Pulse or digital communications – Receivers – Angle modulation
Reexamination Certificate
2001-01-05
2004-11-23
Liu, Shuwang (Department: 2634)
Pulse or digital communications
Receivers
Angle modulation
C375S286000, C375S216000, C375S331000, C375S213000, C375S231000, C375S324000, C370S902000
Reexamination Certificate
active
06823026
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates generally to radio communication systems, and in particular, to baseband detection in such systems.
BACKGROUND OF THE INVENTION
In some radio communication systems, constant-envelop modulation schemes, such as frequency shift keying (FSK), are suitable for low-power wireless communications. At receivers within such systems, non-coherent demodulation can be used to reduce hardware complexity. In addition, direct-conversion receivers (DCRs) are also desirable for portable communications applications where low power is a requirement. A DCR translates a received radio frequency (RF) signal directly into a baseband signal without the need for image-rejection filters and other intermediate frequency (IF) components. However, conventional IF FSK detectors, such as a limiter-discriminator are not well suited for use in DCRs. Therefore, there is a need for an efficient baseband FSK detector for use with DCRs.
Some conventional baseband discriminators can be used with DCRs.
FIG. 1
shows an exemplary prior art baseband discriminator
10
usable with DCRs. The baseband discriminator
10
output is given by:
v
(
t
)
=
I
(
t
)
ⅆ
Q
(
t
)
ⅆ
t
-
Q
(
t
)
ⅆ
I
(
t
)
ⅆ
t
I
2
(
t
)
+
Q
2
(
t
)
,
(
1
)
where I(t)=A cos(&phgr;(t)) and Q(t)=A sin(&phgr;(t)) are, respectively, the in-phase (I) and quadrature-phase (Q) components of the baseband equivalent received signal, A is the received signal amplitude, and &phgr;(t) is the phase function of the FSK signal. The output &ngr;(t) is equal to the instantaneous frequency deviation
ⅆ
φ
(
t
)
ⅆ
t
.
A pair of mixers
11
-
12
and a pair of lowpass filters
13
-
14
provide the I and Q components to the discriminator
10
.
The baseband discriminator
10
shown in
FIG. 1
can be implemented digitally using the circuit
20
shown in FIG.
2
. In this implementation, I(t) and Q(t) are sampled N times per symbol period and digitized by the analog-to-digital converters (ADCs)
21
-
22
. The discriminator
20
includes multipliers
23
-
26
, delay circuits
27
-
28
, an adder
29
, a subtractor
30
, and a divider
31
. A down sampling circuit
32
down samples or decimates the output of the divider
31
.
The derivatives in
FIG. 1
are approximated by the difference between samples that are
T
N
apart in time, where T is the symbol period. The output of the digital baseband discriminator
20
is given by:
v
n
=
Q
(
nT
)
I
(
nT
-
T
N
)
-
I
(
nT
)
Q
(
nT
-
T
N
)
I
2
(
nT
)
+
Q
2
(
nT
)
,
n
=
0
,
1
,
…
(
2
)
The circuit
20
is similar to a delay-and-multiply detector (quadrature detector) in a heterodyne FSK receiver. It is also similar to a differential phase detector because &ngr;
n
is proportional to
sin
(
φ
(
nT
)
-
φ
(
nT
-
T
N
)
)
,
thus when
T
N
is small, &ngr;
n
is approximately proportional to the phase difference
φ
(
nT
)
-
φ
(
nT
-
T
N
)
.
In general, it is desirable to keep the number of samples per sample period N small. However, when the baseband equivalent received signal is fast varying, using a small value for N incurs a significant performance loss with respect to the continuous-time implementation because the finite difference cannot accurately approximate the derivatives. This problem arises when there is a significant amount of frequency offset between the receiver and transmitter oscillators within a communication system, or when a large frequency deviation (high modulation index) is used for FSK modulation.
To overcome this accuracy problem, the number of samples per sample period N can be increased. However, while the finite differences can accurately approximate the derivatives when N is large, high-resolution analog-to-digital converters (ADCs) may be necessary because the signal variations represented by
&LeftBracketingBar;
I
(
nT
)
-
I
(
nT
-
T
N
)
&RightBracketingBar;
and
&LeftBracketingBar;
Q
(
nT
)
-
Q
(
nT
-
T
N
)
&RightBracketingBar;
decrease as N increases. This is especially true for narrow-band signals, such as FSK signals with small frequency deviations (low modulation indexes). Therefore, when used as the baseband detector for FSK, the digital baseband discriminator of
FIG. 2
is very sensitive to frequency offset and frequency deviation.
Another known baseband detector
40
is shown in FIG.
3
. In this circuit, a time-domain received signal is filtered by an IF filter
41
and then converted to the phase-domain using a phase detector
42
. The differential phase is then computed by the delay
44
and subtractor
46
for detecting the information symbol using decision circuitry
47
. The merit of this circuit is that the same architecture can be used to detect FSK and differential phase shift keying (DPSK) signals. However, the input to this circuit is an IF signal, which requires IF circuit components. Thus, depending on the implementation of the phase detector
42
, this architecture is not always suitable for use with DCRs. Furthermore, the mod 2&pgr; ambiguity normally associated with the phase detector
42
makes the circuit
40
sensitive to frequency offset, thus requiring a higher receiver sampling rate.
REFERENCES:
patent: 4499426 (1985-02-01), Parker
patent: 4612509 (1986-09-01), Betts et al.
patent: 4984219 (1991-01-01), Brown et al.
patent: 5377229 (1994-12-01), Wilson et al.
patent: 5420888 (1995-05-01), Davis et al.
patent: 5581579 (1996-12-01), Lin et al.
patent: 5642379 (1997-06-01), Bremer
patent: 5828705 (1998-10-01), Kroeger et al.
patent: 6373888 (2002-04-01), Lindoff
Lee Yumin
Mueller Bruce D.
Haas Kenneth A.
Liu Shuwang
Motorola Inc.
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