Direct Sequence Spread Spectrum in Mobile Computing

1. DirectSequenceSpread
Spectrum

2. Direct sequence transmission
• Another spread spectrum technique .
• Basic approach is to spread the narrow RF energy over a wider
band in a controlled way.
• Each device does the transmission over a fixed channel.
• Basic high level approach is shown below:
DirectSequence
SpreadSpectrum

3. Direct sequence transmission
✔The Spreader employs an encoding scheme (Barker or
Complementary Code Keying (CCK).
✔The spread signal is then modulated by a carrier.
✔The Correlator reverses this process in order to recover the
original data
DirectSequence
SpreadSpectrum

4. DirectSequence
SpreadSpectrum
 Basic steps of DSSS technique:
Narrowband radio signal is processed by a spreader
1. spreader applies a mathematical transform which convert
narrowband input to a flat amplitude and spread frequency band.
2. On a carrier, to a narrow band receiver it looks like a low level
noise.
3.The original signal can be recovered with a correlator, which
inverse of the spreading process

5. DirectSequence
SpreadSpectrum
 Basic steps of DSSS technique:
• Direct-sequence modulation works by applying a chipping
sequence to the data stream -> original input.
• Chip is a binary data used in spreading process to encode the
original signal.Chipping streams are also called pseudorandom
noise codes (PN codes).
Advantages –
-reduces frequency selective fading
-in cellular networks
• base stations can use the same
frequency range
• several base stations can detect
and recover the signal

6. DSSS operation
• Higher spreading ratios
• improve the ability to recover the transmitted signal
• Require a higher chipping rate and a larger frequency band.
• Require more expensive RF components operating at the higher
frequency
DirectSequence
SpreadSpectrum