US-12621064-B1 - Detection of a skywave symbol

US12621064B1US 12621064 B1US12621064 B1US 12621064B1US-12621064-B1

Abstract

A method for detection of a skywave symbol, the method may include receiving, by a skywave propagation receiver, received signals that are within a skywave frequency range that comprises legal subcarriers and illegal subcarriers; and detecting a reception of the skywave symbol based on at least one energy parameter related to at least two out of (a) received signals at the illegal subcarriers, (b) received signals at the legal subcarriers, (c) received signals at any of the subcarriers.

Inventors

Nir Halay
Tamir Ostfeld
Elad Segalis

Assignees

Raft Technologies Ltd.

Dates

Publication Date: 20260505
Application Date: 20190821

Claims (12)

1 . A method for detection of a skywave symbol, the method comprises: receiving, by a skywave propagation receiver, received signals that are within a skywave frequency range that comprises legal subcarriers and illegal subcarriers; and detecting a reception of the skywave symbol based on an energy parameter related to received signals at the illegal subcarriers that are received during a time window, and to received signals at the legal subcarriers that are received during the time window.
2 . The method according to claim 1 wherein the skywave symbol lacks a synchronization preamble.
3 . The method according to claim 1 wherein the skywave symbol is an orthogonal frequency division multiplexing (OFDM) skywave symbol.
4 . The method according to claim 1 wherein the skywave symbol is transmitted over an asynchronous skywave communication channel.
5 . The method according to claim 1 wherein a duration of the detecting is less than a length of the skywave symbol.
6 . The method according to claim 1 wherein a duration of the detecting is less than 6 microseconds.
7 . The method according to claim 1 wherein a duration of the detecting is less than duration of a synchronization preamble based detection of the skywave symbol.
8 . The method according to claim 1 wherein the time window has a duration that substantially equals a duration of the skywave symbol.
9 . The method according to claim 1 wherein the time window rejects sidelobes once applied on the entire symbol.
10 . The method according to claim 1 wherein the detecting comprises detecting an end of a reception of the skywave symbol.
11 . A non-transitory computer readable medium that stores instructions for: receiving, by a skywave propagation receiver, received signals that are within a skywave frequency range that comprises legal subcarriers and illegal subcarriers; and detecting a reception of the skywave symbol based on an energy parameter related to (a) received signals at the illegal subcarriers that are received during a time window, and (b) received signals at the legal subcarriers that are received during the time window.
12 . A skywave propagation receiver that comprises a receiving unit that is configured to receive signals that are within a skywave frequency range that comprises legal subcarriers and illegal subcarriers; and a processor that is configured to detecting a reception of the skywave symbol based on an energy parameter related to (a) received signals at the illegal subcarriers that are received during a time window, and (b) received signals at the legal subcarriers that are received during the time window.

Description

CROSS REFERENCE This application claims priority from U.S. provisional patent 62/720,461 filing date Aug. 21, 2018. BACKGROUND The atmosphere is the gaseous envelope surrounding the planet Earth and comprising a mixture of gases. The ionosphere is the upper part of the Earth's atmosphere. Scientifically, above the stratosphere, which means beyond an altitude of 60 km up to 1000 km, the atmosphere is characterized by a high density of free electrons and free ions, mostly produced by the energetic photo-ionization of UV and X-rays arriving from the sun, and to a minor extent over high latitudes by corpuscular ionization. A high or relevant density of free electrons and free ions is not a clearly defining characteristic because electrons and ions are present at every altitude in the lower and upper atmosphere. Therefore, a more practical definition, originating from the first application of long distance radio communications, is that part of the atmosphere in which the density of ionization is sufficient to deflect (Deflected means it bounces back on the plane surface when it is bent by a gravitational force) radio waves in the 2-30 MHz range. This part of the atmosphere, the ionosphere, includes the following layers: the mesosphere, thermosphere, and exosphere. The radiation of the sun ionizes gasses in the ionosphere. Several ionospheric layers (regions) can be identified, each layer having its particular composition and being ionized by specific wavelengths in the solar radiation. Modern experimental and theoretical investigations divide the ionosphere into three regions: D, E, and F. The real heights of the ionospheric layers vary with solar zenith angle time, time of day, seasons, solar cycles, and solar activity. The F-layer (160-1000 km height) is the layer with the highest electron density, which implies signals penetrating this layer will escape into space atomic. By daylight, the F-layer is split into a lower F1-layer (160-210 km height) and a higher F2-layer (above 210 km height). The E-layer (90-160 km height) can only reflect radio waves having frequencies lower than about 10 MHz. The D-layer (60-90 km height), is responsible for high attenuation at the lower HF frequencies, disappears almost completely at night. Therefore allowing frequencies which are not usable during the day to propagate successfully at night. The F1-layer always disappears during the night and sometimes in winter even during the day. The F2 layer is present 24 hours a day under all solar terrestrial conditions, making it the most important layer of the ionosphere. The main characteristics of the F2 layer are its high variability, on timescales ranging from the 11 years of a solar cycle and even longer, to a few seconds during strong interactions with the plasmasphere above (at altitudes >1,000 km) depending on solar-terrestrial conditions. The Sun does affect the electron density of the F2 layers causing a rapid increase after sunrise, with maximum values occurring at any time during the day. A sporadic E layer occurs at altitudes from 90 to 140 km (the E region). Usually, it is considered independent of the normal E layer of the ionosphere. Most importantly, sporadic E layers can have an electron density similar to the F region. However, its random time of occurrence and presence at any particular place makes Es layer prediction very difficult. Electromagnetic waves entering the ionosphere may be refracted back to Earth, depending on the operating frequency. The High Frequency (HF) band is defined by the International Telecommunication Union (ITU) as radio waves with frequencies between 3 MHz and 30 MHz. HF radio signals can propagate via 3 different ways: a. Ground waves: near the ground for short distances, up to 100 km over land and 300 km over sea.b. Direct waves: available through line-of-sight. Available only in distances with line-of-sight.c. Skywaves: reflected by the Ionosphere, all distances. For long distances (more than 1000 km) between the transmitter and the receiver, only skywaves are applicable for HF transmission. Not all HF waves are reflected by the Ionosphere. If the frequency is too high, the wave will penetrate through the Ionosphere. If it is too low, it will be absorbed by the D region. Furthermore, the Ionosphere is usually not stable. It variates during solar cycles (roughly 11 years), seasons and even during each day. These variations cause difficulties in HF radio transmission. For example, the range of usable frequencies will vary throughout the day, with the seasons, and with the solar cycle. For long distances between the transmitter and the receiver, one reflection from the Ionosphere is not enough. The radio wave is returned to earth and refracted from the Ionosphere once again. This double “hop” causes significant deterioration in signal power due to the refraction from earth and the double propagation in the D region. Furthermore, multiple number of “hops” can co-exist in a single transmiss