SE506622C2 - Direct Sequence Banding (DSSS) method of a data sequence - Google Patents

Abstract

The present invention relates to a method for direct sequence spreading of a data sequence (d(t)) or a modulated data sequence (s(t)). The method yields better spectral properties than conventional methods for data transmission using direct sequence spread spectrum (DSSS) and is also suitable for systems where one wants to conceal the existence of transmission, so-called stealth radio. This is achieved by one or more spreading sequence generators (21) in the transmitter and the corresponding sequence generators (31) in the receiver emitting new output data at a rate which essentially exceeds the one required for obtaining in the transmitter a certain desired spread bandwidth, the spreading sequence or spreading sequences (c(t)) being caused to bandspread the data sequence (d(t)) or the modulated data sequence (s(t)), and the desired spread bandwidth being obtained by filtering in band-limiting filters (22).

Description

506 622 2 10 15 20 25 30 35 The spreading codes in DSSS systems are often created by binary feedback shift registers.

The codes are of the binary pseudo-random sequences (PN sequences) type and are available in several different designs. The so-called maximum length sequences have the best properties. The problem with them is that for a given length of sequence, there are a limited number of different sequences. To have more sequences to choose from, combinations of two or more maximum length sequences, so-called gold sequences, are usually used.

However, these sequences have significantly poorer correlation properties.

To increase the amount of sequences with good correlation properties, chaos-generated sequences have recently been proposed for communication systems, see Heidari-Bateni and McGillem, "A Chaotic DSSS Communication System", IEEE Transaction on Communication, Vol. 42, Feb./Mars/Apr. 1994. The generated chaos sequences are non-binary, ie they can assume many different values within a range. An advantage of chaos-generated sequences is that a very large number of different sequences with low cross-correlation can be achieved, which is good in CDMA applications and for stealth radio. Another advantage is that a chaos sequence is very easy to generate. In several cases you only need to save a constant and the previous value to generate the next value, see H.G. Schuster, "Deterministic Chaos, An Introduction", Physick-Verlag GmbH., 1984, to which reference is made. In the above-mentioned article by Heidari-Bateni and McGillem, the values of the chaos sequence have been allowed to amplitude-modulate a carrier and the resulting signal has proven to be better from an LPI point of view than traditional DSSS.

Pulse shaping of the data pulses and the spreading code have been proposed in eg Mark A Wickert et al., "Practical Limitations in Limiting the Rate-Line Detectability of Spread Spectrum LPI Signals", Proceedings of MILCOM, Monterey, 1990, to reduce the detection probability of the DSSS signal in LPI context. It has been found that it is above all the frequency components over half the scattering sequence rate that make the signal easily detected by means of a squaring detector or Delay-and-Multiply Detector, which are the most probable detectors. for signal reconnaissance against DSSS signals. Attempts are therefore made to filter out these higher frequency components.

The present invention provides another way to improve the spectral properties of various applications in the fields of sneak radio (LPI) and frequency division in multi-user systems, but is not limited to these applications but is intended to cover all areas where similar problems set. The features of the invention are set out in the claims.

The invention will be described in more detail below with reference to the accompanying drawings, which schematically show a known principle of BPSK DSSS, schematically show an embodiment of a transmitter according to the invention Fig. 1 Fig. 2 and Fig. 3 schematically show an embodiment of a receiver according to the invention.

Fundamental to the invention is that the spreading sequence is created at a rate which significantly exceeds the need to obtain a certain desired spreading bandwidth, and that the sequence or the modulated signal is subsequently filtered to the desired spreading bandwidth.

In the proposed invention, filtering is very powerful with the aim that the scatter symbols after the filtering should assume values between the sequence values generated by the scatter code generator and create a dependency between many consecutive sequence values. This is something completely different from the moderate filtering of the transmitted signal commonly used in known DSSS systems, which is performed to improve the spectral properties. This moderate filtering only means that the emitted scattering pulses are slightly rounded without being too deformed.

