NZ272086A

NZ272086A - Digital transmission quality estimation: impulse response coefficient comparison

Info

Publication number: NZ272086A
Application number: NZ27208695A
Authority: NZ
Inventors: Christophe Mourot
Original assignee: Alcatel Australia
Priority date: 1994-05-11
Filing date: 1995-05-08
Publication date: 1997-02-24
Also published as: CA2148874A1; AU1791995A; FR2719961B1; JPH07321853A; EP0682420A1; FR2719961A1; FI952200A0; FI952200A; AU698680B2

Description

<div class="application article clearfix" id="description"> 2720 o Priority Dato(s): i. I. J.. |. 5).htr. Complete Specification F-'ilsJ: ..5L|.S.).3.$?.... Class: (6).HQMr.&.1.1 o©s;..Wo.vf.\ a.o Publication Date:.. lifEB.M P.O. Journal No: TRUE COPY NEW ZEALAND PATENTS ACT 1953 ; f COMPLETE SPECIFICATION j - - MAY ton? j i " DIGITAL TRANSMISSION QUALITY;" j WE, ALCATEL AUSTRALIA LIMITED, CA&oo T^53(3) A Company of the State ofNew South Wales, of 280 Botany Road, Alexandria, New South Wales, 2015, Australia, hereby declare the invention for which we pray that a patent may be granted to ;s, and the method by which it is to be performed, to be particularly described in and by the following statement: 1 This invention relates to digital data transmission, and in particular to a device for estimating the quality of a transmission channel for a receiver of a digital transmission system. !n a digital transmission system, in particular a radio system, a transmitter sends a sequence of symbols to a receiver over a transmission channel. The sequence transmitted is degraded by the transmission channel with the result that the sequence of symbols received by the receiver is no longer identical to that transmitted. The main form of degradation is inter-symbol interference caused by the fact that a symbol transmitted can propagate along more than one path in the transmission channel. If at least two paths have a path difference greater than the distance between two symbols transmitted in succession, a symbol on one of these paths will interfere with a subsequent symbol on another, shorter path. An equalizer is used in the receiver to correct inter-symbol interference. To function correctly it must know the impulse response of the transmission channel. To this end a learning sequence is transmitted comprising symbols which are known, unlike the data symbols transmitted which, it can be assumed, are not known to the receiver. It is therefore standard practise for a packet of symbols addressed to a particular receiver to include in succession data symbols, a learning sequence and further data symbols, i.e. the learning sequence is in the middle of the packet. The learning sequence is chosen to suit the characteristics of the transmission channel, and in particular its length. Given that the symbols are transmitted regularly and therefore periodically, with a period known as the symbol period, the length of the channel is defined as the number of symbol periods which is equivalent to the difference between the longest path and the shortest path of that channel. A device in the receiver estimates the impulse response of the channel and generates a replica of the learning sequence which it correlates with the corresponding sequence of symbols received. The result of this correlation is a set of impulse response coefficients h; where i varies from O to L, L being the length of the channel. In this set of coefficients, intended to be used by the equalizer, the most direct path in the channel is represented by hQ while the others represent longer paths which cause interference with the first. In the most general case these coefficients are complex coefficients as the symbols received have two orthogonal components, called the "phase" component and the "quadrature" component. in known receivers, transmission channel quality is estimated by measuring the received signal level or by calculating the bit error rate from the received symbols. However, the quality estimates obtained in this way do not indicate if ail the treatments available at the receiving end are really necessary. 2720 The prior art also includes a device for monitoring activation of an equalizer on the basis of a set of impulse response coefficients supplied by a device for estimating the impulse response of the channel and addressed to the equalizer. This device operates in the following manner: all coefficients are eliminated for which the square of the modulus is less than a threshold and the equalizer is not used if all the coefficients are eliminated except for the first. In this case the transmission channel quality is deemed to be very good and the equalizer is of no utility. Indeed, its use would even tend to degrade the quality of the received data, by attempting to eliminate inter-symbol interference where there would in fact be none present. A device of this kind cannot provide an estimate of the transmission channel quality, but merely take a binary decision as to whether the equalizer should be used or not. A knowledge of the estimated transmission channel quality is needed in many applications, and in particular to determine whether all the treatments available at the receiving end are really necessary. An object of the invention is to alleviate these various drawbacks of the prior art. To be more precise, one object of the present invention for estimating the quality of a transmission channel for a receiver in a digital transmission system. Another object of the present invention is to provide a device of this kind which indicates if the treatments available at the receiving end (such as equalization, for example) are of benefit or not. According to the invention, there is provided a device for estimating the quality of a transmission channel for a receiver of a digital transmission system, said receive." including means for estimating the impulse response of the transmission channel in the form of a set of impulse response coefficients h; where i varies from O to L and where L is the length of the transmission channel expressed as a number of symbol periods, and means for calculating an estimated value of the quality of the transmission channel by comparing first information characteristic of the first impulse response coefficient h0 and second information characteristic of other impulse response coefficients hj where j varies from 1 to L. The general principle of the invention is therefore to compare the first impulse response coefficient to all the other coefficients to estimate the transmission channel quality. The first coefficient h0 represents the weighted wanted information while the other coefficients h, through hL represents the weighted inter-symbol interference. A symbol y, received at time t can be analysed as follows: - the result (h0.x,) of weighting by the first coefficient h0 of the corresponding transmitted symbol x,; - inter-symbol interference, i.e. the result (h,.x,., + ... + hL.x,.