CS 414 Indian Institute of Technology, Bombay 1 CS 414 Signals & Transmission Wireless Propagation Basics

CS 414 Indian Institute of Technology, Bombay 2 Over the next ~3 classes ● Signals and Transmission ● Multiplexing and Modulation ● Wireless propagation basics – Antennas, RF propagation, radiation patterns, interference

CS 414 Indian Institute of Technology, Bombay 3 Data ● Analog Data – Continuously generated data – Examples: ● Digital Data – Data present/generated at discrete instances – Examples:

CS 414 Indian Institute of Technology, Bombay 4 Data as Signals ● Analog Signals – Continuously time-varying signal intensity ● Digital Signals – Signal intensity varies periodically

CS 414 Indian Institute of Technology, Bombay 5 Analog Signaling ● Analog Data via Analog Signals – Audio (20 Hz to 20 Khz) ● Practical: 300 to 3400 Hz – Telephone ● Translate speech to frequency and amplitude pattern ● Digital data via Analog Signals – Modems ● Modulate carrier frequency according to data ● Demodulate at receiver

CS 414 Indian Institute of Technology, Bombay 6 Digital Signaling ● Analog Data via Digital Signals – Coder encodes analog signal ● Period sampling – Decoder decodes ● Digital Data via Digital Signals – Signal levels to represent data bits

CS 414 Indian Institute of Technology, Bombay 7 Sine Wave ● Fundamental analog signal ● s(t) = A sin(2πft+ Ø) – A : amplitude, f: frequency, Ø :phase shift ● Any waveform can be appromixated / constructed using a set of sinusoids – Different amplitudes and frequencies – Example (board)

CS 414 Indian Institute of Technology, Bombay 8 Bandwidth & Data Rate ● Bandwidth = Range of frequencies in a signal – Spectrum of signal – Overloaded with data rate – Unit: Hertz ● Data rate is bits per second transmitted

CS 414 Indian Institute of Technology, Bombay 9 Bandwidth vs Data Rate ● Higher the bandwidth => Higher the data rate ● Examples (board) ● Theortically infinite bandwidth (data rate) – Limited by transmission medium – Cost and Hardware complexity – Higher frequencies have little amplitude ● Higher data rates at lower bandwidths more susceptible to distortions and errors

CS 414 Indian Institute of Technology, Bombay Multiplexing ● Wireless is a shared medium – how to share with no conflicts? ● multiplex between multiple users ● Four dimensions – Space (SDM) – Time (TDM) – Frequency (FDM) – Code (CDMA)

CS 414 Indian Institute of Technology, Bombay Multiplexing Example ● Cars on highway – in different lanes – in same lane, but following each other

CS 414 Indian Institute of Technology, Bombay Space Divison Mulitplexing ● FM stations Source: Mobile Communications, Jochen Schiller

CS 414 Indian Institute of Technology, Bombay Time Division Multiplexing ● Clients use all bandwidth at different times – When to use channel? ● Time synchronization Source: Mobile Communications, Jochen Schiller

CS 414 Indian Institute of Technology, Bombay Frequency Division Multiplexing ● Each sender has separate frequency – Overlapping radio stations – Tune to 92.1Mhz! Source: Mobile Communications, Jochen Schiller

CS 414 Indian Institute of Technology, Bombay TDM+FDM ● Different frequencies for different users and different times ● More robust, more complex Source: Mobile Communications, Jochen Schiller

CS 414 Indian Institute of Technology, Bombay CDMA ● Code = Language ● Different users speak in different languages ● Can decipher if you know language ● Tx/Rx pairs use unique codes ● More when studying cellular systems Source: Mobile Communications, Jochen Schiller

CS 414 Indian Institute of Technology, Bombay Modulation/Keying ● to change/adjust/adapt ● to convert a signal based on another data signal/stream for transmission ● Digital modulation – Convert digital data/signal to analog signal ● Analog modulation – Convert analog data to analog signal

CS 414 Indian Institute of Technology, Bombay Modulation ● Digital signal as analog signal – if data rate is B bps (strictly signals/sec) – BW of analog signal multiples of B Hz ● Transmitting a signal at B Hz directly may not be possible – transmission medium effects – antenna properties medium – not suitable for FDM ● Solution: Convert signal (baseband signal) to appropriate frequency signal (carrier signal)

