AU668243B2

AU668243B2 - Automatic gain control method and device for a digital receiver, especially a time division multiple access receiver

Info

Publication number: AU668243B2
Application number: AU52627/93A
Authority: AU
Inventors: Frederic Gourgue
Original assignee: Alcatel Radiotelephone SA
Current assignee: Alcatel CIT SA
Priority date: 1992-12-23
Filing date: 1993-12-22
Publication date: 1996-04-26
Anticipated expiration: 2013-12-22
Also published as: JPH06276171A; FI935755A0; DE69315417D1; ES2109455T3; FR2699768A1; NO304959B1; NO934737L; FR2699768B1; AU5262793A; EP0604295B1; DE69315417T2; NO934737D0; EP0604295A1; FI935755A; ATE160657T1

Abstract

Method of automatic gain control of a digital receiver (1) comprising amplifier means (2). The method comprises the following steps: - sampling at a predetermined sampling frequency of the output signal (s(t)) from the said amplifier means; - processing of the samples obtained including, for each sample, a comparison of the absolute value of this sample respectively to a predetermined maximum and minimum threshold; and - adjustment of the gain (G) of the amplifier means (2) carried out on completion of each sample processing, in such a way that: * if a sample is outside a maximum bracket [-SMAX, +SMAX], this gain (G) is reduced by a predetermined reduction level, * if several successive samples, of a predetermined number, lie within a minimum bracket [-SMIN, +SMIN], this gain (G) is increased by a predetermined increase level. Use especially in mobile telephony. <IMAGE>

Description

AUSTRALIA

Patents Act COMPLETE SPECIFICATION

(ORIGINAL)

Class Int. Class Application Number: Lodged: Complete Specification Lodged: Accepted: Published: Priority Related Art: r

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Name of Applicant: Societe Anonyme dite Alcatel Radiotelephone Actual Inventor(s): Frederic Gourgue Address for Service: PHILLIPS ORMONDE FITZPATRICK Patent and Trade Mark Attorneys 367 Collins Street Melbourne 3000 AUSTRALIA Invention Title: AUTOMATIC GAIN CONTROL METHOD AND DEVICE FOR A DIGITAL RECEIVER, ESPEYIALLY A TIME DIVISION MULTIPLE ACCESS RECEIVER Our Ref 352369 POF Code: 1501/213663 The following statement is a full description of this invention, including the best method of performing it known to applicant(s): AUTOMATIC GAIN CONTROL METHOD AND DEVICE FOR A DIGITAL RECEIVER, ESPECIALLY A TIME DIVISION MULTIPLE ACCESS

RECEIVER

The present invention concerns an automatic gain control method and device for a digital receiver, especially a time division multiple access (TDMA) receiver.

The received digital frames are made up of data packets sent turn and turn about by a set of transmitters at a greater or lesser distance. Two packets received in succession can differ greatly in level by 90 dB, for example. The gain of the receiver amplifier must be set to produce a signal level within the dynamic range of the processing system connected to the amplifier output.

A data packet comprises, for example: an increasing power ramp, possibly with slight *ringing, of 28 ps duration; a period during which the power is constant, of 542.8 ps duration, during which the information is 20 transmitted (this period is referred to as the "active period" hereinafter); and a reducing power ramp of 28 ps duration.

The reducing power ramp of a data packet may partially overlap the increasing power ramp of the next packet. Nevertheless, the gain of the receive system must be set as each data packet arrives in order for the signal to be within the dynamic range of the processing system connected to the receiver output during the active period. This gain must remain constant during reception in the active period in order to optimize baseband processing.

Until now, a first way to solve this problem has been to have a plurality of receive amplifiers with different gains working in parallel. The output of each amplifier is sampled and stored and a selector unit 2 selects the best stored result. Another way has been to use a delay line followed by an amplifier controlled by a device sampling the received signal on the input side of the delay li to assess the power of this signal and to adjust the gain of the amplifier in order to amplify the signal appropriately at the output of the delay line.

The selection technique using a plurality of amplifiers is a costly solution to the problem. Using a delay line followed by n amplifier generates noise and a time-delay.

An object of the present invention is to ameliorate these drawbacks.

According to the invention, the automatic gain control method for a digital receiver, especially a time division multiple access digital receiver, including variable gain amplifier means receiving an input signal, is characterized in that it includes the following steps: sampling the output signal of said amplifier means at a predetermined sampling frequency, processing each sample obtained by comparing its absolute value with a predetermined maximum threshold and a predetermined minimum threshold respectively associated 20 with a maximum range and a minimum range, and adjusting the gain of the amplifier means after processing each sample so that: if a sample is outside the maximum range the gain is reduced by a predetermined amount; if a predetermined number of successive samples lie in the minimum range the gain is increased by a predetermined amount.

