GB2298551A - Determining ideal received signal strength power - Google Patents

Abstract

A limiting receiver circuit 4 provides a measured received signal strength signal (RSSIm) 6 and a measured baseband output power signal 11, and a processor receives these signals 6, 11 as inputs together with at least one system coefficient 7, the processor output being the ideal (or true) received signal strength power (RSSIi) of the required carrier. An unwanted signal at the input 15 of a limiter 12 of the receiver may cause an erroneous increase in the measured signal RSSIm together with a decrease in the measured baseband power 8. The processor provides a corrected value RSSIi. The system coefficients input to the processor may be the ideal baseband power and modulation type, such as GMSK. The measured baseband power may be determined from the sum of the squares of I and Q signals. The processor may be an analog function, a digital processor, or a look up table.

Description

RECEIVER CIRCUIT Field of the Invention This invention relates in general to communications systems, and more particularly to a circuit for a receiver in a communications system.

Background to the Invention The measurement of received signal strength (RSSI) is used by radio systems to determine the power incident at the antenna of the radio system.

Modern cellular systems require the reporting of received signal strengths to accurately control up link and down link power. This allows the minimum power to be transmitted to make a successful call, giving the dual benefits of minimising interference to adjacent cells and minimising the power consumption of the systems transmitters. For cellular radio systems the signal strength must be reported in the presence of high power interferers, which unless high selectivity filter are used will cause large errors in the signal strength reported.

Currently when operating a limiting receiver under high interference conditions a high selectivity filter must be used if accuracy is not to be degraded. High selectivity filters will cause a significant cost impact on the system. It will generally necessitate the use of a number of peripheral components such as passive tuning components and gain blocks to compensate for the filters' losses. This will increase the parts count allowing more opportunities for error and hence possibly reducing the product quality.

Another alternative is to accept the loss of performance due to the interferers and the subsequent loss of system capacity. The result being temporary loss of radio channels due to interference, or those channels being barred from use in a given geographic areas. Hence capacity will be lost in either time or space. The disadvantages are unacceptable in today's communications systems. Thus, a means for deriving an ideal received signal strength power is desired that overcomes the above disadvantages.

Summarv of the Invention According to the present invention, there is provided a circuit for determining an ideal received signal strength power RSSIi for a limiting receiver system in a communications system including a limiting receiving circuit having as an input an RF signal and outputting a measured baseband output power and a measured received signal strength RSSIm; and a processor having as input the measured baseband output power, the measured received signal strength RSSIm and at least one system coefficient and outputting an ideal received signal strength power RSSIi.

In a preferred embodiment, the system coefficients include ideal inphase and quadrature amplitude for the RF signal and the modulation type.

Brief Description of the Drawing FIG. 1 shows a circuit for determining an ideal received signal strength power (RSSIi) for a limiting receiver system according to a preferred embodiment of the present invention.

Detailed Description of the Preferred Embodiment FIG. 1 shows a circuit 10 for determining an ideal received signal strength power RSSIi 5 for a limiting receiver system in a communications system including a limiting receiving circuit 4 and a processor circuit 3. The limiting receiver circuit 4 has as an input an RF signal 2 and outputs a measured baseband output power 8 and a measured received signal strength RSSIm 6. The processor circuit 3 has as input the measured baseband output power 8, the measured received signal strength RSSIm 6 and at least one system coefficient 7 and outputs an ideal received signal strength power RSSIi 5.

In a preferred embodiment the system coefficients 7 are the ideal baseband power and modulation type. For example, the modulation type may be GMSK The processor may be an analog function, a digital processor or a look up table. For example, a look up table where particular system coefficients 7, measured baseband output power 8 and measured RSSI 6 correspond to a particular output or an ideal received signal strength power RSSIi 5.

The baseband power 11 is determined by the sum of the squares of inphase and quadrature signals of the baseband signal 8.

In the limiting receiver circuit 4 when a high power interferer is incident on the receiver it may not be possible to fully filter out the unwanted signal before the limiter 12 by the band pass filter 13. Any unwanted signal incident at the input 15 of the limiter 12 will cause the RSSI output 6 to be inaccurate and will cause the baseband I and Q signals to be less than the desired level 8.

