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2.7.1. Adaptación al canal: control de potencia y control de tasa

El control de potencia que tradicionalmente se usa en comunicaciones móviles ajusta la potencia en el transmisor para compensar la degradación del canal, de manera que la potencia transmitida varíe de forma inversamente proporcional a la calidad del canal. El efecto de estos ajustes es mantener en el receptor una probabilidad de error dada para ese enlace. Este ajuste de potencia funciona adecuadamente en aplicaciones basadas en conmutación de circuitos, ya que permite mantener un cierto umbral de probabilidad de error, una tasa de transmisión constante y por encima de un mínimo dado. Sin embargo, en aplicaciones basadas en conmutación de paquetes lo más importante no es tanto que la tasa de datos sea constante sino ofrecer la mejor tasa posible (best effort), manteniendo por encima de ciertos umbrales otros parámetros de calidad de servicio (latencia, retardo, etc.). Por ello, en contraposición al control de potencia, se define el control de tasa, cuyo objetivo no es mantener constante la tasa de datos, sino ajustar la tasa en función de las condiciones del canal. La diferencia se muestra en la figura 2.24, donde en la parte superior (a) se representa el efecto del control de potencia, considerando que de forma ideal la potencia de la señal transmitida es inversamente proporcional a la calidad del canal, lo cual se traduce en una tasa de datos constante. En la parte inferior (b), se representa el efecto del control de tasa, parte donde se supone que la potencia transmitida se mantiene constante y es la tasa de transmisión la que varía siguiendo la variación del canal. El papel que juega en este caso el controlador de tasa es, ante un desvanecimiento del canal, recalcular qué esquema de modulación y tasa de codificación debe usarse para mantener la potencia de transmisión de acuerdo al nuevo estado del canal. La tasa de codificación dependerá del número de bits de redundancia que se inserten en el codificador de canal por cada bit de información, lo cual puede expresarse como:

$\"\"/$

En condiciones adversas del canal será necesario usar tasas de codificación bajas (más bits de redundancia) y modulaciones más robustas como QPSK, mientras que en condiciones de canal favorables será posible emplear tasas de codificación altas (menos bits de paridad) y modulaciones más eficientes como 16QAM o 64QAM.

**2.7.2. Scheduling dependiente del estado del canal**

En las redes de acceso radio, y en particular en LTE, la tarea de ordenación temporal de las transmisiones entre los diferentes recursos radio disponibles se asocia a una entidad denominada scheduler. El scheduler controlará en cada instante de tiempo a qué usuario debe asignar cada recurso compartido para maximizar la eficiencia del sistema. También determinará la tasa de transmisión que debe usar cada uno de los enlaces, de forma que la adaptación de tasa o rate matching queda integrada en el scheduler. En general, el scheduler determinará en buena parte las prestaciones del sistema en enlace descendente, sobre todo en redes con elevada carga de tráfico.

$\"\"/$

Figura 2.24. (a) Control de potencia.(b) Control de tasa.

La base del esquema de transmisión en LTE es el uso de canales compartidos, en los cuales el recurso tiempo-frecuencia se comparte de forma dinámica entre los usuarios. Este concepto de funcionamiento es similar a la aproximación que se implementa en High Speed Downlink Packet Access (HSDPA), sólo que los recursos en el caso de HSDPA son tiempo-código en lugar de tiempo-frecuencia.

Este esquema de uso compartido en tiempo y frecuencia permite una mejor adaptación a las variaciones de las condiciones de demanda de recursos que aparecen en el acceso radio en redes de conmutación de paquetes y al mismo tiempo permite habilitar otras técnicas básicas en LTE.

Cuando se tienen en cuenta las condiciones del canal, entonces se habla de channel-dependent scheduling. Como se ha comentado antes, en el caso particular de LTE, el scheduling no sólo tiene en cuenta la distribución de recursos en el dominio del tiempo sino también en el dominio de la frecuencia, lo cual es posible dadas las características del acceso radio OFDMA.

La ampliación al dominio de la frecuencia del scheduling dependiente del canal es particularmente interesante en terminales con baja movilidad (velocidad), ya que el canal varía entonces más lentamente con el tiempo. Para servicios que requieran tiempo real, o restricciones en los retardos máximos admisibles, un esquema de scheduling que sólo considere el dominio temporal no podrá maximizar la calidad ofrecida. En cambio, la posibilidad de explotar la variación en el dominio de la frecuencia permite mejorar sensiblemente las condiciones para dicho tipo de usuarios.

