This document looks at the different multiplexing techniques used in optical communications

1. MULTIPLEXING
Multiplexing is a technique, in which multiple users transmit data over a single
channel. The channel may be co-axial cable, a fiber cable, radio or satellite.
Multiplexing is useful as to increase the channel utilization and the transmission
capacity.
Multiplexing has two devices called multiplexer (MUX) and De multiplexer (DEMUX)
Multiplexer combines the diffèrent signals into a single signal. De- multiplexer
performs the inverse operation of multiplexer (Disbands them).
Types Of Multiplexers
i) Optical multiplexer and ii) Electrical multiplexer.
In electrical multiplexer, Multiplexing is done before the Electrical to optical conversion
(E/O). And demultiplexing is done after Optical to electrical signal. And a reverse for
optical Multiplexers as illustrated below

2. General Multiplexing Techniques
 Time domain Multiplexing(TDM)
In TDM, narrow pulses from multiple modulators/sources and/or channels under the
control of a common clock are interleaved and transmitted sequentially, thus
enhancing the bandwidth utilization of a single-fiber link.
Sources emits light with the same wavelength
TDM sync techniques
Bit interleaving(bit by bit)
Packet interleaving(in blocks)
 Frequency Domain Multiplexing(FDM)
In FDM the optical channel bandwidth is divided into a number of nonoverlapping
frequency bands and each signal is assigned one of these bands of frequencies. The
individual signals can be extracted from the combined FDM signal by appropriate
electrical filtering at the receive terminal.
Similar to WDM

3. Cont’d
 Wavelength Division Multiplexing(WDM)
Utilize a number of optical sources, each operating at a different wavelength to
share the single-fiber link.
The separation and extraction of the multiplexed signals (i.e. wavelength separation)
is performed with optical filters (e.g. interference filters, diffraction grating filters, or
prism filters).
 Space Division Multiplexing (SDM)
Does not involve the application of several message signals onto a single fiber.
In SDM, each signal channel is carried on a separate fiber within a fiber bundle or
multifiber cable form.
This technique necessitates an increase in the number of optical components
required (e.g. fiber, connectors, sources, detectors) within a particular system and
therefore has not been widely used to date.

4. OPTICAL MULTIPLEXING STRATEGIES
 Optical Time Domain Multiplexing (OTDM)
In OTDM, several optical signals at a bit rate of B per second share the same
carrier frequency and are multiplexed to for a composite bit stream at a bit rate
nB per second , where n is the number of channels/and or sources
The bit rates are controlled by a constant frequency synchronous clock by
providing time delays for every signal.
All sources emits at the same wavelength.

5. 4 channel OTDM with a baseband channel rate of 40 Gbit/s and comp
160Gbit/s

6.  Optical Subcarrier multiplexing(OSCM)
In OSCM, the microwave frequency or RF electrical subcarriers are modulated with an
optical carrier and then are transmitted using a single wavelength signal
Microwave subcarrier multiplexing enables multiple broadband signals to be
transmitted over single-mode fiber and can be particularly attractive for video
distribution systems

7. OSCM Cont’d
The modulated microwave subcarrier signals are obtained by frequency conversion from
the baseband using voltage-controlled oscillators (VCOs). These subcarrier signals fi are
then summed in a microwave power combiner prior to the application of the composite
signal to an injection laser which is dc. biased at around 5 mW in order to produce the
desired intensity modulation. The IM optical signal is then transmitted over single-mode
fiber and directly detected using a wideband photodiode before demultiplexing and
demodulation using a conventional microwave receiver.
 Orthogonal frequency division multiplexing(OFDM)
OFDM is a multicarrier transmission technique which is based on frequency division
multiplexing (FDM)
In conventional FDM multiple-frequency signals are transmitted simultaneously in
parallel where the data contained in each signal is modulated onto subcarriers and
therefore the subcarrier multiplexed signal typically contains a wide range of
frequencies. Each subcarrier is separated by a guard band to avoid signal overlapping.
The subcarriers are then demodulated at the receiver by using filters to separate the
frequency bands.
.

8. OFDM Cont’d
OFDM in contrast employs several subcarrier frequencies orthogonal to each other
(i.e. perpendicular) and therefore they do not overlap. Hence this technique can
squeeze multiple modulated carriers tightly together at a reduced bandwidth
without the requirement for guard bands while at the same time keeping the
modulated signals orthogonal so that they do not interfere with each other.
As shown below

9.  Optical Wavelength division multiplexing(OWDM)
OWDM involves the transmission of a number of different peak wavelength optical
signals in parallel on a single optical fiber. Although in spectral terms optical WDM is
analogous to electrical FDM, it has the distinction that each WDM channel effectively
has access to the entire intensity modulation fiber bandwidth.

10. WDM Cont’d
Categories of WDM
 Coarse WDM (CWDM)
 Dense WDM(DWDM)
Both categories use the same concept of multiple-wavelength channels on a single
fiber, they differ in the channel spacing they employ. CWDM as implied by the
terminology uses wider channel spacing and hence provides significantly fewer
channels than DWDM.
Optical code division multiplexing(OCDM)
Optical code division multiplexing (OCDM), sometimes termed optical code division
multiple access (OCDMA) is a digital technique where, instead of each channel
occupying a given wavelength, frequency or time slot, the information is
transmitted using a coded sequence of pulses. Each channel employs a specific
code to transmit and recover the original signal. It utilizes the basic principle of
spread spectrum transmission where all users share the fiber channel bandwidth
simultaneously.

11. OCDM Cont’d
A decoder is then used at the receiving end to recover the particular channel
employing autocorrelation with the original chip sequence.
• In order to enable an increasing number of OCDM channels to be
transmitted on a single fiber, however, ultrashort pulses (i.e. 10−15 second
pulse duration) are required to be used in OCDM systems. Furthermore, the
larger the number of channels, the longer the code sequences needed to
provide a unique channel code.

12. Hybrid Multiplexing
When two (or more) different multiplexing techniques are combined to allow
optical signal multiplexing for several optical signals, the resultant is
referred to as hybrid multiplexing. This combination of different multiplexing
techniques can be used to overcome the problems associated with a
specific technique.
A hybrid multiplexing system can comprise either optical or electrical
domain multiplexing, or combination of both signal types.
Common examples of optical hybrid multiplexing are WDM being combined
with OTDM, OCDM or SCM.
Hybrid WDM/OTDM systems can support terabit per second transmission
rates when several WDM channels are combined with OTDM technology.