Mac-Phy Cross-Layer analysis and design of Mimo-Ofdm Wlans based on fast link adaptation

catalaLa darrera versio de l’estandard deWLAN, anomenada IEEE 802.11n, ha augmentat la seva capacitat notablement en relacio als sistemes anteriors gracies a la incorporacio de la tecnologia de multiples antenes en transmissio i recepcio (MIMO). No obstant aixo, la nova proposta, al igual que les anteriors, segueix sense especificar com s’han d’implementar elsmecanismes de configuraciomes crucials, un dels quals es l’algoritme de codificacio imodulacio adaptativa (AMC). Aquests algoritmes ja han demostrat la seva importancia a l’hora demaximitzar el rendiment del sistema tenint en compte les condicions canviants del canal. En aquesta tesis s’ha proposat un algoritme AMC de llac tancat, anomenat adaptacio rapida de l’enllac (FLA), que selecciona eficientment l’esquema demodulacio i codificacio adaptativa per xarxes WLAN basades en arquitectures multiportadora multiantena. L’algoritme FLA determina el mode de transmissio capac de maximitzar el throughput per les condicions de canal actuals, mentre satisfa un requisit de qualitat de servei en forma de taxa d’error per paquet (PER). FLA utilitza una metodologia de prediccio de PER basada en l’estimacio de la relacio senyal renou (SNR) efectiva exponencial (EESM). El rendiment de l’algoritme FLA ha estat avaluat en sistemes IEEE 802.11n, ja que aquests, gracies a la incorporacio d’unmecanisme de realimentacio demodes de transmissio, poden adoptar solucions AMC de llac tancat. En una primera part, l’estudi s’ha centrat a la capa fisica i despres s’ha estes a la subcapa MAC. A la capa fisica s’ha demostrat l’efectivitat de l’algoritme FLA aconseguint un rendiment molt proper al que ens proporcionaria un esquema AMC optim en termes de throughput, alhora que es satisfan els requisits de PER objectiu. L’algoritme FLA tambe ha estat avaluat utilitzant informacio imperfecte del canal. S’ha vist que l’algoritme FLA proposat es robust en front dels efectes d’estimacio imperfecte del canal, i nomes en canals altament selectius en frequencia, la informacio imperfecte del canal provoca una davallada en el rendiment en termes de throughput. A la subcapa MAC, l’algoritme FLA ha estat complementat amb una estrategia de temps d’espera que disminueix la dependencia amb la informacio de canal disponible a mesura que aquesta va quedant desfassada respecte de l’estat actual. Aquesta informacio de canal desfassada es consequencia de la subcapa MAC que degut a la multiplexacio d’usuaris introdueix grans retards entre que es determina el mode de transmissio mes adequat i la seva utilitzacio per a l’acces al canal. Els resultats obtinguts han demostrat la superioritat de FLA respecte d’altres algoritmes de llac obert en condicions de saturacio i de no saturacio, i independentment de la longitud de paquet, nombre d’usuaris, protocol (CSMA/CA i CSMA/E2CA) i esquema d’acces (Basic Access i RTS/CTS). Ames, s’han desenvolupat diversosmodels analitics per tal d’estimar el rendiment del sistema a la subcapa MAC. Aquests models consideren tots els detalls de funcionament de la subcapaMAC del 802.11n, comper exemple un nombre finit de retransmissions de cada paquet, l’slot anomal o els errors introduits pel canal. Inicialment s’ha proposat unmodel semi-analitic que determina el throughtput en condicions de saturacio, considerant el rendiment dels algoritmes AMC. Despres s’ha presentat un model analitic que estima el rendiment del sistema per condicions de no saturacio, mitjancat elmodelat de cues finites a cada estacio. Aquestmodel consideramodes de transmissio fixes i es capac de determinar de manera molt precisa metriques de rendimentmolt importants comson la probabilitat de bloqueig de cada estacio, el retard mitja del paquets, la probabilitat de descart o la mesura del goodput. Finalment, el model analitic de no saturacio s’ha utilitzat per definir un model semi-analitic per tal d’estimar el rendiment del sistema quan es considera l’us d’algoritmes AMC. EnglishThe latestWLAN standard, known as IEEE 802.11n, has notably increased the network capacity with respect to its predecessors thanks to the incorporation of the multipleinput multiple-output (MIMO) technology. Nonetheless, the new amendment, as its previous ones, does not specify how crucial configuration mechanisms, most notably the adaptive modulation and coding (AMC) algorithm should be implemented. The AMC process has proved essential to fully exploit the system resources in light of varying channel conditions. In this dissertation, a closed-loop AMC technique, referred to as fast link adaption (FLA) algorithm, that effectively selects themodulation and coding scheme (MCS) for multicarriermultiantennaWLAN networks is proposed. The FLA algorithm determines the MCS that maximizes the throughput while satisfying a quality of service (QoS) constraint, usually defined in the form of an objective packet error rate (PER). To this end, FLA uses a packet/bit error rate prediction methodology based on the exponential effective SNRmetric (EESM). The FLA algorithm performance has been evaluated under IEEE 802.11n systems that thanks to the incorporation of a feedbackmechanismare able to implement closed- loop AMC mechanisms. Initially, this AMC technique relies only on physical layer information but it is subsequently extended to also take into account themediumaccess control (MAC) sublayer performance. At the physical layer, the FLA algorithm has demonstrated its effectivity by performing very close to optimality in terms of throughput, while satisfying a prescribed PER constraint. The FLA algorithm has also been evaluated using imperfect channel information. It has been observed that the proposed FLA technique is rather robust against imperfect channel information, and only in highly-frequency selective channels, imperfect channel knowledge causes a noticeable degradation in throughput. At the MAC sublayer, the FLA algorithm has been complemented with a timeout strategy that weighs down the influence of the available channel information as this becomes outdated. This channel information outdate is caused by the MAC sublayer whose user multiplexing policy potentially results in large delays between acquiring the instant in which the channel state information is acquired and that in which the channel is accessed. Results demonstrate the superiority of FLA when compared to open-loop algorithms under saturated and non-saturated conditions and irrespective of the packet length, number of users, protocol (CSMA/CA or CDMA/E2CA) and access scheme (Basic Access or RTS/CTS). Additionally, several analytical models have been developed to estimate the system performance at the MAC sublayer. These models account for all operational details of the IEEE 802.11n MAC sublayer, such as finite number of retries, anomalous slot or channel errors. In particular, a semi-analytical model that assesses the MAC layer throughput under saturated conditions, considering the AMC performance is first introduced. Then, an analytical model that allows the evaluation of the QoS performance under non-saturated conditions is presented. This model focuses on single MCS and it is able to accurately predict very important system performance metrics such as blocking probability, delay, probability of discard or goodput thanks to the consideration of the finite queues on each station. Finally, the previous non-saturated analytical approach is used to define a semi-analytical model in order to estimate the system performance when considering AMC algorithms (i.e. whenmultiple MCSs are available)