There are ample benefits to deploying a wireless backhaul or access system in the unlicensed 2.4 GHz or 5.8 GHz bands:

1. The cost of the equipment, often based on mass-market WiFi chipsets, is generally very low.
2. There is no licensing cost, hence both the initial deployment costs and operating expenditure can be kept in check, leading to a very low Total Cost of Ownership (TCO).
3. As no license needs to be applied for and granted, a link can be deployed almost instantaneously; as soon as the equipment arrives. 
4. Most sub-6 GHz systems utilize OFDM modulation that, coupled to the comparatively low propagation losses and decent (building) penetration, allow for non-line of sight operation, thereby enabling wireless links that would otherwise be blocked.

Unfortunately, there are substantial limitations to an unlicensed wireless backhaul or access system, too:

1. Dependability – an unlicensed system may work fine on day one, but over time, as more and more consumer WiFi devices are deployed in the very same unlicensed bands, interference will develop, causing the unlicensed system performance and throughput to be compromised. As interference and non-line of sight propagation conditions will vary dynamically, the result can be an unreliable link, performing well at some times, and poorly at others.
2. Performance – Capacity – Due to limited spectrum in the lower, unlicensed frequency bands, systems operating in those bands are constrained to roughly 100-200 Mbps of full-duplex throughput. As this capacity can’t scale much, the unlicensed system will become a bottleneck with growing capacity demand.
3. Performance – Latency – Due to the time-division duplex (TDD) nature of most of the inexpensive sub-6 GHz unlicensed links, the link cannot transmit and receive simultaneously. Furthermore, due to the limited channel spectrum available, data rates will be comparatively low. Those two factors lead to typical latencies in the order of 2-10 milliseconds, where frequency division duplex links in higher frequency bands can easily cater for latencies <0.1 ms. Unlicensed sub-6 GHz links are therefore not well suited for latency sensitive applications.

The Licensed Wireless Solution

Any of the aforementioned limitations may convince an organization to consider an alternate backhaul or access solution to address their network needs. An excellent upgrade path is a licensed wireless system in the 6-42 GHz frequency bands.

In this spectrum range, a user can exclusively license a frequency channel between points A and B; there will be no interference and no associated performance degradation. System performance is therefore very predictable and link availability can be engineered to meet 99.999%.

Licensed systems, like the Harmony Enhanced and Harmony Enhanced^MC operate very spectrum-efficiently at high modulations (up to 2048 or 4096 QAM), and have larger communication channels at their disposal than sub-6 GHz unlicensed systems (up to 112 MHz per channel versus up to 40 MHz per channel). This pushes link capacities beyond a gigabit per second, compared to 100 to 200 Mbps for sub-6 GHz unlicensed solutions.

Addressing Impediments to Licensed Microwave Deployment

In the past, there were concerns with licensed microwave systems compared to unlicensed systems. Most, if not all, of those concerns can be addressed:

• Initial Cost – The cost of licensed microwave equipment has dropped significantly over recent years. Current carrier-grade licensed systems are available on the market at costs similar to that of high-quality unlicensed equipment.
• Install Cost – In the past, microwave systems were associated with large antennas, and tall towers. This has changed with increased system gain and adaptive modulation, enabling 1’ and 2’ antennas for long hop lengths and simple, quick deployment on buildings, and even street furniture such as light poles.
• Licensing Costs – In some markets outside of the U.S., spectrum charges can be high. With the latest microwave systems, spectral efficiency has been improved drastically, enabling high capacity links in small channel sizes, such as 14 MHz, and therefore reducing the amount of spectrum required and the associated recurrent spectrum costs. Please note that the licensing cost provides an insurance against performance variations and link failure due to spectrum congestion. It’s often a price worth paying.
• Line of Sight – Line of sight is still required to install a reliable licensed wireless link.  However, the benefits are so significant, that using an intermediate (passive) repeater site, or looking for an alternate mounting point to achieve line of sight will often be warranted.

There are compelling reasons to deploy an unlicensed access or backhaul network. Despite that, a user may want to evolve to a licensed microwave network for achieving the significant dependability, capacity and latency benefits mentioned above. Licensed and unlicensed microwave technologies are not mutually exclusive.

Licensed wireless backhaul technology has evolved and developed considerably over recent years. Leveraging that progress will help the user build cost-effective high-performance wireless data transport for many years to come.