JP2739318B2 - Maximum likelihood receiver - Google Patents

Description

The present invention is used for receiving and demodulating an encoded signal. In particular, the present invention relates to a maximum likelihood receiver that receives a signal via a wireless digital transmission path having delay waveform distortion.

[Conventional technology]

In general, when performing signal transmission, a signal transmitted from a transmitter is subjected to linear distortion due to band characteristics of a transmission device and a transmission medium arranged on a transmission path. Therefore, in digital signal transmission, intersymbol interference occurs, and the performance of the receiver is reduced. Various equalizers have been conventionally proposed to reduce transmission characteristic degradation due to such linear distortion.
A maximum likelihood receiver is known as having particularly high performance.

FIG. 6 is a block diagram of a first conventional maximum likelihood receiver.

The received signal is supplied to an input terminal 1 and further passed through a matched filter 61 and a sampling circuit 62 to a maximum likelihood code sequence estimator 63.
Supplied to The output of the maximum likelihood code sequence estimator 63 is an output terminal
Connected to 11.

The operation of the maximum likelihood receiver will be described with reference to a case where carrier modulation is used as a modulation method and quadrature detection is performed before performing equalizer processing, using a complex number representation of a baseband detection waveform.

Code sequence {a _n} the linear modulation signal for transmission at a rate of 1 / T complex envelope S (t), using the basic waveform g (t), It is represented by The impulse response h (t) corresponding to the linear distortion of the transmission line acts on this. Therefore, the received wave R (t) at the input terminal 1 is Becomes Here, “*” represents a convolution operation, and N (t) represents noise.

The matching filter 61 matches the impulse signal of g (t) * h (t), and is a filter having the characteristic that the signal-to-noise ratio of the output waveform is highest. The sampling circuit 62 samples the output waveform of the matched filter 2 every time T in synchronization with the complex envelope S (t). The maximum likelihood code sequence estimator 63 internally generates a matched filter ideal output corresponding to a possible combination of code strings {a _i }, and converts the waveform into a complex envelope S
The signal space distance between the sampled signal and the actually input signal is calculated in synchronization with (t), and the combination of code strings {a _i } corresponding to the waveform having the shortest distance is output to the output terminal 11. I do.

FIG. 7 is a block diagram of a second conventional maximum likelihood receiver.

When a non-linear modulation wave is used, a linear matched filter cannot be formed, so that the modulation wave is approximately represented by a combination of several carrier waves. For example, TFM (T
amed FM) and Gaussian baseband filter normalization 3
GMSK (Gaussian-filtere) with a dB bandwidth B _b T (one side) of 0.21
To d Minimum Shift Keying) generating a modulated wave of approximately 1 time 1 and the carrier frequency f _c of the signal waveform of the time slot, a wave of five carrier wave having a frequency offset of ± Delta] f and ± 2.DELTA.f Connection is made so that the phase is continuous for each slot.

FIG. 7 illustrates an example, in which a received wave input from an input terminal 1 is supplied to multipliers 71-1 to 71-5. Multiplier 71−
1～71-5 further, _{respectively, e 1 = cos2π (f c} + 2Δf) t e 2 = cos2π (f c + Δf) t e 3 = cos2πf c te 4 = cos2π (f c -Δf) t e 5 = cos2π (f _c -2Δf) t is input. Outputs of the multipliers 71-1 to 71-5 are supplied to integrators 72-1 to 72-5, respectively. These multipliers 71
-1 to 71-5 and integrators 72-1 to 72-5 determine the correlation between the received wave and the five carrier waves, respectively. These multipliers 71-1 to 71-5 and integrators 72-1 to 72-5
Is used to form a matched filter corresponding to the waveform of one time slot, and its output is sampled by sampling circuits 62-1 to 62-5. The maximum likelihood code sequence estimator 63 includes a sampling circuit 62
A code sequence is estimated based on the values sampled by -1 to 62-5. When the constraint length of the code sequence is long, the hardware scale of the maximum likelihood code sequence estimator 63 increases exponentially with respect to the constraint length. Therefore, the signal is usually estimated using a Viterbi algorithm. The maximum likelihood code sequence estimator 63 is
The estimated code string is output from the output terminal 11.

[Problems to be solved by the invention]

However, when linear distortion is added to the non-linear modulation wave due to the band characteristics of the transmission path, the received wave cannot be represented by a linear combination as in equation (2), and it is difficult to obtain a matched filter. Met. Therefore, even if the conventional maximum likelihood receiver for the nonlinear modulation signal is used as it is, there is a defect that the error rate characteristics are not improved.

An object of the present invention is to solve the above problems and to provide a maximum likelihood receiver capable of receiving and demodulating a nonlinear modulation signal in which linear distortion has occurred.