Figure 1 schematically shows an example of a system with known technology for direct sequence band spreading. The data source 12 supplies binary value d (t) to a modulator 13 which BPSK modulates a carrier. The modulated signal s (t) is multiplied by a spreading sequence in a multiplier 14. The spreading sequence, consisting of the symbols 11, is generated by a PN generator 11 at a rate which roughly gives the spreading bandwidth WSS for the system. The band-spread signal x (t) is transmitted on an arbitrary channel to the receiver, where the received signal y (t) is multiplied in a multiplier 16 by a sequence generated in a PN generator 19, which is identical to the spreading code. The effect of the spreading code on the signal then disappears because c2 (t) = 1. The spread signal r (t) is demodulated in a demodulator 17 and reproduced data which is an estimate of original data is fed to the data sink 18. soe 622 4 10 15 20 If the known DSSS system is performed in analogue technology, the BPSK modulator 13 and the multipliers 14 and 16 can be constituted by diode ring mixers. To the modulator 13 is supplied, in addition to data, the desired carrier generated in an oscillator. The receiver's demodulator 17 is also applied to a sine signal which is phase locked to the carrier in y (t). In addition, the demodulator can also consist of a diode ring mixer, an integrator and a decision circuit. The PN generators 11 and 19 can be designed with digital circuits as feedback shift registers. The feedback pattern and the starting value are the same for the transmitter's and receiver's PN generators.

Alternatively, the signal processing of the system in the units 11, 13, 14, 16, 17 and 19 can be performed in digital technology, with eg a digital signal processor (DSP) or with an application-specific integrated circuit (ASIC). With such an implementation, digital / analog conversion of x (t) and analog / digital conversion of y (t) are added. An example of a circuit that contains most of the functions of a receiver as above is PA-100 "Spread Spectrum Demodulator ASlC" from Loral Corporation.

In general, the signal x (t) must be amplified and possibly filtered before it is applied to the channel 15. The receiver amplifies and frequency filters forward y (t) before it is spread in 16.

A transmitter according to the invention can in principle be designed as in figure 2. Data to be transmitted from the data source 23 modulates a carrier in a modulator 24. The modulator can be designed for various common modulation forms, eg BPSK, QPSK or MSK (Minimum Shift Keying). The signal s (t) has its phase changed in the phase rotor 25 controlled by the signal c (t). The phase change is within the interval n. The signal x (t) thus modified is transmitted via some medium to the receiver. The bandwidth of the signal c (t) is significantly larger than s (t), which leads to the desired band spread. The spreading signal c (t) is created by the interaction of the sequence generator 21 and the filter 22. The sequence generator 21 may, but need not, be designed as a chaos generator. The sequence generator leaves a new value at a rate which is k times greater than the rate corresponding to the desired spreading bandwidth Wss. The filter 22 low-pass filters the sequence and forms the signal c (t), which has a bandwidth corresponding to the spreading bandwidth Wss. Regardless of whether the sequence generator leaves binary or multi-level sequences, the signal c (t) will have many possible levels after the filtering.

If the transmitter is made as above and the modulation method in 24 is selected to someone with a constant amplitude, for example BPSK or MSK, then the modulated and banded signal will come from the CZ) O \ O \ N) N x (t), which is to be transmitted also to have a constant amplitude. This is a feature that is highly desirable in contexts where you want to use low-power nonlinear amplifiers. If the ratio between the bandwidth of the signals s (t) and c (t) is large (large scattering factor), then the bandwidth of x (t) will be largely determined by the transmission function in filter 22. This design is very suitable in cell radio contexts.

The receiver can in principle be designed as in figure 3. Sequence generator 31 and filter 32 are identical to the transmitter's corresponding units 21 and 22. The sequence generator 31 also has the same code key as the transmitter's sequence generator 21. The sequence generated in the sequence generator 31 is synchronized to y (t ) which is the received signal x (t) received and delayed in the transmission medium. The generated replica of the signal c (t) is reversed in the inverter 33 before it is allowed to control the phase rotor 36. Alternatively, the inverter 33 is included in the phase rotor 36. The spread spectrum signal y (t) is spread in the phase rotor 36 and the signal r (t) contains only the original modulated carrier. In the demodulator 35, data is recreated with conventional techniques for the selected modulation method and fed to the data sink 34.

