Initial Investigations of the Bluetooth link Gert Frølund Pedersen and Patrick Eggers Center for PersonKommunikation Aalborg University, 9220 Aalborg, Denmark Gfp@cpk.auc.dk, Pe@cpk.auc.dk Abstract Measurements using a live Bluetooth link are performed to establish initial knowledge about the link performance and the payload rate that can be obtained in indoor and outdoor environments. The measurements are conducted using two notebooks each with a Bluetooth PC-card and some specially developed S/W capable of making a duplex link and displaying various error rates as well as payload rate. Two modes are available one using the Bluetooth frequency hopping mode and one mode without frequency hopping. Initial results show that for the packet types allowing for the highest payload in duplex mode, which is 433 Kb/s, the payload rate is higher than 200 Kb/s within a distance of 10 to 25 meters for the indoor case and some 150 meters for the outdoor case. No improvements by using frequency hopping over non- frequency hopping were seen, it was concluded that this issue needs further investigations. 1. Introduction Bluetooth is a wireless system for short-range communication between many types of devices. The target coverage for a 0 dBm transmitting device is from some 10 centimeters to 10 meters. The devices will be small and the orientation will often not be known or common for the individual devices. It is therefore not possible to specify “normal” antenna requirements such as gain, directivity or polarization. The requirements for the antennas need therefore to be “best average” communication performance as is somewhat the case for other portable wireless equipment, e.g. pagers and phones. Another concern for wireless devices is the radio channel, which exhibits fast and large changes. The amplitude of a narrow band radio channel can easily change by 20 dB within a few milliseconds even for a slow moving device. To combat both the fast fading and the interference from other equipment working in the same frequency band the Bluetooth system has included fast frequency hopping. The idea behind the frequency hopping is that if the radio channel between the transmitter and receiver at one frequency is in a deep fade this is not very likely to be the case for another frequency if the distance between the two frequencies is large enough. The frequency separation needed depends strongly on the physical distance between the transmitter and the receiver. For a mobile phone having a distance of, say, a kilometer to the basestation in a city a frequency separation of less than one MHz is sufficient to obtain two independent radio channels. In Bluetooth up to 79 different channels each with 1 MHz separation will be used but the distance is only a few meters. For such a small distance even in an indoor environment the coherence bandwidth can be higher than the entire Bluetooth band. In such a case the frequency hopping will only combat the interference. Another way to obtain different or uncorrelated radio channels is to use more than one antenna and then switch to the best antenna at any given time or even combine the signals from the two antennas to obtain an even better signal. Having two antennas which, if designed properly can give two independent channels make it possible to combat a fading radio channel. If such an arrangement is necessary for Bluetooth depend on the radio channel, which is very different from what is known from other wireless systems. Some measurements have been conducted with frequencies and distances matching the ones for Bluetooth indicating that the coherence bandwidth will be larger than the total Bluetooth band (80 MHz) in 20 to 50 % of the cases. 1.1 Propagation environment of Bluetooth The Bluetooth channel is new in the sense that it is short range and communication between relatively slowly moving or stationary terminals. The frequency of 2.4 GHz is also new compared with most previous studies at 900 and 1800 MHz. The instantaneous bandwidth is around 220 KHz, and the frequency hopping bandwidth is around 80 MHz.