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Reader Forum: All-indoor microwave – LTE’s best backhaul solution for North American operators

Editor’s Note: Welcome to our weekly Reader Forum section. In an attempt to broaden our interaction with our readers we have created this forum for those with something meaningful to say to the wireless industry. We want to keep this as open as possible, but we maintain some editorial control to keep it free of commercials or attacks. Please send along submissions for this section to our editors at: dmeyer@rcrwireless.com.

There’s a lot of buzz in the microwave industry about the trend toward all-outdoor radios, but those who haven’t been through LTE deployments may be surprised to learn that based on our experience deploying LTE backhaul for some of the world’s largest LTE networks, all-indoor is actually the best radio architecture for LTE backhaul.

We can debate today’s LTE backhaul capacity requirements, but one thing we do know is that with new advances in LTE technology, the capacity needed is going to grow. This means that microwave radios installed for backhaul will likely have to be upgraded with more capacity over time. Although people are experimenting with compression techniques and very high QAM modulations and other capacity extension solutions, the most proven way to expand capacity is to add radio channels because it represents real usable bandwidth independent of packet sizes, traffic mix and the RF propagation environment.

All-indoor radios are more expensive initially in terms of capital expenditures, but they’re cheaper to expand and (as electronics are accessible without tower climb) are more easily serviced. While an outdoor radio connects to the antenna with Ethernet or coax cable, indoor radios usually need a more expensive waveguide to carry the RF signal from the radio to the antenna. So you pay more up front with an all-indoor radio but as the radio’s capacity grows you save money. There are several reasons.

When everything related to the radio is indoors, you just have a waveguide and an antenna up on the tower. To add radio channels with an all-indoor radio you go into the cabinet and add an RF unit. With an outdoor radio, you have to climb the tower, which can cost as much as $10,000. Also, when you add a new outdoor RF unit you may have to swap out the antenna for a larger one due to extra losses incurred by having to combine radio channels on tower.

This is important for LTE because a lot of people don’t know how much capacity they’ll need. They may deploy one radio channel now for 150 megabits per second of service and need to add another 150 Mbps in six months. When you’re putting in a system you want it to be future-proof. When you add radios in indoor system, you can run multiple channels over the same waveguide and antenna. When you add radios in an outdoor system many scenarios will require a coupler, which introduces loss and necessitates a larger antenna. Operators pay lease costs based on vertical space on the tower, so the bigger the antenna, the higher the cost. Tower leases run approximately $100 per antenna foot per month.

According to Aviat’s total cost of ownership analysis, for most mobile operators the tower lease is 55% of the cost of an LTE deployment over ten years. Upgrading an outdoor radio increases lease costs, whereas with an indoor radio you can expand the radio without having to touch the tower equipment or change the lease costs.

Another point is that all-indoor radios are easy to access in all kinds of weather. In many cases, climbing a tower to service an outdoor radio may be problematic due to weather or the tower owner’s own priorities.

One objection raised when it comes to indoor radios is that there isn’t room for them. Telco cabinets are tight for space, and the theory is that an indoor radio takes up too much real estate to be practical. But this objection is based on old concepts of what an indoor radio looks like. While an indoor radio may have occupied a whole telco rack in the early 90s, today’s radios require less than 10 vertical inches of rack space.

Despite the lower TCO of an all-indoor radio, there are a lot of split-mount and all-outdoor radios are being made and sold. Here’s a summary of appropriate applications for each type of radio:

–All-indoor radios offer the lowest total cost of ownership for high-capacity applications in the 6 to 11 GHz frequency bands. If you’ve got a high-capacity application or you don’t know what your capacity is going to be, the lowest TCO comes with using an all-indoor radio.

–Split-mount radios (indoor mounted baseband unit and outdoor mounted RF unit) are ideal for 18 to 23 GHz applications, where high waveguide losses make all-indoor installation impractical, or low-capacity 6 to 11 GHz applications.

–All-outdoor radios are best considered when there is no cabinet space available. No baseband unit means all-outdoor radios have lower initial capex and lower installation costs than split mount radios. However lack of baseband unit limits flexibility and in many circumstances is not worth the cost savings. Deploying all-outdoor radios in rings, with TDM and IP, or in high power applications generally will require another device, and at that point you may as well have used split mount. For these reasons, we generally recommend all-outdoor radios only in cases where there is no place for anything indoors.

While vendors selling only all-outdoor and split-mount radios would have us believe that all-indoor radios are dinosaurs, the truth is that they provide the easiest maintenance, the lowest-cost upgrades and the lowest TCO in LTE applications. That’s why we are seeing our U.S. customers with LTE deployments using primarily all-indoor radios.

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