AU661563B2

AU661563B2 - Gain control device in signal receiver

Info

Publication number: AU661563B2
Application number: AU35486/93A
Authority: AU
Inventors: Pierre Lopez; Thierry Podolak
Original assignee: Alcatel NV
Current assignee: Alcatel Lucent NV
Priority date: 1992-03-27
Filing date: 1993-03-25
Publication date: 1995-07-27
Anticipated expiration: 2013-03-25
Also published as: FR2689348B1; EP0562963B1; ES2092241T3; DE69304760T2; CA2092790A1; JPH0661884A; FI105302B; EP0562963A1; CA2092790C; AU3548693A; FR2689348A1; DE69304760D1; FI931315A; FI931315A0

Description

66156 3 1 P/00/011 28/5/91 Regulation 3.2

AUSTRALIA

Patents Act 1990 o t *6 4

ORIGINAL

COMPLETE SPECIFICATION STANDARD PATENT Invention Title: GAIN CONTROL DEVICE IN SIGNAL RECEIVER The following statement is a full description of this invention, including the best method of performing it known to us:- S. *S *r 4 4 This invention relates to information signal receivers, and in particular, to high dynamic range information signal receivers, whereby the signal received requires amplitude adjustment before it can be processed.

The invention is applicable in particular to receivers in TDMA systems (Time Division Multiple Access). A TDMA system is for instance applied to the digital cellular communications system referred to as GSM. In such a system, mobile stations are used for communications via radio links with fixed stations, called base stations, which are connected to the standard telecommunications network. Communication between a fixed station and mobile stations located within a coverage area occurs over a 2-way frequency channel shared between several mobile stations by means of time division, one time slot of a repetitive frame being allocated to each mobile station. The set of time slots in these repetitive frames, which are thus allocated to a mobile station, is often called o time channel. When transmitting from mobile to fixed station, each mobile 15 station transmits an information packet during each time slot it is allocated, at a level specific to that station. Upon reception in the fixed station, the packets coming from the various mobile stations, which follow one another in the time slots of each frame, do not all have the same amplitude. Due to different propagation conditions (different distances, obstacles, etc.) their amplitude falls Ge a.

20 within what is conventionally called a high dynamic range, which may reach up 8GeS to 90 dB for instance.

It is necessary that such differences in level be compensated as soon as the signal reaches the fixed station receiver input, in order to prevent saturation of the equipment which, further down the line in the fixed station, processes the 25 signal received. A reasonable objective may be to bring down the dynamic from dB to 20 dB approximately.

This compensation effects a constant amplitude correction during each time slot of the time channel in cases such as the GSM system where the amplitude of the signal in a packet carries information used for reception. Though, in other cases, amplitude correction may vary during a time slot.

A first known device which may be used for this purpose includes a variable gain amplifier whereby the gain is controlled by a level feedback loop: the amplifier output level is compared with a reference level and any difference generates a gain control signal applied to the variable gain amplifier and tending to maintain the output level of the latter close to the reference level.

Such a device is not suitable in cases such as TDMA type systems where significant level variations between the end of a time slot and the start of the next one would require a very large pass band, hence presenting unstable conditions which would require expensive precautions.

Another known device also uses a variable gain amplifier, but the gain control signal, instead of depending on the difference between the level of the signal received and a reference level, is rather of the predictive type: it depends on the time difference between the moment a packet is received within a time slot and the moment when typically this packet should be received. In a few words, the later the signal, the further it is, hence the weakest its level is assumed to be, and the gain is adjusted accordingly. It is obvious that such a principle is only suitable if the mobile stations do not set their synchronisation so as to eliminate the effects of distance and if the transmission only occurs in open space.

Therefore, it cannot be applied to the GSM system (except for certain packets called 'access packets' which are not entirely synctironised).

An object of the present invention is to provide a gain control device in an information signal receiver, without the above mentioned drawbacks or 99** 20 limitations.

