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Article on
Base Station Scheduling

by
Samir Amberkar
(originally published on 19-Nov-2011)




Abstract: The article states a generic resource scheduling problem faced by base station scheduler and attempts an incremental solution for it.


Background

Take GPRS, UMTS, LTE, WiMAX/802.16 or possibly any other packet based bearer services - where resources are limited and are to be shared, the question that standards and specifications don't answer is "How are the allocations to be made ?" or "Which resource should go to which user at certain point in time ?" !! Specification do specifiy the way resources are arranged and protocols for managing (allocation & deallocation) these resources, but it is left to the implementor to design appropriate "logic", "method" or "algorithm" to decide dynamic allocations.

Allocation functions are (usually) implemented in base station (BSS in GPRS, RNC in UMTS, eNB in LTE). In this article, we will analyse these functions and come out with a generic design of scheduler in base station based on certain assumptions.

Introduction

Even though resource allocation logic is not mentioned in standards, standards do mention possible classes of services or types of bearers that can be or are to be supported. Best example would be IEEE 802.16 standard. 802.16 (Elements of 802.16 - 2) specifiy 5 types of allocations. These types are rightly called "scheduling mechanisms"; even though each type is capable of supporting bearer for particular service or application, when implemented in base station, it is basically a scheduling mechanism !! For example, 802.16 UGS (Unsolicited Grant Service) supposed to provide "periodic" and "fixed" allocations. This service is suitable for constant bit rate services like voice call, video call (without any compression or silent suppresion intelligence). So to support this service, 802.16 base station has to implement scheduling methods for fixed and periodic allocations.

Another example is UMTS which calls it "bearer service". UMTS goes a step ahead by distinguising between end-to- end service (called Teleservice) and bearer service. Refer article UMTS - Services for more details. Here bearer service is specified in terms of certain parameters, called "QoS parameters". The QoS parameter of our importance is "Traffic Class". UMTS specifies four traffic classes - Conversational, Streaming, Interactive, and Background. Conversational class maps to UGS in 802.16.

This separation of bearer services in terms of classes or services helps scheduling mechanisms by limiting the possible combinations of allocation requests. We will see more of it later.


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