Microwave links are critical components of modern communication systems that are used for telecom carrier backhaul and government applications. They are designed to transmit and receive data at high speeds over long distances, and their reliability is of utmost importance.
The antennas used in microwave links are classified based on their specifications such as frequency, polarization, and radiation patterns. These specifications play a critical role in determining the link's performance, including its range, capacity, and signal quality.
To maintain link uptime as close to 100% as possible, microwave link designers and operators employ various redundancy methods. For example, some microwave links have a backup path in case the primary path fails. This backup path can be either a secondary microwave link or a wired connection. Additionally, some microwave links use multiple frequencies or channels to ensure that if one channel fails, the other channels can still transmit data.
One of the main challenges of maintaining high link uptime is protecting against problems with the radio chain, which is the part of the system that handles the transmission and reception of signals. Many radio components are located outdoors and are often mounted directly to the back of the antenna. As a result, Outdoor Unit (ODU) failures can take a long time to repair or replace, as time climbs are needed to access the equipment.
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To mitigate this risk, backup schemes are implemented to keep the link up and running during repair times. For example, some microwave links have a redundant ODU that can take over if the primary ODU fails. Additionally, some microwave links use an automatic switching mechanism to switch between different antennas or frequencies to ensure continuous link uptime.
In summary, microwave links play a critical role in modern communication systems, and their reliability is essential. Redundancy methods are employed to maintain link uptime, and particular attention is paid to protecting against problems with the radio chain. By implementing backup schemes, microwave link operators can minimize downtime and ensure that critical communications remain uninterrupted.
1+1 Configuration:
In radio communication systems, redundancy is crucial to ensure continuous operation in case of failures or disruptions. One way to increase redundancy is by using a 1+1 configuration, which involves two separate modems in a single Indoor Unit (IDU), with each modem connected to its own Outdoor Unit (ODU).
The ODUs are mounted on a "coupler" device, which is directly mounted on the antenna, similar to a 1+0 configuration. This eliminates the need to mount the coupler-ODU fixture separately, simplifying the installation process.
However, the ODU coupler introduces coupling losses, which can affect the system's performance. There are two types of couplers available: symmetric and asymmetric. The symmetric version introduces equal losses in both the primary and redundant paths, typically about 3.5 dB. The asymmetric version favors the primary path, resulting in about 1 to 1.5 dB loss, while the redundancy path experiences about 6.5 to 7 dB loss.
Depending on the user's preference regarding the losses, they can choose between the two couplers. Some users prefer to lose as little as possible in the primary link and set up the redundant path at a lower modulation (lower speed) to compensate for the 7dB loss. In this way, the redundant link is just as robust as, if not more so, the primary link, albeit at a lower performance level. This allows the user to maintain the link while working on repairing the primary link.
The 1+1 configuration described above is often referred to as HSB (hot standby). In recent times, the concept has been taken to the next level by monitoring the Tx power of the redundant ODU periodically. This is referred to as MHSB (Monitored Hot Standby). This way, the user can make sure that the redundant ODU transmitter is working correctly while remaining unused for data transmission. The redundant ODU power is kept at a low level not to interfere with the primary transmitter.
In a typical 1+1 configuration, only one transmitter (primary Tx) is active at a given time, but both receivers are active. The receiver chain in the IDU then selects the better of the two received signals, i.e., establishes Receive diversity. This improves the system's reliability by reducing the effect of fading or interference in one of the received signals.
Overall, a 1+1 configuration with its redundant modem and ODU offers an effective way to increase system redundancy in radio communication systems, ensuring continuous operation and reducing downtime due to equipment failures or disruptions.
1+1 Space Diversity:
Under certain circumstances, the conditions of RF links can pose considerable challenges due to long link distances or the presence of a body of water obstructing the link path. Long link distances limit the available link budget, while bodies of water can create strong reflections, leading to multipath interference at the receiver. In instances where a large body of water is present, the water's surface may be irregular, resulting in inconsistent performance due to fluctuating RF conditions at the receiver.
To enhance link performance, a "space diversity (SD)" configuration can be employed, wherein a separate antenna is installed for each ODU. The receiver can then select the better of the two signals received at the two antennas, significantly improving link conditions in situations involving multipath issues or weak RF signals. The SD setup also eliminates the coupler and associated coupling losses. However, the additional cost of a second set of antennas and increased tower rental fees due to greater antenna space requirements are potential drawbacks.
1+1 configuration vs. 2+0 configuration:
Some users choose to implement a 2+0 configuration instead of the 1+1 configuration. In a 2+0 configuration, there are two primary links simultaneously running from A to B without any redundancy, which doubles the link capacity from the 1+0 or 1+1 case. If one link malfunctions, the other link will be functioning, which gives inherent protection against any one path failure.
However, there are some drawbacks with 2+0. To achieve 2+0, users need to attain spectrum licenses for two separate RF channels from the governing authorities in their country, which may not be practical due to the lack of RF channels or the high cost of spectrum licenses. In addition, antennas must support dual polarization with good cross-polarization rejection, such as the HP series from most manufacturers, which are significantly more expensive than single-polarization antennas and do not have provisions for direct mounts. This lack of slip-fit mount options forces the user to mount ODUs separately and run a short waveguide, which is cumbersome and costly. The remote mount, needed for dual pol antennas, needs separate mounting of ODUs as well as running a short waveguide. It should be noted that remote mounting needs to be done for both ODUs in the case of dual pol antennas. The direct mount shown in the left of the diagram makes the deployment very simple and introduces very little loss compared to the remote mount.
2+0 “Co-Channel” Deployment:
To save money on additional spectrum licensing fees, a user can utilize two RF channels in a "co-channel" configuration. This involves using one RF path for vertical polarization and the other RF path for horizontal polarization with a dual pol antenna. However, there are two additional steps that need to be taken to ensure minimal cross-talk between the two paths.
- The first step is to use an ultra high performance (UHP) antenna, which has a cross pol rejection value of up to 50 dB, significantly reducing cross-talk. However, this also increases hardware costs.
- The second step is to use cross polarization interference canceller (XPIC) technology at the modem level. This cancels out interference between the two data paths through signal processing. This is particularly useful in co-channel or adjacent-channel deployments and the cost is relatively minor compared to the cost increase with UHP antennas.