Multiplex communications – Generalized orthogonal or special mathematical techniques – Plural diverse modulation techniques
Reexamination Certificate
1998-03-12
2001-08-21
Ngo, Ricky (Department: 2731)
Multiplex communications
Generalized orthogonal or special mathematical techniques
Plural diverse modulation techniques
C370S207000, C370S208000, C370S478000, C375S261000
Reexamination Certificate
active
06278686
ABSTRACT:
1. FIELD OF THE INVENTION
1.1 General field
The field of the invention is that of the transmission or broadcasting of digital data, or of analog and sampled data, designed to be received in particular by mobile receivers. More specifically, the invention relates to signals produced by means of new forms of modulation as well as the corresponding techniques of modulation and demodulation.
For many years now, it has been sought to build modulation schemes adapted to highly non-stationary channels, such as channels for transmission towards mobile receivers. In such channels, the signal sent out is affected by phenomena of fading and multiple paths. The work carried out by the CCETT within the framework of the European project EUREKA 147 (DAB: Digital Audio Broadcasting) has shown the value, for this type of channel, of multicarrier modulation (MCM) and especially of OFDM (Orthogonal Frequency Division Multiplexing).
OFDM has been chosen within the framework of this European project as the basis of the DAB standard. This technique can also be envisaged as a modulation technique for the broadcasting of television programs. However, it has been observed that there are a certain number of limitations (specified hereinafter) in dealing with the problem of modulation encoded with high spectral efficiency such as the modulation required for digital television applications.
1.2 Possible applications
The invention can be applied in many fields, especially when high spectral efficiency is desired and when the channel is highly non-stationary.
A first category of applications relates to terrestrial digital radio-broadcasting, whether of images, sound and/or data. In particular, the invention can be applied to synchronous broadcasting which intrinsically generates long-term multiple paths. It can also advantageously be applied to broadcasting toward mobile receivers.
Another category of applications relates to digital radiocommunications. The invention can be applied especially in systems of digital communications at high bit rates with mobile receivers, in the framework for example of. the UMTS (RACE project). It can also be envisaged for high bit rate local radio networks (of the HIPERLAN type).
A third category of applications is that of underwater transmission. The transmission channel in underwater acoustics is highly disturbed because of the low speed of transmission of acoustic waves in water. This leads to a major spread of the multiple paths and of the Doppler spectrum. The techniques of multicarrier modulation, and especially the techniques that are an object of the present invention, are therefore well suited to this field.
2. PRIOR ART
2.1 Theoretical observations on the representation of the signals
Before presenting the signals according to the invention, a description is given here below of the known signals. This description is based on a general approach to multicarrier signals defined by the inventors. This approach is novel per se. This general approach has indeed no equivalent in the prior art and is no way obvious to those skilled in the art. It must therefore be considered to be a part of the invention and not as forming part of the prior art.
The signals of interest are real signals (an electrical magnitude for example), that have finite energy and are a function of time. The signals may therefore be represented by real functions of L
2
(R). Furthermore, these signals are limiteband w signals and their spectrum is contained in
[
f
c
-
w
2
,
f
c
+
w
2
]
,
f
c
being the “carrier frequency” of the signal. It is therefore possible, in an equivalent manner, to represent a real signal a(t) by its complex envelope s(t) with:
s
(
t
)=
e
−2iwƒ
c
t
F
A
[a
](
t
) (1)
where F
A
designates the analytical filter.
The signal s(t) belongs to a vector subspace
(
characterized
by
the
band
limitation
to
±
w
2
)
of the space of the complex functions of a real variable with a summable square L
2
(R). This vector space can be defined in two different ways, depending on whether the construction is done on the field of the complex values or on the field of the real values. With each of these spaces, it is possible to associate a scalar product that takes values in C or in R to build a Hilbertian space. H designates the Hilbertian space built on the field of the complex values and H
R
designates the Hilbertian space built on the field of the real values.
The corresponding scalar values are written as follows:
(
x
|
y
)
=
∫
R
x
(
t
)
y
*
(
t
)
ⅆ
t
(
2
)
in the case of H and
(
x
|
y
)
R
=
ℜe
∫
R
x
(
t
)
y
*
(
t
)
ⅆ
t
(
3
)
in the case of H
R
The associated standards are obviously identical in both cases:
&LeftDoubleBracketingBar;
x
&RightDoubleBracketingBar;
=
[
∫
R
&LeftBracketingBar;
x
(
t
)
&RightBracketingBar;
2
ⅆ
t
]
1
/
2
(
4
)
2.2 General principles of the OFDM
The general principles of the OFDM are presented for example in the French patent FR-86 09622 filed on Jul. 2, 1986. The basic idea of the technique is that of transmitting encoded signals as coefficients of elementary waveforms that are confined as far as possible in the time-frequency plane and for which the transmission channel may be considered to be locally stationary. The channel then appears to be a simple multiplier channel characterized by the distribution of the modulus of the coefficients which follows a law of Rice or of Rayleigh.
EProtection is then provided against fading phenomena by means of a code. This code can be used in soft decision mode in association with time and frequency interleaving that ensures that the signals playing a part in the minimum meshing of the code are affected, to the utmost possible extent, by independent fading phenomena.
This technique of encoding with interleaving in the time-frequency plane is known as COFDM. It is described for example in the document [23] (see Appendix 1 (to simplify the reading, most of the prior art references are listed in Appendix 1. This Appendix as well as Appendices 2 and 3 must of course be considered to be integral parts of the present description)).
There are two types of known OFDM modulation. The terms applied in the literature are often ambiguous. Here we introduce new appellations that are more precise while recalling their correspondence with the exiting literature. We shall use the generic name OFDM followed by a suffix specifying the type of modulation within this group.
2.3 OFDM/QAM
2.3.1 Theoretical principles
A first category of modulation is considered to be a multiplex of QAM (Quadrature Amplitude Modulation) modulated carriers or possibly QPSK (Quadrature Phase Shift Keying) modulated carriers in the particular case of binary data elements. Hereinafter, this system shall be called OFDM/QAM. The carriers are all synchronized and the carrier frequencies are spaced out in reverse to the symbol time. Although the spectra of these carriers overlap, the synchronization of the system makes it possible to ensure orthogonality between the symbols sent out by the different carriers.
The references [1] to [7] give a good idea of the literature available on this subject.
For greater simplicity in the writing, and according to the novel approach of the invention, the signals will be represented by their complex envelope described here above. Under these conditions, the general equation of an OFDM/QAM signal is written as follows:
s
(
t
)
=
∑
m
,
n
a
m
,
n
x
m
,
n
(
t
)
(
5
)
The coefficients a
m,n
take complex values representing the data sent. The functions x
m,n
(t) are translated into the time-frequency space of one and the same prototype function x(t):
x
(
t
)
=
{
1
τ
0
if
&LeftBracketingBar;
t
&RightBracketingBar;
≤
τ
0
0
elsewhere
(
6
)
x
m
,
n
(
t
)
=
ⅇ
ⅈ
(
France Telecom & Telediffusion de France
Kinney & Lange , P.A.
Ngo Ricky
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