OFDM is a variation of Frequency Division
Multiplexing. It uses multiple carriers to support
Figure 2. Some allocations for different cellular
networks. The current 4G networks are based on some
combination of 700, 800, and 2,600 MHz.
4G 2500–2600 MHz
2G
700–800 MHz
1G
ISM band (unlicensed)
ISM band (unlicensed) 902–928 MHz
2400–2483.5 MHz
4G
800–900 MHz
3G
1800–2000 MHz
1800–2200 MHz
3G
Figure 3. The early vision of a 4G network. Devices
could access multiple wireless networks.
Wireless LAN
Overlay
network
CAP CAP
CAP
CAP: Common Access Point
Ad hoc network
Cellular/3G
Satellites
computer.org/ITPro 37
higher bit rates per user. The frequencies of carriers
are chosen by standards committees to be
orthogonal, so there’s no cross-talk or interference.
Figure 5 shows how multiple carriers are
created and then multiplexed to create a single
carrier for transmission.