Pulse or digital communications – Receivers – Angle modulation
Reexamination Certificate
1999-04-22
2004-08-31
Tse, Young T. (Department: 2634)
Pulse or digital communications
Receivers
Angle modulation
C375S272000
Reexamination Certificate
active
06785347
ABSTRACT:
FIELD OF INVENTION
The present invention relates to digital communications. More specifically, it relates to efficient reception of a Frequency Shift Key (FSK) signal.
BACKGROUND OF THE INVENTION
Frequency Shift Keying (FSK) is a data modulation scheme commonly used in digital communications. FSK has been recommended as a low speed modulation format for numerous standards for the Public Switched Telephone Network (PSTN) by both national and international standards organizations. Examples of standards that recommend FSK are Bell 103, Bell 202, ITU-T V.21, and ITU-T V.23.
FSK possesses several characteristics that make it useful for communication systems. It is a simple modulation method that is a commonly used approach to the exchange of additional low speed information in high speed communication systems. For instance, FSK is used when call waiting caller ID information is transmitted during a voice or data connection. FSK is also used to exchange ITU-T V.8bis messages during a voice connection. In these applications, the computational complexity and memory requirements for demodulating of the signal are very important. A signal encoding scheme that requires a large amount of computation and memory for demodulation and detection will not be suitable for many Caller ID and other types of devices that have limited computing resources.
Because it is a simple signal encoding scheme, FSK is also often used as a start-up scheme for connections that use complex higher density signal encoding schemes, such as Quadrature Amplitude Modulation (QAM) signaling, and is recommended in ITU-T V.32 and ITU-T V.34.
It is well understood in the art that FSK can be detected using either a non-coherent method employing a frequency detector or a coherent method that utilizes a pair of product detectors.
FIG. 1
illustrates a generalized coherent FSK receiver
10
. The receiver
10
receives a received signal r(t) that is composed of a data signal s(t) and a noise signal n(t). The data signal s(t) is composed of mark (binary 1) and space (binary 0) signals s
1
(t) and s
0
(t), respectively. The mark signal s
1
(t) can be described as s
1
(t)=A cos(&ohgr;
1
t+&thgr;
c
), where &ohgr;
1
=2&pgr;f
1
and f
1
is the frequency that represents a binary 1 in the FSK encoding scheme. The space signal s
0
(t) can be described as s
0
(t)=A cos(&ohgr;
0
t+&thgr;
c
), where &ohgr;
0
=2&pgr;f
0
and f
0
is the frequency that represents a binary 0 in the FSK encoding scheme. In the FSK scheme described, f
1
>f
0
and the data frequency shift of s(t) is &Dgr;F=f
1
−f
0
around a center frequency f
c
of the FSK data signal s(t).
The receiver
10
includes an upper channel product detector
12
and a lower channel product detector
14
, which each receive r(t). The upper channel product detector
12
receives a first coherent reference signal 2 cos(&ohgr;
1
t+&thgr;
c
) at one input terminal and r(t) at a second input terminal. The lower channel product detector
14
receives a second coherent reference signal 2 cos(&ohgr;
0
t+&thgr;
c
). Note here that, in coherent FSK detection, it is necessary to know the phase &thgr;
c
of the received sinusoidal signal in order to build the coherent reference signals 2 cos(&ohgr;
1
t+&thgr;
c
) and 2 cos(&ohgr;
0
t+&thgr;
c
).
The output of the lower channel product detector
14
is input to a negative terminal of summer
16
and the output of the upper channel product detector
12
is input to a positive terminal of summer
16
. Summer
16
will subtract the lower channel output from the upper channel output in order to produce a difference signal at an output terminal of the summer. This difference signal is then input to a low pass filter (LPF)
18
.
LPF
18
, which can also be viewed as a matched filter, when combined with the frequency translation performed by product detectors
12
and
14
, as dual bandpass filters. Thus, the input noise n(t) that affects the output of receiver
10
consists of two narrowband components n
1
(t) and n
0
(t) centered at f
1
and f
0
, respectively. The bandwidth B of LPF
18
is less than the frequency difference &Dgr;F, such that 2&Dgr;F>2B, where the effective bandwidth B
p
is 2B, and the filtering action of LPF
18
separates the mark and space signals s
1
(t) and s
0
(t) in order to produce a baseband analog output signal r
o
(t).
FIG. 2
illustrates the resulting power spectra for receiver
10
.
The baseband analog output signal r
o
(t) is {+A for a binary 1; −A for a binary 0}+n
o
(t). This signal is input to sample and hold
20
that obtains a discrete signal r
o
(t
o
) that is then input to threshold comparator
22
for comparison to threshold V
T
. Because of the symmetry of the baseband analog output signal r
o
(t), i.e. a binary 1 is +A and a binary 0 is −A, and because the noise in each of the upper and lower channels is similar, i.e. white Gaussian noise, the optimum threshold for V
T
is 0. The threshold comparator
22
then outputs a digital output signal m(t) that reflects the data signal s(t) along with the noise signal n(t).
Theoretically, the performance of the coherent method is better than the non-coherent method. However, the coherent method requires that a coherent reference signal be obtained. The coherent reference is often extracted from the noisy received FSK signal so that the reference itself contains noise. Also, the reference recovery circuitry required to recover the coherent reference is typically complex and expensive. As a consequence, the non-coherent method is often used to avoid the coherent reference recovery circuitry.
FIG. 3
illustrates a generalized noncoherent FSK receiver
30
. Receiver
30
receives signal r(t) that is input to an upper channel detector
32
and a lower channel detector
42
. The upper channel detector
32
is configured to detect the mark signal and is composed of a bandpass filter
34
centered on f
1
connected in series with an envelope detector
36
. Envelope detector
36
outputs an upper channel output signal v
U
(t) that is input to a positive input terminal of summer
38
. The summer
38
outputs analog output signal r
o
(t) to a sample and hold
50
that obtains a discrete signal r
o
(t
o
) that is then input to threshold comparator
52
for comparison to a threshold voltage.
The lower channel detector
42
is configured to detect the space signal and is composed of a bandpass filter
44
centered on f
0
connected in series with an envelope detector
46
. Envelope detector
46
outputs a lower channel output signal v
L
(t) that is input to a negative input terminal of summer
38
.
The analog output signal r
o
(t) from summer
38
is positive when the upper channel output signal v
U
(t) exceeds the lower channel output signal v
L
(t). A mark signal can be viewed as the signal value of v
U
(t) for a mark less the signal value of v
L
(t) for a mark or {+A+n
1
(t)}−{0+n
0
(t)}. Similarly, r
o
(t) from summer
38
is negative when the lower channel output signal v
L
(t) exceeds the upper channel output signal v
U
(t). Thus, a space signal can be viewed as the signal value of v
L
(t) for a space less the signal value of v
U
(t) for a space or {−A+n
0
(t)}−{n
1
(t)}.
Noncoherent FSK detection requires only 1 dB of additional signal-to-noise ratio (E
b
/N
0
) over coherent FSK detection. However, in noncoherent FSK detection, it is unnecessary to generate a coherent reference signal and it is therefore unnecessary to know the phase &thgr;
c
of the received sinusoidal signal. An estimate of the phase information can be obtained using a phase locked loop (PLL) circuit. However, it is difficult to obtain a phase estimate in a digital receiver, particularly when the number of samples per binary bit is small because a phase estimate typically requires more than ten samples in order to converge. The Bell 202 and ITU-T V.23 standards, for example, set forth a requirement of only six samp
3com Corporation
Lugo David B.
McDonnell Boehnen & Hulbert & Berghoff LLP
Tse Young T.
LandOfFree
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