An Explanation of Diversity Reception Techniques for the...

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An Explanation of Diversity Reception Techniques for the...

An Explanation of Diversity Reception Techniques for the FLEX-5000

  System Dependencies
  Minimum PowerSDR Version:     1.14.0 
  Applicable Hardware:     FLEX-5000 

Content provided by: FlexRadio Systems Engineering

Diversity reception is a set of techniques using multiple receivers connected to separate antennas for improving the intelligibility of received signals by improving the signal to noise ratio and mitigating signal fading and drop outs.  Below are descriptions of several diversity reception techniques that can be employed using the FLEX-5000 and the RX2 (second independent receiver).

These techniques can be used in combination for increasing levels of signal reception enhancement.  Below are three different combinations of diversity reception techniques that are possible with the FLEX-5000 and the RX2.
  • Stereo or Dual Diversity Reception - Good
  • Stereo/Dual Diversity + Antenna or Space Diversity Reception - Better
  • Stereo/Dual Diversity + Antenna/Space Diversity + Polarization Diversity Reception - BEST!

Stereo or Dual (Receiver) Diversity Reception
To accomplish stereo or dual diversity reception, it requires two receivers that are frequency and phased locked on the same signal outputting the demodulated signal to two discrete audio channels.  Optimally both receivers need to be virtually identical in hardware and configuration (demodulation mode, filter characteristics, preamp, AGC setting, etc) in order to create a sense of "stereo spatial depth" for the listener.  Humans are very good at localizing sounds and quickly estimating their direction and distance because we have two ears that allow for multi-path reception of audio frequencies. The end result is by being able to hear stereophonically, our brains can use this information to extract intelligibility from weak and noisy signals.

When experimenting with diversity reception, establishing the ability to achieve stereo or dual diversity is the prerequisite for being able to utilize all of the other diversity reception techniques listed in this article.

Stereo or dual diversity is easily accomplished with the FLEX-5000 with the optional RX2 (second independent receiver). The RX2 is identical to the primary receiver (RX1) and using SDR software their local oscillators can be frequency and phase locked to enable stereo or dual diversity reception.

Space or Antenna Diversity Reception

Space or antenna diversity is accomplished by simultaneously receiving on two or more antennas separated in distance by several wavelengths of the received signal.  Even though it mat be difficult to have two antennas separated in distance by multiple wavelengths on the longer wavelength frequencies (hundreds of meters), good results can be achieved when the antennas are separated in distances as small as a quarter wavelength (1/2 λ).  This diversity reception techniques is used to mitigate the effects of ionospheric multi-path fading, shadowing and signal drop-outs (path loss) which serves to enhance the overall signal-to-noise ratio and readability of received signals.

A transmitted radio signal can be reflected along multiple paths before finally being received.  Each of these "bounces" or reflections can introduce phase shifts, time delays and signal strength attenuation that can destructively interfere with one another at the receiving antenna.  Antenna or space diversity is especially effective at mitigating these multi-path situations.  This is due in part because multiple antennas afford a receiver multiple reception points in 3-D space of the same signal.  Each antenna will experience a different interference environment.  Thus, if one antenna is experiencing a deep fade, it is likely that another has a sufficient signal for adequate reception.  It should be noted that unidirectional antennas are best suited for this purpose since directional antennas receive only a fraction of all scattered RF energy.

Polarization Diversity Reception
As noted with stereo or antenna diversity, signals reaching the multiple receiving antennas will not necessarily be in-phase with one another, therefor they will have different degrees of  polarization from vertical to horizontal.  It is well known that receiving vertically polarized signals with a horizontally polarized antenna (and visa versa) will result in a lower signal strength. In a linearly polarized system, a misalignment of polarization of 45 degrees will degrade the signal up to 3 dB and if misaligned 90 degrees the attenuation can be 20 dB or more!  This is a significant amount of signal loss.

You can minimize these polarization losses by using two antennas that are of different polarization.  Therefore  polarization diversity is the technique of simultaneously receiving on horizontally and vertically polarized antennas.  If one of your antennas is a vertical loop (fed to have vertical polarization) or a multi-band vertical antenna and the other one a dipole, vertical loop (fed for horizontal polarization), horizontal loop or a broad beam width yagi it will increases the statistical probability that a selective fade will only affect one, but not both receiver systems.

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Last Modified:Monday, November 29, 2010
Last Modified By: Administrator
Type: INFO
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