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Technological advancements to decrease cost and increase capacity
Microwave backhaul networks continue to scale with increased capacity to meet the data demands mobile users are placing on networks. Advancements in technology, higher capacity configuration, such as 2+0, and increased spectrum have all played a roll in this mobile network expansion. Yet, at the same time, the cost of annual backhaul spectrum has gone up as well, with most countries outside of the United States seeing prices between $1,000-$4,000 per year for a 28-megahertz channel that is, in most practical applications, limited to about 200 megabits per second. What’s more, in many countries backhaul spectrum is so congested even acquiring a channel can often be difficult. As a baseline, 28-megahertz channel spectrum lease charges account for between 50% to 75% in a seven-year total cost of ownership equation.
Clearly, there are spectrum challenges with current 200 Mbps links, and scaling to 400 Mbps or higher utilizing traditional technology is problematic and costly. When additional spectrum is available, the options are to use a wider 56-megahertz channel or deploy a 2+0 system that consumes 2×28-megahertz channels. However, both of these options result in a doubling of already very expensive spectrum lease charges.
The question operators are asking is: what advances in the spectral efficiency of microwave backhaul systems can enable greater scalability? In the past, the most common technological improvement for spectral efficiency has been modulation. Most systems today offer 256 QAM, which can deliver up to 200 Mbps in the 28-megahertz channel. Newer systems on the market are now offering 2,048 QAM, which provide about a 35% improvement over 256 QAM. While there is some link budget reduction, it can typically be managed through the use of adaptive modulation, which will switch back down to 256 QAM or lower during a link fade event. Recent advancements in microwave backhaul spectral efficiency are also being considered.
Some microwave systems now offer compression techniques that further improve spectral efficiency. Utilizing header optimization, which removes common fields from headers, can provide a 10% to 20% throughput improvement. More significantly, more advanced systems are incorporating bulk payload compression. This technological advancement analyzes the traffic, looks for bit patterns, and replaces them with shorter symbols, and has been found to offer up to 150% throughput improvement. When both these technologies are deployed, speeds greater than 500 Mbps are achievable in a single 28-megahertz channel. In comparison, traditional microwave systems would require 3×28-megahertz channel and three separate microwave systems that would require two antennas. As a result, an operator would incur three-times the annual spectrum costs, double the tower lease costs, and increase the capital expense costs by up to 75%.
These same spectral efficiency techniques can also improve total cost of ownership on existing, lower capacity links. For example, with these new features in play, 200 Mbps to 250 Mbps can be delivered in a single 14-megahertz channel rather than a 28-megahertz to 56-megahertz channel that would currently be used. This provides up to a 75% decrease in the recurring annual spectrum costs.
Another emerging technique that improves microwave spectral efficiency is multiple-input, multiple-output. MIMO can double the capacity in a single channel through spatial separation and by transmitting two signals over separate antennas. MIMO can be used in combination with 2,048 QAM and acceleration to provide further spectral efficiency improvement.
MIMO’s challenge is that the separation of the antennas is very dependent on the link details, including frequency band link length. The antenna separation for many links often needs to be up to 10 meters. This is a difficulty for operators, as it requires them to fin two exact mounting locations on what is typically rented tower space. Each mounting location will incur monthly lease charges. Equipment cost and installation costs are also doubled versus a 1:0, because MIMO requires twice the radios and antennas. These factors and deployment challenges make it unlikely that MIMO will be deployed on a network wide scale. However, it may be used in combination with 2,048 QAM and compression on unique problem links to provide very high capacity, such as one gigabit per second in a 28-megahertz deployment or in locations where only a 14-megahertz channel is available to deliver up greater than 500 Mbps.
Another spectral efficiency technique often discussed is XPIC. XPIC allows microwave systems to use both the vertical and horizontal polarization of the same channel on the same link. However, once the polarizations are used on a link, they cannot be used on adjacent links where they would normally be deployed, so that the adjacent links would require different channels. As a result, XPIC does not provide any network-wide spectral efficiency benefits. And, because the telecom regulator typically bills each polarization of a frequency channel separately, there is no spectrum cost savings. That said, XPIC can be useful in a network in select single link cases where only a single dual-polarized channel is available. These cases are fairly rare and deployment of XPIC has typically been limited to a small percentage of links in networks.
Spectrum costs represent a significant portion of total backhaul network costs, and it is clear that more spectrally efficient technologies are crucial to cost effectively scale networks to meet capacity demands. While the cases above all assume more spectrum would be available, in reality many operators are restricted by the amount of available spectrum. In that case, improving spectral efficiency becomes paramount in order to avoid negative impacts on service revenue. High order modulation and compression will be key technologies in enabling this much-needed spectral efficiency. MIMO and XPIC can also be useful on special network scenarios, although they are much more difficult to be deployed on a network wide basis.
Greg Friesen is the VP of product management at DragonWave, responsible for global product management responsibilities across the company’s complete portfolio of products. This role includes regular interaction with customers to understand their evolving network requirements.
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