|Publication number||USRE39983 E1|
|Application number||US 11/203,648|
|Publication date||Jan 1, 2008|
|Filing date||Aug 12, 2005|
|Priority date||Sep 10, 1999|
|Also published as||US6606357|
|Publication number||11203648, 203648, US RE39983 E1, US RE39983E1, US-E1-RE39983, USRE39983 E1, USRE39983E1|
|Inventors||Raymond F. Cobb, Michael B. Luntz|
|Original Assignee||Harris Corporation|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (53), Non-Patent Citations (1), Classifications (20), Legal Events (2)|
|External Links: USPTO, USPTO Assignment, Espacenet|
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.
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.
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.
At the transmit site 10, quadrature channel data symbols dI and dQ, that have been encoded with some form of forward error correction (FEC) code, are modulated in mixers 11I and 11Q onto respective phase-quadrature components of a carrier signal fC. 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
At the receiver 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
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 29I/Q, to which the received I/Q channel data plus carrier and the regenerated carrier signals are supplied. The output of the phase detector 29I/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
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 rate ½, wherein two coded symbols are transmitted for every one information symbol, thus doubling the transmission rate and hence the occupied bandwidth. One way to gain efficiency at the expense of bandwidth is to use even lower-rate codes, such as rate ⅓ codes. Another common technique is to concatenate two codes. This most often takes the form of concatenating a convolutional code with a block code, such as a Reed-Solomon code. These two types of codes have good synergy, and significant power gains can be realized with relatively little additional band-spreading.
A significant problem with these types of codes is that they do not necessarily work well at a very low energy per bit-to-noise density ratio (Eb/N0)—on the order of one to zero dB. While these codes are capable of yielding ultra-low error rates at moderate Eb/N0 values, they do not produce a significant drop in required power for moderate error rates (their efficiency falls off rapidly below about 4 dB Eb/N0). Commercially available demodulators are built with this limitation in mind, and do not provide carrier tracking below about 4 dB Eb/N0.
Demodulation with a low-rate code, such as rate ½ or rate ⅓, at a very low Eb/N0 is difficult for two reasons: first-carrier phase and symbol timing are very difficult to recover; second-maintaining soft decision thresholds is also a problem. A high ratio of the symbol rate to the data rate implies a very low ESN0. For example, for a rate ⅓ code, ES/N0 is about 5 dB less than Eb/N0. The demodulator must have sufficient bandwidth to pass these high symbol rates, and thus must work at an extremely low signal to noise ratio (S/N). As discussed above, carrier recovery generally involves a nonlinear operation, such as raising the signal to a power (e.g., fourth power in the carrier regenerator circuit 26 of FIG. 3). When carried out at a very low signal to noise ratio, the signal to noise ratio is reduced even further, making the demodulator's task of recovering the data extremely difficult.
In accordance with the present invention, the above discussed power and bandwidth utilization problems associated with conventional carrierless PSK modulation schemes employed for satellite communication systems are effectively obviated by means of a modulation waveform that 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, for example at about 15 dB below the signal. This injected carrier component serves to facilitate detection and recovery (rather than non-linear regeneration) of the carrier at the receiver.
In addition, the injected carrier-based modulation scheme of the invention exploits the substantially improved performance of modern forward error correction coding schemes, such as but not limited to turbo codes, so as to significantly reduce the signal power required for achieving a relatively low bit error rate. Namely, using a prescribed ‘carrier-injecting’ waveform in combination with advanced coding techniques enables the invention to double the number of small-terminal users on a satellite transponder, thus effectively halving their leasing costs. Conversely, the waveform of the invention allows use of a smaller antenna for more flexible siting or mobility.
Pursuant to a first embodiment, a DC offset or bias is applied to the encoded data stream upstream of the QPSK modulation circuitry, which serves to shift reference levels for the encoded in-phase data symbol stream to values that cause the spectral waveform of the QPSK signal to contain a prescribed amount of carrier signal energy. This readily discernible injected carrier obviates the need for a non-linear carrier regeneration circuit in the carrier recovery path of the receiver, as in a conventional QPSK demodulator. Instead, the carrier may be directly extracted by a carrier recovery phase locked loop.
