question: How do devices reconstruct a piece of information that has been sent by radio waves?
comment: There is a lot of error correction in digital communication! There's two basic techniques: We can send a checksum along with the data. If I want to send you 01001001, I can also send "btw there are 3 1s in the data I just sent." That way, on the off chance that one of those 1s gets flipped to a 0 (or vice versa), you'll say "Oh weird, I only see 2 1s. Can you send that again please?" We can also send redundant data - say, repeat every digit 3 times. Thus, our message of "01001001" becomes "000 111 000 000 111 000 000 111" (spaces added for clarity). Only 000 and 111 are valid groups, so if a bit gets flipped and we wind up with 010 or 100 or 110 or something, we know there's an error. We can say "2 out of 3 wins, 010 is likely to really be 000" or we can request a new transmission. Please note that both these examples are dumbed-down ELI5 versions of the methods used - in reality, checksums and redundancies are much more sophisticated.
passages: Many techniques such as carrier recovery, clock recovery, bit slip, frame synchronization, rake receiver, pulse compression, Received Signal Strength Indication, error detection and correction, etc., are only performed by demodulators, although any specific demodulator may perform only some or none of these techniques.
  
  Many things can act as a demodulator, if they pass the radio waves on nonlinearly.
  
  Section::::AM radio.
  
  An AM signal encodes the information into the carrier wave by varying its amplitude in direct sympathy with the analogue signal to be sent. There are two methods used to demodulate AM signals:
   A simple example of how this works using metadata is transmitting the word "HELLO", by encoding each letter as its position in the alphabet. Thus, the letter "A" is coded as 1, "B" as 2, and so on as shown in the table on the right. Adding up the resulting numbers yields 8 + 5 + 12 + 12 + 15 = 52, and 5 + 2 = 7 calculates the metadata. Finally, the "8 5 12 12 15 7" numbers sequence is transmitted, which the receiver will see on its end if there are no transmission errors. The receiver knows that the last number received is the error-detecting metadata and that all data before is the message, so the receiver can recalculate the above math and if the metadata matches it can be concluded that the data has been received error-free. Though, if the receiver sees something like a "7 5 12 12 15 7" sequence (first element altered by some error), it can run the check by calculating 7 + 5 + 12 + 12 + 15 = 51 and 5 + 1 = 6, and discard the received data as defective since 6 does not equal 7.
   The technician injects a test signal into the device under test. Then, by using the signal tracer, the tech can follow the signal through the various circuits of the radio receiver. So long as the signal can be heard, the circuitry up to that point is (at least minimally) functional. If the signal disappears, however, a fault can be assumed to be present in the stage of the circuit just passed.
   Section::::Description.
   Section::::Forensic process.:Acquisition.
  
  The second step in the forensic process is acquisition, in this case usually referring to retrieval of material from a device (as compared to the bit-copy imaging used in computer forensics).
   Section::::History.
  
  The term "digital receiver" was coined in 1970 by a researcher at a United States Department of Defense laboratory. A laboratory called the Gold Room at TRW in California created a software baseband analysis tool called Midas, which had its operation defined in software.
   BULLET::::- Demodulation: After the radio signal is filtered and amplified, the receiver must extract the information-bearing modulation signal from the modulated radio frequency carrier wave. This is done by a circuit called a demodulator (detector). Each type of modulation requires a different type of demodulator
   Section::::Precoding in Simple Words.
  
  Precoding is a technique which exploits transmit diversity by weighting the information stream, i.e. the transmitter sends the coded information to the receiver to achieve pre-knowledge of the channel. The receiver is a simple detector, such as a matched filter, and does not have to know the channel state information. This technique will reduce the corrupted effect of the communication channel.
  
  For example you are sending the information formula_1
  
  and it will pass through the channel, formula_2,
  
  and add Gaussian noise, formula_3.
  
  The received signal at the receiver front-end will be formula_4;
   In order to find material defects, a test probe, attached or in a small distance, is moved over the surface of the device under test. This can be done manually or automatically. The test probe transmits and receives microwaves.
  
  Changes of the dielectric properties at surfaces (e. g. shrinkage cavities, pores, foreign material inclusion, or cracks) within the interior of the device under test reflect the incident microwave and send a part of it back to the test probe, which acts as a transmitter and as a receiver.
   Section::::Techniques.
  
  There are several ways of demodulation depending on how parameters of the base-band signal such as amplitude, frequency or phase are transmitted in the carrier signal. For example, for a signal modulated with a linear modulation like AM (amplitude modulation), we can use a synchronous detector. On the other hand, for a signal modulated with an angular modulation, we must use an FM (frequency modulation) demodulator or a PM (phase modulation) demodulator. Different kinds of circuits perform these functions.
   When alternating current flows in a conductor it radiates an electromagnetic wave (radio wave). In multi-element antennas, the fields due to currents in one element induce currents in the other elements. Antennas are self-interacting in this respect; the waves reradiated by the elements superimpose on the original radio signal being studied. NEC calculates the field resulting from these contributions, adds it to the original radio signal, and then runs the entire calculation again with this modified field. Because the reradiated signal is normally small compared to the original signal, it only produces a small change, or perturbation, in the resulting element currents. The program then repeats the calculation again with the new element currents, getting new radiation fields. This process is repeated until resulting values converge.
   There are several common types of FM demodulators:
  
  BULLET::::- The quadrature detector, which phase shifts the signal by 90 degrees and multiplies it with the unshifted version. One of the terms that drops out from this operation is the original information signal, which is selected and amplified.
  
