SECX1061 SPREAD SPECTRUM COMMUNICATION
UNIT I INTRODUCTION 10 hrs.
Spread spectrum signal - Spread spectrum digital communication system model
-Positive Performance features - Processing gain and other fundamental
parameters – Jamming methods - Pseudo Noise sequences-generation of PN
codes-PN code properties - Correlation properties - classification of SS.
A Short History
Spread-spectrum communications technology was first described on
paper in 1941 Hollywood actress Hedy Lamarr and pianist George Antheil
described a secure radio link to control torpedos. They received U.S. Patent
#2.292.387. The technology was not taken seriously at that time by
the U.S. Army and was forgotten until the 1980s, when it became active. Since
then the technology has become increasingly popular for applications that
involve radio links in hostile environments.
Theoretical Justification for Spread Spectrum
Spread-spectrum is apparent in the Shannon and Hartley channel-capacity
theorem:
C = B × log2 (1 + S/N)
In this equation,
C is the channel capacity in bits per second (bps), which is the maximum data
rate for a theoretical bit-error rate (BER).
B the required channel bandwidth in Hz.
S/N the signal-to-noise power ratio.
To be more explicit, one assumes that C, which represents the amount of
information allowed by the communication channel, also represents the desired
performance. Bandwidth (B) is the price to be paid, because frequency is a
limited resource. The S/N ratio expresses the environmental conditions or the
physical characteristics (i.e., obstacles, presence of jammers, interferences, etc.).
Definitions
Different spread-spectrum techniques are available, but all have one idea
in common: the key (also called the code or sequence) attached to the
communication channel. The manner of inserting this code defines precisely the
spread-spectrum technique. The term "spread spectrum" refers to the expansion
of signal bandwidth, by several orders of magnitude in some cases, which
occurs when a key is attached to the communication channel.
,The ratio (in dB) between the spread baseband and the original signal is called
processing gain. Typical spread-spectrum processing gains run from 10dB to
60dB.
To apply a spread-spectrum technique, simply inject the corresponding spread-
spectrum code somewhere in the transmitting chain before the antenna
(receiver). (That injection is called the spreading operation.) The effect is to
diffuse the information in a larger bandwidth. Conversely, you can remove the
spread-spectrum code (called a despreading operation) at a point in the receive
chain before data retrieval. A despreading operation reconstitutes the
information into its original bandwidth. Obviously, the same code must be
known in advance at both ends of the transmission channel. (In some
circumstances, the code should be known only by those two parties.)
How Spread Spectrum Works
Spread Spectrum uses wide band, noise-like signals. Because Spread
Spectrum signals are noise-like, they are hard to detect. Spread Spectrum signals
are also hard to Intercept or demodulate. Further, Spread Spectrum signals are
harder to jam (interfere with) than narrowband signals. These Low Probability
of Intercept (LPI) and anti-jam (AJ) features are why the military has used
Spread Spectrum for so many years. Spread signals are intentionally made to be
much wider band than the information they are carrying to make them more
noise-like.
Spread Spectrum signals use fast codes that run many times the information
bandwidth or data rate. These special "Spreading" codes are called "Pseudo
Random" or "Pseudo Noise" codes. They are called "Pseudo" because they are
not real Gaussian noise.
Spread Spectrum transmitters use similar transmit power levels to narrow band
transmitters. Because Spread Spectrum signals are so wide, they transmit at a
much lower spectral power density, measured in Watts per Hertz, than
narrowband transmitters. This lower transmitted power density characteristic
gives spread signals a big plus. Spread and narrow band signals can occupy the
same band, with little or no interference. This capability is the main reason for
all the interest in Spread Spectrum today.
To qualify as a spread spectrum signal, two criteria should be met:
The transmitted signal bandwidth is much greater than the information
bandwidth
Some function other than the information being transmitted is employed
to determine the resultant transmitted bandwidth
, What Spread Spectrum Does?
The use of these special pseudo noise codes in spread spectrum (SS)
communications makes signals appear wide band and noise-like. It is this very
characteristic that makes SS signals possess the quality of Low Probability of
Intercept. SS signals are hard to detect on narrow band equipment because the
signal's energy is spread over a bandwidth of maybe 100 times the information
bandwidth.
The spread of energy over a wide band, or lower spectral power density,
makes SS signals less likely to interfere with narrowband communications.
Narrow band communications, conversely, cause little to no interference to SS
systems because the correlation receiver effectively integrates over a very wide
bandwidth to recover an SS signal. The correlator then "spreads" out a narrow
band interferer over the receiver's total detection bandwidth. Since the total
integrated signal density or SNR at the correlator's input determines whether
there will be interference or not. All SS systems have a threshold or tolerance
level of interference beyond which useful communication ceases. This tolerance
or threshold is related to the SS processing gain. Processing gain is essentially
the ratio of the RF bandwidth to the information bandwidth.
A typical commercial direct sequence radio, might have a processing gain
of from 11 to 16 dB, depending on data rate. It can tolerate total jammer power
levels of from 0 to 5 dB stronger than the desired signal. Yes, the system can
work at negative SNR in the RF bandwidth. Because of the processing gain of
the receiver's correlator, the system functions at positive SNR on the baseband
data.
CHIP: The time it takes to transmit a bit or single symbol of a PN code.
CODE: A digital bit stream with noise-like characteristics.
CORRELATOR: The SS receiver component that demodulates a Spread
Spectrum signal.
DE-SPREADING: The process used by a correlator to recover
narrowband information from a spread spectrum signal.
