WO1998027802A2 - Technique de densite spectrale de puissance - Google Patents

Technique de densite spectrale de puissance Download PDF

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Publication number
WO1998027802A2
WO1998027802A2 PCT/US1997/023296 US9723296W WO9827802A2 WO 1998027802 A2 WO1998027802 A2 WO 1998027802A2 US 9723296 W US9723296 W US 9723296W WO 9827802 A2 WO9827802 A2 WO 9827802A2
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WIPO (PCT)
Prior art keywords
sequence
signal
emichment
enrichment
spreading
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PCT/US1997/023296
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English (en)
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WO1998027802A3 (fr
Inventor
Robert J. Davis
Dale Edward Reiser
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Sanderford, Hugh, Britton
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Application filed by Sanderford, Hugh, Britton filed Critical Sanderford, Hugh, Britton
Priority to AU70972/98A priority Critical patent/AU7097298A/en
Publication of WO1998027802A2 publication Critical patent/WO1998027802A2/fr
Publication of WO1998027802A3 publication Critical patent/WO1998027802A3/fr

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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B1/00—Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
    • H04B1/69—Spread spectrum techniques
    • H04B1/707—Spread spectrum techniques using direct sequence modulation
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04J—MULTIPLEX COMMUNICATION
    • H04J13/00—Code division multiplex systems
    • H04J13/10—Code generation

Definitions

  • the present invention relates to spectral enrichment, and in particular to an enrichment technique that produces a spread spectrum modulated signal from a short length spreading sequence with characteristics similar to a spreading sequence of much greater length.
  • the preferred embodiment of the present technique allows for shorter length spreading sequences to be utilized with similar Power Spectral Density (PSD) and probability of reception properties to that of longer spreading sequences.
  • PSD Power Spectral Density
  • Other advantages of the present technique, with shorter spreading sequences includes shorter acquisition times due to shorter spreading sequences, preamble signal identification, and on-air transmit time reduction.
  • FCC Federal Communications Commission
  • C.F.R. Code Of Federal Regulations
  • Power spectral density is a measure of the power a spread spectrum transmitter produces over a given frequency range (bandwidth), and thus a measure of it's capability to interfere with other (i.e. licensed) wireless systems that co-exist in neighboring frequency bands.
  • Spread spectrum systems differ from normal wireless communications systems by spreading their tiansmitted signal over many frequencies simultaneously, instead of concentrating all of their transmitted power at a single frequency.
  • This technique has many advantages over traditional methods of wireless transmission, one of which is it's ability to reduce it's probability of reception by traditional wireless receivers.
  • the number of frequencies a spread spectrum transmitter occupies at any given time is directly proportional to the power transmitted in each one of the frequencies transmitted.
  • the more frequencies occupied in that band of frequencies has the tendency to lower the power in each frequency component transmitted.
  • the transmitted carrier of a spread spectrum wireless system can be realized utilizing various modulation means which include Binary Phase Shift Keying (BPSK), Fast Frequency Shift Keying (FFSK, or SFSK and MSK in some references), Gaussian Minimum Shift Keying (GMSK), Frequency Hopping (FH), and Frequency Ramp techniques that move the transmitted carrier over an ascending or descending band of contiguous frequencies in a predetermined amount of time (Chirp).
  • Spread spectrum transmission may be realized by other types of modulation means that will also produce a transmitted frequency spectral density similar to those listed which would need to comply with the requirements stated in C.F.R. Part 15.247, paragraph D.
  • This technique uses theory based on Fourier Analysis, and may be applied to any spread spectrum system where a reduction in power spectral density is desired to either comply with FCC limitations or reduce the system's probability of intercept by other non-system users.
  • the technique also allows for a unique identification of a system transmitter before reception of the main tiansmitted message.
  • This identification would allow a system receiver to selectively reject transmitters not related to it's mission at an earlier time during the transmitted signal interval, improving the receiver's probability of intercept of other system transmitters that contain the correct encoding for the receiver's intended purpose providing another level of Code Division Multiple Access (CDMA) to the system.
  • CDMA Code Division Multiple Access
  • the following discussion describes implementation of the enrichment method utilizing techniques for BPSK spread spectrum modulation of a transmitted wireless or Radio Frequency (RF) signal (or “carrier” as customarily referred to in the art.).
  • RF Radio Frequency
  • Binary Phase Shift Keying is a technique which imparts information to an RF carrier by changing the phase of the carrier from 0 degrees to 180 degrees dependent on the state (on or off, "1" or "0") of an input signal (modulation).
  • the input signal may be generated by digital means (i.e. an microprocessor, logic circuitry, ASIC, etc.), or analog means (Digital to Analog Converter, Pulse Shaper, filtered data stream, etc.).