9 According to the present invention there is provided a gain control device on an information signal receiver, comprising a signal input receiving an input .4 information signal; amplification means with adjustable amplification factor, coupled with the said input to receive the said input information signal and to 25 alter the level in relation with a gain control signal received at one gain control input, in order to obtain an output information signal; a signal output coupled with the said amplification means and supplying an output information signal, wherein said device further includes evaluation means coupled with the said input, in order to receive the said input information signal and obtain an amplitude signal being dependent on the level of the latter signal, and processing means coupled with the said evaluation means, receiving the said amplitude signal and evolving a gain control signal according to a set gain control law, the gain control signal is transmitted to the said gain control input of the said amplification means, these devices being arranged such that the level of the said output information signal has a reduced dynamic range compared with that of the said input information signal.

In the case where the said input information signal is coming from remote stations, each transmitting within its own time slot in a repetitive frame, in accordance with the known TDMA system (Time Division Multiple Access), the said processing means, responding to a time slot synchronisation signal, has the advantage of operating on a time slot basis, maintaining the same gain control almost throughout the time slot, in order to preserve the amplitude information which may be contained in a packet received during this time slot.

The device of the invention has the further advantage of including a delay means installed between the said signal input and the said amplification means, the said delay means supplying the said amplification means an input information signal delayed by a time lag such that the said gain control signal is 15 available for gain control within the said amplification means at the time when the latter receives the said delayed input information signal.

:.,°Furthermore, the gain control device of the invention includes a filter arranged such that the said input information signal is filtered before being supplied to the said evaluation means, by either a band pass or low pass filter.

This filter is located either between the said signal input and the said amplification means, or between the coupling point of the said signal input with the said amplification means, and the said evaluation means.

According to one method of implementation of the said amplification means, the latter include at least one variable attenuator whose attenuation is 25 controlled by the said gain control signal.

In order that the invention may be readily carried into effect, embodiments thereof will now be described in relation to the drawings, in which: Figure 1 represents the location of mobile stations in relation with a fixed station, in a TDMA type system.

Figure 2 represents the TDMA frame received by the fixed station regarding Figure 1.

Figure 3 represents a method of implementation of the gain control device for information signal receiver according to the present invention, this method of implementation includes two alternatives.

Figure 1 represents a transmission system including a fixed station and two mobile stations (11) and (12) located at different distances from the base station. All of them may for instance belong to a cellular network such as the GSM network.

In the following text, we shall deal only with transmissions from mobile stations to fixed station, Transmission from mobile stations to fixed station occurs in packets within a repetitive time slot frame, such as illustrated in Figure 2 and showing a frame made up of consecutive time slots (IT1, IT2, etc.), and the start of frame (T2) where time slots IT1 and IT2 are also found. During the first time slot (IT1) of frame mobile station (11) transmits packet (P1 while during time slot (IT2), mobile station (12) transmits packet (P21).

During the first time slot (IT1) of frame mobile station (11) transmits a new packet (P12) which is followed by a packet (P22) transmitted by mobile station and so on.

Each transmitted packet is in fact a signal at a frequency allocated to the communication between the mobile stations studied and the fixed station, modulated by the information to be transmitted, it being digital information in the case of a GSM type system.

Figure 2 illustrates the signals as received by the fixed station. Some arrangements, which will not be dealt with herein since they are outside the Vpresent invention, may be used such that the time position of the successive packets be appropriately defined upon their arrival in the fixed station.

The important point, with regard to the present invention, is that the 25 packets received from the various mobile stations have different amplitudes.

Indeed, mobile station (11) is relatively close to fixed station The amplitude of the signal received by fixed station (10) within time slot (IT1) is therefore relatively strong. In comparison, mobile station (12) is further away and the amplitude of the signal received by fixed station (10) within time slot (IT2) is relatively weak.

Of course distance is not the only factor affecting the amplitude of the signals received by fixed station we can also mention the presence of obstacles impairing propagation, fading by multiple paths, etc.

Reception of signals thus having a high dynamic range poses the problem of reducing this dynamic range, before the signals are used, for instance to recover the analog or digital information they contain. Indeed, these signals will have to travel through circuits containing active components, transistors in particular, whose amplitude response is not linear. Thus, if the signals are too weak they might be unusable, conversely if the signals are too strong they may be distorted by saturation, which may lead to loss of information.

It is therefore necessary to perform a sufficient amplification of the weak signals, and a lesser amplification of the strong signals, i.e. reduction of the dynamic range of the receiver input signals.

The invention proposes some means to meet this end, illustrated in Figure 3 as an example of implementation.

