DIRECT SEQUENCE DIRECT SEQUENCE SPREAD SPREAD

SPECTRUM WITH SPECTRUM WITH FREQUENCY FREQUENCY

HOPPINGHOPPINGAlso known as

HYBRID SPREAD SPECTRUM

BUT FIRST, LET’S

REFRESH…

PROCESSING GAINPROCESSING GAIN

• 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.

DIRECT SEQUENCEDIRECT SEQUENCE

• 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.

DIRECT SEQUENCEDIRECT SEQUENCE

PSEUDO RANDOM NOISE CODE PSEUDO RANDOM NOISE CODE (PNcode)(PNcode)

• 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

PSEUDO RANDOM NOISE CODE PSEUDO RANDOM NOISE CODE (PNcode) (PNcode)

• 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

EXAMPLE…EXAMPLE…

• 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.

DIRECT SEQUENCEDIRECT SEQUENCE

DIRECT SEQUENCEDIRECT SEQUENCE• 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

DIRECT SEQUENCEDIRECT SEQUENCE

• 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.

FREQUENCY HOPPINGFREQUENCY HOPPING• the carrier frequency is “hopping” according to a

unique sequenceGp(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.

FREQUENCY HOPPINGFREQUENCY HOPPING

FREQUENCY HOPPINGFREQUENCY HOPPING

FREQUENCY HOPPINGFREQUENCY HOPPING

FREQUENCY HOPPINGFREQUENCY HOPPING• 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.

Frequency HoppingFrequency Hopping• 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.

FREQUENCY HOPPINGFREQUENCY HOPPING• 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

HOPPINGAlso known as

HYBRID SPREAD SPECTRUM

HYBRID SPREAD SPECTRUMHYBRID 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

HYBRID SPREAD SPECTRUMHYBRID SPREAD SPECTRUM

HYBRID SPREAD SPECTRUMHYBRID SPREAD SPECTRUM• 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).

Hybrid spread spectrum superior qualities

• 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