Downlink performance analysis for broadband wireless systems with multiple packet sizes☆
Introduction
Current wireless systems (e.g., WiFi, WiMAX, LTE, DVB-S2, etc.) have air interfaces characterized by Adaptive Modulation and Coding (ACM), [1], [2], [3], [4], [5], entailing different bandwidth efficiency levels, and supporting IP traffic with packets of different lengths [6], [7]. The air interface typically has a time structure (called frame or Transmission Time Interval, TTI, depending on the wireless technology) according to which resources are allocated for transmissions; for the sake of simplicity, we will denote by such interval. We expect that within this time, more packets can be allocated according to resources of allocation. The focus is here on the downlink of WiMAX, where the Base Station (BS) has to transmit to Subscriber Stations (SSs). Moreover, we will provide some details about the corresponding LTE case.
In the aim of a cell planning study, we consider a single traffic class and a single Medium Access Control (MAC) downlink queue at the BS, collecting the packets for the different SSs. This queue could be divided into different (logical) sub-queues, one per user in the cell, to support the scheduling of different traffic flows. However, we do not consider the presence of multiple downlink queues, since all the flows belong here to the same traffic class. This is motivated by the fact that our aim is to provide a simple tool for cell planning. Hence, we investigate the mean packet delay assuming a downlink queue at the BS with First Input First Output (FIFO) discipline.1 If packets have a fixed size of one resource, this problem can be solved by means of a classical queuing model, accounting for resources available per frame [8], [9]. The interest is in modifying such approach, considering both different packet sizes and a frame-based resource allocation.
In WiMAX, MAC layer fragmentation can be adopted to achieve an efficient use of frame resources, while in LTE IP packets are segmented at the Radio Link Control (RLC) layer and padding is used to fit certain allowed sizes at MAC layer. On the basis of these considerations, we will evaluate the performance with and without fragmentation. On one side, fragmentation allows a better utilization of resources; on the other side, fragmentation requires extra processing, signaling overhead (fragmentation header), and a fragmented packet cannot be used until all its fragments have been received. However, no fragmentation jointly with FIFO service and a frame-based allocation of radio resources can lead to Head-Of-Line (HOL) problems, when long packets exceeding the remaining capacity in the frame are at the top of the service list. HOL issues have been widely studied in the literature for what concerns buffer management with FIFO service discipline, especially in the context of switch architectures. The interest is here to assess the impact of fragmentation/no-fragmentation on queue behavior at the BS and to investigate cell planning issues.
The rest of this paper is organized as follows: the next section surveys resource allocation issues in LTE and WiMAX; Section 3 provides the analysis of the downlink queue in the most general case with resources, considering different packet sizes and approaches with and without packet fragmentation; Section 4 applies a theoretical approach to cell planning for a WiMAX case; finally, Section 5 draws conclusions. Appendices provide details on queue analysis, theoretical methods adopted, and possible generalizations.
Section snippets
Resource allocation in broadband wireless systems
In this section, we survey two important Orthogonal Frequency Division Multiplexing (OFDM)-based air interfaces to highlight the resource granularity and the frame structure. The basic parameters are: the FFT size, , the number of data sub-carriers, , the cell bandwidth, , the oversampling factor, , and the cyclic prefix parameter, . From these main parameters it is possible to compute the sampling frequency, , the carrier spacing, , the useful symbol time ,
Approaches for downlink queue analysis
Our interest is to study the general case of a MAC frame with transmission resources and packets of length resources, with . The case is not relevant here because long IP packets can be segmented in order to fit the MAC frame capacity.
In the following study, is typically related to a higher resource granularity level than RBs or symbol/slots, referring to the capacity of MAC PDUs per frame. Since in some theoretical approaches, the complexity increases with , it is
Application to WiMAX cell planning
On the basis of the study made in the previous section, we concentrate on fixed WiMAX cell planning by adopting the equivalent length approach, since it represents the simplest method. In the following analysis, we consider: a realistic IP packet size distribution, inter-cell interference due to resource reuse, and a suitable propagation model to determine the use of different modulation and coding combinations in the cell. Of course, a refined cell planning taking many system details into
Conclusions
Future broadband wireless access systems will support heavy traffic loads, especially in the (base-to-mobile station) downlink direction. Therefore, cell planning needs to be based on theoretical tools, suitable for describing the air interface “multi-resource” characteristics. Referring to OFDM(A)-based systems, we have studied the performance of a MAC downlink transmission queue with packets requiring different numbers of PHY-layer resources, considering both fragmentation and
Giovanni Giambene ([email protected]) was born in Florence, Italy, in 1966. He received the Dr. Ing. degree in Electronics in 1993 and the Ph.D. degree in Telecommunications and Informatics in 1997, both from the University of Florence, Italy. From 1994 to 1997, he was with the Electronic Engineering Department of the University of Florence, Italy. He was Technical External Secretary of the European Community COST 227 Action (“Integrated Space/Terrestrial Mobile Networks”). From 1997 to 1998,
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Giovanni Giambene ([email protected]) was born in Florence, Italy, in 1966. He received the Dr. Ing. degree in Electronics in 1993 and the Ph.D. degree in Telecommunications and Informatics in 1997, both from the University of Florence, Italy. From 1994 to 1997, he was with the Electronic Engineering Department of the University of Florence, Italy. He was Technical External Secretary of the European Community COST 227 Action (“Integrated Space/Terrestrial Mobile Networks”). From 1997 to 1998, he was with OTE of the Marconi Group, Florence, Italy, where he was involved in a GSM development program. In 1999, he joined the Information Engineering Department of the University of Siena, Italy, first as a research associate and then as an assistant professor and aggregate professor. He teaches the master level course on Networking at the University of Siena. From 1999 to 2003 he participated in the project “Multimedialità”, financed by the Italian National Research Council (CNR). From 2000 to 2003, he contributed to the “Personalised Access to Local Information and services for tourists” (PALIO) IST Project within the EU FP5 programme. He was vice-Chair of the COST 290 Action for its whole duration 2004–2008, entitled “Traffic and QoS Management in Wireless Multimedia Networks” (Wi-QoST). He participated in the SatNEx I & II network of excellence (FP6 programme, 2004–2009) as work package leader of two groups on radio access techniques and cross-layer air interface design for satellite communication systems and in the FP7 Coordination Action “Road mapping technology for enhancing security to protect medical & genetic data” (RADICAL) as work package leader. At present he is involved in the ESA SatNEX III research project, the COST Action IC0906 “Wireless Networking for Moving Objects” (WiNeMO), and the FP7 Coordination Action ‘Responsibility’.
Snezana Hadzic-Puzovic received the Dipl. Ing. degree in telecommunications from the Faculty of Electrical Engineering, University of Belgrade, Serbia in 2006 and the Ph.D. degree in telecommunications from the Faculty of Information Engineering, University of Siena, Italy in 2011. Her main research interests include resource management in broadband wireless and heterogeneous networks, MAC protocol analysis, quality of service and cross-layer optimizations.
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This study has been carried out within the framework of the EU COST IC906 Action - WG2; part of this work has been supported by an Alcatel-Lucent grant and by a CNIT research grant.