Hence, Figure 3 represents a gain control device designed to be used in an information signal receiver (not shown), this gain control device including: a signal input (31) receiving an input information signal.

amplification means (33) with adjustable amplification factor, coupled with the said input (31) to receive the said input information signal and alter its level in relation with a gain control signal received at the gain control input in order to obtain an output information signal.

20 a signal output (52) coupled with the said amplification means (33) and *e 9.

supplying the said output information signal.

9 evaluation means (37) coupled with the said input in order to receive the said input information signal and obtain an amplitude signal being dependent on the level of the latter signal.

25 processing means (38) coupled with the said evaluation means (37), receiving the said amplitude signal and evolving a gain control signal according to a set gain control law, the gain control signal is transmitted to the said gain control input (51) of the said amplification means (33).

Figure 3 also represents a coupler (36) sampling the input information signal 3o from the branch containing the amplification means (33) in order to supply it to the one containing the evaluation means and the processing means (37 38).

Evaluation means (37) may be simple envelope detectors, while processing means (38) may come in various forms as will be seen later.

The amplification means with adjustable amplification factor, may take the form of an amplification stage chain into which are inserted controllable attenuation stages. In this case, the gain is adjusted by attenuator control.

These means are arranged such that the level of the said output information signal has a reduced dynamic range compared with that of the said input information signal as illustrated in Figure 2. The input information signal being that of Figure 2, the output information signal is a similar signal but such that the difference between the amplitudes of packets P11, P12 and P21, P22 is reduced. To this end, evaluation means (37) determine the amplitude of the input signal, this amplitude undergoes processing which will be mentioned later in the processing means (38) which produce a gain control signal. Applied at the gain control input (51) of the amplification means this gain control signal determines the more or less important amplification of the input information signal and achieves the effect sought that is reduction of the dynamic range of 15 this signal, the amplification means (33) thus supplying at output (52) an output information signal with reduced dynamic range.

In Figure 3 gain control device, the processing means (38) may be organised to respond to a time slot synchronisation signal, supplied by a time base (not shown) and applied at the synchronisation input to operate on a i" 20 time slot basis and to maintain the same gain control almost throughout the time slot. For instance, processing means (38) observe the maximum or V06% minimum signal amplitude during time slot (IT) of frame (T1) in Figure 2, to determine a gain control signal which will be applied and maintained at the gain control input (51) of the amplification means (33) throughout time slot (IT) of 25 frame and so on, frame after frame and for each time slot. Of course, this 00 1: assumes that the level of the signal received from a mobile station does not change too much between two consecutive frames. According to another example, the packet may include a preamble of relatively short duration used for level evaluation in the evaluation means (37) and for processing in the processing means such that a gain control signal be available nearly at the start of an information packet transmitted during a time slot, this gain control signal being then maintained throughout the time slot duration. In this other case, it is assumed that the preamble in question is not to be processed in circuits requiring a low dynamic range, that it is for instance assigned to gain control for the remainder of the packet. According to a further example, a mixed solution is also possible, whereby the signal received during a given time slot is divided into time slices of preset duration and level evaluation in each slice determines the gain control in the next slice.

According to a preferred method of implementation, the gain control device in accordance with the inverion further includes delay means (34), shown in a dotted line in Figure 3 since they are optional; they are installed between the said coupler (36) and the said amplification means (33) and supply the latter an input information signal delayed by a time lag such that the said gain control signal is available for gain control in the said amplification means (33) at the time where the latter receive the said delayed input information signal.

In all gain control cases previously mentioned, such an arrangement 15 allows for evaluation of the input information signal amplitude, in the said evaluation means followed by amplitude processing in the said processing means by allocating all the time required, during which time the input signal goes across the said delay means Then, at the time when the packet, or more generally, the information signal received during a time slot, a, a.

20o reaches the said amplification means the gain control signal is supplied at their gain control input (51) by the said processing means such that this 0 signal can be applied at the very start of the information signal from which is S was derived. It is thus possible to adjust the amplitude of any packet from one or several packet samples, which are sampled, evaluated and processed before 25 the packet reaches the amplification means.

In a preferred method of implementation, evaluation of a packet reception level occurs in a sample sampled after a short time from the start of the pack, of a duration sufficient to eliminate interference by integration, thus supplying a gain control signal used to adjust the amplitude of the entire signal received during the time slot.

