digital communication fundamentals
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Chapter 1
Digital CommunicationsFundamentals
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1. Digital Communication Fundamentals
1.1 Communication Systems (Fig 1.1)
Mobile any terminal that moves rapidly.
Portable hand held terminal that moves slowly.
Base Station (BS)
a station that is fixed andcommunicates with the mobile.
Classes of transmission systems (Fig 1.2)
Simplex one way communication, Eg. Paging,
Broadcast etc. Half duplex two way communication, either
transmit or receive at any instant, Eg. Walkie
Talkie
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Channel is time shared (uses time slots).
Single channel, simultaneoustransmission and reception not possible.
A channel transmitting at higher data rates
appears to be transmitting and receiving
data simultaneously. Used in digital communication systems
and is prone to timing error and jitter.
TDD
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1.2 Digital Mobile Systems
Figure 1.2 A typical
Communication System
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Example: ARQ (Automatic Repeat Request).
1.3 Problems associated with MCS
Fading Occurs due to motion.
Losses could be as high as tens of dBs.
Fading rate proportional to the Carrier
Frequency and speed of the mobile.
Imbedding error correction protocols in data terminals
insures data fidelity.
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At high data rates, frequency selective fading
causes higher BER and Inter SymbolInterference (ISI).
Noise a general problem encountered in
communication channels resulting in higher BER.
can be reduced by error correction schemes. Additive White Gaussian Noise (AWGN) is
assumed.
Noise, fading, path loss complicate analysis.
Requires separate circuits handle each.
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1.4. Modulation and Coding requirements
a. Maximize transmission bit rates
b. Minimize error probability, .
c. Minimize power - minimize bit energy to noise
power spectral density, .
d. Minimize system BW.
e. Maximize system utilization.
f. Minimize system complexity, cost and load.
B eP P
0/NEb
R
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Maximizing rate requires larger BW.
Minimizing BER requires maximizing
power.
Trade offs required.
Theoretical considerations that necessitate
trade offs are: Nyquist theoretical BW.
Shannon capacity theorem.
Goals (a) contradicts (d) and (b) contradicts (c).
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BW efficiency represents the measure of data
throughput per hertz of BW.
Units are bits/s/Hz.
Shows how efficiently modulation (signaling)
technique utilizes BW.
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64
Theoretical max BW efficiency with out ISI2 (without ISI)
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k
M PAM
symbols / s / Hz
BWE bps / s / Hz
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Example
16ary PSK with a data rate of 9600 bps.
2
16 2 4 /
/
4 /9600
2400 /4
k
k
s
M
k bits symbol
Rbits sR
bits symbol
symbols s
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1.5.3 Shannons capacity theorem
The capacity of a channel in the presence
of AWGN is a function of average received
signal power.
2 lo 1g 1 .3S
C WN
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Unattainable
Physical Systems
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1.5.5 Entropy (H)
Defined as the average amount of information
per source output.
piis the probability of the ithoutput and
H in the case of two sources having
probabilities p and q = (1-p)is
2
1
log 71.n
i i
i
H p p
1ip
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2 2log log 1.8 H p p q q
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1.5.6 Bandwidth
BW-Range of positive frequencies over
which signal exists
Null to Null BW Range of positive
frequencies within the main lobe
3-db BW range of positive frequencies
from the maximum to the frequency where
the magnitude drops to 1/2 its peak value.
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