Antenna diversity uses two or more antennas that fade independently, so that a deep fade on one is unlikely to occur on the other. Receive diversity places the antennas at the receiver and combines or selects their signals, while transmit diversity places them at the transmitter and spreads the signal across them. Both make a link more reliable without raising the data rate.
This page explains transmit and receive diversity in practical terms: how much fade margin they save, and how to compare them. The wider setting, including array gain and capacity, is in MIMO and multiple antenna systems.
What is a diversity antenna?
A diversity antenna is one of a set of antennas arranged to receive or transmit independent versions of the same signal. The aim is not more gain, as in a directional antenna, but a lower chance that all paths fail at once. A Wi-Fi router, a car radio with two antennas and a wireless microphone receiver all use this idea.
Multipath is the reason it works. Reflected waves add with different phases, and at some positions they nearly cancel. These fades are often only a fraction of a wavelength wide, so moving a short distance can restore the signal. Two antennas that see different multipath patterns will rarely fade at the same moment, as Figure 1 shows.
Diversity in wireless communication: the main types
Diversity in wireless communication means sending information through more than one independent fading path. Those paths can be in space, frequency, time or polarisation.
| Type | Independent paths from | Rule of thumb | Cost |
|---|---|---|---|
| Spatial diversity (space diversity) | Separated antennas | About half a wavelength or more, with scattering all around | Extra antennas and chains |
| Polarisation diversity | Orthogonal polarisations | Co-located antennas | Needs depolarising scatter |
| Frequency diversity | Separated carriers or subcarriers | Spacing above the coherence bandwidth | Extra bandwidth |
| Time diversity | Repeats, or coding with interleaving | Spacing above the coherence time | Latency |
| Pattern diversity | Different antenna patterns | Differing arrival angles | Antenna design |
OFDM systems get frequency diversity through coding across subcarriers. Interleaving provides time diversity, but it does little for a stationary user because the channel stays still.
Diversity gain and diversity order
Diversity order L is the number of independent fading branches. At high SNR, error rate falls as SNR-L, so each 10 dB of SNR gives L decades of improvement. Diversity gain is the SNR saved at a target error rate compared with no diversity. Unlike array gain, it is not fixed: the lower the target error rate, the bigger the saving.
Receive diversity
Receive diversity processes the signals from several antennas at the receiver. There are three standard methods, all of which need no transmit CSI.
- Selection combining picks the branch with the best SNR. With a fast RF switch it needs only one receive chain.
- Equal gain combining aligns the phases and adds the branches with equal weights.
- Maximal ratio combining (MRC) aligns the phases and weights each branch by its amplitude. The output SNR is the sum of the branch SNRs, which is the best possible with independent noise.
Antenna diversity vs true diversity
These terms are common on wireless microphones and similar receivers. Antenna (switched) diversity uses two antennas and one receiver, and it switches when the signal drops. True diversity uses two complete receivers and picks the better output. True diversity can act before the audio or data suffers, at the price of a second receive chain.
Transmit diversity
Transmit diversity protects a link when the receiver has only one antenna, as is common on small devices. The transmitter cannot simply send the same signal from two antennas, because the two would interfere and could cancel at the receiver, which is the effect that beamforming controls on purpose. The signals need to be coded or steered.
Open-loop transmit diversity needs no CSI. The classic example is the Alamouti code, from the 1998 paper “A simple transmit diversity technique for wireless communications”. Two symbols s1, s2 are sent from antennas 1 and 2, then -s2*, s1* in the next symbol period. The receiver’s linear combination separates them completely. This gives diversity order 2 with a single receive antenna, at one symbol per period.
Closed-loop transmit diversity uses channel state information to pick or phase the antennas. With full CSI it matches MRC at the receiver, as the MISO comparison shows.
Worked example: BPSK in Rayleigh fading
The bit error rate of BPSK with L-branch MRC in Rayleigh fading has a closed form. With mean Eb/N0 of γ per branch and μ = √(γ/(1+γ)):
Pb = [(1-μ)/2]L Σk=0L-1 C(L-1+k, k) [(1+μ)/2]k
Alamouti with two transmit antennas uses the same formula with L = 2 and γ/2, because the same total power is shared by two antennas. The table gives the Eb/N0 needed at each receive antenna.
| Scheme | For BER 10-3 | For BER 10-5 | Diversity gain at 10-3 | Diversity gain at 10-5 |
|---|---|---|---|---|
| No diversity | 24.0 dB | 44.0 dB | None | None |
| Transmit diversity, Alamouti 2×1 | 14.1 dB | 24.3 dB | 9.9 dB | 19.6 dB |
| Receive diversity, 2-branch MRC | 11.1 dB | 21.3 dB | 12.9 dB | 22.6 dB |
| Receive diversity, 4-branch MRC | 4.0 dB | 10.0 dB | 19.9 dB | 34.0 dB |
A simulation of 4 million BPSK symbols at 15 dB agrees with the formula: 1.74 × 10-4 against 1.78 × 10-4 for 2-branch MRC, and 6.68 × 10-4 against 6.77 × 10-4 for Alamouti.
