• Resistance to jamming (interference). DS is better at resisting continuous-time narrowband jamming, while FH is better at resisting pulse jamming. In DS systems, narrowband jamming affects detection performance about as much as if the amount of jamming power is spread over the whole signal bandwidth, when it will often not be much stronger than background noise. By contrast, in narrowband systems where the signal bandwidth is low, the received signal quality will be severely lowered if the jamming power happens to be concentrated on the signal bandwidth.
• Resistance to eavesdropping. The spreading code (in DS systems) or the frequency-hopping pattern (in FH systems) is often unknown by anyone for whom the signal is unintended, in which case it "encrypts" the signal and reduces the chance of an adversary's making sense of it. What's more, for a given noise power spectral density (PSD), spread-spectrum systems require the same amount of energy per bit before spreading as narrowband systems and therefore the same amount of power if the bitrate before spreading is the same, but since the signal power is spread over a large bandwidth, the signal PSD is much lower, often significantly lower than the noise PSD, therefore the adversary may be unable to determine if the signal exists at all. However, for mission-critical applications, particularly those employing commercially available radios, spread-spectrum radios do not intrinsically provide adequate security; "...just using spread-spectrum radio itself is not sufficient for communications security".

Some of the most powerful attacks involve an adversary manipulating the RF spectrum while a victim cognitive radio is in its learning phase. By skewing the radio’s performance in certain situations, the attacker can make a victim radio believe things that are untrue, and consequently cause it to behave in a suboptimal or malicious manner. Any radio that can learn from its environment can also be taught by its environment. So the HYBRID SS is just a good compromise to take into account what has been described above.

best regards

• Resistance to jamming (interference). DS is better at resisting continuous-time narrowband jamming, while FH is better at resisting pulse jamming. In DS systems, narrowband jamming affects detection performance about as much as if the amount of jamming power is spread over the whole signal bandwidth, when it will often not be much stronger than background noise. By contrast, in narrowband systems where the signal bandwidth is low, the received signal quality will be severely lowered if the jamming power happens to be concentrated on the signal bandwidth.
• Resistance to eavesdropping. The spreading code (in DS systems) or the frequency-hopping pattern (in FH systems) is often unknown by anyone for whom the signal is unintended, in which case it "encrypts" the signal and reduces the chance of an adversary's making sense of it. What's more, for a given noise power spectral density (PSD), spread-spectrum systems require the same amount of energy per bit before spreading as narrowband systems and therefore the same amount of power if the bitrate before spreading is the same, but since the signal power is spread over a large bandwidth, the signal PSD is much lower, often significantly lower than the noise PSD, therefore the adversary may be unable to determine if the signal exists at all. However, for mission-critical applications, particularly those employing commercially available radios, spread-spectrum radios do not intrinsically provide adequate security; "...just using spread-spectrum radio itself is not sufficient for communications security".

Some of the most powerful attacks involve an adversary manipulating the RF spectrum while a victim cognitive radio is in its learning phase. By skewing the radio’s performance in certain situations, the attacker can make a victim radio believe things that are untrue, and consequently cause it to behave in a suboptimal or malicious manner. Any radio that can learn from its environment can also be taught by its environment. So the HYBRID SS is just a good compromise to take into account what has been described above.

best regards

Thank You Andrea Piroddi

in fact I prefer FHSS .. do you think that's a good choice ؟

 Please , what's your opinion ?

Regards...

Ameer, I think hat Andrea's answer is quite complete. I might only add that FHSS involves tuning a transmitter and receiver to new frequencies, which is inherently going to be slower (at spreading the spectrum) than DS. It does take some time to have the transmitter and receiver settle on each new frequency channel. Whether this has any credible effect on security is another question. Probably not.

Thank You Albert Manfredi 

yes I read it , in fact we have some question must answer when we are compare (FHSS & DSSS) within security concept :

1- What is the capability to operate when other radio signals are present in the same band ?

2- how can protect the transmission against eavesdropping , what we need ?

Answer : Q1-

- FHSS systems operate with SNR(Signal to Noise Ratio) of about 18 dB

- DSSS systems operate with SNR 12 dB because it is using more efficient modulation techniques, called Phase-shift keying (PSK)

All band interference

a- Noise

Because DSSS systems operate with lower SNR, for same level of transmitted energy, DSSS systems can operate over longer distances than FHSS

b- Interference

Since whole spectrum used by the radio is 83.5 MHz in FHSS, while it is 22 MHz in DSSS, there is a greater chance of having interference in DSSS than FHSS

Answer : Q2-

DSSS systems use spreading sequence of 11 chips, thus can modulate 14 channel defined in the IEEE 802.11 standard.

Since the carrier frequency is fixed for a given DSSS system and possible frequencies is limited, it would be easier for a listener to “tune in” on the DSSS transmission This requires message encryption at the application layer.

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In FHSS, frequencies may be selected by the user

In order for the malicious node to “tune in” on the FHSS transmission, a malicious node should know:

1- # of frequencies selected in the system

2- The actual frequencies

3- The hopping sequence

4- The dwell time

FHSS modulation acts as a layer 1 encryption process, therefore, there is no need for application level encryption.

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I will summarize reasons to use FHSS :

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- DSSS provides higher (11Mbps) capacity than FHSS, but is sensitive to many environmental factors such as (system collocation, multipath effects, near/far problem, and Bluetooth)

- For long distance applications, use point to point topologies

- For short distance applications, use point to multiple topologies

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- FHSS provides only 3 Mbps, but it is very robust technology ( works better in harsh environment that is characterized by large areas of coverage multiple collocated cells, noises & interference, multipath, Bluetooth, and etc.)

- Best to use for big coverage areas with a big number of collocated systems and where directional antennas are impossible to implement. Therefore, it is more suitable for fixed broadband wireless access (BWA)

My Best Regards .. Ameer