  • Detection circuitry in an RF receiver monitors the phase of the transmitted signal and utilizes it in a variety of fashions to extract the original data imparted to the tiansmitter carrier (demodulation).
  • Spread Spectrum transmission occurs when two pieces of information are modulated simultaneously on a single RF carrier.
  • the first is a spreading sequence that has time related properties such that when the sequence is compared with a copy of itself that is aligned in time with the original, a "window" is opened and system operation is normal. When the two sequences are not aligned in time, or are not precise copies of each other, the "window" is closed, and system operation cannot be realized.
  • the second piece of information is the actual system data to be utilized for system operation.
  • the data can be digital or analog information, such as computer "bits", or voice.
  • spreading sequences operate at speeds many times higher that the speed of the data information, conditions exist, based on information theory, where the spreading sequence provides an advantage for system RF carrier reception over other RF carriers that may be present.
  • other system transmitters that are not time aligned with the spreading sequence of the receiver, possessing similar spreading sequences, operating on identical RF carrier frequencies, are held at a disadvantage when compared to system transmitters whose spreading sequences are time aligned with the receiver's. These effects are referred to as Process Gain.
  • the time dependent characteristics of both the data and spreading information have a direct effect on the frequency and amplitude (or power) characteristics of the modulated signal.
  • the relationships between the time related characteristics (time domain) and frequency related characteristics (frequency domain) of these signals can be analyzed for interdependency by Fourier Analysis techniques. Although the underlying theory that is the basis for Fourier Analysis is far beyond the scope of this discussion, several results based on it are important to understanding the techniques described later. The results that are important are outlined below. To begin with, eveiy signal that may be represented in the time domain has a corresponding representation in the frequency domain. Fourier analysis allows one to "bridge the gap" between these two domains.
  • the signal of greatest importance to this discussion is that of a repetitive rectangular pulse in the time domain 101 as shown in Figure 1.
  • the pulse depicted repeats every time period T and remains at it's maximum magnitude (or amplitude) for time period /.
  • the time period Twill be equal to 50 microseconds (50 one-millionths of a second) and the time period of/ will be equal to 1 microsecond (one one-millionth of a second).
  • the magnitude of the time domain signal will be normalized to a value of " 1 " for ease of discussion.
  • the frequency domain representation of the time domain signal 104 that results from the time domain signal 101 is also shown if Figure 1. Fourier analysis shows that the frequency domain signal manifests itself as a [sin(x)/(x)] 2 107 function.
  • the [sin(x)/(x)] 2 frequency domain signal 104 is complex in structure, with many individual frequency components that are spaced at ⁇ IT Hertz (F) 105 intervals.
  • the signal also has areas where there are no frequency components present (nulls) that are at frequencies separated by N*( ⁇ 1//) ( ) Hertz 106.
  • the individual frequency components are spaced at 1/(50 * IO "6 seconds) or 20 KHz., and the nulls are spaced at ⁇ 1 MHZ intervals.
  • the number of individual frequency components and their frequency spacing for this complex signal is important in understanding the techniques employed in the enrichment technique. Each one of these frequency components shares the total magnitude (power) of the entire frequency domain signal.
  • the individual spectral component's (“lines") 108 powers sum to produce a level equal to that if all the signal energy were concentrated at one frequency (as in traditional systems that do not employ spread spectrum techniques). Since the total power transmitted is "shared" by all the individual frequency components, the number of frequency components is directly related to the power each one of the components contains. The more components in a given frequency bandwidth, the less total power each one shares with the others, the fewer frequency components in a given frequency bandwidth, the more of the total power each one shares, and so on.
  • a spreading sequence has a similar frequency domain representation as the simple pulse depicted above.
  • Figure 2 shows the time 205 and frequency domain 208 representation of a typical spreading sequence.
  • the time T 201 is the rate at which the sequence repeats (for pmposes of this example 50 microseconds), the time / 202 is the duration of the smallest period the sequence rests in the "on" or one state (for purposes of this example one microsecond). Note that the individual frequency components 206 are spaced at 20 KHz intervals and that the "nulls" 207 are spaced at N* ⁇ 1 MHZ intervals. If the time duration for either the sequence repetition 201 or smallest sequence period 202 is changed, there will be a related change to the frequency spectra of the signal generated 208. For a given smallest sequence period time 202 (or "chip" as commonly referred to in the art), the number of chips in the sequence determines the time for the sequence to repeat.