Transmit and receive diversity have the same order here, and the curves are parallel. The 3.01 dB between them is the array gain that receive combining has and open-loop transmit diversity lacks.
Fade margin with antenna diversity
Link budgets use fade margin: the mean SNR needed above the threshold for a chosen outage. In Rayleigh fading, a single antenna needs 20.0 dB for 1% outage. Diversity reduces this sharply, and this saving goes straight into the link budget.
| Scheme | Fade margin for 1% outage | Saving |
|---|---|---|
| Single antenna | 20.0 dB | None |
| Transmit diversity, Alamouti 2×1 | 11.3 dB | 8.7 dB |
| 2-branch selection | 9.8 dB | 10.2 dB |
| 2-branch MRC | 8.3 dB | 11.7 dB |
| 4-branch selection | 4.2 dB | 15.8 dB |
| 4-branch MRC | 0.8 dB | 19.1 dB |
The first extra antenna gives the largest step. Note also how close selection is to MRC at two branches, at 1.5 dB apart, which explains why a simple switch is often enough on low-cost devices.
Antenna spacing and correlation
The branches must fade independently for diversity to work fully, and correlated branches lose part of the gain. For 2-branch MRC at 1% outage, using the envelope correlation between branches:
| Envelope correlation | Fade margin | Loss against uncorrelated |
|---|---|---|
| 0 | 8.3 dB | None |
| 0.25 | 8.9 dB | 0.6 dB |
| 0.5 | 9.7 dB | 1.4 dB |
| 0.7 | 10.7 dB | 2.4 dB |
| 0.9 | 12.8 dB | 4.5 dB |
Diversity tolerates moderate correlation well, and an envelope correlation of 0.5 is a common design limit. Half a wavelength is 167 mm at 900 MHz, 62 mm at 2.4 GHz and 26 mm at 5.8 GHz. Base stations need far greater spacing, often several wavelengths or more, because the scatterers around them lie in a narrow range of angles. On a small device with less space, polarisation or pattern differences do the same job. Antenna parameters covers the terms used for those antennas.
Spatial diversity and spatial multiplexing
Spatial diversity and spatial multiplexing use the same antennas for opposite goals. Diversity sends one stream through many paths to raise reliability, while multiplexing sends several streams through separate paths to raise rate. A channel with L independent paths cannot give the maximum of both at once, and real systems trade one against the other through rank adaptation. Spatial multiplexing covers the rate side in detail. A MIMO link can be used in either mode, and at low SNR or high mobility, diversity is usually the better choice.
Frequently asked questions
What is diversity in an antenna system?
It is the use of two or more antennas with independent fading, so that the receiver or transmitter can rely on whichever signals are strong. It improves reliability rather than peak gain. In Rayleigh fading, two receive antennas with MRC cut the fade margin at 1% outage from 20.0 dB to 8.3 dB.
What is the difference between transmit diversity and receive diversity?
Receive diversity uses several antennas at the receiver, and transmit diversity uses several at the transmitter. They can have the same diversity order, but open-loop transmit diversity is 3 dB worse, since it does not have the array gain. Transmit diversity is useful when the receiver cannot fit more antennas.
What is space diversity?
Space diversity, also called spatial diversity, uses antennas placed far enough apart to fade independently. Microwave links use it with antennas separated vertically on a tower. Handsets and routers use it with antennas a fraction of a wavelength to a few wavelengths apart.
Does antenna diversity increase speed?
Not directly, because it sends one stream. It does raise average SNR and cut deep fades, which lets rate adaptation hold a higher modulation order for longer. For a true speed increase through parallel streams, the link needs spatial multiplexing.
Related topics
See MIMO and multiple antenna systems for array gain and capacity, SIMO vs MIMO for how receive diversity compares with extra streams, and Antennas and Propagation for Radio Design for the channel. The antennas hub has more on antenna design.