GB2405296A

GB2405296A - Pre-spreading a carrier signal to avoid frequency spurs caused by carrier leakage

Info

Publication number: GB2405296A
Application number: GB0319492A
Authority: GB
Inventors: Christopher John Ward
Original assignee: Thales Holdings UK PLC
Current assignee: Thales Holdings UK PLC
Priority date: 2003-08-19
Filing date: 2003-08-19
Publication date: 2005-02-23
Anticipated expiration: 2023-08-19
Also published as: GB2405296B; GB0319492D0

Abstract

A spread spectrum signal generator 1 includes a carrier signal generator 2, first and second spreading signal generators 4, 5, 9 and first and second mixers 6, 3. A carrier signal is pre-spread by the first spreading signal at the mixer 6 to generate a first spread carrier signal. The second mixer 3 mixes the second spreading signal with the first spread carrier signal to produce a second spread carrier signal having a spectrum spread according to both the first and second spreading signal. By pre-spreading the carrier signal, carrier leakage e.g. from imperfect mixing of the second spreading signal, or an offset in the spreading signal, results in leakage of a part of the pre-spread carrier signal. This avoids a frequency "spur" (see fig. 3) in the second spread carrier signal. The pre-spread carrier signal may undergo filtering 11 to remove undesired spectral components such as side lobes associated with the carrier signal and low-frequency spectrum leakage signals arising from the first spread carrier signal. Preferably the first spreading signal is cancelled from the content of the second spread carrier signal. Application is to communications or positioning systems.

Description

Apparatus and method for signal generation The present invention relates

to methods and apparatus for generating a spread-spectrum signal by spreading the spectral bandwidth of a signal(s). The present invention relates particularly, though not exclusively, to methods and apparatus for spread-spectrum generation for use in communications or positioning using electromagnetic signals.

In both communications applications and positioning systems employing electromagnetic (e/m) signal transmitters, it is generally desirable to operate such transmitters over a wide signal frequency spectrum. A wide signal frequency spectrum provides the versatility of communication over any number of a wide range of signal frequencies (in the case of communication systems), or of greater positioning accuracy by virtue of having a greater range of positioning resolution (in the case of positioning systems). For example, typical Global Positioning Systems (GPS) operate over an e/m signal frequency spectrum having a spread of approximately lOMHz.

It is often difficult or impossible to find an unbroken e/m frequency spectrum which is sufficiently wide yet does not overlap with signal frequency bands that should preferably be, or must be, avoided such as e/m frequency bands dedicated to the transmission of emergency signals.

Consequently, in order to generate an e/m transmission frequency spectrum of sufficient spread but which avoids specified frequency bands, some existing spread-spectrum signal generators are arranged to generate a multiplicity of different e/m signals each having one of a corresponding multiplicity of different frequencies.

Each such different e/m signal typically only has a very narrow bandwidth and thus the combined multiplicity of such narrow-bandwidth signals has the effect of producing a spread-spectrum comprising a comb of consecutive narrow spectral spikes which avoid predetermined regions of frequency as desired. However, such a comb generally has insufficient spectral spikes to fill the broken spread- spectrum. Were it possible to produce an unlimited number of such spectral spikes then a spread-spectrum could be completely filled thereby at all but those regions of frequency to be avoided. However, it is generally not possible to generate a sufficient number of such spikes.

Accordingly, existing systems typically fill the regions of a spreadspectrum intermediate neighbouring e/m spectral spikes simply by broadening the spectrum of each such spectral spike. Fig. l schematically illustrates this process in terms of six separate e/m spectral frequency spikes generated by five unspread transmitter signals (signals Pl to P6, at frequencies fl to f6), and the effect of spreading each such spike on the regions of the spectrum intermediate successive spikes. l

As can be seen from Figure. l the effect of spreading each spike Pl of the spectral comb is to broaden the respective spike into an e/m spectral peak So of a finite width sufficient to fill the spectral regions intermediate the frequencies f1 (other then the region to be avoided). The result is a spread spectrum which is broken to avoid predetermined e/m frequency bands yet is sufficiently spread elsewhere to provide continuous frequency coverage at the other desired e/m frequency bands.

