802.11n PHYs
What is the PHY in IEEE 802.11n?
The PHY (Physical Layer) in IEEE 802.11n defines how bits are transmitted and received over both 2.4 GHz and 5 GHz using enhanced modulation, coding, and multiple antenna (MIMO) techniques.
What modulation does 802.11n use?
802.11n uses OFDM modulation (like 802.11a/g) combined with advanced modulation and coding schemes (MCS) to adapt between BPSK, QPSK, 16-QAM, and 64-QAM depending on channel conditions.
What is OFDM in 802.11n?
OFDM (Orthogonal Frequency Division Multiplexing) splits the signal into many subcarriers transmitted in parallel to combat multipath delay spread. In 802.11n PHY it is used for high throughput and more efficient spectrum use.
How many subcarriers are used in 802.11n OFDM?
It uses the same number of OFDM subcarriers as 802.11a/g: 52 (48 data + 4 pilot). This is used in both 20 MHz and 40 MHz channel width modes.
What are pilot subcarriers in OFDM?
Pilot subcarriers are fixed, known signals inserted in OFDM symbols used for phase reference, synchronization, and channel estimation in 802.11n.
What coding technique is used in 802.11n PHY?
Convolutional coding with various coding rates (e.g. 1/2, 2/3, 3/4) is used, along with advanced MCS combinations. Additionally, there is interleaving and error correction to improve reliability.
What is interleaving in 802.11n?
Interleaving rearranges bits before transmission into different subcarriers/time to spread out bursts of errors and improve the performance of error‑correction.
What data rates are supported by 802.11n PHY?
802.11n supports data rates from about 6.5 Mbps up to 600 Mbps depending on number of spatial streams, channel width (20 or 40 MHz), guard interval, and modulation/coding scheme.
Which modulation corresponds to the lowest rate in 802.11n?
The lowest rates (e.g. 6.5 Mbps) use BPSK or QPSK with low coding rate (e.g. ½) in 20 MHz channels.
Which data rate uses 64‑QAM in 802.11n?
Higher data rates (e.g. in the 40 MHz channel width, short guard interval, and with multiple spatial streams) employ 64‑QAM in the MCS tables.
What is the OFDM symbol duration in 802.11n?
802.11n uses similar OFDM symbol timing as a/g, but can optionally use a short guard interval (400 ns) instead of 800 ns to reduce symbol duration and increase throughput.
What is a guard interval?
Guard interval is a gap inserted between OFDM symbols to avoid inter-symbol interference due to multipath. In 802.11n, standard GI is 800 ns; short GI (400 ns) is optional for higher throughput.
What is the FFT size used in 802.11n?
The FFT size is 64 for the standard OFDM modes (20 and 40 MHz channels as used by 802.11n), inherited from earlier OFDM standards.
How wide are the 802.11n channels in MHz?
Channels used are 20 MHz by default; optional 40 MHz channel bonding/doubling for more throughput.
Does 802.11n PHY support MIMO?
Yes — Multiple Input, Multiple Output (MIMO) is a core feature of 802.11n, allowing multiple spatial streams for increased throughput.
What is the function of the PHY preamble in 802.11n?
The PHY preamble contains training sequences, synchronization field, HT‑related information, channel estimation etc., needed for receivers to lock onto the transmission.
What is the SIGNAL (or HT‑SIG) field in the PHY header?
It contains information about the modulation, coding scheme, channel width, number of spatial streams, guard interval, etc., so the receiver knows how to interpret the rest of the frame.
What is the service field or tail field in 802.11n PHY?
The service field initializes some scrambler and tail bits are used to bring encoders (e.g. convolutional) to known state at the end of a frame, for decoding accuracy.
Can PHY parameters change dynamically in 802.11n?
Yes — 802.11n supports dynamic adaptation of modulation & coding (MCS), number of spatial streams, guard interval etc., depending on channel conditions.
How does 802.11n handle multipath effects?
OFDM, guard intervals, MIMO diversity techniques are used to mitigate multipath fading & delays.
What limits the range of 802.11n PHY?
The range is limited by higher frequencies (especially in 5 GHz), attenuation, wall penetration, and environmental factors; also higher modulation & wider channels require good signal quality.
Does PHY impact latency in 802.11n?
Yes — symbol durations, guard interval, number of spatial streams, etc., all influence latency along with rate adaptation.
What is spectral efficiency in PHY terms for 802.11n?
Spectral efficiency refers to the amount of data transmitted per unit bandwidth. With MIMO + 40 MHz + high modulation, 802.11n significantly increases spectral efficiency versus older standards.
How does PHY relate to MAC in 802.11n?
The PHY handles the raw modulation, coding, spatial streams etc.; the MAC handles framing, medium access (CSMA/CA), rate adaptation, ACKs/Block ACK etc., with enhancements to take advantage of the PHY improvements.
Is 802.11n PHY still relevant today?
Yes — it’s used widely in many devices (especially “Wi‑Fi 4” certified devices), and for backward compatibility & dual‑band routers/access points.
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