Although transmitting a carrier at about 15 dB below the signal results in very little performance reduction, the bias loss in QPSK causes degradation of the signal to noise ratio. For −15 dB in carrier leakage, the loss is about 0.3 dB at low ES/N0 values, and becomes progressively worse at higher signal levels. It would be very desirable to avoid this loss; in addition, the transmission of an unmodulated carrier component may result in violation of spectral density requirements, such as international communication standards that limit the energy density that may be irradiated upon the earth from a satellite transponder.
To satisfy these objectives a second embodiment of the invention injects into the QPSK waveform a ‘spread’ carrier, that is functionally equivalent to that achieved by the direct insertion of a DC bias voltage in the first embodiment. Since the loss incurred with QPSK is caused by a difference in level between a ‘1’ and a ‘0’, the difference can be made to average to zero over a symbol time by chopping the transmitted carrier with a pseudo random square wave having values +1 and −1. If the carrier is spread at the symbol rate, with an edge occurring at mid-bit, for example, the signal level will be averaged within the demodulator matched filter and no degradation will result. This restores the QPSK loss to be the same as for BPSK.
If the carrier is spread using a square wave (i.e., an alternating 1-0 pattern, the transitions are aligned so as to occur at the midpoint of the data symbols. This ensures that the carrier bias averages to zero over a data symbol time. However, if pseudo-random spreading is used, transitions do not occur at every symbol. This permits some data symbols to be biased. By using Manchester (bi-phase) coding on the spreading sequence, a transition is guaranteed at each symbol and the bias is removed.
To recover the carrier, the demodulator of the second embodiment uses a despreading sequence generator identical to that used in the modulator since the spreading operation is synchronous with symbol timing. The received signal is bandpass-filtered and down-converted to a complex baseband signal. To extract the carrier, the complex baseband signal is coupled to a despreading mixer along with a despreading PN waveform supplied by a phase locked loop tuned to the symbol rate. The extracted carrier is filtered using a phase lock loop which provides a coherent carrier reference to each of a pair of in-phase and quadrature channel mixers, to which the complex baseband signal is applied. The outputs of the mixers are filtered in matched filters, symbol timing for which is derived by filtering the data component of the chopped spectrum using a phase lock loop. The recovered data samples and the symbol clock are coupled to a downstream decoder.
In addition to injecting a prescribed amount of carrier energy into the transmitted QPSK waveform, the present invention takes advantage of more powerful modern codes, such as but not limited to ‘turbo’ codes, which are capable of extending moderate error rate performance down to low values of Eb/N0. High performance codes tend to be directed more toward achieving ultra-low bit error rates at moderate Eb/N0 values, rather than moderate bit error rates at ultra-low Eb/N0, so that there is no real savings in transmitter power in using commercial modems, which are generally designed to hold in no lower than about 4 dB Eb/N0. To realize a sufficient transmitter power savings, the modem must be capable of recovering a coherent carrier reference at negative ES/N0, which cannot be achieved by a conventional demodulator.
Before describing in detail the new and improved carrier injection-based modulation scheme in accordance with the present invention, it should be observed that the invention resides primarily in a prescribed arrangement of conventional communication circuits, associated signal processing components, and attendant circuitry that controls the operation of such circuits and components. As a result, the configuration of such circuits and components, and the manner in which they interface with other communication system equipment have, for the most part, been illustrated in the drawings by readily understandable block diagrams. The block diagram illustrations show only those details that are pertinent to the present invention, so as not to obscure the disclosure with details which will be readily apparent to those skilled in the art having the benefit of the description herein. Thus, the block diagrams are primarily intended to show the major components of the system in a convenient functional grouping, whereby the present invention may be more readily understood.
As discussed above, although QPSK (and also BPSK) has been a favored modulation method for satellite communication systems—because it transmits no separate (energy consuming) carrier reference—the use of signal to noise ratio degrading non-linear components in the carrier regeneration process effectively prevents successful carrier and phase recovery for a very low value of Eb/N0 (e.g., less than four dB). This problem is rendered still more difficult by the use of lower rate codes, such as rate ⅓, which expand the bandwidth of the signal and hence reduce the signal-to-noise ratio still further relative to that for higher rate codes.
