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Spread Spectrum 9/2/15 SPREAD SPECTRUM 37 Spread Spectrum q Frequency-dependent fading bad for narrowband signals Ø Narrowband interference can wipe out signals q “Spread” the narrowband signal into a broadband signal Ø Receiver “de-spreads” signal (“spreads” narrowband interference) dP/df dP/df dP/df dP/df (iii) (iv) (i) (ii) f f f f sender receiver dP/df user signal broadband interference (v) narrowband interference f 38 © Jasleen Kaur 2015 1 9/2/15 Spread Spectrum: Multiple Channels q Resistance to narrowband interference q Coexistence of multiple signals without coordination Ø No need for frequency planning Ø Resistance to frequency-selective fading Ø Tap-proof (with secret code and CDM) Ø Characteristics like background noise channel channel quality quality 2 2 2 2 2 1 5 6 2 3 1 4 frequency narrow band guard space spread frequency signal spectrum narrowband channels spread spectrum channels 39 Direct Sequence Spread Spectrum q XOR of the signal with “chipping sequence” Ø Chipping sequence is a pseudo-random number q Many chips per bit è higher signal bandwidth Ø By the factor, s = tb / tc tb Ø Civil applications, s of 10 – 100 user data Ø Military applications, up to 10,000 0 1 XOR tc q IEEE 802.11 uses Barker codes chipping sequence Ø Good robustness against 0 1 1 0 1 0 1 0 1 1 0 1 0 1 interference = resulting Ø Insensitivity to multi-path signal propagation 0 1 1 0 1 0 1 1 0 0 1 0 1 0 tb: bit period tc: chip period 40 © Jasleen Kaur 2015 2 9/2/15 Direct Sequence Spread Spectrum q Receiver has to perform correlation: Ø Synchronize to identify bit boundaries Ø Do XOR with the chipping sequence q Complication: multi-path propagation Ø Different delays, distortion spread spectrum transmit user data signal signal q Rake receiver: X modulator Ø Uses n correlator for the n chipping radio transmier strongest paths sequence carrier Ø Output of correlators are combined and fed into correlator decision unit lowpass sampled received filtered products sums signal signal data demodulator X integrator decision radio chipping carrier sequence receiver 41 DSSS q Advantages Ø Reduces frequency selective fading Ø In cellular networks § Base stations can use the same frequency range – Several base stations can detect and recover the signal – Soft handover q Challenges Ø Precise power control necessary Ø Precise synchronization necessary 42 © Jasleen Kaur 2015 3 9/2/15 Frequency Hopping Spread Spectrum q Discrete changes of carrier frequency Ø Sequence of frequency changes is pseudo-random Ø Slow Hopping: several user bits per frequency Ø Fast Hopping: several frequencies per user bit tb user data 0 1 0 1 1 t tb: bit period f t : dwell time td d f3 slow f2 hopping (3 bits/hop) f1 t f td f3 fast f2 hopping (3 hops/bit) f1 t 43 FHSS Implementation narrowband spread signal transmit user data signal modulator modulator frequency hopping synthesizer sequence transmier narrowband received signal signal data demodulator demodulator hopping frequency sequence synthesizer receiver 44 © Jasleen Kaur 2015 4 9/2/15 DSSS vs. FHSS q FHSS: Ø Simpler to implement Ø Uses only small portion of the spectrum at any time Ø Used by Bluetooth, GSM q DSSS: Ø Always uses the total bandwidth available Ø More resistant to fading and multi-path effects Ø Signals are much harder to detect § Virtually impossible without knowing the spreading code Ø Used by IEEE 802.11a 45 CELLULAR SYSTEMS Frequency Planning 46 © Jasleen Kaur 2015 5 9/2/15 Cell Structure q Cellular systems implement space division multiplexing Ø Each transmitter (base station) covers a certain area (cell) Ø Mobile stations communicate only via the base station q Cell characteristics: Ø Cell radii can vary § Tens of meters (buildings) § Hundreds of meters (cities) § (say) tens of kilometers in country-side (GSM) Ø Cells are never perfect circles or hexagons, depend on: § Environment (buildings, mountains, valleys), § Weather conditions, § And even system load 47 Small Cells vs. Huge Cells q Advantages of small cells: Ø Higher capacity: SDM allows re-use of (scarce) frequency § Allows higher users per km2, very small cells used in cities Ø Less transmission power needed § Energy is a serious problem for mobile handheld devices Ø Local interference only § Base station deals with interference for only local stations Ø More robust to failure of single components § If one antenna fails, it only influences local communication q Disadvantages: Ø Huge infrastructure needed for connecting base stations Ø Handover (changing from one cell to another) needed Ø Careful frequency planning needed 48 © Jasleen Kaur 2015 6 9/2/15 Frequency Planning q Goal: never use same frequency at same time within the interference range Ø Frequency re-used only with certain distant base stations q Standard models: f3 f3 f3 f2 f3 f7 f f f f Ø All cells within a cluster 2 2 5 2 f1 f1 f1 f4 f6 f5 f3 f3 f1 f4 use disjoint frequencies f2 f2 f2 f3 f7 f1 f f Ø Limited transmission f1 1 f2 3 f3 f3 f3 f6 f5 f2 power used 3 cell cluster 7 cell cluster q Dynamic frequency assignment Ø Base station chooses frequencies § Depending on frequencies already used in neighbor cells § Based on interference measurements Ø More capacity in cells with more traffic 49 Cell Breathing q CDM systems do not need elaborate frequency planning or channel allocation q However, cell size depends on current load Ø Cells are said to “breathe” Ø Why? § Additional traffic appears as noise to other users § If noise level is too high, farther away users drop out of cells 50 © Jasleen Kaur 2015 7 .
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