The time lag introduced by the delay means may have any value as long as it exceeds the duration required to receive the packet sample used for evaluation, increased by the duration required for the operation of the processing circuits (38) and the gain control in the amplification means (33).

Furthermore, Figure 3 also illustrates filtering means 4 2 43), shown in a dotted line since they are optional.

Indeed, in some cases, the input information signal includes interference signals out of band which would affect amplitude evaluation if they were not suppressed. The solution is to filter them, using a band pass or low pass filter.

Such a filter (42) may be installed between input (31) and the coupling point of the said signal input with the said amplification means and the said evaluation means. Another solution consists of installing a filter (43) in the evaluation branch only. Indeed, respecting only the amplitude information imposes filter specifications a lot less stringent than in the previous solution. Such a filter would therefore be less expensive.

It should finally be mentioned that, although from the beginning of the description we spoke of amplification means with adjustable amplification 15 factors, a worthwhile solution to implement such amplification means consists of an amplifier chain including at least one variable attenuator, the attenuation being controlled by the said gain control signal. Therefore we could namely use controllable attenuators which would not generate distortion and would tolerate the still significant dynamic range of the input signal.

S* S

S*

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Claims

1. A gain control device in an information signal receiver, including a signal input receiving an input information signal, amplification means with adjustable amplification factor, coupled with the said input to receive the said input information signal and alter its level in relation with a gain control signal received at a gain control input of the amplification means, in order to obtain an output information signal, a signal output coupled with the said amplification means and supplying the said output information signal, wherein said device further includes evaluation means coupled with the said input via a coupling point, in order to receive the said input information signal and obtain an amplitude signal being dependent on the level of the latter signal, processing means coupled with the said evaluation means, receiving the said amplitude signal and evolving a gain control signal according to a set gain control law, the gain control signal is transmitted to the said gain control input of the said 15 amplification means, whereby the level of the said output information signal is controlled to have a reduced dynamic range compared with that of the said input information signal.

2. A gain control device as claimed in claim 1, wherein said input information :..*signal is coming from remote stations, each transmitting within its own time slot in a 20 repetitive frame, in accordance with known TDMA system, said processing means, responding to a time slot synchronisation signal, operates on a time slot basis and maintains the same gain control substantially throughout the time slot.

3. A gain control device as claimed in claim 1 or 2, further including a delay Smeans installed between the said coupling point and the said amplification means, the said delay means supplying the said amplification means an input information signal delayed by a time lag such that the said gain control signal is available for gain control within the said amplification means at a time when the latter receives the said delayed input information signal.

4. A gain control device as claimed in any one of claims 1 to 3, including a filter arranged such that the said input information signal is filtered before being supplied to the said evaluation means. 0

5. A gain control device as claimed in claim 4 wherein said filter is a band 11 pass filter.

6. A gain control device as claimed in claim 4 wherein said filter is a low pass filter.

7. A gain control device as claimed in claim 5 or 6, wherein said filter is located between the said signal input and the said coupling point.

8. A gain control device as claimed in claim 5 or 6, wherein said filter is located between the said coupling point of the said signal input with the said amplificatic, means and the said evaluation means.

9. A gain control device accordance with any of claims 1 to 8, wherein said amplification means includes at least one variable attenuator whose attenuation is controlled by the said gain control signal. A gain control device substantially as herein described with reference to Figures 1 3 of the accompanying drawings. DATED T IhS FIFTH TH DAY OF APRIL 1995 ALCATEL N. V. a .0. o*o ABSTRACT Gain control device in an information signal receiver, including: a signal input (31) receiving an input information signal. amplification means (33) with adjustable amplification factor, coupled with the said input (31) to receive the said input information signal and alter its level in relation with a gain control signal, in order to obtain an output information signal. a signal output (52) supplying the said output information signal. evaluation means (37) coupled with the said input in order to receive the said input information signal and obtain an amplitude signal being dependent on the level of the latter signal. processing means (38) coupled with the said evaluation means (37), receiving the said amplitude signal and evolving a gain control signal according to a set gain control law, the gain control signal is transmitted to the said gain control input (51) of the said amplification means such that the level of the said output information signal has a reduced dynamic range compared with that of the said input information signal. FIGURE 3.