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Sunday, August 14, 2011

Mobile microwave backhaul: Point - to – Point (PtP) Vs Point - to - Multipoint (PtMP)- Which way to go?

Microwave radios play a key role in today’s telecommunication networks and are much more common than many may suspect. Though sometimes hidden from the eye, point - to - point (PtP) microwave radios make up almost half of the cellular backhaul connections worldwide and comprise more than two - thirds of worldwide connections outside of the United States. Radios are also common in carriers’ long - haul connections and metropolitan networks serving as fiber replacements.

Private enterprise networks, public safety and military networks, and utility companies are also utilizing microwave systems. Still, the most common deployment scenario of PtP radios remains the backhauling of cellular networks. When talking about mobile backhaul, let us have a look on the market size and its growth: The number of worldwide mobile subscribers was 3.3 billion in 2007 and was anticipated to grow to 5.2 billion by 2011. According to a report by Infonetics Research, the number of backhaul connections was expected to grow from 2.6 million in 2007 to 4.3 million by 2011.

The microwave radio is so widely deployed mainly because fiber cannot be made available everywhere. In addition, microwave makes for fast and cost-effective deployment compared to fiber.

Mobile technologies such as 3G, WiMAX, and LTE deliver high data rates and lead to a rapid increase in bandwidth - consuming applications. This requires operators to significantly expand their networks or to risk not being able to provide an adequate service quality. The main problem facing operators today is that the demand for bandwidth is not accompanied by a proportional revenue growth. Users demand data bandwidth but are not willing to pay a premium for it as they have for voice services. Instead, they expect a fl at - rate model like they get for their residential high - speed data services like DSL or cable.

Point - to - point (PtP) radio links consists of two symmetrical terminals transmitting and receiving signals between the two sites. Point - to - multipoint (PtMP) system, on the other hand, contains a central site (sometimes called “hub”) that is connected to number of remote terminals. Consider the illustration here to distinguish between these two backhaul solutions.
The illustration indicates a hub site connected to four edge sites by using four PtP radio links, or one PtMP 90 ° sector serving the four users.

Using a pure equipment - count comparison, it’s plain to see that PtMP has an edge. The PtMP sector will require 4 +1 equipment units and antennas, whereas the equivalent PtP solution requires 2 +4 units and antennas. But equipment count may not be the only consideration, and we should also look at the radio link budget. Since the PtMP sector antenna has a very wide beam width as compared to the PtP link that uses two directional antennas, PtP will have a significant advantage in system gain. Thus higher capacities can be delivered, or, in turn, an operator can use lower - power and smaller antennas to achieve similar results as in PtMP. For example, a typical sector antenna gain is 10 – 15 dBi, whereas a typical gain of a PtP antenna in use today is around 40 dBi and even higher. So, assuming that the remote radio antenna is identical, the difference in system gain can be around 30 dB in favor of PtP. This big difference is a major limit in reaching high capacities with high availably using PtMP radios. Another major difference between the PtP and PtMP is the use of the radio-frequency spectrum as a shared media versus dedicated media. The PtMP sector shares the same frequency channel among several users to allow more flexibility and enable statistical multiplexing between users. The use of a dedicated channel for each PtP link, however, does not allow using excess bandwidth of one user and pass it to another. Shared media PtMP also offers the benefi t of statistical multiplexing; but in order to enjoy it, operators must deploy a wide sector antenna that has a downside of interferences between other links at the same channel, thus poor spectrum reuse. In contrast, PtP antennas make it easier to reuse frequency channels while avoiding interferences from one radio link to another due to its pencil beam antennas. The wide - angle coverage of PtMP makes frequency planning more complex and limited, and frequency reuse is not trivial and hard to regulate with interferences between different hubs, especially with large coverage deployments such as mobile backhaul. Therefore it is uncommon to fi nd PtMP architecture in these applications, unlike radio access networks asWiMAX where PtMP is more commonplace.

As highlighted above each configuration has its benefits and downside. This can pose a great challenge to mobile operators as they contemplate on which solution they should consider for their networks.

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