JP2891444B2 - Delay detection method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a demodulation system in a digital modulation / demodulation technique, and more particularly to a demodulation system for a differentially encoded digital modulation signal.

[0002]

2. Description of the Related Art In a digital modulation / demodulation system, there are roughly two methods for detecting a modulation signal. One is synchronous detection that reproduces a carrier and removes a carrier component from a modulated signal to detect an information signal, and the other is asynchronous detection that performs demodulation without reproducing a carrier. Generally, asynchronous detection includes envelope detection in amplitude modulation,
In the present application, only the differentially coded modulation signal is targeted, and therefore, only the conventional technique of delay detection among asynchronous detection will be described. In general, if the propagation environment is static, the characteristics of synchronous detection are better than those of delay detection, but if the propagation environment fluctuates drastically as in mobile communications, it is difficult to completely reproduce the carrier. Differential detection is suitable.

Until now, differential detection systems have been widely used for differentially encoded QPSK and π / 4 shift QPS.
This is a phase modulation method represented by K. That is, the information to be sent is modulated as the phase difference of the transmission signal. This method of delay-detecting a transmission signal uses the phase of a reception signal point one symbol before as a reference phase when detecting the next reception symbol. D. Divsalar et al. (Dariush Divsalar and Marvin K. Simon, "Multip
le-Symbol Differential Detection of MPSK ", IEEE Tr
ansactions on Communications, Vol.38, No.3, pp.300
-307, March, 1990).

As a differential modulation method for further improving the band efficiency as compared with the QPSK or π / 4 shift QPSK method, there is a differential amplitude phase modulation method for modulating both the phase and the amplitude. The differential detection system to which the present invention is directed is a technique applied to demodulate an amplitude and phase modulated signal whose amplitude and phase are differentially encoded. Here, a star 16QAM (16DAPSK) system is taken as an example of the amplitude and phase modulation system, a differential coding system will be described first, and then a conventional delay detection system will be described with reference to FIG. For the conventional technology, refer to the literature (WTWebb, L. Hanzo, R. Steele, "Bandwidth
efficient QAMschemes for Rayleigh fading channe
ls ", IEE Proceedingd-I, Vol.138, No.3, June 1991
) Is described in detail. In the star 16 QAM modulation method, three bits of four bits of transmission information of one symbol are subjected to differential encoding phase modulation (8DPSK), and the remaining one bit is subjected to differential encoding amplitude modulation (DASK). Assuming that two possible amplitude levels of the DASK-modulated signal are A1 and A2 (A1 <A2), the amplitude level of the signal to be modulated is changed (A1 → A2 or A2 → A1)
If they are the same, the amplitude level remains the same. Let S _{k be the} transmitted symbol at time k

[0005]

Let S _k = a _k exp (jφ _k ).

FIG. 3 is a block diagram of a conventional differential detection system for a signal modulated by star 16QAM. The received waveform is sampled in synchronization with the symbol timing, and the received symbol 01 (r _k ) at discrete time k is input to the envelope detector 200 and the phase detector 190. Where rk is a complex number,

[0007]

It is assumed that r _k = | r _k | exp (jθ _k ).

[0008] The phase detector 190, the received symbols r _k and 1-symbol preceding received symbol r _k-1 of the phase difference [Delta] [theta] _k =
(Θ _k −θ _k−1 ) is obtained, and eight possible phase differences Δφ _k (= φ _k −φ _k−1 = 0, ± π / 4, ± π / 2,
The closest phase difference Δφ ′ _k among ± 3π / 4, π) is output.

In the envelope detector 200, the amplitude of the received symbol
10 (| r _k |) is output. In the decision unit, the amplitude 10
(| R _k |) and two thresholds γ _L , such that the ratio of the amplitude 11 (| r _k−1 |) one symbol before is γ _L <γ _H ,
compared to γ _H ,

[0010]

When satisfying γ _L <| r _k | / | r _k−1 | <γ _H , the amplitude ratio information 30 at time k (1 bit /
1) is set to “0”, otherwise “1”
And

The following three points are characteristic of the differential detection method of the star 16QAM signal according to the prior art. Amplitude detection and phase detection are performed independently. In the detection of the amplitude, the amplitude | r _k-1 |
Is demodulated with reference to the amplitude ratio information (1 bit / 1 symbol). In phase detection, the phase θ just before the symbol
_The phase difference information Δφ _k (3 bits / 1 symbol) is demodulated based on _k−1 .

