DIGITAL MODULATION & MULTIPLEXING

KrMritunjay

Multiplexing schemes can be divided into three basic categories, Frequency Division Multiplexing
(FDM), Time Division Multiplexing (TDM), and Code Division Multiplexing (CDM), depending on
how users share the available transmission medium.

Frequency division multiplexing is an analog technique that can be applied when the bandwidth of a link is greater than the combined bandwidths of the signals to be transmitted. In FDM, signals generated by each sending device modulate different carrier frequencies. These modulated signals are then combined into a single composite signal that can be transported by the physical link. Carrier frequencies are separated by-enough bandwidth to accommodate the modulated signal. Individual channels must be separated by strips-of unused bandwidth (guard bands) to prevent signals from overlapping in the frequency domain.Example: Cable Television

Time division multiplexing is a digital process that can be applied when the data rate capacity of a transmission medium is much greater than the data rate required by individual sending and receiving devices. In such case, multiple transmissions can occupy a single link by subdividing them and interleaving the portions. This subdivision can vary from one bit for bit-interleaved multiplexers, trough a few bits for character-interleaved multiplexers, to a few thousand bits in the latest types of high bit-rate multiplexers, the Synchronous Time Division Multiplexers (STDM) designed for the synchronous transfer mode. TDM has become a cost effective method that is not only used on trunk circuits between digital switches but is today even starting to be used on local circuits to end customers.

Code Division Multiplexing is a third kind of multiplexing that works in a completely different way than FDM &TDM. Code Division Multiplexing is a spread out over a wider frequency band .This can make it more tolerant of interference, as well as allowing multiple signals from different users to share the same frequency band.Because CDM is mostly used for the latter purpose it is commonly called CDMA ( Code Division Multiple Access).