5G and rural broadband networks are on track to quickly outgrow the capabilities of microwave-only backhaul solutions. Taking a different, more flexible approach to backhaul planning and availability targets will be key to keep pace with capacity demands and control costs.
So what exactly is this atypical approach? Multi-band systems and differentiated availability. Differentiated availability is the key to unlocking the real and significant benefits of Multi-Band backhaul: higher capacities up to 10 Gbps over distances up to as much as 20 km, all with much lower TCO than microwave links or fiber. However, this approach will require a shift in thinking away from traditional microwave link design, but the savings can be significant. For example, an operator in Asia deploying 10 Gbps multi-band instead of fiber over a 10 km path could save as much as US$900,000 over 10 years, just for that one path alone.
The ‘Five-Nines’ Approach to Link Design
Just like the old adage “no one was ever fired for buying IBM,” playing it safe in microwave link design means designing all microwave links for 99.999% (five-nines) availability at maximum capacity. This approach ensures that for all but five minutes during the course of the year the link is carrying full capacity. Five-nines design made good sense in the days when microwave links primarily carried TDM voice traffic, but with the introduction of packet-based transmission in the early 2000s it became less necessary.
Not all Ethernet traffic is of equal importance. While a certain part of the payload is critically important and delay-sensitive, the remainder can tolerate dropped packets that can be retransmitted without affecting the end-user experience. Over-designing links with excessively high availability criteria for the total payload comes with a big cost penalty in the form of shorter, lower capacity links and larger antennas. When multiplied across a network of hundreds of links, these cost penalties have a dramatic impact.
Packet Transport and ACM
To take advantage of the benefits of packet-based microwave systems, adaptive coding and modulation (ACM) was introduced more than 10 years ago and is now widely adopted. ACM introduced a new approach: links can be designed with two levels, or ‘differentiated’ availability – high availability (e.g., 99.995%) for mid-level modulation such as 256 or 512QAM supporting the most important traffic, and a lower availability target for the maximum capacity at 4096QAM.
Multi-Band extends ACM
Multi-Band essentially extends the concept of ACM, by adding a parallel E-Band link that operates at much higher capacity, but with lower, or ‘optimized’ availability. Now, the 10 ACM steps on the microwave channel between QPSK and 4096QAM are augmented by six additional capacity/modulation steps on the E-Band channel (Figure 1). E-Band also introduces adaptive bandwidth (ACMB), which in addition to reducing the modulation, also reduces the radio’s channel bandwidth, further improving system gain.
Time at Modulation/Capacity
Rather than focus on the top-line availability of a link, a better measure of performance is time in modulation, which predicts how long the link is expected to operate at a certain capacity. Figure 1 shows a 10km multi-band link in a high rain-rate zone (N) where, for the vast majority of time (99.8%), the link is operating at multi-Gigabit capacity. At some point, rain-induced attenuation will result in the E-Band channel dropping out of service, after which the microwave link will still seamlessly support 500 Mbps of capacity at 99.995% availability and will remain in service but for two minutes throughout the entire year (Figure 2).
Considering User Quality of Experience
At the end of the day, it is the end user’s experience that counts. This can depend on many factors: the number of users, the type of services being consumed, the time of day and the overall network loading. Some studies have shown1 that in mobile use cases, it is the RAN (Radio Access Network) that is the limiting factor that determines QoE, not the backhaul links. For bandwidth hungry services such as 4K video, user QoE is not impacted until the backhaul availability falls below 99% at maximum capacity, even in conditions of high RAN congestion and user demand.
New Thinking Results in Longer, Higher Capacity Links
According to analyst predictions2, wireless will to continue to connect more than 60% of the world’s base stations for the next five years. However, solutions will need to keep pace with capacity demands not just for shorter urban links, but also for longer suburban and even rural applications. For operators who adopt the differentiated availability approach, Multi-Band promises to be a key (and perhaps the only) viable and cost-effective solution than traditional microwave or fiber to support multi-gigabit link capacities over distances up to 20km.
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