It is known that the magnitude of a resultant vector which will be measured to determine the RSSI may be mainly dependent on the sum of the interferer and carrier. It is also known that the magnitude of the baseband signal may then been significantly reduced from its ideal level.

This is due to power in the limited signal being split between the interfering signal vector and the carrier vector. As the interfering vector can be suppressed by the baseband filtering only the carrier vector is present at the baseband output. Due to the power loss by the interferer being filtered the baseband output will be less than the ideal output level of a single carrier.

Thus the present invention provides a solution by which the received signal strength of the wanted carrier can be determined even when the interferer is sufficiently large to capture the output of the limiter.

In the presence of an interferer the following phenomena will be observed: 1. The indicated "received signal strength" will be larger than the received signal strength of the carrier alone.

2. The baseband inphase and quadrature signal will be less than there expected amplitude, due to a significant fraction of the limited signal power being the interferer.

The present invention correlates the increase in signal strength and the loss in baseband output power under interference to determine the "received signal strength" of the carrier. The present invention could be implemented as either software or hardware.

Although the circuit of FIG. 1 is shown using an IQ baseband demodulator 17 any type of demodulator could be used which allows measurement of the signal amplitude following the filtering that eliminates the interferer.

The present invention allows for the compensating for the presence of interferer which could also be used to extend the range of limiters, when operating near the receivers noise floor. This noise floor will generally be due to the noise generated by preceding stages and hence could be treated as a wideband modulated interferer.

The present invention is shown as a generic limiting radio receiver 4 with a IQ demodulator 17. The receiver 4 consists of a IF bandpass filter 13, a limiter 12 and an IQ demodulator 17 and active baseband filters 19, 21 on both the I and Q branches. All circuits other than the limiter operate linearly.

From the receiver circuit 4 shown the following signals can be measured, Im, Qm 8, RSSIm 6. From the design of the radio system the following will be known (I2+Q2)ideal, M (modulation type). Using the equations shown below the "received signal strength" of the carrier, C can be determined.

C = Carrier power I = Interferer power Im = Measured baseband I voltage Qm = Measured basebad' Q voltage (I2+Q2)ideal = Ideal magnitude of I/Q power RSSI = Measured RSSI signal M = Modulation type (this will influence the limiters suppression of the carrier) The ratio of Carrier to interferer can be determined, by a function C/I=G(Im,Qm(I2+Q2)ideal,M).

The power of the carrier plus the interferer can be determined by a function C + I = H(RSSI).

The carrier power can be determined by C= G(Im,Qm,(I2+Q2)ideal,M).H(RSSI)/1 + G(1m ,Qm,(I2+Q2)ideal,M) The present invention increases the accuracy of received signal strength measurement. It allows operation of a radio system under high interference condition without the need for high selectivity filters and allows the dynamic range of a limiter to be extended by compensation of the noise power at its input.

The present invention allows radio systems to operate at high interference levels thus allowing smaller cells and low transmit powers to be used.

Claims (8)

Claims

1. A circuit for determining an ideal received signal strength power RSSIi for a limiting receiver system in a communications system, comprising: a limiting receiving circuit having as an input an RF signal and outputting a measured baseband output power and a measured received signal strength RSSIm; and a processor having as input the measured baseband output power, the measured received signal strength RSSIm and at least one system coefficient and outputting an ideal received signal strength power RSSIi.

2. The circuit of claim 1 wherein the system coefficients comprise of any one or number of the following: ideal baseband power and modulation type.

3. The circuit of claim 2 wherein the modulation type is GMSK

4. The circuit of any of the above claims wherein the processor is an analog function.

5. The circuit of any of the above claims wherein the processor is a digital processor.

6. The circuit of any of the above claims wherein the processor is a look up table.

7. The circuit of any of the above claims wherein the baseband power is determined by the sum of the squares of inphase and quadrature signals of the baseband signal.

8. A circuit for determining an ideal received signal strength power RSSI for a limiting receiver system as substantially described with reference to FIG. 1 of the drawing.