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Cellular Mobile Communication
Systems
Lecture 4
Engr. Shahryar Saleem
Assistant Professor
Department of Telecom Engineering
University of Engineering and Technology
Taxila
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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
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(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
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 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
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)
• 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
bits
– 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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