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Asstt. Professor Adeel Akram What is signal encoding? In communications systems, the altering of the characteristics of a signal to make the signal more suitable for an intended application, such as optimizing the signal for transmission Modifying the signal spectrum, increasing the information content, providing error detection and/or correction, and providing data security A single coding scheme usually does not provide more than one or two specific capabilities. Different codes have different sets of advantages and disadvantages. Reasons for Choosing Encoding Techniques Digital data, digital signal Equipment less complex and less expensive than digitalto-analog modulation equipment Analog data, digital signal Permits use of modern digital transmission and switching equipment Reasons for Choosing Encoding Techniques Digital data, analog signal Some transmission media will only propagate analog signals E.g., Fax/Modem Analog data, analog signal Analog data in electrical form can be transmitted easily and cheaply Done with voice transmission over voice-grade lines Signal Encoding Criteria What determines how successful a receiver will be in interpreting an incoming signal? Signal-to-noise ratio Data rate Bandwidth An increase in data rate increases bit error rate An increase in SNR decreases bit error rate An increase in bandwidth allows an increase in data rate Comparing Encoding Schemes Signal interference and noise immunity Performance in the presence of noise Cost and complexity The higher the signal rate to achieve a given data rate, the greater the cost Digital Data to Analog Signals Keying is a form of modulation where the modulating signal takes one of two or more values at all times. For example: "on" or "off“ The name derives from the Morse code key used for telegraph signaling Amplitude-shift keying (ASK) Amplitude difference of carrier frequency Frequency-shift keying (FSK) Frequency difference near carrier frequency Phase-shift keying (PSK) Phase of carrier signal shifted Amplitude-Shift Keying One binary digit represented by presence of carrier, at constant amplitude Other binary digit represented by absence of carrier binary 1 A cos2f ct s t 0 where the carrier signal is Acos(2πfct) binary 0 Amplitude-Shift Keying Susceptible to sudden gain changes Inefficient modulation technique On voice-grade lines, used up to 1200 bps Used to transmit digital data over optical fiber Binary Frequency-Shift Keying (BFSK) Two binary digits represented by two different frequencies near the carrier frequency s t A cos2f1t A cos2f 2t binary 1 binary 0 where f1 and f2 are offset from carrier frequency fc by equal but opposite amounts Binary Frequency-Shift Keying (BFSK) Less susceptible to error than ASK On voice-grade lines, used up to 1200bps Used for high-frequency (3 to 30 MHz) radio transmission Can be used at higher frequencies on LANs that use coaxial cable Phase-Shift Keying (PSK) Two-level PSK (BPSK) Uses two phases to represent binary digits binary 1 A cos2f ct s t binary 0 A cos 2 f t c A cos2f ct A cos2f ct binary 1 binary 0 Phase-Shift Keying (PSK) Differential PSK (DPSK) Phase shift with reference to previous bit Binary 0 – signal burst of same phase as previous signal burst Binary 1 – signal burst of opposite phase to previous signal burst Phase-Shift Keying (PSK) Four-level PSK (QPSK) Each element represents more than one bit s t A cos 2f c t 4 3 A cos 2f c t 4 3 A cos 2f c t 4 A cos 2f c t 4 11 01 00 10 Quadrature Amplitude Modulation QAM is a combination of ASK and PSK Two different signals sent simultaneously on the same carrier frequency st d1 t cos 2f ct d 2 t sin 2f ct Quadrature Amplitude Modulation Analog Data to Analog Signal Modulation of digital signals When only analog transmission facilities are available, digital to analog conversion required Modulation of analog signals A higher frequency may be needed for effective transmission Modulation Techniques Amplitude modulation (AM) Angle modulation Frequency modulation (FM) Phase modulation (PM) Outline Cellular Concept Cellular Architecture Frequency Reuse Multiple Access Methods FDMA, TDMA, and CDMA In particular, we focus on CDMA. Different Generations 1G analog 2G digital 3G higher data rate for multimedia applications 1G Cellular Systems Many Different Standards: AMPS (US) NMT (Northern Europe) TACS (Europe) NTT (Japan) many others... Spectrum around 800 and 900 MHz Frequency Division Duplex (FDD) Forward Link mobile Reverse Link base station Two separate frequency bands are used for forward and reverse links. Typically, 25 MHz in each direction. AMPS: 824-849 MHz (forward) 869-894 MHz (reverse) Frequency Division Multiple Access (FDMA) The spectrum of each link (forward or reverse) is further divided into frequency bands frequency bands Each station assigned fixed frequency band idle idle idle Number of User Channels in AMPS Bandwidth allocated to each user in each link (forward or reverse) is 30 KHz. No. of user channels = Total bandwidth / user bandwidth = 25 MHz / 30 kHz = 833 Is it enough? Frequency Reuse Radio coverage, called a cell. f f The same frequency can be reused in different cells, if they are far away from each other Cellular Architecture MS MS – Mobile Station BSC – Base Station Controller MSC or MTSO– Mobile Switching Center PSTN – Public Switched Telephone Network BSC MSC PSTN segmentation of the area into cells Geometric Representation Cells are commonly represented by hexagons. Why hexagon? How about circle? How about square, or triangle? Hexagon vs Circles Notice how the circles below would leave gaps in our layout. Still, why hexagons and not triangles or rhomboids? Hexagonal Cells Cell site and Cell The cell site is a location or a point, the cell is a wide geographical area Cells site covers a portion or a sector of each cell, not the whole thing. Antennas from other cell sites cover the other portions. The covered area, if you look closely, resembles a sort of rhomboid In reality, the cell is the red hexagon Channel Reuse The total number of channels are divided into N groups. N is called reuse factor. Each cell is assigned one of the groups. The same group can be reused by two different cells provided that they are sufficiently far apart. Example: N=7 Reuse Distance How far apart can two users share the same channel? It depends on whether signal quality is acceptable or not. The larger the distance between the two users, the better the signal quality. How to measure signal quality? Nyquist Bandwidth Given a bandwidth of B, the highest signal transmission rate for binary signals (two voltage levels) is: C = 2B Ex: B=3100 Hz; C=6200 bps With multilevel signaling C = 2B log2 M M = number of discrete signal or voltage levels Signal Quality The signal quality depends on the ratio between signal power and interference (noise) power. S S I Ii i Interference from the i-th interfering BS. This is called signal-to- noise (interference) ratio (SNR or SIR). Signal-to-Noise Ratio Ratio of the power in a signal to the power contained in the noise that’s present at a particular point in the transmission Typically measured at a receiver Signal-to-noise ratio (SNR, or S/N) signal power ( SNR) dB 10 log 10 noise power A high SNR means a high-quality signal, low number of required intermediate repeaters SNR sets upper bound on achievable data rate Shannon Capacity Formula Equation: C B log 2 1 SNR Represents theoretical maximum that can be achieved In practice, only much lower rates achieved Formula assumes white noise (thermal noise) Impulse noise is not accounted for Attenuation distortion or delay distortion not accounted for Classifications of Transmission Media Transmission Medium Physical path between transmitter and receiver Guided Media Waves are guided along a solid medium E.g., copper twisted pair, copper coaxial cable, optical fiber Unguided Media Provides means of transmission but does not guide electromagnetic signals Usually referred to as wireless transmission E.g., atmosphere, outer space Propagation Model The received signal power depends on the distance between the transmitter and the receiver d Pr P0 d0 P0 is the power received at a reference distance d0 is called the path loss exponent Typically, 2 ≤ ≤ 6 * Typical values of α Table : Path Loss Exponents for Different Environments Propagation Environment Path Loss Exponent Free Space 2 Urban Area 2.7 to 3.5 Shadowed Urban Area 3 to 5 In-Building Line-of-Sight 1.6 to 1.8 Obstructed In Building 4 to 6 Obstructed In Factory 2 to 3 As shown in Table typical values for the path loss exponent are between 2 to 6 2G Cellular Systems Four Major Standards: GSM (European) IS-54 (later becomes IS-136, US) JDC (Japanese Digital Cellular) IS-95 (CDMA, US) Example: GSM Frequency Band 935-960, 890-915 MHz Two pieces of 25 MHz band (same as AMPS) AMPS has 833 user channels How about GSM? Time Division Multiple Access (TDMA) The mobile users access the channel in roundrobin fashion. Each station gets one slot in each round. Slots 2, 5 and 6 are idle FDMA/TDMA, example GSM f 960 MHz 124 200 kHz 1 935.2 MHz 20 MHz 915 MHz 124 1 890.2 MHz t 1 2 3 7 8 Each freq. carrier is divided into 8 time slots. Number of channels in GSM Freq. Carrier: 200 kHz TDMA: 8 time slots per freq carrier No. of carriers = 25 MHz / 200 kHz = 125 No. of user channels = 125 * 8 = 1000 Capacity Comparison Reuse factor 7 for AMPS 3 for GSM (why smaller reuse factor?) What’s the capacity of GSM relative to AMPS? A. one half of AMPS B. the same C. 3 times larger D. 10 times larger Answer AMPS reuse factor = 7 no. of users / cell = 833 / 7 = 119 GSM reuse factor = 3 no. of users / cell = 1000 / 3 = 333 almost 3 times larger than AMPS! Multiple Access Methods Three major types: Frequency Division Multiple Access (FDMA) Time Division Multiple Access (TDMA) Code Division Multiple Access (CDMA) Frequency hopping (FH-CDMA) Direct sequence (DS-CDMA) Frequency-Time Plane Frequency Partition of signal space into time slots and frequency bands Time FDMA Frequency Different users transmit at different frequency bands simultaneously Time TDMA Frequency Different users transmit at different time slots Each user occupy the whole freq. spectrum Time Frequency Hopping CDMA Frequency At each successive time slot, the frequency band assignments are reordered Time Each user employs a code that dictates the frequency hopping pattern Assignment Write note on 3G Mobile technology Write note on 3.5G Mobile technology Write note on 3.75G Mobile technology Write note on 4G Mobile technology Give an Overview of GSM network Architecture Difference between CDMAOne and CDMA2000 Questions ???????????????? ???????????????? ??