All-digital non-conventional chaotic communication system for elastic communication and signaling

Abstract

An all-digital spread-spectrum communication system employing chaotic symbol modulation includes a transmitter having a symbol mapper that converts a sequence of information bits into a sequence of bit symbols, a digital chaos modulator that employs an M-ary chaos shift keying (M-CSK) architecture to chaotically spread the bit symbols in the digital domain, including a separate chaos generator for each M-CSK symbol, and a digital-to-analog converter (DAC) that converts the chaotically modulated bit symbols into an analog signal for transmission. The system also includes a receiver responsive to an analog signal from the transmitter and generating a received signal from the analog signal. The receiver performs signal acquisition on the received signal using a lookup table and a transmitter ID in the received signal, performs tracking on the received signal using a receiver ID in the received signal, performs despreading and demodulation on the received signal, and performs bit removal from the symbols in the received signal.

Description

Technical Field

[0001]

[0001] This disclosure generally relates to variants of spread spectrum communication systems employing chaos modulation and coding, and more particularly to the implementation of an all-digital spread spectrum communication system including a transmitter having a digital chaos modulator that employs digital chaos symbol modulation and coding and has a chaos generator for each symbol. Prior Art

[0002]

[0002] Digital communication systems typically map or transform a stream of information bits encoded for transmission into a constellation of symbols, where each symbol defines a group of bits. For example, a bit mapper can employ M-ary phase shift keying (M-PSK) that supplies in-phase and quadrature-phase bits for each symbol to be transmitted. The mapped symbols are then modulated onto a waveform, filtered, and converted / upconverted into an analog signal for transmission. When this analog signal is received by a receiver, the signal is converted into a digital signal to remove the carrier wave, and this digital signal is demodulated to reproduce the bit symbols. Knowledge of the time and position of the individual symbols in the signal is required to correctly determine the value of each symbol. The information bits are then extracted from the bit symbols.

[0003]

[0003] For certain applications, it is desirable to transmit data or communication signals without being detected by any other party such as an enemy. That is, conventionally, various low probability of interception / low probability of detection (LPI / LPD) communications are applied so that the enemy does not know that a signal is being transmitted. One technique is to spread the energy of the transmission signal, which is usually transmitted over a relatively narrow frequency band, over a wide frequency band or spectrum, known in the art as direct sequence spectrum spreading processing, so that the signal energy disappears in the background and cannot be detected readably. Conventionally, spectrum spreading systems use a pseudo-noise (PN) sequence to spread information bits in combination with conventional modulation techniques such as M-PSK, M-ary quadrature amplitude modulation (M-QAM), etc. for transmission purposes. These techniques bury the signal below the noise floor, but cannot hide its characteristics and the enemy can detect it. Sub-techniques of spectrum spreading processing include spreading the signal with a chaotic sequence to spread out the energy of the transmission waveform. The modulation techniques employed for chaotic spread signals are usually conventional modulation and coding such as M-PSK, M-QAM, etc., and simple synchronization is enabled using conventional demodulation and decoding between the modulated bits transmitted by the transmitter and the bits received by the receiver. However, when conventional modulation and coding techniques are employed in a digital communication system, the effectiveness of chaotic spreading of the transmission signal is reduced. Therefore, in these types of communication systems, advantages can be obtained by providing full chaotic spreading and modulation of the information signal.

Brief Description of the Drawings

[0004]

Figure 1

Figure 2

Figure 3

Figure 4

Best Mode for Carrying Out the Invention

[0005]

[0008] The following discussion of embodiments of the present disclosure directed to all-digital communication systems that employ chaos spreading, modulation, and coding is merely exemplary in nature and is not intended to limit the present disclosure in any way, nor to limit its application or use.

[0006]

[0009] Figure 1 is a schematic block diagram of an all-digital chaos communication system 10. The all-digital chaos communication system 10 includes a transmitter 12 that transmits encoded data and / or information signals through a communication channel 14, such as a wireless communication channel, and a receiver 16 receives it. The communication system 10 is intended to be used for any application that can obtain the effect of spread-spectrum signal processing. The transmitter 12 includes an encoder 20, such as a forward error correction (FEC) encoder. The encoder 20 corresponds to a channel coding method, such as convolutional coding, Reed-Solomon coding, low-density parity-check (LDPC) coding, turbo coding, etc., and adds redundant bits to the information bits supplied on line 18 for transmission for the purpose of error correction, and supplies a stream of encoded information bits. The encoded information bits are sent to a symbol mapper 22, and the symbol mapper 22 converts these bits into bit-symbol constellations, such as 2 bits per symbol, 4 bits per symbol, etc., in a manner that is correctly understood by those skilled in the art.

