Pulse or digital communications – Receivers – Angle modulation
Reexamination Certificate
2002-12-23
2004-08-31
Tran, Khai (Department: 2631)
Pulse or digital communications
Receivers
Angle modulation
C375S341000
Reexamination Certificate
active
06785348
ABSTRACT:
BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
The invention relates to a method and a device for coherently demodulating a frequency-modulated signal with a continuous phase.
A large number of digital modulation types are known, which are based on Amplitude Shift Keying (ASK), Frequency Shift Keying (FSK) or Phase Shift Keying (PSK) methods as well as mixed forms of them. For frequency economy reasons, so-called CPM (Continuous Phase Modulation) modulation types with a continuous phase are frequently used in digital communications systems. FSK with a continuous phase is referred to as CPFSK (Continuous Phase FSK). One example of this is Gaussian Minimum Shift Keying (GMSK), which is used in the pan-European GSM (Global System for Mobile Communictions) mobile radio standard.
Coherent or incoherent methods may be used for demodulating the CPFSK signal. Incoherent demodulation may be carried out either by using an analog FM demodulator or digitally by using a differential demodulator. One disadvantage is that relatively high losses in the region of 3 dB occur with incoherent demodulation. Furthermore, drops in power occur, since the Inter Symbol Interference (ISI) cannot be taken into account.
CPFSK modulation, which is primarily non-linear, can be described approximately as linear modulation. The linear approximation on which this characteristic is based is described in the article “Exact and Approximate Construction of Digital Phase Modulations by Superposition of Amplitude Modulated Pulses (AMP)” by Pierre A. Laurent, IEEE Trans. Commun., Volume COM-34 (1986), pages 150-160. This characteristic of CPFSK-modulated signals provides the capability for coherent demodulation.
The book “Nachrichtenübertragung” [Information transmission] by K. D. Kammeyer, B. G. Teubner Verlag, Stuttgart 1996, Section 12.1.5, pages 422 and 423, which represents the closest prior art, describes a coherent demodulator for CPFSK signals with a modulation index &eegr;, which is equal to 0.5 or to a multiple of 0.5. The in-phase and quadrature branches of the received signal are sampled alternately (because of the 90° phase offset between these branches), and the sample values obtained are compared with the corresponding complex-value representations of the CPFSK substitute symbols (on which the linear approximation is based) for the input data symbols used at the transmitter. Among the possible input data symbols, the input data symbol that is actually transmitted is defined as the one whose complex-value substitute symbol comes closest to the two measured sample values (real and imaginary part).
This coherent demodulation method for CPFSK signals can be generalized to rational modulation indices &eegr;=M/N (where M and N are integers). With rational modulation indices, there are always a finite number of substitute symbol states, so that the demodulation can still be carried out just by comparing the sample values with the finite modulation alphabet of substitute symbols.
There is no longer any finite modulation alphabet of substitute symbols for non-rational modulation indices &eegr;. The result of this is that the conventional method for coherent CPFSK demodulation can no longer be used in these situations.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a method and a device for demodulating a CPFSK-modulated received signal, which overcome the above-mentioned disadvantages of the prior art methods and apparatus of this general type.
In particular, it is an object of the invention to provide a method and a device for demodulating CPFSK received signals, which enables good reception and enables the CPFSK received signal to be demodulated even when the modulation indices are not rational.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for demodulating a CPFSK-modulated signal. The method includes steps of: obtaining an estimate of an n−1-th substitute symbol, which occurs in a linear approximation of the CPFSK modulated signal, as a function of a previously determined n−1-th input data symbol; and determining an n-th input data symbol on which the CPFSK modulated signal is based by using the estimate of the n−1-th substitute symbol occurring in the linear approximation of the CPFSK modulated signal.
In accordance with an added feature of the invention, the step of obtaining the estimate of the n−1-th substitute symbol includes using an equation â
n−1
=â
n−2
exp{j&pgr;&eegr;{circumflex over (d)}
n−1
}, where âhd n−
1
is the estimate for the n−1-th substitute symbol, â
n−2
is an estimate for an n−2-th substitute symbol, {circumflex over (d)}
n−1
is the n−1-th input data symbol that has been determined, and &eegr; denotes a modulation index.
In accordance with an additional feature of the invention, the step of determining the n-th input data symbol includes determining the n-th input data symbol d
n
based on a phase angle of a currently obtained n-th complex-value sample symbol y
n
relative to a phase angle of the estimate of n−1-th substitute symbol â
n−1
that has been estimated for an n−1-th time step.
In accordance with another feature of the invention, the step of determining the n-th input data symbol d
n
includes obtaining a determined value of the n-th input data symbol d
n
using an equation:
d
^
n
=
{
1
arg
(
y
n
)
>
arg
(
a
^
n
-
1
)
-
1
arg
(
y
n
)
<
arg
(
a
^
n
-
1
)
;
where {circumflex over (d)}
n
is the determined value of the n-th input data symbol d
n
.
In accordance with a further feature of the invention, the method includes using an equalizer, and in an even more preferred embodiment, the method includes using a Viterbi equalizer for performing the step of determining the n-th input data symbol d
n
.
In accordance with a further added feature of the invention, the step of obtaining the equalization includes basing the equalization on a trellis state diagram, in which an i-th channel state relating to a time step n is described by an L-tuple Z
n
i
=(z
n
L−1,(i)
, . . . , z
n
1,(i)
, z
n
0,(i)
). In the equation, z
n
L−1,(i)
, . . . , z
n
1,(i)
, z
n
0,(i)
can each assume possible values of input data symbols d
n
, and L denotes a channel memory.
With the foregoing and other objects in view there is also provided, in accordance with the invention, a device for demodulating a CPFSK-modulated signal. The device includes: an input data symbol decision device for determining an n-th input data symbol on which the CPFSK-modulated signal is based; and a substitute symbol estimation device for estimating an n−1-th substitute symbol occurring in a linear approximation of the CPFSK-modulated signal as a function of a previously determined n−1-th input data symbol. The input data symbol decision device uses the n−1-th substitute symbol that has been estimated for determining the n-th input data symbol.
In accordance with an added feature of the invention, the substitute symbol estimation device is constructed for estimating the n−1-th substitute symbol using the equation â
n−1
=â
n−2
exp{j&pgr;&eegr;{circumflex over (d)}
n−1
}, where â
n−1
is an estimate of the n−1-th substitute symbol, â
n−2
is an estimate of an n−2-th substitute symbol; {circumflex over (d)}
n−1
is the n−1-th input data symbol that has been determined; and &eegr; denotes a modulation index.
In accordance with an additional feature of the invention, the input data symbol decision device is constructed for determining the n-th input data symbol d
n
based on a phase angle of a currently obtained n-th complex-value sample symbol y
n
relative to a phase angle of the n−1-th substitute symbol â
n−1
that has been estimated for an n−1-th time step.
In accordance with another feature of the invention, the input data symbol estimation device i
Hammes Markus
Madden Michael
Neubauer Andre
Speth Michael
Greenberg Laurence A.
Infineon - Technologies AG
Locher Ralph E.
Stemer Werner H.
Tran Khai
LandOfFree
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