Transcript DIRECT SEQUENCE SPREAD SPECTRUM WITH FREQUENCY HOPPING

DIRECT SEQUENCE
SPREAD
SPECTRUM WITH
FREQUENCY
HOPPING
Also known as
HYBRID SPREAD SPECTRUM
BUT FIRST,
LET’S
REFRESH…
PROCESSING 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 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 SEQUENCE
PSEUDO RANDOM NOISE CODE
(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
(PNcode)
• When the length of such a shiftregister 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) = N
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 SEQUENCE
DIRECT 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 jammingsignal in the radio channel will be spread
before data-detection is performed.
DIRECT 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 HOPPING
• 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.
FREQUENCY HOPPING
FREQUENCY HOPPING
FREQUENCY HOPPING
FREQUENCY 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 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 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 carrierfrequencies.
FINALLY…
DIRECT SEQUENCE
SPREAD
SPECTRUM WITH
FREQUENCY
HOPPING
Also known as
HYBRID SPREAD SPECTRUM
HYBRID SPREAD SPECTRUM
• combination of direct-sequence and
frequency-hopping.
• One data bit is divided over frequencyhop 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 SPECTRUM
HYBRID 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 frequencyhop sequences are chosen in such a way
that two transmitters with different FHsequences 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/lowprobability-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