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The Squeeze in the Middle

December 03, 2009

The Squeeze in the Middle

The onrush of bandwidth hungry smart phones and downloadable apps has created a crisis in the backhaul segment, or middle mile, of the mobile network. Around the world operators are coming to the realization that the two technologies that have the capacity, performance and cost per bit to deliver the ever increasing flood of mobile internet, streaming video and interactive gaming are packet microwave and fiber.

The reality is that in most countries the fiber penetration is below 20% of the base stations and so packet microwave will be used in the majority of 3G+, such as WiMAX (News - Alert), HSPA+ and LTE, base station deployments. This is good news for the packet microwave suppliers and they have responded to the challenge with higher capacity systems, reduced cost points and networkable IP features. There is another issue, however, that is still to be addressed.
Considerations such as spectrum availability, usage, and efficiency are becoming as important in the backhaul as they are in the radio access network. The middle mile is getting squeezed.
In regions such as Western Europe, where microwave is widely deployed, the backhaul spectrum is already congested and as a result it is difficult to get large channel allocations from the regulator. Indeed, most of the preferred channel allocations are already given out and it is a requirement that new base station deployments re-use the existing narrow channel allocations.
Yet, the requirement is to deliver at least an order of magnitude higher capacity when upgrading to 3G+ networks. In regions such as the United States where copper leased lines has been the predominant backhaul technology, spectrum availability is greater, but even here the major metros are starting to become congested and the situation is worsening as more roll-outs continue.
One response is to move to the higher frequency portions of the spectrum – above 38 GHz – that are not widely used at present in order to gain access to more spectrums. The issue here becomes one of propagation physics. The achievable reach reduces as the frequency of operation increases due to higher losses in the atmosphere.
Meanwhile, the spacing between base stations increases as one expands out from the metro core. The net result is that while these higher frequencies may have a role to play, they are far from being the panacea that will solve this problem.
Another means to address the issue is to increase the spectral efficiency. This can be achieved through the increasing the modulation depth: going from QPSK to 256 QAM will give about a four-fold increase in throughput. This does not come for free of course, as there is a reduction in system gain that accompanies this increase in modulation complexity. The response to this has been to deploy techniques such as adaptive modulation which allow the system to operate at the higher modulation rates, and then automatically step down to the lower more robust modulation rates when the path loss increases due to rain or some other atmospheric disturbance. This modulation shifting allows high value services to be maintained while still providing best effort bandwidth for activities such as internet surfing or video download. Even this best effort bandwidth is available more than 99.9% of the time.
Unfortunately, increasing the modulation depth in itself does not provide adequate throughput to support 4G services. Increasing the modulation index further is possible, but is a game of diminishing returns. Going from 256 QAM to 512 QAM easily doubles the complexity but only provides a 12 percent increase in throughput. The traditional response to this has been to deploy polarization multiplexing to use both the vertical and horizontal polarizations of the RF channel.
While this provides a 2 fold increase in throughput, it requires more than double the hardware and so actually results in an increase in the cost per bit. A much more efficient means to increase the spectral efficiency is to use base band techniques to eliminate white space between packets, compress the redundant information in the payload and headers and eliminate protocol conversion by having IP-based mobile networks. This combination of techniques can lead to another 2 to 4 fold increase in the throughput, depending on the traffic characteristics.
As an example, a 256 QAM radio with these bandwidth accelerating techniques operating in a 7 MHz channel can deliver up to 100 Mbps – the sweet spot for a single 3G+ base station. This approach enables the deployment of the new services while eliminating the costly and time-consuming effort of identifying and procuring new larger channels which would otherwise be required.
The increase in bandwidth hungry applications is indeed squeezing the middle mile, and is in fact driving transformation throughout the network. The good news is that the techniques to deal with this influx of bandwidth efficiently and cost effectively are available.
Perhaps it is time to measure the efficiency of our mobile networks in Mbits/MHz just like we do the gasoline efficiency of our automobiles. Spectrum (News - Alert), like gasoline, is a non renewable resource. Once it is used up there is no more so we would be best served to use it wisely.

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Edited by Kelly McGuire
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