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Chapter 10. Wireless Ethernet > Orthogonal Frequency Division Multiplexing

10.5. Orthogonal Frequency Division Multiplexing

A 64-point complex number IFFT/FFT operation forms the modulation mechanism for this 802.11a wireless Ethernet. Among 64 subcarriers, 48 of them are used for BPSK, QPSK, 16) Quadrature Amplitude Modulation (QAM), or 64QAM, 4 of them are used for pilot tone. Complex time domain signals are modulated onto in-phase and quadrature channels of an RF carrier separately. A rate 1/2 convolution code is used for error protection. Rate 2/3 and 3/4 codes are derived through puncturing. An interleaving process is used to randomize subcarrier and bit location distribution among each OFDM symbol.

10.5.1. Summary of 802.11a Standards

The packet format for the OFDM implementation of wireless Ethernet at 5 GHz is shown in Figure 10.53. An OFDM packet consists of a preamble, a header, and a MAC frame. The OFDM preamble has a Sync field of 10 short symbols and 2 long symbols. The OFDM header has 1 Signal symbol including a Rate field of 4 bits, followed by a reserved bit, a Length field of 12 bits, followed by a parity bit and 6 tail bits. The MAC frame is further encapsulated with a Service field of 16 bits in the beginning and 6 tail bits as well as some necessary PAD bits at the end.


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