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What will be the impact on network planning when the next generation of E-band radios (those that operate in the 80 GHz band) hit the market? Some have predicted they will become the mainstay radios in new and enhanced mobile networks, but a closer look at the facts indicates that won’t entirely be the case. While they are certain to play a role in future network deployments, a number of mitigating factors will determine just how and when they will be deployed.
Initially, E-band radios didn’t have many features, but were the only real way to push gigabit-per-second capacities over a microwave link. They were also limited to very simple modulation schemes that result in very poor spectral efficiency. Yet, as spectrum allocation has become more of an emergent issue, regulatory bodies around the world continue to draft rules that encourage the development of radios with better spectral efficiency. As a result, E-band radio makers adopted digital modulation schemes, enabling many packet-based features, such as forward error correction, adaptive modulation, quality of service and others common to lower frequency microwave radios.
Just as advancements were being made with E-band radios, improvements to lower frequency radios led to dramatic increases in capacity. These included higher order modulation (up to 2048 QAM most recently), wider channel sized, the addition of lossless packet compression, statistical multiplexing, multi carrier radios, cross polarization operation and multiple antennas on a single link. However, it should be noted that there is a trade off, as many of these techniques require additional radios and/or antennas, thus increasing the cost per link.
Advantages and disadvantages
So, as E-band radios have been improving and adopting technological features and achieving performance similar to lower frequency radios, operators have another option to consider when planning their networks. Depending on the deployment scenario, there are both advantages and disadvantages to consider.
Firstly, the E-band spectrum remains largely unpopulated, so that in areas where there is a good deal of microwave deployment, it is much easier to find frequency availability for E-band when the lower frequencies are heavily occupied. And, because there is more spectrum available at E-band, regulators have allocated larger channels – 250 megahertz versus the more common 28 or 56 megahertz channels at lower frequencies. Yet, while E-band can deliver more capacity per radio than lower frequency radios when using the same level of modulation, this advantage is offset somewhat because of the difficulties in achieving a higher order of modulation due to the low noise and linearity performance required. One must also consider that RF propagation loss increases as the frequency increases, meaning that E-band radios have a much shorter reach than the lower frequency radios. What’s more, the E-band spectrum isn’t available everywhere. For example, India has yet to finalize regulations for E-band spectrum use.
Macrocell versus small cell backhaul
When considering E-band for macrocell backhaul the most important limiting factor is the radios’ reach, which is typically gauged at 1,500 meters. The radius of the macrocells drives the spacing of cell sites so that it is smaller in dense urban areas and larger in suburban or rural areas. Looking at a typical large metro deployment area in North America, the reach of E-band radios would cover less than 20% of those link distances currently found.
Increasing the reach of E-band radios using techniques such as higher power, adaptive modulation and/or frequency compression will enable a higher percentage of links to be addressed by the E-band radios. However, when a mobile operator runs out of available RAN spectrum at a site due to increased user demand, they will need to decide between splitting the macrocell or deploying small cells to increase capacity.
By splitting the macro cells operators can utilize more E-band radios because it will reduce the average cell site spacing. However, outdoor small cells require a regulated form factor in order to be deployed on streetlights and lampposts, which also rules out the use of conventional, large parabolic antennas. In fact, many jurisdictions focused on spectral reuse have already limited the minimum allowable antenna size for E-band radios in order to maintain narrow beam widths and minimize interference between links. In effect, this precludes the use of E-band radios in most outdoor small cell deployments. Currently, radios operating under area licensed regulatory regimes (e.g. sub 6 GHz or 24 and 28 GHz in North America) or 60 GHz license exempt radios do not have these antenna restrictions, making them a much better fit for small cell backhaul for the time being, although regulatory changes could have future impacts.
E-band radios have definitely been on an evolutionary track and are emerging as a more viable option for network operators and planners. By achieving near parity in feature sets with lower frequency radios, the next generation of E-band radios will be able to address many more deployment scenarios and will be given greater consideration by operators. That said, it is more likely they will join lower frequency radios as part of mobile operator’s broader toolkit, rather than be an all-encompassing solution to the many complexities that will be involved in upcoming microwave backhaul deployments.