[0007]

[0010] Then, the symbol is modulated by a digital chaos modulator 24. For example, the digital chaos modulator 24 employs an M-ary chaos shift keying (M-CSK) architecture, represents the symbol as a chaotic sequence of values, and spreads the energy of the symbol over a wider spectrum to be lower than the noise floor. Here, an m-bit mapping per symbol corresponds to M-CSK, where M = 2 mrepresents the number of symbols. In this embodiment, the modulator 24 employs a separate chaos generator 26 for each of the M symbols. For example, when mapping 2 bits per symbol, the modulation is performed as 4-CSK (provide), and four chaos generators 26 are employed. Each of the chaos generators 26 generates, for example, a unique symbol for 4-CSK, symbol 00 is supplied by one of the chaos generators 26, symbol 01 is supplied by another one of the chaos generators 26, symbol 10 is supplied by another one of the chaos generators 26, and symbol 11 is supplied by another one of the chaos generators 26. A chaos diffusion coefficient is selected to set how many chaos (diffusion) bits are used to represent one symbol. The chaos diffusion coefficient determines, for the generator 26, how much the symbol is diffused, i.e., the number of chips or samples per symbol. Specifically, with a diffusion coefficient of length L, the symbol is represented by L chaos bits or samples. For example, with a diffusion coefficient of length 512, each symbol is represented by 512 chaos bits or samples. When the next symbol is repeated, the generator 26 starts from chaos bit (r - 1)·L + 1. Here, r is the symbol repetition count. When the symbol is first represented, the generator 26 starts from chaos bit or sample 1, and when this symbol is represented the second time, the generator 26 starts from chaos bit or sample L + 1, and so on. The chaos sequence is selected so that they are never repeated for a given application. The chaos bits or samples are configured in frame units, and preamble bits and a synchronization function are packed in. The synchronization function assists the receiver 16 in determining the chaos state used for transmission, and thus helps with symbol synchronization for the restoration of the transmitted data. Since the synchronization period is short, conventional synchronization techniques can also be employed, if desired, and the risk of being detected is low. A selector (see Figure 2) selects which of the symbols diffused by the modulation process to output from the modulator 24 at any given point in time.

[0008]

[0011] Next, the chaotic modulated symbol is sent to a digital / analog converter (DAC) 30, such as a high-speed interpolation DAC or a delta-sigma DAC. The digital / analog converter 30 modulates the digital signal onto an analog waveform and utilizes the available Nyquist zone to establish an offset carrier prior to transmission. Note that if desired, the analog signal can be upconverted to a higher frequency, but this is not necessary if the DAC and the Nyquist zone are appropriately selected. Next, the analog signal is filtered by an image rejection filter 32. The image rejection filter 32 removes a replica that is generated by the DAC 30 and transmitted on channel 14 by an antenna 34, such as an omnidirectional antenna, e.g., a whip or dipole antenna, or a directional antenna, e.g., an AESA or a reflector antenna.

[0009]

[0012] The transmitted signal on channel 14 is received by the corresponding antenna 40 in the receiver 16. If it was up-converted in the transmitter 12, it is down-converted to a lower frequency and then converted to a digital signal by an analog / digital (ADC) converter 42 to extract the transmitted symbols. The receiver 16 first performs signal acquisition based on a local look-up table and the transmitter ID, and then performs tracking using the receiver ID. In the correlation processor 44, despreading and demodulation operations are performed on the received signal. The correlation processor 44 includes a plurality of correlators 46 corresponding to a desired resolution. For example, for a fast search at the beginning of a frame, it includes three or more correlators in parallel. Each correlator 46 receives a digital sequence or sample sent to a chaos generator 48 similar to the chaos generator 26 to remove the chaos sequence. Then, by the correlation process, the digital sequence or sample is filtered by a band-pass filter 50, multiplied by a multiplier 52, and integrated by an integrator 54 in a known manner. The correlated bits from each of the correlators 46 are then added in an adder 56. A soft or hard decision processor 58 removes these bits from the symbols, and a decoder 60 removes redundant bits and supplies information bits on line 62 using a process known from the literature.