[Means for solving the problem]

The maximum likelihood receiver of the present invention comprises: means for detecting a component of a specific code sequence included in a received digital signal to obtain a delay component included in the signal; and means for generating a fundamental wave corresponding to each of the delay components. And characterized in that:

The means for obtaining the delay component desirably includes a component extraction correlator for measuring the delay time and the amplitude of the delay component based on the correlation between the received digital signal and the code sequence stored in advance. Further, it is desirable that the means for generating a fundamental wave is configured to generate a fundamental wave with a time slot length of a delay component.

The maximum likelihood receiver further includes, as maximum likelihood code sequence estimating means, a correlation memory for storing the output of the correlating means over a predetermined time slot, and a state up to a plurality of current states estimated on the receiving side. A state transition memory for storing transition history data, a plurality of degenerate paths that extract a relative relation between state transition histories, and a relation between inner product values of signal waveforms of respective time slots in each path; A degenerate path memory that stores the relationship between the next degenerate path that is continuous with the degenerate path;
A distance for calculating a signal space distance in a time slot to a next state that is continuous with a plurality of current states, based on an output of the means for obtaining a delay component, the storage contents of the state transition memory, and the storage contents of the degenerated path memory. The operation unit and Viterbi
It is desirable to include a state determiner determined by an algorithm, and means for outputting a code of a code sequence corresponding to a state history of a state where the cumulative signal spatial distance of the plurality of determined current states is the smallest.

(Operation)

The maximum likelihood receiver of the present invention generates a fundamental wave corresponding to each delay component included in a received digital signal, and obtains a correlation between the fundamental wave and the received digital signal. For this reason, even if linear distortion due to the delay component occurs, the code sequence can be satisfactorily reproduced.

Further, the maximum likelihood receiver of the present invention is configured to recursively calculate a signal space distance for each time slot, and can simplify state determination using a degenerated path table.

〔Example〕

FIG. 1 is a block diagram of a maximum likelihood receiver according to an embodiment of the present invention.

Here, the modulation wave is described a GMSK signal of TFM signal or B _b T = 0.21 as an example. These two signals are almost equivalent, It is represented by Here, 2πf _c = ω _c is the angular frequency of the carrier wave, E (t) is the complex amplitude.

FIG. 2 shows a trellis representing the change in φ (t). Here, a trellis for four time slots is shown. Each grid point represents a state, and the state is indicated by a number in the figure. The same number represents the same state, and there are 16 states. The time slot III and the time slot IV are the same trellis as the time slot I and the time slot II. Also, I
The envelope represented by (t) = cosφ (t) has the waveform shown in FIG.

Returning to the description of the embodiment of FIG. 1, the input terminal 1 is supplied with a modulated wave represented by the equation (3). The modulated wave includes, as a frame signal, a signal modulated with a specific code sequence at regular intervals. The component extraction correlator 2 correlates the received signal with a previously stored code sequence,
A specific code sequence is extracted from the received signal. At this time, if a plurality of delay wave components are superimposed on the received signal, complex delay profiles (α ₀ , α ₀₎ corresponding to the delay time difference of each component and the amplitude of the delay wave are added to the output of the component extraction correlator 2. ₁ ,
α ₂ ,...) are output. Here, α _i represents a complex coefficient of the i-th delay component. This delay profile is supplied to the basic waveform generator 3. The basic waveform generator 3 generates a basic waveform for one time slot as shown in FIG. 3 with respect to the delay time of the delay profile. This basic waveform is supplied to each corresponding correlator in the basic waveform correlator 4.

The modulated wave at the input terminal 1 further includes a signal of a code sequence portion for transmitting information, which is supplied to the basic waveform correlator 4. The basic waveform correlator 4 correlates each fundamental wave from the basic waveform generator 3 with a code sequence portion. This correlation output is stored in a corresponding area in the correlation memory 5. Each area has a storage capacity corresponding to the constraint length of the signal. In the case where the modulated wave has the waveform shown in FIG.
Since it is a time slot, four stages of memory are required.

The distance calculator 7 includes a correlation memory 5, a degenerated path memory 6,
And a signal space distance based on the storage contents of the state transition memory 9. The signal space distance is an index indicating the similarity between the complex envelope R (t) of the received signal and some expected signal waveforms U _k (t), It is represented by Here, the complex envelope R (t) is a direct wave E
(T) and E (t−T) obtained by delaying this wave by time T, R (t) = α ₀ E (t) + α ₁ E (t−T) + N (T) ...
... (6) Here, N (t) represents noise. Also, U _k (t)
Is expressed as follows: U _k (t) = α ₀ E _k (t) + α ₁ E _k ′ (t) (7) Here, E _k (t) and E _k ′ (t) are basic waveforms output by the basic waveform generator 3.

Therefore, the integral term F (t) is, F (t) = | R (t) | 2 + | α 0 | 2 | E k (t) | 2 + | α 1 | 2 | E k '(t) | ² + 2Re _{[α 0 R (t) E k} * (t) ] + 2Re _{[α 1 R (t) E k} '* (t) ] + 2Re _{[α 0 α 1 E k (t} ) E k' (t) ] (8) The first, second, and third terms are signal levels, and their integral values can be easily obtained. Clauses 4 and 5
The term is a correlation between the received signal and the fundamental wave, and can be obtained from the contents stored in the correlation memory 5. However, which one of the stored contents is used depends on the history of the state, so the state transition memory 9 storing the history of the state transition
See Since it is determined recursively by the signal space distance (5), the state transition memory 9 also stores the signal space distance D _s to (m-1) T. The sixth term represents an inner product value between the basic signal waveforms. For this value, a value calculated in advance is stored in the degenerated path memory 6 as a degenerated path table.

