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DIRECT SEQUENCE SPREAD SPECTRUM WITH FREQUENCY HOPPING

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DIRECT SEQUENCE SPREAD SPECTRUM WITH FREQUENCY HOPPING

Also known as

HYBRID SPREAD SPECTRUM

BUT FIRST, LET’S REFRESH…

- the ratio of transmission and information bandwidth…
Gp = BW1 / BW2

- determines the number of users that can be allowed in a system,
- the amount of multi-path effect reduction,
- the difficulty to jam or detect a signal
- it is advantageous to have a processing gain as high as possible.

- The data signal is multiplied by a Pseudo Random Noise Code (PNcode)
- Signals generated with this technique appear as noise in the frequency domain. The wide bandwidth provided by the pseudo noise code allows the signal power to drop below the noise threshold without losing any information.

- a binary signal which is produced at a much higher frequency then the data that is to be transmitted
- Since this has a higher frequency, it has a large bandwidth, which spreads the signal in the frequency plain (ie. it spreads its spectrum).

- a sequence of chips valued -1 and 1 (polar) or 0 and 1 (non-polar) and has noise-like properties
- results in low cross-correlation values among the codes and the difficulty to jam or detect a data message
- A usual way to create a PNcode is by means of at least one shift-register

- When the length of such a shift-register is n, the following can be said about the period NDS:
NDS = 2n - 1

- In direct-sequence systems, the length of the code is the same as the spreading-factor with the consequence that:
Gp(DS) = NDS

- the PNcode is combined with the data-signal
- The bandwidth of the data signal is multiplied by a factor NDS
- The power contents however stays the same, with the result that the power spectral density lowers.

- In the receiver, the received signal is multiplied again by the same (synchronized) PNcode.
- Since the code existed of +1s and -1s, this operation completely removes the code from the signal and the original data-signal is left.
- the despread operation is the same as the spread operation.
- The consequence is that a possible jamming-signal in the radio channel will be spread before data-detection is performed.
- So jamming effects are reduced

- Near-Far effect
- This effect is present when an interfering transmitter is much closer to the receiver than the intended transmitter.
- The result is that proper data detection is not possible.

- the carrier frequency is “hopping” according to a unique sequence
Gp(FH) = NFH

- a broad bandwidth in the spectrum which is divided into many possible broadcast frequencies to which the data will be sent over.
- there exists a code which determines at any particular moment in time what frequency it will transmit at, hopping from frequency to frequency. Hence, the only way to obtain the transmission is to have an identical code that knows which frequency it will jump to next.

- The faster the "hopping-rate'' is, the higher the processing gain.
- The signal would stay at any one frequency for less then 10 milliseconds, hence there is minimal effects on narrow band signals, as well as due to the large number of frequencies used (and quick hops) deciphering of the code is next to impossible.

- Two kinds of Frequency Hopping Techniques.
- Slow Frequency Hopping (SFH)
- one or more data bits are transmitted within one Frequency Hop.
- An advantage is that coherent data detection is possible.
- A disadvantage is that if one frequency hop channel is jammed, one or more data bits are lost. So error correcting codes are required.

- Fast Frequency Hopping (FFH)
- In this technique one data bit is divided over more Frequency Hops.
- error correcting codes are not needed.
- An other advantage is that diversity can be applied. Every frequency hop a decision is made whether a -1 or a 1 is transmitted, at the end of each data bit a majority decision is made.
- A disadvantage is that coherent data detection is not possible because of phase discontinuities.
- The applied modulation technique should be FSK or MFSK.

- Slow Frequency Hopping (SFH)

- Advantage
- Frequency-Hopping sequences have only a limited number of "hits'' with each other.
- if a near-interferer is present, only a number of "frequency-hops'' will be blocked in stead of the whole signal.
- From the "hops'' that are not blocked it should be possible to recover the original data-message.

- Disadvantage
- obtaining a high processing-gain is hard.
- There is need for a frequency-synthesizer able perform fast-hopping over the carrier-frequencies.

FINALLY…

DIRECT SEQUENCE SPREAD SPECTRUM WITH FREQUENCY HOPPING

Also known as

HYBRID SPREAD SPECTRUM

- combination of direct-sequence and frequency-hopping.
- One data bit is divided over frequency-hop channels (carrier frequencies).
- In each frequency-hop channel one complete PN-code of length is multiplied with the data signal
- Using the FFH scheme in stead of the SFH scheme causes the bandwidth to increase, this increase however is neglectable with regard to the enormous bandwidth already in use

- As the frequency hop sequence and the pseudo noise codes are coupled, an address is a combination of pseudo noise codes and frequency hop sequence.
- To bound the hit-chance (the chance that two users share the same frequency channel in the same time) the frequency-hop sequences are chosen in such a way that two transmitters with different FH-sequences share at most two frequencies at the same time (time-shift is random).

- Multipath-rejection capabilities
- Improved data integrity/security
- Better low-probability-of-detection/low-probability-of-interception (LPD/LPI) properties
- Lower link delay (latency) figures
- Superior narrowband/wideband jamming resistance
- Fast synchronization, higher user density
- Less mutual interference among users in a given area or frequency band
- Near-far reception properties of FH
- Lower overall peak occupied