The technical implementation of proposed methods can, to a large extent, be performed with digital signal processing in digital signal processors or ASICs. In the same way as in the description of the known system, digital / analog conversion of x (t) is added, analog / digital conversion of y (t) and that the signal x (t) is amplified and possibly filtered before it is applied to the channel. . The receiver amplifies and frequency filters forward y (t) before spreading in the phase rotor 36.

The chaos generators 21 and 31 can, in e.g. a DSP, is performed using the so-called logistical function xn-l-l = kxn _xr |) 'see the above-mentioned book by H.G. Schuster. The computational accuracy of the DSP used determines how long the sequence may be before it repeats itself. The filters 22 and 32, which determine the spread bandwidth Wss, can be digital low-pass filters of the type FIR (Finite Impulse Response) or llR (Infinite Impulse Response). It is important that the transmitter's and receiver's chaos generators and filters are designed in exactly the same way and with exactly the same numerical precision. The phase rotation in 25 and 36, respectively, can be performed mathematically as a complex multiplication of the I-Q split signal s (t) and y (t), respectively. Alternatively, the carrier signal applied to the modulator 24 and the demodulator 35, respectively, can be phase changed. The required sine signal can be created in a Direct Digital 'Synthesis (DDS) circuit and where its phase changes controlled by c (t). An example of a circuit that has the desired functions of complex multiplication and phase controllable signal generation is HSP45116 "Numerical | y Controlled Oscillator / Modulator 'from Harris Semiconductor.

An alternative embodiment of the transmitter is to create two independent spreading signals c (t) and c '(t), respectively, which are allowed to modulate the real part of the s (t) and the magnetic part (I-Q modulation) in a complex multiplier. This can be done by doubling the sequence generator 21 and the filter 22 so that two signals c (t) and c '(t) are created. The phase rotator 25 is replaced by a complex multiplier where the complex signal s (t) is multiplied by c (t) as real value and c '(t) as imaginary value. In this embodiment, the signal x (t) will not have a constant amplitude, but the possibilities of forming the spectrum of x (t) will be even better. Wss is unambiguously determined by the transfer function in the filters 22.

This design, especially with chaos-type sequence generators, is particularly suitable for LPI systems.

Claims (6)

10 15 20 25 30 35 7 506 622 Patent claims: A method for direct sequence bandwidth spreading of a data sequence (d (t)) or a modulated data sequence (s (t)), with one or more spreading sequence generators (21) in the transmitter and corresponding sequence generators (31) in the receiver, k ä indicating that the sequence generators provide new output data at a rate substantially in excess of that required to obtain in the transmitter a certain desired spreading bandwidth, that the spreading sequence or the spreading sequences (c (t)) are caused to spread the data sequence (d (t)) or the modulated data sequence (s (t)), and that the desired spread bandwidth is obtained by filtering so strongly in band-limiting filters (22) that a dependency between many consecutive sequence values is created. Method according to claim 1, characterized in that the spreading sequence or the spreading sequences (c (t)) are chaos sequences generated by a chaos generator (21, 31). Method according to claim 1 or 2, characterized in that it uses only one sequence generator (21) in the transmitter, the generated sequence of which, after filtering in said band-limiting filter (22), is caused to modulate the phase of the data sequence carrier, after which the receiver correspondingly sequence-controlled phase shifting takes place with the opposite sign. Method according to claim 1 or 2, characterized in that it uses only one sequence generator (21) in the transmitter, the generated sequence of which is caused to modulate the carrier phase of the data sequence, whereupon the modulated data sequence (s (t)) is filtered in said band-limiting filter ( 22), after which in the receiver the corresponding sequence-controlled phase shift takes place with the opposite sign. Method according to claim 1 or 2, characterized in that it uses two sequence generators in the transmitter, whose generated sequences, after filtering in said band-limiting filter, are caused to modulate the real part and imaginary part (1 and Q modulation) of the data sequence, respectively, after which in the receiver sequence-controlled phase shifting takes place with the opposite sign. Method according to Claim 1 or 2, characterized in that it uses two sequence generators in the transmitter, the generated sequences of which are caused to modulate the real part of the data sequence of the data sequence and the imaginary part (1 and Q modulation), respectively, 506 622 whereupon the modulated data sequence (s (t )) is filtered in said band-limiting filter, after which the receiver corresponding sequence-controlled phase shifting takes place with opposite signs.