|J of weighting by the other coefficients hi through hL of the symbols x j e [1 ,L] transmitted after the corresponding symbol x,; and - Gaussian additive white noise n,. Thus: Y, = h0.x, + (h^x,., + ... + hL.x,.L ) + n,. Said estimated value of transmission channel quality is advantageously the ratio of the square of the modulus of the first impulse response coefficient h0 to the sum of the squares of the moduli of the other impulse response coefficients h;. In this way the estimated value of transmission channel quality is defined as a power ratio. Said set of impulse response coefficients preferably includes at least one null coefficient, a null coefficient being a coefficient whose modulus has previously been eliminated by the impulse response estimating means because its square was below a predetermined threshold for the square of the modulus. In this way, while obtaining the benefit of a long channel length, the number of correlation coefficients is reduced by siiminating those which give no information on the transmission channel. This means, for example, coefficients corresponding to paths which have the worst signal/noise ratio. Also, the probability of the impulse response estimating device generating a correlation coefficient h; equal to zero is virtually nil, if only for reason of calculation noise. Thus in reality there is a non-null probability of a zero coefficient value. The invention forces to zero any coefficient with a very low value, and in this way prevents detection of inter-symbol interference where there is none. Said predetermined threshold value for the square of the modulus is advantageously a function of an estimate of the signal/noise ratio. Thus if the signal/noise ratio is sufficiently high, a decision can be taken to reduce the threshold value for the square of the modulus (or even to eliminate it altogether) in order to act on low coefficients which actually represent inter-symbol interference. The invention also concerns the use of the transmission channel quality estimating device for monitoring activation of an equalizer receiving said set of impulse response coefficients h;. A range of activation of the equalizer is advantageously defined between minimal and maximal acceptable quality threshold values, said equalizer being activated is said estimated transmission channel quality value is within said range of activation. In this way the equalizer is not used when the transmission channel quality is very good (in which case the equalizer degrades the quality by finding inter-symbol interference where there is none), or when it is very bad (in which case the equalizer cannot achieve anything). Finally, the invention also concerns the use of the transmission channel quality estimating device in an application selected from the group comprising: - estimating the performance of an equalizer receiving said set of impulse response coefficients h:; - monitoring the power at which a signal is transmitted; and - monitoring a change of cell in the case of a digital cellular mobile radio system. Estimating the performance of an equalizer, for example, enables dynamic rating of the equalizer or any other unit on the downstream side of the receiver system. The transmission power to be monitored is either that of a remote transmitter (for example, in frequency division duplex (FDD) mode) or that of the local transmitter associated with the receiver and/or a distant transmitter (for example, in time division duplex (TDD) mode). Other features and advantages of the invention will emerge from a reading of the following description of a preferred embodiment of the invention given by way of illustrative and non-limiting example only and from the appended drawings, in which: Figure 1 is a diagram showing a transmission channel to which is connected a receiver including a device in accordance with the invention for estimating transmission channel quality. Figure 2 shows a curve of the variation in the bit error rate in the receiver from figure 1 as a function of transmission channel quality as measured by the device of the invention. Referring to figure 1, the transmission channel receives the transmitted symbols 2 and supplies the received symbols 3 to the receiver 4. The receiver 4 includes: - means 5 for estimating the impulse response of the transmission channel 1; - an equalizer 6; - a channel decoder 7; and - a device 8 in accordance with the invention for estimating the quality of the transmission channel 1. The symbols 2 are transmitted in packets, each packet including data symbols and a learning sequence made up of symbols known to the receiver 4. For each packet of received symbols 3, the means 5 for estimating the impulse response of the transmission channel generate a replica of the learning sequence and correlate it with the sequence of received symbols 3. The result of this correlation is a set 9 of impulse response coefficients h;, with i varying from 0 to L, where L is the length of the transmission channel 1 expressed as a number of symbol periods. A sequence of received data symbols 3 may not be identical to the sequence of transmitted data symbols 2. The transmission channel 1 degrades the data, and in particular causes inter-symbol interference. The function of the equalizer 6 is to reduce this inter-symbol interference. To this end it uses the set of 9 impulse response coefficients h,. For each received data symbol 3 the equalizer 6 supplies to the channel decoder a decision 10 (i.e. what it deems to be the most correct value) and an assessment 11 of this decision. It is evident that the receiver 4 includes many other processor modules (in particular on the output side of the channel decoder 7) which, for simplicity, are not shown in figure 1 and not discussed here. The device 8 of the invention for estimating the quality of the transmission channel 1 includes means for calculating an estimated value Q of the transmission channel quality from the set 9 of impulse response coefficients h;. This estimated value Q of transmission channel quality is defined as a ratio between the power assigned to the first coefficient h0 (i.e. to the inter-symbol interference). This value Q is, for example, the ratio of the square of the modulus of the first impulse response coefficient h0 to the sum of the squares of the moduli of the other impulse response coefficients hjf with {varying from 1 to L. Coefficients h,- with a very low value can be forced to 0 to prevent the system from finding inter-symbol interference where there is none. The probability of the means for estimating the transmission channel impulse response supplying a coefficient h; equal to zero is practically nil (in particular because of calculation noise). In reality, there is a non-null probability of a zero coefficient value. For example, it is possible to eliminate coefficients for which the square of the modulus is less than a threshold value, this threshold value possibly depending on an estimate of the signal/noise ratio. Of course, other techniques can be used to limit the number of non-null 2720 8 coefficients, if necessary. Figure 1 also shows one example of use of the device 8 of the invention. The receiver 4 includes means 12 for monitoring activation of the equalizer 6 as a function of the estimated value Q of transmission channel quality. The range of activation of the equalizer 6 is defined between minimal and maximal acceptable quality threshold values, for example. Figure 2 shows clearly the feasibility of defining a range of activation in this way; this figure shows a curve of the variation of the bit error rate in the receiver 4 as a function of the estimated value Q of transmission channel quality. For an acceptable performance level, i.e. a value Y of the bit error rate (BER) which is acceptable given the quality constraints for the system, there are two threshold values Qm and QM of transmission channel quality. The range of activation 21 of the equalizer is therefore defined between these two threshold values Qm and QM. For an estimated value Q below the minimal threshold value Qm, the quality of the transmission channel is too bad for the equalizer 6 to be able to correct the receive signals 3 adequately. For an estimated value Q above the maximal threshold QM, the quality of the transmission channel is very good and the use of the equalizer 6 could only degrade the symbols received. The minimal and maximal threshold values Qm and QM are determined by 11 simulation or by field trials, for example. Because it supplies an estimated value Q or transmission channel quality, the device of the invention can also be used for other applications, including: - estimating the performance of the equalizer 6; - monitoring the power at which a signal is transmitted; and - monitoring a change of cell (handover) in the case of a cellular digital mobile radio system. It is obvious that the device 8 for estimating transmission channel quality described herein above with reference to figure 1 can be used for many other applications without departing from the scope of the invention. Any type of action can be taken in response to an estimated value of transmission channel quality Q, for example to maintain a given quality, to optimize the symbol transmission format, to optimize receive end processing or to improve power consumption. 12 </div>

