1 Interference Management and Rate Adaptation in OFDM-based UWB Networks Raed T. Al-Zubi and Marwan Krunz Department of Electrical and Computer Engineering University of Arizona, Tucson, AZ 85721 E-mail:{alzubi, krunz}@ece.arizona.edu Abstract—Ultra-wideband (UWB) communications has emerged as a promising technology for high data rate wireless personal area networks (WPANs). Several proposals for UWB-based WPANs have been made. One widely popular proposal was standardized by ECMA-368, and is based on OFDM. In this paper, we address one of the important aspects that impact the performance of this standard, namely interference management between different uncooperative beacon groups that operate simultaneously over the same area. We first propose an interference management distributed reservation protocol (IM-DRP) for OFDM-based UWB communications. IM-DRP aims at improving the throughput of an UWB WPAN by reducing interference between uncooperative beacon groups. We then integrate IM-DRP into the design of a rate adaptation strategy that exploits the multi-rate capability of OFDM-based UWB systems. Besides maintaining a target packet error rate, our proposed strategy attempts to reduce the required reservation time over a link, hence allowing more links to be simultaneously activated. This improves the overall network throughput. Simulations are used to demonstrate the performance gain of our proposed schemes. Index Terms—OFDM-based UWB WPANs, interference management, multi-rate capability, rate adaptation schemes . ✦ 1 I NTRODUCTION In 2002, the FCC issued its First Report and Order, which permitted the deployment of ultra-wideband (UWB) devices [7] over the spectrum from 3.1 GHz to 10.6 GHz. This report motivated researchers to exploit the interesting features of UWB for various applications, including short-range wireless communications, wireless sensor networks, imaging and radar systems, and pre- cision location tracking systems. In general, UWB tech- niques can be categorized into two classes: single-band or impulse UWB (I-UWB), which is based on sending ultra-short pulses without frequency carriers, and multi- band or multi-carrier UWB (MC-UWB), which is based on sending data using multiple simultaneous OFDM sub-carriers. In this paper, we focus on the latter class, as standardized by ECMA-368 [6]. Industry advocates of this class formed an organization called the Multi- band OFDM Alliance (MBOA) [16]. This alliance even- tually evolved into a large industrial alliance known as WiMedia, which defines, certifies, and supports wireless technology for multimedia applications. WiMedia mem- bers approached and asked the European Computer Manufacturers Association (ECMA) to consider WiMe- dia specifications for OFDM-based UWB and undertake the system standardization process. OFDM-based UWB has since emerged as a promising technology for high data rate WPANs. An UWB WPAN uses the concept of a beacon group of a node, which consists of the neighbors of that node An abridged version of this paper was accepted for presentation at the IEEE GLOBECOM Conference, New Orleans, LA, USA, November 2008. that operate on the same time-frequency code (TFC) (see Fig. 1). The union of overlapped beacon groups is called an extended beacon group. Nodes in an extended beacon group are time-synchronized, and they use a specific TFC for data transmission. For simplicity, in this paper, we use the term virtual network (VN) to refer to a group of nodes that are time-synchronized and use the same TFC. Using different TFCs for different VNs (ECMA-368 defines 10 TFCs) facilitates channel reuse over the same area. The ECMA-368 standard supports both random access and time-based reservations. The latter mode, known as the distributed reservation protocol (DRP), is particularly suited for real-time (voice and video) traffic. According to this protocol, devices that want to communicate with each other reserve their required medium access slots (MASs) from the available MASs that are not reserved by their neighbors in the same VN. A B C Nodes A, B, and C use the same TFC and they are time- synchronized Fig. 1: Concepts of beacon group and extended beacon group in ECMA-368. A solid circle around a node indicates the beacon group of that node, whereas the union of these beacon groups (dashed line) indicates an extended beacon group.