Method for estimating the frequency shift of a CPFSK signal
View Patent ↗A method for estimating the frequency shift of a CPFSK signal includes scanning the CPFSK signal. Intermediate signal values are determined for the scanned CPFSK signal. An estimated value (ν) is determined for the frequency shift of the CPFSK signal by assessing a predefined number of L 0 consecutive intermediate signal values. An integer delay parameter D is specified. Intermediate signal values for intervals k·T+τ are determined in each case from scanning values of the CPFSK signal obtained for intervals k·D·T+τ and [k−1]·D·T+τ, whereby T designates a scanning period of the scanned CPFSK signal, k is a scanning index and τ is a delay constant. The estimated value (ν) for the frequency shift is determined from the intermediate signal values for intervals i·D·T+τ with i=0 . . . L 0 −1. The integer delay parameter D is variable and is selected depending on the type of CPFSK modulation used for the CPFSK signal.
1. A method for estimating the frequency shift of a continuous phase-frequency-shift keying (CPFSK) signal, comprising the steps of:
a) scanning the CPFSK signal;
b) determining intermediate signal values for the CPFSK signal scanned in step a); and
c) determining an estimated value (v) for the frequency shift of the CPFSK signal by assessing a predefined number of L 0 consecutive intermediate signal values obtained in step b),
whereby an integer delay parameter D is specified,
whereby in step b) the intermediate signal values for intervals k·T+τ are determined in each case from scanning values of the CPFSK signal obtained for intervals k·D·T+τ and [k−1]·D·T+τ, whereby T designates a scanning period, with which the CPFSK signal is scanned in step a), k is a scanning index and τ is a delay constant, and whereby in step c) the estimated value (ν) for the frequency shift is determined from the intermediate signal values thus obtained in step b) for intervals i·D·T+τ with i=0 . . . L 0 −1, wherein the integer delay parameter D is variable and is selected depending on the type of CPFSK modulation used for the CPFSK signal.
2. The method according to claim 1 , wherein the CPFSK signal x(k·T+τ) scanned in step a) is present in complex form, and wherein the intermediate signal values z(k·T+τ) of step b) are determined according to the equation z·(k·T+τ)={x(k·D·T+τ)·x([k−1]·D·T+τ)} 2 , whereby x* designates the conjugated complex form of the CPFSK signal, and wherein the estimated value ν for the frequency shift is determined according to the equation
v
=
1
4
·
π
·
D
·
T
·
arg
{
∑
i
=
0
L
0
-
1
z
(
i
·
D
·
T
+
τ
)
}
.
3. The method according to claim 1 , wherein the phase of the CPFSK signal during its modulation is changed in harmony with a predetermined phase function, the phase function reaches its end value after a predetermined number L of symbols of the CPFSK signal, and the value D=L is selected for the delay parameter.
4. The method according to claim 3 , whereby the phase function allocated to the CPFSK signal is such that L>1 applies.
5. The method according to claim 1 , further comprising the step of passing the CPFSK signal through a low-pass filter before being scanned in step a).
6. The method according to claim 1 , wherein the CPFSK signal is a transmitted signal sent over a digital mobile radio system, and the step of determining the estimated frequency shift (ν) of the CPFSK signal is carried out in a receiver of the digital mobile radio system, in order to correct the incoming CPFSK signal accordingly, depending on the estimated frequency shift (ν).