  • Shorter length sequences will repeat at faster intervals, with longer length sequences repeating at longer intervals in time 201. For example, a sequence with 15 chips @ one microsecond per chip duration would repeat every 15 microseconds, a sequence with 63 chips @ 1 microsecond per chip would repeat every 63 microseconds and so on. For a given constant chip time of 1 microsecond, the 15 chip sequence would have a frequency component 206 every 66 KHz, and the 63 chip sequence would have a frequency component 206 eveiy 15.9 Khz. Over a certain frequency span, the 63 chip spreading sequence would produce more spectral components than the 15 chip spreading sequence.
  • the frequency span being measured was 66 KHz
  • the number of frequency components produced by the 15 chip spreading sequence would be one and the number produced by the 63 chip spreading sequence would equal 4.2.
  • the transmitted power for the RF carrier were equal
  • the power in each one of the 63 chip frequency components would be lower than that in each one of the 15 chip frequency components by 4.2: 1.
  • the device used to measure the power in each one of these frequency components was configured to measure energy in a narrow "window" of frequency span (narrow bandwidth filter) that was narrower than the frequency spacing between the individual frequency components of the two signals, the signal that was generated by the 15 chip spreading sequence would show a 4.2: 1 (6.2 dB) increase in power relative to that signal generated by the 63 chip sequence.
  • the method the FCC uses to measure the power of each individual carrier generated by a spread spectrum system is equivalent to that mentioned late in the last paragraph.
  • longer chipping sequences give lower power readings for individual frequency components as they produce more frequency components per given frequency span than shorter chipping sequences.
  • the measurement name that is given to this technique is Power Spectral Density, as it is a measure of the number of individual frequency components per frequency span, and the relative magnitude (or power) of each one of the components.
  • the measurement itself is either performed on a measuring receiver with a narrow (3 KHz) detector bandwidth, or a spectrum analyzer with similar detector characteristics.
  • the power in each one of the lines decreases by the original power divided by N. If the power is measured in decibels (a standard power measurement unit for wireless systems) the power in decibels (dB) decreases by a factor of 10*log H) (N) dB. For a 2: 1 decrease, the power in dB decreases by 3 dB, for a 10: 1 decrease, the number is 10 dB and so forth.
  • a standard measurement unit used for power pertaining to RF wireless systems is dBm.
  • the 15 chip sequence transmitter would obtain power spectral density readings that would be 6.2 dBm (10*log H) (4.2)) higher than the 63 chip transmitter, with identical chip periods.
  • the power spectral density of the transmitted spread spectrum signal measures out of specification to the FCC requirements, why not simply increase the number of chips in the spreading sequence until compliance is reached for a given output power? The main reason is time.
  • the chipping sequences at the transmitter and receiver must be the same and that they must also have the same time alignment to provide acceptable system performance .
  • One process for time alignment of the chipping sequences between the transmitter and receiver involves keeping the tiansmitter chipping sequence start and stop times constant and moving the start and stop times for the receiver chipping sequence until the receiver chipping sequence is aligned to the transmitters in time.
  • This process called serial co ⁇ elation, requires as many time periods of sequence repetition as the number of steps performed to obtain alignment. For example, if a 63 chip sequence was searched in one chip time steps on the receiver side of the system, 63 sequence repetition times would be needed to traverse the total number of combinations required.
  • the search (or correlation) process needs to take an energy "snapshot" equal to the time period of the entire chipping sequence to satisfy all the system requirements to determine co ⁇ elation or non-co ⁇ elation (time alignment) to the tiansmitter' s chipping sequence.
  • the nrinimum time for a 63 chip sequence @ 63 microseconds per sequence repetition would be a minimum of 63 * 63 microseconds or 3.969 milliseconds.
  • a chipping sequence with a length of 127 chips, at the same chipping rate would take twice as long as the 63 chip case, a 255 chip length sequence four times as long, and so on.
  • Another process for time alignment of the chipping sequences between the transmitter and receiver involves presenting the received, transmitted signal and applying it to a matched filter.
  • the matched filter has identical frequency and time characteristics to that of the transmitted signal.
  • the signal is presented to the matched filter for at least one sequence repetition time.
  • the filter passes the signal energy for the time that the tiansmitted signal retains the filters characteristics. This usually occurs for one chip time per sequence repetition.
  • This process called parallel correlation, allows for faster signal acquisition than the serial correlation process mentioned above, as theoretically only one spreading sequence repetition time needs to be expended for signal acquisition.
  • the use of shorter spreading sequences has time advantages for both serial and parallel correlation time alignment techniques that are directly related to the number of chips in the spreading sequence and their time duration's.
  • the on-air time for a system transmitter is directly related to the amount of time needed for signal acquisition. In many applications, the ability for the transmitter to get on and off the air as quickly as possible is paramount. The longer the transmitter transmits directly relates to battery life, intercept potential, packet rate, and transmitter collision statistics for many transmission protocols.