In a first embodiment of the method according to the invention each sample is compared with four 9* 9 9 e*.

JMN CJWINWORDIJENNYINODELETE/52627C93 C- I thresholds: a maximum threshold and a minimum threshold and their respective opposite values.

In a second embodiment of the method according to the invention the absolute value of each sample is determined and then compared with the maximum threshold and the minimum threshold.

According to another aspect of the invention the automatic gain control device for a digital receiver, especially a time division multiple access digital receiver, said receiver variable gain amplifier means receiving an input signal, implementing the method according to the invention, characterized in that it-Gple s means for sampling the output signal of said amplifier means at a predetermined sampling frequency, means for comparing the absolute value of each sample obtained with a maximum threshold and a minimum i threshold and for generating threshold detection information, and means for adjusting the gain of the amplifier means, said adjustment means cooperating with 20 the comparator means to reduce said gain by a predetermined amount if a sample is detected outside a maximum range and to increase said gain by a too. predetermined amount if a predetermined number of successive samples lie in a minimum range.

The present invention therefore has the advantages of enabling integration of the automatic gain control to. device into an integrated circuit and of using a very simple detector in the feedback loop. Also, the stability of the system improved by the control logic implemented with the method according to the invention.

Moreover, in the GSM a correction of 90 dB 9- obtained during a bit period and the method according to the invention operates in the voltage domain rather than the power domain, which helps to improve the speed of the automatic gain control.

4 Other features and advantages of the invention emerge in the following description given by way of nonlimiting example with reference to the appended drawings: figure 1 is a block diagram showing an automatic gain control device according to the invention; figure 2A is a timing diagram for the situation in which the initial level of the signal is above the maximum threshold; figure 2B is a timing diagram for the situation in which the initial level of the signal is below a minimum threshold; and figure 3 is a table showing the respective probabilities of a sample overshoot and at least one overshoot in two successive samples required for 15 adjustment of the algorithm and the sampling frequency using the method according to the invention in the case Sof a sinusoidal input signal of amplitude A.

One embodiment of the method in accordance with the invention and the associated device are described iiext.

In an automatic gain control device 1 according to the invention an intermediate frequency (IF) amplifier 2 receives an analog signal e(t) in the form of a succession of data packets. Its output is connected to a •Dsignal amplitude detector 4 which uses two thresholds (minimum and maximum) with a sampling unit 5 connected to i its input. An analog/digital converter 6 is connected to the output of the amplifier 2 and supplies a set of digitized data N which is processed on its output side.

The samples are fed from the output of the detector 4 to a control logic device 3 which controls the gain of the amplifier 2 in such a way as to prevent any risk of oscillation.

A control algorithm embodying the method according to the invention and implemented in the control logic device 3 may comprise the following steps, for example:

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if the absolute value of a sample exceeds a maximum threshold SMAX, i.e. if the sample is outside the range [-SMAX, +SMAX], the gain is reduced by 3 dB; if two successive samples lie within the minimum range [-SMIN, +SMIN] the gain is increased by 3 dB.

The signal e(t) at the input of the amplifier 2 can he modelled in the following form: e(t)=A(t).sin[w 0 where: A(t) conforms to predetermined power masks, in particular the masks specified in a technical recommendation such as "GSM 05.05"' WO is the angular frequency equivalent to the intermediate frequency IF; and 9(t) is the phase modulated by the flow of data.

15 To give a numerical example of the invention, the variations in A(t) are slow (less than 0.4 dB between two samples, for example) and the intermediate frequency is 9.75 MHz, for example.

The sampling unit 5 operates at a frequency Fe of 13 MHz, which is 48 times the symbol frequency in the GSM case. The output samples are of the following type: u(kTe)=sin[w 0 kTe+9(kTe)].

As the phase 9(t) varies by at least n/2 between two symbols in the GSM case, it varies less than n/96 between two samples. The phase variation due to the carrier frequency is much larger, however, as it is equal to w 0 /Fe or The phase variation between two successive samples is (taking the samples k and for example): w 0 Te 9((k+l)Te)-9((k-l)Te) Note that this comprises two terms: variation due to the intermediate frequency: w 0 Te, or w 0 /Fe; variation due to the modification by the flow of data: The sampling frequency Fe and the intermediate frequency (angular frequency wo) are chosen so that the ratio w 0 /Fe is an odd multiple of n/2.

The sampling frequency Fe is also chosen to be much higher than the symbol frequency of the input signal received so that the variation of phase due to modulation by the flow of data between two successive samples is negligible in comparison with n/2.

It is also desirable for the sampling frequency Fe to be chosen sufficiently high for the amplitude A(t) of the signal as defined hereinabove to vary slowly between two samples.

