WO2001022575A1

WO2001022575A1 - Method and apparatus for providing a constant loop bandwidth of a power control loop system at different power levels

Info

Publication number: WO2001022575A1
Application number: PCT/US2000/025236
Authority: WO
Inventors: Chia-Sam Wey; Jukka-Pekka Neitiniemi; Kim Anh Tran
Original assignee: Wey Chia Sam; Neitiniemi Jukka Pekka; Kim Anh Tran
Priority date: 1999-09-23
Filing date: 2000-09-14
Publication date: 2001-03-29
Also published as: AU7487600A

Abstract

A method and apparatus for providing a constant loop bandwidth of a power control loop system at different power levels. The present invention generates a power control loop output signal using series attenuators. The power output signal is processed in a power control loop to generate an attenuator control signal, wherein the attenuator control signal providing a constant loop bandwidth that is independent of the power level of the power output signal. To provide a constant loop bandwidth that is independent of the power level, a processor circuit such as a square root circuit is coupled between the output of the controller and the two series attenuators. The processing circuit processes the control signal to generate an attenuator control signal. The attenuator control signal provides a constant loop bandwidth that is independent of the power level of the power control loop output power signal, Pout. For example, the square root circuit receives the output from the controller Vc and the output Vs of the square root circuit controls the PIN diode attenuators, where Vs=Vc1/2. Alternatively, the square root circuit may be positioned between the detector/linearizer and the comparator.

Description

METHOD AND APPARATUS FOR

PROVIDING A CONSTANT LOOP BANDWIDTH OF A POWER CONTROL

LOOP SYSTEM AT DIFFERENT POWER LEVELS

BACKGROUND OF THE INVENTION

1. Field of the Invention.

This invention relates in general to radio transceivers, and more particularly to a method and apparatus for providing a constant loop bandwidth of a power control loop system at different power levels.

2. Description of Related Art. Cellular mobile radio provides the technology that enables everyone to communicate anywhere with anybody. This technology has created an entire industry in mobile telecommunications which is rapidly growing and which has become a backbone for business success and expanding economies.

Telecommunication devices are designed to move information from one place to another over channels, and a radio channel is an extraordinarily hostile medium on which to establish and maintain reliable communications. A channel is particularly noisy and unruly between mobile radios. If the number of channels available for all the uses of a radio system is less than the number of all possible users, then such a system is called a trunk radio system. Trunking is a process whereby users share a limited number of channels in some orderly manner. Time division multiple access (TDMA) is one method for sharing channels. TDMA implies the use of digital voice compression techniques which allows multiple users to share a common channel on a schedule. Modern voice encoding greatly shortens the time it takes to transmit voice messages by removing most of the redundancy and silent periods in speech communications. Thus, multiple users can share the same channel during the intervening time slots. Accordingly, all users share the physical resource by having their own assigned, repeating time slot within a group of time slots called a frame. For this reason, a time slot assignment is often called a channel.

Within a single cell, mobile stations can be found at different distances from a base station. In a cellular communications system, mobile stations are dispersed at varying distances relative to a base station. Depending on the distance to the base station, the delayed time and the attenuation of an individual's mobile signal is likely to be different from the delay and attenuation of any of the other mobile stations. However, TDMA techniques rely heavily on the proper timing of transmission bursts. Thus, a base station performs measurements on the time and delay of each mobile station. The base station then commands these mobile stations to modify their transmission bursts. This feature is called timing advance.

To compensate for attenuation over different distances within the cell, at the same time the base station is making timing adjustments on mobiles, the base station commands the mobiles to use different power levels in such a way that the power arriving at the base station's receiver is approximately the same for each time slot. This power control is typically performed in steps of 2 dB. To perform this power measurement process, the base station gives the mobile station a list of mobile stations on which to perform power measurements. The mobile station performs continuous measurements on the quality and the power level of the serving cell, and of the power levels of the adjacent cells. The measurement results are put into a measurement report, which are sent back to the base station. The base station may also perform measurements on the quality and power of the link to the mobile station. If a base station discovers that a mobile station is not receiving its signal at a sufficient power level for reliable downlink communication, the base station may apply a power control on its own by commanding a transmitted power level in each time slot.

