Microwave radio architectures variants can be used to ensure improved link availability.
An additional benefit of using more advanced microwave radio architectures is the ability to design longer distance links than would be achieved using a 1+0 microwave radio architecture.
Overview of typical microwave radio architectures: Radio Solution Redundancy – Ring, 1+1 Hot Standby, 1+1 Space Diversity, 1+1 Frequency Diversity, N+1
There are many microwave radio architectures to ensure improved link reliability (availability) for microwave radio backhaul solutions.
The diagrams below show various radio topologies.
Split-System Topology Microwave Radio Architecture Examples
All-Outdoor (AO) Topology Microwave Radio Architecture Examples
All-Indoor (AIS) Topology Microwave Radio Architecture Examples
With reference to the above diagrams, ‘protection’ solutions include and can be described as follows:
1+1 HSB (Hot Standby – most often ‘Hitless’) Microwave Radio Architecture
Here, a waveguide coupler is used, often either as 3dB/3dB or 1dB/6dB splits.
Two independently-powered Outdoor Units are each connected to a waveguide coupler. The common interface of the waveguide coupler connects to the antenna.
Both ODUs can transmit through the waveguide coupler to the antenna port. The penalty is a loss of 6dB system gain for a 3dB/3dB coupler or a 2dB / 12dB loss of system gain for a 1dB/6dB coupler.
If one ODU fails, the other ODU immediately takes over in a ‘hitless’ manner, which means that the TDM stream is not affected i.e. hundreds or thousands of calls are not simultaneously ‘dropped’.
A ‘hit’ entails loss of synchronisation in a TDM-oriented voice solution.
Either way, to main 99.999% link availability, the link distances must be shorter, or larger antenna diameters must be used.
1+1 SD (Space Diversity) Microwave Radio Architecture
Here, an Indoor unit connects to two Outdoor Units. However in this case, each ODU is connected to its own antenna.
The antennas are mounted at different vertical positions on the tower.
One of the radios transmits and at the remote side, the best receive signal is chosen (on the basis of transmission stream frame error performance – monitoring of FEC).
The paths to each remote antenna are affected differently by fading and multipath reflections, so this implementation provides ‘diversity gain’ that also helps when longer-distance 99.999% availability links are needed.
1+1 FD (Frequency Diversity) Microwave Radio Architecture
Here, an Indoor unit connects to two Outdoor Units.
The ODUs can be interconnected through a coupler to one antenna. Each radio transmits using a different frequency. The transmission from the best-performing frequency channel is chosen by monitoring and comparing each transmission stream’s frame errors.
Again, being able to select between the better of two links, means there is a ‘diversity gain benefit’, allowing longer-distance links. However, since a waveguide coupler is used, there is also a loss of system gain (as per the 1+1 HSB case)
N+1 FD and SD Microwave Radio Architecture
This solution is often implemented using an All-Indoor-System (AIS).
Many streams (N) of GbE or STM-1 are ‘protected’ by a separate (+1) channel frequency.
As an example, there can perhaps be 3 traffic streams that are protected by an independent ‘protection’ channel. If any of the 3 traffic streams is affected (e.g. equipment failure), the ‘protection’ channel takes over on behalf of the affected channel. Each stream operates on its own frequency channel.
Furthermore, to ensure higher link availability over longer distances, SD (two antennas) can be implemented. A ‘branching unit’ connects each of the channel transmitters and receivers (including the SD receive interface if necessary) for each of the N+1 radios together – the combined signals are connected to the antenna(s) using low-loss waveguide.
These links are popular for long distance backhaul, lower-frequency links at 6, 7 and 8GHz.
Ring Microwave Radio Architecture
Here, the microwave radios will be connected in a ring. The Ethernet or TDM traffic will either circulate clockwise or anti-clockwise. Or, some traffic will flow clockwise and some anticlockwise if there is a problem on the ring at a non-master ring protection unit.
‘Ring Protection Equipment’ is placed at the interface between the microwave radios (here, egress or ingress ‘East’ or ‘West’ terminology is used to describe traffic ring flow).
Within a ring, there is one ‘Master’ unit which will have one port ‘blocked’ and the other port ‘forwarding’. All other ring protection equipment has their East and West ports ‘forwarding’. So, for normal operation, the traffic can completely traverse the ring by exiting the forwarding port and eventually getting back to the Master Unit’s ‘blocked’ port. If there is a problem with traffic traversing the ring, the ‘Master’ can decide to ‘forward’ rather than ‘block’ the previously blocked port.
The methods to monitor and control the ring entail either:
- sending and monitoring receipt of ‘ring packets’
- monitoring ‘East’ or ‘West’ interconnect port state-changes (e.g. if an Ethernet port gets switched-off in the event that a radio link’s equipment stops working).
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