Pulse or digital communications – Spread spectrum – Direct sequence
Reexamination Certificate
2000-05-31
2004-06-01
Ghebretinsae, Temesghen (Department: 2631)
Pulse or digital communications
Spread spectrum
Direct sequence
C375S144000, C375S146000, C375S147000
Reexamination Certificate
active
06744807
ABSTRACT:
INTRODUCTION AND BACKGROUND
THIS invention relates to a multi-dimensional, direct sequence spread spectrum (DSSS) communication system and method.
In a DSSS system, the spectrum spreading is accomplished before transmission through the use of a spreading sequence that is independent of the data signal. The same spreading sequence is used in the receiver (operating in synchronism with the transmitter) to despread the received signal, so that the original data may be recovered.
In some multiple-access communication systems, a number of independent users are required to share a common channel. It is known to use bipolar phase shift keying spread spectrum (BPSK-SS) or quadrature phase shift keying spread spectrum (QPSK-SS) modems in such systems. However, the data rate or bit error rate (BER) of these systems for a given bandwidth is not always satisfactory for many applications, for example multi-user applications and multimedia applications including voice, data and video data streams.
OBJECT OF THE INVENTION
Accordingly, it is an object of the present invention to provide a multi-dimensional DSSS modem, a modulator and a demodulator for such a modem and associated methods with which the applicant believes the aforementioned disadvantages may at least be alleviated.
SUMMARY OF THE INVENTION
According to the invention there is provided a multi-dimensional spread spectrum modulator comprising:
a spreading sequence generator for generating a set of substantially mutually orthogonal spreading sequences;
a first frequency spreading means comprising a first input for a first set of parallel input data streams; a second input for the set of spreading sequences, respective input data streams being associated with respective spreading sequences in the set of spreading sequences; and an output;
a second frequency spreading means comprising a first input for a second set of parallel input data streams; a second input for the set of spreading sequences, respective input data streams being associated with respective spreading sequences in the set of spreading sequences; and an output; and
a quadrature modulator comprising first and second inputs and an output, the first input of the quadrature modulator being connected to the output of the first frequency spreading means and the second input of the quadrature modulator being connected to the output of the second frequency spreading means.
The set of spreading sequences may be derived from at least one complex sequence comprising a real part and an imaginary part.
Each spreading sequence in the set of spreading sequences may comprise a combination of at least two parts of at least one complex sequence. The combination is preferably an arithmetic sum of a real part and an imaginary part. For example, a first spreading sequence in the set may comprise the sum of the real part of the complex sequence and a negative of the imaginary part of the complex sequence and a second spreading sequence in the set may comprise a negative sum of said real part and said imaginary part of the complex sequence.
The first input of the first frequency spreading means may comprise n/2 branches for n/2 input data streams, the first spreading means may comprise n/2 spreading multipliers; and the set of spreading sequences may comprise n/2 spreading sequences connected to respective multipliers, to spread the n/2 data streams.
The first input of the second frequency spreading means may comprise n/2 branches for n/2 input data streams, the second frequency spreading means may comprise n/2 spreading multipliers and respective ones of the n/2 spreading sequences may be connected to the n/2 multipliers of the second frequency spreading means, to spread the n/2 data streams at the first input of the second frequency spreading means.
The n/2 branches of the first frequency spreading means and the n/2 branches of the second frequency spreading means may be connected to an output of a serial-to-parallel converter.
Output signals of the n/2 multipliers of the first frequency spreading means are preferably added by a first adder, to provide a first spreaded signal; and output signals of the n/2 multipliers of the second frequency spreading means are preferably added by a second adder, to provide a second spreaded signal.
The first and second spreaded signals may be modulated on first and second quadrature carriers respectively, to provide first and second modulated spreaded signals.
The first and second modulated spreaded signals may be added by a third adder to provide an output signal of the modulator.
In another embodiment of the invention the first set of data streams may be duplicated as the second set of data streams.
Also included within the scope of the present invention is an n-dimensional spread spectrum demodulator, the demodulator comprising:
an input for a received spread spectrum signal comprising n data signals;
the input being connected to a circuit for recovering quadrature carriers in the received signal;
a local despreading sequence generator for generating a set of n/2 mutually orthogonal despreading sequences;
the input also being connected to a tracking and synchronizing circuit for tracking spreading sequences in the received signal and synchronizing the locally generated despreading sequences with the tracked spreading sequences; and
circuitry connected to the carrier recovery circuitry and the spreading sequence synchronization circuitry for extracting the n data signals from the received signal, utilizing the recovered quadrature carriers and the n/2 despreading sequences.
The despreading sequences may comprise combinations of real and imaginary parts of complex sequences.
The carrier recovery circuitry may comprise an n-dimensional decision-directed complex Costas carrier recovery loop having a first input for the received spread spectrum signal, a second input for the n/2 despreading sequences, a first output for the n data signals and a second output for the recovered quadrature carriers.
The second output may be taken from a voltage controlled oscillator in the loop and which oscillator is driven by a first error signal derived from n-dimensional decision-directed circuitry of the Costas carrier recovery loop.
The tracking and synchronizing circuit may comprise an n-dimensional decision-directed complex delay-locked-loop having a first input for the received spread spectrum signal, a second input connected to the second output of the carrier recovery circuitry for receiving the recovered carriers; a third input connected to the first output of the carrier recovery circuitry for inputting the recovered data signals; a fourth input for data relating to late and early replicas of the despreading sequences; a first output for the synchronized despreading sequences which is connected to the second input of the carrier recovery circuitry; and a second output for said data relating to late and early replicas of the despreading sequences and which second output is connected to said fourth input.
The first and second outputs are preferably taken from a voltage controlled code generator which is driven by a second error signal derived from n-dimensional decision-directed circuitry of the complex delay-locked-loop circuitry.
Yet further included within the scope of the invention is a modem comprising a modulator as hereinbefore defined and a demodulator as hereinbefore defined.
Still further included within the scope of the invention is a method of frequency spread modulating n data streams, the method comprising the steps of:
dividing the n data streams into first and second groups;
utilizing respective substantially mutually orthogonal spreading sequences to frequency spread each of the streams in the first group, to provide a first group of frequency spreaded signals;
utilizing respective ones of the spreading sequences to frequency spread each of the streams in the second group, to provide a second group of frequency spreaded signals; and
modulating the first group of frequency spreaded signals and the second group of frequency spreaded signals on first
Linde Louis Philippus
Marx Frans Engelbertius
Ghebretinsae Temesghen
Jones Day
University of Pretoria
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