gher throughput of useful not used. If all the subcarriers were occupied, the signal data) are created by puncturing the output of the rate would be approximately 64 X 312.5 kHz = 20 MHz wide 1/2 code, that is, some of the output bits are simply (63 spacings between subcarriers + half of the width of ignored. Such a procedure is roughly equivalent to a subcarrier on each side); the actual (ideal, undistorted) intentionally introducing some bit errors in the data signal is about 16.6 MHz wide, thus fitting nicely within stream, though it is easier to correct for because the the spectral mask in Figure 5.1-16. location of the missing bits is known; if done cleverly, it Recall that to use OFDM, the time over which a symbol allows for higher throughput with little performance loss. is integrated must be an integer number of cycles for all The use of the resulting output bits is significantly the subcarriers in order that orthogonality is maintained. different from PBCC, because instead of single-carrier The native symbol time of the 802.11a OFDM symbol is modulation, the 802.11a standard uses orthogonal fre- thus one cycle of the lowest frequency subcarrier, or quency-division multiplexing (OFDM). A data symbol is 3.2 usec. A cyclic prefix is appended to the symbol to composed of 48 subcarriers, which carry data, and 4 allow for a guard interval that eliminates the effects of additional pilot subcarriers, which transmit a known multipath. The cyclic prefix is 0.8 usec long (about 20% pseudo-random sequence to assist the receiver in main- of the total symbol), SO that the total symbol length is taining synchronization with the transmitter. The 4 usec. Therefore, the OFDM symbol rate is 250 Ksps. subcarriers are assigned to 64 frequency slots, separated Each symbol contains 48 active subcarriers, SO this is by 312.5 KHz; the slots at the band edges and the slot in equivalent to sending 12 million single-carrier symbols per the center of the band (i.e., at the carrier frequency) are second. to nearest frequency reuse: t