Download FDM and TDM in Telecommunications and more Exercises Network Analysis in PDF only on Docsity!
MultiplexingMultiplexing
• Multiplexing is the name given to techniques, which allow more
than one message to be transferred via the same
communication channel. The channel in this context could be a
transmission line, e.g. a twisted pair or co-axial cable, a radio
system or a fibre optic system etc.
• A channel will offer a specified bandwidth, which is available for
a time t , where t may . Thus, with reference to the channel
there are 2 ‘degrees of freedom’, i.e. bandwidth or frequency
and time.
MultiplexingMultiplexing
CHANNEL BL BH freq BH BL Time t Frequency Multiplexing is a technique which allows k users to occupy the channel for the duration in time that the channel is available.
Now consider a signal v^ s ( t^^ )^ ^ Amp^ cos(^ ^ t^ )
The signal is characterised by amplitude, frequency, phase and time. 2
MultiplexingMultiplexing
These two basic methods are illustrated below. M 1 M 2 M 3 M 4 M 5 time freq BL BH B time freq M 1 M^2 M 3 M^4 M 5 t BL BH M 1 M 2 M 3 M 4 M 5 BL BH B M 1 M 2 M 3 M 4 M 5 t^ t FDM TDM t BL BH 4
- (^) FDM is widely used in radio and television systems ( e.g.
broadcast radio and TV) and was widely used in
multichannel telephony (now being superseded by digital
techniques and TDM).
- The multichannel telephone system illustrates some
important aspects and is considered below. For speech,
a bandwidth of 3kHz is satisfactory.
- (^) The physical line, e.g. a co-axial cable will have a
bandwidth compared to speech as shown next
Frequency Division Multiplexing FDM Frequency Division Multiplexing FDM 5
In order to use bandwidth more effectively, SSB is used i.e. m ( t ) cos( (^) ct ) carrier fc freq SSB Filter SSBSC We have also noted that the message signal m ( t ) is usually band limited, i.e.
m ( t )
cos( (^) ct ) SSB Filter
SSBSC
Band
Limiting
Filter
Speech
300Hz–3400Hz Frequency Division Multiplexing FDM Frequency Division Multiplexing FDM 7
Frequency Division Multiplexing FDMFrequency Division Multiplexing FDM The Band Limiting Filter (BLF) is usually a band pass filter with a pass band 300Hz to 3400Hz for speech. This is to allow guard bands between adjacent channels. 10kHz 300Hz 3400Hz 300Hz 3400Hz f f f Speech m ( t )^ Convention 8
Frequency Division Multiplexing FDMFrequency Division Multiplexing FDM Consider now a single channel SSB system. m ( t ) BLF SSB Filter fc DSBSC SSBSC 300Hz 3400Hz m ( t ) DSBSC freq freq freq fc fc The spectra will be 10
Frequency Division Multiplexing FDMFrequency Division Multiplexing FDM Consider now a system with 3 channels f f f BLF BLF BLF SSB Filter SSB Filter SSB Filter f c f c f c f 1 f 2 f 3 FDM Signal M ( t ) Bandlimited m 1 ( t ) m 2 ( t ) m 3 ( t ) FDM Transmitter or Encoder 11
Frequency Division Multiplexing FDMFrequency Division Multiplexing FDM Note that the baseband signals m 1 ( t ), m 2 ( t ), m 3 ( t ) have been multiplexed into adjacent channels, the channel spacing is 4kHz. Note also that the SSB filters are set to select the USB, tuned to f 1 , f 2 and f 3 respectively. (^) A receiver FDM decoder is illustrated below: SSB Filter SSB Filter SSB Filter LPF LPF LPF M ( t ) FDM Signal f 1 f 2 f 3 f c f c f c m 1 ( t ) m 2 ( t ) m 3 ( t ) Band Limited Back to baseband 13
Frequency Division Multiplexing FDMFrequency Division Multiplexing FDM
- (^) The SSB filters are the same as in the encoder, i.e. each one
centred on f 1 , f 2 and f 3 to select the appropriate sideband and reject
the others. These are then followed by a synchronous demodulator,
each fed with a synchronous LO, fc1 , fc2 and fc3 respectively.
- (^) For the 3 channel system shown there is 1 design for the BLF (used
3 times), 3 designs for the SSB filters (each used twice) and 1
design for the LPF (used 3 times).
- (^) A co-axial cable could accommodate several thousand 4 kHz
channels, for example 3600 channels is typical. The bandwidth used
is thus 3600 x 4kHz = 14.4Mhz. Potentially therefore there are 3600
different SSB filter designs. Not only this, but the designs must
range from kHz to MHz.
Basic 12 Channel GroupBasic 12 Channel Group
The diagram below illustrates the FDM principle for 12 channels (similar to 3 channels) to a form a basic group. m 1 ( t ) m 2 ( t ) m 3 ( t ) m 12 ( t ) Multiplexer 12kHz 60kHz freq i.e. 12 telephone channels are multiplexed in the frequency band 12kHz 60 kHz in 4kHz channels basic group. 16
Basic 12 Channel GroupBasic 12 Channel Group
A design for a basic 12 channel group is shown below: 300Hz 3400kHz 4kHz 300Hz 3400kHz 4kHz 300Hz 3400kHz 4kHz Band Limiting Filters DSBSC 8.6 15.4kHz 12.6 19.4kHz 52.6 59.4kHz f 1 = 12kHz f 1 = 16kHz f 12 = 56kHz Increase in 4kHz steps FDM OUT 12 – 60kHz 12.3 15.4kHz 16.3 19.4kHz 56.3 59.4kHz CH m 1 ( t ) CH m 2 ( t ) CH m 12 ( t ) SSB Filter 17
Super GroupSuper Group
5 basic groups multiplexed to form a super group, i.e. 60 channels in one super group. Note – the channel spacing in the super group in the above is 48kHz, i.e. each carrier frequency is separated by 48kHz. There are 12 designs (low frequency) for one basic group and 5 designs for the super group.
The Q for the super group SSB filters is Q^
kHz
kHz
12 - which is reasonable
Hence, a total of 17 designs are required for 60 channels. In a similar way, super groups may be multiplexed to form a master group, and master groups to form super master groups… 19
Time Division Multiplexing TDMTime Division Multiplexing TDM
TDM is widely used in digital communications, for example in the form of pulse code modulation in digital telephony (TDM/PCM). In TDM, each message signal occupies the channel ( e.g. a transmission line) for a short period of time. The principle is illustrated below: Transmission Line Tx Rx SW1 SW 1 2 3 4 5 1 2 3 4 5 m 1 ( t ) m 2 ( t ) m 3 ( t ) m 4 ( t ) m 5 ( t ) m 1 ( t ) m 2 ( t ) m 3 ( t ) m 4 ( t ) m 5 ( t ) Switches SW1 and SW2 rotate in synchronism, and in effect sample each message input in a sequence m 1 ( t ), m 2 ( t ), m 3 ( t ), m 4 ( t ), m 5 ( t ), m 1 ( t ), m 2 ( t ),… The sampled value (usually in digital form) is transmitted and recovered at the ‘far end’ to produce output m1 ( t )… m5 ( t ). 20
