A central feature of OFDM is that one wider frequency band is divided into multiple narrow subchannels; each subchannel then carries a proportional fraction of the total information signal, modulated onto a subchannel-specific carrier. All the subchannels can be allocated to one transmission at a time (time-division multiplexing, 6.2 Time-Division Multiplexing), or disjoint sets of subchannels can be allocated to different transmissions that can then proceed (at proportionally lower data rates) in parallel. The latter is known as frequency-division multiplexing.
In many settings OFDM comes reasonably close to the Shannon-Hartley limit. Perhaps more importantly, OFDM also performs reasonably well with multipath interference, below, which is endemic in urban and building-interior environments with their many reflective surfaces. Multipath interference is, however, not necessarily comparable to the Gaussian noise assumed by the Shannon-Hartley theorem. We will not address further technical details of OFDM here, except to note that implementation usually requires some form of digital signal processing.
WiFi Signal 4.1.3
The picture above shows two transmission paths from A to B. The respective carrier paths may interfere with or supplement one another. The longer delay of the reflecting path (red) wil also delay its encoded signal. The result, shown at right, is that the line-of-sight and reflected data symbols may overlap and interfere with each other; this is known as intersymbol interference. Multipath interference may even change the meaning of the data symbol as seen by the receiver; for example, the red and black low data-signal peaks above at the point of the vertical dashed line may sum together so as to be received as a higher peak (assuming the underlying carriers are in sync).
The picture above is from a mathematical simulation intended to illustrate multipath fading. The walls of the room reflect 40% of the signal from the transmitter located in the orange ball at the lower left. The transmitter transmits an unmodulated carrier signal, which may be reflected off the walls any number of times; at any point in the room the total signal intensity is the sum over all possible reflection paths. On the right-hand side, the small-scale blue ripples represent the received carrier strength variation due to multipath interference between the line-of-sight and all the reflected paths. Note that the ripple size is about half a wavelength.
In comparison to this simulated intensity map, real walls tend to have a lower reflectivity, real rooms are not two-dimensional, and real carriers are modulated. However, real rooms also introduce scattering, diffraction and shadowing from objects within, and significant (3 to 10) multipath-fading signal-strength variations are common in actual wireless settings.
Generally, multipath interference is a problem that engineers go to great lengths to overcome. However, as we shall see in 4.2.3 Multiple Spatial Streams, multipath interference can sometimes be put to positive use by allowing almost-adjacent antennas to transmit and receive independent signals, thus increasing the effective throughput.
The channel width is increased by adding additional 5 MHz channels. For example, the 65 Mbps bit rate above for 802.11n is for a nominal frequency range of 20 MHz, comparable to that of 802.11g. However, in areas with minimal competition from other signals, 802.11n supports using a 40 MHz frequency band; the bit rate then goes up to 135 Mbps (or 150 Mbps if a smaller guard interval is used). This amounts to using two of the three available 2.4 GHz Wi-Fi bands. Similarly, the wide range in 802.11ac bit rates reflects support for using channel widths ranging from 20 MHz up to 160 MHz (32 5-MHz official channels).
We looked extensively at the 10 Mbps Ethernet collision-handling mechanisms in 2.1 10-Mbps Classic Ethernet, only to conclude that with switches and full-duplex links, Ethernet collisions are rapidly becoming a thing of the past. Wi-Fi, however, has brought collisions back from obscurity. An Ethernet sender will discover a collision, if one occurs, during the first slot time, by monitoring for faint interference with its own transmission. However, as mentioned in 4.1.2 Collisions, Wi-Fi transmitting stations simply cannot detect collisions in progress. If another station transmits at the same time, a Wi-Fi sender will see nothing amiss although its signal will not be received. While there is a largely-collision-free mode for Wi-Fi operation (4.2.7 Wi-Fi Polling Mode), it is not commonly used, and collision management has a significant impact on ordinary Wi-Fi performance.
Finally, we note that, unlike Ethernet collisions, Wi-Fi collisions are a local phenomenon: if A and B transmit simultaneously, a collision occurs at node C only if the signals of A and B are both strong enough at C to interfere with one another. It is possible that a collision occurs at station C midway between A and B, but not at station D that is close to A. We return to this below in 4.2.1.4 Hidden-Node Problem.
A variety of newer rate-scaling algorithms have been proposed; see [JB05] for a summary. One, Receiver-Based Auto Rate (RBAR, [HVB01]), attempts to incorporate the signal-to-noise ratio into the calculation of the transmission rate. This avoids the confusion introduced by collisions. Unfortunately, while the signal-to-noise ratio has a strong theoretical correlation with the transmission bit-error rate, most Wi-Fi radios will report to the host system the received signal strength. This is not the same as the signal-to-noise ratio, which is harder to measure. As a result, the RBAR approach has not been quite as effective in practice as might be hoped.
Similarly, in simple MISO, the transmitter picks whichever of its antennas that gets a stronger signal to the receiver. The receiver is unlikely to be in a dead zone for both transmitter antennas. Note that for MISO the sender must get some feedback from the receiver to know which antenna to use.
We can do quite a bit better if signal-processing techniques are utilized so the two sender or two receiver antennas can be used simultaneously (though this complicates the mathematics considerably). Such signal-processing is standard in 802.11n and above; the Wi-Fi header, to assist this process, includes added management packets and fields for reporting MIMO-related information. One station may, for example, send the other a sequence of training symbols for discerning the response of the antenna system.
