DE4212300A1 - Simultaneous channel evaluation for digital communications network - using correlator to evaluate channel responses to time shifted test signals obtained from equal periodic base signal - Google Patents

Abstract

The channel evaluation method is used for a number of time discrete real or complex channel pulse responses provided by a number of digital communication channels in response to real or complex test signals. The test signals are generated by providing time shifted versions of an equal periodic base signal allowing the received signals to be filtered using a single filter with an inverse response to the base signal. Pref. the evaluation is effected via a correlator with a shift register (5) for the received signal supplied via a switch (6) having a second input coupled to the shift register output. The shift register outputs are combined with respective weighting factors and fed to an adder (7). USE - For testing radio channels in CDMA cellular mobile communications network.

Description

The invention is based on a method for simultaneously estimating the discrete-time impulse responses of digital message channels after the Preamble of claim 1.

With digital message transmission via time variant and frequency Selective channels are required in the receivers for signal equalization Information about the current channel status / 1 /. This information can be obtained in that the transmitter in addition to the signals for Send additional test signals that are not modulated with data, that after transmission over the channels for receiver-side channel estimation can be used, see e.g. B. / 2 /.

In the uplink of mobile radio systems, those different from K should mobile transmitters and each via a different from K Channels arriving at the receiver of the base station in the Base station can be evaluated. The multiple access that occurs here problem can be solved by the multiple access methods FDMA (frequency division multiple access), TDMA (time division multiple access) or CDMA (code division multiple access) / 3 /. With the multiple access method CDMA are the signals of the individual mobile transmitters neither in the frequency nor disjoint in the time domain. For channel estimation, this means that in The receiver's responses to the test signals are not separated Form, but as a sum. The known methods of the channel estimate / 4,5,6,7 /, in which a frequency or temporal separation of the responses of the channels to the test signals is required therefore do not use at CDMA. These known methods are like this designed so that only a single channel can be estimated, but not Simultaneously several channels that are neither in the time nor in the frequency domain are disjoint.

The invention has for its object the problem of channel estimation in Solving CDMA systems at low cost. The approach according to the invention is set out below. Here, time-discrete signals  that can be real or complex. Vectors and matrices are identified by underlining. i is the discrete time.

Fig. 1 shows the K channels ( 1 ), their impulse responses

{h ^(k) (i)} = {h ^(k) (1), h ^(k) (2). . . h ^(k) (W _k ′)}, k = 1. . . K (1)

should be estimated with the sizes

W _k ′ ∈ IN, k = 1. . . K (2)

are the lengths of the impulse responses. With the sizes

W _k ∈ IN, W _k W _k ′, k = 1. . . K (3)

the impulse responses (h ^(k) (i)) according to Eq. (1) as vectors

write. The vectors h ^(k) according to Eq. (4) can become a vector

h = [ h ^{(1) T} , h ^{(2) T.} . . h ^{(K) T} ] ^T (5)

be united.

A test signal is placed in each of the channels (1) to be estimated, see FIG. 1

{a ^(k) (i)} = {. . . a ^(k) (1), a ^(k) (2), a ^(k) (3),. . .}, k = 1. . .K (6)

fed. The channel output signals caused by these test signals are summed up in a summer ( 2 ) to form the total signal (s (i)). An interference signal (n (i)) is added to the sum signal (s (i)) in the adder ( 3 ). (n (i)) represents the received noise, the noise of the receiver input stage etc. At the output of the summer ( 3 ) you have the signal

{e (i)} = {s (i)} + {n (i)}. (7)

From this signal, the impulse responses (h ^(k) (i)), k = 1. . . K, see Eq. (1) to be estimated.

There are endless possibilities to test signals (a ^(k) (i)) according to Eq. (6) to choose. The choice according to the invention, which is not obvious, consists in all K test signals from one and the same periodic basic signal

{p (i)} = {. . . p (P), p (1), p (2). . . p (P), p (1). . .} (8th)

the period

according to the regulation

to generate. The main claim relates to this special choice of test signals (a ^(k) (i)), k = 1. . . K.

