31-05-2012, 01:48 PM
EE4512 Analog and Digital Communications-Multiplexing Techniques
Multiplexing Techniques.pdf (Size: 818.07 KB / Downloads: 64)
Time division multiplexing (TDM) combines several low,
fixed and predefined bit rate sources into a single high
speed bit stream for transmission over a single digital
communication channel:
The TDM time slots have to be chosen properly. If the time
slots are too small (for example, 1 bit) then the multiplexer
and demultiplexer must switch rapidly. If the time slots are
too large (for example, 64 Kb) then the data must be
buffered and delay would be produced. TDM is used for
baseband (not bandpass) data transmission.
If data arrives asynchronously from variable rate sources, a
statistical multiplexer with input buffers is used. The design
is performed by observation and tested in simulation.
Frequency division multiplexing (FDM) divides the total
bandwidth available to the system into non-overlapping
frequency sub-bands for transmission over a single digital
communication channel.
FDM usually utilizes guard bands to separate the digital
data transmissions. FDM is used for bandpass (not
baseband) data transmission.
TDM is used for baseband and FDM for bandpass data
transmission. FDM requires the assignment and
coordination of carrier frequencies which can be
problematic. Code division multiple access (CDMA) utilizes
spread spectrum modulation over the same frequency
band. CDMA is considered in EE4542 Telecommunications
Engineering.