The extent of traditional microwave spectrum usage, at frequencies of 4GHz to 42GHz, depends on the availability of such spectrum in the specific area where the link will be established.
Networks grow denser for multiple reasons. For example a cellular network may evolve from a plan-to-coverage to a plan-to-capacity scheme and therefore require more cell sites, or an enterprise-connectivity service may gain momentum and cause the number of connected customers to grow in a given area. This leads to increasing strain on wireless backhaul spectrum availability.
The main cause of this is the limited reuseability in microwave spectrum. An angular separation of at least 90 degrees is required in order to reuse the same frequency band in two adjacent links. Hence, the denser the network, the less likely it is you will be able to do so and the more likely you are to get a negative response from your regulator saying that there is no more spectrum available for your required link.
The implications of this lack of spectrum are severe as the common alternatives – deploying fiber or leasing capacity from another operator – are both problematic in terms of cost and time-to-market.
Moreover, this worrying trend will accelerate as the capacity-density in networks grows. In particular, this will be the case in 5G networks as both the higher capacity-density and the higher frequency-bands used for radio-access-networks will dictate a larger number of cell sites and a much denser network grid.
What’s the solution?
There are several solutions for the lack of wireless backhaul spectrum in traditional microwave bands.
One solution involves using higher bands such as millimeterwave including E-Band, V-Band and, in the future, W-Band and D-Band. These bands offer significantly higher spectrum availability as the offered channels are much wider than in microwave bands, as well as a narrower beam implementation to enable a higher reuse of the same frequency.
The main limitation of mmW applications is the range in which they can be used due to the high attenuation of signals in those frequency bands.
Other solutions can help achieve higher capacity with existing, narrowband, microwave channels, and can significantly improve the reuse capabilities of such bands. These capacity-boosting techniques for standard microwave bands include XPIC and 4x4 LoS MIMO. In addition, unique technologies such as Advanced Frequency Reuse offer a double reuse scheme in microwave frequencies as they enable frequency reuse with adjacent link as close as 15 degrees apart.