3F4 Data Transmission
Introduction
Dr. I. J. Wassell
Pre-requisites
• Familiarity with IB courses
– Signal and Data Analysis (Paper 7)
– Linear Systems and Control (Paper 6)
– Communications (Paper 6)
Booklist
• Couch, L. W, “Digital and Analog Communication
Systems,” Prentice Hall (5th Edition). Covers all
except DFE.
• Shanmugam, K. S. Digital and Analog
Communication Systems,” Wiley. All except DFE.
• Proakis, J. G, “Digital Communications,”
McGraw Hill.
• Wicker, S. B., “Error Control Systems for Digital
Communication and Storage,” Prentice Hall, 1995.
Applications
• Data transmission over copper cables and
optical fibres, e.g.,
– computer local area networks (LANs)
– integrated services digital network (ISDN)
connections, e.g., Basic rate (2B+D) and primary
rate (30B+2D) channels. B=64kBit/s, D=16kBit/s.
– 30 channel pulse code modulation (PCM), i.e., 30
telephony channels, 2.048MBit/s.
– Asynchronous transfer mode (ATM) in wide area
networks (WANs), e.g., 25MBit/s, 155MBit/s
Applications
• Data storage,
– magnetic disk drives
– magnetic tape drives
– optical disk drives
Topics Covered
• Communications System Model
• Pulse Amplitude Modulation (PAM) for
baseband data transmission
• Intersymbol Interference (ISI), noise and bit
error rates (BER)
• Pulse Shaping for bandwidth control and
elimination of ISI
Topics Covered
• Line coding schemes
• Optimum Transmit and Receive Filtering
• Equalisation to compensate for undesirable
channel characteristics
• Error control coding (ECC)
Baseband Transmission
• The transmitted signal is limited to a range
from -B Hz to +B Hz.
|X(f)|
0
-B
B
Baseband
|X(f)|
f
-fc
0
fc
Bandpass
• Example baseband channels include
– copper cable, magnetic disk, CD-(ROM)
f
Comms System Model
• Transmission of digital data in
communications channels
– True digital data, eg, comms in a computer
network
– Analogue information which has been
converted to a digital format, eg anti-alias LPF
followed by A/D conversion
Transmission Model
Error
Digital
Source
Source
Encoder
Control
Coding
Line
Coding
Modulator
(Transmit
Filter, etc)
Hc(w)
Transmitter
N(w)
Digital
Source
Sink
Decoder
Error
Control
Decoding
Receiver
X(w)
Noise
Demod
Line
Decoding
Channel
(Receive
Filter, etc)
+
Y(w)
Components of the Model
• Assume input source is in the form of
symbols, eg bytes from a PC, or 16-bit audio
samples
• Source encoding- Transforms digital symbols
into a stream of binary digits (BITS), eg PCM
• Error Control Coding- Adding extra bits
(redundancy) to allow error checking and
correction.
Components of the Model
• Line Coding- Coding of the bit stream to
make its spectrum suitable for the channel
response. Also to ensure the presence of
frequency components to permit bit timing
extraction at the receiver.
• Transmit Filtering- Generation of
analogue pulses for transmission by the
channel.
Components of the Model
• Channel- Will affect the shape of the
received pulses. Noise is also present at the
receiver input, eg thermal noise, electrical
interference etc.
• We will concentrate on the components to
the right of the vertical dashed line.
Channel Response
Assumptions
– Linear time-invariant (LTI) frequency response,
ie, the channel frequency response Hc(w) is
fixed, known and linear.
– Additive Gaussian noise- The channel noise has
a Gaussian amplitude distribution (pdf) is often
assumed to be uncorrelated (ie white, flat power
spectral density) and additive.
Channel Response
Thus the received signal may be expressed as,
Y(w)= Hc(w) X(w)+N(w)
Where,
Hc (w) is the channel frequency response
X(w) is the transmitted signal spectrum
N(w) is the noise spectrum
In practice,
– Channel response may be non-linear, time-varying or
unknown
– Noise may be non-Gaussian, particularly interference.