Spread Spectrum

9/2/15

SPREAD SPECTRUM

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

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

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

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

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

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

CELLULAR SYSTEMS Frequency Planning

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

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

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 use disjoint frequencies f3 f3 f1 f4 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

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