Transmit-power reduction for class-1 Bluetooth-enabled indoor cordless phones

Abstract

Class-1 Bluetooth devices support a transmission range of about 100m and are useful for indoor cordless telephony with advantages of wider coverage, greater user mobility, and more convenience. To minimize the transmit power of class-1 devices, feedback power control specified in the Bluetooth specification can be used. This paper shows that further transmit-power reduction is possible by reducing the Golden Receive Power Range (GRPR) from the specified value of 20dB. For typical indoor log-normal-shadowing channels, more than 4dB reduction in the mean transmit power can usually be obtained by reducing the GRPR to 10dB. However, using a smaller GRPR increases the frequency of making power-adjustment requests through the Link Manager Protocol (LMP), thereby pre-empting more voice packets and affecting the voice quality. We compute the overhead cost due to power control, defined as the percentage of the total number of packets used for power-adjustment requests, when the GRPR is set at 10dB ± 6dB, wherein 6dB is the tolerance allowed in implementation. It is found that the overhead cost is less than about 1% but becomes close to 1% as the GRPR approaches 4dB, indicating that the link performance would become barely acceptable for voice transmission in some situations. We also consider utilizing the reserved byte in LMP power-control commands to convey the preferred number of power-adjustment steps to the transmitter in order to reduce the overhead cost. With this arrangement, the (worst-case) overhead cost is reduced to about 0.3%, so that the voice quality can be maintained acceptable even if the GRPR is reduced to 10dB ± 6dB for transmit-power reduction.