System to generate switching criteria and equalize adaptively in data spectrum

Abstract: The system according to the invention for detecting data spectrum distortion in transmission, in particular the distortion caused by selective fading, is based on identifying in-band dispersion, i.e an amplitude distortion and for this aim it substantially includes: two blocks (B₁,B₂) in parallel, the module behaviour of which have equal slopes but different signs upon a change of frequencies; two detectors of average power (R₁,R₂), each being associated to one of said blocks; and only one detected average power comparison means (A₀). This system allows to detect the assymmetries of the data spectrum and by adding an AGC and a reference voltage, also the information related to the central notch is obtained. The distortion measurement obtained in this manner allows to generate a switching criterion or control, as countermeasure for selective fadings. The criterion is obtained at intermediate frequency and generates an alarm whenever the distortions caused by fading evolve towards an error condition for the type of modulator-demodulator that is used. With reference to the "signature" drawn for a BER equal to 10⁻⁶ a curve (IF signature) is drawn representing the activation of an alarm whenever fading parameters exceed prefixed values. The IF signature is much more extended in frequency than the BER = 10⁻⁶ signature. Adaptive equalization is carried out at intermediate frequency on amplitude only, leaving group delay distortions unaltered.

Description: The present invention concerns a system for detecting data spectrum distortion in transmission, e.g. distortions caused by selective fading; the present invention refers in particular to a system for detecting possible in-band dispersions, wherein dispersion means an amplitude dispersion. Until today, the measurement of amplitude dispersion has been made by measuring the power of the data spectrum around 'n' frequencies; generally this measurement is carried out by 'n' frequency selective filters. The obtained information is much more complete as 'n' increases; but this functional requirement conflicts with many other requirements, in particular with the complex its involved in manufacturing frequency selective filters (band-pass filters, with a very narrow bandwidth), and with the need to save space and to limit costs. Owing to these and other requirements, the number 'n' of filters that are used is typically limited to two or three. If two filters are used, or in any case in which 'n' is an even number, information regarding the dissymmetry of the data spectrum with respect to the central frequency (fc) is obtained which is incomplete, as the channel bandwidth is sampled only in some points, hence said information is not "all band". If there are three filters, or in any case in which 'n' is an odd number, it is possible to detect also the presence of fading with a notch frequency equal to the frequency of the central band, but also in this case, there is still the invonvenience of obtaining a measurement which extends not over the whole band. From EP-A-0 257 526, an amplitude equalizer is known which sums up the output of two circuit means having opposite characteristics. That is, use is made of a positive slope first order amplitude equalizer, and a negative slope first order amplitude equalizer is provided in parallel thereto. Further, a feedback means is added to enable automatic amplitude equalization. The aim of the invention is to provide a particularly simple and efficient system that can be easily embodied and allows to eliminate the inconvenience of the methods using several filters and in particular permits to obtain information over the "whole band", i.e. a measurement of the dispersion over the whole band, that is less sensitive to the interference frequencies. This and further objects are achieved by the system according to the invention defined in claim 1, which is substantially based on identifying a dispersion over the whole band, or an amplitude distortion over the whole band (that is less sensitive to interference frequencies). The received signal is submitted to two distortions applied in parallel, having substantially equal slopes and opposite signs; each applied distortion is suitable to establish a variation of the average power spectrum in transient condition, these two average powers are measured and the signals resulting from these measurements are compared. The equivalence or difference of the signals indicates the absence or presence of fading, respectively. According to a first feature of the invention, the results of the measurement of the two average powers are different in the presence of fading, even if the notch frequency (fo) is out of band. According to another feature of the invention, if the fading notch frequency (fo) coincides with the central frequency of the data spectrum, the voltages related to the power level according to a certain characteristic curve are compared. The difference between the voltages is taken as a quality index for the in-band distortion: the higher the voltage difference, the higher the distortion and vice versa. According to a further feature of the invention, a switching criterion or control signal is retrieved from the IF signature providing the information for the distortion measurement. From the IF signature, a leading frequency may be derived