JPH06232938A - Delay detection method - Google Patents

Abstract

(57) [Abstract] [Purpose] An error rate is detected by detecting the envelope level of two or more received symbols in a delay detection method for an amplitude / phase modulation signal whose amplitude and phase are differentially encoded. To improve. [Configuration] In scheme amplitude and whose phase delay detection the amplitude phase modulated signal differentially encoded, received waveform is sampled in synchronization with the symbol timing, as the received symbol 01 (r _k) at discrete time k Envelope detector
Input to 200. In FIG. 2, the envelope level (amplitude) 10 of the received symbol output from the envelope detector 200
Are input to the plurality of delay circuits 300 and 301, and the output signals 11 and 12 from the delay circuits are input to the determination 500. Judge
At 500, the amplitude 10 of the received symbol 01 (r _k ) at time k is detected based on the envelope levels 11 and 12 of the received symbols that are temporally continuous by the number of delay circuits, and the amplitude information 30 is output. .

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 digital modulation / demodulation technology, and more particularly to a demodulation system for differentially encoded digital modulation signals.

[0002]

2. Description of the Related Art There are roughly two methods for detecting a modulated signal in a digital modulation / demodulation system. One is a synchronous detection in which a carrier wave is reproduced, a carrier wave component is removed from a modulated signal and an information signal is detected, and the other is an asynchronous detection in which a carrier wave is demodulated without being reproduced. Generally, asynchronous detection includes envelope detection in the amplitude modulation method,
In the present application, since only the modulation signal that is differentially encoded is targeted, only the conventional technique of differential detection among asynchronous detection will be described. In general, if the propagation environment is static, the characteristics of synchronous detection are superior to those of differential detection, but when the propagation environment fluctuates drastically like in mobile communications, it is difficult to completely reproduce the carrier wave. Delay detection is suitable.

The differential detection method has been widely used so far because of differentially encoded QPSK and π / 4 shift QPSK.
This is a phase modulation method represented by K. That is, this is a method of modulating the information to be transmitted as the phase difference of the transmission signal. This method of differentially detecting the transmission signal uses the phase of the reception signal point one symbol before as the reference phase when detecting the next reception symbol. In addition, a method for improving the error rate performance by using not only the immediately preceding received symbol but the phase of multiple received signal points as the reference phase has been proposed by D. Divsalar et al. (Dariush Divsalar, 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 in which both phase and amplitude are modulated. The differential detection method targeted by the present application is a technique applied to demodulate an amplitude / phase modulation signal in which amplitude and phase are differentially encoded. Here, the star 16QAM (16DAPSK) system is taken as an example of the amplitude / phase modulation system, the differential encoding system is described first, and then the conventional differential detection system is described with reference to FIG. For the prior art, refer to the literature (WTWebb, L.Hanzo, R.Steele, "Bandwidth
efficient QAM schemes for Rayleigh fading channe
ls ", IEE Proceedingd-I, Vol.138, No.3, June 1991
) Is described in detail. In the star 16QAM modulation method, 3 bits of 4 bits of transmission information of 1 symbol are differentially encoded and phase modulated (8DPSK), and the remaining 1 bit is differentially encoded and amplitude modulated (DASK). If the two amplitude levels that a DASK-modulated signal can have are A1 and A2 (A1 <A2), the amplitude level of the modulated signal will change (A1 → A2 or A2 → A1) let
If they are the same, the amplitude levels remain the same. Let S _{k be the} transmitted symbol at time k

[0005]

## EQU1 ## 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 16 QAM. The received waveform is sampled in synchronization with the symbol timing, and the received symbol 01 (r _k ) at the discrete time k is input to the envelope detector 200 and the phase detector 190. Where rk is a complex number,

[0007]

## EQU00002 ## Let r _k = | r _k | exp (jθ _k ).

In the phase detector 190, the phase difference between the received symbol r _k and the received symbol r _k-1 one symbol before is Δθ _k =
(Θ _k −θ _k−1 ) is obtained, and eight phase differences Δφ _k (= φ _k −φ _k-1 = 0, ± π / 4, ± π / 2, which the modulated signal can take are obtained.
± 3 π / 4, and outputs the closest phase difference [Delta] [phi _'k out of [pi).

