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Efficient channelisation code allocation in UMTS

IP.com Disclosure Number: IPCOM000004839D
Original Publication Date: 2001-Jul-05
Included in the Prior Art Database: 2001-Jul-05
Document File: 5 page(s) / 99K

Publishing Venue

Motorola

Related People

Peter Legg: AUTHOR [+2]

Abstract

This paper addresses an efficient, low complexity, allocation of channelisation codes for downlink shared channel in the UMTS Third Generation cellular telecoms system. The cumulative code weight is tracked as users are added to the transmission schedule, the required codes are then sorted by spreading factor, and allocated in a simple direct manner.

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Efficient channelisation code allocation in UMTS

by Peter Legg and Eric Villier

Abstract

This paper addresses an efficient, low complexity, allocation of channelisation codes for downlink shared channel in the UMTS Third Generation cellular telecoms system. The cumulative code weight is tracked as users are added to the transmission schedule, the required codes are then sorted by spreading factor, and allocated in a simple direct manner.

Introduction

This paper is written within the context of the Third Generation cellular telecommunication system known as UMTS and specified by 3GPP. The system is divided into a radio network (the UTRAN) and a core network. Within the UTRAN, packet data are scheduled for transmission on the downlink (i.e. the forward link) by the Radio Network Controller (RNC). More specifically, scheduling is the responsibility of the MAC protocol. The MAC schedules packet data onto both dedicated channels (DCH's) and onto the downlink shared channel (DSCH). Here we are concerned with the DSCH.

The DSCH represents a section of the channelisation code resource, which can be reassigned frame by frame to different mobiles. The MAC runs every frame and determines which mobiles should be granted which code in the next scheduled frame.

An exhaustive search for DSCH codes is easy to conceive. The spreading factor (SF) of the desired code is taken and codes at that SF are tested, one by one, until one is found that satisfies the three conditions:

1) the code is not already in use,

2) the subtended codes are unoccupied,

3) all codes of smaller SF on the path to the root of the code-tree are unoccupied.

Prior art: exhaustive search

This is computationally intensive. For example, in testing on a PowerPC 750 processor, scheduling users into 1/4 of the complete OVSF code tree takes 11000 instructions per 10ms frame.

Problem(s) To Be Solved

In UMTS, the OVSF codes of the downlink shared channel (DSCH) transport channels may be reallocated to different users every 10ms radio frame. Selecting codes in a computationally efficient manner is therefore important.

Proposed Solution to the Problem(s)

The solution described below is applicable when:

1) the DSCH codes are taken from a reserved part of the complete OVSF code tree,

2) for every user, all codes within the DSCH reservation are addressable, in principle.

Furthermore, the method requires the codes to be allocated at the end of the scheduling algorithm, rather than during the algorithm. In other words, in the course of the scheduling algorithm, we identify the users who are to receive data (i.e. are scheduled) and the SF of the code they require, but we do not allocate codes. The algorithm uses the weight of the code tree to test whether there is remaining code resource as the algorithm progresses. The weight of a code is 1 SF.

Thus, at the end of the algorithm, we have a list of scheduled users and the SF allotted to each. We now need to identify precisely which code each should be given. To do this a simple me...