Cellular Mobile Communication Systems Lecture 4

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Cellular Mobile Communication SystemsLecture 4

• Engr. Shahryar Saleem
• Assistant Professor
• Department of Telecom Engineering
• University of Engineering and Technology
• Taxila
• TI -1011

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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. gt
  lower power needed
• 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
• FSK or PSK

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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 other
• 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 Keying)
• Two symbols used 0 and 1 are two sinusoids
  with 180-degree phase difference
• Phase shifts according to the voltage level of
  the baseband signal
• very simple PSK
• low spectral efficiency
• robust, used e.g. in satellite systems

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Advanced Phase Shift Keying (cont)

• QPSK (Quadrature Phase Shift
• Keying)
• 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 Voltage
• 0 by a Ve Voltage
• Then the QPSK symbol is decided as follows.
• 01 cos(2pfct p/4), 45
• 11 cos(2pfct 3p/4), 135
• 10 cos(2pfct 5p/4), 225
• 00 cos(2pfct 7p/4), 315
• Why do we choose this mapping?
• cos(AB) cos(A)cos(B) sin(A)sin(B)

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p/4 - QPSK

• p/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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p/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
• correlated
• Usually deployed in all base stations but
  harder at the mobile

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Performance Degradation and Diversity
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Error Control

• BER in wireless networks
• Several orders of magnitude worse than
  wireline
• networks
• 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 1s
• Add a parity bit1 if number of 1s in data is
  odd
• add parity bit0 if number of 1s in data is
  even
• If any bit is in error, the received codeword
  will
• have odd number of 1s
• 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
• length)
• 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
  efficient)
• Ck/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 bits
• e.g., v1011011 v2110001 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 k-bit 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)