Pursuant to the present invention this problem is solved by modifying the conventional QPSK modulation process described above, so as to inject into the resultant QPSK waveform a prescribed amount of carrier energy, that serves to facilitate detection and recovery (rather than non-linear regeneration) of the carrier at the receiver. In addition, the injected carrier-based modulation scheme of the invention may exploit high performance forward error correction (FEC) coding schemes, to significantly reduce the signal power required for achieving a desired bit error probability.
At the transmit site 40, in-phase (I) channel and quadrature-phase (Q) channel data symbols dI and dQ are encoded in an encoder 41 with a prescribed forward error correction code, such as a high performance code (e.g., a turbo, as a non-limiting example), and coupled to associated mixers 42I and 42Q, to which a carrier signal fC is also applied. The encoded data symbol streams are typically defined as excursions between prescribed voltage levels, shown in
This is diagrammatically illustrated in
It should be noted that the addition of a bias is not limited to a single data channel. Either of both channels may be biased. This has the effect of changing the phase of the transmitted carrier relative to the data streams. For example, if equal bias is applied to both data channels, the carrier phase will be at 450 relative to the I channel.
As shown in
At the receive site 50, signals received by an antenna 52 and associated low noise amplifier circuitry 53 are coupled to a single demodulator loop, which is shown at 58. To demodulate the data, the received signal is coupled to a carrier recovery path 55 and a data recovery path 57. Since a prescribed amount of discrete carrier energy is contained in the transmitted QPSK waveform, that carrier may be readily extracted by a phase locked loop 58, without the need for an upstream signal to noise degrading nonlinear carrier regenerator circuit. The data recovery path 55 includes a pair of phase detectors 59I/Q, to which the received I/Q channel data plus carrier and the extracted carrier signals are supplied. The output of the phase detector 59I/Q represents the encoded data symbols, which are detected using matched filters and are applied to data detection and error correction recovery circuitry 61, to recover the original data.
As discussed above, in addition to injecting a prescribed amount of carrier energy into the transmitted QPSK waveform, the present invention takes advantage of more powerful forward error correction codes which are capable of extending moderate error rate performance down to low values of Eb/N0. A natural limitation exists on how much improvement can be made in power efficiency, no matter how much bandwidth is used. This limit, known as the Shannon Limit, and shown in graphical form in
In the graph of
However, since conventional systems provide carrier tracking only down to about 4 dB Eb/N0, as pointed out above, there is room for more than 4 dB improvement in efficiency at reasonable bandwidths. Even a 3 dB improvement will double the number of signals that can be handled by a transponder, or will reduce antenna gain requirements by half. From the graph of
Because high performance codes tend to be directed more toward achieving ultra-low bit error rates at moderate Eb/N0 values, rather than moderate bit error rates at ultra-low Eb/N0, there is no real savings beyond present data values in transmitter power in using commercial modems, which are generally designed to hold in no lower than about 4 dB Eb/N0. To realize a sufficient transmitter power savings, the modem must be capable of recovering a coherent carrier reference at negative ES/N0. As pointed out above, conventional demodulators do not have this performance. The invention takes full advantage of the power of high performance codes by demodulation at low Eb/N0.
As described above, the spectral waveform of the composite QPSK signal of
In accordance with a second embodiment of the invention, diagrammatically illustrated in
To facilitate PN timing recovery in the demodulator, the PN sequence may be relatively short. Manchester or bi-phase coding of the PN sequence—i.e., multiplying the sequence values by an alternate +1/−1 volt signal—guarantees a transition at each mid-symbol. This, in turn, ensures that the carrier and data signals are time-orthogonal. As a consequence, the carrier does not interfere with the data signal (except for the small power loss described above), and the data bits do not produce phase jitter in the carrier recovery loop.
