Diversity in Microwave Links

Example of a 1+0 Unprotected Microwave Link

Diversity schemes are used to improve signal reliability by transmitting data across multiple channels with differing characteristics. These techniques help counter fading, co-channel interference and error bursts, using the fact that different channels experience varying levels of fading and interference. Signals can be transmitted and/or received in multiple versions and combined at the receiver, or redundant forward error correction codes can be used to send parts of the message over different channels. Diversity techniques often utilise multipath propagation to achieve a diversity gain.

The following classes of diversity schemes are typical in Terrestrial Microwave Links:

Diversity is important in Long Haul Microwave Backhaul in Cellular Networks

Unprotected (1+0): A single set of microwave equipment with no diversity or backup, offering no protection against failures.
Hot Standby (HSB, 1+1): Two sets of microwave equipment share the same antenna and frequency, with one active and the other in standby mode (receiver active, transmitter muted). If the active unit fails, the standby unit activates.
Frequency diversity (N+1): The signal is transmitted over multiple frequency channels, including one protection channel to automatically replace a faded channel, mitigating frequency-selective fading.
Space diversity: The signal travels through multiple propagation paths, achieved via multiple wires or multiple transmitter and/or receiving antennas (transmit/ reception diversity).
Polarisation diversity: Signals are transmitted and received using antennas with different polarisations, combined at the receiver using a diversity combining technique.

Diverse Path Resilient Failover

In terrestrial point-to-point microwave systems (11 GHz to 80 GHz), a parallel backup link can be installed alongside a high-bandwidth primary link prone to rain fade. For example, a primary link such as an 80 GHz, 1 Gbit/s full-duplex microwave bridge with 99.9% availability may be down for 10+ hours annually due to heavy rain. A secondary, lower-bandwidth link, such as a 5.8 GHz, 100 Mbit/s bridge, can be installed in parallel, with routers enabling automatic failover to the backup link during rain fade. This setup allows high-frequency links (23 GHz+) to serve distant locations with greater reliability, achieving up to 99.99% uptime annually.

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