Antenna diversity

Also known as: spatial diversity, receive diversity, diversity reception

Antenna diversity uses two or more antennas that receive independently faded copies of the same signal and combines or selects among them to counter multipath fading, improving link reliability without any increase in transmit power.¹ The premise is statistical: fades are localized, so when one antenna sits in a deep null another — spaced far enough apart to fade independently — is likely in a strong spot. Taking the better copy, or a weighted sum, makes a simultaneous deep fade on all branches rare, which is what raises reliability.

Two independently faded branches feed a combiner; selecting the stronger branch or summing them in phase makes a simultaneous deep fade on both unlikely.

How it works

The branches must be decorrelated so their fades are independent. That is achieved by spacing antennas apart (spatial diversity — roughly half a wavelength or more indoors, farther outdoors), by using orthogonal polarizations, or by different radiation patterns (pattern diversity). Given decorrelated branches, several combining rules trade complexity against gain:

Selection combining picks the single branch with the highest instantaneous signal (or SNR). One receiver chain suffices if a switch samples both; cheap and effective against dropouts.
Switched diversity stays on the current branch until it fades below a threshold, then switches — even cheaper, with a small penalty.
Equal-gain combining (EGC) co-phases the branches and adds them with equal weight, needing a receiver per branch but no amplitude weighting.
Maximal-ratio combining (MRC) co-phases the branches and weights each by its own SNR before summing. It is optimal for additive noise: the output SNR is the sum of the branch SNRs, so two equal branches gain 3 dB even with no fade, and much more when one branch is deep in a fade.

The theoretical benefit is measured as diversity order — with N independent branches the probability of a deep fade drops roughly as the Nth power of a single branch’s fade probability, which is why even two antennas dramatically cut dropout rates in a Rayleigh-fading channel.

Variants

Diversity is not only spatial. The same “combine independently faded copies” idea applies across any dimension where fades decorrelate:

Spatial diversity — separated antennas, as above; the most common form.
Polarization diversity — one antenna per orthogonal polarization, useful where physical separation is impractical (a compact handset) because a multipath channel scrambles polarization.
Frequency diversity — sending the same information on frequencies spaced beyond the channel’s coherence bandwidth, so a frequency-selective fade cannot hit both. Frequency hopping and wideband spreading achieve this implicitly.
Time diversity — repeating or interleaving data across time beyond the coherence time, so a temporal fade only damages part of a codeword; interleaving with FEC is the everyday example.

The branches need not be equally strong for combining to pay off — that is MRC’s strength, since a weak branch still contributes its share of the summed SNR rather than being discarded.

In practice

The engineering questions are how far apart to place antennas and how many to use. Spacing must exceed the channel’s spatial coherence: roughly half a wavelength suffices in a rich-multipath indoor environment, but a line-of-sight outdoor link with little scattering may need several wavelengths to decorrelate the branches, because correlated branches give little diversity gain no matter how they are combined. Returns also diminish with count — the jump from one to two antennas removes most deep fades, while each further antenna adds progressively less, so two- and four-branch systems dominate in practice. Cost and complexity scale with the method: selection needs the least hardware, MRC the most (a full coherent receiver per branch plus channel estimation), and real systems pick the point on that curve that their power and size budget allows.

Relevance to SDR

Antenna diversity is everywhere in modern wireless: Wi-Fi access points, cellular handsets and base stations, DECT phones, and vehicular receivers all use it, and it is the receive-side foundation that MIMO generalizes into spatial multiplexing. In land-mobile radio, base-station voting receivers are a form of selection diversity across geographically separate sites. For an SDR listener, diversity requires multiple coherent (or at least independently sampled) front ends; a phase-coherent array such as KrakenSDR can implement MRC in software. GopherTrunk is a single-stream decoder — it processes one I/Q capture at a time and does not combine multiple antennas — so diversity is relevant to the broader RF context and to hardware choices rather than something GT itself performs.

Sources

Antenna diversity — Wikipedia, on spatial diversity and its role against multipath fading. ↩