En el caso de LTE, en enlace descendente cada terminal móvil mide la calidad del canal a través de las señales de referencia enviadas por la estación base y esta medida es devuelta a la estación base como referencia para el funcionamiento del scheduler. En principio, la mínima asignación por parte del scheduler es de 180 kHz (12 subportadoras) durante 1 ms, lo que permite pensar en que se podrán compensar en parte variaciones relativamente rápidas del canal.

El enlace ascendente LTE se basa en la separación ortogonal de usuarios, por lo que el scheduler debe, en este caso, gestionar recursos. Las decisiones de asignación se toman cada milisegundo y controlan qué usuario puede transmitir en una celda en cada intervalo de tiempo, en qué recursos en frecuencia transmitirá y con qué tasa de transmisión (formato de transporte). Como consecuencia del uso de un esquema de portadora única para la transmisión en enlace ascendente, las asignaciones en frecuencia que haga el scheduler deben ser contiguas. Una de las dificultades añadidas que plantea el scheduling en enlace ascendente es la medida de las condiciones del canal, que no resulta tan evidente como en enlace descendente. Esto hace que en el sistema se incorporen también mecanismos de diversidad en enlace ascendente para paliar situaciones en las que el scheduling no se pueda usar.

Los detalles de la implementación de estas técnicas de scheduling se ven con detalle para el sistema LTE en el capítulo 7.

2.7.3. Coordinación de interferencias inter-celda

El esquema de acceso radio de LTE proporciona ortogonalidad entre los usuarios de una misma celda, tanto en enlace ascendente como descendente. En general, y por ese motivo, las prestaciones de LTE en términos de eficiencia espectral y tasas de transmisión de pico están más limitadas por las interferencias provenientes de celdas vecinas, en comparación con lo que sucedía en las tecnologías Tercera Generación Móvil (3G). Cualquier técnica que permita reducir las interferencias entre celdas será, por tanto, una mejora en las prestaciones de LTE, sobre todo en términos de la calidad de servicio que se puede ofrecer a usuarios en los límites (bordes) entre celdas.

En realidad, lo que aquí se denomina coordinación de interferencias entre celdas es una técnica integrada en el grupo de funciones del scheduler, mediante la cual se controla el nivel de interferencia que reciben los usuarios en los bordes de las celdas para mejorar parámetros de calidad de servicio, en particular su tasa de transmisión. Como se verá con detalle en el capítulo 7, la coordinación de interferencias implica aplicar ciertas restricciones en las funciones de scheduling en el dominio de la frecuencia, tanto en enlace ascendente como descendente, configurando el equivalente a esquemas de reúso en frecuencia distintos para el interior y exterior de las celdas. La figura ilustra este concepto que se tratará con detalle en el capítulo 7.

$\"\"/$

Figura 2.25. Esquema de coordinación de interferencias mediante reúso de frecuencia fraccionado.

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[13] S. Verdu, Multiuser Detection. Cambridge University Press, 1998.

[14] P. Wolniansky, G. Foschini, G. Golden, y R. Valenzuela, “V-BLAST: An architecture for realizing very high data rates over the rich-scattering wireless channel”, en URSI International Symposium on Signals, Systems, and Electronics, Pisa, Italia, Septiembre 1998.

[15] S. M. Alamouti, “A simple transmit diversity technique for wireless communications”, IEEE Journal on Selected Areas in Communications, vol. 16, no. 8, pp. 1451–1458, 1998.

[16] J. Olmos, S. Ruiz, M. Garcia-Lozano, y D. Martin-Sacristan, “Link abstraction models based on mutual information for LTE downlink”, COST 2100, TD (10)11052, 2010.

[17] 3GPP, “E-UTRA UE Radio Transmission and Reception”, 3GPP, TS 36.101 V8.4.0, 2008.

[18] T. Sorensen, P. Mogensen, y F. Frederiksen, “Extension of the ITU channel models for wideband (OFDM) systems”, en IEEE Vehicular Technology Conference, Dallas, USA, Fall 2005.

[19] F. Adachi y M. Sawahashi, “Performance analysis of various 16 level modulation schemes under Rayleigh fading”, IEEE Electronics Letters, vol. 28, no. 17, pp. 1579–1581, 1992.

[20] 3GPP, “Evolved universal terrestrial radio access (E-UTRA); multiplexing and channel coding”, 3GPP, TR 36.212 V8.3.0, 2008.