Since the loss incurred with QPSK is caused by a difference in level between a ‘1’ and a ‘0’, the difference can be made to average to zero over a symbol time by chopping the transmitted carrier with a square wave having values +1 and −1. If the carrier is chopped at the symbol rate, with an edge occurring at mid-bit, for example, the signal level will be averaged within the demodulator matched filter and no degradation will result. This restores the QPSK loss to be the same as for BPSK. It should be noted that the QPSK mechanism of the invention differs significantly from forming a no-carrier BPSK signal and then summing in the carrier in quadrature, as discussed in an article by J. Bussgang et al, entitled: “Phase Shift Keying with a Transmitted Reference,” IEEE Transactions on Communication Technology, Vol. COM-14, NO. 1, February 1966, pp 14-22. This latter approach results in a carrier-containing signal with identical ONE and ZERO values and hence no bias loss. This method cannot be carried out with QPSK, since both quadrature channels are used to send data.
As shown in the demodulator diagram of
The received signal shown at 111T and 111F is filtered in a bandpass filter 113, and then down-converted to a complex baseband signal in an I/Q downconverter 115 (which may comprise quadrature mixer, and A/D converter). The complex baseband signal shown at 115F is coupled to a despreading mixer 121, to which a despreading PN waveform is supplied by a phase locked loop 123, tuned to the symbol rate, so as to produce the carrier signal spectrum 121F shown also in the spectral diagram of
The outputs of the mixers 131 and 133, one of which is shown at 131T, are then filtered in a pair of matched filters 135 for optimum detection, as in a conventional demodulator. As a non-limiting example, the matched filter outputs may be quantized to three or more bits, to provide good decoder performance. Optimum performance of the decoder requires accurate quantized decision levels. The use of a coherent automatic gain control (AGC) circuit 23 i.e., AGC-derived from the reference carrier—provides very accurate AGC and hence quantization levels by virtue of the relatively high signal-to-noise ratio realized by eliminating the non-linear carrier recovery circuit.
Symbol timing for the matched filters 135 shown at 123F is derived by filtering the data component of the chopped spectrum using phase lock loop 123. The recovered data samples shown at 135T and the symbol clock 123F are coupled to a downstream decoder 141.
Further reduction of the spectral lines in the transmitted signal may be accomplished by spreading the carrier spectrum with a more complex sequence, such as a pseudo-random noise (PN) sequence. The carrier is restored by multiplying the received signal by the identical PN sequence at the demodulator. Since the sequences must be identically aligned, a search procedure is implemented to obtain the correct PN sequence phase, as is done for code division multiple access (CDMA) signals in other systems.
As will be appreciated from the foregoing description, power utilization shortcomings of regeneration-based carrierless PSK modulation schemes, such as those employed for satellite communication systems, are effectively obviated by means of a QPSK modulation waveform that usurps a relatively limited portion of the available transmitter power, and injects in its place a prescribed amount of carrier energy, to facilitate recovery of the carrier at the receiver. Moreover, the injected carrier-based modulation scheme of the invention may employed high performance forward error correction coding schemes, to significantly reduce the signal power required for achieving a relatively low bit error rate.
While we have shown and described several embodiments in accordance with the present invention, it is to be understood that the same is not limited thereto but is susceptible to numerous changes and modifications as known to a person skilled in the art, and we therefore do not wish to be limited to the details shown and described herein, but intend to cover all such changes and modifications as are obvious to one of ordinary skill in the art.
