Modulation and Data Rates in 802.11ac

IEEE 802.11ac is a WiFi standard used for indoor Access Points, Routers and outdoor Radios including the CableFree AC Range. Chipsets using 802.11ac technology includes support for 256QAM modulation, MIMO antennas and wide channels, which can combine to offer high data rates for users.

Installers need to be careful with 802.11ac especially regarding channel planning and interference mitigation.

Responsible use of the 5GHz spectrum also needs to be considered, to avoid congestion for current and future users.

Data Rates

The table lists theoretical airside data rates for a single spatial stream (1×1 SISO). For 2×2 MIMO, multiply the listed rates by two. Actual net (user) data rates depend on traffic type and protocol. For typical WiFi usage, approximately 50% of the theoretical data rate is achievable. With advanced streaming protocols like CableFreeMAC, up to 80% may be available.

Advertised Data Rates

New technologies

802.11ac introduced new technologies, including:

• Wider Channel Bandwidths:
  • Mandatory 80 MHz channels (vs. 40 MHz in 802.11n), doubling throughput by allowing more data per transmission.
  • Optional 160 MHz channels (contiguous or non-contiguous 80+80 MHz), quadrupling throughput, though limited by spectrum availability, primarily for residential use.
• More MIMO Spatial Streams:
  • Supports up to eight spatial streams (vs. four in 802.11n), increasing throughput proportionally in multipath environments.
  • Typical deployments use three streams, with Wave 2 products supporting up to four streams for enhanced performance.
• Downlink Multi-User MIMO (MU-MIMO):
  • Optional feature enabling an access point (AP) to transmit to up to four clients simultaneously using Space Division Multiple Access (SDMA).
  • Improves network efficiency in dense environments by spatially separating streams, reducing interference via beamforming.
  • Requires clients with 802.11ac Wave 2 radios for full benefits.
• Advanced Modulation:
  • Introduces 256-QAM (3/4 and 5/6 coding rates) as optional modes, encoding 8 bits per symbol (vs. 6 bits with 64-QAM in 802.11n), boosting data rates by 20–33% (e.g., from 65 Mbps to 78–86.7 Mbps in a 20 MHz channel).
  • Some vendors offer non-standard 1024-QAM, providing ~25% more bandwidth than 256-QAM, but requiring higher signal-to-noise ratios (SNR) and not universally supported.
• Standardised Beamforming:
  • Employs explicit feedback with a compressed V matrix for accurate channel state information (CSI), ensuring vendor interoperability.
  • Improves SNR and supports MU-MIMO by steering signals to intended clients, unlike 802.11n’s inconsistent beamforming implementations.
• MAC Enhancements:
  • Increases frame aggregation limits (A-MPDU to 1,048,576 bytes, A-MSDU to 11,426 bytes), reducing contention and improving throughput for high-rate applications like video streaming.
  • Introduces optional Galois Counter Mode Protocol (GCMP) for faster encryption, complementing CCMP for high-speed data.
  • Adds VHT TXOP power save, allowing clients to doze during irrelevant transmit opportunities, identified by partial AID or Group ID, enhancing battery efficiency.
• Coexistence and Backward Compatibility:
  • Supports 20/40/80/160 MHz channels with coexistence mechanisms for 802.11a/n devices, using legacy-readable preambles (L-STF, L-LTF, L-SIG).
  • Adds four Very High Throughput (VHT) fields to the PPDU header to distinguish 802.11ac frames, with the first three fields decodable by legacy devices.
  • Uses RTS/CTS protection and dynamic bandwidth operation to adjust channel width based on interference, ensuring efficient spectrum use.

Mandatory and optional features

• Mandatory features (carried over from 802.11a/802.11g)
  • 800 ns regular guard interval: Standard guard interval to prevent inter-symbol interference
  • Binary Convolutional Coding (BCC): Error-correcting code for reliable data transmission.
  • Single spatial stream: Support for one spatial stream for basic MIMO operation.

• New mandatory features (introduced in 802.11ac)
  • 80 MHz channel bandwidths: Wider channels to increase data throughput.

• Optional features (carried over from 802.11n)
  • Two to four spatial streams: Support for multiple spatial streams to enhance throughput via MIMO.
  • Low-Density Parity-Check Code (LDPC): Advanced error correction for improved reliability.
  • Space-Time Block Coding (STBC): Improves signal robustness in multipath environments.
  • Transmit Beamforming (TxBF): Focuses signal towards the receiver for better performance.
  • 400 ns Short Guard Interval (SGI): Reduces guard interval to increase data rate in stable environments.

• Optional features (introduced in 802.11ac)
  • Five to eight spatial streams: Expands MIMO capabilities for higher throughput.
  • 160 MHz Channel Bandwidths: Supports contiguous 160 MHz channels for increased data rates.
  • 80+80 MHz Channel Bonding: Allows non-contiguous 80+80 MHz channels for flexibility in spectrum use.
  • MCS 8/9 (256-QAM): Higher modulation (256-QAM) for greater data density per symbol.

For more information

For More Information on CableFree WiFi products, please Contact Us and our team will be delighted to advise on a precise solution to match your exact requirements.