WO2002054625A2 - Method and apparatus for adaptive gain control of rf telecommunication system - Google Patents

Abstract

A system and method are disclosed, the system and method being designed for adaptively controlling multiple parameters within an RF telecommunications system to address the problem of interference while maximizing coverage and capacity. In one embodiment of the invention, there is provided a variable gain stage comprising an RF filter (20), amplifier (21) and a controller (23) responsive to a plurality of system parameters, on the base station receiver prior to or in place of the system RF front end. The system parameters of interest can include dynamic parameters such as received signal strength indication, idle channel disturbance, bit error rate, frame error rate and SINAD (signal-plus-noise-plus-distortion). The algorithm used to determine optimum gain at the receiver can also take into account a model of the traffic and dropped call rates as a function of time of day or day of week, adjacent cell site traffic and the like. In another embodiment of the invention, a variety of system parameters are measured and used to determine the optimum gain on the base station transmitter-side amplifiers. In yet another embodiment of the invention, the duration, power and frequency of interfering signals are dynamically sampled and the parameters used to adjust the filter characteristics of the base station receiver front-end filters.

Description

METHOD AND APPARATUS FOR ADAPTIVE CONTROL OF RF TELECOMMUNICATION SYSTEM

This application is being filed as a PCT international patent application in the name of Graham Y. Mostyn, a U.S. citizen and resident, on 18 December 2001, designating all countries.

Background of the Invention

Field of the Invention

The present invention relates to telecommunications systems. More specifically, the present invention relates to a system and method for dynamically optimizing the performance of a telecommunications system by monitoring a plurality of system input parameters and dynamically adjusting selected output parameters.

Description of Related Art The use of wireless telecommunications has evolved in the past decade from analog systems for voice communication carrying a small number of signals, to digital systems using time division multiple access ("TDMA"), code division multiple access ("CDMA") or gateway mobile switching ("GSM") protocols to handle large volumes of both voice and data communications. As demand has grown, especially in urban settings, additional RF spectrum ranges have been assigned to these telecommunications uses, and the assigned spectrum has become more crowded. Increasingly, wireless telephony is utilized for Internet access, point- to-point data transmission and similar data-intensive uses. The wireless telecommunications industry is increasingly moving toward CDMA, wide-band ("WCDMA") and GSM protocols to accommodate the increased demands. A major problem with CDMA and WCDMA is a high sensitivity to interference. Interference may be out-of-band, i.e., from signals at frequencies outside of the service provider's assigned RF spectrum but causing interference with signals within the assigned frequency due to modulation products or failure of the filter to remove the out-of-band signals. They may also be in-band interferors, i.e., signals within the service provider's assigned frequency range. In-band interference may be due to a number of sources, including signals from mobile transmitters controlled by distant base stations, rogue transmissions, harmonics of transmissions from other parts of the RF spectrum. Both in-band and out-of-band interfering signals vary widely in RF frequency, power level and duration. This variation can occur over short periods of time.

With CDMA systems, the signals received at the base station from each mobile unit must be of similar intensity. Typically, the mobile switching center, at a central location, monitors background noise levels and the signal strength of each mobile unit. It then sends a signal, through the base station, to each mobile unit, adjusting the transmit strength of mobile unit to provide a sufficient signal to noise ratio, and to equalize all signals. Klein et al., U.S. Patent No. 5,265,119, describes a method for mobile switching center control of mobile transmit power based on received signal power. Muszinski, U.S. Patent No. 5,623,484, describes a closed- loop method of controlling the mobile transmit power of a plurality of mobile units communicating with a plurality of base stations controlled by a single mobile switching center by controlling the average signal-to-interference plus noise ratio (SINR) of all uplink CDMA signals close to a predetermined threshold value. There is a problem with relying on active control of the mobile transmit power to control signal quality at the base station. It is known that as in-band interference and noise increase — causing the required output signal from mobile units to increase ~ the required transmit power from mobile units will exceed the unit's maximum transmit power. As a result, the call will be dropped. This is most likely to happen at time of heavy traffic on the system, and first affects distant and low-powered units. This phenomenon is commonly referred to as "breathing." The coverage area expands and contracts with changes in interference and demand. Recently, we observed a more dramatic demonstration of this breathing effect. Under certain conditions, interfering signals can increase the required mobile transmit power so greatly that the coverage area collapses almost completely. Even mobile units very near the base station cannot transmit sufficient power to remain in contact with the base station. Simple reduction in required transmit power from all mobile units would not provide a solution, because it would result in a greatly increased bit error rates and frame error rates. This, in turn, would degrade the quality of the communication. Thus, a system and method are needed to actively reduce interfering signals and optimize desirable signals. The dual problems of interference and diminished system capacity and range are currently partially addressed at the base station IF chain where adjustments can be made in amplification of the incoming signal from the mobile unit. RF front ends of current cellular radio systems employ the architecture shown in figure 1. Signals of multiple radio channels received by the antenna 1 pass through an RF filter 2 and low-noise amplifier 3. The RF signal is then multiplexed by a multicoupler 4 to many independent radio channel cards, one per channel. Each card 5 contains a mixer 6 to convert the wanted channel to an intermediate frequency (IF). Next, the signal passes through an IF filter 7. The IF signal is passes through an automatic gain control amplifier 8 and presented to quadrature mixer, where it is brought to baseband, digitized and processed.