|Cited Patent||Filing date||Publication date||Applicant||Title|
|US3569626||Jul 28, 1967||Mar 9, 1971||Kokusai Denshin Denwa Co Ltd||Reference carrier wave synchronizing system|
|US3815034||Sep 28, 1972||Jun 4, 1974||Nippon Electric Co||Demodulator for phase-modulated carrier waves|
|US3828138||May 10, 1973||Aug 6, 1974||Nasa||Coherent receiver employing nonlinear coherence detection for carrier tracking|
|US4216542||Mar 6, 1979||Aug 5, 1980||Nasa||Method and apparatus for quadriphase-shift-key and linear phase modulation|
|US4313205||Jul 7, 1980||Jan 26, 1982||Communications Satellite Corporation||Carrier synchronization and symbol synchronization for offset-QPSK burst communications|
|US4466108||Oct 6, 1981||Aug 14, 1984||Communications Satellite Corporation||TDMA/PSK Carrier synchronization without preamble|
|US4472817||Aug 9, 1982||Sep 18, 1984||Communications Satellite Corporation||Non-PLL concurrent carrier clock synchronization|
|US4501002||Feb 28, 1983||Feb 19, 1985||Auchterlonie Richard C||Offset QPSK demodulator and receiver|
|US4620159||Aug 27, 1985||Oct 28, 1986||Nec Corporation||Demodulator for multiphase PSK or multilevel QAM signals|
|US4674103||Jul 22, 1986||Jun 16, 1987||International Business Machines Corp.||Method and arrangement for detecting the presence of a training signal in a modem receiver|
|US4787096||Mar 4, 1987||Nov 22, 1988||National Semiconductor Corp.||Second-order carrier/symbol sychronizer|
|US4795986||Nov 16, 1987||Jan 3, 1989||Gte Telecommunicazioni, S.P.A.||Method and circuit for carrier synchronism recovery in PSK coherent demodulators|
|US4814719||Dec 14, 1987||Mar 21, 1989||Ford Aerospace & Communications Corporation||Unsymmetrical QPSK demodulator|
|US4847872||Jan 7, 1988||Jul 11, 1989||Ant Nachrichtentechnik Gmbh||Method and arrangement for synchronizing a receiver in digital transmission systems|
|US4953181||Mar 8, 1989||Aug 28, 1990||Lear Siegler Jennings Corp.||Apparatus and method for detecting digital carrier synchronization problems|
|US4980648||Oct 24, 1989||Dec 25, 1990||Amp Incorporated||Local oscillator signal phase acquisition system for digital demodulator|
|US5073904||May 22, 1989||Dec 17, 1991||Hitachi, Ltd.||Digital signal processing type demodulation method and demodulation circuit|
|US5077531||Dec 18, 1990||Dec 31, 1991||Kokusai Denshin Denwa Co., Ltd.||PSK signal demodulation system|
|US5081645||Aug 6, 1990||Jan 14, 1992||Aware, Inc.||Novel spread spectrum codec apparatus and method|
|US5084903||Dec 21, 1990||Jan 28, 1992||First Pacific Networks||Modulation and demodulation system employing AM-PSK and QPSK communication system using digital signals|
|US5097487||Oct 24, 1989||Mar 17, 1992||Walp Patrick K||Local oscillator signal generating system for digital demodulator|
|US5115454||May 12, 1987||May 19, 1992||Kucar Andy D||Method and apparatus for carrier synchronization and data detection|
|US5150383||Mar 7, 1991||Sep 22, 1992||Matushita Electric Industrial Co., Ltd.||Asynchronous quadrature demodulator|
|US5212814||Apr 18, 1991||May 18, 1993||Mitsubishi Denki Kabushiki Kaisha||Automatic output power controller|
|US5214674||Jan 25, 1991||May 25, 1993||Comsat||Method and apparatus for carrier synchronization acquisition in a digital burst mode communication system|
|US5229997||Mar 20, 1992||Jul 20, 1993||Nec Corporation||Carrier recovery control with sequential fec sync detection and frame sync detection|
|US5247543||Aug 15, 1991||Sep 21, 1993||Nec Corporation||Carrier aquisition apparatus for digital satellite communication system|
|US5260974||May 10, 1991||Nov 9, 1993||Echelon Corporation||Adaptive carrier detection|
|US5355114||Mar 16, 1994||Oct 11, 1994||Echelon Corporation||Reconstruction of signals using redundant channels|
|US5412620||Oct 1, 1993||May 2, 1995||Micrilor, Inc.||Hydroacoustic communications system robust to multipath|
