Interference Mitigation Technique
Original Publication Date: 2003-Dec-25
Included in the Prior Art Database: 2003-Dec-25
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The UK third generation (3G) spectrum allocation has been split into 12 paired frequency division duplex (FDD) bands and 4 unpaired time division duplex (TDD) bands, each band being 5 MHz wide, as shown in Fig. 1. Due to the close frequency spacing between adjacent bands there is considerable concern that adjacent carrier interference will lead to "dead zones" appearing in areas that suffer from high levels of adjacent channel interference and poor coverage from the serving network. An example of a dead zone scenario is illustrated in Fig. 2. In this scenario two networks are operating on adjacent bands and a mobile station (MS) of the first network is operating at the cell edge of a base station (BS) of the first network. In this case the first BS may not be able to transmit sufficient power in the downlink to compensate for the additional adjacent channel interference the MS is receiving from a nearby second network BS. Thus a dead zone appears in the first network's coverage centered on the second network's BS. The same mechanism that leads to adjacent channel interference in the downlink can also lead to significant adjacent channel interference in the uplink. Considering the same scenario as shown in Fig. 2, but assuming the first MS is not dropped, but transmitting at or near maximum power. As the separation between the first MS and the second BS becomes small, the interference experienced by the second BS from the first MS will become significant and could dominate the second BS causing a reduction in the cell's capacity. This is potentially a far more serious problem than the downlink interference case as this could result in dramatic fluctuations in the cell's capacity. There are a number of mitigation techniques that could be used to prevent an adjacent channel MS causing high levels of interference to a BS, the simplest being to switch the MS off. However, before any such action can be taken the MS or its network must first detect that the MS is operating in a manner that is causing significant interference to the adjacent channel operator. This is not a simple task as the victim BS will only be aware that the noise level within its cell has risen, but will not be able to ascertain the source and the MS will be unaware of the impact it is having on the adjacent channel.