The spreading of each spike PI of the unspread spectral comb of Figure l into a spectral peak Sl is typically performed according to a process schematically illustrated in Figure 2.

In a spread-spectrum signal generator a given spike PI Of a spectral comb is represented by a continuous wave (COO) carrier signal of frequency f, (i.e. an extremely narrow bandwidth centred on frequency value fl). The carrier signal is input to a signal mixer together with a spreading signal having a frequency spectrum extending across a predetermined frequency bandwidth. The result of mixing the spreading signal with the carrier signal P is to impose the broad frequency spectrum of the spreading signal upon the narrow-spectrum carrier signal P1 so as to spread the latter and produce a spectral peak S1 representing the spread carrier signal. The spreading signal, spectral comb, carrier signal and spread carrier signal are typically electrical signals, and the electrical spread carrier signal is used to drive an e/m signal transmitter.

The spreading signal typically comprises a pseudo-random binary sequence (PRBS) comprising a binary signal which oscillates randomly between values of +1 (plus one) and -1 (minus one) so as to produce a sequence of square pulses of randomly varying duration. As is well known in the art, the frequency spectrum of a PROS is a "sine" function the spectral bandwidth of which is determined by the duration of the shortest waveform element.

Consequently, each spread carrier signal S1 comprises a sine function centred on a frequency value fl.

At e/m wavelengths suitable for wide bandwidth positioning systems, signal mixers in such spread- spectrum signal generators typically produce between - 20dB and -30dB continuous wave (COO) leakage relative to the carrier level, P1, if no correction measures are taken. Carrier leakage results from imperfect mixing of the CW carrier signal with an associated spreading signal, with the result that a portion L1 of the carrier spike P1 remains unspread when mixing with the spreading signal. The unspread portion of L1 of the carrier signal then leaks into the spread carrier signal S. as a small leakage spike or "spur" such as the leakage spike L illustrated in Figure 3.

If the mixing of the carrier signal P1 and the spreading signal were perfect, then the amplitude of the leakage I spike L1 would be zero and no "spur" would appear in the spread carrier signal S1 output from the mixer. However, in reality, carrier leakage can be higher than the desired amplitude of the spread-spectrum of which the spread carrier signal S1 forms a part. This often forces the use of a low frequency modulation where leakage can be better controlled, followed by heterodyne conversion to the operating frequency.

Heterodyne conversion such as this can be inconvenient and costly to implement when wide bandwidth operation is necessary. Leakage varies with temperature, worsens with increasing frequency, and can be phase shifted relative to the main modulation component. These factors restrict the degree of cancellation that can be achieved by the application of, for example, an adjust-on-test DC bias to the modulation waveform as is often attempted in prior art systems to combat this problem.

An additional source of spectral spurs (as illustrated in Figure 3) is the imperfect switching of the PRBS spreading signal generator between the binary values +l and -1. In practice, the binary values between which a typical PRBS system switches are often not exactly balanced (i.e. of equal magnitude but opposite sign).

Rather, a DC offset is often to be found in a PRBS signal such that the signal becomes unbalanced and oscillates between values of opposite sign and differing magnitude.

For example, a 5% offset in a PRBS signal would result in oscillations between the values +1.05 and -0.95, as opposed to the desired values of + 1.0 and -1.0 respectively. This DC offset also results in a spectral spike or "spur" leaking into the spread carrier signal S from the spreading signal itself.

Furthermore, the square-wave pulses of a PROS sequence only approximate a true square wave-form. Typically the rising and falling edges of a squarewave pulse suffer from overshoot and undershoot respectively which causes S the corners of the pulse to carry ripples or transients.

Where the falling and rising edges of a pulse differ in shape (i.e. are not symmetrical) leakage results from this loss of square symmetry.