The above three characteristics all cause deterioration of the error rate characteristics after demodulation. The reason for this is first characterized in that amplitude and phase error events are not independent. Next, the reason why the characteristics and
To detect the next received symbol r _{k based} on the amplitude and phase of only the previous received symbol r _k-1 ,
This is because both the noise component received symbol r _k is received trying to detection and the noise component received before one symbol is superimposed. For this reason, there is a drawback that the noise component is about twice that of the ideal synchronous detection in the sense that the carrier can be completely reproduced. However, if the average value of the noise component is zero, deterioration of characteristics due to noise can be suppressed to a minimum by observing the received signal for a long time.

[0013] From such a viewpoint, as described above, in the phase modulation system, a differential detection system over a plurality of received symbols has been proposed in a paper by D. Divsalar et al. This technique can be applied to an amplitude and phase modulation system represented by a star 16QAM system. That is, it is possible to improve the deterioration of the error rate characteristic caused by the feature of the above-described conventional delay detection method. That temporally continuous with N received symbol sequence _{(r k-N + 1,} r k-N + 2, ......, r k)
From the transmission phase difference sequence (Δφ _{k−N + 2} , Δφ _{k−N + 3} ,.
, Δφ _K ) is selected so as to maximize the value A according to the equation (1).

[0014]

(Equation 4)

Equation (1) is derived on the assumption that all transmission symbols have the same amplitude. In addition, the improvement of the characteristic deterioration caused by the characteristic can be achieved by the conventional technique as long as the method is limited to the differential detection method based on only the received symbol one symbol before. However, no method has been invented for improving the deterioration of the error rate characteristic caused by the feature. The feature is a factor of deterioration of the error rate characteristic of the amplitude ratio information, and improvement is desired.

[0016]

In a delay detection system in which only a received symbol before one symbol is used as a reference for detecting a next received symbol, noise is about twice as large as that in an ideal synchronous detection system. The characteristics increase. That is, both the noise component received one symbol before and the noise component received by the next received symbol to be detected are superimposed. In addition, a known multi-symbol delay detection method in the phase demodulation method can be applied to the detection of phase information in the amplitude / phase modulation / demodulation method, but the detection characteristic of the amplitude information is not improved.

The delay detection system of the present invention overcomes the above-mentioned drawbacks, and in particular, aims to improve the detection characteristics of the amplitude section, reduce the deterioration of the characteristics due to noise components, and improve the error rate characteristics.

[0018]

SUMMARY OF THE INVENTION The present invention is characterized in that when differentially coded amplitude-phase modulated signals are subjected to delay detection, detection is performed based on the amplitudes of two or more received symbols.

[Principle] A method of detecting an amplitude component signal over a plurality of received symbols will be described below. First, assume that the number of received symbols is three for simplicity. Time series 3
The transmitted symbol sequences are represented by S = (S _k−2 , S _k−1 ,
S _K ) and the received symbol sequence is represented by r = (r _K−2 , r _k−1 , r
_k ), and S _i = a _i exp (jφ _i ), r _i = | r _i |
expressed as exp (jθ _i ). a _i represents the transmission amplitude, φ _i represents the transmission phase, θ _i represents the reception phase, and S _i and r _i represent the transmission and reception symbols of the ith symbol, respectively. Here, the amplitude sequence of the transmission symbol is represented by a vector a = (a _k−2 , a _k−1 ,
a _k ), the phase difference sequence of the transmission symbol
(Δφ _k−1 , Δφ _k ), = (φ _k−1 −φ _k−2 , φ _k −φ
_k-1 ), and the amplitude ratio sequence of the received symbol is represented by a vector γ = (γ
_k−1 , γ _k ) = (| r _k−1 / r _k−2 |, | r _k / r _k−1
|) The phase difference sequence of the received symbol is represented by a vector Δθ = (Δ
θ _k−1 , Δθ _k ) = (θ _k−1 −θ _k−2 , θ _k −θ _k−1 ), and the vector γ and the vector γ under the condition that the vector a and the vector Δφ have been sent The conditional probability density function p (vector γ, vector Δθ
| Vector a, vector Δφ) is given by equation (2).

