SPREAD SPECTRUM 37 Spread Spectrum Frequency-dependent ading bad or narrowband s Ø Narrowband intererence can wipe out s Spread the narrowband into a broadband Ø Receiver de-spreads ( spreads narrowband intererence) dp/d dp/d dp/d dp/d (i) (ii) (iii) (iv) sender user broadband intererence narrowband intererence receiver dp/d (v) 38 Jasleen Kaur 015 1
Spread Spectrum: Multiple Channels Resistance to narrowband intererence Coexistence o multiple s without coordination Ø No need or requency planning Ø Resistance to requency-selective ading Ø Tap-proo (with secret code and CDM) Ø Characteristics like background noise channel quality 1 narrow band 3 4 guard space 5 6 requency narrowband channels channel quality 1 spread spectrum requency spread spectrum channels 39 Direct Sequence Spread Spectrum XOR o the with chipping Ø Chipping is a pseudo-random number Many chips per bit è higher bandwidth Ø By the actor, s = t b / t c Ø Civil applications, s o0 100 Ø Military applications, up to 10,000 IEEE 80.11 uses Barker codes Ø Good robustness against intererence Ø Insensitivity to multi-path propagation t b 0 1 t c 0 1 1 0 1 0 1 0 1 1 0 1 0 1 0 1 1 0 1 0 1 1 0 0 1 0 1 0 XOR chipping = resulting t b : bit period t c : chip period 40 Jasleen Kaur 015
Direct Sequence Spread Spectrum Receiver has to perorm correlation: Ø Synchronize to identiy bit boundaries Ø Do XOR with the chipping Complication: multi-path propagation Ø Dierent delays, distortion Rake receiver: Ø Uses n correlator or the n strongest paths Ø Output o correlators are combined and ed into decision unit received demodulator X chipping lowpass iltered X spread spectrum correlator products radio carrier integrator modulator sampled sums transmit transmi4er decision data radio carrier chipping receiver 41 DSSS Advantages Ø Reduces requency selective ading Ø In cellular networks Base stations can use the same requency range Several base stations can detect and recover the Sot handover Challenges Ø Precise power control necessary Ø Precise synchronization necessary 4 Jasleen Kaur 015 3
Frequency Hopping Spread Spectrum Discrete changes o carrier requency Ø Sequence o requency changes is pseudo-random Ø Slow Hopping: several user bits per requency Ø Fast Hopping: several requencies per user bit t b 0 1 t d 0 1 1 t t b : bit period t d : dwell time slow (3 bits/hop) t d t ast (3 hops/bit) t 43 FHSS Implementation modulator narrowband modulator spread transmit transmi4er requency synthesizer received demodulator narrowband demodulator data requency synthesizer receiver 44 Jasleen Kaur 015 4
DSSS vs. FHSS FHSS: Ø Simpler to implement Ø Uses only small portion o the spectrum at any time Ø Used by Bluetooth, GSM DSSS: Ø Always uses the total bandwidth available Ø More resistant to ading and multi-path eects Ø Signals are much harder to detect Virtually impossible without knowing the spreading code Ø Used by IEEE 80.11a 45 CELLULAR SYSTEMS Frequency Planning 46 Jasleen Kaur 015 5
Cell Structure 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 Cell characteristics: Ø Cell radii can vary Tens o meters (buildings) Hundreds o meters (cities) (say) tens o kilometers in country-side (GSM) Ø Cells are never perect circles or hexagons, depend on: Environment (buildings, mountains, valleys), Weather conditions, And even system load 47 Small Cells vs. Huge Cells Advantages o small cells: Ø Higher capacity: SDM allows re-use o (scarce) requency Allows higher users per km, very small cells used in cities Ø Less transmission power needed Energy is a serious problem or mobile handheld devices Ø Local intererence only Base station deals with intererence or only local stations Ø More robust to ailure o single components I one antenna ails, it only inluences local communication Disadvantages: Ø Huge inrastructure needed or connecting base stations Ø Handover (changing rom one cell to another) needed Ø Careul requency planning needed 48 Jasleen Kaur 015 6
Frequency Planning Goal: never use same requency at same time within the intererence range Ø Frequency re-used only with certain distant base stations Standard models: Ø All cells within a cluster use disjoint requencies Ø Limited transmission power used 3 cell cluster 7 5 4 6 5 4 7 6 5 7 cell cluster Dynamic requency assignment Ø Base station chooses requencies Depending on requencies already used in neighbor cells Based on intererence measurements Ø More capacity in cells with more traic 49 Cell Breathing CDM systems do not need elaborate requency planning or channel allocation However, cell size depends on current load Ø Cells are said to breathe Ø Why? Additional traic appears as noise to other users I noise level is too high, arther away users drop out o cells 50 Jasleen Kaur 015 7