that extends over a frequency range much wider than the conventional 10⁻⁶ range. The circuit for the embodiment of the distortion measurement according to the invention includes two parallel distortion blocks with equal slopes, but opposite signs, two average power testers and a comparator. The different characteristics and advantages of the invention appear better in the description of the preferred embodiments represented in the attached drawings, in which Figure 1 shows a circuit of the device according to the invention and Figures 2 to 11, 13 and 14 illustrate different configurations of the circuit. Figures 12 and 15 to 17 represent advantageous applications. The circuit of Figure 1 includes: A) a transmission means symbolically represented by block H (ω) (10), B) two blocks in parallel B1(1) and B2(2), C) two detectors of average power R1 and R2 and D) a comparator A0(3) of the signals obtained from said detectors R1 and R2. Signal Eu at the output from A0(3) indicates the measure of the dissymmetry due to selective fading. Signal e'(t) is the transmitted signal and forms the input of the common block, representing the transmission channel H(ω) (10), and the output of this block e(t) is the input signal of the system that measures dissymmetry. In the absence of fading, H(ω) remains constant upon a frequency variation, hence, there will be no amplitude distortion. In this case, if we consider the transfer function formed by H(ω) and block B1, the two cascaded blocks of Figure 2 will be obtained. Inside the blocks, the trend of the module versus frequency is represented. If the transfer function formed by H(ω) and block 2 is considered,the two cascaded blocks of Figure 3 will be obtained. Therefore, if a data spectrum in transmission without distortion and with a certain average power (Figure 4) arrives at the input of block B1 and at the input of block B2, the distortions caused by block B1 and block B2 establish an average power variation of the spectrum in transient condition; however, as the spectrum, because it is undistorted, is symmetrical with respect to f C (central frequency of the data spectrum) and as the slopes P1 and P2 of the two blocks are equal and have opposite signs and therefore the variation determined by block B1 is equal to the variation caused by block B2, detectors R1 and R2 installed downstream of the devices B1, B2 (Figure 1) measure an average power each that is identical. These two measurements, compared by means of the comparator, e.g. the operational amplifier A0, indicate the absence of fading (Eu= 0). On the contrary, in the presence of fading and if frequency fo (notch frequency) is not within the system bandwidth, the module H(ω) has an in-band trend that can be represented by a slope assumed to be the slope of H(ω) in Figure 5 and Figure 6. It can be demonstrated that the average power of the data spectrum B1 is higher than the average power of the spectrum B2, hence the power measured by the two detectors R1 and R2 is different. Therefore, these two measurements, compared by the simple operational amplifier, A0 indicate the presence of fading (Eu≠0). The power measurements of the two detectors are also different if fo is held within the band (Figure 9 and Figure 10). In fact, one block tends to reduce distortion, whereas the other block tends to increase it. Therefore, a schematization similar to that indicated in Figure 8 can be made, as shown in Figure 11. In this case, the data spectrum in transmission arrives at the input of block B1 and B2 (Figure 7) already distorted, block B1 increases and block B2 reduces this distortion. This situation can be represented by the scheme illustrated in Figure 8. It can be demonstrated that the average power of data spectrum 1 (SD1) is higher than the average power of data spectrum 2 (SD2). Therefore, the power measurements of the two detectors R1 and R2 are again different. The only case in which the device does not indicate presence of fading, when it is present, is the situation wherein fo = fc. The output of the detectors is a voltage that is related, according to a certain characteristic curve, to the power level at the input; the higher the power level, the higher the voltage value at the output and vice versa; in this manner, it is clear that, if the average power of data spectrum 1 (SD1) is higher than the average power of data spectrum 2 (SD2), the output voltage of detector R1 is higher than the output voltage of detector R2. The difference of the two voltages not only indicates whether fading is present or not, but also whether fo is located on the right-hand or on the left-hand side of f C ; further this difference is also a quality index of the value of band distortion, i.e. the higher the difference between the two voltages, the greater the distortion is, and vice versa. The system according to the invention has further advantages, developments and applications. In particular, by processing the voltages detected by the dissymetry tester, it is possible to extract also the information related to the central notch, thus obtaining a dispersion tester (the dispersion is considered as amplitude distortion). The block diagram shown in Figure 12 represents once again the dissymmetry tester, at the output of which are supplied two voltages V1 and V2 related to the dissymmetry of the data spectrum. The information related to fading with a central notch is