Deploying a broadband access network in the unlicensed band where the spectrum is free, is very tempting. So why are companies paying large amounts of money to buy licensed spectrum? The main reason is interference and its impact on the reliability of the network. The unlicensed band attracts many users with a variety of devices which all operate in the same frequency band causing potential interference. There are many misconceptions about interference, such as: because the allowed power levels are relatively low, interference is not a serious problem. To see why this is not necessarily true consider the following example.
Assume we have a basestation communicating with a user 1 mile away in an urban area in the 900 MHz unlicensed band. The pathloss between the user and the basestation (computed using the Hata model) is 140 dB. Next consider an interfering device located within line of sight of the basestation at a distance of 0.1 miles. The pathloss between the interferer and the basestation (computed using free space propagation) is 70 dB. We will say that the interferer has a “power advantage” of 70dB over the user. In other words, if the user and the device use the same power (more precisely the same effective radiated power, or ERP), the interfering signal at the basestation antenna will be 70dB stronger than the user’s signal. This is assuming the same antenna gain in the direction of the user and the interferer.
Next consider user located 10 miles from the basestation. The pathloss between that user and the basestation (computed using the Hata model) is 178 dB. In this case the interferer has a power advantage of 108dB!
We see that the interfering signal may be enormously more powerful than the desired signal because of the difference in pathloss. This difference is true regardless of the power levels used, as long as we assume that the user and interfere use the same power.
The situation can be somewhat ameliorated by the processing done at the receiver which may enhance the desired signal relative to the interfering signal. We will refer to this as the “processing gain” of the receiver. We will therefore say that the interference to signal ratio after processing (in dB) equals the pathloss difference (in dB) minus the processing gain (in dB).
The processing gain will depend on the characteristics of the communication systems as well as of the interference. In some cases the gain will be small or non-existent. In other it may be large. For example, if the communication system has a bandwidth of 20MHz and the interference is narrowband, the receiver may offer a large process gain. If the communication system and the interferer use exactly the same kind of signals, there may be no processing gain at all. Even if the processing gain is significant, it will generally be much smaller than the pathloss differences shown in the examples above, causing the interfering signal to overwhelm the desired signal from the user.
The point of this discussion is to show that an interferer near the basestation can “jam” the basestation and make it unable to communicate with the users. Similarly, an interferer located in close proximity to the user may “jam” that user and make it unable to communicate with the basestation. Here we focused on the basestation “jammer” because its effects are potentially more severe in that it can cause shutting down a complete cell, rather than just one or a few users.
Deploying a cellular network in the unlicensed band is therefore risky business. A single interferer in the wrong place can effectively disrupt an entire cell. One can hope that this is going to happen very infrequently, but when it happens the consequences can be quite unpleasant. So if you are committed to deploying in the unlicensed band be prepared to live with the sword of Damocles hanging over your head …