COMM 1208 Unit 5 Baseband Communications
Baseband Communications
1.
Bandwidth ............................................................................................................................ 2
1.1
Bandwidth definitions (Google definitions) .................................................................... 2
1.2
Minimum Bandwidth Requirements .............................................................................. 3
2. Line Coding .......................................................................................................................... 4
2.1
Requirements .................................................................................................................. 4
2.2
Analogue Telephone Line Considerations ...................................................................... 4
2.3
Digital Signalling Formats ............................................................................................. 4
2.3.1
Unipolar Non Return to Zero (NRZ) ........................................................................ 4
2.3.2
Bipolar NRZ ............................................................................................................. 5
2.3.3
Unipolar Return to Zero (RZ) .................................................................................. 5
2.3.4
Bipolar RZ ................................................................................................................ 5
2.3.5
Alternate Mark Inversion (AMI) RZ Signalling ...................................................... 6
2.3.6
Manchester Coding .................................................................................................. 6
2.3.7
Coding comparison ................................................................................................... 7
3. M-Ary Line Coding ............................................................................................................. 8
4. Line Transmission Systems .............................................................................................. 9
4.1
Equaliser ....................................................................................................................... 10
5. Transmission Line Impairments ................................................................................... 10
5.1
Amplitude distortion and phase distortion .................................................................. 11
5.2
Inter Symbol Interference ............................................................................................. 11
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COMM 1208 Unit 5 Baseband Communications
1. Bandwidth
1.1 Bandwidth definitions (Google definitions)

A measure of the capacity of a communications channel. The higher a channel's
bandwidth, the more information it can carry.
www.tamu.edu/ode/glossary.html

A relative range of frequencies that can carry a signal on a transmission medium.
www.adaptivedigital.com/services/serv_definitions.htm

A measure of spectrum (frequency) use or capacity. For instance, a voice transmission
by telephone requires a bandwidth of about 3000 cycles per second (3 KHz). A TV
channel occupies a bandwidth of 6 million cycles per second (6 MHz) in terrestrial
Systems. In satellite based systems a larger bandwidth of 17.5 to 72 MHz is used to
spread or "dither" the television signal in order to prevent interference.
www.spidersat.net/glossary/glossary_b.htm

The range of frequencies, expressed in hertz (Hz), that can pass over a given
transmission channel. The bandwidth determines the rate at which information can be
transmitted through the circuit.
www.ssloral.com/html/products/glossary.html

The complete range of frequencies over which a circuit or electronic system is allocated
to function. In transmission, the US analog and digital television channel bandwidth is
6 MHz.
www.wgcu.org/watch/hdtv_glossaryofterms.html

The range of frequencies a channel can carry. The higher the frequency, the higher the
bandwidth and the greater the capacity of a channel. In Internet terms, higher
bandwidth means a higher ability to transmit and receive data.
www.7designavenue.com/glossary.htm
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1.2 Minimum Bandwidth Requirements
Any signal can be treated as if it were made up of an infinite number of frequency
components. We have seen the spectrum of a square wave already in which case the
(significant) frequency components include much higher frequencies than the fundamental
frequency. To transmit the waveform without significant distortion would therefore require a
channel with a considerable bandwidth, together with a suitable phase change characteristic.
A pulse train does not have to be
received undistorted in order to make
+V
correct decisions about its binary states.
Time
In fact, in the case illustrated, so long as
the fundamental component, at f=l/(T)
Hz, of the square wave corresponding to
Period = T
the bit stream ...01010101… can be
Bit Interval B = T/2
T
B
transmitted, then correct decisions can
be made about the binary states. It is
possible, in theory at least, to transmit 2/T symbols per second over a channel of bandwidth
2/(T) Hz.
B=2xf
Bits per second = 2 x bandwidth
For example a 64 kbit/s data stream, could be sent and recovered over a 32 kHz bandlimited
channel.
This is an important general rule (due to Nyquist) for digital waveforms.
Example:
A primary ISDN signal has a bit rate of 2.048 Mbit/s. What would be the
minimum theoretical bandwidth required to transmit this signal?
Answer:
Minimum bandwidth = 2.048 Mbit/s = 1.024 MHz
Example:
A spectrum analyser and antenna is used to record the radiation pattern
from a TDM system which contains clock generators. There are two peaks, at
153.088 MHz and 154.112 MHz.
(a) What is the frequency of the clock generator which is responsible for these peaks?
