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Time required to transmit the PLCP header.

TABLE 6.5 Default time parameters in IEEE 802.11 frequency-hopping spread spectrum physical layer

TABLE 6.5 Default time parameters in IEEE 802.11 frequency-hopping spread spectrum physical layer band. The IEEE 802.11 DSSS physical layer uses a particularly simple form as shown in Figure 6.76: Each binary data bit results in the transmission of plus or minus the polarity of the 11-chip Barker sequence. The term chip is used to distinguish the time required to transmit a +1 or — 1 signal element from the time required to transmit a data bit (=11 chip times). The Barker sequence provides good immunity against interference and noise as well as some protection against multipath propagation.

The DSSS transmission system in 802.11 takes the 1 Mbps data signal and converts it into an 11 Mbps signal using differential binary phase shift keying (DBPSK) modulation. Eleven channels have been defined to operate in the 2.4 GHz ISM band in the United States. Nine channels have been defined to operate in the 2.4 GHz band in Europe. Channels can operate without interfering with each other if their center frequencies are separated by at least 30 MHz. The 802.11 DSSS physical layer also defines an option for 2 Mbps operation using differential quaternary PSK (DQPSK).

Figure 6.77 shows the format of the PLCP frame. The PLCP preamble starts with 128 scrambled bits of synchronization that the receiver uses to detect the presence of a signal. The preamble ends with a 16-bit start frame delimiter (hF3A0) that is used for bit synchronization. The PLCP header consists of an 8-bit signal field that indicates to the physical layer the modulation that is to be used for transmission and reception of the MPDU (h0A for 1 Mbps DBPSK, h14 for 2 Mbps DQPSK); an 8-bit service field that is reserved for future use; a 16-bit field that indicates the number of bytes in the MPDU, from 4 to 216; and a 16-bit CRC using the CCITT-16 generator polynomial. The PLCP header is always transmitted at the base rate of 1 Mbps.

11-chip Barker sequence:

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