In TDMA, transient response and loop speed are important in the design of the control loop. However, loop stability and loop speed are two contradictory design parameters in control loop theory. For example, the wider the loop bandwidth, the faster the loop speed, but the lesser the loop stability margin. RF output-power control can be performed with a closed-loop approach. The RF power is sensed at an amplifier output using a directional coupler or capacitive divider and is detected across a fast Schottky diode. The resulting signal represents a direct measurement of the peak RF output voltage. This resulting signal is then compared to a reference voltage. The error signal is then used as an input to a controller. The controller provides a control signal to attenuators at the power-amplifier to force the measured voltage and the reference voltage to be equal. Power control is accomplished by changing the reference voltage. However, the output power envelope during ramp-up and ramp-down does not closely follow the reference signal.

It can be seen then that there is a need for a power control loop method and apparatus which provides a constant loop bandwidth at different power levels.

SUMMARY OF THE INVENTION To overcome the limitations in the prior art described above, and to overcome other limitations that will become apparent upon reading and understanding the present specification, the present invention is a power control loop method and apparatus which provides a constant loop bandwidth at different power levels.

The present invention solves the above-described problems by including a square root circuit between the generation of the control voltage and attentuators in the transmit path. A method in accordance with the principles of the present invention includes generating a power output signal using one or more series attenuators, sampling the power output signal and processing the power output signal in a power control loop to generate an attenuator control signal, the attenuator control signal providing a constant loop bandwidth that is independent of the power level of the power output signal.

Other embodiments of a system in accordance with the principles of the invention may include alternative or optional additional aspects. One such aspect of the present invention is that the processing further includes comparing the sampled power output signal to a power control reference signal, generating an error control signal in response to the comparing, producing a control signal in response to the error control signal and taking the square root of the control signal to produce the attenuator control signal. Another aspect of the present invention is that the processing may alternatively include taking the square root of the sampled power output signal to produce a square root output signal, comparing the square root output signal to a power control reference signal, generating an error control signal in response to the comparing and processing the error control signal to produce the attenuator control signal.

Another aspect of the present invention is that the output power level of the output signal, P_ou„ is characterized according to P_out = P_in*m*V_c ^x, where m is the slope of the one or more attenuators, x represents the output power response, and Pin is an input power signal level.

Another aspect of the present invention is that the one or more attenuators are configured to obtain a value for x equal to 4.

Another aspect of the present invention is that the taking the square root of the control signal results in the output power level, P_out, being characterized by P_out = P_in ^*m*V_c ².

Another aspect of the present invention is that the one or more series attenuators include ^> type pin diode attenuators coupled in series.

These and various other advantages and features of novelty which characterize the invention are pointed out with particularity in the claims annexed hereto and form a part hereof. However, for a better understanding of the invention, its advantages, and the objects obtained by its use, reference should be made to the drawings which form a further part hereof, and to accompanying descriptive matter, in which there are illustrated and described specific examples of an apparatus in accordance with the invention.

BRIEF DESCRIPTION OF THE DRAWINGS Referring now to the drawings in which like reference numbers represent corresponding parts throughout:

Fig. 1 illustrates a transmit time slot for a TDMA burst; Fig. 2 illustrates a block diagram of a base transceiver system;

Fig. 3 illustrates a power control loop such as may be used in the transceiver illustrated in Fig. 2;

Fig. 4 illustrates a t type pin diode attenuator;

Fig. 5 illustrates a power control loop according to the present invention; Fig. 6 illustrates one embodiment of a square root circuit according to the present invention; and

Fig. 7 illustrates the a graph of the output power ramping provided by the square root circuit compared with the raise cosine signal.

