Robust digital modulation and demodulation scheme for radio...

Details Robust digital modulation and demodulation scheme for radio... Robust digital modulation and demodulation scheme for radio...

1998-11-19

2002-02-26

Chin, Stephen (Department: 2634)

Pulse or digital communications

Systems using alternating or pulsating current

Plural channels for transmission of a single pulse train

C375S261000, C375S298000, C375S316000, C370S335000, C370S529000

Reexamination Certificate

active

06351498

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a digital modulation and demodulation scheme for radio communications between a terminal and a base station such as those of the mobile communication, and more particularly, to a digital modulation and demodulation scheme for radio communications which is robust against variation of receiving signal strength due to fading and capable of reducing occurrences of errors.
2. Description of the Background Art
There are various known schemes for digital modulation and demodulation for radio communications between a terminal and a base station such as those of the mobile communication, and the M-ary modulation and demodulation scheme using orthogonal codes is one such scheme which is attracting much attentions recently.
FIG. 1
shows a configuration of a conventional M-ary modulation and demodulation system, which comprises an M-ary encoder
101
, a PSK (Phase Shift Keying) modulator
102
, a synchronous detector
103
, and an M-ary decoder
104
. Digital signals entered at a modulation input terminal are divided into blocks of L bits size each (where L is a natural number greater than 1) in advance such that each L bits data series can be regarded as an L bits code, and the M-ary encoder
101
generates orthogonal codes of M=2
L
bits length each which are uniquely defined with respect to respective codes. For example, in an exemplary case of L=2 shown in
FIG. 2
, M=4 so that 2 bits (a
1
, a
2
) of the entered data are converted into one of the 4 bits length orthogonal codes C
1
to C
4
according to a conversion rule shown in
FIG. 2
, and then outputted. The PSK modulator
102
applies the bi-phase shift keying (BPSK) to a carrier by using this orthogonal code. At the receiving side, a signal is detected by the synchronous detector
103
, and the M-ary decoder
104
calculates cross-correlation value by multiplying the detected signal with each one of the four possible orthogonal codes C
1
to C
4
, and determines the orthogonal code with the highest cross-correlation value as a received signal. In addition, the M-ary decoder
104
outputs 2 bits of the original data corresponding to the determined orthogonal code according to the conversion rule shown in FIG.
2
.
As is well known, biorthogonal codes can be used instead of ordinary orthogonal codes, and M=2
L−1
(L≧3) in the case of using biorthogonal codes.
In the M-ary modulation and demodulation scheme, mutually orthogonal codes are used for different input data so that the cross-correlation between signals becomes zero, and therefore it has a characteristic of having a small interference in the identical channel. This characteristic makes it convenient as a modulation and demodulation scheme in the case of multiplexing a plurality of signals at the identical frequency as in the CDMA (Code Division Multiple Access) scheme.
However, in the mobile communication environment, in general, bursty errors occur due to thermal noise and abrupt carrier phase rotation that are caused by frequent dropping of receiving power due to fading.
FIG. 3
shows a situation of an occurrence of an error due to fading in the example (L=2) shown in FIG.
2
. In
FIG. 3
, when a time Tf of receiving power dropping due to fading extends over a plurality of bits, the orthogonal code that encountered this receiving power dropping due to fading (which appear shaded in
FIG. 3
) is difficult to detect using the correlation because of thermal noise, and therefore it has a higher probability of being decoded erroneously as another orthogonal code, and when such an erroneous decoding occurs, the bursty error of about L bits length will be caused.
Thus the conventional M-ary modulation and demodulation scheme has been associated with the problem that the bursty error is caused by the receiving power dropping due to fading.
Next, the operation of a decoder in the conventional modulation and demodulation system will be described in further detail.
FIG. 4
shows a configuration of a transmitting side of the conventional modulation and demodulation system. Here, it is assumed that orthogonal code generators
203
-
1
to
203
-
4
employ 4-ary orthogonal codes obtained from 4×4 Hadamard matrix, and therefore a shift register
202
stores data in 2 bits length. It is also assumed that a frequency modulator
205
employs the 4-valued frequency modulation (G=2, 2
G
=4).
In
FIG. 4
, the input data entered from an input terminal
201
is stored in the shift register
202
. When the input data in a prescribed length of 2 bits (a
1
, a
2
) is stored in the shift register
202
, a processor
204
reads lout these bits from the shift register
202
, selects a corresponding one of the orthogonal code generators
203
-
1
to
203
-
4
according to a conversion rule shown in
FIG. 5
, and outputs a bit sequence outputted from the selected orthogonal code generator to the frequency modulator
205
. Then, the frequency modulator
205
converts the entered 2 bits (b
1
, b
2
) or (b
3
, b
4
) into 4-valued symbols (S
1
, S
2
, S
3
, S
4
) because G=2, and outputs four frequencies (&ohgr;c±&ohgr;
1
, &ohgr;c±&ohgr;
2
, where &ohgr;c is the carrier frequency) corresponding to these four symbol values, which are transmitted from an antenna
205
a
. The relationship between the input bits, the 4-valued symbols and the transmission frequencies in this case is shown in FIG.
6
.
FIG. 7
shows a configuration of a receiving side of the conventional modulation and demodulation system. The desired radio signals received at a reception unit
211
are entered into four band-pass filters
212
-
1
to
212
-
4
having the central frequencies ranging from &ohgr;c+&ohgr;
2
to &ohgr;c−&ohgr;
2
. Each band-pass filter outputs only a signal having a frequency component in a prescribed bandwidth. The output signals of the band-pass filters are entered into corresponding envelope detectors
213
-
1
to
213
-
4
, where voltage values or power values of the signals are obtained. Here, it is assumed that the voltage values are to be obtained, and the voltage values obtained in correspondence to the symbols (S
1
, S
2
, S
3
, S
4
) will be denoted as (e
1
, e
2
, e
3
, e
4
).
These voltage values are entered into a code judgement device
214
, where the symbol with the largest voltage value among four voltage values entered therein is selected, and bits corresponding to the selected symbol are obtained according to a correspondence table shown in FIG.
6
. The obtained bits are then multiplied with output bits of orthogonal code generators
223
-
1
to
223
-
4
at multiplexers
224
-
1
to
224
-
4
and entered into integrators
215
-
1
to
215
-
4
. These integrators integrate the entered values over a transmission time required for 2 bits that are stored into the shift register at the transmitting side, and enters the obtained values into a decoding unit
216
. This decoding unit
216
selects one input with the largest value among the four inputs, obtains 2 bits corresponding to one of the orthogonal code generators
223
-
1
to
223
-
4
that is used in multiplication of that input, and outputs these bits at an output terminal
217
.
In this conventional decoder, there is a problem in that the code judgement device
214
converts the entered values into bit sequences once so that the output signals of the envelope detectors
213
-
1
to
213
-
4
cannot be directly utilized at the decoder
216
and consequently not much coding gain can be obtained.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a digital modulation and demodulation scheme for radio communications which is capable of reducing errors due to fading while maintaining the characteristic of the M-ary modulation and demodulation scheme that it is robust against interferences.
It is another object of the present invention to provide a decoder for decoding orthogonal codes which is capable of realizing high quality s

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