Pulse or digital communications – Systems using alternating or pulsating current – Angle modulation
Reexamination Certificate
1999-09-10
2003-08-12
Vo, Don N. (Department: 2631)
Pulse or digital communications
Systems using alternating or pulsating current
Angle modulation
C375S308000, C375S332000, C329S304000, C332S103000
Reexamination Certificate
active
06606357
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates in general to communication systems, and is particularly directed to a new and improved modulation scheme that is especially suited for QPSK-based satellite communication systems. The modulation scheme is effective to rob a relatively limited portion of the available transmitter power, and inject into the QPSK waveform a prescribed amount of carrier energy, through which detection and recovery of the carrier at the receiver may be achieved without incurring a signal to noise degradation penalty. In addition, the injected carrier-based modulation scheme of the invention exploits the substantially improved performance of modern forward error correction codes, and significantly reduces the signal power required for achieving a relatively low bit error rate.
BACKGROUND OF THE INVENTION
A major concern of both providers and users of satellite communication systems is how to maximize the use of system resources. The most important resources are considered to be transponder bandwidth and effective isotropic radiated power (EIRP), since some portion of each is employed by every signal sent through the transponder. Because satellite resources are expensive (for example, a single transponder may cost hundreds of thousands of dollars per month in leasing fees), for the case where satellite power is the scare resource, minimizing the amount of power required for each signal allows more signals to be sent through the transponder, and thereby reduces leasing fees. An alternative application is to reduce the aperture size of the receiver antenna for the same transponder power. More recently developed low-cost systems that use small aperture antennas tend to be power-limited as they have lower G/T values, and therefore require more power from the satellite.
FIGS. 1 and 2
are examples of simulated spectrum analyzer displays of satellite transponder utilization. In the spectral utilization of
FIG. 1
, wherein the available passband is essentially ‘packed’ with signals, optimum utilization is obtained when all of the transponder's available EIRP is employed. The utilization diagram of
FIG. 2
, on the other hand, shows the case where more than half of the transponder's bandwidth goes unused. This condition indicates that money is wasted, since users are paying for the entire transponder, but utilizing -only a portion of its available capacity. Such power-limited utilization of the transponder may be due to the receiving antennas on the ground having relatively small apertures, so that more power is required for adequate signal quality.
Earth terminals of commercial satellite communication systems have historically employed relatively large, and therefore large gain-to-noise temperature (G/T) ratio, antennas. Since these systems tend to be bandwidth-limited, considerable effort has gone into developing more bandwidth-efficient modulation techniques, such as using some form of M-ary phase shift keying (MPSK) and quadrature amplitude modulation (QAM). Much less work has been carried out in improving power efficiency than in improving bandwidth efficiency. If more power-efficient modulation techniques were available, then each signal would require less power, and a larger number signals could be sent through a power-limited transponder. Alternatively, if the amount of power a given signal requires can be minimized, the required earth terminal EIRP and hence transmitter and/or antenna aperture size can be minimized. This is a third major benefit to small-aperture systems, which enjoy: 1- reduced satellite power usage; 2- reduced transmitter power or antenna aperture for the ground terminal; and 3- reduced antenna aperture for the receive terminal. The first and second benefits go together, while the third may be considered a trade-off against the first and second.
FIG. 3
diagrammatically illustrates the modulation and demodulation signal schemes employed by respective transmitting and receiving earth stations
10
and
20
that are linked by a satellite transponder
30
of a typical QPSK system. Historically, QPSK (and also BPSK) has been a preferred modulation scheme for satellite communications since, among other advantages, no additional energy is required to transmit a discrete carrier reference. Instead, the demodulator is responsible for restoring or ‘regenerating’ the carrier based on the received signal.
At the transmit site
10
, quadrature channel data symbols d
I
and d
Q
, that have been encoded with some form of forward error correction (FEC) code, are modulated in mixers
11
I and
11
Q onto respective phase-quadrature components of a carrier signal f
C
. As will be discussed in detail below, the use of forward error correction encoding of the data serves to trade bandwidth for power. The phase quadrature modulated signals are then summed in a summer
13
into a composite QPSK signal. This QPSK signal, a spectral waveform for which is shown in
FIG. 4
, is transmitted via amplifier-feed circuitry
14
coupled to an antenna
15
.
At the receive site
20
, signals received by an antenna
22
and associated low noise amplifier circuitry
23
are coupled to a demodulator loop, which supplies both I and Q carrier references. To demodulate the data, the received signal is coupled to a carrier recovery or regeneration path
25
and a data recovery path
27
. As shown in the spectral diagram of
FIG. 4
, since no discrete carrier component is separately transmitted from the transmit site
10
, the carrier must be ‘regenerated’ at the receive site
20
.
For QPSK signals this is usually accomplished by means of a relatively complex circuit
26
, such as a Costas loop, or a fourth-power circuit, so as to provide a carrier reference. Its output drives a phase locked loop
28
, so as to provide a carrier reference for the data recovery path. The data recovery path
27
includes a phase detector
29
I/Q, to which the received I/Q channel data plus carrier and the regenerated carrier signals are supplied. The output of the phase detector
29
I/Q represents the encoded data symbols, which are applied to downstream error correction recovery circuitry to recover the original data.
As described above with reference to the transponder utilization diagram
FIG. 2
, a large percentage of transponder bandwidth often goes unused, so that improving power efficiency will allow more signals to be transmitted through the same transponder. In fact, using more bandwidth to gain power efficiency is a good trade in many systems, as there will still be sufficient bandwidth to support additional users. One way to trade bandwidth for power is to avoid the use of modulation waveforms, such as QAM, that give up power efficiency for bandwidth. As shown in system diagram of
FIG. 3
, described above, another technique is to use forward error correcting codes. In addition to the use of FEC codes, error detection and retransmission can be used to minimize transponder power usage.
Forward error correcting codes trade bandwidth for power by sending redundant symbols in order to enable errors to be corrected at the receive site. Forward error correction has a long history in satellite communication systems and many types of decoders are available as inexpensive chips. Some codes employ check bits to verify that no errors were made in the reception. If an error is detected, then the receiving site requests that the transmitter site re-send the block of data where the error appeared. This can be a difficult technique for communication over geosynchronous satellites, due to the long time delays involved. Protocols have been developed with these delays in mind, and many systems now employ both error detection and retransmission. Still, in heavy fading conditions, as can occur during rainstorms, the system may often become clogged with retransmissions. As a result, performing all error correction at the receiver is highly desirable, even if retransmission is used.
At present, the most commonly used error correcting codes are convolutional codes, typically running at ra
Cobb Raymond F.
Luntz Michael B.
Allen Dyer Doppelt Milbrath & Gilchrist, P.A.
Harris Corporation
Vo Don N.
LandOfFree
Carrier injecting waveform-based modulation scheme for... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Carrier injecting waveform-based modulation scheme for..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Carrier injecting waveform-based modulation scheme for... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3075297