With Long-Term Evolution (LTE) cellular networks forthcoming and WiMAX already expanding, it is now very clear where the weakness in 4G systems lies: backhaul. It is one thing to provide air interfaces that can deliver very high bit rates to and from mobile handsets, but another problem to aggregate all those high-speed data calls to and from the carriers’ switch and network.

Right now about 85% of all backhaul in the U.S. is by multiple T1/E1 wirelines with roughly 10% fiber and 5% microwave links. While T1 lines are basically adequate for 3G, they can no longer deliver the data throughput necessary for the new 4G systems. And it is often difficult to get the right of way, much less afford the cost of fiber.

The trend is to follow the European model where over 70% of backhaul is microwave. But even that is getting difficult. The traditional microwave bands devoted to backhaul (6 to 38 GHz) are filling up, and in big cities and metro areas there is already an interference problem or lack of additional space and licenses. The solution is to push higher into the spectrum. That is why there are multiple 60-GHz backhaul solutions on the market and a growing presence of 80-GHz systems. Yes, 80 GHz.

I recently spoke with the executives of BridgeWave, a microwave backhaul equipment manufacturer. They are pioneers in the 80-GHz space, having introduced systems in this range as far back as 2006. They are now beginning to experience significant growth in their 80-GHz products as carriers begin to work on their 4G infrastructure. In the past, most of the BridgeWave systems were short-range links used in enterprise networks. Today, the mobile backhaul market share is growing, and as much as 60% to 70% of new systems is expected to go to that application.

The 80-GHz spectrum is divided into two general segments: 71 to 76 GHz and 81 to 86 GHz. It is a licensed spectrum segment, unlike the unlicensed 60-GHz spectrum. License fees are a low $75 compared to thousands of dollars for a license in the lower bands. The 60-GHz segment is also used for backhaul, but the oxygen absorption level at that frequency restricts it. This causes huge path losses in the air, far greater than the well-known Friis formula predicts.

But with sufficient power, radios in the 60-GHz space can easily achieve a range of up to about 2 miles without difficulty. Beyond that, they are not too useful. That is why the 80-GHz segment has become the go-to place for backhaul.

A good example of an 80-GHz backhaul solution is BridgeWave’s 80-GHz FlexPort system (see the figure). It can achieve a maximum data rate of 1.2 Gbits/s. The neat thing about this system is that it can carry both legacy time-domain multiplexing (TDM) traffic as well as IP packets natively. That means that carriers who adopt this system can address their forthcoming 4G upgrade needs while supporting their existing backhaul networks.

Many assume that there will also be more LTE basestations needed to cover the same area with the proposed data rates, which means even more backhaul will be needed. An 80-GHz solution seems to offer the cost and low-power needs.

Some of the key features of FlexPort are configurable 1-GHz bandwidth RF channels including frequency agility, which aids in network planning and design as well as the ability to switch on the fly to another channel if interference or other conditions warrant. The modulation is reliable quadrature phase-shift keying (QPSK), which is extremely efficient in the use of spectrum. Binary phase-shift keying (BPSK) is used for lower data rates and more difficult link conditions. FlexPort also supports four Sonet/synchronous digital hierarchy (SDH) ports and five Ethernet ports. Because of its built-in gigabit Ethernet layer 2 switch, the FlexPort provides the low latency necessary to provide quality of service (QoS), virtual local area network (VLAN), and jumbo packets as required by Carrier Ethernet.

As for power levels, it is very low, in the tens of milliwatt range up to 100 mW and adjustable. But thanks to antenna sizes of one and two feet in diameter, the gain is in the 44- to 51-dBi range, giving huge effective radiated power increases with beam widths of 0.9° and 0.4°, respectively. That gives the FlexPort units a range of about 5 to 6 miles (8 to 9.5 km), line of sight. The narrow beamwidths also provide inherent security.

If you are looking for a way to upgrade your mobile system backhaul or add new links, provide WiMAX backhaul, or add backhaul for video surveillance systems, consider an 80-GHz system. Pricing is low compared to some of the other alternatives.