AWS auction spawns spectrum shuffle
Last year’s auction of advanced wireless services, or AWS, spectrum (MRT, October 2006) has generated great enthusiasm on the part of commercial mobile phone carriers hoping to deploy third-generation services. To potential AWS licensees, the new 1700 MHz and 2100 MHz spectrum represents the additional capacity that is so desperately needed for high-speed wireless data services. However, for those mobile radio incumbents already using this spectrum for microwave backhaul applications, tough decisions lie ahead.
The spectrum auctioned by the FCC comprises the 1710 MHz to 1755 MHz band, currently utilized by federal government organizations, and the 2110 MHz to 2150 MHz band, allocated to non-government organizations such as rail companies and commercial carriers, though some state governments also use this band. It is likely that the new AWS licensees will want to utilize the spectrum right away, particularly in key markets. This means mobile radio incumbents will need to find new backhaul routes — and quickly — in order to vacate the spectrum without disrupting existing services. Speed of deployment, then, will be a key factor in the choice of new backhaul mechanism.
This all might seem like hard luck for the mobile radio incumbents. However, the cost of the relocation will be borne by the new AWS licensees, and with change can come opportunity. The ability to reassess backhaul options and upgrade to new digital systems should compensate for much of the inconvenience.
Incumbents have several backhaul relocation options: lease of existing T-1 lines, deployment of fiber networks, utilization of existing microwave backhaul systems in neighboring bands, or relocation of wireless backhaul services to different bands. It will most likely come down to each of these options being considered case-by-case. Each link will be assessed in terms of geography, interference, available infrastructure and available capacity.
The leasing of T-1 lines may seem a convenient option because they already exist — especially in major cities — and therefore come unhindered by major capital expenditure. However, there is significant and ongoing operational expenditure. In fact, taking into account the leasing cost and accompanying interest rates, utilizing T-1 infrastructure arguably can be the most expensive backhaul system option over the long term.
Incumbents also should be mindful that T-1 networks in rural areas may not be readily available, and quality of service (QOS) may not be reliable. Remoteness may engender poor response times when maintenance and servicing is required. As a “tenant,” carriers have no control over the network they are using. Worse, T-1 providers usually are carriers themselves, meaning that tenants are pumping money directly into the pockets of their competition.
Fiber networks similarly have their own pros and cons. On the plus side, they have very high capacity, and costs are diminishing. However, cost is directly proportional to distance and can become prohibitive for long links. Also on the negative side, fiber is difficult to deploy across rugged terrain. Moreover, if fibers are broken — which can happen because of natural disasters — it can be a long time before services are restored, especially in more remote locales.
Of the available options, microwave radio links remain the most flexible. They can cover great distances without incurring extra cost, can be deployed quickly and offer payback periods of as little as two years. Offering superior reliability, microwave links also are relatively easy to restore in the event of misalignment because of natural disasters. Furthermore, with microwave backhaul, users have total control over the system.
The frequency bands specified by the FCC for microwave backhaul relocation are illustrated in the figure on page 32. Federal government incumbents currently using the 1710 MHz to 1755 MHz band can opt to use existing systems in the 1750 MHz to 1850 MHz band or deploy new systems in the 4 GHz or 7 GHz bands. Similarly, non-government mobile radio incumbents (2110 MHz to 2150 MHz) can use existing 2450 MHz to 2483 MHz systems or relocate to the 6, 10, 11 or 18 GHz bands.
Again, decisions regarding band selection for radio link networks will be made on a case-by-case basis, taking into account geography and distance to be covered, available infrastructure, interference issues and capacity.
The most expedient radio link solution, at first glance, appears to be the reuse of existing microwave antenna systems in neighboring bands: 1750 MHz to 1850 MHz for government applications or 2450 MHz to 2483 MHz for non-government. Yet such a solution would be makeshift at best because significant modifications to the antenna system would be required to support the new frequencies and provide optimum performance. Moreover, many existing networks are comprised of aging, analog equipment with limited capacity. In any case, the 2450 MHz to 2483 MHz band is capacity-rated to 8 DS-1s (about 12 Mb/s), a substantial limitation.
These limiting factors provide the justification to upgrade to new radio link systems utilizing digital microwave technology, where capacity is generally much less of an issue. Nevertheless, in selecting the appropriate band for deploying a new microwave backhaul network, incumbents still need to work within the constraints of FCC capacity regulations, available spectrum and the laws of physics.
