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Cellular Mobile Communication Systems

Lecture 4

Engr. Shahryar SaleemAssistant Professor

Department of Telecom EngineeringUniversity of Engineering and Technology

TaxilaTI -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. => 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 ofFSK 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 inGSM 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 ShiftKeying):– 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 Voltage0 by a +Ve Voltage

• Then the QPSK symbol is decided as follows.01 : cos(2πfct + π/4), 4511 : cos(2πfct + 3π/4), 13510 : cos(2πfct + 5π/4), 22500 : 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 tonot simultaneously have fading• Error control coding!– Frequency diversity – repeat information in frequencychannels that are spaced apart• Frequency hopping spread spectrum, OFDM– Space diversity – use multiple antennas spaced sufficientlyapart so that the signals arriving at these antennas are notcorrelated• 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 wirelinenetworks – Channel errors are random and bursty, usuallycoinciding 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 evennumber of 1’s– Add a parity bit=1 if number of 1’s in data is oddadd parity bit=0 if number of 1’s in data is even– If any bit is in error, the received codeword willhave odd number of 1’s– Single parity can detect any single bit error (butnot 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 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)