The threshold detector 4 assesses the absolute value of the amplitude of each sample relative to two 15 thresholds: a maximum threshold which is 6 dB (for example) S"below the saturation level of the baseband part of the receiver detects signals which are too strong and thus gains of the amplifier 2 which are too high; a mrainimum threshold SMIN which is 10 dB (for 9. example) below the maximum threshold determines if the signal is of insufficient amplitude and thus if the gain of the amplifier 2 is too low.

The control logic device 3 interprets the information from the detector 4 in order to adjust the gain of the amplifier 2 whilst ensuring that the loop is stable, using the following algorithm: if the absolute value of a sample exceeds the maximum threshold the gain is reduced by 3 dB; if the absolute value of two successive samples is below the minimum threshold the gain is increased by 3 dB; otherwise the gain is not changed.

An example of simulating the aforementioned algorithm is now described in the case of a sinusoidal input signal: e(t)=A.sin(w 0 taking into account the various possible detection situations.

Referring to figure 2A, if the initial level of the signal above the maximum threshold SMAX the probability of a threshold overshoot depending on the ratio between the peak voltage A and the maximum threshold is as given by the figure 3 table. The probability values given in this table are optimal values of the ratio between the carrier frequency and the sampling frequency which enables the phase to change by n/2 (modulo u) between two samples.

Consequently, for a value of I sin[(w 0 kT)] close to 0 there is a corresponding value of I sin[(O 0 I close to 1, which explains the very S 15 high probabilities in the last column of the table. Note that the effect of a high value of the sampling frequency is to render negligible the phase difference between two samples as compared with the phase difference n/2 due to the carrier. Moreover, the variations in the amplitude 20 A(t) between two samples are also negligible.

In the example shown in figure 2A, corresponding to a carrier frequency of 9.75 MHz and a sampling frequency of 13 MHz, it is assumed that at the initial time t=0 the gain G of the amplifier 2 is such that the amplitude of the output signal is greater than SMAX and that the first sample to be sampled EO is virtually null (this is by way

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of non-limiting example). At the time t=Te the sample El lies between the thresholds SMIN and SMAX and the gain of the amplifier 2 is unchanged (gain variation AG=0). At the time t=2Te the modulus of the sample E2 is greater than SMAX, which causes the gain of the amplifier 2 to be reduced by 3 dB and a consequential reduction in the amplitude of the output signal. The sample E3 lies between the minimum threshold SMIN and the maximum threshold SMAX. The gain is therefore unchanged. The -1 I I 8 subsequent samples E4, E5, E6, E7 also have absolute values in the range SMIN-SMAX. A sample may lie in the range [-SMIN, +SMIN] but this can never occur for two successive samples. The gain G then remains constant for all of the duration of the packet.

Referring to figure 2B, if the initial level of the signal is below the minimum threshold SMIN the first sample FO will of course be in the range [-SMIN, +SMIN] but the automatic gain control device according to the invention will not react for reasons of stability. If the second sample F1 is also in the range [-SMIN, +SMIN] the gain G is increase by 3 dB for each new sample Fl below the minimum threshold SMIN. When the absolute value of a sample crosses the minimum threshold SMIN, as is the case with samples F2 and F3, this increase in the gain G is halted. There are then two possible situations: if the peak value of the signal received is just above the minimum threshold SMIN then there is a very 20 high probability that the absolute values of two succes sive samples will lie below the threshold, as is the case with the successive samples F4 and F5, and the gain G of the amplifier 2 is then increased by a further 3 dB; if the signal is already well above the minimum threshold SMIN, by at least 3 dB, for example, the gain of the amplifier 2 is held constant.

In the figure 2B situation a third adjustment of the gain of the amplifier 2 is still required to stabilize the amplitude of the output signal as the absolute values of two successive samples F8, F9 are below the minimum threshold SMIN.

If the initial level of the signal is between the minimum and maximum thresholds SMIN and SMAX, there is zero probability of a threshold overshoot and the probability of falling below the minimum threshold is as l~ IIIIAIP--s _II given by the figure 3 table. If the peak value of the signal is at least 3 dB above the minimum threshold SMIN the gain will not be varied because there is a zero probability that the absolute values of two successive samples are under the threshold. If the peak value is at least 3 dB above the minimum threshold SMIN it is possible for the absolute values of two successive samples to be under the threshold, which by the method according to the invention leads to an increase in the gain by 3 dB, taking the system into a stable area.

The simulation results described hereinabove show that the automatic gain control method according to the invention can compensate a variation in level without or introducing any significant time-delay. It can correct a 15 power jump of 90 dB within the time to receive one bit.

Of course, the invention is not limited to the examples that have just been described, which can be modified in many ways without departing from the scope of the invention.