It is the aim of the present invention to overcome at least some of the aforementioned deficiencies in the

prior art.

At its most general the present invention proposes generating a spread carrier signal by applying a pre- spreading signal to a carrier signal in order to pre- spread the carrier signal and subsequently applying another spreading signal to the pre-spread carrier signal such that leakage during application of the spreading signal, due to an imperfect mixer or due DC offset in a spreading signal, results in the leakage of a pre-spread carrier signal. This effectively results in a pre- spreading of a leakage signal prior to the subsequent mixing stage. The present invention also proposes cancelling the pre-spreading signal from the content of the spread carrier signal.

Accordingly, in a first of its aspects, the present invention may provide a spread-spectrum signal generator including: carrier signal generator means; spreading signal generator means for generating a first spreading signal and a second spreading signal; and, mixer means for mixing the first spreading signal with a carrier signal generated by the carrier signal generator means thereby to produce a first spread carrier signal, and for mixing the second spreading signal with the first spread carrier signal thereby to produce a second spread carrier signal having a spectrum being spread according to the first spreading signal and the second spreading signal.

Thus by mixing the first spreading signal with the carrier signal the present invention may produce a pre- spread carrier signal which is subsequently mixed with a second spreading signal. Carrier leakage resulting from the imperfect mixing of the second spreading signal with the pre-spread carrier signal, or from a offset in the second spreading signal, or from another source, results in leakage of a part of the (pre-spread) first spread carrier signal. Leakage of an unspread carrier signal would result in a frequency "spur" in the second spread carrier signal (output from the spread spectrum signal generator), while leakage of the first spread carrier signal avoids such spurs.

The first spread carrier signal (namely the pre-spread carrier signal) may undergo signal filtering prior to subsequently being mixed with the second spreading signal in order to remove undesired spectral components of the first spread carrier signal such as frequency spectrum side lobes associated with the carrier signal. Most preferably, a high-pass filter is employed being arranged to remove low-frequency spectrum leakage signals (arising from the first spreading signal) from the first spread carrier signal.

The spread-spectrum signal generator may be arranged to generate the first and second spreading signals in any suitable form such as would be readily apparent to the skilled person. For example, one or both of the spreading signals so generated may be pseudo-random binary sequences (BRBS), or may be any other suitable form of spreading signal. For example, complex modulatin may be used in which an "in-phase" and "quadrature-phase" signals are mixed with the carrier signal (pre-spread or otherwise) instead of (or in addition to) a PRBS signal, as a spreading signal.

The first and the second spreading signal are each preferably wide-band noise-like oscillating signals.

Preferably, the mixer means includes a spreading signal cancellation means arranged to fully cancel or substantially cancel the first spreading signal from the content of the second spread carrier signal. In this way, while the spreading effect of the first spreading signal may remain in the second spread carrier signal, the first spreading signal itself may be removed from the second carrier signal such that, of the first and second spreading signals, only the second spreading signal remains in the second spread carrier signal. Thus, when a spreading signal cancellation means is provided, the presence of the first spreading signal within the content of a carrier signal is only temporary and need only be present for pre-spreading the carrier signal but may be removed upon or after subsequently spreading the pre spread carrier signal with the second spreading signal.

The spreading signal cancellation means may be arranged to substantially cancel the first spreading signal from the content of the second spread carrier signal when mixing the second spreading signal with the first spread carrier signal, or after mixing the second spreading signal with the first spread carrier signal.

For example, the spreading signal generator means may be arranged to generate a third spreading signal and to generate the second spreading signal according to the first spreading signal and the third spreading signal.

The first spreading signal and the third spreading signal may be combined or utilised, when generating the second spreading signal, in such a way that when the second spreading signal is mixed with the (pre-spread) first spread carrier signal, that very act of mixing simultaneously cancels the first spreading signal from the content of the second spread carrier signal which results from such mixing. Preferably the first and the third spreading signals are each input into a Logic Gate or circuit which generates the second spreading signal according to the first spreading signal and the third spreading signal together with the logic employed by the Logic Gate.

