Link Engineering

Cross-Band Diversity

Q: How much band separation is needed for the fading to be uncorrelated?

For multipath fading the fades on two carriers decorrelate once the separation exceeds the coherence bandwidth of the channel, which is roughly the inverse of the delay spread. A microwave hop with 5 ns of delay spread has a coherence bandwidth near 40 MHz, so carriers spaced by 100 MHz or more are largely independent. For rain-induced fading on millimeter-wave links the dominant decorrelation comes from the strong frequency dependence of specific attenuation, so pairing a lower band such as 18 GHz with a higher band such as 38 GHz gives very different fade depths during the same storm cell. In practice a band ratio of 1.5 to 2 or greater, or carriers in entirely different licensed allocations, is used so the correlation coefficient drops below about 0.3 and the diversity gain approaches the ideal.

Q: How does cross-band diversity differ from frequency diversity within one band?

Conventional frequency diversity, such as an N+1 protection channel, uses two carriers inside the same licensed band a few tens of MHz apart. Because both carriers see almost identical rain attenuation and similar multipath, their fades are highly correlated during deep events and the protection benefit is modest. Cross-band diversity deliberately places the two carriers in widely separated bands, for example 6 GHz paired with 23 GHz, so that rain attenuation, atmospheric ducting, and multipath statistics differ sharply between them. The lower band rides through heavy rain while the higher band carries traffic in clear air, which gives far lower correlation and much larger availability improvement than same-band frequency diversity.

Q: What combining method is used and what diversity gain does it provide?

Most cross-band links use selection combining at the packet or trunk level, switching traffic to whichever band currently meets the bit-error-rate threshold, because the two radios use independent modems and local oscillators that are hard to combine coherently. Where baseband combining is available, maximal-ratio combining adds the post-detection SNR of both branches. For two branches with correlation coefficient near zero, selection combining typically yields about 8 to 10 dB of diversity gain at the 99.99 percent availability point, while maximal-ratio combining adds roughly another 2 to 3 dB. The resulting outage probability for two independent branches is the product of the individual outage probabilities, so two links each at 99.9 percent can reach about 99.9999 percent combined.

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A link-hardening technique that carries the same payload over two or more widely separated frequency bands so the statistically uncorrelated fading on each path keeps an aggregate link alive when any single band is degraded. Unlike same-band frequency diversity, pairing carriers such as 6 GHz with 23 GHz exploits the sharp frequency dependence of rain attenuation and multipath: the lower band rides through a storm cell while the higher band carries traffic in clear air. A selection or maximal-ratio combining stage picks or merges the surviving branch, and because the branch outage probabilities multiply, two 99.9% links can reach roughly 99.9999% combined availability.

Category: Link Engineering

Typical band ratio: 1.5 to 4×

Diversity gain: 8 to 12 dB

How Separated Bands Decorrelate Fading

Cross-band diversity attacks the single largest failure mode of fixed microwave and millimeter-wave links: a deep fade that drives one carrier below the receiver threshold. By sending the same trunk over two bands that are far apart in frequency, the design ensures the two carriers rarely fade at the same instant. The improvement comes entirely from the low correlation of the impairments, so the engineering effort is spent confirming that the chosen bands really do fade independently for the dominant propagation mechanism on that hop.

For multipath-limited paths over water or flat terrain, the relevant metric is coherence bandwidth, approximately the inverse of the channel delay spread. A hop with 5 ns of delay spread has a coherence bandwidth near 40 MHz, so carriers separated by 100 MHz or more decorrelate. For rain-limited millimeter-wave links the mechanism is different: specific attenuation rises steeply with frequency, so a 38 GHz carrier can be 6 to 10 dB deeper into fade than an 18 GHz carrier during the same cell. The link availability is then governed by the same outage statistics used in any link budget, but applied to two branches whose outage events are nearly independent.

Because the two radios run independent local oscillators and modems, coherent combining is usually impractical. Operators instead switch traffic to whichever band currently meets the bit-error-rate threshold, a selection-combining approach that is robust and modem-agnostic. Where a common baseband is available, maximal-ratio combining adds the post-detection SNR of both branches for a further 2 to 3 dB of gain.

Correlation and Combined Availability

The diversity benefit is set by the branch correlation coefficient. When it falls below about 0.3 the two branches behave almost independently and the combined outage probability approaches the product of the individual branch outages. Designers target a band ratio of at least 1.5 to 2, or carriers in entirely different licensed allocations, to push correlation into this regime.

Governing Equations

Combined outage (independent branches):
P_out,div ≈ P_out,1 × P_out,2

Selection-combining diversity gain (two branches):
G_div ≈ 10·log₁₀(1 / P_out) − 10·log₁₀(1 / √P_out)

Coherence bandwidth (multipath decorrelation):
B_c ≈ 1 / (5 × σ_τ)

Maximal-ratio combined SNR:
γ_MRC = γ₁ + γ₂

Where P_out = single-branch outage probability, σ_τ = RMS delay spread, γ_i = per-branch SNR. Example: two branches each at P_out = 10⁻³ (99.9%) give P_out,div ≈ 10⁻⁶, near 99.9999% availability.

Diversity Scheme Comparison

Scheme	Separation domain	Decorrelates against	Typical diversity gain	Extra hardware	Best application
Cross-band	Widely separated bands (e.g. 6 / 23 GHz)	Rain fade, multipath, ducting	8 to 12 dB	Two full radios + antennas	High-availability backhaul
Frequency (same band)	Two carriers, tens of MHz apart	Multipath only	3 to 6 dB	Second carrier on same radio	N+1 protection trunks
Space	Vertically spaced antennas	Multipath	5 to 9 dB	Second antenna + receiver	Over-water and flat-terrain hops
Polarization	Orthogonal H / V	Multipath (limited rain)	3 to 6 dB	Dual-pol feed	Space-constrained towers
Angle	Separate elevation lobes	Multipath	2 to 5 dB	Multi-feed antenna	Long over-water paths

Common Questions

Frequently Asked Questions

How much band separation is needed for the fading to be uncorrelated?

For multipath, fades decorrelate once separation exceeds the coherence bandwidth, roughly 1/(5·σ_τ); a 5 ns delay spread gives B_c ≈ 40 MHz, so 100 MHz separation is enough. For rain, the steep frequency dependence of specific attenuation means pairing bands such as 18 GHz and 38 GHz gives very different fade depths in the same cell. A band ratio of 1.5 to 2 or greater drops the correlation coefficient below about 0.3, where diversity gain approaches the ideal.

How does cross-band diversity differ from frequency diversity within one band?

Same-band frequency diversity places two carriers tens of MHz apart, so both see nearly identical rain attenuation and their deep fades stay correlated. Cross-band diversity uses widely separated bands, for example 6 GHz with 23 GHz, where rain attenuation, ducting, and multipath statistics differ sharply. The lower band survives heavy rain while the higher band runs in clear air, giving far lower correlation and a much larger availability gain than same-band schemes.

What combining method is used and what diversity gain does it provide?

Most cross-band links use selection combining at the trunk or packet level because the two radios use independent oscillators and modems. With correlation near zero, selection combining yields about 8 to 10 dB of gain at the 99.99% availability point, and maximal-ratio combining adds another 2 to 3 dB where a common baseband exists. The combined outage equals the product of branch outages, so two 99.9% links reach roughly 99.9999%.

Related Terms

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