  • the on-air time for a system transmitter is directly related to how many transmitters a given system receiver can process in a given amount of time without the possibility of one transmitter corrupting the reception of another transmitter. This of biggest concern in systems that do not employ Time Division Multiple Access (TDM A) techniques such as ALOHA protocols. TDMA techniques would also benefit however as shorter time periods for transmitter operations would be needed due to reduced on-air transmitter time.
  • TDM A Time Division Multiple Access
  • the method of the present invention allows for utilization of shorter chipping sequences, while maintaining necessary power spectral density for FCC compliance requirements at power levels equal to or greater than those that may be utilized previously only for systems utilizing longer length sequences.
  • the technique also provides minimal impact to system operation by it's use.
  • Other benefits to implementation are shorter transmitter on-times and higher transmitter output power than previously obtainable with more traditional methods. It is therefore an object of the present invention to provide an enrichment technique that produces a spread spectrum modulated signal from a short length spreading sequence with characteristics similar to a spreading sequence of much greater length. It is another object of the present invention to provide an enrichment technique which allows for shorter length spreading sequences to be utilized with similar Power Spectral Density (PSD) and probability of reception properties to that of longer spreading sequences.
  • PSD Power Spectral Density
  • Figure 1 illustrates time and corresponding frequency domain representations of a simple radio frequency pulse.
  • Figure 2 illustrates time and corresponding frequency domain representations of a typical spreading sequences.
  • Figure 3 illustrates an exemplary time domain signal of the preferred embodiment of the power spectral density technique of the present invention.
  • Figure 4 illustrates the power spectral density technique of Figure 3, illustrating the application of the emichment sequence through modulo two addition of both enrichment and spreading sequences via modular two adder, illustrated in the form of a XOR gate, addition algorithm, or hardware.
  • Figure 5 illustrates the power spectral density technique of Figure 4, graphically illustrating the modulo addition of the spreading and emichment sequences via exemplary XOR gate logic sequence.
  • Figure 6 illustrates the process of how the emichment sequence transmissions of Figure 5 are related to the spreading sequence transmissions, and the spreading sequence start and stop interval (Epoch).
  • Figure 7 illustrates an exemplary frequency spectrum not generated by the technique of the present invention.
  • Figure 8 illustrates a BPSK frequency domain signal generated by the time domain Modulo two addition of the two sequences of Figure 4, as affected by the by enrichment sequence transition of Figure 6, relative to the spreading sequence epoch, with the emichment sequence transition placed at the beginning of the code Epoch.
  • Figure 9 illustrates a BPSK frequency domain signal generated by the time domain Modulo two addition of the two sequences of Figure 4, as affected by the by enrichment sequence transition of Figure 6, relative to the spreading sequence epoch, with the emichment sequence transition placed later in the code Epoch.
  • Figure 10 is a chart illustrating the improvement in power spectral density versus enrichment sequence transition relative to chipping sequence Epoch.
  • Figure 11 illustrates the composite spectrum of the preferred embodiment of the present invention of Figure 4, illustrating the spreading sequence multiplied with the emichment sequence frequency domain,
  • the frequency component spacing 206 and number of frequency components 108 present in a given frequency span As discussed earlier, this could be accomplished by changing the length of the spreading sequence to provide the correct result. If a method existed to produce the extra number of frequency components 108 at the correct frequency spacing, the frequency spacing needed to comply to the FCC requirements using the example above would be:
  • a method for producing specttal components spaced at 5.5420 Khz would be needed to comply with the 8 dBm / Hz requirement if the transmitter output power desired was 1 Watt.
  • a time domain signal could be constructed that, when modulating the transmitted carrier, would produce the necessary 5.542 Khz spectral components.
  • This signal could be a square wave that has a frequency of 5.542 Khz, an impulse of an arbitrary length that repeats each 5.542 Khz, or some other signal that would have a spectral line spacing at 5.542 Khz.
  • a time domain signal is needed that will produce frequency components spaced at 5.55420 KHz increments.
  • This signal depicted in Figure 3 will be referred to as the enrichment sequence.
  • the enrichment sequence 301 is applied to the spreading sequence through modulo two addition of both the enrichment and spreading sequences..
  • the modulo two addition adds the necessary frequency components to the transmitted frequency domain spectra as it performs a convolution in the time domain with the spreading sequence via the modulo two addition circuitry employed, resulting in a multiplication of the two spectra in the frequency domain.
  • Modulo two addition may be realized by adding the time domain spreading sequence to an enrichment time domain signal (emichment sequence) utilizing an exclusive or gate or similar hardware or by performing the modulo two addition in the spreading sequence generating hardware or software (i.e. algorithmically if the spreading sequence is generated in software, or by constructing the spreading sequence hardware (either discrete or in integrated circuit fo ⁇ n) to modulo two add the two sequences together.).
