802.11a PHYs
What is the PHY in IEEE 802.11a?
The PHY (Physical Layer) is responsible for transmitting and receiving raw data bits over the wireless medium using radio signals.
What modulation does 802.11a use?
IEEE 802.11a uses Orthogonal Frequency Division Multiplexing (OFDM) with multiple modulation types such as BPSK, QPSK, 16-QAM, and 64-QAM.
What is OFDM in 802.11a?
OFDM is a modulation scheme that splits a high-speed signal into multiple lower-speed signals, transmitted simultaneously over different frequencies.
How many subcarriers are used in 802.11a OFDM?
802.11a uses 52 subcarriers: 48 for data and 4 as pilot subcarriers for synchronization and tracking.
What are pilot subcarriers in OFDM?
Pilot subcarriers are known reference signals inserted into OFDM symbols for phase tracking and channel estimation.
What coding technique is used in 802.11a PHY?
802.11a uses convolutional coding with coding rates of 1/2, 2/3, or 3/4 to provide forward error correction.
What is convolutional coding?
Convolutional coding is an error-correction technique that encodes data using a shift register and generator polynomials to increase transmission reliability.
What is interleaving in 802.11a?
Interleaving rearranges bits before transmission to protect against burst errors by spreading errors over multiple codewords.
What data rates are supported by 802.11a PHY?
802.11a supports data rates of 6, 9, 12, 18, 24, 36, 48, and 54 Mbps.
Which modulation corresponds to the 6 Mbps rate?
6 Mbps uses BPSK modulation with a 1/2 coding rate in the 802.11a PHY.
Which data rate uses 64-QAM?
54 Mbps in 802.11a uses 64-QAM with a 3/4 coding rate for high throughput.
What is the OFDM symbol duration in 802.11a?
Each OFDM symbol is 4 microseconds long, including a 0.8 microsecond guard interval.
What is a guard interval?
The guard interval is a brief time added between OFDM symbols to reduce inter-symbol interference caused by multipath delay.
What is the FFT size used in 802.11a?
802.11a uses a 64-point Fast Fourier Transform (FFT) to modulate and demodulate OFDM signals.
How wide is the 802.11a channel in MHz?
The channel width in 802.11a is 20 MHz, accommodating the OFDM spectrum.
Why is OFDM suitable for 5 GHz PHY?
OFDM handles multipath interference well, which is common in indoor 5 GHz environments, enhancing robustness and throughput.
Is the 802.11a PHY half-duplex or full-duplex?
802.11a PHY operates in half-duplex mode—devices cannot transmit and receive simultaneously on the same channel.
Does 802.11a PHY support MIMO?
No, MIMO (Multiple Input, Multiple Output) is introduced in later standards like 802.11n. 802.11a uses single-antenna systems.
What is the function of the PHY preamble in 802.11a?
The PHY preamble includes training sequences used for synchronization, channel estimation, and signal detection by the receiver.
What is the SIGNAL field in the PHY header?
It contains information such as modulation type, data rate, and length of the payload, allowing the receiver to decode the frame properly.
What is the service field in 802.11a PHY?
The service field includes initialization bits for scramblers and reserved bits for future use.
What is the tail field in PHY?
The tail field is used to reset the convolutional decoder to a known state, improving decoding accuracy.
Can PHY parameters change dynamically in 802.11a?
Yes, devices can dynamically switch modulation and coding schemes based on channel quality using rate adaptation algorithms.
How does 802.11a handle multipath effects?
OFDM and guard intervals help minimize the impact of multipath delay spread in the 5 GHz band.
What limits the range of 802.11a PHY?
The higher frequency (5 GHz) has less wall penetration and shorter range compared to lower-frequency protocols like 802.11b/g.
Does PHY impact latency in 802.11a?
Yes, PHY layer efficiency and symbol timing can affect transmission latency, especially at lower data rates.
What is spectral efficiency in PHY terms?
Spectral efficiency refers to how efficiently data is transmitted per unit of bandwidth. Higher modulations like 64-QAM improve it.
How does PHY relate to MAC in 802.11a?
PHY transmits the data bits over the air, while MAC handles framing, addressing, and access control—both layers work together.
Is the 802.11a PHY backward compatible?
No, 802.11a is not backward compatible with 802.11b/g due to operating on different frequency bands.
Is 802.11a PHY still used today?
While newer standards have replaced it, the 802.11a PHY remains foundational and is still used in backward compatibility modes of modern devices.
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