Preferably, the Logic Gate is either Exclusive OR logic or is Exclusive NOR logic, the latter being simply the inverse of the former. It is to be noted that such Exclusive OR or Exclusive NOR logic gates are mixers in logic form. That is to say, the Exclusive OR or Exclusive NOR logic gates do to logic signals what other mixers do to waveforms in effect.

When using such logic gates, it is preferable that the first and third spreading signals are each bi-polar signals which oscillate between values of equal magnitude but opposite sign or, alternatively, that the first and third spreading signals are pre-processed prior to being received as inputs at the Logic Gate such that the pre processed spreading signals are either rendered bi-polar or are rendered mono-polar (i.e. oscillating between values of 1 and 0) for subsequent processing by the logic gate or circuit. Preferably the logic gate/circuit implements an Exclusive NOR logic whereby the output of the gate/circuit logic is "high" when its input are both "low" or both "high", and is logic "low" when the inputs differ (i.e. one is high and the other is low). The "high" state may be a value of 1 (one) and the logic state "low" may be a value of -1 (minus one) for bi-polar logic or 0 (zero) for mono-polar logic respectively.

Alternatively, Exclusive OR logic may be employed by the logic gate in which case the logic values would simply be the inverse of those referred to above in respect of Exclusive NOR.

The spreading signal cancellation means may include a logic gate/circuit, preferably an Exclusive NOR logic gate or an Exclusive OR logic gate, arranged to receive the first spreading signal and the third spreading signal as logic inputs thereto and to generate the second spreading signal from the logic output of the logic gate/circuit, and a signal mixer for multiplying the second spreading signal with the first spread carrier signal thereby to substantially delete the first spreading signal from the content of the second spread carrier signal when generating the latter (i.e. as a result/consequence of the aforesaid act of multiplying).

The net result is differential cancellation of the first spreading signal from the second spread carrier signal, the latter then having only the second spreading signal imposed upon it as desired.

Where the logic gate initially produces a mono-polar output (i.e. values 1 or 0) then the overall logic circuit may also include a means of converting the mono- polar output into a bi-polar final output (i.e. oscillating between values of 1 and -1). Such mono-polar to bi-polar conversion means may include an inverted push-pull drive for receiving mono-polar input signals and converting those signals into bi-polar signals.

Alternatively, the first spread signal may be cancelled from the content of the second spreading signal after generating the latter. Accordingly, the spreading signal cancellation means may include a signal mixer for multiplying the first spreading signal with the second spread carrier signal to substantially delete the first spreading signal from the content of the second spread carrier signal. This may be done without the use of the aforementioned third spreading signal and logic gate/circuit and may require only a first and second spreading signal which may be generated independently of each other.

The present invention may also be implemented in a variant form by simply replaced the Exclusive NOR (or EXOR) gate with a simple phase inverter/mixer to which the first and third spreading signals are input for mixing. The result would be that the mixing signal input for mixing with the first spread carrier signal would be a mixture of the first and third spreading signals. When this mixed spreading signal (i.e. the second spreading signal) is then mixed again with the first spreading signal during mixing of the former with the first spread carrier signal, the result is that the second mixing of the first spreading signal cancels out the first spreading signal present in the first spread carrier signal. However, the use of an Exclusive OR or Exclusive NOR logic circuit is preferred.

The first spreading signal and the second spreading signal are preferably binary sequences, e.g. generated from, or consisting of, pseudo-random binary sequences (PROS), and the first spreading signal preferably has a chip rate which is lower than that of the second spreading signal. The first spreading signal, and preferably both the first and the second spreading signal, is/are bi-polar and oscillate between values of equal size but opposite magnitude. Preferably the first, or the first and second, spreading signals oscillate between values substantially equal to 1 (one) and -1 (minus one). A lower chip rate in the first spreading signal means that substantial portions (e.g. many chip transitions) of the second spreading signal may be mixed with the first spread carrier signal while the first spreading signal remains at a constant value between chip transmissions. Consequently, the effects upon the frequency spectrum of the second spread carrier signal resulting from chip transitions in the first spreading signal (e.g. the side lobes associated with the frequency spectrum of the first spreading signal) have reduced influence/effect on the second spread carrier signal.