The performance of such a radio is frequently described by its spurious-free dynamic range (SFDR), the range over which an accurate representation of the received signals can be produced. The lowest level, termed sensitivity, is set primarily by the LNA and mixer noise figures, as well as the IF bandwidth and method of processing. The highest level, a measure of large signal handling capability, is known as the degradation level, and is quantified by the compression point or third-order intercept point - it is set by distortion in the signal chain. The SFDR is commonly characterized by the difference between these two levels.

Automatic gain control (AGC) amplifiers 8 are employed prior to stages of limited dynamic range to maximize the dynamic range of the system as a whole. Such AGC systems sample, at their input, the signal level within the channel bandwidth and adjust their gain to produce an output with lesser variation.

In figure 1, each AGC control operates within the associated narrowband IF section, monitoring the level and controlling the gain for that particular radio channel. In the prior art architecture illustrated, the two parameters of the SFDR for any given radio channel are fixed by the component stages, the AGC algorithm, and are independent of activity on adjoining radio channels or bands.

In this prior art architecture, the performance of the wideband amplifier and mixer is a compromise in performance between different radio link installations and at different times of day. This is because both the number and strength of in-band mobile signals on the radio channels vary greatly, as well as the strength of out-of- band interfering signals.

Summary of the Invention In accordance with principles of invention, a system and method are designed to adaptively control multiple parameters within an RF telecommunications system to address the problem of interference while maximizing coverage and capacity.

In one embodiment of the invention, there is provided a variable gain stage comprising an RF filter, amplifier and a controller responsive to a plurality of system parameters, on the base station receiver prior to or in place of the system RF front end. The system parameters of interest may include dynamic parameters such as received signal strength indication, idle channel disturbance, bit error rate, frame error rate and SLNAD (signal-plus-noise-plus-distortion). The algorithm used to determine optimum gain at the receiver may also take into account a model of the traffic and dropped call rates as a function of time of day or day of week, adjacent cell site traffic and the like.

In another embodiment of the invention, a variety of system parameters are measured and used to determine the optimum gain on the base station transmitter- side amplifiers.

In yet another embodiment of the invention, the duration, power and frequency of interfering signals are dynamically sampled and the parameters used to adjust the filter characteristics of the base station receiver front-end filters. Brief Description of the Drawings

Figure 1 illustrates prior art receiver architecture. Figure 2 illustrates the relationship between signal degradation and noise level at differing traffic levels. Figure 3 illustrates a receiver architecture of the invention.

Figure 4 illustrates an alternative receiver architecture of the invention

Description of Preferred Embodiments

It is beneficial, under low traffic conditions, to improve the overall sensitivity of the system, dropping the noise level, at the expense of decreasing the degradation level and reducing the SFDR. In this case, there are few strong signals, and intermodulation interference (which is reduced with a high degradation level) is less likely to occur. The decrease in degradation level is therefore less important than improving the noise figure, which enables coverage to be increased and additional mobiles received that would otherwise be lost. Under high traffic conditions, there is increased likelihood of strong signals from nearby mobile stations, both in-band and out-of-band, and it is desirable to maximize the degradation level, at the expense of sensitivity and again of SFDR.

