LTE is a long term evolution of the technique standard of the Universal Mobile Telecommunications System (UMTS) established by the 3rd Generation Partnership Project (3GPP) organization, and is approved regularly and started in the meeting of 3GPP TSG RAN #26 in December, 2004, in Toronto. An LTE system introduces critical transmission techniques such as Orthogonal Frequency Division Multiplexing (OFDM) and Multiple-Input Multiple-Output (MIMO) of multiple antennas, and increases the frequency spectral efficiency and data transmission rate significantly. The peak data rate can reach 50 Mbit/s for uplink and 100 Mbit/s for downlink. Further, the system supports various distribution of bandwidth, and thus the distribution of frequency spectral is more flexible, with the capacity and coverage of the system significantly increased. The architecture of the LIE wireless networks is more flattened, reducing the system delay and reducing the costs for network building and maintaining.
In addition, in LTE-A, in order to acquire sufficient transmission bandwidth (such as 100 MHz) to support higher uplink and downlink peak data rate (such as 1 Gbit/s for downlink and 500 Mbit/s for uplink), the carrier aggregation technique can be adopted to aggregate a plurality of component carriers to obtain transmission of high bandwidth.
In LTE R12, small cells are introduced. A small cell is a wireless access node with low power, operates in an unlicensed or licensed frequency spectral, and can cover a range of 10 m to 200 m. In contrast, the coverage range of a macro cell can reach as much as several kilometers. The small cell fuses the techniques of femtocell, picocell, microcell and distributed wireless techniques. The small cell has a very small volume and can be used indoor and outdoor. Generally, the small cell can cover the indoor space of about 10 m or a range of 2 kilometers in the open air. As shown in FIG. 1, where the multiple of smaller circles represent small cells and the three relatively larger circles represent macro cells, it can be seen that the small cells are characterized in that they are densely disposed with the coverage ranges crossed mutually. In the scenario of LTE-A heterogeneous networks, the macro cells realize wide area coverage and the small cells realize Hotpoint coverage.
Considering that a large amount of small cells would appear in the future applications, it becomes more important to enable more, effective utilization of energy and network resources in the networks. Of course, it is also necessary to guarantee the communication quality of the user terminals accessing into the cells. The “cells” in the context includes both macro cells and small cells if not specified otherwise.
The small cells can be divided into different clusters. In order to avoid serious interferences in uplink and downlink between different small cells, the small cells in the same cluster generally adopt the same uplink/downlink configuration. However, even this, users especially edge users in adjacent small cells may produce interferences when transmitting data. In addition, users of macro cells may also be subject to interferences from communications of other macro cells or small cells.
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