[0010]

[0013] As described above, in order for the receiver 16 to be able to extract the transmitted symbol as discussed in this specification, for example, chaos state and symbol synchronization 28 are used, and synchronization of the transmitter and receiver, as well as data transmission tracking, are required. As discussed below, the synchronization between the DAC 30 and ADC 42 and the hardware clock is achieved by transmitting an acquisition or preamble synchronization pulse from the transmitter 12 to the receiver 16 and phase-locking the DAC 30 and ADC 42 using, for example, clock synchronization 36. The preamble synchronization pulse can be generated by conventional techniques such as using quadrature phase shift keying (QPSK) for a short time period, by chaos techniques such as using differential chaos shift keying (DCSK) synchronization pulses for a short time, or by inverse chaos techniques using RF analog synchronization pulses. The chaos state synchronization of the chaos generator 26 can be achieved by transmitting a synchronization pulse from the transmitter 12 to the receiver 16. For data transmission tracking, the correlation processor 44 can use a threshold detector to determine whether a signal is present or not.

[0011]

[0014] Figure 2 is a schematic block diagram of a chaos waveform generation system 70 that shows the synchronization and tracking of this type. The system 70 supplies transmitter and receiver ID bits as well as preamble bits into the transmission message. The system 70 includes a synchronization block 72 that represents synchronization 28. The synchronization block 72 supplies, in block 74, a time of day (TOD) signal converted from a GPS signal if the GPS signal is available on line 76, or supplies, in block 78, a local transmitter time or a known constant as well as the transmitter and receiver IDs. The transmitter and receiver IDs supply the receiver preamble and ID bits shown in block 80, as well as the transmitter ID, preamble, and data bits shown in block 82. These are sent to a symbol mapping block 84 that represents the symbol mapper 22. The TOD signal and the transmitter and receiver ID bits are also supplied to a digital chaos sequence generator 86 that represents the generator 26 in a chaos state synchronization block 88. The chaos state synchronization block 88 supplies a chaos bit sequence, together with a clock signal on line 92, to a multiplexer 90. Here, the symbol mapping block 86 selects the output of the multiplexer 90. If desired, the chaos sequence selected by the multiplexer 90 can be filtered by a baseband bandpass filter 94, and then the filtered chaos sequence can be converted to an analog signal by a DAC 96 that represents the DAC 30 and filtered by an image rejection filter 98 that represents the filter 32. If the baseband bandpass filter 94 is not used, the chaos sequence selected by the multiplexer 90 is converted to an analog signal by the DAC 96 and filtered by the image rejection filter 98. The combination of a unique chaos waveform and a strong orthogonality property for every symbol, and, if desired, filtering for each symbol, minimizes inter-symbol interference (ISI) and correlates a spectrum limited die with the bandpass filtered signal to reduce energy loss due to filtering

[0012]

[0015] One approach for coping with chaotic states and symbol synchronization 28 involves determining a sequence of chaotic bits, x n+1 = f1(x n ) + a · f1(x n ) by identifying a chaos generating function. To generate a matching pair in the transmitter 12 and the receiver 16, they need to have the same initial state x0 and the same appropriately selected bifurcation parameter a, and the parameter a should be updated less frequently. Both the initial state x0 and the bifurcation parameter a depend on the output of a keying function that depends on known constants, namely, in the case of no TOD from the TOD or GPS, i.e., in a GPS denied environment, as well as on the transmitter and receiver IDs, and are loaded into a lookup table. The chaos initial key generation function is denoted by the transmitter ID as n x and the receiver ID as n y . The initial state key and the chaos function parameter are generated by the function g(·) for the signal acquisition phase [a, x0] = g(t GPS , n y ) when GPS-aided TOD is available, and in the absence of GPS, they are generated for [a, x0] = g(C, n y ). Tracking during signal transmission is indicated by [a, x0] = g(t x , n x ) and does not require GPS. The resulting synchronization structure includes preamble bits, the transmitter ID, a constant in the absence of the transmitter TOD or GPS, thus the TOD, and the end of the pulse signal.

