FDM

FDM is a technique where all signals are transmitted simultaneously , but over different frequency bands. FDM obeys the rule " some bandwidth all of the time," where the broadband is divided into a number of narrowbands, and every subband is called a channel. Special requirements must be met to prevent interference between the subbands (channels) and to reduce the number of erroneous transmissions. Typical implementations of FDM include voice, AM and FM bands, cable TV, and TV tuners. The bandwidth of a FM channel obeys Carson's rule, which is explained through the following equation:

BW(FM) = 2( D f + fm)

If the frequencies are separated from each other and don't overlap or interfere, a number of signals can be carried simultaneously. Separation is achieved through a guard band, which uses the unused portion of the spectrum. In voice channels that require a 0.3 to 3.4-kHz channel, the band is 4-kHz wide, where 3.1 kHz are used for the voice channel and 900 Hz are available for the guard channel. To divide the band , one pair of a multiplexor/demultiplexor is installed on both ends of the transmission facility. For North America, a standard voice multiplexing scheme is twelve 4-kHz voice channels from 60 to 108 kHz (f1, f2, f3, ... f12). Figure 3-1 shows how the input is transformed from serial to parallel and how the encoding is sent to 12 modulators. The lower part of the figure shows how the bandwidth is used.

Figure 3-1. Frequency-Division Multiplexing (FDM)

FDM works best with analog signals. In the U.S., common terminology includes the following terms: A group with 12 analog voice channels is called a N-carrier. A supergroup is composed of five groups with 12 channels each, totaling 60 channels. Ten supergroups are combined to produce a mastergroup of 600 voice channels, and a jumbogroup is comprised of six mastergroups with 3600 voice channels. The grouping usually is available over coaxial cables and it forms a family that is referred to as the E-carrier.