DETAILED DESCRIPTION OF THE INVENTION In the following description of the exemplary embodiment, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration the specific embodiment in which the invention may be practiced. It is to be understood that other embodiments may be utilized as structural changes may be made without departing from the scope of the present invention.

The present invention is a power control loop method and apparatus which provides a constant loop bandwidth at different power levels. The present invention includes a square root circuit between the generation of the control voltage and attentuators in the transmit path.

Fig. 1 illustrates a transmit time slot for a TDMA burst 100. The transmit time slot 100 includes both ramp-up and ramp-down periods 110, 112, during which periods a transmitter's power amplification is gradually increased and then gradually decreased. During the modulation period 114, the transmitter carrier signal is modulated with applied data. The ramp-up and ramp-down periods 110, 112 are critical during the control of the transmitter's power transition so as to prevent frequency spreading.

Fig. 2 illustrates a block diagram of a base transceiver system 200. In Figure 2, the base transceiver system 200 receives transmission bursts via the air interface 210, which are filtered in an input filter 212. A transceiver 218 provides a high frequency receiver 220 and a high frequency transmitter 260. The signal from the input filter 212 is processed by the high frequency receiver 220 and then processed by digital signal processors in a transmitter/receiver module 230. The transceiver/receiver module 230 consists of a low frequency part for digital signal processing and a high frequency part for modulation and demodulation. These process signals are then provided to the Abis interface 240 via transmission system 250. Signals from the Abis interface 240 are received at the transmission system 250 and then forwarded to the transmitter/receiver module 230. The signals are then processed and provided to the high frequency transmitter 260 which are filtered by the output filter 270 and sent out over the air interface 210 to mobile stations (not shown). An operations and maintenance module 280 may be provided to administer the functionality of the base transceiver system and to provide clock distribution.

Fig. 3 illustrates a power control loop 300 such as may be used in the transceiver 218 illustrated in Fig. 2. In Fig. 3, a power amplifier 310 receives the input signal 312 and under control of the loop controller 320 provides the output signal 330. A directional coupler 340 samples the output signal 330 and a detector/linearizer 350 produces a filter output voltage 372, V_f. A reference voltage 380, V_r, is generated, wherein the voltage waveform of the reference voltage 380 is selected such that the switching spectrum will meet the GSM (Group Speciale Mobile) specifications. The filter output voltage 372, V_f, is compared with the reference voltage 380, V_r, and a resulting output error voltage 382, V_e, is sent to the controller 320 to produce a control signal V_c 322 for controlling the PIN diode attenuators 314, 316. The control signal 322 is also adjusted by a blanking and offset module 360.

Eventually, the control loop 300 causes the error voltage 382 to be zero or close to zero. Accordingly, the control signal 322 is a ramp function and is synchronized with the reference voltage 380. Typically, the reference voltage 380 is a raised cosine signal. However, a raised cosine signal has been proven to be insufficient in some applications. A different reference signal, i.e., the Blackman window signal, may therefore be preferred.

If the original bandwidth of the control loop 300 varies from 70 kHz to 2.2 MHz and it is desired to reduce the loop bandwidth down to a range of 350 kHz to 1.1 MHz, the time constant of the controller 320 is adjusted so that the loop bandwidth varies from 35 kHz to 1.1 MHz in the original power control loop. The gain of the amplifier 310 is set to ramp from 1 to 10. The bandwidth of the control loop 300 is proportional to the gain of the amplifier 310 thus the loop bandwidth is then varying from 35 ^* 10=350 kHz to 1*1.1 MHz.

A function of the power control loop 300 is to control the transient response, ramping-up or ramping-down, within 14 microseconds regardless of the variation of gain, temperature, and many other variables. Of course this control loop 300 has to be stable under any condition, e.g. temperature, power level, etc. Since the loop bandwidth varies as a function of power level due to the non-linear characteristics of the different loop components, the output power envelope during ramping-up and ramping-down cannot closely follow the reference signal if the a low power loop bandwidth is used. A low power loop bandwidth is too slow to follow the reference signal at each end of the ramping.

