August 22, 2011

What Spread Spectrum Does?

 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.
A spread spectrum system is one in which the transmitted signal is spread over a wide frequency band, much wider, in fact, than the minimum bandwidth required to transmit the information being sent. Spread spectrum communications cannot be said to be an efficient means of utilizing bandwidth. However, it does come into its own when combined with existing systems occupying the frequency. The spread spectrum signal being “spread” over a large bandwidth can coexist with narrow band signals only adding a slight increase in the noise floor that the narrow band receivers see.
This technique is used as a way to reduce the power density of radio transmission. Spread spectrum waveforms can also be used to primarily improve performance in the area of interference tolerance.

 The spread of energy over a wide band, or lower spectral power density, makes SS signals less likely to interfere with narrow band 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 correlate 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 jammier 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 correlative, the system functions at positive SNR on the base band data.

Besides being hard to intercept and jam, spread spectrum signals are hard to exploit or spoof. Signal exploitation is the ability of an enemy (or a non-network member) to listen in to a network and use information from the network without being a valid network member or participant. Spoofing is the act of falsely or maliciously introducing misleading or false traffic or messages to a network. SS signals also are naturally more secure than narrow band radio communications. Thus SS signals can be made to have any degree of message privacy that is desired. Messages can also, be cryptographically encoded to any level of secrecy desired. The very nature of SS allows military or intelligence levels of privacy and security to be had with minimal complexity.

 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 narrow band 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 transmits 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 narrow band 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.

  In today’s commercial spread spectrum systems, bandwidths of 10 to 100 times the information rates are used. Military systems have used spectrum widths from 1000 to 1 million times the information bandwidth. There are two very common spread spectrum modulations: frequency hopping and direct sequence. At least two other types of spreading modulations have been used: time hopping and chirp.
  Direct sequence systems—Direct sequence spread spectrum systems are so called because they employ a high-speed code sequence, along with the basic information being sent, to modulate their RF carrier. The high-speed code sequence is used directly to modulate the carrier, thereby directly setting the transmitted RF bandwidth. Binary code sequences as short as 11 bits have been employed for this purpose, at code rates from under a bit per second to several hundred megabits per second.

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