802.11ax PHYs
What is the PHY in IEEE 802.11ax?
The PHY (Physical Layer) in 802.11ax is responsible for transmitting and receiving data over the wireless medium, supporting higher efficiency, multiple antennas, and advanced modulation techniques.
What modulation does 802.11ax use?
802.11ax uses OFDMA with advanced modulation schemes including BPSK, QPSK, 16-QAM, 64-QAM, 256-QAM, and up to 1024-QAM for higher throughput.
What is OFDMA in 802.11ax?
Orthogonal Frequency Division Multiple Access (OFDMA) divides channels into smaller subchannels (Resource Units), allowing multiple users to transmit simultaneously, improving efficiency and reducing latency.
How many subcarriers are used in 802.11ax OFDM?
802.11ax uses up to 256 subcarriers in a 20 MHz channel, compared to 52 in 802.11a, enabling finer frequency granularity and more efficient spectrum use.
What are pilot subcarriers in OFDM?
Pilot subcarriers are reference signals embedded within OFDM symbols to assist in synchronization, channel estimation, and phase tracking, similar to previous standards but optimized in 802.11ax.
What coding technique is used in 802.11ax PHY?
802.11ax uses Low-Density Parity-Check (LDPC) codes which provide more powerful error correction and improve data reliability over convolutional codes used in 802.11a.
What is convolutional coding?
802.11ax generally replaces convolutional coding with LDPC, which offers improved error correction performance and efficiency.
What is interleaving in 802.11ax?
Interleaving is used in 802.11ax to spread bits over multiple subcarriers to mitigate the effect of burst errors, similar to earlier standards but optimized for higher subcarrier counts.
What data rates are supported by 802.11ax PHY?
802.11ax supports a wide range of data rates from under 1 Mbps up to several Gbps, depending on channel width, number of spatial streams, and modulation (up to 1024-QAM).
Which modulation corresponds to the lowest data rate?
The lowest rates in 802.11ax use BPSK with low coding rates on narrow Resource Units.
Which data rate uses 1024-QAM?
The highest data rates in 802.11ax use 1024-QAM combined with multiple spatial streams and wide channels for maximum throughput.
What is the OFDM symbol duration in 802.11ax?
The OFDM symbol duration is extended to 12.8 microseconds with a 0.8 or 3.2 microsecond guard interval, to improve robustness especially in dense and multipath environments.
What is a guard interval?
A guard interval in 802.11ax reduces inter-symbol interference by providing time buffers between OFDM symbols; 802.11ax supports both standard (0.8 µs) and extended (3.2 µs) guard intervals.
What is the FFT size used in 802.11ax?
802.11ax uses a 256-point FFT for 20 MHz channels, allowing for increased subcarrier count and spectral efficiency.
How wide is the 802.11ax channel in MHz?
802.11ax supports flexible channel widths of 20, 40, 80, and 160 MHz, enabling scalable throughput.
Why is OFDM suitable for 5 GHz PHY in 802.11ax?
OFDM, combined with OFDMA and longer guard intervals, effectively handles multipath and interference common in dense 5 GHz deployments, improving efficiency.
Is the 802.11ax PHY half-duplex or full-duplex?
802.11ax PHY is still half-duplex, but it introduces uplink and downlink OFDMA for simultaneous transmissions on different subchannels.
Does 802.11ax PHY support MIMO?
Yes, 802.11ax supports MU-MIMO (Multi-User MIMO) for both uplink and downlink, allowing multiple clients to communicate simultaneously using multiple antennas.
What is the function of the PHY preamble in 802.11ax?
The preamble includes multiple training fields for synchronization, channel estimation, and spatial stream training, optimized for OFDMA and MU-MIMO.
What is the SIGNAL field in the PHY header?
The SIGNAL field conveys modulation, coding, resource allocation, spatial streams, and length information, allowing dynamic adaptation per user.
What is the service field in 802.11ax PHY?
The service field is used for scrambler initialization and reserved for future enhancements, similar to prior standards.
What is the tail field in PHY?
The tail field terminates the LDPC decoder state, aiding in accurate error correction.
Can PHY parameters change dynamically in 802.11ax?
Yes, 802.11ax features dynamic modulation, coding, resource unit assignment, and spatial stream adjustments based on real-time channel conditions.
How does 802.11ax handle multipath effects?
Longer OFDM symbols and guard intervals, combined with advanced channel estimation, reduce multipath interference significantly.
What limits the range of 802.11ax PHY?
Like earlier 5 GHz standards, range is limited by higher frequency propagation characteristics, though advanced coding and MIMO improve effective range.
Does PHY impact latency in 802.11ax?
PHY efficiency improvements, especially OFDMA and MU-MIMO, reduce latency by allowing simultaneous transmissions and better resource allocation.
What is spectral efficiency in PHY terms?
Spectral efficiency is greatly enhanced in 802.11ax through higher order modulations, OFDMA, MU-MIMO, and flexible channel widths.
How does PHY relate to MAC in 802.11ax?
PHY transmits data over the air, while MAC manages resource allocation, scheduling, and coordination to leverage PHY’s advanced features like OFDMA and MU-MIMO.
Is the 802.11ax PHY backward compatible?
Yes, 802.11ax PHY is backward compatible with earlier standards (802.11a/b/g/n/ac), allowing operation on the same bands with fallback modes.
Is 802.11ax PHY still used today?
Yes, 802.11ax is the current mainstream Wi-Fi PHY, widely deployed for its improvements in throughput, efficiency, and dense environment performance.
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