As a specific example, consider the diagram above, with two sending antennas A1 and A2 at the left and two receiving antennas B1 and B2 at the right. Antenna A1 transmits signal S1 and A2 transmits S2. There are thus four physical signal paths: A1-to-B1, A1-to-B2, A2-to-B1 and A2-to-B2. If we assume that the signal along the A1-to-B2 path (dashed and blue) arrives with half the strength of the other three paths (solid and black), then we have
The antennas are each more-or-less omnidirectional; the signal-strength variations come from multipath interference and not from physical aiming. Similarly, while the diagonal paths A1-to-B2 and A2-to-B1 are slightly longer than the horizontal paths A1-to-B1 and A2-to-B2, the difference is not nearly enough to allow B to solve for the two signals.
After the mode and channel are set, Wireshark will report the 802.11 management-frame headers, and also the so-called radiotap header containing information about the transmission data rate, channel, and received signal strength.
6.0. In this exercise we outline the two-ray ground model of wireless transmission in which the signal power is inversely proportional to the fourth power of the distance, rather than following the usual inverse-square law. Some familiarity with trigonometric (or complex-exponential) manipulations is necessary.
Sender and receiver are shown at equal heights above the ground, for simplicity. We assume 100% ground reflectivity (this is reasonable for very shallow angles). The phase of the ground signal is reversed 180 by the reflection, and then is delayed slightly more by the slightly longer path.
Kindle device software update is the most important task to do otherwise when you try to download a book my kindle won't connect to wifi error occurs so first download the latest version of the software on your PC then transfer that file from PC to kindle via USB then install the file also to know more about it read this article kindle wont connect to wifi
The uncalibrated daughterboards have very serious signal distortion! Users should follow Device Calibration to perform the self-calibrations for EACH daughterboard. Pursuing the best signal quality, the frequency range of the calibration should cover the range of your measurement.
A session SHALL NOT be extended past the guidelines in Sections 4.1.3, 4.2.3, and 4.3.3 (depending on AAL) based on presentation of the session secret alone. Prior to session expiration, the reauthentication time limit SHALL be extended by prompting the subscriber for the authentication factor(s) specified in Table 7-1.
This pinout reference card comes with Teensy 4.1. Pinout Card Files:Front Side (PDF) /Back Side (PDF)Cards printed before September 2021 incorrectly showed pin 53 with PWM.A larger, more detailed pinout chart by KurtE is also available on the forum.Digital PinsDigital Input PinsDigital pinsmay be used to receive signals. Teensy 4.1 pins default toINPUT most with a "keeper" resistor.Teensy 4.1 pins accept 0 to 3.3Vsignals. The pins are not 5V tolerant. Do not drive any digital pin higherthan 3.3V.Input Pullup / Pulldown / Keeper ResistorsAll digital pins have optional pullup, pulldown, or keeper resistors.These are used to keep the pin at logic HIGH or logic LOW or the samelogic level when it is not beingactively driven by external circuity. Normally these resistors are usedwith pushbuttons & switches.The pinMode function with INPUT_PULLUP or INPUT_PULLDOWN must be used toconfigure these pins to input mode with the built-in resistor.Pin Change InterruptsAll digital pins can detect changes. Use attachInterrupt to causea function to be run automatically. Interrupts should only be used forclean signals. TheBounce libraryis recommended for detecting changeson pushbuttons, switches, and signals with noise or mechanical chatter.Digital Output PinsAll digital pins can act at output. The pinMode function withOUTPUT or OUTPUT_OPENDRAIN must be used to configure these pins to outputmode. The digitalWrite and digitalToggle functions are used to controlthe pin while in output mode. Output HIGH is 3.3V. The recommendedmaximum output current is 4mA.Pulse Width Modulation (PWM)35 of the digital pins supportPulse Width Modulation (PWM),which can be used to controlmotor speed, dim lights & LEDs, or other uses where rapid pulsingcan control average power. PWM is controlled by the analogWrite function.22 groups of PWM can have distinct frequencies, controlled by theanalogWriteFrequency function.Slew Rate LimitingThis optional feature greatly reduces high frequency noise whenlong wires are connected to digital output pins. The rate of voltagechange on the pin is slowed. The extra time is only nanoseconds, whichis enough to lower undesirable high frequency effects which can causetrouble with long wires.Variable Drive StrengthThe output impedance of each digital output may be controlled in7 steps, ranging from 150 ohms (weakest) up to about 21 ohms (strongest).Adjustable Output BandwidthDigital output bandwidth is also programmable, in 4 steps: 50, 100,150 and 200 MHz.LED PinPin 13 has an orange LED connected. The LED can be veryconvenient to show status info. When pin 13 is used as an input,the external signal must be able to drive the LED when logic HIGH.pinMode INPUT_PULLUP should not be used with pin 13.Analog PinsAnalog Inputs18 pins can be used an analog inputs, for reading sensors or otheranalog signals. Basic analog input is done with the analogRead function.The default resolution is 10 bits (input range 0 to 1023), but can beadjusted with analogReadResolution. The hardware allows up to 12 bitsof resolution, but in practice only up to 10 bits are normally usabledue to noise.More advanced use is possible with the ADC library.Analog RangeThe analog input range is fixed at 0 to 3.3V. On Teensy 4.1,the analogReference() function has no effect. The analog pinsare not 5V tolerant. Do not drive any analog pin higher than 3.3 volts.Analog ComparatorsThese comparators allow an analog signal to be converted todigital, with a precisely defined voltage threshold for logiclow versus high.CommunicationTools > USB Type menu configures the type of USB device Teensy will implement. 2ff7e9595c
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