[0020]

(Equation 5) Where σ _N is the standard deviation of Gaussian noise per symbol. Maximizing p in equation (2) is equivalent to maximizing p 'in equation (3) under the condition that σ _N is sufficiently small.

[0021]

(Equation 6) In general, when demodulating over the number of received symbols of Na (Na ≧ 3), equation (3) is extended to equation (4). (4)
formula

[0022]

(Equation 7)

[0023]

[Embodiment 1] FIG. 1 is a block diagram of a differential detection system for realizing likelihood over three symbols realized by the present invention. Envelope detector 200, two 1-symbol delay circuits 300
, 301, 7 phase detectors 100, 101, 102, 110, 11
2, 120, 121, a decision unit 400 and a selector 800. This system is a configuration for detecting a modulated signal having two amplitude levels. In the following description,
Two amplitude levels and A _1, A _2. The received waveform is sampled in synchronization with the symbol timing, and the received symbol 01 (r _k ) at discrete time k is detected by the envelope detector 200.
And phase detectors 100, 101, 102, 110, 112, 120, 1
Entered in 21. Here r _k is expressed by a complex number. From the envelope detector 200, the amplitude 10 of the received symbol 01 (r _k ) is _obtained.
(| R _k |) is output. The reception amplitude 10 is input to a delay circuit 300 which delays the output by one symbol timing. The output 11 of the first-stage delay circuit 300 is | r _k-1 | _{, and} the output 12 of the second-stage delay circuit 301 is | r _k-2 |. Judge
400 is the received amplitude of three consecutive symbols 10 (|
r _k |), reception amplitude 11 (| r _k-1 |), reception amplitude 12 (|
r _k−2 |).

Phase detectors 100, 101, 102, 110, 112
, 120 and 121 output the phase difference with the highest likelihood and the likelihood value based on the different likelihood calculation methods. The likelihood value 41 _, 42 _, 43 _, 44 _, 45 _, 46 _, 47 of the phase detector is the decision unit 400.
Phase outputs 21 _, 22 _, 23 _, 24 _, 25 _, 26 _, 27
Are input respectively. The seven phase detectors receive three temporally consecutive received symbol sequences (rk _-2 , rk _-1 , r).
_k ) to (5), the likelihood of all possible phase difference sequences (Δφ _k−1 , Δφ _k ) is calculated, and the phase difference information having the largest likelihood value and the likelihood value are calculated. Output.

[0025]

(Equation 8)

In the equation (5), the transmission amplitude sequence vector a =
(A _k-2 , a _k-1 , a _k ) is different for each phase detector,
Takes the following values: In the phase detector 100, (A ₂ , A ₂ , A ₂ ), in the phase detector 101, (A ₁ , A ₁ , A ₂ ), the phase detector 102
In _{(A 2, A 2, A} 1), in the phase detector _{110 (A 1, A 2,} A 2), in the phase detector 112 (A _1,
A ₂ , A ₁ ), and in the phase detector 120, (A ₂ , A ₁ ,
A ₁ ), in the phase detector 121, (A ₂ , A ₁ , A ₂ ) The decision unit 400 provides the maximum likelihood value output from these seven phase detectors and the reception amplitude (| r
_k−2 |, | r _k−1 |, | r _k |), the likelihood calculation based on the equation (3) is performed for seven possible transmission amplitude sequences, and the transmission amplitude sequence with the highest likelihood is determined. judge.

A selection signal 50 for outputting as output phase difference information 40 phase difference information of a phase detector that performs likelihood calculation of a phase difference sequence based on a transmission amplitude sequence that maximizes equation (3). Are output from the decision unit 400 to the selector 800. For example, if the transmission amplitude sequence that maximizes equation (3) is (A ₂ , A ₁ ,
If A ₁ ), the output phase difference information 26 of the phase detector 120 is selected as the phase difference information 20. The amplitude ratio information 30 is output from the transmission amplitude sequence determined by the determiner 400. Here, the amplitude ratio information is uniquely determined by whether or not the amplitude of two temporally continuous symbols has changed. For example, if the transmission amplitude sequence is (A ₂ , A ₁ , A ₁ ) For example, the amplitude ratio information is “10”.

