Choosing the Right Frequency for Point-to-Point Radio Connections

Fortunately, the Federal Communication Commission (FCC) has opened new unlicensed frequency bands in the 24 GHz and 60 GHz bands that could solve the problem. By using these bands for backhaul and the existing bands for wireless data transfers, designers can solve potential interference headaches. Let's see how.

Point-to-Point and Point to Multipoint: Living in Harmony
PWLAN hotspots and distributed/multi-location LANs may employ wireless technology within the access layer as well as in the inter-LAN-segment and backhaul layers (Figure 1). The primary difference between LANs and hotspot lies in the user traffic and applications involved. Generally, a LANs traffic is concentrated within work groups which are generally serviced by isolated LAN segments whereas within a hotspot, very little of the traffic is peer-to-peer within the LAN...it is mostly destined for the Internet.

In these scenarios unlicensed operation is attractive in the user access layer simply because of three things

• The unlicensed nature of the technology
• The costs of the associated equipment involved (access points and PC NIC cards)
• The standardized interfaces employed (i.e. IEEE 802.11 a/b/g)

Therefore, the deployment of the network relies on applying the maximum amount of capacity to the access layer users since this is where the revenue is being made (or where the applications are being used in the case of a corporate LAN). This then dictates the desire to apply all available unlicensed RF channels to the access points (APs) within the access layer of the network.

The point-to-point links in the LAN interconnect and backhaul layers are therefore deployed on other frequencies since it is not desirable to consume valuable unlicensed spectrum on these functions. The fixed-outdoor, longer-range nature of these point-to-point links makes them subject to detrimental interference. In contrast to the user access layer, where mobility enables the user to "fix" RF blockage and locational interference problems, the fixed outdoor nature of these point-to-point links makes use of the 2.4, 5.2, and 5.8 GHz ranges a poor choice for medium to long term high capacity applications that depends on interference free operation.

Therefore, moving from the very short range, indoor environment—where the user essentially fixes any link problems—to the longer-range outdoor environment—where "fixes" are network-affecting and costly to address—changes the way the service provider tends to think about using unlicensed technologies. There is a notion that interference can be dealt with through coordination, but the fact is that many sources of interference are not coordinatable or are not bothered by it and are therefore not overly motivated to do anything differently.

In the case of hotspot deployments, the user access layer and backhaul layer may be parts of a larger, layered metropolitan network construct. As shown in Figure 2, the lower layers (user access) generally have much lower availability and reliability demands than the higher layers. Interference susceptibility needs to be carefully examined in the context of the whole network. A number of factors may affect deployment solutions, including:

• Revenue dependencies
• Number of users impacted by an outage
• MTTR of any network segment

Choices
With the assumption that the 2.4, 5.2, 5.8 GHz bands are optimally deployed/consumed in the user access layers of the network, designers need to understand what the choices are for deploying the point-to-point links that interconnect the LAN segments in various buildings or operate as the backhaul for a hotspot.

Unlicensed broadband wireless offers a quick and easy deployment option without the associated regulatory process or fees associated with licensed frequencies. The downside of the unlicensed frequencies is that other users can become uncontrolled sources of radio interference, which causes all of the interfering systems to degrade in performance and can impact or even destroy a given broadband link. This degraded performance can unpredictably change the transmission performance of the system, including significant changes/reductions in transmission bandwidths. It is important to note that the other "users" of the spectrum may or may not be "coordinatable" entities—you cannot coordinate with a home users handi phone or microwave oven.

Figure 3 conceptually illustrates the major differences between the radio engineering rules allowed for the "broadcast centric" point to multipoint non-line of sight access models typically used in the 2.4, 5.2, and 5.8 GHz unlicensed bands compared to the point-to-point, line-of-sight access model intended for the 24 GHz, and 60 GHz unlicensed bands. These differences make 24 GHz and 60 GHz unlicensed bands well suited to high capacity high bandwidth high performance applications (for example direct to enterprise voice and data, traffic aggregation and/or backhaul applications).

When interference strikes a fixed broadband radio link, it can be permanent or transient in nature. Debugging these types of interference problems requires a high degree of uncommon RF field expertise and this can easily become time consuming and costly to fix. Determining that a problem exists in a fixed wireless link is only the first step. The next step is to determine where the source of interference is coming from. This in itself is a challenge since the location determination process requires two-dimensional angle and range information whilst the debugging activity only really produces the needed angular data.

Focusing on the operation of the point-to-point links in the network, it is then imperative to consider the interference attributes of the available spectral allocations to ensure that the exposure to potential interference is well understood. When applying these technologies to a given network, the relationship between interference, availability and operating frequency shown in Figure 4 is applicable.

Spectral Allocations
There are essentially four spectral allocations that are used for broadband point-to-point applications in North America:

• 2.4, 5.2, 5.8 GHz
• Licensed operation (i.e. 18 GHz or 23 GHz)
• 24 GHz
• 60 GHz
• 80 GHz/95 GHz

Let's look at each of these in more detail starting with 2.4, 5.2, and 5.8 GHz.