Indeed, the spread-spectrum signal generator may include output controls means arranged such that chip transitions of the PRBS of the first spreading signal do not occur during periods when a second spread carrier signal is being output from the spread-spectrum signal generator.

The spread-spectrum signal generator preferably generates a spreadspectrum extending over a range of up to about lGHz in frequency (including any gaps therein required to avoid specified frequency bands).

The invention in its first aspect preferably generates the first, second and third spreading signals, the carrier signal, and the first and second spread carrier signals as electrical (or optical) signals. The second spread carrier signal ultimately output from the spread- signal generator may be used to drive a signal transmitter such as an e/m signal transmitter arranged to transmit e/m signals at e.g. radio or microwave frequencies.

In a second of its aspects, the present invention may provide a signal transmitter, such as an electromagnetic signal transmitter, comprising a spread-spectrum signal generator according to the invention in its first aspect, and including none, some or all of the variants and preferred features described above. Preferably, the signal transmitter also includes a signal transmission control means arranged to control signal transmissions such that transitions between the binary states of the first spreading signal do not occur during periods of signal transmission. The result is that the undesired effects of chip transitions in the first spreading signal (e.g. side-lobes in the frequency spectrum thereof) do not appear in the output/transmitted e/m second spread carrier signal since such chip transitions are not "seen" at the output.

In a further aspect, the present invention may provide a positioning system comprising a signal transmitter according to the invention in its second aspect, and may provide a communication system comprising such a signal transmitter in a yet further aspect.

It is to be understood that the aforementioned spread- signal generator of the invention in its first aspect, and the signal transmitter of the invention in its second aspect each implement a method of spread-spectrum signal generation and signal transmission respectively which methods are encompassed by the present invention.

Accordingly, in a third of its aspects the present invention may provide a method of spread-spectrum signal generation including: generating a carrier signal; generating a first spreading signal and a second spreading signal; and, mixing the first spreading signal with a carrier signal generated by the carrier signal generator means thereby to produce a first spread carrier signal, and mixing the second spreading signal with the first spread carrier signal thereby to produce a second spread carrier signal having a spectrum being spread according to the first spreading signal and the second spreading signal.

The method may include substantially cancelling the first spreading signal from the content of the second spread carrier signal. The cancelling the first spreading signal from the content of the second spread carrier signal may be done when or after mixing the second spreading signal with the first spread carrier signal.

The method may include generating a third spreading signal and generating the second spreading signal according to the first spreading signal and the third spreading signal.

Accordingly, the method may include applying Exclusive NOR (or Exclusive OR) logic to the first spreading signal and the third spreading signal as logic inputs to generate the second spreading signal as logic output, and multiplying the second spreading signal with the first spread carrier signal to substantially delete the first spreading signal from the content of the second spread carrier signal when generating the latter.

Alternatively, the method may include multiplying the first spreading signal with the second spread carrier signal to substantially delete the first spreading signal from the content of the second spread carrier signal.

The first spreading signal and the second spreading signal are preferably binary sequences and may comprise, or be generated according to, pseudorandom binary sequences (PRBS) and the first spreading signal has a chip rate which is preferably lower than that of the second spreading signal. The first spreading signal preferably oscillates between the binary states substantially equal to +1 (plus one) and -1 (minus one).

Preferably, the second spreading signal also oscillates in this way.

The invention in a fourth of its aspects may provide a method of signal transmission comprising a method of spread-spectrum signal generation according to the invention in its third aspect including none, some or all of the aforementioned variant/preferable features thereof.

