TI Cellular Mobile Communication Systems Lecture 4 Engr. Shahryar Saleem Assistant Professor Department of Telecom Engineering University of Engineering and Technology Taxila 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 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 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 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 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 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), : cos(2πfct + 3π/4), : cos(2πfct + 5π/4), : cos(2πfct + 7π/4), 315 Why do we choose this mapping? cos(A+B) = cos(A)cos(B) – sin(A)sin(B)

TI 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 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 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 Performance Degradation and Diversity

TI 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 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 Data packet and messaging systems requires error rates up to 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 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 Single Parity (cont)

TI 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 Block Codes (cont)

TI 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 (7,4) Hamming Code

TI 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 FEC (cont) Forward Error Correction Process

TI 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 Convolution Encoder

TI 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 What Does Coding Get You? (cont)