|US5440266||Jan 27, 1994||Aug 8, 1995||Fujitsu Limited||Demodulating apparatus having quick pull-in synchronization facility at instantaneous disconnection of input signal|
|US5497160 *||Sep 17, 1993||Mar 5, 1996||Motorola, Inc.||Method and apparatus for improved auto-correlation and range correlation in pseudo-random noise coded systems|
|US5533060||Dec 16, 1994||Jul 2, 1996||Satellite Communication Laboratory Co., Ltd.||Multi-phase PSK signal decoder including phase display generator with phase correcting feedback|
|US5541965||Jan 12, 1994||Jul 30, 1996||U.S. Philips Corporation||Carrier frequency synchronization device using two different B/W filters|
|US5563550||Aug 28, 1995||Oct 8, 1996||Lockheed Martin Corporation||Recovery of data from amplitude modulated signals with self-coherent demodulation|
|US5574399||Oct 30, 1995||Nov 12, 1996||Hideto Oura||Coherent PSK detector not requiring carrier recovery|
|US5623511 *||Nov 28, 1994||Apr 22, 1997||Lucent Technologies Inc.||Spread spectrum code pulse position modulated receiver having delay spread compensation|
|US5691980||Jun 7, 1995||Nov 25, 1997||General Electric Company||Local communication network for power reduction and enhanced reliability in a multiple node tracking system|
|US5694389||Feb 22, 1996||Dec 2, 1997||Kabushiki Kaisha Toshiba||OFDM transmission/reception system and transmitting/receiving apparatus|
|US5715521||Apr 20, 1995||Feb 3, 1998||Oki Electric Industry Co., Ltd.||Method of controlling synchronization signal power in a communication system|
|US5748623||Sep 2, 1994||May 5, 1998||Ntt Mobile Communications Network, Inc.||Code division multiple access transmitter and receiver|
|US5748677||Jan 16, 1996||May 5, 1998||Kumar; Derek D.||Reference signal communication method and system|
|US5790602||Dec 15, 1995||Aug 4, 1998||E-Systems, Inc.||Receiver synchronization using punctured preamble|
|US5799047||Sep 12, 1996||Aug 25, 1998||Nec Corporation||Carrier synchronization unit and synchronization method|
|US5815115 *||Dec 26, 1995||Sep 29, 1998||Lucent Technologies Inc.||High speed wireless transmitters and receivers|
|US5828694||Jul 1, 1996||Oct 27, 1998||Trimble Navigation Limited||Determination of multipath tracking error|
|US5838736||Jan 16, 1996||Nov 17, 1998||Alcatel N.V.||Method and apparatus for generating carriers for phase demodulation having at least two phase states, and a corresponding demodulation stage|
|US5875215||Aug 21, 1996||Feb 23, 1999||Nec Corporation||Carrier synchronizing unit|
|US5892798 *||Nov 3, 1997||Apr 6, 1999||Trw Inc.||Modulated clock MSK modulator for high data rate applications|
|US5914950||Apr 8, 1997||Jun 22, 1999||Qualcomm Incorporated||Method and apparatus for reverse link rate scheduling|
|US6094450||Jun 20, 1997||Jul 25, 2000||Cincinnati Electronics Corporation||Spread spectrum chip shift keying modulation/demodulation system and method|
|US6396880||Apr 17, 1998||May 28, 2002||Analog Devices Inc||π/4 DQPSK encoder and modulator|
|US6618096 *||Sep 29, 1997||Sep 9, 2003||Scientific-Atlanta, Inc.||System and method for adaptively balancing quadrature modulators for vestigial-sideband generation|
|1||*||"Phase Shift Keying with a Transmitted Reference" by J.J. Bussgang and M. Leiter, IEEE Transactions on Communication Technology, vol. Com-14, No. 1, Feb., 1966 pp. 14-22.|
|U.S. Classification||375/281, 332/103, 329/304, 375/332, 375/308|
|International Classification||H04L27/00, H04L25/06, H04L27/10, H04L27/227, H04L27/20, H03D3/00, H04L1/00|
|Cooperative Classification||H04L1/004, H04L25/06, H04L27/206, H04L27/2273, H04L2027/0067|
|European Classification||H04L27/20D2B, H04L27/227A3, H04L25/06|
|Feb 14, 2011||FPAY||Fee payment|
Year of fee payment: 8
|Feb 12, 2015||FPAY||Fee payment|
Year of fee payment: 12