In the envelope detector 200, the amplitude of the received symbol
10 (| r _k |) is output. Amplitude 10
Two thresholds γ _L such that the ratio of (| r _k |) to the amplitude 11 (| r _k-1 |) one symbol before is such that γ _L <γ _H ,
compared to γ _H ,

[0010]

## EQU3 ## When γ _L <| r _k | / | r _k-1 | <γ _H is satisfied, the amplitude ratio information 30 (1 bit /
1 symbol) value is "0", otherwise "1"
And

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

All of the above three characteristics cause deterioration of error rate characteristics after demodulation. The reason for this is first described in terms of features, since the amplitude and phase error events are not independent. Next, the characteristics and causes the deterioration of characteristics,
In order to detect the next received symbol r _{k based} on the amplitude and phase of only the received symbol r _k−1 one symbol before,
This is because both the noise component received one symbol before and the noise component received by the received symbol r _k to be detected are superimposed. Therefore, it has a drawback that the noise component becomes about twice as large as that of the ideal synchronous detection in the sense that the carrier wave 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.

From such a viewpoint, as described above, in the phase modulation method, a differential detection method over a plurality of received symbols has been proposed in a paper by D. Divsalar et al. It is possible to apply this technique to the amplitude phase modulation system represented by the Star 16 QAM system. That is, it is possible to improve the deterioration of the error rate characteristic due to the characteristics of the differential detection method according to the related art described above. That is, N received symbol sequences (r _{k-N + 1} , r _{k-N + 2} , ..., r _k ) that are temporally continuous.
From the transmission phase difference sequence (Δφ _{k-N + 2} , Δφ _{k-N + 3} , ...
..., those chosen to method for estimating the [Delta] [phi _K) is to maximize the value A by (1).

[0014]

[Equation 4]

Equation (1) is derived under the assumption that all transmitted symbols are of equal amplitude. Further, the improvement of the characteristic deterioration due to the characteristics can be achieved by the conventional technique as long as it is the differential detection system in which only the received symbol one symbol before is used as a reference. However, no method has been invented for improving the deterioration of the error rate characteristic due to the characteristics. The feature is a factor of deterioration of the error rate characteristic of the amplitude ratio information, and improvement is desired.

[0016]

In the differential detection method, in which only the received symbol one symbol before is used as the reference when detecting the next received symbol, the noise is approximately doubled as compared with the ideal synchronous detection method. Increased characteristics deteriorate. That is, the noise component received one symbol before and the noise component received by the received symbol to be detected next are both superimposed. It is also possible to apply the already known multi-symbol delay detection method in the phase demodulation method to the detection of the phase information in the amplitude phase modulation / demodulation method, but the detection characteristic of the amplitude information is not improved.

The differential detection system of the present invention overcomes the above-mentioned drawbacks, and particularly aims to improve the detection characteristic of the amplitude part, reduce the deterioration of the characteristic due to noise components, and improve the error rate characteristic.

[0018]

The present invention is characterized in that, when differentially detecting an amplitude-phase modulated signal that has been differentially encoded, it is detected with reference to 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, the number of received symbols is set to 3 for simplicity. 3 consecutively in time
The number of transmitted symbol sequences is S = (S _k-2 , S _k-1 ,
S _K ), the received symbol sequence is r = (r _K-2 , r _k-1 , r
_k ) and S _i = a _i exp (jφ _i ), r _i = | r _i |
It is expressed as exp (jθ _i ). a _i is the transmission amplitude, φ _i is the transmission phase, θ _i is the reception phase, and S _i and r _i are the transmission and reception symbols of the i-th symbol, respectively. Here, the amplitude sequence of transmission symbols is represented by the vector a = (a _k-2 , a _k-1 ,
a _k ), the phase difference sequence of the transmitted symbols is represented by the vector Δφ =
(Δφ _k-1 , Δφ _k ), = (φ _k-1 −φ _k-2 , φ _k −φ
_k-1 ), the amplitude ratio sequence of the received symbol is the 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 the vector Δθ = (Δ
If θ _k-1 , Δθ _k ) = (θ _k-1 −θ _k-2 , θ _k −θ _k-1 ), the vector γ and the vector γ are transferred under the condition that the vector a and the vector Δφ are sent. 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] Generally, when demodulating over the number of Na (Na ≧ 3) received symbols, the expression (3) is expanded to the expression (4). (4)
formula

[0022]

[Equation 7]

[0023]

[Embodiment 1] FIG. 1 is a block diagram of a differential detection system realized by the present invention for obtaining a likelihood over three symbols. Envelope detector 200, two 1-symbol delay circuits 300
, 301, 7 phase detectors 100, 101, 102, 110, 11
2, 120, 121, a judgment device 400 and a selector 800. This system has a structure for detecting a modulated signal having two amplitude levels. In the following explanation,
The two amplitude levels are A _{1 and} A ₂ . The received waveform is sampled in synchronization with the symbol timing, and the received symbol 01 (r _k ) at the 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 represented by a complex number. From the envelope detector 200, the amplitude of the received symbol 01 (r _k ) is 10
(| R _k |) is output. The reception amplitude 10 is input to the 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 has a reception amplitude of 10 (|
r _k |), reception amplitude 11 (| r _k-1 |), reception amplitude 12 (|
r _k-2 |) is input.

