Graph coloring based physical-cell-ID assignment for LTE networks

Abstract

Autoconfiguration of the radio parameters is a key feature for next generation mobile networks. Especially for LTE the NGMN Forum has brought it up as a major requirement. It is indispensable that algorithms used for autoconfiguration terminate quickly and do not cause infinite iterative reconfigurations within the network.Reference signal sequences are among the most important radio parameters for LTE, which are comparable to scrambling codes in 3G networks. In LTE they additionally serve as Cell Identifiers on the Physical Layer. Each cell is assigned one of the 504 available Physical Cell Identifiers. For proper operation the assignment has to be as well collision as also confusion free. Due to the high number and the layered structure of the cells within the network such an assignment is a complex task.In addition to this complexity each change of the Physical Cell ID of an operational cell causes a service interruption in the cell, which has to be avoided. The approach presented maps the ID assignment problem to the well known and well understood problem of graph coloring. It is shown that an efficient initial assignment even for complex networks is possible. Cells added during the subsequent network growth, can already be confused when inserted into the network. In this case the IDs of the operational cells causing the confusion must be changed.As a next logical step the incremental approach shows how the properties of the colored graph can be used for extending the network with new cells, with only minimal interruption while still retaining the properties of a colored graph.

Neural networks based Physical Cell Identity assignment for self organized 3GPP Long Term Evolution

“…Various types of cases in the PCI assignment issue have been explored [8], according to which it is quite clear that two types of conflicts can occur while assigning the PCIs in a cellular network; when two cells in the vicinity of each other or two cells having a common neighbor are assigned the same PCI. This causes collision in the former, and confusion in the later case.…”

Dynamic radio configuration of Self-Organizing base stations

“…All class B3 parameters need to be configured collision free (TABLE I), which means that the configured value of the parameter needs to be different from the values configured in all the neighboring cells. An example for a B3 parameter is the Physical Cell ID (PHY-CID) assignment [6]: LTE reference signal sequences, which are comparable to 3G scrambling codes, are of special interest for self-configuration. They serve as a regionally unique identifier on the physical level because reading and decoding a reference signal takes less than 5 ms.…”

“…3 c). This case cannot be avoided a priori and requires a specific PHY-CID reassignment algorithm [6].…”

“…Evaluation in [6] has shown that for a first estimation of the required number of PHY-CIDs the well-known coloring algorithm of Powell and Welsh gives a worst case assumption. But in all cases the number of required PHY-CIDs was very close to the maximum number of first-degree neighbors of a single cell within the cluster.…”

Dynamic Reservation Scheme of Physical Cell Identity for 3GPP LTE Femtocell Systems

“…The PCI conflict occurs when the same PCIs are selected in two adjacent cells. Also, the PCI assignment scheme using graph coloring was proposed to avoid both PCI conflicts and PCI confusion [15]. However, these schemes do not include the femtocells, which are deployed in the conventional macrocell network.…”