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Due to the rapid adoption of tablets and new bandwidth intensive applications, mobile data traffic doubled in 2011 and user demand is expected to increase 18-times by 2016. This tremendous capacity growth is causing many analysts in the industry to predict fiber will be deployed at all cell sites, and that microwave backhaul will not be able to keep up and therefore start to decline. A quick survey of microwave vendor’s collateral and press releases shows they haven’t given up yet. Certainly, fiber will be at many sites, and can easily meet the backhaul capacity requirements. However, with new technologies being rapidly introduced, there is a path to up to a 20-times increase in microwave backhaul capacities, ensuring this will be a viable backhaul solution for the foreseeable future.
The good news is that there are many vendors in the microwave industry focussing on this capacity challenge, resulting in a lot of scalability innovation. Most packet and hybrid microwave systems available today can deliver up to 400 Megabits per second of full duplex throughput, utilizing a 50 MHz or 56 MHz channel. While this is sufficient for today’s needs, operators are asking for scalability beyond a gigabit. Some of the innovations that are being discussed and announced at tradeshows include higher order modulations, MIMO, multi-channel transmission, header and bulk compression and wider spectrum usage. Let’s take a look at each of these, the network impact and the capacity effect.
The first and most commonly discussed capacity tool is modulation. Today, most systems operate up to 256 QAM. Many vendors have already announced systems up to 1024 QAM, and there is discussion about further scaling to 4096 QAM. The introduction of 4096 QAM would represent about a 50% capacity increase over existing 256 QAM systems, however it does have the effect of reduced link budget, which may result in larger antennas or lower frequency bands being required.
MIMO, which has been used in access systems in the past, is now being considered for point-to-point microwave backhaul. MIMO can allow the system to double the capacity, without any additional spectrum. However, MIMO requires a second transmitter and antenna, and requires enough room on the tower to allow the antennas to be separated by a distance that varies with the link characteristics.
Another important innovation that is being introduced in the microwave industry is compression. There are two types of compression. The first is header compression, which optimizes traffic based on the frame structure and common information. Header compression can offer up to 20% bandwidth improvement depending upon the underlying traffic. The second type of compression is bulk compression, which optimizes the throughput based on patterns in the traffic payload. Bulk compression offers up to a 100% throughput improvement depending on the traffic mix. The benefit of both bulk and overhead compression is that they require no additional hardware, and can be used to improve spectral efficiency. Higher order modulations, MIMO and compression are very suitable for existing frequency bands that are congested are expensive to acquire new channels in. With these three techniques, an operator can drastically improve their throughput in a single channel. If all three are combined, capacity in a single polarization of a 50/56 MHz channel could increase up to 5-times up to 2 Gbps.
There is also more spectrum being released in some of the higher bands, such as 26 GHz, 28 GHz, 38 GHz and 42 GHz. Some vendors are introducing new technologies to take advantage of this increased spectrum. For example, 112-megahertz channels are now commonly being considered, which could provide a doubling in capacity, with no additional hardware. This requires twice the spectrum, but has little link budget effect.
Another solution used to increase capacity is multi-carrier technology, which allows a single radio to transmit 2 to 3 channels without the addition of new hardware. Multi-carrier can also provide up to a 3-times increase in throughput, with minimum link budget impact, but a corresponding increase in spectrum utilization. In most cases, these two technologies would likely not be used together, as they each achieve similar results.
It should be noted that current systems and any of the technologies above can be used with the existing technology, cross polarization interference cancellation (XPIC), to offer a second polarization in the same channel, doubling the link bandwidth.
While it is clear that there is a significant bandwidth challenge for microwave backhaul, it is also clear that the vendors up to the challenge to innovate with new higher-capacity solutions. While the innovation isn’t done, the combination of the advancements discussed above can provide a path to much greater than 2 Gbps, with scale in the 5 Gbps to10 Gbps range when spectrum is available. With this level of performance, from a technology that is often one-tenth the cost of fiber-based solutions, we will surely see microwave as a viable backhaul solution for quite some time.