S 20 For example, the values of the minimum and maximum *thresholds have been given purely by way of numerical example and in practise are chosen to suit constraints specific to the receiver concerned. Also, the number of successive samples below the minimum threshold before the gain of the amplifier is increased may be greater than 2.

Additionally, the number of detection and comparison thresholds can be increased by the value of the adjustment increments of the amplifier, in order to obtain various modes, such as a "catch-up" mode and a "tracking" mode. Other ratios between the carrier frequency and the sampling frequency or between the bit frequency and the sampling frequency can also be used.

The automatic gain control method according to the invention may be used with advantage in a receiver for the GSM and TETRA systems.

Claims

1. Automatic gain control method for a digital receiver, especially a time division multiple access digital receiver, including variable gain amplifier means receiving an input signal, characterized in that it includes the following steps: sampling the output signal of said amplifier means at a predetermined sampling frequency, processing each sample obtained by comparing its absolute value with a predetermined maximum threshold and a predetermined minimum threshold respectively associated with a maximum range and a minimum range, and adj sting the gain of the amplifier means after processing each sanmple so that: if a sample is outside the maximum range the gain is reduced by a predetermined amount; if a predetermined number of successive samples lie in the minimum range the gain is increased by a predetermined amount.

2. Method according to claim 1 characterized in that each sample is compared with four thresholds: said maximum threshold and said minimum threshold and their respective opposite values. 0

3. Method according to claim 1 characterized in that the absolute value of each sample is determined and then compared with the maximum threshold and .00.0 0o. the minimum threshold.

4. Method according to any one of claims 1 to 3 characterized in that the predetermined number of successive samples in the minimum range for which the Sco:. gain is increased is equal to 2. 25

5. Automatic gain control device for a digital receiver, especially a time division multiple access digital receiver, said receiver including variable gain :amplifier means receiving an input signal, implementing the method according to 0 0: any one of the preceding claims, characteriz.. in that it includes means for sampling the output signal of said amplifier means at a predetermined sampling frequency, means for comparing the absolute value of each sample obtaineJ with a maximum threshold and a minimum threshold and for generating threshold RA4/ detection information, and means for adjusting the gain of the amplifier means, IF said adjustment means cooperating with the comparator means to reduce said S JMN C:jWINWORDIJENNYNODELETE52627C93 -11 gain by a predetermined amount if a sample is detected outside a maximum range and to increase said gain by a predetermined amount if a predetermined number of successive samples lie in a minimum range.

6. Device according to claim 5 characterized in that the predetermined amounts by which the gain is reduced and increased are substantially equal to 3 dB.

7. Device according to claim 5 characterized in that the minimum threshold is substantially 10 dB below the maximum threshold.

8. Device according to any one of claims 5 to 7 characterized in that the maximum threshold is substantially 6 dB below a saturation level of a baseband part of the receiver.

9. Device according to any one of claims 5 to 8 characterized in that the input signal is supported by a carrier at an intermediate angular frequency and conveys a flow of data at a symbol frequency, the ratio of the intermediate angular frequency to the sampling frequency being an odd multiple of n/2 so that the variation in phase due to the carrier between two samples is an odd multiple of 7/2, said sampling frequency being further chosen to be very much higher than the symbol frequency of the input signal received so that between two sample. the contribution of the modulation of the phase by the flow of data is negligible in 2C comparison with 7c/2.

10. Automatic gain control method for a digital receiver substantially as heir-,~ described with reference to the accompanying drawings.

11. Automatic gain control device for a digital receiver substantially as herein described with reference to the accompanying drawings. DATED 16 January, 1996 PHILLIPS ORMONDE FITZPATRICK •Attorneys For: SOCIETE ANONYME DITE: ALCATEL RADIOTELEPHONE b C.\WINVORIDANDREA\ELE'2627CLDOC I ABSTRACT Automatic gain control method for a digital receiver comprising amplifier means The method comprises the following stages: sampling the output signal of said amplifier means at a predetermined sampling frequency, processing each sample obtained by comparing its absolute value with a predetermined maximum threshold (SMAX) and a predetermined minimum threshold (SMIN) respectively associated with a maximum range [-SMAX, +SMAX] and a minimum range [-SMIN, +SMIN], and adjusting the gain of the amplifier means (2) after processing each sample so that: if a sample (E2) is outside the maximum range S: -SMAX, +SMAX] the gain is reduced by a predetermined amount; if a predetermined number of successive samples (FO, F1;F4, F5;F8, F9) lie in the minimum range [-SMIN, +SMIN] the gain is increased by a predetermined amount. Application to mobile telephones, for example. F* Figure to publish: Figure 1. 4* *4 4 44