achieved by summing the two detected voltages and comparing the sum with a reference voltage representing absence of fading. An Automatic Gain Control (AGC) is presumably provided at the input of the distortion detector. The AGC is absolutely necessary in this embodiment because the module is inserted after a locking device. This causes a level stability upon a fading parameters change, which makes processing in the central band of the data spectrum inefficient . To overcome this drawback, an AGC with a selective filter Fs is added. If the module is not used after a locking device, the AGC becomes redundant. Therefore, the dispersion tester has three voltages V1, V2, V3; these voltages supply the information related to the amplitude distortion caused by selective fading. The distortion measurement thus obtained allows to generate a switching criterion or control, to oppose selective fadings that can create dispersions, i.e. amplitude distortions on the data spectrum modulated on a transmission carrier wave. Until today, the generation of the switching control was based on "BER" measurements (Bit Error Rate) made at the base-band. The method used is the parity method, i.e. the number of even ones or odd ones, counted in each receive frame, are compared during a certain time period with the number of even ones or odd ones, counted in the same transmit frames. When the number of parity discordances tested in a certain time slot exceed a certain number, the measurement block for the error rate supplies an alarm. There are two alarms: the alarm generated when the BER is equal to 10⁻³ and the alarm generated when the BER is equal to 10⁻⁶; the first alarm clearly has higher priority than the second alarm. If the so-called signatures FR1 and FR2 are drawn (i.e. the graphics that put in relation together the parameters of the model representing selective fading and an error rate) related to a BER =10⁻³ and a BER = 10⁻⁶ for a given modulation-demodulation system, e.g. for an amplitude modulation system in quadrature 4 QAM/34 Mbit, the curves of Figure 13 are obtained. It can be noticed that in these curves FR1 and FR2 have very similar behaviours, so that the BER = 10⁻⁶ and BER = 10⁻³ alarms often are almost simultaneous. If we consider that BER = 10⁻³ represents the out of service threshold, it can be understood that it is absolutely necessary to generate a further alarm criterion which is a frequency leading or in anticipation with respect to the signature drawn for BER = 10⁻⁶. Moreover, it must be observed that for generating the above described switching controls, it is absolutely necessary that errors are detected at the base band in order to establish an error rate. According to an advantageous feature, the invention is also useful for the generation of the switching criterion which should not have any of the above mentioned inconveniences and supply well spaced and enlarged signatures. The invention also allows to generate the desired leading alarm criterion with respect to the signature drawn for BER = 10⁻⁶. The invention allows for advantageous applications in the system structure. In fact,if the device is supposed to be used in a (1+1) switching system having a transmission channel and a standby channel, switching to the standby radio beam is considerably leading or in anticipation with respect to the error conditions. As regards this matter, field trial tests were made to verify that the preceding statement is true, evidencing that the only case in which the system looses efficiency is when the standby beam is not available, because it is affected by equal or major error rates. In case a switching system (n+1)/(n+2) is used; the use of the device may be of great importance, because before switching, it is necessary to align the standby channel to the channel that is going towards an error condition. This alignment clearly requires a certain time period, and for this reason it is extremely important to use a device that allows to perform fast switching before detecting errors at the receive end. The device according to the present invention makes switching more efficient and prevents that it is carried out in error conditions. Limiting the error conditions improves the performances of a digital radio link, and more precisely reduces the error criteria (CCIR recommendations): Severely Errored Seconds (SES, BER = 10⁻³), Degraded Minutes (DM, BER = 10⁻⁶), Errored Seconds (ES, seconds with at least one error), Residual Bit Error Ratio (RBER). For the switching criterion implementation, an IF measurement of the distortions in the data spectrum is performed, and the information of this measurement is processed for drawing a curve (IF signature) with reference to a BER signature equal to the conventional rate 10⁻⁶; this IF signature represents the curve of the values corresponding to the distortions due to selective fading, which evolve towards an error condition for the adopted modulation-demodulation type. The IF signature generates a frequency leading or in anticipation by extending over a frequency range much more extended than the range of the 10⁻⁶ signature, with respect to the amplitudes of which it shows wider amplitudes with a considerable gap between the two curves. The device of the present invention is characterized in that a circuit consisting of at least two triggers is added to the distortion measurement circuit. The different features