Theoretical Analysis of Interference under Site Sharing

Here I write about a simulation analysis of the system performance deterioration caused by the following four types of interference: eNodeB->UE, UE->eNodeB, eNodeB->eNodeB, and UE->UE when two systems share the eNodeBs. During simulation, it is assumed that the time percentage of the interference occurrence is 100%, that is, interference is always there.

From simulation result with no filtering, when eNodeBs are co-located, the most serious interference is the eNodeB->eNodeB interference. The network coverage and capacity losses caused by the other three types of interference are smaller than 2% even if the guard band is not provided. Here i mainly describes the eNodeB->eNodeB interference and UE->UE interference.

Analysis of eNodeB->eNodeB Interference

The eNodeB->eNodeB interference can cause serious performance deterioration when the isolation between systems is not good. As Adjacent Channel Interference Ratio increases, network coverage shrinkage and capacity reduction also decreases.

Assuming the situation that eNodeB of the interfering system is in transmitting state and the eNodeB of the interfered system is in receiving state. Due to the “spill over” of transmitted signal into the receiving band, uplink coverage and capacity losses will occur. When total Adjacent Channel Interference Ratio of radio equipment (eNodeB, Filter, Combiners etc) is greater, the network coverage reduction and capacity losses are limited.

Based on our analysis, most installations can meet the interference requirement under a 5MHz guard band setting. Therefore, if interference exists within the entire network, a 5 MHz guard band should ensure the network coverage and capacity losses are limited.

Analysis of UE->UE Interference

When two systems are sharing the same site location, UE->UE interference does not seriously affect the network coverage and capacity. UE->UE interference comes about due to timing misalignment and one UE becomes the interfering system in transmitting state while another UE of the interfered system is in the receiving state. Hence, the downlink coverage and capacity losses occur as the interfered UE cannot perform in the best fashion.

As specified in 3GPP TS 36.101, both Adjacent channel selectivity  and Adjacent Channel Leakage power Ratio are required to be at least 33dB for a Class 3 and Class 4 mobile at +/- 5MHz from centre frequency. In addition, it is difficult to control the positions of the UEs through network planning. When two UEs of two systems are very close to each other, the isolation between the UEs is in the worst scenario, but only a small portion of the subscribers are affected and the influence is only temporary.

 Impact of UE interference to the network coverage

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