Capacity Evaluation of Frequency Hopping Based Ad-hoc Systems Apurva Kumar and Rajeev Gupta IBM India Research Lab Block 1, liT Delhi New Delhi, India-110016 {kapurva, grajeev}@in.ibm.com ABSTRACT The IEEE 802.15 Wireless Personal Area Networks (WPAN) study group has been working on evolving a standard for short- range wireless connectivity between low complexity and low power devices operating within the personal operating space (POS). The scenarios envisioned for WPANs are likely to involve a large number of POSs operating in an indoor environment. Among short-range wireless technologies, Bluetooth TM t based M- hoe connectivity comes closest to satisfying the WPAN requirements. Bluetooth provides a gross rate of 1 Mbps per network and allows several such networks to overlap using frequency hopping. The 'aggregate throughput' thus achieved is much higher than 1 Mbps. In the absence of external interfering sources, aggregate throughput is limited by self interference which depends upon, (i) physical layer parameters like hopping rate, hopping sequences, transmitted power, receiver sensitivity, modulation, forward error correction (ii) channel characteristics like coherence bandwidth and coherence time (iii) spatial characteristics. In this work we consider the problem of finding the capacity of Bluetooth based ad-hoc systems by accurately modeling the Bluetooth physical layer and the indoor wireless channel. We predict the throughput in Bluetooth based M-hoe systems as a function of a generalized set of parameters using realistic scenarios and assumptions. Keywords Bluetooth technology, GFSK, frequency hopping, forward error correction, capacity, throughput, M-hoe networks, bit error rate. 1. INTRODUCTION The Wireless Personal Area Network (WPAN) Study Group was formed by the IEEE 802.11 Working Group to investigate the need for a supplemental wireless network standard in the 2.4 GHz Industrial Scientific Medicine (ISM) band specifically targeted to provide very low power consumption, low complexity, wireless Permission to make digital or hard copies of all or part of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advan- tage and that copies bear this notice and the full citation on the first page, To copy otherwise, to republish, to post on servers or to redistribute to lists, requires prior specific permission and/or a fee. ACM SIGMETRICS 2001 6/01 Cambridge, MA., USA © 2001 ACM ISBN 1-58113-334-0/01/06...$5.00 connectivity among devices within or entering a Personal Operating Space (POS). This includes devices that are carried, worn, or located near the body. The IEEE 802.15 Working Group for WPAN was formed in March 1999. The primary WPAN design criteria are different from those of the 802.11. The WPAN functional requiretments are simpler, yet there is a much greater concern over power consumption, size, and realizable product cost. This is due to the WPAN focus on the requirements of wearable computing and peripherals. Since Bluetooth -a newer addition to the wireless space- comes closest to satisfying WPAN requirements, the Physical (PHY) and medium access control (MAC) functions of 802.15 are likely to derive heavily from the RF and baseband specifications of Bluetooth [ 1]. Bluetooth enables the design of low-power, small sized low cost radios, operating in 2.4 GHz band, which can be embedded in portable devices like mobile phones, modems, headsets, printers, computers etc. The Bluetooth system provides a point-to-point or a point-to-multipoint connection. In point-to-multipoint connection the channel is shared among several Bluetooth units. Two or more units sharing the same channel form a piconet. Piconet is the basic networking unit in Bluetooth consisting of a master and up to seven active slaves. The medium access is time division duplex (TDD) over Time Division Multiple Access (TDMA) within a piconet and slow Frequency Hopped Multiple Access (FHMA) between piconets. Each piconet uses a different pseudo-random hopping sequence with a maximum hopping rate of up to 1600 hops/see. The 2.4 GHz ISM band is divided into 79 or 23 frequency bands of 1MHz each. All devices operating in a piconet are time and frequency hop synchronized to the channel. Different piconets are not time or fi'equency synchronized with each other. For the scenarios envisioned in Bluetooth, it is likely that a large number of such M-hoe networks will co-exist in the same area and share the medium in the same location in an uncoordinated fashion. We define aggregate throughput for a scenario as the sum of individual piconet throughputs operating simultaneously in that scenario. We also define capacity of the system as the maximum number of piconets in a given area while guaranteeing a certain quality of service (QoS). The Bluetooth modulator has a gross rate of 1Mbps. However due to non-orthogonality of the hop sequences the theoretical aggregate gross rate of 79Mbps (in the 79 frequency hop system) is unachievable. The aggregate i Bluetooth is a trademark owned by Telefonaktiebolaget L M Ericsson, Sweden. 133