The most common techniques for combining diversity signals are selection, feedback, maximal ratio, and equal gain.

Selection combining used in spatial diversity systems involves the sampling of MM size 12{M} {} antenna signals, and sending the largest one to the demodulator. Selection-diversity combining is relatively easy to implement but not optimal because it does not make use of all the received signals simultaneously.

With feedback or scanning diversity, the MM size 12{M} {} signals are scanned in a fixed sequence until one is found that exceeds a given threshold. This one becomes the chosen signal until it falls below the established threshold, and the scanning process starts again. The error performance of this technique is somewhat inferior to the other methods, but feedback is quite simple to implement.

In maximal-ratio combining, the signals from all of the MM size 12{M} {} branches are weighted according to their individual SNRs and then summed. The individual signals must be cophased before being summed.

Maximal-ratio combining produces an average SNR γM¯γM¯ size 12{ {overline {γ rSub { size 8{M} } }} } {} equal to the sum of the individual average SNRs, as shown below:

γ M ¯ = ∑ i = 1 M γ i ¯ = ∑ i = 1 M Γ = MΓ γ M ¯ = ∑ i = 1 M γ i ¯ = ∑ i = 1 M Γ = MΓ size 12{ {overline {γ rSub { size 8{M} } }} = Sum cSub { size 8{i=1} } cSup { size 8{M} } { {overline {γ rSub { size 8{i} } }} } = Sum cSub { size 8{i=1} } cSup { size 8{M} } {Γ} =MΓ} {}

where we assume that each branch has the same average SNR given by γi¯=Γγi¯=Γ size 12{ {overline {γ rSub { size 8{i} } }} =Γ} {}.

Thus, maximal-ratio combining can produce an acceptable average SNR, even when none of the individual i γγ size 12{γ} {} is acceptable. It uses each of the MM size 12{M} {} branches in a cophased and weighted manner such that the largest possible SNR is available at the receiver.

Equal-gain combining is similar to maximal-ratio combining except that the weights are all set to unity. The possibility of achieving an acceptable output SNR from a number of unacceptable inputs is still retained. The performance is marginally inferior to maximal ratio combining.

Content actions

Add module to:

My Favorites Login Required (?)

'My Favorites' is a special kind of lens which you can use to bookmark modules and collections. 'My Favorites' can only be seen by you, and collections saved in 'My Favorites' can remember the last module you were on. You need an account to use 'My Favorites'.

| A lens I own Login Required (?)

Definition of a lens

Lenses

A lens is a custom view of the content in the repository. You can think of it as a fancy kind of list that will let you see content through the eyes of organizations and people you trust.

What is in a lens?

Lens makers point to materials (modules and collections), creating a guide that includes their own comments and descriptive tags about the content.

Who can create a lens?

Any individual member, a community, or a respected organization.

What are tags?

Tags are descriptors added by lens makers to help label content, attaching a vocabulary that is meaningful in the context of the lens.

| External bookmarks

Footer

This work is licensed by Sinh Nguyen-Le and Tuan Do-Hong under a Creative Commons Attribution License (CC-BY 2.0), and is an Open Educational Resource.

Last edited by Tuan Do-Hong on Nov 15, 2007 7:21 am -0600.