(b) What is the order of these harmonics?
(c) How might these peaks be reduced without affecting the performance of the system?
Answer:
(a)
A clock generator outputs a square wave which contains only odd
harmonics. Therefore these frequencies are odd multiples of the clock frequency, and the
difference between them is twice the clock frequency (or perhaps 4 or six times - but
much less likely). The difference is 154.112 - 153.088 MHz = 1.024 MHz. Therefore the
clock is 512 kHz.
(b)
154.122 MHz is the 301st harmonic (154.122/.512 = 301). The other one is the
299th harmonic
(c)
The output from the clock generator should be filtered e.g. by using a ferrite
bead in series, or a small capacitor to earth. Track layout is important, keeping all
tracks as short as possible. Or the circuit could be screened - effectively put in a metal
box.
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2. Line Coding
2.1 Requirements
Digital data can be transmitted by various pulse waveforms, also called line codes. The
following properties are desirable for a line code:
 It is important that the pulses stream to be transmitted does not have a DC component. It
can case baseline wander or Galvanic Corrosion.
 It should be relatively easy to recover the data clock.
 The line coding scheme should be bandwidth efficient.
 The line code should be robust in the presence of noise.
 It should be possible to recognise a line coding error, sometimes called a line violation. (In
some signalling protocols, a line violation is deliberately generated to mark the start of a
frame)
2.2 Analogue Telephone Line Considerations
To review the telephone line. At the local exchange
a voltage is applied, via inductors and resistors to
R
the copper pair. This allows the transmitting
+
equipment to sink current. The variations in
C L
current correspond to change in the voltage signal
Telephone
RX
TX
on the line. The receiver terminal reads this
Line
voltage. In most cases the receiving terminal is
R
Terminal
allowed to take a DC feed from the line itself.
+
L
The bandwidth of an analogue telephone line
connection is 300 Hz to 3.4 kHz. A square wave or
Local Exchange
any pulse train with very fast rise times will be
distorted if it is sent along a telephone line. Therefore an analogue telephone line is not
suitable for sending digital pulses as all frequency components outside the 300 - 3 kHz range
will be removed. This bandwidth limitation is not caused totally by the copper pair but by the
filters in the local exchange which are part of the analogue to digital process. In the past some
analogue telephone lines also had loading coils (inductors) on the line which were intended to
give a flat frequency response.
On a digital telephone line all analogue filters are removed so the usable bandwidth of the
copper pair itself is much greater and can extend to a few Megahertz. These lines are suitable
for pulse transmission e.g. ISDN.
2.3 Digital Signalling Formats
2.3.1 Unipolar Non Return to Zero (NRZ)
Symbol 1 is represented by transmitting a pulse of constant amplitude for the entire duration
of the bit interval, and symbol 0 is represented by no pulse. NRZ indicates that the assigned
amplitude level is maintained throughout the entire bit period. This allows for long series
without change, which makes synchronization difficult (difficult to recover the clock).
Unipolar also contains a strong DC component.
.
From www.wikipedia.com
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In telecommunication, a non-return-to-zero (NRZ) line code is a binary code in which "1's" are
represented by one significant condition and "0's" are represented by the other significant
condition, with no other neutral or rest condition. The pulses have more energy than a RZ
code, but it does not have a rest state, which means a synchronization signal must also be
sent alongside the code.
For a given data signaling rate, i.e., bit rate, the NRZ code requires only half the bandwidth
required by the Manchester code.
When used to represent data in an asynchronous communication scheme, the absence of a
neutral state requires other mechanisms for data recovery, to replace methods used for error
detection when using synchronization information when a separate clock signal is available.
2.3.2 Bipolar NRZ
Pulses of equal positive and negative amplitudes represent symbols 1 and 0. (e.g. ± 5 volts, ±
12 volts) In either case, the assigned pulse amplitude level is maintained throughout the bit
interval. Because of the positive and negative levels the average voltage will tend towards
zero and hence little DC component. Again synchronisation will be difficult.
2.3.3 Unipolar Return to Zero (RZ)
Symbol 1 is represented by a positive pulse that returns to zero before the end of the bit
interval and symbol 0 is represented by the absence of pulse.
