Demodulators – Frequency shift keying or minimum shift keying demodulator
Reexamination Certificate
1999-11-19
2001-11-27
Grimm, Siegfried H. (Department: 2817)
Demodulators
Frequency shift keying or minimum shift keying demodulator
C329S315000, C375S324000, C375S334000, C375S336000
Reexamination Certificate
active
06323727
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to detection of digital frequency modulated (FM) data symbols.
BACKGROUND OF THE INVENTION
Digital communication requires the transmission of data symbols. For frequency modulation (FM), each symbol defines a change in the frequency of the transmitted signal or, as indicated in
FIG. 1
to which reference is now made, as a set of changes in the frequency.
FIG. 1
shows two Manchester encoded bits. When using FM, a “1” bit has the frequency of ƒ
0
+&Dgr;ƒ during the first T/2 seconds (from t
0
to t
1
of
FIG. 1
) of the bit and the frequency of ƒ
0
−&Dgr;ƒ during the second T/2 seconds (from t
1
to t
2
) of the bit. A “0” bit has the opposite shape: during the first T/2 seconds (from t
2
to t
3
) of the bit the frequency is ƒ
0
−&Dgr;ƒ and during the second T/2 seconds (from t
3
to t
4
) of the bit the frequency is ƒ
0
+&Dgr;ƒ. For advanced mobile phone service (AMPS) wideband data transmission, the bit duration T is 0.1 msec (i.e. bit rate is 10 KHz) and &Dgr;ƒ is 8 KHz.
Attempts have been made to decode the symbol values using the in-phase i(t) and quadrature q(t) components of the baseband version of the complex transmitted signal, where:
i(t)=A cos(2&pgr;ft+&thgr;),
q(t)=A sin(2&pgr;ft+&thgr;)tm Equation 1
where f is the instantaneous frequency+&Dgr;ƒ or −&Dgr;ƒ of the baseband signal and &thgr; is the instantaneous phase of the baseband signal.
FIG. 2
, to which reference is now made, illustrates a unit circle
14
within an i-q coordinate system and the movement of the signal of
FIG. 1
along circle
14
. At the beginning (t
0
), the signal of
FIG. 1
has a first phase &eegr;
0
, shown in
FIG. 2
as somewhere within quadrant I.
When the frequency is +&Dgr;ƒ, the signal moves counter clockwise along the circle
14
and when the frequency is −&Dgr;ƒ, the signal moves clockwise along the circle
14
. Thus, during the first portion of the 1 bit, the signal moves counter clockwise along the circle
14
until it reaches the position indicated by t
1
. In this example, t
1
is in quadrant II. During the second portion of the 1 bit, the signal moves clockwise, arriving, at time t
2
, to a position close, if not equal to, the position at time t
0
. During the first portion of the 0 bit, the signal moves clockwise, moving to quadrant IV by time t
3
and then, during the second portion of the 0 bit, the signal moves counter clockwise back to a position close, if not equal to, the position at time t
2
.
U.S. Pat. No. 4,322,851 to Vance describes a decoder which utilizes the in-phase and quadrature information to decode binary FSK bits. U.S. Pat. No. 5,469,112 to Lee describes a decoder which also utilizes the in-phase and quadrature information but to decode multi-level FSK bits. Neither of U.S. Pat. No. 4,322,851 or U.S. Pat. No. 5,469,112 will successfully decode general FM modulated symbols (e.g. Manchester encoded bits) since both assume that each has a constant value.
SUMMARY OF THE PRESENT INVENTION
It is an object of the present invention to provide a decoding method and apparatus for decoding FM modulated symbols given in-phase and quadrature signals.
There is therefore provided, in accordance with a preferred embodiment of the present invention, a symbol detector for frequency modulated (FM) symbols which includes a section determiner, a direction of movement determiner and a data symbol determiner. The section determiner receives a sample of in-phase and quadrature signals associated with a baseband transmitted FM symbol and determines the section value of a unit circle in an in-phase—quadrature coordinate system in which the sample lies. The direction of movement determiner receives the section value of a current sample and the section value of a neighboring sample and generates a positive direction of movement value if the direction of movement along the unit circle from the neighboring sample to the current sample is counterclockwise, a negative direction of movement value if the direction of movement is clockwise and 0 otherwise. The data symbol determiner receives the direction of movement values and decodes the transmitted FM symbol by m asking N direction of movement values.
Additionally, in accordance with a preferred embodiment of the present invention, the section determiner includes a plurality of comparators for generating at least the signs of the following data: the in-phase sample and the quadrature sample. Other information, such as the sum of the in-phase and quadrature samples and the difference of the in-phase and quadrature samples, can also be utilized.
Moreover, in accordance with a preferred embodiment of the present invention operative for Manchester bit decoding, the data symbol determiner includes a mask which adds together the direction of movement values associated with generally the first half of the data symbol and subtracts the direction of movement values associated with generally the second half of the data symbol.
The present invention incorporates the methods performed by the decoder and its elements.
REFERENCES:
patent: 4322851 (1982-03-01), Vance
patent: 5453715 (1995-09-01), Lee
patent: 5469112 (1995-11-01), Lee
“Zero-Crossing Baseband Demodulator ”, E.K.B. Lee, 0-7803-3002-1/95—IEEE 1995, pp. 466-470.
Burshtein David
Rainish Doron
DSPC Technologies Ltd.
Fish & Richardson P.C.
Grimm Siegfried H.
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