Definition of spread spectrum: SS is a transmission technique in which a pseudo-noise code, independent of the information data, is used as a modulation waveform to “distribute” the energy of the signal in a bandwidth much greater than the bandwidth of the signal information. At the receiver, the signal is “broadcast” using a synchronized replica of the pseudo-noise code.
-A technical transmission
-PN code independent of data
-Dividing the energy on the transmitter side (by means of a propagation signal, often called a code signal, which is independent of the data)
-Distribute the energy on the receiver side (correlating the received propagation signal with the synchronized replica of the propagation signal)
• Input fed to channel encoder
Produces a narrow bandwidth analog signal around the center frequency
• Signal modulated by sequence of digits
-Diffusion code / sequence
-Typically generated by pseudo-sound / pseudo-random number generator
• Increase bandwidth significantly
-Spreads spectrum
• The receiver uses the same sequence to demodulate the signal
• Demodulated signal fed to channel decoder
=> Benefits of interference suppression: immunity to various noise and multipath distortions
-Including jamming
=> Can hide / encrypt signals
-Only the receiver that knows the extension code can retrieve the signal
=> Multiple access: multiple users can share the same higher bandwidth with little interference Ex.
-Cellphones
-Code Division Multiplexing (CDM)
-Code Division Multiple Access (CDMA)
=> Energy density reduction: Energy density reduction may be required to comply with national allocation regulations. Since by distributing the downlink power over a wider bandwidth, the total transmitted power can be increased and thus improved performance.
=> Fine time resolution: SS signals can be used to measure or determine position location. This can be done by measuring the delay time of the pulse as it passes through the channel.
Propagation techniques: for signals of bandwidth W and duration T, the dimensionality of the signaling space is approximately 2WT. To increase it, we can increase W (by spectrum spread) or T (by time extension, called time jump).
In spectrum spread, the signal is spread out in the frequency domain.
Over time propagation —- the signal propagates in time
Types of techniques:
(i) Direct sequence modulation (DS) (ii) Frequency hopping modulation (FH)
(iii) Time Skip Modulation (TH) (iv) Hybrid (SS) / FH, DS,
• DSSS:
-A pnt pheno-noise sequence generated in the modulator, is used in conjunction with an M matrix PSK modulation to change the phase of the PSK signal pseudo-randomly.
• FHSS
-A pseudo-noise sequence pn generated in the modulator is used in conjunction with an M matrix FSK modulator to change the carrier frequency of the FSK signal pseudo-randomly.
Direct Sequence Spread Spectrum (DSSS): => Each bit represented by multiple bits using spreading code
=> The spreading code spreads the signal across a wider frequency band
-In proportion to the number of bits used
-The 10-bit spreading code distributes the signal in 10 times the bandwidth of a 1-bit code
=. Performance similar to FHSS
In the modulator:
=> Spread:
=> Multiply x
On the demodulator:
=> Detaching: The received baseband signal is multiplied with the synchronized replica of the spreading code signal g
• A filter with BW R is used to remove any spurious high frequency components.
• Any unwanted signal at the receiver is propagated by g
The spread spectrum approach called transmitted reference (TR) can use a truly random code signal for spreading and de-spreading. These codes have properties of randomness.
Essence behind SS interference rejection ability:
=> Multiplication by the propagation signal once it extends the signal bandwidth.
=> Multiplication by the propagation signal twice, followed by filtering, recovers the original signal.
=> The desired signal is multiplied twice, but the interfering signal is multiplied only once.
DS / SS Performance and Gain Processing:
A fundamental problem in the SS system is how much protection the dispersion can provide against interfering signals with finite power.
Since the approximate dimensionality is 2WT, we can express the processing gain as: Gp = N / D
Where Wss – spread spectrum bandwidth, Wmin- min. data bandwidth, (taken at data rate R)
Wss is roughly the Rch code chip rate. So, Gp = Rch / R
The chip is the shortest uninterrupted waveform in the system.
Frequency Hopping Spread Spectrum (FHSS):
• The carrier frequency is determined pseudo-randomly.
• FH allows gigahertz bandwidth, thus allowing high throughput gain compared to DS.
STURDINESS:
=> The greater the diversity (multiple transmissions, at different frequencies, distributed over time), the more robust the signal will be against random interference.
Assume four symbols: ABCD. Using diversity N = 8, then
AAAAAAAABBBBBBBBCCCCCCCCDDDDDDDD
Then the repeating symbols, called chips.
=> Each chip is transmitted at a different hopping frequency.
=> Analogy with diversity: An analogy with target shooting is that a pellet from a burst of shotgun pellets has a greater chance of hitting the target, compared to the action of a single bullet.
FAST JUMP VERSUS SLOW JUMP:
=> SFH: several modulation symbols per hop
=> FFH: Several frequency hops per modulation symbol.
=> For SFH, the chip is the shortest uninterrupted waveform in the system is that of the data symbol
=> For FFH the chip is the shortest uninterrupted waveform which is the jump waveform.
