e sample time may be as long as 250 nsec, and a signal with a bit rate of 1/fs. In 802.11a OFDM, the sampling frequency is 10 Mbps (period of 100 nsec) can be completely over- 20 MHz and N = 64, SO f1 = 312.5 kHz. Symbol time is lapped by its reflection. When there are several re- Nts = 64/fs = 3.2 usec. flections, arriving at the receiver at different times, the In order to prevent intersymbol interference, 802.1 signal may be mutilated beyond recognition. The OFDM inserts a guard time of 0.8 usec in front of each symbol, transmission system goes a long way to solving the prob- after the IDFT conversion. During this time, the last lem. It does this by sending the data partitioned into 0.8 usec of the symbol is copied, SO the guard time is also symbols whose length in time is several times the called a circular prefix. Thus, the extended symbol time expected reflected path length time differences. The in- that is transmitted is 3.2 + .8 = 4 usec. The guard time is dividual data bits in a symbol are all sent in parallel on deleted after reception and before reconstruction of the separate subcarrier frequencies within the transmission transmitted data. channel. Thus, by sending many bits during the same Although the previous equation, where N = 64, in- time, each on a different frequency, the individual trans- dicates 64 possible subcarriers, only 48 are used to carry mitted bit can be lengthened SO that it won't be affected data, and four more for pilot signals to help the receiver by the multipath phenomenon. Actually, the higher bit phase lock to the transmitted carriers. The remaining rates are accommodated by representing a group of data carriers that are those at the outside of the occupied bits by the phase and amplitude of a particular trans- bandwidth, and the DC term (m = 0 in Eq. (5.2.1)), are mitted carrier. A carrier modulated using quadrature null. It follows that there are 26 ((48 + 4)/2) carriers on phase shift keying (QPSK) can represent two data bits each side of the nulled center frequency. Each 