Cellular Mobile Communication Systems Lecture 4

1. Cellular Mobile Communication SystemsLecture 4 Engr. Shahryar Saleem Assistant Professor Department of Telecom Engineering University of Engineering and Technology Taxila TI -1011 TI - 1011

2. 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 TI - 1011

3. 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 TI - 1011

4. 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 TI - 1011

5. 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 TI - 1011

6. 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 TI - 1011

7. 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) TI - 1011

8. 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 TI - 1011

9. 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. TI - 1011

10. 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 TI - 1011

11. Performance Degradation and Diversity TI - 1011

12. 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 TI - 1011

13. 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) TI - 1011

14. 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) TI - 1011

15. Single Parity (cont) TI - 1011

16. 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) TI - 1011

17. Block Codes (cont) TI - 1011

18. 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: TI - 1011

19. (7,4) Hamming Code TI - 1011

20. 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 TI - 1011

21. FEC (cont) Forward Error Correction Process TI - 1011

22. 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 TI - 1011

23. Convolution Encoder TI - 1011

24. 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 TI - 1011

25. What Does Coding Get You? (cont) TI - 1011