and advantages of the invention are better shown in the description of the set up represented in Figures 14 and 15. Figure 14 clearly shows that the comparison is no longer between the BER 10⁻⁶ (FR2) signature and the BER 10⁻³ (FR1) signature(as shown in Figure 13, they are generally too near to each other), but instead between the BER 10⁻⁶ (FR'2) signature and a reference FRR (IF) signature drawn at IF, i.e. where the selective fading parameters exceed the threshold that causes the alarm activation. In other words, the FRR curve at IF is the curve corresponding to the selective fading distortions evolving towards an error condition for the adopted type of modulation-demodulation (in this case 4 QAM/34 Mbit). Figure 14 indicates by A the new alarm criterion frequency leading or in anticipation with respect to the FR'2 signature drawn for BER = 10⁻⁶. Figure 15 shows a preferred embodiment of the device according to the invention. The first part of the device, that is the distortion detector, is substantially the same circuit as shown in Figure 12: it includes a transmission means symbolically represented by block H (ω) (10), an AGC (B0) with amplifier AM (11), selective filter Fs (12) and detector (13), and the two branches, each one of which includes block B1 or B2, respectively, with curves having equal slopes, but opposite signs, and detector R1 or R2, respectively. Signals A and B at the output from R2 and R1, respectively, are subtracted in S1 and summed up in A1. Signal S2 at the output from S1 is applied to the two blocks B3 and B4, the voltage frequency characteristics of which are represented inside the rectangle of the related block; said characteristics (V vs f) are equal but symmetrical with respect to the central frequency (fc) of the data spectrum. Also a reference voltage VRef is applied to adder A1 of A and B, and an output signal S3 is applied to block B5 that has the characteristic V (f) centered in f C . Enabling AB for block B5 is given by circuit 15 that receives outputs 18 and 19 from blocks B6 fed by A, and B7 fed by B, respectively. Signals 20 at the output from B3, 21 at the output from B4 and 22 at the output from B5 are applied to another circuit 25 (nNAND, nNOR or similar, as in 15) that supplies an alarm signal ALL. The distortion measurement also allows to embody an adaptive equalizer using dissymmetry detection of the data spectrum; more particularly, the invention refers to a system for obtaining an efficient countermeasure to selective fading by means of adaptive equalization of amplitude only at intermediate frequency (IF), i.e. equalizing any distortion of the transmission channel module and leaving the group delay distortions unaltered. Traditionally, adaptive equalization is made with the distortion measurement, i.e. the amplitude dispersion carried out by the power measurement of the data spectrum around 'n' frequencies. According to an important feature of the invention, dispersion detection over the whole band is made by using dissymmetry measurement, and this measurement drives an equalization structure in a critical manner. The related device includes a delay line and a multiplier in the equalizer and a rotation block of the equalizer signal, two adder notches, two detectors and one comparator in the dissymmetry tester. The different characteristics and advantages shall appear better from the following description of the two preferred equalizer embodiments represented in the attached drawing, in which Figures 16 and 17 are both circuit schemes, but with different structures of adaptive IF equalizers controlled by a measurement system for dissymmetries of the data spectrum. Figure 16 shows an equalizer made up of a delay line T1 installed in parallel to multiplier CM having a multiplying coefficient A1, while the measurement system for the data spectrum dissymmetries works on the signal u(t) at the output from the equalizer. The input signal e(t) to be equalized is applied to T1 and CM, the output signals 1 and 2, respectively, of which are summed up in adder S1, from where the equalized signal u(t) is outputted, on which, according to an important feature of the invention, the measurement system of dissymmetries works after the equalizer. Said signal u(t), as regards dissymmetry detection, is summed up in two different adding notches NS1 and NS2; in the first notch NS1, the signal u(t-T'₁) coming from delay line T'₁ (connection 3) and the signal u(t) (connection 6) are summed up. The second adder NS2 sums up the signal u(t-T'₁) (connection 5) and signal - u(t) drawn from the 180° end of rotation block B1 (line 7). The signals 9 and 8, respectively, outputted from NS1 and NS2, respectively, are each applied to the detector R1 or R2, respectively and, the two measured voltages V1 and V2 are compared in notch D that calculates the difference. The comparison signal D₁₂ is sent back to the multiplier CM, that supplies the equalizing factor to S1. Figure 17 shows a circuit which is substantially equal, except that the equalizer is made up of a feedback structure shown on branch 4 and comprising delay line T1 in feedback and multiplier CM with multiplying coefficient A1. The measurement system for the dissymmetries driving said equalizer is made in a similar way as that shown in Figure 16, including the phase shift block B1, the two summing notches NS1 and NS2, the two detectors R1 and R2 and the comparator D.

