Interference management techniques are critical to the performance of heterogeneous cellular networks, which will have dense and overlapping coverage areas, and experience high levels of interference. Fractional frequency reuse (FFR) is an attractive interference management technique due to its low complexity and overhead, and significant coverage improvement for low-percentile (cell-edge) users. FFR sets restrictions on RB allocation between the different UEs in each cell.
FFR method separates the frequency bands allocated to the areas near a base station where no signal interference from adjacent base stations occurs from the frequency bands allocated to the areas far from the base station where signal interference from an adjacent base station can occur. The transmit power is reduced and the frequency-reuse factor is set to 1 for the frequency band allocated to the cell area where no signal interference from adjacent base stations occurs, and conversely, the transmit power is increased and the frequency-reuse factor is set to 3 for the frequency band allocated to the cell area where signal interference from an adjacent base station can occur. This improves the signal to interference plus noise ratio (SINR) and throughput for users located at the cell edge without degrading spectral efficiency.
To implement FFR, the frequency band allocated to a user at a cell edge must be different from that allocated to another user in the adjacent cell, as shown in Figure above . The LTE standard specifies an inter-base-station interface that enables adjacent base stations to exchange information on bands generating large interference in other cells and on bands that are affected by large interference from other cells.
The following describes interference-coordination signals that can be used for implementing ICIC in the downlink and uplink.
1) Downlink
The signal used for interference coordination in the downlink is called relative narrowband transmit power (RNTP). This signal can take a value of 0 or 1 and is sent to multiple base stations serving adjacent cells for each resource block (RB).note 2) Specifically, this value is set to 0 if the ratio between the transmit power of the downlink signal allocated to the RB and the average transmit power of the system frequency band is guaranteed to be under a certain threshold and to 1 otherwise.3) This scheme enables a base station to learn about an RB that may be transmitting at high power in an adjacent cell and to reduce interference by avoiding allocating that RB to a user experiencing poor reception. Increasing the transmit power above the system average for a user experiencing poor reception should also improve the quality of that users reception.
2) Uplink
There are two types of signals for interference coordination in the uplink: high interference indicator (HII) and interference overload indicator (OI). The HII signal is used by a base station to notify to multiple base stations serving adjacent cells of the uplink RB it has allocated to a cell-edge user. This enables cell-edge users in adjacent cells to be allocated different bands, the same as in the downlink approach, which means that improved throughput can be expected for these cell-edge users. The OI signal, on the other hand, is used by a base station to notify to multiple base stations serving adjacent cells the results of measuring interference power for each RB and classifying those results into multiple levels. Thus, the base station of a cell that receives notification of high interference power from an adjacent cell can reduce the transmit power of its users and thereby reduce the amount of interference created in the adjacent cell.
Reference
FFR method separates the frequency bands allocated to the areas near a base station where no signal interference from adjacent base stations occurs from the frequency bands allocated to the areas far from the base station where signal interference from an adjacent base station can occur. The transmit power is reduced and the frequency-reuse factor is set to 1 for the frequency band allocated to the cell area where no signal interference from adjacent base stations occurs, and conversely, the transmit power is increased and the frequency-reuse factor is set to 3 for the frequency band allocated to the cell area where signal interference from an adjacent base station can occur. This improves the signal to interference plus noise ratio (SINR) and throughput for users located at the cell edge without degrading spectral efficiency.
To implement FFR, the frequency band allocated to a user at a cell edge must be different from that allocated to another user in the adjacent cell, as shown in Figure above . The LTE standard specifies an inter-base-station interface that enables adjacent base stations to exchange information on bands generating large interference in other cells and on bands that are affected by large interference from other cells.
The following describes interference-coordination signals that can be used for implementing ICIC in the downlink and uplink.
1) Downlink
The signal used for interference coordination in the downlink is called relative narrowband transmit power (RNTP). This signal can take a value of 0 or 1 and is sent to multiple base stations serving adjacent cells for each resource block (RB).note 2) Specifically, this value is set to 0 if the ratio between the transmit power of the downlink signal allocated to the RB and the average transmit power of the system frequency band is guaranteed to be under a certain threshold and to 1 otherwise.3) This scheme enables a base station to learn about an RB that may be transmitting at high power in an adjacent cell and to reduce interference by avoiding allocating that RB to a user experiencing poor reception. Increasing the transmit power above the system average for a user experiencing poor reception should also improve the quality of that users reception.
2) Uplink
There are two types of signals for interference coordination in the uplink: high interference indicator (HII) and interference overload indicator (OI). The HII signal is used by a base station to notify to multiple base stations serving adjacent cells of the uplink RB it has allocated to a cell-edge user. This enables cell-edge users in adjacent cells to be allocated different bands, the same as in the downlink approach, which means that improved throughput can be expected for these cell-edge users. The OI signal, on the other hand, is used by a base station to notify to multiple base stations serving adjacent cells the results of measuring interference power for each RB and classifying those results into multiple levels. Thus, the base station of a cell that receives notification of high interference power from an adjacent cell can reduce the transmit power of its users and thereby reduce the amount of interference created in the adjacent cell.
Reference
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