Typically, commercially available attenuators have a characteristic P_out (dB) = P_min (dB) + mS3V_c, where m is the slope (dB/volt). If a log amplifier is inserted between the control voltage V_c 322 and the linear attenuators 314, 316, a constant loop bandwidth may be obtained, but since the slope is not constant the power level will vary. The slope may vary by a substantial margin, e.g., 10:1. Further, these type of attenuators are relatively expensive.

Alternatively, a k type pin diode attenuator may be used. Fig. 4 illustrates a I type pin diode attenuator 400. This type of attenuator 400 is a low cost solution for controlling the output power level. However, to obtain sufficient attenuation, two I type pin diode attenuators are needed in series in the transmitter path (as shown in Fig. 3). The attenuation behavior may be expressed as:

P_out = P_in V_c ^x Equation 1

The power "x" can vary from 3.5 to 5. However, with careful design, "x" may be made to equal 4. Nevertheless, to achieve a loop bandwidth independent of power, "x" needs to be 2, which is impossible to attain. As a result, the loop bandwidth will vary from 20 to 50. For example, if the maximum allowable loop bandwidth is 2 MHz and the variation is 30 to 1 , then the minimum loop bandwidth is 65 kHz. However, this is too slow, for example, for GSM applications. The loop bandwidth and stability are dependent upon the AC small

signal gain (& ^>). Specifically, β is the ratio between an incremental change in the detected voltage (dV_f) due to an incremental change in the control voltage

(dV_c) to the PIN diode attenuator. Accordingly,

_a dV_f dV_f dPou, β= - — = - — • Equation 2 dVc dPou, dVc

For a linear detector, the relationship of power to voltage is:

dV_f 1

; and dPout 2 -SF - JPo

dPout - ' £__! _

— = x ^■ (P_in ■ m -P_{oul *} Equation 3

If P_out is to be canceled, that is if the gain, 3, is to be independent of P_out, then"x" needs to be 2. As indicated above, the attenuators may be designed such that two in series will result in "x" being equal to 4. Thus, if a square root circuit is inserted between the control voltage V_c and the two series attenuators in the transmitter path, "x" may be made to equal 2 so that the loop bandwidth is independent of the power level. Fig. 5 illustrates a power control loop 500 according to the present invention. In Fig. 5, a power amplifier 510 receives the input signal 512 and under control of the loop controller 520 provides the output signal 530. A directional coupler 540 samples the output signal 530 and a detector/lineahzer 550 produces a filter output voltage 572, V_f. A reference voltage 580, V_r, is generated, wherein the voltage waveform of the reference voltage 580 is selected such that the switching spectrum will meet the GSM specifications. The filter output voltage 572, V_f, is compared at comparator 576 with the reference voltage 580, V_r, and an output error voltage 582, V_e, is sent to the controller 520 to produce a control signal V_c 522 To provide a constant loop bandwidth that is independent of the power level, a processor circuit 526 such as a square root circuit is coupled between the output of the controller 520 and the two series attenuators 514, 516. The processing circuit 526 processes the control signal 522 to generate an attenuator control signal 528. The attenuator control signal 528 provides a constant loop bandwidth that is independent of the power level of the power control loop output power signal, Pout 530. For example, the square root circuit 526 receives the output from the controller V_c 522 and the output V_s 528 of the square root circuit 526 controls the PIN diode attenuators 514, 516, where V_s = v Those skilled in the art will recognize that the square root circuit 526 may alternatively be positioned between the detector/lineahzer 550 and the comparator 576 with appropriate selection of the reference voltage 580, V_r, such that the switching spectrum will meet the GSM specifications.