[0028]

[Embodiment 2] FIG. 2 is a configuration diagram of a differential detection method for obtaining likelihood over three symbols, implemented according to the present invention. Envelope detector 200, two 1-symbol delay circuits 30
0, 301, one phase detector 190 and a decision unit 500. The difference from the first embodiment is that there is only one phase detector. That is, in the first embodiment, the determiner determines the amplitude ratio information and the phase difference information, whereas in the present embodiment, the determiner 500 determines only the amplitude ratio information. Thus, the configuration is simplified as compared with the first embodiment. This system is a configuration for detecting a modulated signal having two amplitude levels, as in the first embodiment, and the two amplitude levels are A ₁ and A ₂ .

From the envelope detector 200, the received symbol 01
| R _k | which is the amplitude 10 of (r _k ) is output. The reception amplitude 10 (| r _k |) is input to a delay circuit 300 that delays the output by one symbol timing. First stage delay circuit 30
The output 11 of 0 is | r _k-1 | The output 12 of the second stage is | r _k-2 |
It is. Three reception amplitudes 10 (| r _k |), 11 (| r _k-1 |), and 12 (| r _k-2 |) reception amplitudes, which are _{consecutive in} time, are simultaneously input to the decision unit 500. . The received symbol 01 is input to the phase detector 190. Phase detector 190
Outputs the phase difference information 20. As the phase detector 190, a differential detector for a differential phase modulation signal which can be realized by a conventional technique can be used as it is. The output from the phase detector 190 is output as the phase difference information 20 and the
Entered into 500. The decision unit 500 removes (A ₁ , A ₁ , A ₁ ) from the received amplitude sequence (| r _k−2 |, | r _k−1 |, | r _k |) and the phase difference information 20 over three symbols. The likelihood is calculated for all possible transmission amplitude sequences ( _ak-2 , _ak-1 , _ak ) based on equation (3), the transmission amplitude sequence having the highest likelihood is determined, and the amplitude ratio is determined. Output as information 30.

[0030]

As described above, according to the present invention,
The error rate after demodulation can be improved by performing delay detection on the amplitudes of a plurality of symbols under the condition that white noise is added to a communication channel using the differential amplitude phase modulation / demodulation method. In general, the more the number of symbols to be subjected to likelihood calculation, the more the error rate characteristics are improved. However, as in the first or second embodiment, the bit error rate is improved by about 1 dB at 10 ^-3 even with about 3 symbols. .

[Brief description of the drawings]

FIG. 1 is a first embodiment of a differential detection system configured according to the present invention.

FIG. 2 is a second embodiment of the differential detection system configured according to the present invention.

FIG. 3 shows a configuration of a conventional differential detection system.

[Explanation of symbols]

The amplitude of the received symbol r _k in the received symbols r _k 10 time k at 01 time k | r _k | 11 time k-1 received symbol r _k-1 of the amplitude at | r
_k-1 | 12 Amplitude of received symbol r _k-2 at time k-2 | r
_k-2 | 20,21,22,23,24,25,26,27 Phase difference information 30 Amplitude ratio information 41,42,43,44,45,46,47 Phase difference information likelihood 50 Selection signal 100,101,102,110,112,120,121 phase Detector 190 Phase detector (prior art) 200 Envelope detector 300,301 One symbol delay circuit 400,500,700 Likelihood determinator 800 Selector

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int. Cl. ⁶ , DB name) H04L 27/00-27/38

Claims (2)

(57) [Claims] 1. A delay detection system for demodulating an amplitude and phase modulated signal whose amplitude and phase are differentially encoded without the need of reproducing a carrier, comprising means for detecting an envelope level and a phase of a received symbol. , Based on the envelope level of two or more symbols,
A differential detection method that estimates the most likely transmission symbol. 2. The differential detection method according to claim 1, wherein a most probable transmission symbol is estimated by simultaneously considering a phase amount of two or more symbols.