  • an enrichment time domain signal emichment sequence
  • the enrichment sequence 401 is modulo two added to the spreading sequence 402 with an exclusive or gate 403.
  • the enrichment sequence 401 is presented to one input of the exclusive or gate 403 and the spreading sequence 402 is presented to the gate's other input.
  • the exclusive or gate's function (modulo two addition) is to invert the one- zero state of the spreading sequence 402 dependent on the one-zero state of the emichment sequence 401. If both the emichment sequence and the chipping sequence are at the same logical state the output of the modulo two adder 403 will produce a logic zero output. If the two sequences are at different logical states (for example, the chipping sequence is at logical state one 405 and the enrichment sequence is at logical state zero 408) the output of the modulo two adder 404 will be a logical one.
  • FIG. 5 shows more graphically the modulo two addition of the two signals.
  • An enrichment sequence 501 is applied to one input of an exclusive or gate 507.
  • the enrichment sequence is at a logical one state 508 for 60.01 microseconds 503, and at a logical zero state 509 for a period of 120.02 microseconds 502, and repeats in this fashion for the entire time the spreading sequence is produced and the transmitted signal is being produced.
  • the spreading sequence 504 applied to the other input of the exclusive or gate 507 will appear on the output of the exclusive or gate 507 inverted 505.
  • the output of the exclusive or gate 507 will transform that one state to a zero state.
  • the output of the exclusive or gate will transform that zero state to a one state.
  • the spreading sequence will appear on the exclusive or gate's 507 output unchanged in state from that input. If the transmitted signal is On-Off Keyed, or any other non-constant envelope data modulation method is utilized, the enrichment and spreading signals will still continue to be generated and modulo two added (or a record will be kept to produce an enriched signal that will have similar properties) during the time the transmitted signal is off-air to preserve the spectral properties of the enriched signal and not to introduce any impulse distortion when the transmitter keys on due to a discontinuous phase shift from the earlier signal.
  • the enrichment sequence time domain signal above is modulo two added to the 63 chip spreading sequence time domain signal time domain convolution of the two signals will multiply their frequency domain signals together resulting in the composite spectrum shown in Figure 11.
  • the enrichment spectrum 1102 will "borrow" energy from the original spectrum generated by the spreading sequence 1101, theoretically reducing the overall power from each one of the spectral components (spectral "lines) by 4.5 dB.
  • the composite, multiplied signal will have spectral lines at both 15.9 KHz 1104 and 5.55420 KHz 1105 spacings.
  • Another way to explain the frequency multiplication effects would be to say that the enrichment sequence synthesizes the frequency characteristics of a longer spreading sequence length from a given shorter sequence.
  • the enrichment allows the system to meet the required 8 dBm level as set forth by the FCC.
  • Construction of the enrichment sequence to produce a certain spectral signature with a given spreading sequence requires care to produce the desired effects mentioned above.
  • This sequence has characteristics identical to the original spreading sequence as far as how the signal translates from the time domain to the frequency domain (refer to Figure 3).
  • the period of the minimal signal duration of the enrichment sequence is important. Too short a time period per enrichment sequence length 302 and the spectra will be dominated by the spreading sequence amplitude characteristics (lower level of enrichment), too long of a period would have similar effects.
  • the period / 303 of the emichment sequence therefore needs to be balanced with the period T 302 of the emichment sequence repetition to realize the maximum benefit from the technique. Furthermore, these two periods need to be balanced with the periods of the spreading sequence minimal period 202 and repetition period 201 for maximum effect. It is at first intuitive that longer enrichment sequence lengths T 302 would produce greater numbers of specttal lines at narrower frequency spacings (thus improving spectral density readings), this is not the case. As stated above, if the time duration of the minimum period of the enrichment sequence is too long, the frequency characteristics of the emiched spectra will be dominated by the original spreading sequence and not as a combination of both the spreading and emichment sequences.
  • the logical state of the emichment sequence should be only be changed at the beginning of the spreading sequence interval. This is mainly due to the fact that the system receiver needs to integrate the transmitted signal power over the entire period of the chipping sequence. If any disturbance in the chipping sequence occurs during this integration time, it manifests a discontinuity in the integration, degrading system fidelity.
  • the time domain properties of the spreading sequence require that the enrichment sequence only be asserted during certain periods during the spreading sequence to assure that spurious frequency domain artifacts are not unnecessarily generated as a result of modulo two adding the enrichment sequence to the spreading sequence. This is due to the fact that the spreading sequence is rich in harmonic content in the frequency domain and the number of one to zero transitions in a given area of the spreading sequence, occurring at the chipping frequency, produce spectral components across the entire frequency spectrum it occupies.
