The upcoming 700 MHz auction creates an opportunity to create a cost-effective way to bridge the digital divide to the nation's rural communities. However, this radio access spectrum is useless without an equally cost-effective backhaul solution to connect it to the rest of the World Wide Web.

Erik Boch

One of the virtues of the Web is it has the ability to shrink distance between people and businesses across the planet. So, fundamentally, with the presence of broadband connectivity, the Web could be an important impacting factor for rural residents, communities and businesses...that is, if broadband connectivity is available.

A fundamental limiting factor in the deployment of rural broadband has been the existence of broadband access network infrastructure. Wireless technologies have long been seen as one of the best ways to overcome this problem, but there has been a lack of available spectrum for broadband at low enough frequencies to overcome range and foliage losses.

Segments of 700 MHz spectrum are being freed up in the United States and are making their way into the hands of telecommunications operators. This spectrum has some interesting propagation attributes that originally made it interesting to the TV broadcast community and now make it well-suited to rural broadband deployments, including:

• long-range operation: large cell sizes, reduced base station costs
• low-loss penetration of foliage: small, low cost antennas
• low-loss penetration of building materials: indoor CPE
• small channel sizes: suitable for low subscriber density applications

As broadband penetration rates in metro areas near saturation, rural broadband (including fixed and mobile) may be the source of another wave of growth for operators. So, all this sounds like a great fit, but what are the problems? One problem is backhauling customer traffic from the rural base station site to the nearest metro where it can be tied into the WWW. Lack of high-speed fiber or copper and the cost-penalizing distances involved create a significant challenge for operators.

Figure 1

In most cases, cost-effective access to the WWW requires getting traffic to/from major metro centers where high bandwidth fiber-optic access to the WWW can be acquired. Generally, access to high bandwidth fiber or copper decreases as the distance from the (central) metro high bandwidth "core" networks increases. This also proportionally drives up backhaul costs and deployment time scales.

A solution to this is to employ high bandwidth microwave radio equipment to bridge larger distances between the metro and outlying 700 MHz cellular hub (base station) aggregation sites.

In order to achieve high bandwidth and high availability in backhaul, good choices for backhaul may be to operate in the 6 GHz or 11 GHz common carrier bands. In these bands, the FCC leases spectrum cost-effectively on a link-by-link basis for point-to-point microwave operation.

Figure 2

Using WiMax equipment to deliver access services means the traffic is essentially all connectionless Ethernet-based. This data-centric service payload is more efficiently and more cost-effectively backhauled on a native Ethernet backhaul network.

Ethernet-based backhaul allows the operator to consider availability-enhancing network topologies such as rings and constrained meshes. These topologies allow the operator to further enhance overall backhaul network availability and reach through the exploitation of angle-diversity and geographic-separation-diversity. (See Figure 1.)

Additionally, Ethernet-friendly wireless equipment features such as adaptive-modulation can allow further backhaul availability and reach improvements through the adaptive conversion of throughput-gain to system-gain within the radio equipment. (See Figure 2.) The link will operate at higher throughput rates during clear weather conditions (typically ~ 99.9% of the time), down-speeding adaptive during heavy rain. This enables the link designs to be initially conceived for operation over much longer link distances.

Combining the benefits of Ethernet, constrained meshes and adaptive modulation enable very long range, resilient, self-healing backhaul networks to be implemented to support large cell, rural and semi-rural 700 MHz access network deployments.

Boch is chief technical officer and vice president of
Engineering for Dragonwave. - eboch@dragonwaveinc.com