This concept of dynamically adjusting system performance parameters based is illustrated in figure 2. The conventional architecture of figure 1 has a baseline SFDR, often optimized to a maximum level, shown on the left-hand side of the figure. With lower traffic levels, the low-noise option in the center of figure 2 may be preferable, and under high traffic conditions, the increased degradation level shown on the right-hand side may be preferable. These conditions can be realized with the using the system and method of the invention. In one embodiment of the invention, shown schematically in figure 3, an adaptive stage comprising a first wideband filter 20, and a low-noise linear amplifier 21, with adjustable gain controlled dynamically by system parameters are positioned prior to the wideband filter 2 of the prior art system. A controller 23, which may be a dedicated circuit within the receiver, a computer located in the base station, or a remotely located computer, processes system parameter data and adjusts system gain. Optionally, the RF filter characteristics, such as band edge and internal notch f equency, width and depth are also dynamically controlled by controller 23 in response to measured system parameters.

In a second embodiment, shown in figure 4, the wideband filter 2 and amplifier 3 of the prior art are replaced with a wideband filter 20, low-noise linear amplifier 21 with adjustable gain. In other respects, the embodiment is similar to the first embodiment.

Receiver hardware The architecture of one aspect of the invention is shown in figure 3. The conventional receiver architecture of figure 1 is preceded by a high-performance amplifier and filter, followed by an adjustable gain block controlled dynamically by system parameters.

Dynamic parameters used to determine optimal gain include the SINAD, received signal strength indication, idle channel disturbance, bit error rate and frame error rate. The system would preferably respond to changes in these parameters within a single frame, or in less than 20 milliseconds. These parameters are obtained by measuring and digitizing the received signal strength in a separate RF receiver, or the signal strengths on each channel of a narrow-band scanning receiver. The monitoring methods are known in the art.

Other parameters may also be included for determining optimal gain. They include "minutes of use" and dropped call rate as a function of day and time, and dropped calls. "Minutes of use" data may be obtained from the traffic switch/billing equipment. The interfering signal data may be used to adjust the RF filter parameters in the receiver front end. Center frequency may be adjusted, and in-band bandstop filters added to block strong in-band interferers. The system gain parameters to be optimized to achieve a high traffic level (to maximize revenue), or quality of service, or a combination of the two. One example of a "Quality of service" parameter would be "dropped call rate," again commonly monitored in cellular telephone systems. The data used to adjust the RF front end is passed to adaptive software algorithms, which produce an output controlling the current system gain. The digital output of the software is used to drive a variable gain attenuator following the first amplifier. In figure 4, an alternative architecture is shown. Only a single wideband filter 20 and linear low-noise amplifier 22 are used. These are dynamically controlled by a controller 23.

Data on interfering signals, either in-band or out-of-band, may be used to adjust the wideband filter parameters in the receiver front end. Parameters of interest include frequency, power and amplitude of the interfering signal. Center frequency may be adjusted, and in-band bandstop filters added to block strong in- band interferors.

In the preferred embodiment, the wideband filter 21 is a very high quality filter. Preferably, it is a superconductive filter. Superconductive filters are known in the art, and are commercially available. Such filters have low loss within the pass band and very sharp skirts. Adjustment of the center passband and addition of in- band notches may be done using any known means. One example is described in Scarpa, et al. US Patent No. 5,325,204.

Software implementation

The algorithm described is intended as an example to indicate how the system may work. A very large number of alternative software algorithms may be used.

Develop traffic level model for the cell site and adjacent sites channels The minutes of use in each hour, representing traffic level, undergo a rolling average over several days of data, to develop a continually updated traffic level model as a function of day of week and time of day.

Measure signal strength and interfering signals, both in-band and out-of-band The output of the RF receiver or scanning receiver is also measured. Develop adaptive algorithm to control system gain Let the two traffic level models be A(t) for wanted channels, where A is the minutes of use as a function of time t, and B(t) for unwanted channels, where B represents the wideband signal level, again as a function of time.

Let Y(t) be the RF system gain at time t A simple algorithm would be

Y(t) = m A(t) + n B(t) + p where m, n and p are constants.

The software adopts initial values of m and b, set by the programmer, calculates Y(t) and adjusts the system gain. Consequently, the system gain is increased during reduced traffic periods to improve coverage, and reduced during increased traffic periods to increase large-signal degradation level.

The software employs iterative techniques well known in the prior art to continually adjust m, n and p to maximize the desired metric.