Phase detectors 100, 101, 102, 110, 112
, 120 and 121 output the phase difference with the highest likelihood and its likelihood value based on different likelihood calculation methods. Likelihood values 41 _, 42 _, 43 _, 44 _, 45 _, 46 _, 47 of the phase detector are
To the phase output 21 _, 22 _, 23 _, 24 _, 25 _, 26 _, 27 selector 800
Respectively input to. The seven phase detectors are three time-sequential received symbol sequences (r _k-2 , r _k-1 , r
from _k) (5) based on the equation of all possible retardation sequence (Δφ _k-1, perform likelihood calculation of [Delta] [phi _k), the largest likelihood value and the phase difference information and the likelihood value becomes Output.

[0025]

[Equation 8]

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
(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 ), in the phase detector 120 (A ₂ , A ₁ ,
A ₁ ), in the phase detector 121 (A ₂ , A ₁ , A ₂ ), the maximum likelihood value output from these seven phase detectors and the received amplitude (| r
_{Based on k-2} |, | r _k-1 |, | r _k |), likelihood calculation based on equation (3) is performed for seven possible transmission amplitude sequences, and the transmission amplitude sequence with the highest likelihood is calculated. judge.

A selection signal 50 for outputting the output phase difference information of the phase detector which performs the likelihood calculation of the phase difference sequence based on the transmission amplitude sequence maximizing the equation (3) as the phase difference information 40. , Judgment device 400 outputs to selector 800. For example, 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. Also, the amplitude ratio information 30 is output from the transmission amplitude sequence judged by the judging device 400. Here, the amplitude ratio information is uniquely determined by whether or not the amplitudes of two symbols that are continuous in time have 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 block diagram of a differential detection system which is implemented by the present invention and which calculates a likelihood over three symbols. Envelope detector 200, two 1-symbol delay circuits 30
0, 301, one phase detector 190 and a decision device 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. This simplifies the configuration as compared with the first embodiment. Similar to the first embodiment, this system is a configuration for detecting a modulated signal having two amplitude levels, and the two amplitude levels are A ₁ and A ₂ .

Received symbol 01 from envelope detector 200
│r _k │ which is the amplitude 10 of (r _k ) is output. The reception amplitude 10 (| r _k |) is input to the delay circuit 300 which delays the output by one symbol timing. First stage delay circuit 30
The output 11 of 0 is | r _k-1 |, and the output 12 of the second stage is | r _k-2 |
Is. To the determiner 500, three reception amplitudes 10 (| r _k |), reception amplitudes 11 (| r _k-1 |), and reception amplitudes 12 (| r _k-2 |) that are temporally consecutive are input simultaneously. . The received symbol 01 is also input to the phase detector 190. Phase detector 190
Outputs phase difference information 20. As the phase detector 190, a delay 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 as well as the judgment device.
Input to 500. The discriminator 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 3 symbols. For all possible transmission amplitude sequences (a _k-2 , a _k-1 , a _k ), the likelihood is calculated based on equation (3), the transmission amplitude sequence with the highest likelihood is determined, and the amplitude ratio is calculated. Output as information 30.

[0030]

As described above, according to the present invention,
The error rate after demodulation can be improved by differentially detecting the amplitudes of multiple symbols under the condition that white noise is added to the communication channel using the differential amplitude phase modulation / demodulation method. Generally, the larger the number of symbols to be subjected to the likelihood calculation, the more the error rate characteristic is improved, but the bit error rate is improved by about 1 dB at 10 ⁻³ even with about 3 symbols as in the first or second embodiment. .

[Brief description of 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 is a configuration of a differential detection system according to a conventional technique.

[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 (conventional technology) 200 Envelope detector 300,301 1-symbol delay circuit 400,500,700 Likelihood detector 800 Selector

Claims (2)

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