Capítulo 3
El Núcleo de Red - EPC

Jose F. Monserrat

Sin contar con unas ciertas nociones del núcleo de red hay partes del funcionamiento del estándar Long Term Evolution (LTE) que no se comprenden. De hecho, son muchos los libros acerca de LTE que omiten esta descripción sin considerar las consecuencias de tal carencia. Sin ánimo de adentrarse en aspectos técnicos muy concretos, este capítulo ofrece una visión sencilla del núcleo de red de LTE, también conocido como Evolved Packet Core (EPC). El objetivo del capítulo es, por tanto, sintetizar los principales aspectos del EPC que ayuden a comprender el funcionamiento completo del sistema. Con este fin, este capítulo cuenta con las siguientes secciones:

Sección 3.1: En esta sección se presentan las funciones del núcleo de red y su evolución histórica desde la Release 99 hasta la Release 9. Este análisis justificará la necesaria simplificación del núcleo de red que se acometió en una serie de etapas progresivas.

Sección 3.2: Se describe la distribución de funcionalidades que se realiza entre las entidades que componen el EPC. Esta sección es muy importante, puesto que esta distribución funcional justificará el intercambio de mensajes seguido en todos los procedimientos que impliquen al EPC.

Sección 3.3: Analiza las distintas alternativas de distribución física de la arquitectura del EPC, tanto desde el punto de vista de entidades funcionales como de nivel de protocolos.

Sección 3.4: En ésta se profundiza acerca del sistema subsistema IP multimedia o IP Multimedia Subsystem (IMS). El IMS es un conjunto de protocolos y entidades encargadas de garantizar una determinada calidad extremo a extremo a lo largo de toda la comunicación. Los protocolos más significativos integrados en IMS son: el protocolo de iniciación de sesión o Session Initiation Protocol (SIP), responsable de la negociación inicial; el protocolo de descripción de la sesión o Session Description Protocol (SDP), que formaliza la descripción de las características de la sesión, y el protocolo de transporte en tiempo real o Real-Time Transport Protocol (RTP) y su correspondiente protocolo de control, el protocolo de control de tiempo real o Real Time Control Protocol (RTCP), encargados de garantizar que la sesión cumple con los parámetros de calidad establecidos.

Sección 3.5: Una de las funciones fundamentales del EPC es garantizar la seguridad y privacidad de las comunicaciones. Esta sección describe los procedimientos de seguridad utilizados en LTE.

Sección 3.6: Muestra cómo se garantiza la calidad extremo a extremo en LTE, analizando los mecanismos de control en cada uno de los enlaces de la arquitectura del sistema.

3.1. Introducción

Las entidades lógicas de una red celular se dividen normalmente en red de acceso radio o Radio Access Network (RAN), que incluye todos los elementos y entidades funcionales relacionadas con el control del enlace radio, y en el núcleo de red o Core Network (CN), que comprende las funcionalidades a nivel de transporte y superiores. Un mismo CN puede interactuar con RAN de distintas tecnologías, como es el caso de la red Global System for Mobile Communication (GSM) y Universal Mobile Telecommunications System (UMTS). Además, el CN es también el responsable de la interconexión con otras redes, incluyendo Internet.

Aunque parezca sencilla, la división de funcionalidades entre red de acceso y red troncal es una tarea compleja y cada tecnología lo aborda de una manera distinta. En las tecnologías estandarizadas por el Third Generation Partnership Project (3GPP) se siguen los siguientes principios:

La estructura de celdas es transparente para el CN.

Los buffers de retransmisiones se sitúan en la RAN por lo que, a todos los efectos, la tecnología radio utilizada es transparente para el CN.

Las funciones propias de la RAN son:Codificación, modulación y otras funciones de nivel físico.Petición automática de reenvío o Automatic Repeat-reQuest (ARQ), compresión de cabeceras y otras funciones típicas de nivel de enlace.Gestión de recursos, handover y control de admisión.Funciones de seguridad–cifrado y protección de la integridad.

Las funciones propias del CN son:Facturación.Roaming, o interconexión con redes operadas por otra compañía.Control de la calidad de servicio dentro del sistema.Interconexión con redes externas.