Claims: A method for detecting data spectrum distortions in transmitted digital signals (e'(t)), in particular distortions due to selective fading, for generating a switching criterion and for adaptatively equalizing said spectrum, characterized in that the received signal (e'(t)) undergoes two parallel distortions (B1, B2) which have upon frequency variation slopes of equal magnitude and opposite sign, each of the distortions causing a variation in the average power of the spectrum in transient condition; that the two average powers resulting from the two parallel distortions are measured; and that the signals arising from these two measurements are compared; wherein the equivalence or difference of the measured average powers indicates the absence or presence of fading, respectively. The method of claim 1, characterized in that, if fading is present and the notch frequency (fo) of the fading is different from the central frequency (fc) of the data spectrum, the voltages related to the power levels are compared, wherein the difference between the two voltages indicates the presence of selective fading and whether the notch frequency (fo) is on the left hand or on the right hand side of the center frequency (fc). The method of claim 2, characterized in that, if fading is present and even if the fading has a notch frequency (fo) which is equal to the central frequency (fc) of the data spectrum, the measured voltages are processed to obtain a distortion measurement. The method of claim 1, characterized in that a switching criterion or control signal is generated to oppose selective fadings that may create distortions of the data spectrum of a transmission carrier wave, wherein the measurement of the distortion of said spectrum is carried out at an intermediate frequency IF, and wherein the information of this measurement is processed to obtain an IF signature with reference to a bit error rate (BER) of 10 ⁻⁶, the IF signature representing a curve corresponding to the distortions due to selective fading. The method of claim 1, characterized in that an adaptive equalization of the distortion is carried out at the intermediate frequency (IF) acting onto the amplitude only, wherein any distortions of the transmission channel module are equalized, the group delay distortions leaving unaltered. The method of claim 5, characterized in that the dispersion detection is carried out at the intermediate frequency (IF) over the whole band without using filters, wherein the related dissymmetries are measured and the signal resulting from this dispersion detection drives the equalizer structure. The method of claim 6, characterized in that the measurement of dissymmetry is carried out on the equalizer signal. A device for performing the method of claim 1, characterized in that the device includes two parallel distortion blocks (B1, B2) for causing variations in the average power of the spectrum in transient condition whereby the distortions upon frequency variation have slopes of equal magnitude and opposite sign; two average power meters (R1, R2), each of which is associated with one of said blocks; and a comparator (A0; S1, D) for the signals obtained from said power meters. The device of claim 8, characterized in that each of the average power meters (R1, R2) supplies a voltage related, according to a certain characteristic curve, to the input power level. The device of claim 8 for performing the method of claim 4, characterized in that, in addition to the distortion measurement circuit, there is provided a circuit consisting of at least two triggers (B3, B4, B5), to one of which, preferably the trigger that sums up the measurement signals, is also applied the reference voltage representing absence of fading. A device for performing the method of claims 5 and 6, characterized in that the device includes a delay line (T1) in the equalizer, a multiplier (CM) and an adder (S1), and that the dissymmetry measuring means comprises a rotation block (B1) for the equalizer signal, two adder notches (NS1, NS2), two detectors (R1, R2), a comparator (D) and a line (D12) for supplying the comparator signal to the multiplier. The device of claim 11, characterized in that the mulitplier and the delay line of the equalizer are provided in parallel, and that a branch of the dissymmetry measuring means includes a second delay line (T'1). The device of claim 11, characterized in that the equalizer has a feedback structure with regard to the multiplier (CM), and that the delay line is provided in series with said multiplier on the feedback branch.

Inventors: CONTI ADELIO, NOBILI ROCCO, TROYER PAOLO, CONTI, ADELIO, NOBILI, ROCCO, TROYER, PAOLO
Applicants: ALCATEL ITALIA, ALCATEL ITALIA SOCIETA PER AZIONI
Application year: 1990
Main classification: H04B7
IPC codes: H04B7, H04B17, H03H21, H04B3, H03H17, H03H15, H04L27