ial bit stream can be turned into a number of parallel bit streams each one of which is encoded onto a separate subcarrier. All available subcarriers are used by a single user to achieve a high data throughput. A bit stream can be spread using a chipping code and then each chip can be transmitted in parallel on a separate subcarrier. Since the codes can allow multiple user access, this system is known as Multi-Carrier CDMA (MC-CDMA). MC-CMDA is under consideration by the WIGWAM project as one of the building blocks of the 1 Gbps wireless LAN (see the Section "Gigabit Wireless LANs, p. 350"). A significant advantage of OFDM is that, since the symbol rate is much lower when spread across multiple carriers than it would be if the same total symbol rate were transmitted on a single carrier, the wireless link much less susceptible to inter-symbol interference (ISI). ISI occurs when, as a result of multi-path propagation, two symbols transmitted at different times arrive together at the receiving antenna after traversing different propagation paths (Figure 4-18). Although OFDM is inherently less Chapter Four Inter-symbol interference Delayed signal Symbol n Symbol n+1 Symbol n+2 Direct signal Symbol n Symbol n+1 Symbol n+2 Figure 4-18: Inter Symbol Interference (ISI) susceptible to ISI, most OFDM systems also introduce a guard interval between each symbol to further reduce ISI. OFDM radios also use a number of subcarriers, called pilot tones, to gather information on channel quality to aid demodulation decisions. These subcarriers are modulated with known training data at the start of each transmitted data packet. Decoding this known data enables the receiver to determine and adaptively correct for the frequency offset and phase noise between the reference oscillators in the transmitter and the receiver and for fading during propagation. Figure 4-19 shows a schematic block diagram of a simple OFDM transmitter and receiver. From the left, the input bit stream at a rate of R bps passes through a series to parallel conver