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OFDM Physical Layer Convergence Procedure
Apr 30,2007 00:00
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OFDM Physical Layer Convergence Procedure The following describes each of the OFDM PLCP frame fields: • PLCP Preamble This field enables the receiver to acquire an incoming OFDM signal and synchronize the demodulator. The preamble consists of 12 symbols. Ten of them are short for establishing AGC (automatic gain control) and the coarse frequency estimate of the carrier signal. The receiver uses the long symbols for fine-tuning. With this preamble, it takes 16 microseconds to train the receiver after first receiving the frame. • Rate The following identifies the data rate based on the value of bits 1, 2, 3, and 4: Bits 1–4 Data Rate 1101 6Mbps 1111 9Mbps 0101 12Mbps 0111 18Mbps 1001 24Mbps 1011 36Mbps 0001 48Mbps 0011 54Mbps • Reserved This field is set to 0. • Length This field identifies the number of octets in the frame. • Parity This field is one bit based on positive (even) parity, based on the first 17 bits of the frame (Rate, Reserved, and Length fields). • Tail This field must be set to all zeros. • Service This field consists of 16 bits, with the first 7 bits as zeros to synchronize the descrambler in the receiver and the remaining 9 bits (all 0s) reserved for future use (and set to zeros). • PSDU The PSDU (PLCP service data unit) is the payload from the MAC layer being sent. • Tail This field consists of six bits (all zeros) appended to the symbol to bring the convolutional encoder to zero state. • Pad Bits This field contains at least six bits, but it is actually the number of bits that make the Data field a multiple of the number of coded bits in an OFDM symbol (48, 96, 192, or 288). The PLCP Preamble and Signal fields are convolutional encoded and sent at 6Mbps using BPSK no matter what data rate the Signal field indicates. A data scrambler using a 127-bit sequence generator scrambles all bits in the data field to randomize the bit patterns in order to avoid long streams of ones and zeros. |