2.3.4 Bipolar RZ
Positive and negative pulses of equal amplitude are used for symbol 1 and symbol 0. In either
case the pulse returns to 0 before the end of the bit interval.
From www.wikipedia.com
Return-to-zero (RZ) describes a line code used in telecommunications signals in which the
signal drops (returns) to zero between each pulse. This takes place even if a number of
consecutive zeros or ones occur in the signal. The signal is self-clocking. This means that a
separate clock does not need to be sent alongside the signal, but suffers from using twice the
bandwidth to achieve the same data-rate as compared to non-return-to-zero format.
The "zero" between each bit is a neutral or rest condition, such as a zero amplitude in pulse
amplitude modulation (PAM).
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2.3.5 Alternate Mark Inversion (AMI) RZ Signalling
Positive and negative pulses (of equal amplitude) are used for alternative symbols 1 .No pulse
is used for symbol 0. In either case the pulse returns to 0 before the end of the bit interval. An
advantage of AMI is that it is easy to recognise a line violation.
From www.wikipedia.com
A binary 0 is encoded as zero volts as in unipolar encoding. A binary 1 is encoded alternately
as a positive voltage and a negative voltage. This prevents a significant build-up of DC, as the
positive and negative pulses average to zero volts. Little or no DC-component is considered an
advantage because the cable may then be used for longer distances and to carry power for
intermediate equipment such as line repeaters. The DC-component can be easily and cheaply
removed before the signal reaches the decoding circuitry.
Bipolar encoding is preferable to non-return-to-zero where signal transitions are required to
maintain synchronization between the transmitter and receiver. Other systems must
synchronize using some form of out-of-band communication, or add frame synchronization
sequences that don't carry data to the signal. These alternative approaches require either an
additional transmission medium for the clock signal or a loss of performance due to overhead,
respectively. A bipolar encoding is an often good compromise: runs of ones will not cause a
lack of transitions, however long sequences of zeroes are still an issue. Long sequences of zero
bits result in no transitions and a loss of synchronization. Where frequent transitions are a
requirement, a self-clocking encoding such as return-to-zero or some other more complicated
line code may be more appropriate, though they introduce significant overhead.
2.3.6 Manchester Coding
Symbol 1 is represented by a positive pulse followed by a negative pulse - with each pulse
being of equal amplitude and duration of half a pulse. For symbol 0 the polarities of these
pulses are reversed. An advantage of this coding is that it is easy to recover the original data
clock.
From www.wikipedia.com
Manchester coding provides a simple way to encode arbitrary binary sequences without ever
having long periods without level transitions, thus preventing the loss of clock
synchronization, or bit errors from low-frequency drift on poorly-equalized analog links (see
ones-density).
If transmitted as a bipolar signal (i.e. where the two signaling levels are of opposite polarity),
the DC component of the encoded signal is zero, again preventing baseline drift of the
repeated signal, making it easy to regenerate and preventing waste of energy.
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


Time is divided into periods, and one bit is transmitted per period
A "0" is expressed by a low-to-high transition, a "1" by high-to-low transition (according
to G.E. Thomas' convention--in the IEEE 802.3 convention, the reverse is true)
The transitions signifying 0 or 1 occur at the midpoint of a period
Manchester codes always have a transition at the middle of each bit period, and depending on
the state of the signal, may have a transition at the beginning of the period as well. The
direction of the mid-bit transition is what carries the data, with a low-to-high transition
indicating one binary value, and a high-to-low transition indicating the other.
2.3.7 Coding comparison
1
0
1
1
0
0
1
0
Unipolar NRZ
Bipolar NRZ
Unipolar RZ
Biplolar RZ
AMI
Manchester
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3. M-Ary Line Coding
The utilisation of bandwidth can be made more efficient by adopting an M-Ary format for the
representation of the input binary data . A Binary code consists of two symbols- '1' and '0'. A
quaternary (i.e. 4 level) code would consist of 4 symbols. The 4 symbols could be assigned to
00, 01, 10 and 11 for example. This would allow us to half the symbol rate on a transmission
line compared to one bit per symbol.
Note that binary data rate is measured in bits/second whereas the symbol rate is measured in
Baud. (Symbols per second).