Fig. 6 illustrates one embodiment of a square root circuit 600 according to the present invention. V_c 610 is the input to the square root circuit 600. The output V_s 620 of the square root circuit 600, where V_{S =} V_C ^,/2, controls the two series attenuators illustrated in Fig. 5 so that: P_out = P_in m V_c ² Equation 4

Fig. 7 illustrates a graph 700 of the output power ramping 710 provided by the square root circuit compared with the raised cosine signal 712. Thus, Fig. 7 illustrates that the loop bandwidth is constant independent of the output power so the output power signal 710 closely follows the raised cosine signal 712. Thus, the inclusion of a square root circuit between the V_c signal and the two series attenuators provides a constant bandwidth independent of the output power level.

The foregoing description of the exemplary embodiment of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be limited not with this detailed description, but rather by the claims appended hereto.

Claims

WHAT IS CLAIMED IS:

1. A method for providing a constant loop bandwidth to a power control loop wherein the loop bandwidth is independent of output power levels, comprising: generating a power output signal using one or more series attenuators; sampling the power output signal; and processing the power output signal in a power control loop to generate an attenuator control signal, the attenuator control signal providing a constant loop bandwidth that is independent of the power level of the power output

signal.

2. The method of claim 1 wherein the processing further comprises: comparing the sampled power output signal to a power control reference signal; generating an error control signal in response to the comparing; producing a control signal in response to the error control signal; and taking the square root of the control signal to produce the attenuator control signal.

3. The method of claim 2 wherein the output power level of the

output signal, P_out, is characterized according to:

P_out = P_in m V_c ^x,

where m is the slope of the one or more attenuators, x represents the output power response, and P_ιn is an input power signal level.

4. The method of claim 3 wherein the one or more attenuators are configured to obtain a value for x equal to 4.

5. The method of claim 4 wherein the taking the square root of the control signal results in the output power level, P_out, being characterized by:

6. The method of claim 1 wherein the processing further comprises: taking the square root of the sampled power output signal to produce a square root output signal; comparing the square root output signal to a power control reference signal; generating an error control signal in response to the comparing; and processing the error control signal to produce the attenuator control signal.

7. The method of claim 6 wherein the output power level of the output signal, P_ou„ is characterized according to:

8. The method of claim 7 wherein the one or more attenuators are configured to obtain a value for x equal to 4.

9. The method of claim 8 wherein the taking the square root of the sampled power output signal results in the output power level, P_out, being characterized by:

10. The method of claim 1 wherein the one or more series attenuators comprise k type pin diode attenuators coupled in series.

11. A power control loop system having a constant loop bandwidth that is independent of output power levels, comprising: at least one amplifier for receiving an input signal and amplifying the input signal to generate a power output signal; one or more attenuators coupled in series with the at least one amplifier for controlling the power level of the power output signal; and a power control loop for sampling the power output signal and processing the power output signal to generate an attenuator control signal, the attenuator control signal providing a constant loop bandwidth that is independent of the power level of the power output signal.

12. The power control loop system of claim 11 wherein the power control loop further comprises: a detector/lineahzer circuit for generating a filter voltage signal; a comparator, coupled to the detector/lineahzer circuit, for receiving the filter voltage signal and comparing the filter voltage signal to a reference signal to generate an error signal; a controller, operatively coupled to the comparator, for generating a control signal to control the one or more series attenuators to ramp-up and ramp-down the power output signal; and a square root circuit, coupled to the controller, the square root circuit producing a square root control signal having a value equal to the square root of the control signal, wherein application of the square root control signal to the one or more attenuators provides the power control loop with a constant loop bandwidth that is independent of the power level of the power output signal.

13. The power control loop system of claim 12 wherein the output power level, P_out, of the power output signal is characterized according to:

where m is the slope of the one or more attenuators, x represents the output power response and P_in is the input power signal level.

14. The power control loop system of claim 13 wherein the one or more attenuators are configured to obtain a value for x equal to 4.

15. The power control loop system of claim 14 wherein the square root device takes the square root of the control signal to obtain the output power signal, the output power signal being characterized according to:

Pou. = P_in m V_c ².

