Multiplex communications – Generalized orthogonal or special mathematical techniques
Reexamination Certificate
2000-09-12
2004-07-27
Vanderpuye, Kenneth (Department: 2661)
Multiplex communications
Generalized orthogonal or special mathematical techniques
C370S210000
Reexamination Certificate
active
06768713
ABSTRACT:
TECHNOLOGICAL FIELD
The subject of this invention is an orthogonal frequency division multiplexing receiver with iterative channel estimation and a corresponding method. It finds application in radio communications and more particularly in Orthogonal Frequency Division Multiplexing or OFDM technology. It may be applied to—among other systems—the European portable radio system HIPERLAN II.
STATE OF THE PRIOR ART
OFDM technology [
1
] is a multi-carrier technology which permits division of users within a time-frequency plane in a simple way. In addition it permits the transmission of signals at a high rate without having to use an equalizer. This method has been widely used in the context of Digital Video Broadcasting or DVB-T, [
2
] and Digital Audio Broadcasting or DAB [
3
]. In a portable radio context, OFDM is present in the HIPERLAN II Standard.
OFDM technology is both a multiple access technology and a modulation technology. The basic principle of OFDM technology is to produce a certain number of narrow band signals all orthogonal to one another. By taking using precautions, these properties of orthogonality are used to recover the transmitted data. The creation of such a system calls upon the use of an inverse Fourier transform on emission and a Fourier transform on reception.
FIG. 1
appended illustrates a traditional OFDM transmission chain with a single sensor. This chain, comprises a serial to parallel conversion circuit
10
receiving symbols A, an inverse Fourier transform circuit
12
, transmission means
14
, reception means
20
, a Fourier transform circuit
22
, a parallel to serial converter
24
and finally a decision means
26
which reconstructs the estimated symbols Â.
The traditional OFDM transmitter processes the flow of data by block. It manages this flow by sequences of N
t
symbols and carries out the inverse Fourier transform on them. This means that the inverse Fourier transform produces N
f
sub-carriers, each carrying one of the symbols of the starting sequence. This block, called an OFDM symbol, contains the data symbols and may also contain pilot symbols which can be used for purposes of synchronization or of channel estimation. In contrast to the case of Code Division Multiple Access or CDMA signals or Time Division Multiple Access or TDMA signals, where one pilot symbol right away occupies the whole of the transmission band, OFDM technology requires the true distribution of the pilot symbols over the whole of the time-frequency plane.
The mobile radio channel taken when there is communication between a transmitter and a receiver is generally of the multi-path type with rapid Rayleigh fading. This phenomenon is due to the conjunction of the movement of the mobile and the propagation of the radio wave along several paths.
The receiver processes the signal received through an OFDM block of symbols (a time-frequency block). The signal is received on a network of L sensors, creating L branches of diversity. The channel estimation is carried out on each of these branches and the results are combined by Maximum Ratio Combining (MRC) to finally estimate the transmitted data.
A receiver with L branches of diversity is shown in FIG.
2
. It comprises L sensors
30
1
,
30
2
, . . . ,
30
L
, L Fourier transform circuits
32
1
,
32
2
, . . . ,
32
L
, L parallel to serial converters
34
1
,
34
2
, . . . ,
34
L
, L channel estimation circuits
36
1
,
36
2
, . . . ,
36
L
, and an adder
38
supplying the estimated symbols Â.
From the point of view of the receiver, after demodulation, the channel allocating a time-frequency block can be shown in the form of a time-frequency matrix, or a surface in time-frequency-amplitude space. However the problem is processed in bi-dimensional space, in contrast to TDMA [
4
] where the problem is uni-dimensional.
The channel estimation is based on the use of pilot symbols. They enable a channel estimation to be provided directly to the locations of the pilots with a view to interpolation in order to estimate the channel allocating the rest of the symbols.
These techniques have disadvantages. In effect, the channel seen by the receiver can vary in a significant manner from one time-frequency block to another. This variation is mainly due to changes in propagation conditions between the transmitter and the receiver. From a physical point of view, the variable character of the channel can be characterized by the product B
d
×T
m
where B
d
represents the width of the Doppler band and T
m
the spread of the delays. The greater the product B
d
×T
m
, the more the channel varies rapidly within the time and frequency domains.
The reception method of the prior art does not seek to optimize the channel estimation. They are content to carry out an estimation of the channel at the positions of the pilot symbols and then to extend this estimation to the data by interpolation. The interpolations are generally carried out in a linear manner. Three of the methods most commonly used may be described:
The first considers the three pilot symbols closest to the symbol at which one wishes to estimate the channel. The plane passing through the-three pilot symbols is calculated and from this the channel is deduced at the point being considered. Even by respecting the Nyquist criterion with respect to the pilot symbols, that is to say, using sufficient pilot symbols and distributing them in a manner that correctly samples the time-frequency plane, this method is sensitive to strong channel variations and does not allow one to carry, out a reliable estimation of the channel, especially in the case where the product B
d
×T
m
is high.
The second method is a simple form of the Minimum Mean Square Error (MMSE) technique: it consists of looking for the constant plane that averages the values of the channel at the pilot 'symbols and of deducing from it the values of the channel allocating the transmitted data. This channel modeling is well suited to channels varying very slightly over the received block, that is to say for low B
d
×T
m
products. However, as soon as the channel becomes more selective, the planar modeling shows its limits and performance is reduced.
The third method is another form of MMSE with a non-constant plane being looked for. This method is therefore better suited to cases where the channel varies slowly, but is less suitable than the second method in the case of channels that are almost constant.
These three methods are suited to highly specific propagation cases, but are in no way suitable for channels of the multi-path type that are selective in time and in frequency.
The precise purpose of this invention is to remedy this disadvantage.
DESCRIPTION OF THE INVENTION
The main aim of this invention is to improve the performance of existing OFDM systems and those to come. This improvement, obtained by optimization of the channel estimation, permits the capacity of the system to be substantially increased. This improvement is brought about by optimization of the operation of the OFDM receiver in the case of slow fading and also in the more complex case of very rapid fading.
It is then possible to prevent the reduction in performance brought about by rapid channel variation over the time-frequency block being considered on reception.
At constant reception quality, the invention enables one to reduce the relative number of and/or the power of the pilot symbols. This aim is achieved by taking into consideration, in an optimum manner, an arbitrary number of pilot symbols from consecutive time-frequency blocks, in the channel estimation. The aim is also achieved by the optimum character, in the channel estimation, of the consideration (of one part or of the totality) of the data symbols of these blocks, which are of course more numerous than the pilot symbols.
The invention may also be used whatever the way in which the pilot symbols are inserted into the stream of transmitted information.
The receiver of the invention carries out a block by block processing ev
Jaffrot Emmanuel
Siala Mohamed
France Telecom
Moore Ian
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Vanderpuye Kenneth
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