Compare multiplexing techniques

Multiplexing is the sharing of one transmission medium among several signals. The main techniques used in telecommunication are FDM, TDM (synchronous and statistical), WDM and CDM.

Frequency Division Multiplexing (FDM)

Each signal modulates a different carrier and occupies its own band, with guard bands between bands; all signals are sent at the same time. Classic use: analogue telephone carrier (12-channel group at 60–108 kHz, 60-channel supergroup), radio and TV broadcasting.

 amplitude
   |  [ch1]  [ch2]  [ch3]  [ch4]
   |  |   |g |   |g |   |g |   |
   +--+---+--+---+--+---+--+---+--> f
      f1     f2     f3     f4
 g = guard band

Time Division Multiplexing (TDM)

Each signal gets a time slot in a repeating frame and uses the full bandwidth in its slot. Synchronous TDM gives fixed slots (T1 = 24 ch at 1.544 Mbps, E1 = 30 ch at 2.048 Mbps); statistical TDM gives slots only to active sources and tags them with addresses.

 |<-------------- one frame ------------->|
 | S1 | S2 | S3 | ... | SN | S1 | S2 | ...
  slot slot              slot
 S_i = time slot of channel i

Wavelength Division Multiplexing (WDM)

Several optical carriers of different wavelengths share one fibre; they are combined and separated by passive optical devices. CWDM and DWDM give from a few to over 100 channels of 10–100 Gbps each.

Code Division Multiplexing (CDM)

All users transmit at the same time in the same band, each multiplied by a unique spreading code; the receiver correlates with the wanted code to extract its signal. Used in CDMA/3G mobile and GPS.

          freq
 FDM:  ch3 |=========|      TDM: | 1 | 2 | 3 | 1 | 2 |
       ch2 |=========|            ------ time ------>
       ch1 |=========|
           +--- time -->     CDM: all users, all time,
                                  all freq, diff. codes

Comparison

Summary

• FDM is simplest for analogue signals but suffers from noise build-up and costly filters; it has been largely replaced.
• TDM fits digital switching and transmission and is the basis of T1/E1, PDH and SDH.
• Statistical TDM is best for bursty data and leads to packet switching.
• WDM gives the highest capacity on fibre and sits under SDH/OTN backbones.
• CDM gives flexible, interference-resistant sharing of radio spectrum. Modern networks combine them, e.g. TDM (SDH) channels carried on WDM wavelengths.