The invention in a fifth of its aspects may provide a S method of signal transmission comprising a method of spread-spectrum signal generation according to the invention in its third aspect including controlling signal transmissions such that transitions between the binary states of the first spreading signal do not occur during periods of signal transmission.

Non-limiting examples of the invention shall now be described with reference to the accompanying drawings of which: Figure 1 illustrates the effect of spreading a comb of continuous-wave carrier frequency spikes in order to generate a broken spread-spectrum; Figure 2 schematically illustrates a spread-spectrum signal generator employing a single spreading signal) Figure 3 illustrates the effect of carrier signal leakage in a spread-carrier signal resulting in a "spur" appearing in the spread carrier signal; Figure 4 schematically illustrates a spreadspectrum signal generator employing a first spreading signal for prespreading a carrier signal and a second spreading signal for further spreading the pre-spread carrier signal, together with spreading signal cancellation means for cancelling the first spreading signal from the content of the final spread carrier signal when and by generating the latter; Figure 5 illustrates a spread-spectrum signal generator employing a first spreading signal for pre- spreading a carrier signal and a second spreading signal for further spreading the pre-spread carrier signal, together with spreading signal cancellation means arranged to cancel the first spreading signal from the content of the final spread carrier signal after generation of the latter.

In the figures like articles are assigned like reference signs.

Referring to Figure 4 there is schematically illustrates a spreadspectrum signal generator 1 including a carrier signal generator 2 arranged to generate a continuous-wave (COO) electrical signal of a fixed predetermined carrier frequency. The spread-spectrum generator 1 also includes a spreading signal generator means comprising a first PRBS generator 5 for generating a first spreading signal, and a second PRBS generator 4 for generating a third spreading signal. Each PRBS generator is arranged to generate a spreading signal being an electrical signal in the form of a pseudo-random binary sequence (PRBS) for use in spreading the carrier signal generated by the carrier signal generator 2. Each such spreading signal comprises a binary sequence of square-wave pulses of pseudo-random duration which oscillate between the bi- polar values of +1 and -1. The chip rate of the binary sequence generated by the first spreading signal generator 5 is lOOK chips/second, while the chip rate of the binary sequence of the third spreading signal generator is between lOM chips/second and 20M chips/second.

The spread-spectrum signal generator 1 further includes mixer means comprising a first mixer 6 arranged to receive the first spreading signal 7 at a first mixer input thereof, to receive the CW carrier signal from the carrier signal generator 2 at a second mixer input thereof, and to multiply the former with the latter and output the result from the output of the mixer 6 as a first spread, or pre-spread, carrier signal.

The first spread carrier signal is subsequently passed through a highpass filter 11 which is arranged to remove from the frequency spectrum of the first spread carrier signal components thereof resulting from lowfrequency leakage from the first spreading signal generator 5. The highpass filtered first spreading signal is subsequently amplified by a buffer 12 to a level suitable for input to a second mixer 3 of the mixer means for mixing thereby with the second spreading signal 25.

The mixer means further comprises a logic circuit in the form of an Exclusive NOR Gate 9 (Exclusive OR may be used alternatively) which is arranged to generate the second spreading signal 25 according to the first spreading signal 8 from the first spreading signal generator 5 and the third spreading signal 23 from the second spreading signal generator 4 as follows.

The logic circuit 9 of the mixer means has a first logic input arranged to receive the third spreading signal 23 from the second spreading signal generator 4, and a second logic input arranged to simultaneously receive a copy 8 of the first spreading signal generated by the first spreading signal generator 5 and being identical to the first spreading signal 7 input to the first mixer 6.

A logic table illustrating the logic applied by the Exclusive NOR Gate 9 is provided below and illustrates that the output 25 of the logic gate 9 possesses a "high" value only when both the first and the second logic input thereof are either "low" or "high", but has a logic output "low"when the logic states at it first and second logic inputs differ. The output 25 of the NOR Gate 9 represents the second spreading signal which, subsequent to low-pass filtering by a low-pass filter lO, is input to a mixer input of a second mixer 3 of the mixing means for mixing thereby with the first spread carrier signal input to the other input of the mixer 3. Where Exclusive OR is used instead of Exclusive NOR, the above logic states will be inverted.

