CS 414 Indian Institute of Technology, Bombay CS 414 RF Propagation Multiplexing and Modulation

CS 414 Indian Institute of Technology, Bombay Announcements ● No class on Friday 8/8/8 – 46 th convocation ● No class on Friday 15/8/08 – Independence day ● Extra class announcement soon ● New TA – Ashish Dhar cse)

CS 414 Indian Institute of Technology, Bombay Path Loss Models ● Free Space Path Loss – Ideal, no reflection, scattering, difraction etc ● Two-Ray (Ground Reflection) Model d T R htht hrhr

CS 414 Indian Institute of Technology, Bombay Path Loss Models ● Knife-edge Diffraction Model ● Multiple Knife-edge Diffraction Model – Account for diffraction effects ● Radar Cross Section Model – Account for scattering ● Indoor propagation models – smaller distances, more unpredicatibility, building material...

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 BER (Bit error rate) ● For any modulation BER is quality metric – Depends on SNR (Signal to noise ratio) – Robustness of different modulation techniques is different for varying SNRs ● In general, – increase in data rate increases BER – increase in SNR decreases BER – increase in bandwidth allows increase in data rate