Channel Bonding: 802.11ac supports 80 MHz and 160 MHz channels, allowing for wider bandwidth and higher data rates.
• Modulation: Uses 256-QAM, which allows the transmission of more data per symbol compared to the 64-QAM in 802.11n.
• Guard Interval: Includes both short (400 ns) and long (800 ns) guard intervals to optimize performance based on environmental conditions.
• Spatial Streams: Supports up to 8 spatial streams, which means data can be transmitted over multiple antennas simultaneously for higher throughput.
• Beamforming: Enables directional signal transmission to increase range and reliability, especially in crowded environments.

Figure-2 depicts WLAN 802.11ac Physical layer transmitter part.

802.11ac MAC layer

The MAC layer in 802.11ac has been enhanced for more efficient data transmission and aggregation.

1. Frame Aggregation:
   • Uses A-MPDU to combine multiple data frames into a single transmission, reducing overhead and improving throughput.
   • Also supports A-MSDU (Aggregated MAC Service Data Unit) for aggregating smaller packets into larger frames.
2. Channel Access:
   • Enhanced Distributed Channel Access (EDCA) is used to prioritize traffic and reduce collisions.
3. Block Acknowledgment (Block ACK):
   • Allows for faster acknowledgment of aggregated frames, improving transmission efficiency.

802.11ac Radio Network Planning

When planning a radio network for 802.11ac, several factors must be considered as follows:

• Channel Planning: The use of 80 MHz or 160 MHz channels may limit the number of non-overlapping channels available. Proper channel assignment is critical in environments with many access points (APs).
• Coverage: Beamforming and MU-MIMO enhance range and coverage. However, planning for proper AP placement is necessary to ensure adequate coverage in high-density environments.
• Capacity: 802.11ac’s MU-MIMO feature enables better handling of multiple clients, but access points should be strategically placed to manage high-traffic areas.
• Backhaul Consideration: Higher data rates require robust backhaul connections (e.g., fiber, high-speed Ethernet) between APs and network infrastructure.

Conclusion

IEEE 802.11ac (Wi-Fi 5) revolutionized Wi-Fi by offering significantly higher data rates, better performance in crowded environments, and advanced features like MU-MIMO and beamforming. Its focus on the 5 GHz spectrum and wide channel bandwidths makes it ideal for high-bandwidth applications such as HD video streaming and gaming. Proper planning for channel usage, AP placement, and capacity is essential for optimizing performance in 802.11ac networks.

This standard laid the foundation for even higher speeds and performance improvements seen in the later IEEE 802.11ax (Wi-Fi 6) standard.

This 802.11 ac tutorial is very useful for beginners who would like to learn MU-MIMO and beamforming features introduced in the 802.11 ac series of standards.