Mobile and Wireless Communications Networks

AN ALTERNATIVE METRIC FOR CHANNEL ESTIMATION WITH APPLICATIONS IN BLUETOOTH SCHEDULING (S. 203-204)

João H. Kleinschmidt, Marcelo E. Pellenz and Luiz A. P. Lima Jr.
Graduate Program in Computer Science, Pontifical Catholic University of Paraná, Curitiba – PR, Brazil. E-mail:{joaohk, marcelo, laplima}@ppgia.pucpr.br

Abstract: Once Wireless Local Networks (WLAN) and Bluetooth devices share the same frequency band (ISM) there is a potential risk of interference if they are supposed to operate close to each other. Additionally, the signal fading effects on mobile Bluetooth networks may deeply affect the overall performance. That is why the use of strategies that minimize transmission on channels with great interference or severe fading is so important. This paper proposes and investigates the use of parameter m of the Nakagami distribution, as the channel estimation metric. We observed that parameter m may provide faster estimates on the channel condition than the bit error rate metric. This metric is applied in a new scheduling algorithm for Bluetooth piconets. Simulation results showing the performance of the algorithm for different traffic conditions are eventually presented. Bluetooth; wireless networks; Nakagami-m fading; scheduling. Key words:

1. INTRODUCTION

Bluetooth is emerging as an important standard1 for short range and lowpower wireless communications. It operates in the 2.4 GHz ISM (Industrial, Scientific and Medical) band employing a frequency-hopping spread spectrum technique. The transmission rate is up to 1 Mbps, using GFSK (Gaussian Frequency Shift Keying) modulation. The Bluetooth MAC protocol is designed to facilitate the construction of ad hoc networks. The devices can communicate with each other forming a network with up to eight nodes, called piconet. Within a piconet, one device is assigned as a master node and the others devices act as slave nodes. Devices in different piconets can communicate using a structure called scatternet. The channel is divided in time slots of A time-division duplex (TDD) scheme is used for full-duplex operation. For data transmission Bluetooth employs seven asynchronous packet types.

Each packet may occupy 1, 3 or 5 time slots. The throughput of Bluetooth links using asynchronous packets was investigated2 for the additive white Gaussian noise (AWGN) channel and for the Rayleigh fading channel. In other work3, we extended the results presented by Valenti2 looking into the performance of Bluetooth links in Nakagami-m fading channels. The sharing of the same frequency band between WLAN and Bluetooth devices may cause interference, if they are operating close to each other. Additionally, may occur mutual interference between different Bluetooth piconets operating in the same area. In Bluetooth networks with node mobility, like in sensor networks applications, the fading effects in the radio signal may significantly decrease the link performance. The use of strategies that minimize the transmission in channels with great interference or severe fading, may substantially improve the piconet performance. Extensive empirical measurements have confirmed the usefulness of the Nakagami-m distribution for modeling radio links13,14. The Nakagami-m distribution4 allows a better characterization of real channels because it spans, via the parameter m, the widest range of multipath fading distributions. For m=1 we get the Rayleigh distribution.

Using m<1 or m>1 we obtain fading intensities more and less severe than Rayleigh, respectively. This work proposes the use of fading parameter m as an alternative channel quality metric. This parameter can be estimated based on the received symbols. In a mobile wireless network, when a node position changes from line-of-sight to non-line-of-sight, for example, the impact in the signal propagation characteristic may be interpreted as a change in the parameter m. This model is interesting when Bluetooth devices are applied to ad hoc sensor networks. Power class one Bluetooth devices can cover ranges up to 100 meters, allowing the formation of large area piconets or scatternets. We also propose a new scheduling algorithm for Bluetooth piconets, which uses the channel quality information in the scheduling policy.