El primer núcleo de red desarrollado por el 3GPP, el de GSM, se caracterizaba por estar basado en la conmutación de circuitos o Circuit Switched (CS). En ese momento histórico, la única funcionalidad de las redes móviles era la comunicación por voz, por lo que este tipo de redes basadas en la conmutación de circuitos y la interconexión con la Red Telefónica Pública Conmutada (RTPC) eran las más adecuadas. Ya en el año 2000, el mercado planteaba nuevos requisitos de transmisión de paquetes y navegación web. Fue, por tanto, necesaria una evolución del núcleo de red compatible con la red GSM existente. La conmutación de paquetes o Packet Switched (PS) se instauró como una red paralela a la CS, añadiendo nuevas entidades funcionales que permitieran la gestión de paquetes y la interconexión con redes externas basadas en protocolo de internet o Internet Protocol (IP). La tecnología resultante no era más que una adaptación de la interfaz radio GSM para permitir la conmutación de paquetes. A esa tecnología se la denominó General Packet Radio Service (GPRS). La siguiente generación móvil, la tercera, también conocida como UMTS, fue diseñada para poder trabajar con el mismo núcleo de red definido para GPRS. Así pues, desde la Release 99 hasta la Release 4 el núcleo de red permaneció inalterado. Fue en 2004, y dentro de la Release 5, cuando se acometió la siguiente revolución tecnológica del núcleo de red con la introducción del IMS (ver sección 3.4). A grandes rasgos, el subsistema IMS permite el establecimiento, control y tarificación de servicios extremo a extremo a partir de un identificador único del usuario independiente de su localización. IMS necesita que el servicio sea IP, es decir, que esté basado en la conmutación de paquetes, aunque dispone de mecanismos de conversión para acceder a la RTPC utilizando voz sobre IP o Voice over IP (VoIP).

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Figura 3.1. Evolución de la arquitectura del núcleo de red del 3GPP.

La nueva tecnología LTE representa una evolución no sólo en el acceso radio, sino también en el núcleo de red. Como se muestra en la figura 3.1, el principal objetivo del nuevo estándar es integrar todas las aplicaciones sobre una arquitectura única más simple. Así pues, el Evolved Packet System (EPS), nombre formal de la tecnología LTE, dispone de un núcleo de red simplificado, compuesto por un único dominio, el de paquetes, que soporta todos los servicios —posiblemente basados en IMS— y las capacidades de interconexión hacia otras redes públicas. El dominio CS desaparece completamente de la red LTE, ya que todas las aplicaciones, incluida la voz y otros servicios de tiempo real, podrán funcionar adecuadamente gracias a la baja latencia del sistema y a los controles definidos a nivel de aplicación. De cara a la interconexión con la RTPC se definen nodos pasarela, o gateways, tanto en el plano de control —IMS— como de usuario, para así permitir la conversión del tráfico de voz IP a una red de transporte basada en CS.

La figura 3.2 muestra la arquitectura de los sistemas 3GPP tal y como está definido en la Release 9 del estándar [1]. Las entidades no sombreadas son las que pertenecen explícitamente a la tecnología LTE. Como se puede observar, LTE se ha especificado para ser totalmente compatible con otras tecnologías 3GPP previas, así como para reducir el número de entidades del núcleo de red. La siguiente sección está explícitamente dedicada a analizar las funciones de cada una de estas entidades.

3.2. Entidades funcionales del EPC

El núcleo de red, o Core Network, incorpora funciones de transporte y de inteligencia. Las funciones de transporte soportan la transmisión de la información de tráfico y señalización, incluida la función de conmutación de llamadas. Las funciones de inteligencia incluyen aspectos tales como el encaminamiento, la gestión de movilidad y el control de los servicios ofrecidos y la calidad de los mismos.

A través del núcleo de red, el sistema LTE se conecta con otras redes de telecomunicación, de forma que sea posible establecer la comunicación no sólo entre usuarios móviles 3GPP, sino también con otros terminales que se encuentren conectados a otras redes.

El núcleo de la red LTE también se conoce como Evolved Packet Core (EPC) y representa la culminación en la evolución de las redes 3GPP hacia la desaparición del modo de conmutación de circuitos en beneficio de un único dominio de paquetes, basado exclusivamente en el protocolo TCP/IP, dando lugar a lo que se conoce como solución “todo IP”, all IP.

A continuación se describen las principales entidades funcionales que constituyen el núcleo de red descrito en la primera fase de estandarización de LTE. Algunos de estos elementos son compartidos con las redes de segunda y tercera generación, mientras que los elementos clave son los recogidos en este texto.

2.7.1. Adaptación al canal: control de potencia y control de tasa

2.7.2. Scheduling dependiente del estado del canal

2.7.3. Coordinación de interferencias inter-celda

Referencias

Capítulo 3 El Núcleo de Red - EPC

3.1. Introducción

3.2. Entidades funcionales del EPC

**2.7.2. Scheduling dependiente del estado del canal**

Capítulo 3
El Núcleo de Red - EPC