What is the degenerate path table? Among the paths determined by the state transition, the paths are relatively grouped.
For example, the paths transitioning to 13, 16, 1, 2, and 9 indicated by thick lines in FIG. 2 are considered to be examples of the degenerate path as shown in FIG. In the degenerate path memory 6, in addition to the above inner product values, paths that can be taken in the next step are stored as a table. From this table, the next state can be determined. In this way, the signal space distances d _k1 and d _k2 from (m−1) T to mT when the state transits to the next two states with respect to the state k are obtained.

The state determiner 8 determines the state according to the Viterbi algorithm since the state transitions from the two states to the state of the transition destination. When a new state is determined by the state determiner 8, the state transition memory 9 regards that a modulated wave corresponding to the state having the smallest signal space distance D _k (n) has been received, and determines a code corresponding thereto. The sequence is output to decoder 10.

FIG. 5 shows the error rate characteristics of the receiver of the present embodiment and the receiver of the conventional example. In this characteristic diagram, "1" component of the direct wave, components of the delayed wave of the delay time 3T indicates an error rate characteristic when e ^j. As shown in this figure, the receiver of this embodiment has significantly improved characteristics, and can receive and demodulate with good characteristics even when a nonlinear modulated wave is received as a discrete delay wave.

〔The invention's effect〕

As described above, the maximum likelihood receiver of the present invention can satisfactorily receive a signal even when linear distortion occurs in a nonlinear modulation wave due to band characteristics of a transmission path. Moreover, the maximum likelihood receiver of the present invention can simplify the calculation using the degenerated path table, and thus has the effect of performing high-speed processing. Further, there is an effect that the calculation process is simplified and power consumption can be reduced.

[Brief description of the drawings]

FIG. 1 is a block diagram of a maximum likelihood receiver according to an embodiment of the present invention. FIG. 2 is a diagram showing a trellis representing a change in φ (t). FIG. 3 is a diagram showing a trellis of an envelope represented by I (t) = cos φ (t). FIG. 4 is a diagram showing an example of a degenerated path. FIG. 5 is a diagram showing error rate characteristics of the receiver of the present embodiment and the conventional receiver. FIG. 6 is a block diagram of a first conventional maximum likelihood receiver. FIG. 7 is a block diagram of a second conventional maximum likelihood receiver. 1 ... Input terminal, 2 ... Correlator for component extraction, 3 ... Basic waveform generator, 4 ... Basic waveform correlator, 5 ... Correlation memory, 6 ... Degenerate path memory, 7 ... Distance calculation Vessel, 8 ...
... A state determiner, 9... A state transition memory, 10.
11 ... output terminal, 61 ... matched filter, 62, 62-1 to 62
-5 ... sampling circuit, 63 ... maximum likelihood code sequence estimator, 71-
1-71-5: multiplier; 72-1 to 72-5: integrator.

 ────────────────────────────────────────────────── ─── Continuation of front page (56) References Technical report of IEICE, Vol. 88, No. 40, p. 7-12 Proceedings of the 1988 IEICE Spring Conference, [Volume B-1], p. 1-476

Claims (2)

(57) [Claims] 1. A fundamental wave generating means for generating a fundamental wave corresponding to a modulated wave of a received digital signal; a correlating means for obtaining a correlation between the fundamental wave generated by the means and the received digital signal; A maximum likelihood code sequence estimating means for obtaining a maximum likelihood code sequence for the output, wherein the fundamental wave generating means detects a component of a specific code sequence included in the received digital signal, and A maximum likelihood receiver comprising: means (2) for obtaining a delay component included; and means (3) for generating a fundamental wave corresponding to each of the delay components. 2. A maximum likelihood code sequence estimating means, comprising: a correlation memory (5) for storing an output of the correlating means over a predetermined time slot; and a state up to a plurality of current states estimated on the receiving side. A state transition memory (9) for storing transition history data, a relation between a plurality of degenerate paths that extract a relative relation between state transition histories, and inner product values of signal waveforms of respective time slots in each path; And a degenerate path memory (6) for storing the relationship between the next degenerate path that is continuous with one degenerate path; storage contents of the correlation memory; output of means for obtaining a delay component; A distance calculator (7) for calculating a signal spatial distance in a time slot to a next state continuous with a plurality of current states based on the stored contents and the stored contents of the degenerated path memory; And a means for outputting a code of a code sequence corresponding to a state history of a state having the smallest cumulative signal spatial distance of a plurality of determined current states. Item 2. The maximum likelihood receiver according to Item 1.