CS 414 Indian Institute of Technology, Bombay Analog Modulation Transmitter Receiver Source: Mobile Communications, Jochen Schiller

CS 414 Indian Institute of Technology, Bombay Digital Modulation Transmitter Receiver Source: Mobile Communications, Jochen Schiller

CS 414 Indian Institute of Technology, Bombay RF Signal ● Three components – amplitude, frequency, phase ● Modulation changes any one of these or combinations of these

CS 414 Indian Institute of Technology, Bombay Terminology ● Signals – Analog elements ● Bits – Data/discrete elements ● Correspondences – one-to-one – one-to-many – many-to-one

CS 414 Indian Institute of Technology, Bombay Amplitude Shift Keying (ASK) ● Amplitude levels represent bits streams ● Low bandwidth ● Very susceptible to noise and path loss Source: Mobile Communications, Jochen Schiller

CS 414 Indian Institute of Technology, Bombay Frequency Shift Keying (FSK) ● Separate frequencies to encode 0s and 1s ● BFSK (Binary FSK) ● Higher BW then ASK Source: Mobile Communications, Jochen Schiller

CS 414 Indian Institute of Technology, Bombay Phase Shift Keying ● Change in phase encodes bits ● BPSK (Binary phase shift keying) – phase shift of 180 ∘ encodes 0s and 1s Source: Mobile Communications, Jochen Schiller

CS 414 Indian Institute of Technology, Bombay QPSK ● Quadrature Phase Keying ● Four phase shifts – 0 ∘ = 00 – 90 ∘ = 01 – 180 ∘ = 10 – 270 ∘ = 11 ● A signal element represents 2-bits of data => more data rate for same bandwidth => complex implementation ● Can we do better? – QAM

CS 414 Indian Institute of Technology, Bombay Analog Modulation ● Amplitude, frequency, phase ● Continuous/Instanteneous modulations Source: Wikipedia

CS 414 Indian Institute of Technology, Bombay Multi-Carrier Modulation ● OFDM – Orthogonal Frequency Division Multiplexing – n symbols/secs – transmitted over c sub-carriers (frequencies) – n/c symbols/s over each carrier – more robust to noise ● Several other variations – MSK, GSK, CCK...

CS 414 Indian Institute of Technology, Bombay Spread Spectrum Techniques ● Spread signal over wider frequency range – Transmitted signal takes more bandwidth than information signal – Tackle narrow-band interference better – Power level much lower than narrowband signal Source: Mobile Communications, Jochen Schiller

CS 414 Indian Institute of Technology, Bombay Spread Spectrum Concept Modulator/ Encoder Modulator PN Sequence generator PN Sequence generator Demodulator De-modulator/ Decoder Input data Analog signal Spread sequence De-Spread sequence Channel Output data Analog signal ● Two types – DSSS, Direct Sequence Spread Spectrum – FHSS, Frequency Hopping Spread Spectrum

CS 414 Indian Institute of Technology, Bombay Direct Sequence Spread Spectrum ● DSSS – Information signal spread based on a “random” sequence ● PN sequence, Chipping sequence, code – Example (on board) – Receiver more complex than transmitter

CS 414 Indian Institute of Technology, Bombay Frequency Hopping Spread Spectrum ● FHSS – Total bandwidth (frequency spectrum) split into many smaller bandwidth channels – PN sequence of k bits acts as index for 2 k frequencies/channels – Random sequence => Frequency hopping – FDM+TDM (?)

CS 414 Indian Institute of Technology, Bombay 33 Next Class ● Antenna basics – beam width, types, gain, path loss ● Reading “The Mistaken axioms of wireless-network research”, D. Kotz, C. Newport, C. Elliott “ Most research on ad-hoc wireless networks makes simplifyingassumptions about radio propagation. The “Flat Earth” model of the world is surprisingly popular: all radioshave circular range, have perfect coverage in thatrange, and travel on a two-dimensional plane.... We then present a set of measurements that clearly demonstrate that these “axioms” are contrary to fact.”