802.11 OFDM Overview

Introduction to 802.11a OFDM

The 802.11a Wireless LAN amendment to the original 802.11 standard was ratified in 1999. The 802.11a standard uses the same core protocol as the original standard, operates in 5 GHz band, and uses a 52-subcarrier orthogonal frequency division multiplexing (OFDM) with a maximum raw data rate of 54 Mbit/s, which yields realistic net achievable throughput in the mid-20 Mbit/s. The raw data rate is reduced to 48, 36, 24, 18, 12, 9 then 6 Mbit/s if required. 802.11a originally had 12 to 13 non-overlapping channels, 12 channels can be used indoor and 4 to 5 of the 12 channels that can be used in outdoor point to point configurations. 802.11a is not interoperable with 802.11b as they operate on separate bands, except if using equipment that has a dual band capability.

802.11a OFDM Signal and Physical Layer Overview

IEEE 802.11a/g and HIPERLAN/2 signals are pulsed (or burst) type signals. The total channel bandwidth is 20 MHz with an occupied bandwidth of 16.6 MHz. A single OFDM symbol contains 52 subcarriers; 48 are data subcarriers and 4 are pilot subcarriers. The center, "DC" or "Null", zero subcarrier is not used. All data subcarriers use the same modulation format within a given burst. However, the modulation format can vary from burst to burst. The possible data subcarrier modulation formats are BPSK, QPSK, 16QAM, and 64QAM. Pilot subcarriers are always modulated using BPSK and a known magnitude and phase. Each OFDM subcarrier carries a single modulated data symbol, or "constellation point", along with its magnitude and phase information . This means that the magnitude and phase will vary for each subcarrier and OFDM symbol in the transmitted burst.

802.11a OFDM Frame Structure

The basic frame structure of an 802.11a burst contains a preamble field followed by a SIGNAL field and multiple data fields. At the start of the burst, a preamble is transmitted at a well-known magnitude and phase. The preamble is used for synchronization and channel equalization. The SIGNAL field (not used in HIPERLAN 2 signals) is transmitted using BPSK, and contains the length, modulation type, and data rate information. Then multiple OFDM symbols containing the input data bits are appended to complete the burst.

802.11a OFDM Signal Timing Parameters

The following table lists the timing parameters associated with the 802.11a signal:

                         802.11a Timing Related Parameters

Parameter Value
Total subcarriers NST 52
Data subcarriers NSD 48
Pilot subcarriers NSP 4  (subcarriers -21, 7, 7, 21)
Subcarrier Frequency Spacing FSP 312.5 KHz   (20MHz/64)
Symbol Interval Time TSYM 4 us  (TGI +TFFT)
Data Interval Time TDATA 3.2 us   (1/FSP)
Guard Interval (GI) Time TGI 0.8 us  (TFFT/4)
IFFT/FFT Period TFFT 3.2 us   (1/FSP)
Preamble TPREAMBLE 16 us   (TSHORT +TLONG)
Short Training Sequence TSHORT 8 us   (10xTFFT/4)
Long Training Sequence TLONG 8 us  (TGI2 + 2xTFFT)
Training symbol GI TGI2 1.6 us  (TFFT/2)
FFT sample size 64 point

See Also

Concepts of OFDM

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