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Cellular Mobile Communication
Lecture 4
Engr. Shahryar Saleem
Assistant Professor
Department of Telecom Engineering
University of Engineering and Technology
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Digital Transmission
• Current wireless networks have moved almost entirely to
digital modulation
• Why Digital Wireless?
– Increase System Capacity (voice compression) more
efficient modulation
– Error control coding, equalizers, etc. => lower power
– Add additional services/features (SMS, caller ID, etc..)
– Reduce Cost
– Improve Security (encryption possible)
– Data service and voice treated same (3G systems)
• Called digital transmission but actually Analog signal
carrying digital data
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Digital Modulation Techniques
• Amplitude Shift Keying (ASK):
– change amplitude with each symbol
– frequency constant
– low bandwidth requirements
– very susceptible to interference
• Frequency Shift Keying (FSK):
– change frequency with each symbol
– needs larger bandwidth
• Phase Shift Keying (PSK):
– Change phase with each symbol
– More complex
– robust against interference
• Most systems use either a form of
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Advanced Frequency Shift Keying
• Bandwidth needed for FSK depends on the distance
between the carrier frequencies
• Special pre-computation avoids sudden phase shifts
• MSK (Minimum Shift Keying)
• Bit separated into even and odd bits, the duration of
each bit is doubled
• Depending on the bit values (even, odd) the higher or
lower frequency, original or inverted is chosen
• The frequency of one carrier is twice the frequency of the
• Even higher bandwidth efficiency using a Gaussian
low-pass filter GMSK (Gaussian MSK), used in
GSM cellular network
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Advanced Phase Shift Keying
• BPSK (Binary Phase Shift
– Two symbols used : 0 and 1 are
two sinusoids with 180-degree
phase difference
– Phase shifts according to the
voltage level of the baseband
– very simple PSK
– low spectral efficiency
– robust, used e.g. in satellite
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Advanced Phase Shift Keying
• QPSK (Quadrature Phase Shift
– 2 bits coded as one symbol
– Four Transmitted symbols assume
four different phase values of 45, 135,
225, 315-degrees
–The difference between the phases is
90- degrees
– Symbol determines shift of sine wave
– Needs less bandwidth compared to
BPSK (high bandwidth efficiency)
– more complex
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QPSK Quick Review
• In QPSK, we use two bits to represent
on one of four phases.
• Example: We represent 1 by a –Ve
0 by a +Ve Voltage
• Then the QPSK symbol is decided as
01 : cos(2πfct + π/4), 45
11 : cos(2πfct + 3π/4), 135
10 : cos(2πfct + 5π/4), 225
00 : cos(2πfct + 7π/4), 315
• Why do we choose this mapping?
• cos(A+B) = cos(A)cos(B) – sin(A)sin(B)
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π/4 - QPSK
• π/4- QPSK is a form of QPSK modulation
• The QPSK signal constellation is shifted by 45 degrees
each symbol interval T
• Phase transitions from one symbol to the next are
restricted to ± 45 degrees and ± 135 degrees
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π/4 – QPSK (Example)
Successive symbols are taken from the two constellations
first symbol (1 1) is taken from the 'blue' constellation
the second symbol (0 0) is taken from the 'green' constellation.
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What is Diversity?
• Idea: Send the same information over several “uncorrelated” forms
– Not all repetitions will be lost in a fade
• Types of diversity
– Time diversity – repeat information in time spaced so as to
not simultaneously have fading
• Error control coding!
– Frequency diversity – repeat information in frequency
channels that are spaced apart
• Frequency hopping spread spectrum, OFDM
– Space diversity – use multiple antennas spaced sufficiently
apart so that the signals arriving at these antennas are not
• Usually deployed in all base stations but harder at the mobile
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Performance Degradation and
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Error Control
• BER in wireless networks
– Several orders of magnitude worse than wireline
– Channel errors are random and bursty, usually
coinciding with deep fast fades
– Much higher BER within bursts
• Protection against bit errors
– Necessary for data
– Speech can tolerate much higher bit errors
(10 -2 depending on encoding/compression algorithm)
• Error Control Coding used to overcome BER
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Error Control Coding
• Diversity scheme that introduces redundancy in the transmitted bits
to correct errors
• If correction not possible, provide the capacity to detect
• For voice the acceptable error rate is 1 in 100 bits or 10 -2
• Data packet and messaging systems requires error rates up to 10-5
• Where this error rate is unachievable, retransmit the data (Automatic
Repeat Request)
• Error detection is the process of determining whether the a block of
data is in error
• Block codes can be used to correct errors and is called Forward
Error Correction (FEC)
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Block Codes
• Block coding involves coding a block of bits into another
block of bits with some redundancy to combat errors
• single parity bit --- even parity code
– Valid codewords should always have even
number of 1’s
– Add a parity bit=1 if number of 1’s in data is odd
add parity bit=0 if number of 1’s in data is even
– If any bit is in error, the received codeword will
have odd number of 1’s
– Single parity can detect any single bit error (but
not correct)
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Single Parity (cont)
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Block Codes (n,k) Blocks
(n,k) block codes
k = number of data bits in block (data word
n-k = number of parity check bits added which apply
parity check to a group of bits in a block of k bits
• n = length of codeword or code block; k + (n-k)= n
• (n-k) /n = overhead or redundancy (lower is more
• C=k/n = coding rate (higher is more efficient)
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Block Codes (cont)
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Block Code Principle
• Hamming distance :
– for 2 n-bit binary sequences, the number of different
– e.g., v1=011011; v2=110001; d(v1, v2)=3
• The minimum distance (dmin) of an (n,k) block code is the
smallest Hamming distance between any pair of
codewords in a code.
– Number of error bits can be detected: dmin-1
– Number of error bits can be corrected t:
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(7,4) Hamming Code
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Forward Error Correction
• FEC Operation
• Transmitter
–Forward error correction (FEC) encoder maps each kbit block into an n-bit block codeword
–Codeword is transmitted;
• Receiver
–Incoming signal is demodulated
–Block passed through an FEC decoder
–Decoder detects and correct errors
• Receiver can correct errors by mapping invalid codeword
to nearest valid codeword
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FEC (cont)
Forward Error Correction Process
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Convolution Coding
• Block Codes treat data as separate Blocks (memory less encoding)
• Convolution codes map a continuous data string into a continuous
encoded string (memory)
• Error checking and correcting carried out continuously
• (n, k, K) code
• Input processes k bits at a time
• Output Produces n bits for every k input bits
• K= Constraint Factor (number of previous bits used in encoding)
• n-bit output of (n, k, K) code depends on:
• Current block of k input bits
• Previous K-1 blocks of k input bits
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Convolution Encoder
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What Does Coding Get You?
• Consider a wireless link
– probability of a bit error = q
– probability of correct reception = p
– In a block of k bits with no error correction
– P (word correctly received) = p k
– P (word error) = 1 – p k
– With error correction of t bits in block of n bits
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What Does Coding Get You? (cont)
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