Space Diversity in Microwave Links

Why use Space Diversity?

Long distance Microwave Links often use Space Diversity to ensure reliable communications between the two end points.

In certain geographic locations, such as over water and in deserts, multipath propagation poses an impediment to long-haul radio performance in the form of intolerable link outages. To compensate, a protection scheme must be applied. Space Diversity is one such widely implemented protection scheme that improves the performance of long-distance microwave radio links.

Microwave links below and above 10GHz

At link frequencies above 10 GHz, the path length of the link is limited by fading due to the occurrence of precipitation, while at link frequencies below 10 GHz rain attenuation has a limited effect on the path length. For this reason, frequencies below 10 GHz are best suited for long-haul communication networks. However, even in these preferred long-haul frequencies, path length and link availability can be
limited by another phenomenon—fading caused by multipath propagation.

The probability of fading due to multipath propagation is dependent upon geographic factors such as the locale, the terrain over which the radio waves propagate and the path inclination (angle). The path length itself also has an effect since the likelihood of multipath propagation increases as the path length increases. In general, multipath propagation is more likely to occur in tropical areas, desert areas and in links over large bodies of water

Multipath Propagation

Multipath propagation occurs as a result of one or more waves that are sent out from the transmitting antenna being reflected or deflected back onto a path that leads to the receiving antenna. The reflected/deflected wave is received in addition to the direct path wave.

Multipath Propagation example

Why use Multiple Antennas?

Spatial diversity employs multiple antennas, usually with the same characteristics, that are physically separated from one another. Depending upon the expected incidence of the incoming signal, sometimes a space on the order of a wavelength is sufficient. Other times much larger distances are needed.

As the multipath transmission is typically caused by fluctual layers in the atmosphere or at ground level, the delay difference between the direct path and the reflected/deflected paths vary over time. Also, the reflection coefficient (strength of the reflection/deflection) varies over time resulting in erratic fading behavior. By putting a second receive antenna on the tower, with a vertical separation from the first antenna, we create a second set of delay combinations. This technique is called Space Diversity. As described below, selective fading will occur at different frequency notches in the two received signals (one at each antenna) due to different delays, resulting in a significantly higher probability of receiving an undistorted signal.

Space Diversity with Multiple Antennas

How to achieve Space Diversity

Space Diversity is usually achieved using two vertically spaced antennas (space diversity), multiple transmitter frequencies (frequency diversity), both space and frequency diversity (quad diversity), or reception using two different antenna patterns (angle diversity). Frequency diversity was the first diversity used by fixed point to point microwave systems. Combining dual‐channel space and frequency diversity produces a powerful diversity improvement receiver configuration. The chapter illustrates the receive signal levels for a quad‐diversity path. The purpose of angle diversity antennas is to mitigate the destructive effects of multipath propagation without using a vertically spaced diversity antenna on the microwave tower.

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High Capacity 11GHz Microwave Links with 1024QAM

CableFree FOR3 Microwave Links offer 881Mbps Full Duplex using 1024QAM, 112MHz Channels in 11GHz Microwave Band

The 11GHz version of CableFree FOR3 is ideal for Long Distance Backbone links for ISPs, Wireless ISPs (WISP), 4G/5G Operator for LTE Backhaul.  In this demonstration you can see CableFree FOR3 Microwave Test Results for operation in the 11GHz Band.  Our demonstration shows 881Mbps Full Duplex net throughput using 1024QAM modulation in 112MHz channels.This demonstration shows the clear advantages of a telecom carrier-grade FDD (Frequency Division Duplex) modem radio, which offers dedicated throughput and genuine Full Duplex capacity.  Many operators use these links to upgrade from existing 5GHz MIMO radios when the capacity available on 5GHz is saturated, or interference levels too high for reliable operation.

Link Aggregation for Higher Capacity

CableFree Microwave Links can be aggregated for higher capacity:  A 2+0 configuration of this radio in 11GHz would offer 1782Mbps Full Duplex net capacity.

Watch the FOR3 11GHz demo:

You can Watch our demo online here on Youtube:

Direct link to Youtube Video:

How the Demo was created:

The CableFree FOR3 radios were placed in a lab with suitable attenuator material between the waveguides to simulate long distance/range & attenuation.  A pair of routers were placed either end of the link, to generate and receive test packet streams.  These routers are capable of saturating a 1Gbps link therefore ideal for this test.  Due to the nature of Telecom Microwave FDD modems, the same full capacity is available at range where suitable antennas are used.  Longer range links require larger antennas to achieve high capacity.

Advantages of Licensed Spectrum

Licensed Spectrum ensures that links can operate without interference from other links within a region – the frequencies are allocated centrally by national government regulators.  The 11GHz band is normally regulated by national governments for long distance links.  Frequency allocations may be narrower than the 112MHz channels shown here.  The FOR3 radio can be configured to operate within whatever channel assignments are offered,with channel widths and centre frequencies set under software control.  Typical channel widths for lower frequency bands are 14MHz, 20MHz, 28Mhz, 30MHz, 40MHz, 56MHz.

About CableFree FOR3 Microwave

CableFree FOR3 Microwave Link

CableFree FOR3 Microwave platform is a high capacity, modern IP microwave radio link offering up to 891Mbps Full Duplex net throughput for diverse applications including 4G/LTE Backhaul, corporate networks, CCTV, Safe Cities and Wireless ISP backbones.
Unique in the wireless industry, CableFree FOR3 is available in both Licensed (2 to 42GHz) and Unlicensed (5GHz, 17GHz and 24GHz) bands, allowing for lower Total-Cost-of-Ownership.
Link to FOR3 Product Page

Zero-Footprint Solution

CableFree FOR3 is a Full Outdoor Radio for Zero-Footprint deployment, eliminating requirement for indoor locations or rack space. The radio is typically mounted on roof-top or tower location with antenna, with Power-over-Ethernet (PoE) connection to the radio using a single Cat5/e/6 cable. An optional SFP optical fibre interface is available for sites where long cable runs or electrical isolation to the radio is required.

Fully Shipping and Available

The CableFree FOR3 is fully shipping and available in all bands listed from 2 to 42GHz.

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Out team has over 21 years experience with real-world deployment of wireless for mission-critical applications, with thousands of commercial deployments worldwide. For further information on Applications and Solutions for the range of CableFree wireless networking products please Contact Us


Microwave Rain Fade Planning ITU-R P.837-6


ITU-R P.837-6 P.0837-01  – Characteristics of precipitation for propagation modelling Radiowave propagation for Terrestrial Microwave Links and Radio Links for Point to Point (P2P, PTP) and Point to Multipoint (P2MP, PTMP) deployments.

Calculations can be made for Link Availability (%) for all frequency bands, to take into account link budgets, transmit power, receive sensitivity, antenna gain, target availability and other factors.  Typical Link Availability Targets are 99.99%, 99.999% and higher.

ITU-R P.837-6 P.0837-01

ITU-R P.837-6 P.0837-01
ITU-R P.837-6 P.0837-01

Recommendation ITU-R P.837 contains maps of meteorological parameters that have been obtained using the European Centre for Medium-Range Weather Forecast (ECMWF) ERA-40 re-analysis database, which are recommended for the prediction of rainfall rate statistics with a 1-min integration time, when local measurements are missing.
Rainfall rate statistics with a 1-min integration time are required for the prediction of rain attenuation in terrestrial and satellite links. Data of long-term measurements of rainfall rate may be available from local sources, but only with higher integration times. This Recommendation provides a method for the conversion of rainfall rate statistics with a higher integration time to rainfall rate statistics with a 1-min integration time.

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