In the present embodiment the logic state "high" is the value +l while the logic state "low" is the value -l, but the latter may be a value O (zero) in alternative embodiments.

Logic Table - Exclusive NOR: INPUT 1 INPUT 2 OUTPUT Low Low High Low High Low High Low Low High High High The low-pass filter 10 filters the second spreading signal 25 so as to remove high frequency transients present in the frequency spectrum of the second spreading signal resulting from the chip transitions in the binary S sequence of the second spreading signal. The second mixer 3 mixes the low-pass filtered second spreading signal 25 with the high-pass filtered, and buffered, first spread carrier signal by multiplying the former with the latter and outputting the result as a second spread carrier signal having a frequency spectrum being spread according to the first spreading signal and the second spreading signal.

The mixer means includes a spreading signal cancellation means comprising the logic circuit 9 in conjunction with the second mixer 3 wherein the Exclusive NOR logic applied by the logic gate 9 is chosen such that when the second spreading signal 25 is mixed with the first spreading signal within the first spread carrier signal, the result is a second spread carrier signal output containing only the third spreading signal and no longer containing the first spreading signal.

This differential cancellation process is illustrated in the following table containing an exemplary 10-chip binary sequence of the first and third spreading signals, 8 and 23 respectively, input to Exclusive NOR Gate 9, together with the corresponding 10-chip binary sequence of the second spreading signal generated by the Gate 9 and the effect of subsequently mixing the binary sequence of the second spreading signal 25 with the binary sequence of the first spreading signal 7 present in the first spread carrier signal both input to the second mixer 3.

Exemplary 10-chip bit sequence: First PRBS (8) 1 -1 1 1 -1 -1 -1 1 - Third PRBS (23) -1 1 1 -1 -1 1 -1 - Second Binary Sequence _1 -1 1 -1 1 -1 1 -1 - Second Binary Sequence _1 1 1 -1 1 1 -1 - Thus, it can be seen from the above table that the effect of mixing the first spreading signal 8 with the second spreading signal 25 generated according to the Exclusive NOR Gate 9 is to simply reproduce (i.e. [First PRBS] [Second Binary Sequence]) the binary sequence of the third spreading signal 23 in the output of the spread spectrum signal generator and to differentially cancel the binary sequence of the first spreading signal 7 from the content thereof.

Referring to Figure 5 there is illustrated and alternative embodiment of the present invention in which the Exclusive NOR Gate 9 is omitted, and the second spreading signal 25 is produced solely by (and is identical to) the PRBS of the "third" spreading signal generator 4. However, in this alternative embodiment, the spreading signal cancellation means comprises a third mixer 20 arranged to receive the second spread carrier signal output from the second mixer 3, to receive the first spreading signal 15 output from the first spreading signal generator 5 (being an exact copy of the first spreading signal 7 input to the first mixer unit 6), to mix the second spread carrier signal with the first spreading signal received thereby, and to output the result as a final second spread carrier signal and as the output from the spread-spectrum signal generator.

Thus, in this way, cancellation of the first spreading signal from the content of the second spread carrier signal may be achieved after generation of the second spread carrier signal as opposed to during its generation as in the embodiment illustrated in Figure 4.

This subsequent cancellation procedure is illustrated in the table below which contains an exemplary 10-chip sequence of the PRBS signals generated by the first and the second spreading signal generators (5 and 4 respectively), and the corresponding 10-chip sequences of the PRBS content of the second spread carrier signal in intermediate form ("SCS1") and in its form as ultimately output from the spread-spectrum signal generator.