Of the bands allocated for backhaul relocation, those at the lower-frequency end (4 and 7 GHz) would naturally be preferred from the perspective of propagation and rain attenuation. These bands are ideal for longer distances.
The government incumbents, allocated the 4 and 7 GHz bands, should have little difficulty in relocating backhaul systems here (approximately 960 links nationwide). The FCC has designated both bands as high capacity; in addition, being government bands, there should be plenty of spectrum available.
The 6 GHz band, allocated to non-government entities, is divided into two bands: 5.925 GHz to 6.425 GHz (OC-3 high-capacity band) and 6.425 GHz to 6.875 GHz (restricted to medium-capacity DS-3). Although the lower half of the band is ideal for long-distance microwave backhaul applications, it is quite congested, and interference analyses will be imperative if new networks are to be added — assuming channels are available. The upper half of the band has a capacity restriction and will only be suited for medium-capacity applications.
This will lead non-government entities — which account for a far larger volume of the backhaul relocation (approximately 5700 links nationwide) — to consider the higher-frequency bands of 10, 11 and 18 GHz.
The 10 and 11 GHz bands both provide a good compromise between rain attenuation, propagation and radio link network congestion (which is low), making them ideal for medium-distance microwave links (10 to 20 miles). Although the 10 GHz band (10.5 GHz to 10.7 GHz) is low capacity — 5 MHz channels restricted to just 16 DS-1s — the FCC rates the 11 GHz band (10.7 GHz to 11.7 GHz) as high capacity, with 40 MHz channels allowing data transfer at 3 DS-3s or OC-3.
Until recently, the stringent FCC Part 101 Category A requirements for radiation pattern control provided an additional challenge for adoption of the 10 GHz and 11 GHz bands, leading to their under-utilization. Many carriers, which haven’t necessarily required the gain of a 4-foot antenna, have been obligated to install one to meet the required radiation patterns (which are easier to achieve with a larger antenna). This has meant additional cost and overly complicated site negotiations and permit applications for the larger-than-necessary antenna.
Breakthrough antenna designs, however, have yielded 3-foot diameter antennas that meet the Part 101 Category A requirements. This will facilitate adoption of the high-capacity 11 GHz band in particular. In addition to the positive cost impact, smaller antenna diameters lead to lighter tower loading and streamlined site negotiations.
The final band available for backhaul is the medium-capacity 18 GHz band (17.7 GHz to 19.7 GHz), which is suitable for short and medium links (roughly 10 miles or less) because of adverse rain impact and overall lower propagation characteristics. Nevertheless, in applications where short distances are practical, this will be a viable option for backhaul relocation.
It is evident that there are a great many options open to those mobile radio incumbents in the newly allocated AWS bands that will need to relocate backhaul services — whether they choose to stick with the flexibility and performance of microwave radio link networks or adopt alternative solutions. As with other recent spectrum shuffles, each case will be heavily arbitrated.
Several things are certain, however. Relocation must be achieved with near-zero downtime to incumbent networks or serious consequences will ensue. Similarly, the replacement backhaul network must operate with the same or better QOS, and the affected parties will need to mutually agree on any decisions regarding the backhaul system. Despite the initial flurry of activity on the part of new licensees, the reality is that incumbents are expected to have up to three years to relocate.
Clearly there is no single solution, and it will be critical for all parties to make informed decisions that encompass both short-term contingencies and long-term strategies. Obtaining expert advice from trusted sources will be essential. The critical issue of fast deployment is likely to favor the adoption of microwave networks, and — although incumbents in the 1700 MHz and 2100 MHz bands are still confronted by many unknowns — these microwave radio link network solutions are ready and available.
Asad Zoberi is area product manager of microwave antenna systems for Radio Frequency Systems , a supplier of cable and antenna systems plus active and passive RF conditioning modules.
Backhaul options
Pro | Con | |
---|---|---|
T-1 lines | ▪ Already exist, resulting in low CAPEX | ▪ On-going leasing costs are high, resulting in high OPEX ▪ Limited in rural areas ▪ Owners have no control over backhaul network |
Microwave backhaul | ▪ Cover great distances at no extra cost ▪ Easy to restore after natural disasters ▪ Easy maintenance ▪ Owners have total control over the network |
▪ Spectrum availability is limited ▪ Upfront CAPEX is required ▪ Resistance to new tower infrastructure |
Fiber-optic backbone | ▪ High capacity | ▪ Cost proportional to distance ▪ Difficult to lay over rugged terrain ▪ Fiber cuts difficult to repair |