The new choice of test signals (a ^(k) (i)), k = 1. . . K according to Eq. (10) suggests a special way of performing the estimation as covered by claim 2.

As shown below, according to Eq. (10) generated test signals (a ^(k) (i)) for particularly simple implementations of the estimator. In Fig. 2 for the case K = 3, W ₁ = 3, W ₂ = 2, W ₃ = 5, the generation of the test signals (a ^(k) (i)), k = 1,2,3, according to Eq . (10) illustrates.

From each of the test signals (a ^(k) (i)) according to Eq. (10) one can have a matrix with P rows and W _k columns

A ^(k) = (A _ÿ ^(k) ), k = 1. . . K, (11) (a)

A _ÿ ^(k) = a ^(k) (W _k = ij) (11) (b)

form. The matrices A ^(k) , k = 1. . . K, according to (11a) can in turn lead to a square matrix of dimension P

A = ( A ⁽¹⁾ , A ⁽²⁾ ... A ^(K) ) (12)

be summarized.

Due to the periodicity of the test signals (a ^(k) (i)) according to Eq. (10) is the sum (s (i)) at the output of the summer ( 2 ), see FIG. 1, also periodically with the period P according to Eq. (9). If one considers the output signal (e (i)) of the adder ( 3 ), see FIG. 1, for the duration P of a period of the sum signal (s (i)), this section of the output signal (e (i)) can be compared with the Vector h according to Eq. (5), from which the interference signal (n (i)), see Eq. (7), interference vector formed

n = [n (1), n (2). . . n (P)] ^T (13)

and with the matrix A according to (12) by the vector

e = A * h + n (14)

to be discribed.

Due to the construction rule given by the equations (10) to (12) of the matrix A , this matrix is right-hand circular. Hence, see e.g. B. / 8 /, is obtained with the right-hand circular matrix

C = A ^-1 (15)

an estimate h of the vector h sought, see Eq. (4) and Eq. (5), according to the relationship

 =C. ·e. (16)

The right circulatory system of the matrixC. according to Eq. (15) allows the investigation from  according to Eq. (16) with a base signal (p (i)) according to Eq. (8) inverse Filter / 6 / to carry out in a particularly simple manner. Such an inverse Filter can, as inFig. 3, advantageously by a Correlator can be realized. The multipliers (4th) of the correlator weight factors to be fed

g = [g (1), g (2). . . g (P)] (17)

be according to the elements

c = (c₁₁, c₁₂ ... c _1P ) (18)

the first line of the matrix C according to Eq. (17) chosen, ie

g = c . (19)

In the shift register (5) of the correlator is first clocked the reception signale according to Eq. (14) a. The switch (6) according to the institution Fig. 3 in position I. Then the shift register (5) of the correlator fed back into position II by flipping the switch (6). One now clocks the correlator P times, for P see Eq. (9), so you get at the exit the correlator summer (7) the estimation  of the searched vectorH, out which the estimates of the impulse responses sought (h^(k)(i)), k = 1. . . K, according to Eq. (5) and Eq. (4) emerge.

From Fig. 3 and the explanations made therefor it is necessary to estimate the channel impulse responses (h ^(k) (i)), k = 1. . . K, is sufficient to have a section of the length P of the signal (e (i)) available. As a result, it is not necessary to send the test signals (a ^(k) (i)) periodically. Rather, it is sufficient to have finite sections of length

to send. This fact is the subject of claim 3.

