Pulse or digital communications – Synchronizers – Phase displacement – slip or jitter correction
Reexamination Certificate
1999-10-12
2003-09-02
Pham, Chi (Department: 2631)
Pulse or digital communications
Synchronizers
Phase displacement, slip or jitter correction
C370S516000
Reexamination Certificate
active
06614864
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to data communication systems and more particularly relates to an apparatus for and method of synchronizing in a spread spectrum communication system.
BACKGROUND OF THE INVENTION
The use of spread spectrum communications techniques to improve the reliability and security of communications is well known and is becoming more and more common. Spread spectrum communications transmits data utilizing a spectrum bandwidth that is much greater than the bandwidth of the data to be transmitted. This provides for a more reliable communication in the presence of high narrowband noise, spectral distortion and pulse noise, in addition to other advantages. Spread spectrum communication systems typically utilize correlation techniques to identify an incoming received signal.
Spread spectrum communications systems are commonly used in military environments to overcome high-energy narrowband enemy jamming. In commercial or home environments it may be used to achieve reliable communication on noise media such as the AC powerline. In particular, certain home electrical appliances and devices can potentially be very disruptive of communications signals placed onto the powerline. For example, electronic dimming devices can place large amounts of noise onto the powerline since these devices typically employ triacs or silicon controlled rectifiers (SCRs) to control the AC waveform in implementing the dimming function.
A communication medium such as the AC powerline may be corrupted by fast fading, unpredictable amplitude and phase distortion and additive noise. In addition, communication channels may be subjected to unpredictable time varying jamming and narrowband interference. In order to transmit digital data over such channels it is preferable to use as wide a bandwidth as possible for transmission of the data. This can be achieved using spread spectrum techniques.
One common type of spread spectrum communications, called direct sequence spread spectrum, is generated by first modulating the digital data and then multiplying the result with a signal having a particularly desirable spectral properties, such as a Pseudo Noise (PN) sequence. The PN sequence is a periodic sequence of bits having a particular period. Each bit in the sequence is termed a chip. The sequence has the property of having very low autocorrelation for delays larger than one chip. In some systems, the PN sequence is replaced by a chirp waveform. Several techniques are available for the transmitter to modulate the data signal, including biphase shift keying also referred to as bipolar phase shift keying (BPSK) and continuous phase modulatation (CPM) techniques. Minimum shift keying (MSK) is a known variation of CPM.
The spread spectrum receiver is required to perform synchronization that is commonly implemented using an acquisition method in combination with a tracking loop or other tracking mechanism. In a noisy unpredictable environment such as the AC powerline, the tracking loop typically fails frequently causing loss of information. Communication systems to overcome these problems are large, complex and expensive. In addition, these systems typically succeed at transmitting only one or more bits per symbol.
Receiving and demodulating signals that have been subject to PN modulation requires that the same PN code sequence be generated in the receiver and correlated with the received signal to extract the data modulation. One type of correlation technique employs a digital matched filter to compare the received digital signal with the locally generated version of the PN code. The digital filter produces an in phase (I) signal and a quadrature (Q) signal from which a digital demodulator such as a DPSK demodulator can derive data values. Another function of the digital matched filter is to produce correlation measurements from which synchronization signals can be generated.
In despreading a spread spectrum signal, the receiver produces a correlation pulse in response to the received spread spectrum signal when the received spread spectrum signal matches the chip sequence to a predetermined degree. Various techniques are available for correlating the received signal with the chip sequence, including those using surface acoustic wave (SAW) correlators, tapped delay line (TDL) correlators, serial correlators, and others.
Synchronization of signals between a transmitter and receiver that are communicating with each other in a spread spectrum communication system is an important aspect of the process of transmitting signals between them. Synchronization between transmitter and receiver is necessary to allow the despreading of the received signals by a spreading code that is synchronized between them so that the originally transmitted signal can be recovered from the received signal. Synchronization is achieved when the received signal is accurately timed in both its spreading code pattern position and its rate of chip generation with respect to the receiver's spreading code.
One of the problems associated with synchronization is that the techniques used to synchronize two signals are relatively expensive to implement. In communication systems having sophisticated and relatively expensive central communication sites which serve a plurality of relatively inexpensive remote communication sites, it is desirable to reduce the cost of synchronization systems in the remote communication sites while not increasing the cost of the central communication sites.
In spread spectrum systems two general areas of uncertainty of the signal exist which must be resolved before a received spread spectrum signal can be recovered. These areas of uncertainty are spreading code phase and carrier frequency. In addition, spreading code clock rate can be a source of synchronization uncertainty. Most of this uncertainty may be eliminated by utilizing accurate frequency sources in both transmitter and receiver that are communicating with each other. However, some uncertainty cannot be eliminated by the use of accurate frequency sources, e.g., Doppler-related frequency errors.
One well-known synchronization technique involves using a sliding correlator. In the sliding correlator, a spreading code generator operates at a rate different from the rate at which a spreading code generator associated with a transmitter that transmitted the signal to be correlated operates. The effect is that the two spreading code sequences slip in phase with respect to each other, and if viewed simultaneously, the spreading codes would seem to slide past each other until the point of coincidence is reached.
More particularly, a sliding correlator receives a spread spectrum signal that is a function of a particular spreading code and generates a signal locally, which is a function of a locally-generated spreading code that is substantially similar to the particular spreading code. Subsequently, the sliding correlator compares the received signal with the locally generated signal. If the two signals are not determined to be aligned, then the sliding correlator phase shifts the local signal with respect to the received signal and loops back to compare the phase shifted local signal with the received signal. This process continues until the sliding correlator determines that the two signals are aligned at which point the total phase shift of the local signal is stored by the sliding correlator for subsequent use. The total phase shift and the locally-generated spreading code are used to despread subsequently received spread spectrum signals which have been spread with spreading codes which are substantially similar to the locally generated spreading code, but phase-shifted.
In general, prior art receivers that utilize standard synchronization techniques such as matched filter detectors or sliding matched filter detectors may not be able to receive if the channel distorts the transmitted waveform sufficiently such that the waveform at the receiver side does not match or badly matches the original sequence.
SUMMARY OF THE I
Matmor Avner
Raphaeli Dan
Itran Communications Ltd.
Nguyen Dung X
Pham Chi
Zaretsky Howard
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