[0013]

[0016] The chaos state synchronization described above can be represented by FIG. 3. This shows a transmitter timeline 110 and a receiver timeline 112. At time t, the transmitter 12 generates and transmits a synchronization pulse 114 using a chaos keying function. The chaos keying function uses the receiver ID and the TOD signal. The synchronization pulse 114 also informs the receiver 16 of the transmitter's TOD. The receiver 16 forms a correlation pair using its own ID and TOD and waits for an incoming signal in time block 116. The receiver 16 discovers the correlation, detects the synchronization pulse, and decodes it as pulse 118. The receiver 16 uses the transmitter ID and the transmitter TOD to form the next correlation pair for data pulses and uses the correlation at the receiver 16 for demodulation. Tracking may be required to search for the start of data with a pilot symbol. This is an example of open-loop synchronization, and there is no handshake between the transmitter 12 and the receiver 16. To ensure synchronization, the initial preamble frame can be repeated multiple times. The number of repetitions varies depending on the operating environment. Alternatively, a closed-loop synchronization scheme can be implemented, in which case the receiver 16 sends either an acknowledgement or a corresponding frame to the transmitter 12.

[0014]

[0017] The chaos generator 26 generates a chaos bit or sample sequence, and each of the generators 26 has a different initial seeding. The initial seeding determines the chaos sequence that is generated, and the next value of one generator 26 becomes the first value of the next generator 26. The generators 26 are only correlated with themselves and appear like white noise.

[0015]

[0018] The communication performance of the proposed chaos communication system closely follows that of an M-FSK (Frequency Shift Keying) communication system. The optimal spreading factor for a given application can be derived by simulation. For example, FIG. 4 is a graph showing the ratio of the energy per bit (Eb) to the spectral noise density (No) in dB on the horizontal axis and the BER on the vertical axis, and shows a simulation indicating that the uncoded BER performance of a 4-CSK system is closest to optimal when the spreading factor (SF) is 512. Graph line 130 is for a 4-CSK system with SF = 64, graph line 132 is for a 4-CSK system with SF = 512, graph line 134 is for a 4-CSK system with SF = 32768, graph line 136 is for a 4-CSK system with SF = 64 and using a convolutional code (CC) with a coding rate of 1 / 2, graph line 138 is for a 4-CSK system with SF = 512 and using a CC with a coding rate of 1 / 2, graph line 140 is for a 4-CSK system with SF = 42768 and using a CC with a coding rate of 1 / 2, graph line 142 is for a 4-CSK system with SF = 64 and using a CC with a coding rate of 1 / 2 and a Reed-Solomon (RS) code 255,171 such that the overall coding rate is 1 / 3, graph line 144 is for a 4-CSK system with SF = 512 and using a CC with a coding rate of 1 / 2 and an RS code 255,171 such that the overall coding rate is 1 / 3, graph line 146 is for a 4-CSK system with SF = 32768 and using a CC with a coding rate of 1 / 2 and an RS code 255,171 such that the overall coding rate is 1 / 3. The coding gain between graph lines 132 and 138 is about 3.7 dB in BER, and the coding gain between graph lines 132 and 144 is about 5.2 dB in BER. No significant performance advantages are obtained even when the spreading factor increases beyond 512. Also, encoding the information bits with either a convolutional code or a combined convolutional-Reed Solomon code can improve the BER performance. -5 in BER is about 3.7 dB, and the coding gain between graph lines 132 and 144 is about 5.2 dB -5 in BER. No significant performance advantages are obtained even when the spreading factor increases beyond 512. Also, encoding the information bits with either a convolutional code or a combined convolutional-Reed Solomon code can improve the BER performance.

[0016]

[0019] What has been described and disclosed above are merely representative embodiments of the present disclosure. It will be readily recognized by those skilled in the art that various changes, modifications, and variations can be made from such descriptions, as well as from the accompanying drawings and claims, without departing from the spirit and scope of the present disclosure as defined in the following claims.

Claims

1. A communication system, comprising: a symbol mapper that converts a series of information bits into a series of bit symbols, a digital chaos modulator that performs chaos spreading modulation on the bit symbols in the digital domain, and a digital-to-analog converter (DAC) that converts the bit stream of the bit symbols subjected to the chaos spreading modulation into an analog signal for transmission without the need for up-conversion; a transmitter; a receiver that generates a received signal from the analog signal from the transmitter, wherein the receiver performs signal acquisition using a look-up table and a transmitter ID in the received signal, performs tracking of the received signal using a receiver ID in the received signal, performs despreading and demodulation of the received signal, and performs bit removal from symbols in the received signal; hardware for synchronizing the hardware clocks in the transmitter and the receiver by transmitting an acquisition or preamble synchronization pulse in the analog signal transmitted by the transmitter; and the receiver forms a correlation pair using the receiver ID to detect and decode the preamble synchronization pulse, and forms a correlation pair using the transmitter ID for demodulation of the received signal. A communication system.