16. The power control loop system of claim 11 wherein the power control loop further comprises: a detector/lineahzer circuit for generating a filter voltage signal; a square root circuit, coupled to the detector/lineahzer, the square root circuit producing a square root signal having a value equal to the square root of the filter voltage signal; a comparator, coupled to the square root circuit, for receiving the square root signal and comparing the square root signal to a reference signal to generate an error signal; and a controller, operatively coupled to the comparator, for generating the attenuator control signal for controlling the one or more series attenuators to ramp-up and ramp-down the power output signal..

17. The power control loop system of claim 16 wherein the output power level, P_out, of the power output signal is characterized according to:

where m is the slope of the one or more attenuators, x represents the output power response and P_ιn is the input power signal level.

18. The power control loop system of claim 17 wherein the one or more attenuators are configured to obtain a value for x equal to 4.

19. The power control loop system of claim 18 wherein the square root device takes the square root of the control signal to obtain the output power signal, the output power signal being characterized according to:

20. The power control loop system of claim 11 wherein the one or more series attenuators comprise Ik type pin diode attenuators.

21. A base station, comprising: input and output filters for filtering signals received and transmitted via an air interface from and to a mobile station; a transceiver, coupled to the input and output filters, for receiving filtered signals from the input filter and for transmitting signals to the output filter; a processor, coupled to the transceiver, for providing digital signal processing to signals provided to and received from the transmitter; a transmission system, coupled to the processor, for providing an interface between the processor and an Abis interface; and an operation and maintenance module, coupled to at least the transceiver and the processor, to administer the functionality of at least the transceiver and the processor and to provide clock distribution to at least the transceiver and the processor;

wherein the transceiver further comprises a power control loop system

having a constant loop bandwidth that is independent of output power levels,

wherein the power control loop system comprises:

at least one amplifier for receiving an input signal and amplifying

the input signal to generate a power output signal;

one or more attenuators coupled in series with the at least one

amplifier for controlling the power level of the power output signal; and

a power control loop for sampling the power output signal and

processing the power output signal to generate an attenuator control signal, the

attenuator control signal providing a constant loop bandwidth that is

independent of the power level of the power output signal.

22. The base station of claim 21 wherein the power control loop

further comprises:

a detector/linearizer circuit for generating a filter voltage signal;

a comparator, coupled to the detector/linearizer circuit, for receiving the

filter voltage signal and comparing the filter voltage signal to a reference signal

to generate an error signal;

a controller, operatively coupled to the comparator, for generating a

control signal to control the one or more series attenuators to ramp-up and

ramp-down the power output signal; and a square root circuit, coupled to the controller, the square root circuit producing a square root control signal having a value equal to the square root of the control signal, wherein application of the square root control signal to the one or more attenuators provides the power control loop with a constant loop bandwidth that is independent of the power level of the power output signal.

23. The base station of claim 22 wherein the output power level, P_ou„ of the power output signal is characterized according to:

24. The base station of claim 23 wherein the one or more attenuators are configured to obtain a value for x equal to 4.

25. The base station of claim 24 wherein the square root device takes the square root of the control signal to obtain the output power signal, the output power signal being characterized according to:

26. The base station of claim 21 wherein the power control loop further comprises: a detector/linearizer circuit for generating a filter voltage signal; a square root circuit, coupled to the detector/linearizer, the square root circuit producing a square root signal having a value equal to the square root of the filter voltage signal; a comparator, coupled to the square root circuit, for receiving the square root signal and comparing the square root signal to a reference signal to generate an error signal; and a controller, operatively coupled to the comparator, for generating the attenuator control signal for controlling the one or more series attenuators to ramp-up and ramp-down the power output signal..

27. The base station of claim 26 wherein the output power level, P_ou„ of the power output signal is characterized according to:

28. The base station of claim 27 wherein the one or more attenuators are configured to obtain a value for x equal to 4.

29. The base station of claim 28 wherein the square root device takes the square root of the control signal to obtain the output power signal, the output power signal being characterized according to:

30. The base station of claim 21 wherein the one or more series attenuators comprise Ik type pin diode attenuators.