RTX successfully implements power control

Reducing power consumption

Reducing power consumption is always in focus when designing battery powered devices like cordless handsets. One obvious way to reduce the power consumption of a cordless handset is to reduce the transmit power delivered to the antenna. If the transmit power is automatically adjusted to the demands of the current system environment, the user should not notice anything - except for prolonged battery life. Full range is still maintained, because the output power will automatically increase when the distance between handset and base station is increased. The functionality of adapting the transmit power level is generally known as 'power control'. For high-density applications like call centres, power control may also reduce interference between users.

Reducing transmit power

In a cordless telephone system, the transmit power is typically 24dBm and the sensitivity limit is typically -93dBm, providing communication at up to 117dB path loss. If the handset is 1 metre away from the base station, the path loss amounts to about 35dB. This gives a link budget margin of 88dB at a distance of 1 metre. In fact, in this ideal situation, the transmit power could be reduced by up to 88dB without compromising link quality. In a standard cordless design, it is not practical to reduce the output power by that much, and generally not necessary when considering current consumption. Figure 1 shows the current consumption vs. output power for a power amplifier (PA) with an efficiency of 45%. According to the graph in figure 1, reducingthe output power by 14dB reduces the power consumption of the PA by 96%. For home or office use, the link budget margin is often much greater than 14dB, allowing a significant reduction in current consumption. This implies that the PA maintains efficiency at lower power levels. The graph in figure 2 shows the approximate path loss in outdoor and indoor environments (multipath effects are not included). Typical for indoor environment is that the radio waves have to travel through walls or ceilings, which is characterised by drops in the link budget.

The challenge to implement power control

According to figure 2, it is reasonable to reduce transmit power in situations where the path loss is much less than the range limit. The challenge is then how to implement power control, or more precisely, how to measure link quality in order to decide the necessary power level.

Measuring the link quality

The best way to measure link quality would be to derive the Signal to Noise Ratio (SNR) of the desired receive signal. For that, the demodulator in the receive chain would have to provide information about the confidence of the detected symbol – also known as soft detection. With the modern CMOS transceivers used in cordless products, soft detection is not provided. In a typical cordless application, there are two measures of the received signal quality: Received Signal Strength Indication (RSSI) and integrity check of the demodulated data by e.g. Cyclic Redundancy Check (CRC).

Measure of signal quality

The RSSI can be used as a measure of signal quality in the case where there is no interference. However, in the case where the receiver experiences an interfering signal in addition to the desired signal, the RSSI may falsely indicate quite sufficient signal strength even if the desired signal is completely jammed by the interfering signal. This is because the RSSI cannot distinguish the desired signal from the interfering signal. In this situation, a data integrity check can be used to detect if the demodulated data is corrupted, and then subsequently adjust the transmit power. Using a CRC check as a measure of link quality implies that a few bit errors occur in the data or voice transmission before the transmit power can be increased. The impact of these bit errors on a voice transmission may not be noticeable, depending on the frequency of occurrence.

Power control in handsets

Implementing power control in handsets is attractive because it increases the talk-time of the product. The base station monitors signal quality from the handset, and sends messages to increase or decrease transmit power as needed.

Power control in base stations

Power control in base stations is most interesting in a call-centre application, where each user has one base station and one handset. In such a situation, the transmit power of each base station should be kept as low as possible, in order to minimise interference since receivers are not perfect. Non-linearity and limited filter performance cause receivers to be sensitive to carriers present in adjacent channels. The effects of non-linearity increase exponentially with the strength of the undesired signal at the receiver input.

Transmit power of base station

A general problem exists when reducing transmit power of a base station with multiple handsets: The base station does not know the necessary transmit power for each handset to stay synchronised to the dummy carrier when the system is idle. One solution would be to transmit a dedicated dummy carrier with full power always. Another solution would be to monitor signal quality continuously for each registered handset, and adjust the base station transmit power accordingly.

Conclusion

The key to successfully implementing power control is being able to reliably measure the quality of the received signal. RTX uses RSSI and CRC information together to determine if the transmit power should be decreased or increased. The power control algorithm carefully decreases transmit power to find the link budget margin, and then adjusts the transmit power to a safe level for optimum performance.