Abstract

  Bluetooth is an increasingly popular and widely deployed standard for wireless personal area networks (WPAN). The Bluetooth physical layer employs Gaussian frequency shift keying (GFSK), which is a particular form of continuous phase modulation (CPM). GFSK provides a favorable trade-off between power and bandwidth efficiency, and allows for low-complexity transmitter and receiver implementations. A simple discriminator detector is used to recover the GFSK modulated data in Bluetooth devices.

    Though structurally and computationally simple, discriminator detectors are very power inefficient. Coherent sequence detectors are significantly more power efficient for modulation schemes with memory, since the memory introduced by CPM is properly taken into account. However, realization of sequence detection (SD) for Bluetooth systems is very difficult because the modulation index h is allowed to vary in a wide interval. This varying modulation index leads to a varying trellis structure for SD with possibly a large number of states. The design of the optimal receiver filter for a sufficient statistic for SD after sampling is also dependent on h. The receiver filter design is further complicated by the operation of Bluetooth systems in a license-free band, thereby, requiring the designed receiver filter to be robust against interference from other devices operating in the same band. Moreover, coherent SD requires phase synchronization, which is a difficult task as well because of the frequency hopping radio of Bluetooth and the allowed local oscillator dynamics.

    Several approaches to a simple and power-efficient receiver design for Bluetooth have been discussed in the literature. The drawbacks of these approaches are that either the achieved power efficiency is insufficient or perfect channel phase estimation at the receiver has been assumed. These designs are restricted to a particular value of h and the effects of high frequency offsets at the receiver oscillator on the performance have not been accounted for. Therefore, the practical applicability of these receivers is limited.

    In the present research work, a novel noncoherent SD (NSD) receiver for Bluetooth systems is proposed. The receiver design is based on the decomposition approach to CPM and the concept of noncoherent sequence detection of CPM. A low-complexity implementation of the receiver is presented with only one receiver filter and NSD on a two-state trellis, which accomplishes significant performance gains of more than 4 dB over the discriminator-based detector. The proposed receiver caters to the requirements of Bluetooth systems comprehensively in that (a) the entire range of possible h is considered and an adaptive solution to account for varying h is provided, (b) a frequency offset compensator is incorporated into NSD to cope with the large local oscillator frequency deviations allowed in Bluetooth systems, and (c) improved decoding methods for the forward error correction (FEC) schemes employed in Bluetooth are devised. Simulation and analytical results verify that the presented NSD receiver operates close to the theoretical limits. The proposed receiver is robust and simple, and therefore, is an attractive solution for Bluetooth devices.