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Automated Assignment of LTE RACH Root Sequences

IP.com Disclosure Number: IPCOM000216057D
Publication Date: 2012-Mar-21
Document File: 5 page(s) / 233K

Publishing Venue

The IP.com Prior Art Database

Related People

Schlangen, Tom: INVENTOR [+3]

Abstract

In mobile telecommunication systems, there are a wide variety of parameters required to maximize the utilization of radio spectrum. In Long Term Evolution (LTE) systems, each eNodeB cell accepts a finite number of preambles on its Random Access Channel (RACH), which enables multiple concurrent User Equipment (UE) connection attempts [1]. The finite number of preambles are generated from a larger pool of root sequence values, which are shared across the system. In order to minimize interference from adjacent eNodeB cells, the root sequence numbers should be allocated carefully. This paper presents an automated method for assigning root sequence numbers, which minimizes inter-cell RACH interference and provides some flexibility for future operational events (e.g., addition or deletion of eNodeB cells, system optimization, unplanned outages).

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LTE RACH Root Sequence Number Assignment

By Tom Schlangen, Michael Bach, Anatoly Andrianov

Motorola, Inc.

Networks Business

 

ABSTRACT

In mobile telecommunication systems, there are a wide variety of parameters required to maximize the utilization of radio spectrum. In Long Term Evolution (LTE) systems, each eNodeB cell accepts a finite number of preambles on its Random Access Channel (RACH), which enables multiple concurrent User Equipment (UE) connection attempts [1]. The finite number of preambles are generated from a larger pool of root sequence values, which are shared across the system. In order to minimize interference from adjacent eNodeB cells, the root sequence numbers should be allocated carefully.

This paper presents an automated method for assigning root sequence numbers, which minimizes inter-cell RACH interference and provides some flexibility for future operational events (e.g., addition or deletion of eNodeB cells, system optimization, unplanned outages).

PROBLEM

Adjacent LTE cells should use unique RACH root sequence numbers in order to avoid interference when multiple UE’s attempt to connect to the adjacent cells simultaneously (i.e., if multiple UEs randomly select the same preamble to access the RACH of adjacent cells, those cells may be unable to distinguish them and one or more of the UE radio access attempts may fail).

The 3GPP standards do not specify a methodology for assigning the RACH root sequence numbers, therefore the current approach is for the system operator to assign them manually (perhaps with the aid of radio planning tools).

Although there are hundreds of root sequence numbers (e.g., LTE defines 838 Zadoff-Chu root sequence numbers for generating the RACH preambles for preamble formats 0-3), LTE networks will be composed of many thousands of eNodeB cells, therefore the finite pool of root sequence numbers must be reused across the network.

The number of root sequence numbers required per cell depends upon both the radius of the cell and the mobility of the UE traffic. For example, a very small cell with low mobility UE traffic can distinguish 64 RACH preambles generated by the UEs using a single root sequence number. Note that the UE’s will randomly select different shifts of the given root sequence in order to create 1 of 64 RACH preambles. Cells with a larger radius and/or higher mobility UE traffic will require multiple root sequence numbers, and cells with a very large radius and very high mobility UE traffic may require the maximum of 64 root sequence numbers (i.e., one preamble per root sequence number).

SOLUTION

One solution to these problems is a method to automatically assign LTE RACH root sequence numbers via the following algorithm, which could be implemented at the eNodeB cells (i.e., distributed algorithm), at a central server (i.e., centralized algorithm), or a hybrid of the two.

1.    eNodeB cell determines the number of RACH root sequence numbers required (per provisioned cell radius and pr...