question: how do radio signals (4G, WiFi etc.) manage to retain their information after passing through trees, buildings and other obstacles? And how are they not mixed up, intertwined?
comment: They are, the key is there are error correction and detection algorithms that work to counter the problem. In bad environments they don't always work. Also, many newer devices actually utilize the attenuation and reflections to improve signal quality. Take a look into [MIMO]( URL_1 ) technology. They do what is called [beamforming]( URL_0 ) where they vary the signal being output on multiple antennae to create hot spots in the signal at the receiving antenna(e) to improve reception. When multiple devices are connected it will actually change the beam forming per device moment to moment depending on the destination. It's really cool stuff. Edits: Links, typos. Edit 2: Figured I could expand as well. When you're dealing with reflections or other signals that could potentially interfere, they are usually a bit weaker than the main signal. This means that a receiver can filter them out by simply ignoring signals that are weaker than a certain level. This is called the noise floor of the radio. Think of it like being in a room full of people talking and ignoring all the background conversations because they're harder to hear and focusing on your current one. Attenuation of the main signal works similarly. As long as the signal isn't attenuated below the noise floor, it will work. If the operating environment for the radios is bad enough, it may be impossible to get a strong enough signal to overcome the noise floor, and then nothing works.
passages: BULLET::::- the radio link used by the phones is a digital link with extensive error correction and detection in the digital protocol
  
  BULLET::::- sufficient operation of mobile phone in tunnels when supported by split cable antennas
  
  BULLET::::- local repeaters inside complex vehicles or buildings
   Many types of radio transmissions depend, to varying degrees, on line of sight (LOS) between the transmitter and receiver. Obstacles that commonly cause NLOS conditions include buildings, trees, hills, mountains, and, in some cases, high voltage electric power lines. Some of these obstructions reflect certain radio frequencies, while some simply absorb or garble the signals; but, in either case, they limit the use of many types of radio transmissions, especially when low on power budget.
   Most materials of the type affecting radio wave transmission over NLOS links are intermediate: they are neither good insulators nor good conductors. Radio waves incident upon an obstruction comprising a thin intermediate material are partly reflected at both the incident and exit boundaries and partly absorbed, depending on the thickness. If the obstruction is thick enough the radio wave might be completely absorbed. Because of the absorption, these are often called lossy materials, although the degree of loss is usually extremely variable and often very dependent on the level of moisture present. They are often heterogeneous and comprise a mixture of materials with various degrees of conductor and insulator properties. Such examples are hills, valley sides, mountains (with substantial vegetation) and buildings constructed from stone, brick or concrete but without reinforced steel. The thicker they are the greater the loss. For example, a wall absorbs much less RF power from a normally incident wave than a building constructed from the same material.
   Also, because of the high frequency, a high data transfer rate may be available. However, in practical last mile environments, obstructions and de-steering of these beams, and absorption by elements of the atmosphere including fog and rain, particularly over longer paths, can greatly restrict their use for last-mile wireless communications. Longer (redder) waves suffer less obstruction but may carry lower data rates. See RONJA.
  
  Section::::Existing last mile delivery systems.:Wireless delivery systems.:Radio waves.
   Separation of the intercepted spectrum and the signals intercepted from each sensor must take place in an extremely small period of time, in order to separate the different signals to different transmitters in the battlefield. The complexity of the separation process depends on the complexity of the transmission methods (e.g., hopping or time division multiple access (TDMA)).
   In an ideal communication scenario, there is a line-of-sight path between the transmitter and receiver that represents clear spatial channel characteristics. In urban cellular systems, this is seldom the case as base stations are located on rooftops while many users are located either indoors or at streets far from base stations. Thus, there is a non-line-of-sight multipath propagation channel between base stations and users, describing how the signal is reflected at different obstacles on its way from the transmitter to the receiver. However, the received signal may still have a strong spatial signature in the sense that stronger average signal gains are received from certain spatial directions.
   BULLET::::- multipath reflection along the street
  
  BULLET::::- diffraction through windows, and attenuated passage through walls, into the building
  
  BULLET::::- reflection, diffraction, and attenuated passage through internal walls, floors and ceilings within the building
  
  The combination of all these effects makes the mobile phone propagation environment highly complex, with multipath effects and extensive Rayleigh fading. For mobile phone services, these problems are tackled using:
  
  BULLET::::- rooftop or hilltop positioning of base stations
  
  BULLET::::- many base stations (usually called "cell sites"). A phone can typically see at least three, and usually as many as six at any given time.
   In practice, the propagation characteristics of these radio waves vary substantially depending on the exact frequency and the strength of the transmitted signal (a function of both the transmitter and the antenna characteristics). Broadcast FM radio, at comparatively low frequencies of around 100 MHz, are less affected by the presence of buildings and forests.
  
  Section::::Impairments to line-of-sight propagation.
   BULLET::::- Combining: In combining, all antennas maintain established connections at all times. The signals are then combined and presented to the receiver. Depending on the sophistication of the system, the signals can be added directly (equal gain combining) or weighted and added coherently (maximal-ratio combining). Such a system provides the greatest resistance to fading but since all the receive paths must remain energized, it also consumes the most power.
   In the radio wave environment for mobile services the mobile antenna is close to the ground. Line-of-sight propagation (LOS) models are highly modified. The signal path from the BTS antenna normally elevated above the roof tops is refracted down into the local physical environment (hills, trees, houses) and the LOS signal seldom reaches the antenna. The environment will produce several deflections of the direct signal onto the antenna, where typically 2–5 deflected signal components will be vectorially added.
   Researchers studying the transmission of multiple signals over different wires in the same cable bundle helped create a theoretical foundation for 4G MIMO. Specifically, techniques for cancelling the effects of crosstalk using knowledge of the source signals were investigated. The “wireline MIMO” researchers included Lane H. Brandenburg and Aaron D. Wyner (1974),
   Section::::Means of achieving non-line-of-sight transmission.:Other methods.
  
