About Interference and Wireless Channel
What is Interference?
In the wireless world, interference, by definition, is a situation where unwanted radio signals operate in the same frequency channels or bands – i.e. they mutually “interfere,” disrupt or add to the overall noise level in the intended transmission. Interference can be divided into two forms based on whether it comes from your own network(s) or from an outside source. If the interfering RF signals emanate from a network under your control, whether it is on the same cell tower or several miles away, it is termed “self-interference.”
If the opposing signals come from a network, device, or other source that is not under your control, it is termed “outside interference.” Thus, the definition of what type of interference is being combated is not based on technology, but ownership. In licensed bands, it is self-interference alone that must be taken into account; however given a more or less known operating environment (the radio spectrum will only have signals transmitting that are under
control by a single entity) proper product design and network deployment can reduce these interferers to a level where they do not impact network performance.
Self-interference is not a phenomenon that is confined to licensed band operations; license-exempt bands must address the same issues.
The techniques and design elements of a given product that serve to reduce and tame self-interference in licensed band operations can be applied directly to the license-exempt systems. Taking advantage of the experience gained in cellular network product design, the Motorola Canopy BWA system embodies many of these same features and deployment guidelines in delivering reliable, self-interference-free operation.
The License-Exempt Challenge
In the license-exempt bands, not only must self-interference be accounted for, but, given the nature of the regulations governing these bands, external interference must be designed for as well. This can be extremely challenging, as there is no way of knowing in advance
where these outside signals may be or will be sourced from, or even how strong the interfering transmissions will be relative to the desired transmission. This aspect of the license-exempt bands represents the possible “downside” to license-exempt network operation.
There are several frequency bands that have been allocated as license-exempt in the US. Internationally, these bands generally cover the same frequency spectrum but with some variations in allowed power limits, channel size, etc. Delving slightly deeper, there are two sets of rules applied to the license-exempt bands:
Industrial, Scientific and Medical (ISM) – three frequency allocations that operate under these rules:
902 MHz to 928 MHz, 2.40 GHz to 2.4835 GHz and 5.725 GHz to 5.85 GHz
Unlicensed National Information Infrastructure (U-NII) -
5.15 GHz to 5.25 GHz, 5.25 GHz to 5.35 GHz and 5.725 GHz to 5.825 GHz.
The primary difference between the rules governing these bands has to do with whether or not the signal must be spread. In the U-NII band spreading is not required, which generally allows greater data rates than equipment in the ISM bands.
The U-NII spectrum has only recently (within the last four years) been allocated in the US (with international adoption slightly behind), while the ISM bands have been accepted for years. Because of this, in many countries around the world, including the US, operation in the
2.4 GHz band can be very difficult, since a great deal of equipment has been deployed, already crowding the air waves. In addition, numerous consumer devices, such as cordless phones, WLANs, garage door openers and even newer fluorescent light bulbs, are designed to operate in this band, adding to the amount of external interference.
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