Spread Spectrum Wifi 802.11

[steve_bank]

A response to Lauren on politics.

It is all mathematical and there is plenty of info on the net.

Frequency hopping goes back to WWII used to remotely guide torpedo. It is credited to the actress Hedy Lamar. Actress and inventor.

Tinkering with Torpedoes How Hedy Lamarr's WWII Invention Led to Wi-Fi and Bluetooth - Evanston RoundTable

What do Clark Gable, Judy Garland, Spencer Tracy, Lana Turner, and James Stewart have in common? They all starred in movies with a clever Austrian

evanstonroundtable.com

Tinkering with Torpedoes How Hedy Lamarr’s WWII Invention Led to Wi-Fi and Bluetooth​

One of the benefits of SS is that you can go the same distance with less power. Another is that there can be many channels in one spectrum. If you looked at SS source with a conventional spectrum analyzer you might see a slight increase over background noise. It can be stealthy.

In one method a signal is spread across a spectrum. f1 - f2, using a pseudo random digital sequence. The recovery processes acts like a very narrow band filter resulting in a high signal to noise ratio and high selectivity. If I remember right digital military radios have very long random sequences.

An older method is analog frequency hopping. Randomly change a carrier frequency in a predetermined way.

One of the benefits of SS is that you can go the same distance with less power. Another is that there can be many chneels in one spectrum. If you looked at SS sourcwith a conventional spectrum analyzer you might see a slight increase over background.

In one method a signal is spread across a spectrum. f1 - f2, using a pseudo random digital sequence. The recovery processes acts like a very narrow band filter resulting in a high signal to noise radi and high selectivity. If I remember right digital military radios have very long random sequnces.

Another method is analog frequency hopping. Randomly change a carrier frequency in a predetermined way. It goes back to WWII.

Wifi 802.11

Direct-sequence spread spectrum - Wikipedia

en.wikipedia.org

Transmission method​

Direct-sequence spread-spectrum transmissions multiply the data being transmitted by a pseudorandom spreading sequence that has a much higher bit rate than the original data rate. The resulting transmitted signal resembles bandlimited white noise, like an audio recording of "static". However, this noise-like signal is used to exactly reconstruct the original data at the receiving end, by multiplying it by the same spreading sequence (because 1 × 1 = 1, and −1 × −1 = 1). This process, known as despreading, is mathematically a correlation of the transmitted spreading sequence with the spreading sequence that the receiver already knows the transmitter is using. After the despreading, the signal-to-noise ratio is approximately increased by the spreading factor, which is the ratio of the spreading-sequence rate to the data rate.

While a transmitted DSSS signal occupies a much wider bandwidth than a simple modulation of the original signal would require, its frequency spectrum can be somewhat restricted for spectrum economy by a conventional analog bandpass filter to give a roughly bell-shaped envelope centered on the carrier frequency. In contrast, frequency-hopping spread spectrum pseudorandomly retunes the carrier and requires a uniform frequency response since any bandwidth shaping would cause amplitude modulation of the signal by the hopping code.

If an undesired transmitter transmits on the same channel but with a different spreading sequence (or no sequence at all), the despreading process reduces the power of that signal. This effect is the basis for the code-division multiple access (CDMA) property of DSSS, which allows multiple transmitters to share the same channel within the limits of the cross-correlation properties of their spreading sequences.

Frequency-hopping spread spectrum - Wikipedia

en.wikipedia.org

​

Frequency-hopping spread spectrum (FHSS) is a method of transmitting radio signals by rapidly changing the carrier frequency among many distinct frequencies occupying a large spectral band. The changes are controlled by a code known to both transmitter and receiver. FHSS is used to avoid interference, to prevent eavesdropping, and to enable code-division multiple access (CDMA) communications.

The available frequency band is divided into smaller sub-bands. Signals rapidly change ("hop") their carrier frequencies among the center frequencies of these sub-bands in a predetermined order. Interference at a specific frequency will affect the signal only during a short interval.[1]

FHSS offers four main advantages over a fixed-frequency transmission:

1. FHSS signals are highly resistant to narrowband interference because the signal hops to a different frequency band.
2. Signals are difficult to intercept if the frequency-hopping pattern is not known.
3. Jamming is also difficult if the pattern is unknown; the signal can be jammed only for a single hopping period if the spreading sequence is unknown.
4. FHSS transmissions can share a frequency band with many types of conventional transmissions with minimal mutual interference. FHSS signals add minimal interference to narrowband communications, and vice versa.

In cell phones the sequences are called Walsh Codes. A code is assigned when you turn on a cell phone and connect to a tower. Your phone runs a continuous mathematical correlation of the digital stream with the code and pulls out the data when there is a correlation. It provides high selectivity and bnose immunity.

Also known as "Walsh-Hadamard code," it is an algorithm that generates statistically unique sets of numbers for use in encryption and cellular communications. Known as "pseudo-random noise codes," Walsh codes are used in direct sequence spread spectrum (DSSS) systems such as Qualcomm's CDMA. They are also used in frequency hopping spread spectrum (FHSS) systems to select the target frequency for the next hop. See CDMA.

Thebasic algorithm

What are walsh codes and how is it implemented in CDMA systems?

Answer (1 of 2): Walsh codes (a.k.a Walsh-Hadamard codes) are a set of perfectly orthogonal codes used to separate users on the downlink (DL) channel in CDMA systems (for example in CDMA2000, UMTS cellular standards etc.). Code orthogonality between two codes W_i and W_j is defined as These co...

www.quora.com

 

[Loren Pechtel]

Yeah, but GPS already exploits the advantages to reduce the transmitter power. You don't get to count them again when resisting jamming.

 

[steve_bank]

If you are close to the reciever you can overload tit, like cell phon jammers in restaurants.

For conventional radio jqmming can conist of high power broad band noise from a distance. During the Cold War Russia and China tried to jam western short wave radio.

With SS hat does not work. De-spreading results in an effective very narrow bandwidth filter that limits broad band noise. A filtering difficult to achieve with analog filters. SS can recover signals burred in noise unlike AM.

I worked on a radar system and am familiar with jamming techniques. For a convetional radar a broad band noise sorcecan be set up at a disance that overliads the radar front end. A countermeasure uses omindirectional antennas off to the sides of the radar antenna. Noise in one antenna is inverted and added to the other, side lobe canceling.

With SS simply transmitting a single jamming frequency at the transmitter frequency does not work, in the direct sequence SS spectrum here is no single frequency to attack.

FM is more reststant to amplitude noise, the received signal is clipped and the detector responds only to frequency shifts.

In WWII American tanks used FM and German tanks used AM. During the invasion of France high power amplitude noise was transmitted from England.

For military SS communications there are techniques that can be added to make it difficult ro send a signal that will be decoded by the reciever. Digital signatures can be encoded into the data stream above regular encryption. Add to that the codes can probably be entered on the fly before a weapon is launched.

 

[barbos]

Yeah, but GPS already exploits the advantages to reduce the transmitter power

SS does not let you reduce power.