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Two-phase PDU segmentation between LLC and RLC to increase overall system throughput

IP.com Disclosure Number: IPCOM000005032D
Original Publication Date: 2001-Jul-24
Included in the Prior Art Database: 2001-Jul-24
Document File: 6 page(s) / 99K

Publishing Venue

Motorola

Related People

Walter Featherstone: AUTHOR [+2]

Abstract

This paper describes a new technique that enables code reselection, in a multi-code data rate system, on a more frequent basis matched with the feed back interval. The technique is based on using a two-phase segmentation of packet data units (PDU) between Logical Link Control (LLC) and Radio Link Control (RLC) rather than the normal segmentation of LLC into RLC blocks using the prevailing code. Such a technique improves the system performance as the probability of receiving the RLC blocks correctly is increased due to finer code matching to the radio channel condition. To illustrate the problem and solution, GPRS/EGPRS (General Packet Radio System / Enhanced GPRS) RLC protocol is used; the technique is, however, equally applicable to other multi-code rate systems employing similar type of protocols.

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Two-phase PDU segmentation between LLC and RLC to increase overall system throughput

Walter Featherstone and Davood Molkdar

Abstract

This paper describes a new technique that enables code reselection, in a multi-code data rate system, on a more frequent basis matched with the feed back interval. The technique is based on using a two-phase segmentation of packet data units (PDU) between Logical Link Control (LLC) and Radio Link Control (RLC) rather than the normal segmentation of LLC into RLC blocks using the prevailing code. Such a technique improves the system performance as the probability of receiving the RLC blocks correctly is increased due to finer code matching to the radio channel condition. To illustrate the problem and solution, GPRS/EGPRS (General Packet Radio System Enhanced GPRS) RLC protocol is used; the technique is, however, equally applicable to other multi-code rate systems employing similar type of protocols.

Introduction

In a multi-code packet data system, the code allocation is the central point of optimising the throughput and delay of a user and the system as a whole. The code allocation technique is also referred to as Link Adaptation (LA). Link adaptation is the process of selecting the best coding scheme that satisfies a target performance based on the prevailing channel conditions. One of the most important factors in enhancing the performance of a system in terms of throughput and QoS is to be able to use the best code at the minimum interval determined by the system specifications. In case of GPRS and EGPRS, this will be at a block interval, i.e. 20ms. However, there are practical constraints that will not allow the code selection/reselection at such a rapid interval. The objective is to be able to change the code as soon as such feedback information becomes available. This paper describes a technique that achieves the aforementioned objective.

Problem(s) To Be Solved

Both GPRS and EGPRS offer a suite of coding schemes (Table 1 and Table 2) that offer varying levels of code protection for RLC blocks transmitted across a radio link. EGPRS also offers a code family structure as shown in Figure 1. Better channel conditions (in terms of radio link performance) enable the system to operate with weaker coding, which facilitates higher data rates. Figure 2 shows the process by which LLC frames are segmented into RLC blocks before being passed to physical layer. The LA process, which selects the optimum coding scheme based on the prevailing channel conditions, governs the size of the RLC blocks. In GPRS and EGPRS, the RLC layer performs the LA process. Typically code selection takes place when a new estimate of the channel conditions becomes available. However, in GPRS code reselection is currently limited to LLC frame periods. This problem has been overcome to an extent in EGPRS by the introduction of code families, which enable intra family code re-selection during the LLC frame period. For example, if a block was originally encod...