2. The system according to claim 1, wherein the digital chaos modulator employs an M-ary chaos shift keying (M-CSK) architecture without any combination with other conventional communications.

3. The system according to claim 2, wherein the digital chaos modulator includes a separate chaos generator for each of the M-CSK symbols.

4. The system according to claim 3, wherein each of the chaos generators has a different initial seeding, the initial seeding determines the chaos sequence generated by the chaos generator, and the next value of one chaos generator becomes the first value of the next chaos generator.

5. The system according to claim 2, wherein the M-CSK architecture is 4-CSK.

6. The system according to claim 1, wherein the preamble synchronization pulse includes chaos state synchronization for the digital chaos modulator.

7. In the system according to claim 6, wherein the chaotic state synchronization uses a chaos generation function that determines a sequence of chaos bits used by the digital chaos modulator, and the chaos generation function uses an initial state and a constant parameter vector, a system.

8. In the system according to claim 7, wherein the preamble synchronization pulse includes the transmitter ID and the receiver ID used by the chaotic state synchronization, a system.

9. In the system according to claim 7, wherein the preamble synchronization pulse includes time-of-day (TOD) information from a GPS signal, or a known value in the absence of GPS used by the chaotic state synchronization, a system.

10. In the system according to claim 1, wherein the transmitter uses a conventional modulation scheme such as quadrature phase shift keying (QPSK) for a short time period, or a chaos technique that uses a differential chaos shift keying (DCSK) synchronization pulse for a short time period, or an inverse chaos technique that uses an RF analog synchronization pulse, to generate the preamble synchronization pulse, a system.

11. A communication system, a symbol mapper that converts a series of information bits into a series of bit symbols, and a digital chaos modulator that employs an M-ary chaos shift keying (M-CSK) architecture to chaotically spread the bit symbols in the digital domain, the digital chaos modulator including a separate chaos generator for each of the M-CSK symbols, the digital chaos modulator, and a digital-to-analog converter (DAC) that converts the bit symbols chaos-spread modulated by the digital chaos modulator into an analog signal for transmission, a transmitter including: a receiver that generates a received signal from the analog signal in response to the analog signal from the transmitter, the receiver performing signal acquisition on the received signal using a look-up table and a transmitter ID in the received signal, performing signal tracking using a receiver ID in the received signal, performing despreading and demodulation on the received signal, and performing bit removal from symbols in the received signal, a receiver. In the analog signal transmitted by the transmitter, hardware for synchronizing between the hardware clocks in the transmitter and the receiver by transmitting an acquisition or preamble synchronization pulse, and comprising a communication system, wherein the receiver forms a correlation pair using the receiver ID to detect and decode the preamble synchronization pulse, and forms a correlation pair using the transmitter ID for demodulation of the received signal.

12. The system according to claim 11, wherein the preamble synchronization pulse includes chaos state synchronization for the digital chaos modulator.

13. The system according to claim 12, wherein the chaos state synchronization uses a chaos generation function that determines a sequence of chaos bits used by the digital chaos modulator, and the chaos generation function uses an initial state and a constant parameter vector.

14. The system according to claim 13, wherein the preamble synchronization pulse includes the transmitter ID and the receiver ID used for the chaos state synchronization.

15. The system according to claim 14, wherein the preamble synchronization pulse includes time (TOD) information from a GPS signal or a known value used for the chaos state synchronization.

16. A communication method comprising: converting a series of information bits into a series of bit symbols; performing chaos spread modulation of the bit symbols in the digital domain; converting the chaos spread modulated bit symbols into an analog signal for transmission by a transmitter; in a receiver, performing signal acquisition of the analog signal using a look-up table and a transmitter ID in the analog signal, and performing tracking of the analog signal using a receiver ID in the analog signal; in the receiver, performing despreading and demodulation of a received signal generated from the analog signal, and performing bit removal from the symbols in the received signal; synchronizing between the hardware clocks in the transmitter and the receiver by transmitting an acquisition or preamble synchronization pulse in the analog signal transmitted by the transmitter; and including Furthermore, in the receiver, the method includes forming a correlation pair using the receiver ID to detect and decode the preamble synchronization pulse, and forming a correlation pair using the transmitter ID for demodulation of the received signal. Communication method. **Claim 17** The method according to claim 16, wherein the step of performing chaos spread modulation of the bit symbol employs an M-ary chaos shift keying (M-CSK) architecture.

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