Frequency hopping spread spectrum (FHSS) works by rapidly switching a carrier among many frequency channels, using a pseudorandom sequence known to both transmitter and receiver. The transmitter hops from one frequency to another, transmitting short bursts of information on each channel in turn. The receiver hops in synch to receive the signals. This makes the signal resistant to interference and jamming as an eavesdropper would need to know the hop sequence to intercept the entire message coherently.

1. FREQUENCY HOPPING SPREAD
SPECTRUM
By:- Tanu Jaswal

2. • Principle of spread spectrum
• Frequency hopping spread
spectrum
• Direct sequence spread spectrum
• Direct sequence CDMA systems

3. •Spread spectrum techniques are methods by
which electromagnetic energy generated in a
particular bandwidth is deliberately spread in the
frequency domain, resulting in a signal with a wider
bandwidth
•Spread spectrum methods:
Frequency hopping spread spectrum
Direct sequence spread spectrum
Time hopping spread spectrum

4. •At the transmitter side:
─Input is fed into a channel encoder
•Produces analog signal with narrow bandwidth
─ Signal is further modulated using sequence of digits
• Spreading code or spreading sequence
• Generated by pseudonoise, or pseudo-random number generator
• Effect of modulation is to increase bandwidth of signal to be
transmitted

5. At the receiving end:
−digit sequence is used to demodulate the spread spectrum
signal
− Signal is fed into a channel decoder to recover data

7. •Signal is broadcast over seemingly random series
of radio frequencies
A number of channels allocated for the FH signal
Width of each channel corresponds to bandwidth of input
signal
•Signal hops from frequency to frequency at fixed
intervals
Transmitter operates in one channel at a time
Bits are transmitted using some encoding scheme
At each successive interval, a new carrier frequency is
selected

8. •The frequency sequence is dictated by the
spreading code
•Receiver should hop synchronously with the
transmitter to be able to recover the message
•Advantages
Eavesdroppers hear only unintelligible blips
Attempts to jam signal on one frequency succeed only at
knocking out a few bits

11. •Slow-frequency-hop spread spectrum
The hopping duration is larger or equal to the symbol
duration of the modulated signal
Tc >= Ts
•Fast-frequency-hop spread spectrum
The hopping duration is smaller than the symbol duration
of the modulated signal
Tc < Ts

14. •Large number of frequencies used
•Results in a system that is quite resistant to jamming
Jammer must jam all frequencies
With fixed power, this reduces the jamming power in any one
frequency band

15. FHSS PMD takes the binary bits of information from
the whitened PLCP service data unit (PSDU) and
transforms them into RF signals
PSDU Data Whitening
Data whitening is applied to the PSDU before
transmission to minimize bias on the data if long strings of
1’s or 0’s appear in the PSDU
The PHY stuffs a special symbol every 4 octets of the
PSDU in a PPDU frame
A 127-bit sequence generator using the polynomial S(x) =
x7 + x4 + 1 and 32/33 bias-suppression encoding algorithm
are used to randomize and whiten the data

16. Modulation
IEEE 802.11 version released in 1997 uses two-level GFSK in the
FHSS PMD to transmit the PSDU at the basic rate of 1 Mbps
The PLCP preamble and PLCP header are always transmitted at 1
Mbps
Four-level GFSK is an optional modulation method defined in the
standard that enables the whitened PSDU to be transmitted at a
higher rate
GFSK is a modulation technique used by the FHSS PMD shifts the
frequency either side of the carrier hop frequency
Four-level GFSK is similar to two-level GFSK and modulator
combines two binary bits into symbol pairs (10, 11, 01, 00)

17. Channel Hopping
For use in the 2.4 GHz frequency band
The channels are evenly spaced across the band over a span 83.5
MHz
Hop channels differs from country to country
Channel hopping is controlled by the FHSS PMD
IR
IR PHY is one of the three PHY layers supported
Differs from DSSS and FHSS because IR uses near-visible light as the
transmission media
IR communication relies on the light energy, which is by line-of-sight
or reflected off objects
Operation is restricted to indoor environments and cannot pass
through walls

18. Modulation
Transmits binary data at 1 and 2 Mbps
Uses PPM modulation to reduce the optical power
required
Specific data rate is dependent upon the type of PPM
Modulation for 1 Mbps operation is 16-PPM, while it is
4-PPM for 2 Mbps
PPM is a modulation technique that keeps the amplitude,
pulse width constant, and varies the position of the pulse
in time

19. 4-PPM symbol map for 2 Mbps
Data bits 4-PPM symbol
00 0001
01 0010
11 0100
10 1000

24. •Signal broadcast over seemingly random series
of frequencies
•Receiver hops between frequencies in sync
with transmitter
•Eavesdroppers hear unintelligible blips
Jamming on one frequency affects only a few
bits

25. f5
f4
f3
f2
f1
1 2 3 4 5 6 7 8 9 10 11 12
TIME