The Problem

Companies from all over the world have invested in spectrum and equipment to deliver portable and/or mobile high bandwidth services to consumers and business alike. Companies have made a clear decision to invest in licensed wireless access technologies for the delivery of these services in order to guarantee quality of service. WiMAX technology solves the last mile access problem, but how do you cost effectively
backhaul the traffic generated by these services?

The available spectrum for these applications is limited, depending upon the region of operation. Typical area licensed WiMAX spectrum allocations are:

• Lower 700 MHz (US)
  2 x 6 MHz channels
• 2.5 GHz (MMDS)
  3 channel blocks totaling 15.5 MHz in US
  72 MHz in Canada
• 3.5 GHz (WLL)
  2 x 28 MHz channel blocks
• 5.8 GHz UNI (license exempt)
  80 MHz allocation
  50% to 75% of spectrum not available due to sharing or interference

The amount of capacity from a given WiMAX base station is limited by the available spectrum and the degree to which the base station and antenna design can re-use the spectrum but is typically 1 to 2 bits per Hz. In many areas the available spectrum will be shared among several carriers, further limiting the effective capacity of a given base station. For an installation with 10 MHz of access spectrum and 4 sectors the total base station capacity will be 50 to 100 Mbps.

In order to be successful a carrier will need to get a reasonable penetration of subscribers to the new service set. For high bandwidth services only a limited number of end users can be supported at a given time off of a single base station due to bandwidth limitations. Thus, in a region of reasonable population density this implies that the WiMAX cell size for a successful carrier will be limited not by WiMAX propagation distances, but rather by the total capacity of the access spectrum.

These considerations will drive network architectures that use alternate technologies to connect the WiMAX base stations to the network core rather than using in-band communication between the WiMAX base stations themselves. First, all of the available WiMAX spectrum will be used for access in order to maximize the cell size and minimize the base station cost. Second, from a pure economic consideration, the value of the WiMAX spectrum as set by various recent auctions, places the value of a 10 MHz channel for a major metro area at tens of millions of dollars. The cost of using alternate technology is much lower than acquiring additional WiMAX spectrum, even assuming it was available and the WiMAX transmission gear zero cost – neither of which is true.

Finally, smaller cell sizes reduce potential blockage or interference issues, increase RF margin and hence increase peak connection capacity resulting in a more robust network with lower latency and high customer satisfaction.

Backhaul Options

The reduction in cell size and increased bandwidth per base station will require out of band backhaul technologies for existing tower locations. Further, as customer penetration increases over time additional base stations will need to be constructed to reduce the cell sizes and increase the spectral re-use in order to deliver the required bandwidth per subscriber. The technologies available for WiMAX backhaul are shown below.

Potential Backhaul Options:
1. Licensed microwave backhaul (i.e. DragonWave)
2. In-band
3. Build fiber or lease lines

Copper technologies are limited in bandwidth and reach resulting in multiple copper pairs being required for a base station backhaul. As leased E1/T1 circuits, the cost per bit is very high, the availability of the service poor (in practice < 99.99%) and the time to order new circuits is 3 to 9 months. When using xDSL technologies, the copper pairs must still be leased from the incumbent local exchange carrier – quite often a direct competitor. The lease cost dominates the lifecycle cost. Even though the capacity per copper pair is higher at shorter distances than leased E1/T1 circuits, at typical distances the capacities are similar. Further the management and fault isolation of these solutions are not yet robust enough for backhaul applications.

Fiber has effectively unlimited bandwidth and once the fiber is deployed has the lowest cost per bit, however the decreasing cell size drives more base stations reducing the percentage of base stations that can be connected via fiber. If fiber construction is required, the cost of construction is $30,000 to $160,000 per mile, depending upon the location of the base station. Further the timescale required to complete the construction is measured in months. Thus, if fiber is already in place it is the best option, however even in urban areas only a small percentage of the base stations are fiber connected.

Wireless backhaul has become the solution of choice for most successful WiMAX deployments. Given the high cost of the WiMAX access spectrum, most carriers are unwilling to accept the uncertain availability of a license exempt backhaul solution. Although the license exempt solutions do have a slightly lower capital cost, the difference in network cost when installation, maintenance, access and switching
equipment are included is negligible.

Licensed wireless provides the best trade-off between time to deployment, network cost and availability. When enhanced with ring/mesh redundancy and fiber backhaul from a small number of currently fibered sites, a viable network architecture can be delivered which meets the carrier’s needs.

WiMAX Backhaul equipment providers like DragonWave’s Horizon and AirPair products provides a high capacity, native Ethernet licensed backhaul that is ideally suited to WiMAX backhaul. The low latency (0.2 ms)enables any voice, data or video application, whether fixed, portable or mobile. The software scalable bandwidth results in an affordable first cost with zero churn scalability. Horizon and AirPair’s Ethernet mesh redundancy provides high availability through equipment and path redundancy at low incremental cost, organic network growth and 100% access to protection bandwidth for silver and bronze services.

Conclusion

As more high bandwidth services are deployed on WiMAX networks the need for an out of band backhaul solution becomes more apparent. In order to avoid downstream churn and a sub-optimal network topology, the backhaul network needs to be planned in conjunction with the access portion. Licensed wireless technology provides the best trade-off between time to deploy, network cost, scalability and availability of any of the candidate technologies. DragonWave’s products provides a high capacity, native Ethernet licensed backhaul that is ideally suited to WiMAX backhaul. For more information on DragonWave’s products, please go to www.dragonwaveinc.com.