  • the emichment may add higher frequency components to the spreading sequence resulting in an overall shortening of the sequence (instead of synthetically lengthening it!).
  • the distortion of spreading sequence vs. emichment sequence application time manifests itself by producing impulse responses in the time domain of very short duration resulting in large amplitude fluctuations (spikes) in the frequency domain which could nullify the effectiveness of the technique or possibly worsen the spectral density results from a system with no enrichment at all. Care must also be taken in the construction of the enrichment sequence to overcome any spectral distortion caused in an OOK system.
  • the chipping rate for the spreading sequence was 1 MHZ with emichment rate at 15.873 KHz.
  • the analysis and simulation showed that optimal placement of the enrichment sequence transition's was dependent on their relationship to the individual spreading sequence properties at the instant in time the enrichment was applied. Transition placement was critical to insure maximum spectral density improvement for the complex signal that was produced from the modulo two addition of the spreading sequence and the emichment sequence. This result mandates generating the cyclic spreading sequence in such a way that it's start time coincides with the optimal period for it's emichment by the enrichment sequence.
  • Figure 6 shows the process of how the enrichment sequence transitions are related to the spreading sequence transitions and the spreading sequence start and stop interval (Epoch).
  • the Figure also shows that the enrichment sequence start and stop intervals (transitions) may be moved in relation to the spreading sequence start and stop intervals (Epoch's).
  • the enrichment sequence 604 transitions at a position that is concurrent with the starting bit that defines the Epoch 602 of the spreading sequence 603 (i.e. the start of the spreading sequence that is utilized in the initial system is defined to be at the locations marked in 602).
  • the emichment sequence transitions are moved in relation to the original spreading sequence Epoch.
  • FIG. 8 An example of how the placement of the enrichment sequence transition 604-608 relative to the spreading sequence Epoch 602 affects the frequency spectrum generated from the process on a BPSK frequency domain signal generated by the time domain modulo two addition of the two sequences is shown in Figures 8 and 9.
  • the spreading sequence 603 for the two Figures was 63 chips 202 in length.
  • Figure 8 placed the emichment sequence 604 transition at the beginning of the code Epoch and Figure 9 placed the enrichment sequence 608 transition later in the code Epoch at chip number 46 after the start of the chipping sequence Epoch 602.
  • the spectral lines 902 in Figure 9 closely follow the [sin(x)/(x)] 2 characteristics of the theoretical spectrum shown in Figure 11, Illustration 1103.
  • Figme 8 shows more of a discontinuous spectrum than Figure 9.
  • the spectral lines 802 in the Figure are markedly less evenly distributed in their individual powers 801 than in Figure 9 (901). Note the 4 spectral lines 802 that are close to the middle of the frequency scale. Therefore, position of the emichment sequence relative to the chipping sequence is important for maximum frequency spectrum continuity.
  • Figures 7, 8, and 9 are frequency spectrums generated by the technique.
  • Figure 7 shows the frequency spectra generated by the spreading sequence 603 without the emichment sequence applied.
  • the Y axis 701 is a measure of power each one of the spectral lines 702 produces due to the chipping sequence 603 by itself.
  • Figure 8 shows the power produced in each spectral line 802 of the combined time domain enrichment sequence and the spreading sequence when modulo two added as discussed earlier.
  • the y axis 801 is a measure of power (referenced to the power shown for the unemiched spectrum in 700) each spectral line 802 contains.
  • Figure 9 shows the power produced in each spectral line 902 of the combined time domain emichment sequence and the spreading sequence when modulo two added together.
  • the y axis 901 is a measure of power (referenced to the power shown for the unemiched spectrum of Figure 7) each specttal line 902 contains.
  • the maximum power obtained by any spectral line in Figure 9 is 4 dB lower than that in any spectral line shown in Figure 7.
  • the maximum benefit realized by the technique as realized in this example is 4 dB.
  • FIG. 10 shows the improvement in power spectral density versus emichment sequence transition relative to chipping sequence Epoch.
  • the y axis value 1001 is the minimal value of power specttal density reduction obtained versus the x axis value of where the enrichment sequence is applied in relationship to the chipping sequence Epoch start.
  • position 46 on the X axis 1002 shows the maximum benefit obtained (the same enrichment sequence was used each x axis measurement). It is clear that the maximal benefit was obtained at position 46.