  Earth–Moon–Earth communication, Meteor burst communications, and Sporadic E propagation are also other methods of achieving communications past the radio horizon.
  
  Section::::How is positioning accuracy affected by NLOS conditions?
   Diffraction phenomena by small obstacles are also important at high frequencies. Signals for urban cellular telephony tend to be dominated by ground-plane effects as they travel over the rooftops of the urban environment. They then diffract over roof edges into the street, where multipath propagation, absorption and diffraction phenomena dominate.
  
  Section::::Modes.:Other effects.:Absorption.
   Section::::Vehicle-to-vehicle.:Conflict over spectrum.
   Ted Compton at Ohio State University developed an adaptive antenna technique for recovering direct sequence spread spectrum signals in the presence of narrowband co-channel interference. Compton’s method, reported in 1974, only requires knowledge of the desired signal’s pseudorandom noise (PN) code—not its direction of arrival. In the late 1970s, Kesh Bakhru and Don Torrieri developed the maximin algorithm for recovering frequency hopping signals in the presence of narrowband co-channel interference.
   Some materials cause absorption of electromagnetic waves, preventing it from reaching the receiver, in other cases, particularly with metallic or conductive materials reflection occurs. This can cause dead zones where no reception is available. Aluminium foiled thermal isolation in modern homes can easily reduce indoor mobile signals by 10 dB frequently leading to complaints about the bad reception of long-distance rural cell signals.
  
  Section::::Properties.:Difficulties.:Multipath fading.
  
  In multipath fading two or more different routes taken by the signal, due to reflections, can cause the signal to cancel out at certain locations, and to be stronger in other places (upfade).
  
  Section::::Properties.:Difficulties.:Hidden node problem.
   Because each individual telecommunication link has to encounter different terrain, path, obstructions, atmospheric conditions and other phenomena, it is intractable to formulate the exact loss for all telecommunication systems in a single mathematical equation. As a result, different models exist for different types of radio links under different conditions. The models rely on computing the median path loss for a link under a certain probability that the considered conditions will occur.
  
  Section::::Development methodology.
   Section::::Means of achieving non-line-of-sight transmission.:Active repeaters.
   Section::::Joint sensing-communications system design and integration.:Coexistence.
  
  Remote sensing and communications systems that implement coexistence methods are forced to coexist with each other and treat each other as sources of interference. This means that unlike non-integration methods, each system tries to perform interference mitigation. However, since both systems are not cooperative and have no knowledge about the other system, any information required to perform such interference mitigation is not shared or known and has to be estimated. As a result, interference mitigation performance is limited since it is dependent on the estimated information.
  
  Section::::Joint sensing-communications system design and integration.:Cooperation.
   In practice, the presence of atmosphere, and especially obstructions caused by terrain, buildings and foliage can greatly increase the loss above the free space value. Reflection, refraction and diffraction of waves can also alter their transmission characteristics and require specialized systems to accommodate the accompanying distortions.
  
  Wireless systems have an advantage over wired systems in last mile applications in not requiring lines to be installed. However, they also have a disadvantage in that their unguided nature makes them more susceptible to unwanted noise and signals. Spectral reuse can therefore be limited.
   If formula_8 the material is a good conductor or a poor insulator and substantially "reflects" the radio waves that are incident upon it with almost the same power. Therefore, virtually no RF power is absorbed by the material itself and virtually none is transmitted, even if it is very thin. All metals are good conductors and there are of course many examples that cause significant reflections of radio waves in the urban environment, for example bridges, metal clad buildings, storage warehouses, aircraft and electrical power transmission towers or pylons.
   See also Non-line-of-sight propagation for more on impairments in propagation.
  
  It is important to take into account the curvature of the Earth for calculation of line-of-sight paths from maps, when a direct visual fix cannot be made. Designs for microwave formerly used  earth radius to compute clearances along the path.
  
  Section::::Mobile telephones.
  
  Although the frequencies used by mobile phones (cell phones) are in the line-of-sight range, they still function in cities. This is made possible by a combination of the following effects:
  
  BULLET::::- propagation over the rooftop landscape
  
  BULLET::::- diffraction into the "street canyon" below
   Section::::Means of achieving non-line-of-sight transmission.:Passive repeaters.
   Because signal0 and signal1 are transmitted at the same time into the air, they add to produce the raw signal
  
  This raw signal is called an interference pattern. The receiver then extracts an intelligible signal for any known sender by combining the sender's code with the interference pattern. The following table explains how this works and shows that the signals do not interfere with one another:
   Antenna combiners are implemented to use the same antenna for several TRXs (carriers), the more TRXs are combined the greater the combiner loss will be. Up to 8:1 combiners are found in micro and pico cells only.
  
presuppositions: Radio signals do not get mixed up and intertwined. Radio signals don't get mixed up or intertwined.
corrections: Radio signals do get mixed up and intertwined, but there are error detection and correction algorithms to fix this. Radio signals do get mixed up or intertwined but error correction and detection algorithms counter these issues.
raw_presuppositions: Radio signals do not get mixed up and intertwined. Radio signals don't get mixed up or intertwined.
raw_corrections: Radio signals do get mixed up and intertwined, but there are error detection and correction algorithms to fix this. Radio signals do get mixed up or intertwined but error correction and detection algorithms counter these issues.