Reexamination Certificate
1999-11-30
2003-09-09
Pham, Chi (Department: 2631)
Reexamination Certificate
active
06616254
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to data communication systems and more particularly relates to a transmitter for use 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 prevalent. Spread spectrum communications transmits data utilizing a spectrum bandwidth much greater than the bandwidth of the data to be transmitted. This provides for 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 noisy 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 switching devices, such as 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 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 use by the transmitter in modulating the data signal, including biphase shift keying (BPSK) and continuous phase modulation (CPM) techniques. Another modulation technique is minimum shift keying (MSK), which 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 Differential Phase Shift Keying (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.
SUMMARY OF THE INVENTION
The present invention is a transmitter apparatus and method for use in a spread spectrum data communications system that utilizes the Differential Code Shift Keying (DCSK) or non-differential Code Shift Keying (CSK) modulation technique. Such communications systems are applicable to relatively noisy environments such as the AC powerline.
In a CSK transmission system, the data is transmitted in the form of time shifts between consecutive circularly rotated waveforms of length T which are referred to as spreading waveforms, i.e., spread spectrum correlator sequence waveforms. The spreading waveforms can comprise any type of waveform that has suitable autocorrelation properties. During each symbol period, referred to as a unit symbol time (UST), a plurality of bits are transmitted. The symbol period is divided into a plurality of shift indexes with each shift index representing a particular bit pattern. The information, i.e., bit pattern, is conveyed by rotating the spreading waveform by a certain amount corresponding to the data to be transmitted. The data is conveyed in the degree of rotation or circular shift applied to the spreading waveform (also referred to as a chirp) before it is transmitted.
In addition to conveying information in the shift applied to the spreading sequence, additional information can be conveyed in the sign of the symbol or more generally in its phase if there is a carrier present.
In a CSK system, the data is conveyed in the absolute shift assigned to the spreading waveform. In a DCSK system, the data is conveyed in the shift differential between consecutive symbols. The synchronization scheme of the present invention is applicable to both CSK and DCSK transmission systems.
Upon reception by the receiver, the signal is input to a matched filter having a template of the spreading waveform pattern to detect the amount of rotation (or circular shift) within the received signal for each symbol. The received data is fed into a shift register and circularly rotated, i.e., shifted. For each bit shift or rotation, the matched filter generates a correlation sum. A shift index is determined for each UST corresponding to the shift index that yields the maximum (or minimum) correlation sum. Differential shift indexes are generated by subtracting the currently received shift index from the previously received shift index. The differential shift index is then decoded to yield the originally transmitted data.
The transmitter transmits data in the form of packets to the receiver. Each packet is preceded by a preamble comprising a predetermined number of symbols. The length of the preamble can be any suitable number of symbols such that the receiver is able to synchronize with the transmitter. The preamble comprises a sequence comprising any number of zero rotated symbols (or symbols with a constant fixed rotation) followed by any number of non-zero rotated symbols having a known predetermined random shift. The rotation that is applied to each symbol is independent of the rotation applied to other symbols.
In a first embodiment, the shift-index, i.e., the data received from a host source, is used to calculate an initial index. The initial index is used in combination with a counter to address a modulation waveform sample Read Only Memory (ROM). The modulation waveform may be any suitable waveform having good correlation properties. Examples of suitable waveforms include chirp waveforms and Pseudo Noise (PN) sequence waveforms.
The waveform samples are clocked out of the sample ROM starting from a position corresponding to the initial index. This yields a rotated symbol having a degree of rot
Matmor Avner
Raphaeli Dan
Itran Communications Ltd.
Pham Chi
Zaretsky Howard
Zaretsky & Associates PC
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