  • the example shown is for only one chipping sequence and one emichment sequence, but extensive simulation and analysis shows this technique is useful for any complex modulation scheme that uses sequences to produce modulated carriers in this manner. Since the chipping sequence is cyclical, the position of where the chipping sequence starts or ends (Epoch) is irrelevant in the amount of benefit it produces when utilized in a spread spectrum system. Only the construction of the chipping sequence is relative, not where it starts or stops. Chipping sequences are usually linear maximal sequences, but this technique is not limited to those types. Barker Codes, Gold Codes, JPL Codes, among others also benefit from this technique.
  • the chipping sequence Epoch 602 Since the start and stop of the chipping sequence Epoch 602 is irrelevant to the effectiveness of the chipping sequence for proper system operation, it's definition is arbitrary and may be moved without effect. Since the enrichment sequence 603 placement is critical to the chipping sequence position to where it is applied for maximum benefit using this technique, the chipping sequence Epoch may be moved to the position where the enrichment sequence provides maximum benefit. Figure 6 shows this method. The original chipping sequence Epoch 602 and it's relationship to the chipping sequence 603 is shown relative to the optimal placement of the enrichment sequence 608. The original Epoch 602 of the chipping sequence 602 is moved to the position shown in 610 to occur at the same point in the chipping sequence where the emichment sequence 608 provides the maximum benefit in power spectral density reduction.
  • the Epoch will then be at the optimal position for power spectral density reduction and minimal system impact due to the enrichment sequence 608 as the system receiver integration start and stop periods will be dependent on the new Epoch 610.
  • any signal perturbations that would affect system performance due to the application of the enrichment sequence will occur when the integration is beginning or ending (at the Epoch time).
  • this integration start and stop time is coincident with the receiver being purged of any signal in it's received path (dumped), and is also coincident with data transition times.
  • the receiver is "blind" to the enrichment sequence at the time it is imposed at the tiansmitter.
  • the technique is also non destructive to the system as it searches for the optimal alignment of the chipping sequence, as when that alignment is achieved, the Epoch will again be aligned to the place where the receiver is blind to the imposition of the emichment sequence on the transmitter signal.
  • spreading sequences are generated by a hardware configuration of shift registers or similar hardware that sometimes limits the instantaneous properties of the spreading sequence to a narrow range of characteristics at start time. For any given sequence, these characteristics are difficult to change for certain hardware configurations.
  • the starting properties of the chipping sequence can easily be moved to any convenient place in the sequence that results in optimal spectral density improvement in conjunction with the enrichment sequence.
  • the apparatus was also used to verify simulation results from Lab View. From the apparatus and testing results, optimal placement of the emichment sequences for a given spreading sequence were deduced. A further step was to construct the spreading sequence generation hardware or software to place these positions at the beginning or ending of the ttansmitted spreading sequence (i.e. moving the spreading sequence Epoch) so that the enrichment sequence can be applied at such a time as to provide the least distortion to system auto-correlation properties and optimal spectral emichment. Results produced by the apparatus confirmed the earlier LAB VIEW and analytical results. Emichment sequence placement versus spreading sequence position and epoch correlated precisely with the earlier work. The apparatus confirmed concept correlation for both BPSK and FSK modulation techniques.
  • the emichment sequence should be applied at the beginning or end of the spreading sequence to maximize system fidelity, it follows that the emichment sequence is harmonically related to the spreading sequence's repetition frequency and thus it's spreading sequence repetition time. This constraint also ties the emichment sequence to the system data rate.
  • Several methods of data modulation are available for spread spectrum systems and most are compatible to this enrichment method.
  • the technique described above utilizes a BPSK emichment technique by modulo two addition of the enrichment sequence to a spreading sequence and then applying the composite signal to a BPSK modulator.
  • the composite signal could also be applied to any other modulator known in the art for producing spread spectrum modulation yielding similar results (for example QPSK, PSK, FM, GSM and others).
  • the enrichment sequence may be removed from the demodulated data signal by subtraction methods, based upon detection of the enrichment sequence used and it's timing during the signal acquisition period (preamble). If the data modulation method is different from the enrichment data modulation method, for example BPSK enrichment modulation, and On-Off Keying (also known as AM or ASK modulation) data modulation, the enrichment modulation is totally transparent to the data modulation and no further processing is needed to reconstruct the data.
  • the system receiver spread spectrum demodulation circuitry may or may not utilize a similar spectral enrichment technique on it's despreading signal.
  • the enrichment sequence transitions are appropriately placed on the transmitted signal at the spreading sequence start and stop times, this may not be necerney. Since the emichment sequence is known however, it may be added to the receiver's despreading signal to improve system fidelity if necerney. This fidelity improvement may improve the cross correlation properties of the system during signal acquisition.
  • the enrichment sequence can also be used to identify or pre-identify a system transmitter before data demodulation begins. For each enrichment sequence length utilized, several different emichment sequences may produce acceptable power spectral density reductions for a given spreading sequence and it's time domain characteristics.
