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.

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.

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.

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

** 802.11a Timing Related Parameters**

Parameter | Value |
---|---|

Total subcarriers N_{ST} |
52 |

Data subcarriers N_{SD} |
48 |

Pilot subcarriers N_{SP} |
4 (subcarriers -21, 7, 7, 21) |

Subcarrier Frequency Spacing F_{SP} |
312.5 KHz (20MHz/64) |

Symbol Interval Time T_{SYM} |
4 us (T_{GIGuard interval} +T_{FFTFast Fourier Transform: A mathematical operation performed on
a time-domain signal to yield the individual spectral components that
constitute the signal. See Spectrum.}) |

Data Interval Time T_{DATA} |
3.2 us (1/F_{SP})_{} |

Guard Interval (GI) Time T_{GI} |
0.8 us (T_{FFT}/4) |

IFFT/FFT Period T_{FFT} |
3.2 us (1/F_{SP}) |

SIGNAL Symbol TIme T_{SIGNAL} |
4 us (T_{GI} +T_{FFT} |

Preamble T_{PREAMBLE} |
16 us (T_{SHORT} +T_{LONG}) |

Short Training Sequence T_{SHORT} |
8 us (10xT_{FFT}/4) |

Long Training Sequence T_{LONG} |
8 us (T_{GI2} + 2xT_{FFT}) |

Training symbol GI T_{GI2} |
1.6 us (T_{FFT}/2) |

FFT sample size | 64 point |

See Also

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