An example of M-Ary Line coding is 2B1Q line code, as above, used on ISDN basic rate
telephone lines between a subscribers premises and the local
telephone exchange. In this case the baud rate will be half the
11
bit rate. (There are 4 possible symbols, each of which requires 2
10
bits. If the probability of each symbol is ¼ then the information
in each symbol is log2 (1/¼) = log2 4 = 2 bits, so that the
Time
information rate is 2 * baud rate = bit rate).
For example, the input binary sequence 11100001 is viewed as a
01
new sequence of dibits (pairs of bits); 11 10 00 01. Each dibit
00
symbol is assigned one of 4 levels. If we increase the number of
levels there will be a trade-off between noise performance and
bandwidth.
Example:
Explain how a ternary line coding system can code 3 bits per symbol.
Answer:
At 3 bits per symbol, 23 = 8, therefore we need at least 8 symbols. A
single ternary pulse would only allow one of 3 symbols to be represented. Two ternary
pulses in a particular order, however would allow for 9 combinations of levels. This code
could be abbreviated as 3B2T. It is not as efficient as 2B1Q but one advantage is that
zero volts is one of the levels and the wave form would resemble a binary bipolar format,
Note that 4B3T coding is also used on ISDN lines in some countries.
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4. Line Transmission Systems
Problems are encountered when a digital signal is sent through a channel. The diagram
shows the basic stages in a digital signal transmission. A simple non-return-to-zero (NRZ)
code is assumed, as already defined. The transmission medium might be a coaxial cable or a
copper twisted pair, as often used in local area networks or digital telephone systems. Similar
principles apply, however, to systems using other transmission media and/or more
complicated codes.
Retiming
Extracot
Noise
Digital Information
clocked at fc
- may be source
and/or channel
coded
Transmitter
E
A
Channel
e.g. Co-ax
Copper pair
Equaliser
B
C
Threshold
Detector
D
Retiming
Sector
Received
Digital
Information
F
Typical waveforms at the points labelled A to F in the system are shown. After passing down
the cable the original waveform A is attenuated and a noise component is added. Also because
of a finite system response time and propagation delays, the clear transition between voltage
levels become indistinct.
To counteract the distortion
illustrated, the system includes an
equaliser which sharpens the
received waveform, so that the
B Received signal from TX channel
relationship of the equaliser output
C to the original binary symbols is
much clearer. The equaliser would
C Output from equaliser
normally consist of an amplifier
stage combined with a filter to
D Output from threshold detector
reduce unwanted frequency
components. For example, it is
quite common for copper cables to
E Output from timing extractor
pick up a 50 Hz noise component
from the mains. It is essential that
F Output from re-timing circuit
the equaliser removes this
1 0 1 1 0 1 0
component. Also, in certain
configurations copper cables will
pick up electromagnetic interference, which must be filtered out. Note also that the input to
the equaliser must be protected from over-voltages such as induced lightning and other
transients.
Passing the equalised waveform through a threshold detector (e.g. a Schmitt trigger)
generates a binary signal very similar to the transmitted one. If the threshold settings are too
small then noise will trigger the detector. If the settings are too large then the data may not
trigger the detector. It is important that the slew rate of the comparator used in the detector
is fast enough for the data rate.
Provided that the noise levels are sufficiently low, and the equaliser and threshold detector
are properly designed, then the only difference between binary waveforms A and D is that the
transitions of the latter are not perfectly in step with those of the former. The transitions of D
will correspond to the threshold-crossings of waveform C which will not precisely mirror those
of the original binary waveform.
The final stage is the re-timing of the received waveform. If this were not carried out, then
the irregularities (jitter) in the waveforms would soon build up to cause error over a long link.
1 0 1 1 0 1 0 0
Binary information
A NRZ Line code from Transmitter
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A regular timing reference signal F - the data clock - is derived from the received waveform
itself by a special circuit, (the timing extraction circuit which is based on a Phase Locked
Loop). The clock signal and the output from the threshold detector are then processed to give
a final regenerated digital signal F whose transitions now coincide with the instants at which
the clock signal goes from low to high.
A comparison of waveforms C and F shows that the combined effect of threshold detection and
re-timing is equivalent to sampling waveform C near its peaks and troughs to determine the
appropriate binary states. So even in the presence of noise, regenerated signal F can be an
almost perfect (delayed) replica of the transmitted signal, provided only that the noise is not
sufficient to cause an incorrect decision to be made at the threshold detector.