  • the receiver may reject similar transmitters with different emichment sequences by demodulating the emichment sequence prior to actual transmitter data demodulation. If a transmitter is received that does not match the appropriate emichment sequence identifier, that tiansmitter could be rejected by the receiver before data demodulation begins, allowing the receiver to pursue reception of other ttansmitters that share the correct enrichment sequence coding without incu ⁇ ing the time delay necessary to fully decode the undesired ttansmitters identification presented in it's normal data packet.
  • the enrichment sequence could also be utilized to algorithmically steer the receiver before data demodulation by identifying the type of system transmitter encountered. This technique could be utilized to set time-out limits (among other things) by identifying system transmitters that are on for different amounts of time due to data packet lengths. It could also be used to tell the receiver if the transmission is to be repeated or processed by the receiver.
  • DEFINITIONS SPREADING SEQUENCE Also referred to a chipping sequence, and spreading means.
  • a sequence of information in binary form whose time domain characteristics produce a frequency domain signal spectra that distributes a narrow band of frequencies over a broader range.
  • Spreading sequences are generated by linear maximal sequences, Gold sequences, Barker sequences, or any other appropriate sequence that produces an auto-correlation function relevant to spread spectrum data transmission techniques.
  • ENRICHMENT SEQUENCE Also referred to as enrichment means.
  • a sequence of information in binary form whose time domain characteristics when modulo two added to a spreading sequence's time domain characteristics produce a frequency domain signal spectra that is the composite of both the spreading sequence and the enrichment sequence for purposes of reducing the transmitted power specttal density of a spread spectrum signal.
  • the invention embodiments herein described are done so in detail for exemplary purposes only, and may be subject to many different variations in design, structure, application and operation methodology. Thus, the detailed disclosures therein should be interpreted in an illustrative, exemplary manner, and not in a limited sense.

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  • Engineering & Computer Science (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • Signal Processing (AREA)
  • Cable Transmission Systems, Equalization Of Radio And Reduction Of Echo (AREA)

Abstract

L'invention concerne une technique d'enrichissement permettant de produire un signal modulé par étalement du spectre à partir d'une séquence d'étalement de faible longueur présentant des caractéristiques similaires à une séquence d'étalement de longueur beaucoup plus grande. La technique présente permet d'utiliser des séquences d'étalement plus courtes pour obtenir une densité spectrale de puissance (DSP) et des propriétés de probabilité de réception similaires à celles obtenues avec des séquences d'étalement plus longues. Les autres avantages que présente cette technique sont, outre des séquences d'étalement plus courtes, des temps d'acquisition plus courts dus aux séquences d'étalement plus courtes, une identification de signaux de préambule, et une réduction du temps d'émission dans l'air.
PCT/US1997/023296 1996-12-18 1997-12-17 Technique de densite spectrale de puissance WO1998027802A2 (fr)

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AU70972/98A AU7097298A (en) 1996-12-18 1997-12-17 Power spectral density technique

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US9619980B2 (en) 2013-09-06 2017-04-11 Immersion Corporation Systems and methods for generating haptic effects associated with audio signals
US9934660B2 (en) 2013-09-06 2018-04-03 Immersion Corporation Systems and methods for generating haptic effects associated with an envelope in audio signals

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USRE35209E (en) * 1990-09-27 1996-04-16 Partyka; Andrzej Spread spectrum communications system
US5661750A (en) * 1995-06-06 1997-08-26 Cellnet Data Systems, Inc. Direct sequence spread spectrum system

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Cited By (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US20150070146A1 (en) * 2013-09-06 2015-03-12 Immersion Corporation Systems and Methods for Generating Haptic Effects Associated With Transitions in Audio Signals
US9619980B2 (en) 2013-09-06 2017-04-11 Immersion Corporation Systems and methods for generating haptic effects associated with audio signals
US9711014B2 (en) * 2013-09-06 2017-07-18 Immersion Corporation Systems and methods for generating haptic effects associated with transitions in audio signals
US9934660B2 (en) 2013-09-06 2018-04-03 Immersion Corporation Systems and methods for generating haptic effects associated with an envelope in audio signals
US10276004B2 (en) 2013-09-06 2019-04-30 Immersion Corporation Systems and methods for generating haptic effects associated with transitions in audio signals
US10388122B2 (en) 2013-09-06 2019-08-20 Immerson Corporation Systems and methods for generating haptic effects associated with audio signals
US10395488B2 (en) 2013-09-06 2019-08-27 Immersion Corporation Systems and methods for generating haptic effects associated with an envelope in audio signals

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AU7097298A (en) 1998-07-17

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