  BULLET::::- The signal is fed into a PLL and the error signal is used as the demodulated signal.
   In recent years a number of hardware/software tools have emerged to recover logical and physical evidence from mobile devices. Most tools consist of both hardware and software portions. The hardware includes a number of cables to connect the mobile device to the acquisition machine; the software exists to extract the evidence and, occasionally even to analyse it.
   Early digital testing was focused on the transmitter to ensure that the waveform transmission characteristics met a standard set of characteristics for parameters such as rise time, amplitude, jitter, and so on. It was assumed that if the signal originated correctly, and was communicated well, that the receiver would interpret it properly and reproduce the correct bits. This was often augmented with modest frame-based checksum approaches for the receiver to finally validate the BER of what it had received.
   Demodulation
  
  Demodulation is extracting the original information-bearing signal from a carrier wave. A demodulator is an electronic circuit (or computer program in a software-defined radio) that is used to recover the information content from the modulated carrier wave. There are many types of modulation so there are many types of demodulators. The signal output from a demodulator may represent sound (an analog audio signal), images (an analog video signal) or binary data (a digital signal).
   All these components are controlled by a Logical Data Viewer simply called viewer. For reconstruction, the viewer starts the UVC and feeds it with the data of the digital object to a format decoder running on top of the UVC. In return it retrieves an LDV and reconstructs a specific representation of the original object’s meaning.
  
  Section::::UVC-based preservation.:Preservation System.:Performance.
   The next level in integration is "software-defined radio", where all filtering, modulation and signal manipulation is done in software. This may be a PC soundcard or by a dedicated piece of DSP hardware. There will be a RF front-end to supply an intermediate frequency to the software defined radio. These systems can provide additional capability over "hardware" receivers. For example, they can record large swaths of the radio spectrum to a hard drive for "playback" at a later date. The same SDR that one minute is demodulating a simple AM broadcast may also be able to decode an HDTV broadcast in the next. An open-source project called GNU Radio is dedicated to evolving a high-performance SDR.
   Despite the standardization there are four tasks before the JTAG device interface can be used to recover the memory. To find the correct bits in the boundary scan register one must know which processor and memory circuits are used and how they are connected to the system bus. When not accessible from outside one must find the test points for the JTAG interface on the printed circuit board and determine which test point is used for which signal. The JTAG port is not always soldered with connectors, such that it is sometimes necessary to open the device and re-solder the access port. The protocol for reading the memory must be known and finally the correct voltage must be determined to prevent damage to the circuit.
   One modulating retro-reflector at the Naval Research Laboratory (NRL) in the United States uses a semiconductor based MQW shutter capable of modulation rates up to 10 Mbit/s, depending on link characteristics. (See "Modulating Retro-reflector Using Multiple Quantum Well Technology", U.S. Patent No. 6,154,299, awarded November, 2000.)
   At the receiving end, the CRC of each submultiframe is calculated locally and compared to the CRC value received in the next submultiframe. If these do not coincide, one or more bit errors is determined to have been found in the block, and an alarm is sent back to the transmitter, indicating that the block received at the far end contains errors.
  
  Section::::E1 frame structure.:Special timeslots.:The CRC-4 procedure.:CRC-4 multiframe alignment.
   BULLET::::- Direct digital synthesis
  
  Section::::Determining the frequency.:Frequency multiplication.
   At the receiver, the oscillating electric and magnetic fields of the incoming radio wave push the electrons in the receiving antenna back and forth, creating a tiny oscillating voltage which is a weaker replica of the current in the transmitting antenna. This voltage is applied to the radio receiver, which extracts the information signal. The receiver first uses a bandpass filter to separate the desired radio station's radio signal from all the other radio signals picked up by the antenna, then amplifies the signal so it is stronger, then finally extracts the information-bearing modulation signal in a demodulator. The recovered signal is sent to a loudspeaker or earphone to produce sound, or a television display screen to produce a visible image, or other devices. A digital data signal is applied to a computer or microprocessor, which interacts with a human user.
   Once the probes are connected, the user programs the analyzer with the names of each signal, and can group several signals together for easier manipulation. Next, a capture mode is chosen, either "timing" mode, where the input signals are sampled at regular intervals based on an internal or external clock source, or "state" mode, where one or more of the signals are defined as "clocks", and data are taken on the rising or falling edges of these clocks, optionally using other signals to qualify these clocks.
   Different from emitter location in SIGINT, frequency analysis MASINT concentrates not on finding a specific device, but on characterizing the signatures of a class of devices, based on their intentional and unintentional radio emissions. Devices being characterized could include radars, communication radios, radio signals from foreign remote sensors, radio frequency weapons (RFW), collateral signals from other weapons, weapon precursors, or weapon simulators (for example, electromagnetic pulse signals associated with nuclear bursts); and spurious or unintentional signals.
   If "m"("t") is the original message, the AM signal can be shown to be 
  
  Multiplying the AM signal by the new set of frequencies yields
  
  After filtering out the component based around cos(4ω"t") and the DC component C, the original message will be recovered.
  
  Section::::A more sophisticated product detector.
  