Pulse or digital communications – Systems using alternating or pulsating current – Plural channels for transmission of a single pulse train
Reexamination Certificate
2000-08-30
2002-11-05
Corrielus, Jean (Department: 2631)
Pulse or digital communications
Systems using alternating or pulsating current
Plural channels for transmission of a single pulse train
C714S786000, C714S796000
Reexamination Certificate
active
06477208
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to communication systems wherein modulated carrier signals convey data in digital form and more particularly to the implementation of coherent demodulators and signal phase estimators for such systems that must accommodate undesired signal-phase values and temporal variations thereof.
2. Description of the Related Art
In many carrier-based communication systems the transmit signals used to convey information are of the form
s
tx
(
t
tx
)=
A
tx
(
t
tx
)·sin[&ohgr;
c tx
·t
tx
+&phgr;
m tx
(
t
tx
)] (1)
where t
tx
represents (relative) transmit time, &ohgr;
c tx
represents the transmit radian carrier frequency in radians per second, &phgr;
m tx
(t
tx
)) represents signal phase variations attributable to any phase modulation effected and A
tx
(t
tx
) represents the signal's amplitude. When the information transmitted is digital in form, the time continuum is generally divided into a succession of contiguous signaling (modulation) intervals and, for each signaling interval, log
2
m (uncoded or coded) data bits select one of m pre-specified options for varying the signal's phase and/or amplitude during the signaling interval. For this invention, the signal's phase may be varied (modulated) to convey digital data using any one of many different modulation methods including m-ary phase shift keying (PSK), m-ary continuous-phase frequency shift keying (CPFSK) and m-ary CPFSK with modulation and convolutional data encoding effected jointly. The signal's amplitude may vary with time as a consequence of implementing an amplitude modulation method such as m-ary amplitude shift keying (ASK) or incidentally, e.g., due to signal filtering effected. For m-ary ASK, modulation phase component &phgr;
m tx
(t
tx
) has a constant value considered herein to be zero radian. This invention also applies to several modulation methods for which signal amplitude and signal phase are varied jointly to convey digital data as for m-ary quadrature amplitude modulation (QAM).
For each of several modulation methods, the transmitted signal can be modeled equivalently as either a phase modulated signal or an amplitude-modulated signal where the signal amplitude is either a real or complex-valued function of time. Such methods are considered to be phase modulation methods herein irrespective of the means used to generate the signals transmitted. Further, for communication systems wherein digital data are transmitted via signaling bursts rather than continuously—as in time division multiple access (TDMA) systems—a transmit signal is modeled by assigning a value of zero to the signal's amplitude whenever signal transmission is disabled.
A transmit signal propagates from its point of origin to one or more receiver locations via one or more communication channels comprised of wire-line, wireless or electronic relay means, or any combination thereof. For a transmit signal representable by Equation 1, a signal received at a receiver location is of the form
s
rx
(
t
)=
s
(
t
)+
n
&Sgr;
(
t
) (2)
where t represents (relative) receive time, s(t) is a delayed, attenuated and generally-distorted version of the transmit signal,
s
(
t
)=
A
(
t
)·sin[&ohgr;
c
·t+&phgr;
m
(
t
)+&phgr;
u
(
t
)], (3)
and n
&Sgr;
(t) represents a sum of noise and undesired signals which exacerbate generation of an exact (delayed) replica of the transmit data at the receiver. In accord with common practice, radian carrier frequency &ohgr;
c
is assumed to be perfectly known at the receiver; frequency uncertainties that result from imperfect frequency synthesis within transmit, relay and receive subsystems and any doppler shift experienced by the signal in propagating to the receiver are considered to affect the value of signal phase &phgr;
u
(t): a phase variation that is unintended and undesired. Signal parameter &ohgr;
m
(t) represents the modulation component of the received signal's phase when phase modulation is effected; otherwise, its value is considered herein to equal zero radian.
For a communication system wherein undesired signal-phase &phgr;
u
(t) can be made to vary slowly relative to and be distinguishable from phase &phgr;
m
(t), the most effective use of signal power can generally be achieved by employing a coherent demodulator to process the received signal. Ideally, a portion of the receiver would generate an exact replica of signal phase &phgr;
u
(t) and subtract this replica from the phase of the received signal to form a signal that exhibits no undesired phase variations; the latter signal would then be demodulated coherently to generate a nominal replica of the transmitted data stream. For many classes of modulation methods, prior art provides means for coherently demodulating signals which exhibit no undesired phase variations that can be implemented effectively whenever time intervals spanned by received modulation symbols, modulation intervals, can be accurately determined at the receiver's location.
In practice, undesired signal-phase &phgr;
u
(t) cannot be replicated exactly at a receiver location, but an estimate of &phgr;
u
(t) can often be generated with sufficient accuracy to allow the implementation of nearly-ideal coherent demodulation—particularly when m-ary PSK is used to generate signal modulation phase &phgr;
m
(t). For ideal (unfiltered) m-ary PSK, the signal's amplitude has a constant value and &phgr;
m
(t) assumes any one of m phases that are equally spaced in a 2&pgr; radians phase range during each modulation interval. As is well known, an m
th
-order nonlinear device having a received m-ary PSK signal applied to its input port and a bandpass filter that rejects undesired components in the nonlinear device's output signal can generate a signal having a radian center frequency of m·&ohgr;
c
and phase m·&phgr;
u
(t) accompanied by a noise signal that derives from the n
&Sgr;
(t) component of the received signal. That is, an m
th
-order nonlinear device provides a means for distinguishing m·&phgr;
u
(t) from &phgr;
m
(t) when m-ary PSK is implemented. The nonlinear device's output signal is typically filtered by a relatively narrow-band phase-lock loop to generate a sufficiently accurate estimate of phase m·&phgr;
u
(t) notwithstanding the presence of noise in the received signal. An ambiguous estimate of phase &phgr;
u
(t) can be determined there from by effectively dividing m·&phgr;
u
(t) by m. An m-fold ambiguity that derives from the divide operation is generally accommodated by either 1) differentially encoding the transmit data stream and differentially decoding the demodulated data stream in a manner appropriated for the value of m implemented or 2) periodically embedding a priori specified symbols within the transmit date stream which replicate properly in the demodulator only when the appropriate one of m ambiguous phase values is selected. Alternatively, phase-lock loops that incorporate other forms of nonlinearities to distinguish &phgr;
u
(t) from &phgr;
m
(t), e.g., a Costas loop, can be used to provide the desired phase estimate when m-ary PSK modulation is employed.
Phase-lock loops that incorporate nonlinearities as described in the preceding paragraph have attributes that limit system performances—particularly when burst signals are used to convey data as in TDMA systems. As is well known, the time required for a phase-lock loop to achieve lock is statistically distributed and occasionally exceeds the mean acquisition time by a considerable amount (such an event is referred to as a phase-lock loop hang-up), and the noise component of the received signal occasionally causes a locked loop to cycle slip, i.e., to lose and regain lock in a manner whereby the accumulated phase of the loop's output signal differs from the accumulated phase of the (multiplied) signal being tracked by one or more cycles. Further, as the nonlinearity order—the value of m—is increase
ComTier
Corrielus Jean
Fenwick & West LLP
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