It is a short a presentation on chirp sread spectrum communication

1. Chirp Spread
Spectrum
Communication
Presented By-
Mahmudul Islam (1506155)
Mohammad Abdul Mobin (1506006)

2. Spectrum
 “spectrum” is a general term to encompass the spatial and temporal
properties of any medium , including fiber optic cable , coaxial cable , and
ambient air.

3. Spread Spectrum
• Spread-spectrum techniques are methods by which a signal
(e.g an electrical , electromagnetic, or acoustic signal generated in
a particular bandwidth is deliberately spread in the frequency
domain , resulting in a signal with a wider bandwidth.)
• These techniques are used for varieties of reasons , including
a . The establishment of secure communications
b. increasing resistance to natural interference, noise
c. jamming , to prevent detection , and
d. to limit power flux density (e.g. in satellite downlinks)

4. Chirp
• A short, high-pitched sound , such as that made by a small bird or an
insect.
• The sound emitted by crickets is commonly referred to as chirping.
• The specificity of bird calls has been used extensively for species
identification.
• The calls of birds have been described using words or nonsense syllables
or line diagrams.
• Common terms in English include words such as quack,chirp

5. Chirp(conc.)
 A chirp is a signal in which frequency increased (‘up-chirp’) or decreases
(‘down-chirp’) with time .
 In some sources , the term chirp is used interchangeably with sweep
signal .
 It is commonly used in sonar and radar , but has other applications , such
as in spread spectrum communications.
 In spread spectrum usage , SAW devices such as RACs are often used to
generate and demodulate the chirped signals .
 In optics , ultrashort laser pulses also exhibit chirp , which , in optical
transmission systems interacts with the dispersion properties of the
materials , increasing or decreasing total pulse dispersion as the signal
propagates .

6. A chirp signal waveform can be written as
S(t) = a(t) cos[θ(t)]
Where θ(t) is the phase and a(t) is the envelope of the chirp signal which is zero outside a
time interval of length T.
The instantaneous frequency is defined as
fM (t) =
1
2𝜋
𝑑θ
𝑑𝑡
The chirp rate is defined by
µ(t) =
𝑑fM
𝑑𝑡
=
1
2𝜋
𝑑2θ
𝑑𝑡2
and represents the rate of change of the instantaneous frequency.
Chirp(conc.)

7.  When µ (t) > 0 then the signal is called up-chirps
 When µ (t) < 0 then the signal is called down-chirps
 For a linear chirp µ (t) is constant, and hence fM (t) is a linear
function of t, and θ(t) is a quadratic function those with.
If we take the waveform to be centered at t = 0 it can be written as
s(t) = a(t) cos [2𝜋f0t + 𝜋µ𝑡2
+ φ0 ]
where fc is the center frequency and a(t) = 0 for |t| > T/2
It is convenient to define the bandwidth B as the range of the instantaneous
frequency, so that
B =| µ |T
Chirp(conc.)

8.  The impulse response of a matched filter for a linear chirp signal is again a linear chirp
signal but with a chirp rate of opposite sign.
 If a chirp waveform is fed into its matched filter the output signal typically has a
narrow IF peak at the chirp center frequency.
 If we consider chirp waveforms with at time domain envelopes and take the matched
filter to be centered at t = 0 we find an analytical expression for the output waveform
g(t) of the matched filter.
Chirp(conc.)

9. g(t) = h(t)*s(t)= φss (t)
Where φss (t) is the autocorrelation function of s(t). It can be shown that φss (t) is given by that
for T < t < T . The envelope has its maximum at t = 0, and its first zeros at t≈±1/B. It is therefore
convenient
to specify the pulse width as 1/B. The ratio of the input and output pulse widths is therefore given by
the time-bandwidth product TB which is known as compression ratio or processing gain.
Another important parameter is the sidelobe rejection, which is about 13 dB for a chirp signal
where a(t) has rectangular shape. A common method of reducing the sidelobes is to apply amplitude
weighting of the chirp signals

10. Characteristics of Chirp pulses
 A chirp pulse is a frequency modulated pulse .
- Its duration is T ; within this time the frequency is changing in a
monotonic manner from a lower value to a higher one .
-The difference between these two frequencies is a good
approximation for the bandwidth B of the chirp pulse

11. The basic chirp signal

12. Chirp Modulation
 Chirp modulation was patented by Sidney Darlington in 1954 with
significant later work performed by Winkler in 1962.
 This type of modulation employs sinusoidal waveforms whose
instantaneous frequency increases or decreases linearly over time . These
waveforms are commonly referred to as linear chirps or simply chirps .
 Hence the rate at which their frequency changes is called the “chirp rate”.
 In birnary chirp modulation , binary data is transmitted by mapping the
bits into chrips of opposite chirp rates. For instance, over one bit period “1”
is assigned a chrip with positive rate a and “0” a chirp with negative rate –
a .

14. Chirp Spread Spectrum (CSS) system
overview:
 Simplicity
-Basically a 2-ary transmission system
-The ‘windowed chirp’ is a linear frequency sweep with a total
duration of 1us.

17. Key Properties of Chirp Spread
Spectrum (CSS)
• High robustness:
Due to the high BT product ,chirp pulses are very resistant
against disturbances
• Multipath resistant:
Due to the broadband chirp pulse , CSS is very immune against
multipath fading; CSS can ever take advantage of RF echoes.
• Low power consumption:
CSS allows the designer to choose an analog implementation ,
which often consumes much less power.
• Low latency :
CSS needs no synchronization ; a wireless connection can be
established very quickly.

18. Mobility Properties of CSS
 Resistance against Doppler effect :
The Doppler effect causes a frequency shift of the chirp pulse ,
Which introduces a negligible shift of the baseband signal on the time axis.

19. Coexistence Properties of CSS
 Immune to in-band interferer:
Scalable processing gain (determined by BT product of the chirp)
enables selection fo appropriate immunity level against in-band interferences
.