Soft Deadline Bounds for Two-Way Transactions in Bluetooth Piconets under co-channel Interference Amre El-Hoiydi, J.-D. Decotignie CSEM, Jaquet-Droz 1, 2007 Neuchâtel, Switzerland T: +41327205111, F: +41327205720 Email: {amre.el-hoiydi, jean-dominique.decotignie}@csem.ch Abstract - This paper presents the delay performances of two- way transactions implemented on top of the Bluetooth HCI layer. The two-way transaction, formally called the Send Data with Acknowledge (SDA) service primitive, is a fundamental building block for real-time protocols. We analyze its delay performances under packet loss probability caused by a number of co-located interfering piconets. We first present a probabilistic treatment of the performance of a Bluetooth piconet under co-channel interference from other Bluetooth piconets. An upper bound on the packet error rate is analytically derived. Simulation results validating the theoretical results are shown. The obtained packet error rate bounds are then applied to the problem of deadline for two- way transactions. The probability that a deadline will be met is introduced, as well as the value of the deadline in function of the desired probability that the deadline will be met. 1 INTRODUCTION Bluetooth is a digital wireless data transmission standard in the 2.4 GHz ISM band aimed at providing a short range wireless link between laptops, cellular phones and other devices [1]. The air interface modulation is Gaussian FSK with a raw bit rate of 1 Mb/s. The communication topology between Bluetooth nodes is a star, where a master communicates in time division duplex with several slaves forming a so-called piconet. Even slots are used for packets from the master to one of the slaves and odd slots are used for the return direction. In order to tolerate interference, which can readily arise in the 2.4 GHz band, a slow frequency hopping scheme is used, where all nodes of a piconet hop together among 79 frequencies at each packet slot. The problem of co-channel interference from other Bluetooth piconets can become of high importance if, for example, a number of machines on a factory floor independently use Bluetooth as their wireless transmission technology. In this paper, we will consider only interference from other Bluetooth piconets. Interference from other sources is not considered. Simulation results addressing the problem of interference between Bluetooth networks can also be found in [2] and [3]. Section 2 presents the problematic of implementing a real time protocol over the Bluetooth protocol stack. RF LC (baseband) LMP HCI Driver SDP TCS RFCOMM Hardware Firmware Software Module Host CPU USB / PC Card / RS232 PCM SCO ACL OBEX HCI Firmware Speech L2CAP Application A/D Figure 1. Bluetooth protocol stack. The impact of the co-channel interference on the packet error probability will be addressed in section 3, deriving an upper bound on the packet error probability. Starting from these probabilistic results, the problem of deadline for two-way transactions is analyzed in section 4. 2 REAL-TIME PROTOCOLS OVER BLUETOOTH Bluetooth is meant to be used through one of its profiles, which specifies the protocol stacks up to the application. Figure 1 shows the Bluetooth stack from physical to application layer. The speech data is transported over synchronous links (SCO), for which slots are reserved. The other services are going through the whole Bluetooth protocol stack, converging at the L2CAP layer, which takes care of multiplexing several connections and segmenting and reassembling packets. L2CAP packets are transported over asynchronous links in the slots not reserved for speech. Only one data packet type (DM1) is allowed to be transmitted over an SCO link, interrupting a voice data stream. The Bluetooth standard has designed the SCO link to transport mainly voice data. The input of the chipset is usually a PCM interface. The data stream is then encoded either in CVSD or log PCM format. In modules available up to now, and to our knowledge, the SCO link is hence not a clean data pipe that can be used for real-time traffic.