Further embodiments of the method are in claims 4 to 7 detected. Claims 8 and 9 relate to the implementation of the method.

literature

/ 1 / Proakis, J.G .: Digital Communications. McGraw-Hill, New York, 1983

/ 2 / Ochsner, H .: The future pan-European digital mobile radio system, Part 1: GSM recommendations and services. Bulletin SEV / VSE 79 (1988) 11, pp. 603-608

/ 3 / Baier, P.W .; Jung, P .; Klein, A .: CDMA - a cheap multiple Access procedure for frequency selective and time variant Cellular channels. Telecommunications, Electronics 41 (1991) 6, p. 223-227 and 234  

/ 4 / Rohling, H .; Borchert, W .: On the mismatched filter design for periodic binary phase encoded signals. ntz archive 10 (1988) 5, Pp. 111-117

/ 5 / Plagge, W .; Poppen, D .: New method for measuring the Channel impulse response and carrier synchronization in digital Mobile radio systems. Frequency 44 (1990) 7-8, pp. 217-221

/ 6 / Ruprecht, J .: Maximum-Likelihood Estimation of Multipath Channels. Hartung-Gorre Verlag, Constance, 1989

/ 7 / Lüke, H.D .: Follow with perfect periodic auto and cross correlation functions. Frequency 40 (1986) 8, pp. 215-220

/ 8 / Marple, S.L .: Digital Spectral Analysis. Prentice-Hall, New Jersey, 1987.

Claims (9)

1. Method for simultaneous estimation of the time-discrete real or complex channel impulse responses (h ^(k) (i)) = (h ^(k) (1), h ^(k) (2)... H ^(k) (W ′ _k ) ), k = 1. . . K, the lengths W ′ _k ε IN of K channels ( 1 ) of the digital message transmission from the discrete-time signal (e (i)) = (s (i)) + (n (i)), where n (i) one Interference signal is and s (i) is the sum of the channel output signals that are generated when a time-discrete real or complex test signal is fed in (a ^(k) (i)) = (... A ^(k) (1), a ^{(k )} (2), a ^(k) (3)...), K = 1. . . K, in each of the channels ( 1 ), characterized in that with the quantities W _k ε IN, k = 1. . . K, the K test signals (a ^(k) (i)) from a periodic, time-discrete base signal (p (i)) = (... P (P), p (1), p (2).. P (P ), P (1)...) Of the period according to the regulation are formed, the channel lengths of W _k 'W _k and the sizes of the BedingungenW _k W _k', k =. 1 . . Ker filling. 2. The method according to claim 1, characterized in that the impulse response of a disturbed replacement channel of length estimates, for which one assumes in the estimation that it reacts to the base signal (p (i)) as input signal with the signal (e (i)), and whose impulse response is achieved by stringing together or not completely stringing together the impulse responses (h ^(k) (i)), k = 1. . . K, the K channels ( 1 ) is created. 3. The method according to one or both of the above claims, characterized in that only finite sections (a ^(k) (1), a ^(k) (2)... A ^(k) (N)), k = 1 . . . K, the length the periodic test signals (a ^(k) (i)) are fed into the K channels ( 1 ), it being possible to send signals before and / or after these sections which are not used for channel estimation but for other purposes, e.g. B. the data transfer serve. 4. The method according to one or more of the above claims, characterized characterized in that the signal (e (i)) is stored before further processing is and can be supplied for further processing, which is not done in real time. 5. The method according to one or more of the above claims, characterized characterized in that the signal (e (i)) is one for the base signal (p (i)) undergoes inverse filtering, d. H. a filter whose response to the base signal (p (i)) apart from a single non-vanishing Value per period that occurs periodically at the interval of the period P is zero. 6. The method according to one or more of the above claims, characterized in that the generation of the test signals (a ^(k) (i)), k = 1. . . K and / or the processing of the signal (e (i)) in the time domain. 7. The method according to one or more of claims 1 to 5, characterized in that the generation of the test signals (a ^(k) (i)), k = 1. . . K and / or the processing of the signal (e (i)) in the frequency domain. 8. The method and device according to one or more of the above claims, characterized in that the processing of the signal (e (i)) with a Correlator is done. 9. The method and device according to one or more of the above claims, characterized in that the generation of the test signals (a ^(k) (i) k = 1 ... K and / or the processing of the signal (e (i)) with Signal processors and / or microprocessors takes place.