As indicated, in some applications, maximizing quality of service may be more appropriate than maximizing traffic. In this case, the traffic level parameter can be supplemented and weighted against a quality of service parameter.

The most general application uses any number of desired high-level system parameters for feedback to the radio front-end and control of the SFDR limitations.

Similarly, the software may be used to adjust the gain on the base station transmit-side amplifier.

There are a number of significant differences between the invention and the conventional AGC system that is well known h the prior art.

Conventional AGC systems generally sample the instantaneous level of the signal to be received on the particular IF channel controlled by the AGC. By comparison, this invention samples the signal on multiple channels within the system's RF band that are close to any given in-use channel and are likely to interfere.

Second, this invention samples signals on adjacent bands, which are outside the system's RF frequency range but likely to interfere due to mtermodulation at the IF level or high power that overwhelms the RF filters. Third, conventional AGC systems operate using the IF signal level, while the present invention considers several parameters. This system is intended to maximize overall system usage and quality of service: the software algorithm is driven by a set of high level system parameters for the wanted channels. Tests have shown that in uplink-limited cellular systems, a reduction in system noise figure (albeit with reduction in the degradation level, and therefore at the expense of SFDR) will increase traffic levels as system coverage and subscriber audio quality improve.

Fourth, conventional AGC systems are memory-less: this invention describes an adaptive system that learns the optimum choice of system performance for traffic conditions, time of day etc.

It will be appreciated by those skilled in the art that a number of variations are possible within the spirit and scope of the invention. For example, the invention may be combined with an adaptively tunable notch filter to remove interfering in- band signals.

While the foregoing disclosure contains many specificities, it should be understood that these are given by way of example only. The scope of the invention should not be limited by the specific examples given above, but only by the appended claims and their legal equivalents.

Claims

1. A radio receiver characterized by a spurious-free dynamic range and at least one other performance parameter, the receiver comprising: a variable gain stage connected to an antenna and configured to receive signals indicative of the at least one other performance parameter and set the gain of the variable gain stage in response to the at least one other performance parameter.

2. The receiver of claim 1 , wherein the variable gain stage comprise: a. an adaptive stage connected to the antenna and configured to filter and amplify a signal from the antenna; and b. a controller connected to the adaptive stage and configured receive signals indicative of the at least one other performance parameter and to adjust the performance of the adaptive stage in response to the at least one other performance parameter.

3. The receiver of claim 2, wherein the at least one other performance parameter includes at least one parameter chosen from the group comprising received signal strength indication, idle channel disturbance, bit error rate, frame error rate and signal-plus-noise-plus-distortion.

4. The receiver of claim 3 wherein the receiver is configured to terminate reception of a signal upon a predetermined condition, and the at least one other performance parameter further comprises the rate at which receptions are terminated.

5. The receiver of any of the claims 2 through 4, wherein the adaptive stage comprises: a. a filter; and b. an amplifier connected to the filter.

6. The receiver of any of the claims 1 through 4, further comprising a plurality of channel modules, each of which is configured to process signals of a range of frequencies substantially different from at least the range of frequencies that another one of the plurality of channel modules is configured to process, wherein the at least one other performance parameter includes performance parameters of at least two of the plurality of channel modules.

7. A method of adjusting a receiver used in wireless communication, the method comprising: a. monitoring at least one parameter of the signal received by the receiver; and b. setting an operating condition of the receiver in response to the at least one parameter.

8. The method of claim 7, wherein the monitoring step comprises monitoring at least one parameter chosen from the group comprising received signal strength indication, idle channel disturbance, bit error rate, frame error rate and signal-plus-noise-plus-distortion.

9. The method of claims 7 or 8, further comprising terminating reception of a signal upon a predetermined condition, wherein the monitoring step further comprises monitoring the rate at which receptions are terminated.

10. The method of claims 7 or 8, wherein the setting step comprises setting the spurious-free dynamic range of the receiver.

11. The method of claim 10, wherein the setting step comprises setting the spurious-free dynamic range of the receiver.

12. The method of claims 7 or 8, wherein the monitoring step comprises monitoring parameters of at least two signals, each of which falls within substantially a frequency range that is different from the frequency range of the other signal.

13. The method of claim 9, wherein the monitoring step comprises monitoring parameters of at least two signals, each of which falls within substantially a frequency range that is different from the frequency range of the other signal.