Pulse or digital communications – Spread spectrum – Direct sequence
Reexamination Certificate
2000-04-18
2003-12-30
Corrielus, Jean B. (Department: 2631)
Pulse or digital communications
Spread spectrum
Direct sequence
C375S130000, C375S150000
Reexamination Certificate
active
06671311
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to data communication systems and more particularly relates to a receiver for use in a code shift keying based direct sequence 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 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 (BPSK) and continuous phase modulated (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.
A common problem encountered in most types of communications systems, particularly spread spectrum communications systems, is the dynamic nature of the channel. In many cases, the characteristics of the channel are not constant and changes with time. Some prior art communications systems employ some form of training sequence at the beginning of the packet before data reception begins. As a result, the receiver is dynamically adjusted for conditions on the channel that exist at the time of the training sequence. Further adaptation to the channel, however, is not performed for the remainder of the packet. In some cases, the packet may have a relatively long duration. In this case, depending on the changes that occur to the channel during reception, the receiver may lose synchronization and from that point onward would not be able to correctly receive data.
Therefore, it would be desirable if the receiver were able to dynamically adapt itself to varying conditions of the channel in such a way that the accuracy of the receiver is maintained. Having the ability to track changes in the channel improves the performance and reliability of the receiver.
SUMMARY OF THE INVENTION
The present invention is a direct sequence spread spectrum receiver for use in a communication system that utilizes a modulation technique referred to as code shift keying (CSK). Use of CSK modulation increases the number of bits transmitted per symbol, decreases synchronization requirements and improves performance. Code shift keying modulation transmits data in the form of circularly rotated spreading waveforms. The spreading waveform may comprise any type of waveform that has suitable auto correlation properties such as a chirp or PN sequence. The receiver of the present invention may be constructed to use any type of spreading waveform including a chirp or PN sequence.
During each symbol period (referred to as a unit symbol time or 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 waveform is rotated by an amount in accordance with the data to be transmitted. The data is conveyed in the amount of rotation applied to the chirp before it is transmitted. Alternatively, the data may be conveyed in the shift differential between consecutive symbols. In addition to the rotation applied to symbols, the phase of the rotated spreading waveform is used to convey an additional bit of information.
The receiver is adapted to receive the signal transmitted through the channel. The receiver operates in either the acquisition mode or the tracking mode. During acquisition, the receiver attempts to acquire synchronization of the signal. It searches for the presence of a preamble and once found tries to synchronize on a synchronization sequence sent by the transmitter. The synchronization sequence is used by the receiver to train the receiver to the conditions on the channel and optionally to generate the initial value for the template to be used during the tracking mode of ope
Matmor Avner
Raphaeli Dan
Corrielus Jean B.
Itran Communications Ltd.
Zaretsky Howard
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