Claims

<div class="application article clearfix printTableText" id="claims"> What we claim is:

1. A device for estimating the quality of a transmission channel for a receiver of a digital transmission system, said receiving including means for estimating the impulse response of said transmission channel in the form of a set of impulse response coefficients h; where I varies from 0 to L and where L is the length of said transmission channel expressed as a number of symbol periods, and means for calculating an estimated value of the quality of said transmission channel by comparing first information characteristic of a first impulse response coefficient hO and second information characteristic of other impulse response coefficients h; where i varies from 1 to L.

2. A device as claimed in claim 1, wherein said estimated value of the quality of said transmission channel is the ratio of the square of the modulus of said first impulse response coefficient h0 to the sum of the squares of the moduli of said other impulse response coefficients h;.

3. A device as claimed in claim 1, wherein said set of impulse response coefficients can include at least one null coefficient, a null coefficient being a coefficient whose modulus has previously been eliminated by said means for estimating said impulse response because the square of that modulus was less than a predetermined threshold value for the square of the modulus.

4. A device as claimed in claim 3, wherein said predetermined threshold value for the square of the modulus is a function of an estimate of the signal/noise ratio. ,7 L-- o

5. An application of the device for estimating the quality of a transmission channel as claimed in any one of claims 1 to 4 to monitoring activation of an equalizer receiving said set of impulse response coefficients h;.

6. An application as claimed in claim 5, wherein a range of activation of the equalizer is defined between minimal and maximal cceptable quality threshold values, said equalizer being activated if said estimated value of the quality of the transmission channel is in said activation range.

7. An application of the device for estimating the quality of a transmission channel as claimed in any one of claims 1 to 4 to an application selected from the group comprising: - estimating the performance of an equalizer receiving said set of impulse response coefficients h; ; - monitoring the power at which a signal is transmitted; and - monitoring a change of cell in a cellular digital mobile radio system.

8. A device substantially as herein described with reference to Figures 1 - 2 of the accompanying drawings. ALCATEL AUSTRALIA LIMITED : - 8 MAY 1395 B. O'Connor Authorized Agent P5/1/1703 14 </div>