4.1 Equaliser
From www.wikipedia.com
An equalization (EQ) filter is a filter, usually adjustable, chiefly meant to compensate for the
unequal frequency response of some other signal processing circuit or system.
An EQ filter typically allows the user to adjust one or more parameters that determine the
overall shape of the filter's transfer function. It is generally used to improve the fidelity of
sound, to emphasize certain instruments, to remove undesired noises, or to create completely
new and different sounds.
5. Transmission Line Impairments
Until now we have assumed that the transmitter sends a rectangular pulse. A transmission
line acts like a filter so the output response of the transmission line to a rectangular pulse can
be quite distorted. The distortion can mean that pulses can become overlapped thus causing
receiver errors. We therefore need to model the effects of transmitting pulses through a
transmission line.
a) Rectangular pulse response of a first order lowpass
V input pulse
filter, where the duration of the pulse is
approximately equal to the filter time constant.
pulse response
b) Response of the same filter to a binary waveform.
The figure shows the response to a single pulse, and
(a)
Time
the superimposed pulse responses corresponding to an
input pulse train (binary waveform). Note that
1 0 1 1 0 1 0 0
because the response to a single pulse takes longer to
decay that the duration of a symbol period, the output
waveform gradually accumulates a DC offset. In the
absence of further processing this would clearly cause
problems for threshold detection. Even in the
(b)
positions corresponding to a binary 0 there can be a
considerable output voltage.
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This figure, on the other hand, shows a much more
desirable overall pulse response for a
telecommunications channel. It shows the possible
pulse response
response of a telecommunications channel to (a) a
rectangular pulse and (b) a binary waveform. Here the
(a)
Time
system response to a bit stream could be decoded
without difficulty, owing to the clear distinction in the
1 0 1 1 0 1 0 0
combined response between binary 1 and 0.
An alternative approach to modelling a linear channel
or component is based on the second definition of
linearity. Any practical message signal can be
described in terms of its frequency content - or, to be
(b)
more precise, modelled as a frequency spectrum.
Similarly, any linear system can be completely specified by its frequency response function,
which is a description of amplitude and phase shifts introduced by the system for all
frequencies.
V
input pulse
5.1 Amplitude distortion and phase distortion
An ideal transmission channel would pass all frequency components of a signal with their
amplitude and phase relationships unchanged The simplest frequency domain model of such
behaviour would be a constant amplitude
ratio and zero phase shift for all frequencies
Transmitted Signal
Time
of interest. For example a square wave
would be unaffected by the transmission
Phase and
Pulse has spread
channel. In practice, the higher order
Amplitude Distortion
due to phase delay
harmonics will be greatly attenuated by the
for harmonics
transmission channel. Also the phase shift
Amplitude Distortion
will be different for each harmonic. For
Only
example, the fundamental harmonic may
have a phase shift of 45 degrees whereas the fifth harmonic could have a phase shift of 80
degrees. This will cause components of the pulse to be delayed or stretched. Specifications for
digital receiver systems usually include limits for phase delay.
5.2 Inter Symbol Interference
t
Intersymbol
Interferencce
Due to the fact that the transmission channels are bandlimited, the
transmitted pulses tend to spread during transmission. This pulse
spreading or dispersion causes overlap of pulses into adjacent pulse time
slots. This signal overlap may result in an error at the point where the
receiver makes a decision as to which pulse has been transmitted,
especially when other impairments are present (such as noise,
interference).
This effect of pulse overlap and the resultant difficulty of discriminating
between symbols at the receiver are termed inter symbol interference
(ISI).
From www.wikipedia.com
In telecommunication, intersymbol interference (ISI) means a form of distortion of a signal
that causes the previously transmitted symbols to have an effect on the currently received
symbol. This is usually an unwanted phenomenon as the previous symbols have similar effect
as noise, thus making the communication less reliable. ISI is usually caused by echoes or non11
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linear frequency response of the channel. Ways to fight against intersymbol interference
include adaptive equalization or error correcting codes.
In a digital transmission system, distortion of the received signal, which is manifested in the
temporal spreading and consequent overlap of individual pulses to the degree that the
receiver cannot reliably distinguish between changes of state, i.e., between individual signal
elements. At a certain threshold, intersymbol interference will compromise the integrity of
the received data.
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