Diverstiy (and multipath distortion) explained.

  • This is a somewhat complicated topic, but I'll try to keep it understandable.

    In order to understand diversity one first needs to understand multipath distortion, so here goes…

    Multipath distortion:

    When a radio frequency (RF) signal is transmitted towards the receiver, the general behavior of the RF signal is to grow wider as it is transmitted further. On its way, the RF signal encounters objects that reflect, refract, diffract or interfere with the signal.
    When an RF signal is reflected off an object, multiple wavefronts are created. As a result of these new duplicate wavefronts, there are multiple wavefronts that reach the receiver.

    Multipath propagation occurs when RF signals take different paths from a source to a destination. One part of the signal goes to the destination while another part bounces off an obstruction, then goes on to the destination. As a result, part of the signal encounters delay and travels a longer path to the destination. Meaning that the receiver sees the same signal twice, but slightly delayed in time.

    Multipath can be defined as the combination of the original signal plus the duplicate wave fronts that result from reflection of the waves off obstacles between the transmitter and the receiver.

    Multipath distortion is a form of RF interference that occurs when a radio signal has more than one path between the receiver and the transmitter. This occurs in cells with metallic or other RF-reflective surfaces, such as furniture, walls, or coated glass.

    Effects of multipath distortion include:

    • Data Corruption—Occurs when multipath is so severe that the receiver is unable to detect the transmitted information.

    • Signal Nulling—Occurs when the reflected waves arrive exactly out of phase with the main signal and cancel the main signal completely.

    • Increased Signal Amplitude—Occurs when the reflected waves arrive in phase with the main signal and add on to the main signal thereby increasing the signal strength.

    • Decreased Signal Amplitude—Occurs when the reflected waves arrive out of phase to some extent with the main signal thereby reducing the signal amplitude.

    A source antenna radiates RF energy in more than one definite direction.
    The RF moves between the source and destination antenna in the most direct path and bounces off RF-reflective surfaces.
    The reflected RF waves normally cause some/all of these conditions to occur:

    • The reflected RF waves travel farther and arrive later in time than the direct RF wave.

    • The reflected signal loses more RF energy than the direct route signal, because of the longer transmission route.

    • The signal loses energy as a result of the reflection.

    • The desired wave is combined with many reflected waves in the receiver.

    • When the different waveforms combine, they cause distortion of the desired waveform and affect the decoding capability of the receiver. When the reflected signals are combined at the receiver, even though the signal strength is high, the signal quality is poor.

    • The reflected wave is also positionally different from the unreflected wave.

    So what happens?

    Multipath delay causes the information symbols represented in 802.11 signals to overlap, which confuses the receiver.
    If the delays are great enough, bit errors in the packet occur.
    The receiver cannot distinguish the symbols and interpret the corresponding bits correctly.
    The destination station detects the problem through the error-checking process of 802.11.
    The cyclic redundancy check (CRC, the checksum) does not compute correctly, which indicates that there is an error in the packet.
    In response to the bit errors, the destination station does not send an 802.11 acknowledgment to the source station.
    The sender eventually retransmits the signal after it regains access to the medium.
    Because of the retransmissions, users encounter lower throughput when multipath interference is significant.
    If the location of the antenna is changed, the reflections are also changed, which diminishes the chance and effects of multipath interference.

    In a multipath environment, signal null points are located throughout the area.
    The distance an RF wave travels, how it bounces, and where the multipath null occurs are based on the wavelength of the frequency.
    As frequency changes, so does the length of the wave.
    Therefore, as frequency changes, so does the location of the multipath null.
    The length of the 2.4 GHz wave is approximately 4.92 inches (12.5 cm). The length of the 5 GHz wave is approximately 2.36 inches (6 cm).

    Now let's talk diversity:

    Diversity is the use of two antennas for each radio, to increase the odds that you receive a better signal on either of the antennas.
    The antennas used to provide a diversity solution must be two separate but equal antennas in the same location.
    Diversity provides relief to a wireless network in a multipath scenario.
    Diversity antennas are physically separated from the radio and each other by the diversity switch (think a relay enabling either one or the other), to ensure that one encounters less multipath propagation effects than the other.
    Dual antennas typically ensure that if one antenna is in an RF null then the other is not, which provides better performance in multipath environments.
    You can move the antenna to get it out of the null point and provide a way to receive the signal correctly.

    Diversity essentially creates robustness where there is multipath distortion.

    Diversity antennas are not designed to extend the coverage range of a radio cell, but to enhance the coverage of a cell.
    The enhanced coverage is an effort to overcome issues that arise from multipath distortion and signal nulls.

    Attempts to use the two antennas on an access point to cover two different radio cells will most of the time result in connectivity issues as it is not designed for using two antennas covering two different coverage cells.
    The problem in using it this way is that, if antenna number 1 communicates to device number 1 while device number 2 (which is in the antenna number 2 cell) tries to communicate, antenna number 2 is not connected (due to the position of the diversity switch), and the communication fails. Diversity antennas should cover the same area from only a slightly different location.

    Recommended distance between center of antennas is 12,5cm or aprox. 5 inces for both 2.4 ang 5.xGhz (alternatively 6cm or 2.3 inches for 5.x GHz operation only). Multiples of these distances are also ok, but if the distance is too large (normally you should keep it under 4*wavelength), you might encounter problems with one of the antennas beeing completely out of reach. Thus resulting in loss of connectivity when the diversity switch enables the "out of reach" antenna.

  • This is brilliant, very useful information. A point that I found particularly useful was about diversity, which others new to the wireless aspects of pfsense might easily be wondering also - A wireless card might have two antennas but each antenna cannot be used at the same time either transmitting or receiving (effectively doubling bandwidth contrary to one antenna) - it doesn't work like that; the two antennas are used separately and purely to create robustness where there is multipath distortion. In single antenna scenarios one should disable diversity and set the tx and rx antennas, available under the wireless configuration pages under the interface. I know I repeat what you posted (thanks again for your help in my previous post, its still working all good!) I repeat it incase others arrive at this page if searching for related issues!

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