PN: Pseudo Noise - a digital signal with noise-like properties.
UNIT I INTRODUCTION 10 hrs.
Spread spectrum signal - Spread spectrum digital communication system model
-Positive Performance features - Processing gain and other fundamental
parameters – Jamming methods - Pseudo Noise sequences-generation of PN
codes-PN code properties - Correlation properties - classification of SS.
A Short History
Spread-spectrum communications technology was first described on
paper in 1941 Hollywood actress Hedy Lamarr and pianist George Antheil
described a secure radio link to control torpedos. They received U.S. Patent
#2.292.387. The technology was not taken seriously at that time by
the U.S. Army and was forgotten until the 1980s, when it became active. Since
then the technology has become increasingly popular for applications that
involve radio links in hostile environments.
Theoretical Justification for Spread Spectrum
Spread-spectrum is apparent in the Shannon and Hartley channel-capacity
theorem:
C = B × log2 (1 + S/N)
In this equation,
C is the channel capacity in bits per second (bps), which is the maximum data
rate for a theoretical bit-error rate (BER).
B the required channel bandwidth in Hz.
S/N the signal-to-noise power ratio.
To be more explicit, one assumes that C, which represents the amount of
information allowed by the communication channel, also represents the desired
performance. Bandwidth (B) is the price to be paid, because frequency is a
limited resource. The S/N ratio expresses the environmental conditions or the
physical characteristics (i.e., obstacles, presence of jammers, interferences, etc.).
Definitions
Different spread-spectrum techniques are available, but all have one idea
in common: the key (also called the code or sequence) attached to the
communication channel. The manner of inserting this code defines precisely the
spread-spectrum technique. The term "spread spectrum" refers to the expansion
of signal bandwidth, by several orders of magnitude in some cases, which
occurs when a key is attached to the communication channel.
,The ratio (in dB) between the spread baseband and the original signal is called
processing gain. Typical spread-spectrum processing gains run from 10dB to
60dB.
To apply a spread-spectrum technique, simply inject the corresponding spread-
spectrum code somewhere in the transmitting chain before the antenna
(receiver). (That injection is called the spreading operation.) The effect is to
diffuse the information in a larger bandwidth. Conversely, you can remove the
spread-spectrum code (called a despreading operation) at a point in the receive
chain before data retrieval. A despreading operation reconstitutes the
information into its original bandwidth. Obviously, the same code must be
known in advance at both ends of the transmission channel. (In some
circumstances, the code should be known only by those two parties.)
How Spread Spectrum Works
Spread Spectrum uses wide band, noise-like signals. Because Spread
Spectrum signals are noise-like, they are hard to detect. Spread Spectrum signals
are also hard to Intercept or demodulate. Further, Spread Spectrum signals are
harder to jam (interfere with) than narrowband signals. These Low Probability
of Intercept (LPI) and anti-jam (AJ) features are why the military has used
Spread Spectrum for so many years. Spread signals are intentionally made to be
much wider band than the information they are carrying to make them more
noise-like.
Spread Spectrum signals use fast codes that run many times the information
bandwidth or data rate. These special "Spreading" codes are called "Pseudo
Random" or "Pseudo Noise" codes. They are called "Pseudo" because they are
not real Gaussian noise.
Spread Spectrum transmitters use similar transmit power levels to narrow band
transmitters. Because Spread Spectrum signals are so wide, they transmit at a
much lower spectral power density, measured in Watts per Hertz, than
narrowband transmitters. This lower transmitted power density characteristic
gives spread signals a big plus. Spread and narrow band signals can occupy the
same band, with little or no interference. This capability is the main reason for
all the interest in Spread Spectrum today.
To qualify as a spread spectrum signal, two criteria should be met:
The transmitted signal bandwidth is much greater than the information
bandwidth
Some function other than the information being transmitted is employed
to determine the resultant transmitted bandwidth
, What Spread Spectrum Does?
The use of these special pseudo noise codes in spread spectrum (SS)
communications makes signals appear wide band and noise-like. It is this very
characteristic that makes SS signals possess the quality of Low Probability of
Intercept. SS signals are hard to detect on narrow band equipment because the
signal's energy is spread over a bandwidth of maybe 100 times the information
bandwidth.
The spread of energy over a wide band, or lower spectral power density,
makes SS signals less likely to interfere with narrowband communications.
Narrow band communications, conversely, cause little to no interference to SS
systems because the correlation receiver effectively integrates over a very wide
bandwidth to recover an SS signal. The correlator then "spreads" out a narrow
band interferer over the receiver's total detection bandwidth. Since the total
integrated signal density or SNR at the correlator's input determines whether
there will be interference or not. All SS systems have a threshold or tolerance
level of interference beyond which useful communication ceases. This tolerance
or threshold is related to the SS processing gain. Processing gain is essentially
the ratio of the RF bandwidth to the information bandwidth.
A typical commercial direct sequence radio, might have a processing gain
of from 11 to 16 dB, depending on data rate. It can tolerate total jammer power
levels of from 0 to 5 dB stronger than the desired signal. Yes, the system can
work at negative SNR in the RF bandwidth. Because of the processing gain of
the receiver's correlator, the system functions at positive SNR on the baseband
data.
CHIP: The time it takes to transmit a bit or single symbol of a PN code.
CODE: A digital bit stream with noise-like characteristics.
CORRELATOR: The SS receiver component that demodulates a Spread
Spectrum signal.
DE-SPREADING: The process used by a correlator to recover
narrowband information from a spread spectrum signal.
PN: Pseudo Noise - a digital signal with noise-like properties.