Exemplary 10-chip bit sequence: (7) 1 -1 1 l _1 -1 -1|1 | Second PRBS - - -- (25) 1 1 1 1 1 1 1 1 1 1 First PRBS -1 -1 1 -1 1 -1 1 -1 -1 1 Second PRBS First PRBS -1 1 1 -1 -1 1 -1 -1 1 1 Second PR:S Thus, applying the first PRBS sequence (15) to the second spread carrier signal, which signal contains the first PRBS (7), has the effect of cancelling out the first PRBS signal (7) from the second spread carrier signal. Thus, multiplying the first spreading signal 7 with its identical copy 15 results in a product having a constant value of 1. The components of the spreadspectrum signal generator illustrated in Figure 5 having the same reference signs as components of the spread-spectrum signal generator of Figure 4 are in fact the same signal generator components.

The mixer units (items 6, 3 and 20) of the spread- spectrum signal generator of Figure 4 or Figure 5 each comprise a phase inverted operating as a balanced modulator. As is well known in the art, such phase inverters comprise three input/output ports, namely: an "L-port", an "I-port", and an "R-port". In the embodiments of the present invention, the mixers 3, 6 and of the mixer means employ the I-port for receiving spreading signals, and employ the L-port for receiving carrier signals (pre-spread or otherwise). Consequently, the R-port is employed as the mixer output. This arrangement and use of the ports of such mixers may also be employed in other embodiments envisaged or encompassed by the present invention.

In variants of the above illustrated embodiments of the present invention, the mixer unit 6 of the mixing means may be merged within the carrier frequency generator 2 as part of a closed loop phase-locked synthesizer. The generator output would therefore be phase modulated.

This alternative has the advantage that amplitude imbalances between the two states of the modulator would not appear on the generator output.

In a further variant of the aforementioned embodiments, the second spreading signal (e.g. output from the low pass filter lo of Fig.4 or 5) may be input to an additional mixer unit into which is simultaneously fed a S fixed-frequency carrier signal whereby the additional mixer unit mixes the second spreading signal with the fixed-frequency carrier signal and outputs the result.

The output of this additional mixer unit would then be mixed with the first spread carrier signal (e.g. at mixer unit 3 of Figs.4 or 5) for use in generating the second spread carrier signal and for use in implementing differential cancellation of the first spreading signal from the content of the second spread carrier signal.

Hence, the differential cancellation occurring is between two IF signals rather than between one IF signal and one baseband waveform as is the case in the embodiments of Fig.4 and Fig.5 (IF = "Intermediate Frequency") .

It is to be appreciated that these embodiments of the invention are given by way of example only, and modifications/variants thereto will be readily apparent to those skilled in the art without departing from the scope of the present invention.

Claims

CLAIMS: l. A spread-spectrum signal generator including: carrier signal

generator means; spreading signal generator means for generating a first spreading signal and a second spreading signal; and, mixer means for mixing the first spreading signal with a carrier signal generated by the carrier signal generator means thereby to produce a first spread carrier signal, and for mixing the second spreading signal with the first spread carrier signal thereby to produce a second spread carrier signal having a spectrum being spread according to the first spreading signal and the second spreading signal.
2. A spread-spectrum signal generator according to Claim l wherein the mixer means includes a spreading signal cancellation means arranged to substantially cancel the first spreading signal from the content of the second spread carrier signal.
3. A spread-spectrum signal generator according to Claim 2 wherein the spreading signal cancellation means is arranged to substantially cancel the first spreading signal from the content of the second spread carrier signal when mixing the second spreading signal with the first spread carrier signal.
4. A spread-spectrum signal generator according to Claim 2 wherein the spreading signal cancellation means is arranged to substantially cancel the first spreading signal from the content of the second spread carrier signal after mixing the second spreading signal with the first spread carrier signal.
5. A spread-spectrum signal generator according to Claim 3 wherein the spreading signal generator means is arranged to generate a third spreading signal and to generate the second spreading signal according to the first spreading signal and the third spreading signal.
6. A spread-spectrum signal generator according to Claim wherein the spreading signal cancellation means includes an Exclusive OR logic gate or an Exclusive NOR gate arranged to receive the first spreading signal and the third spreading signal as logic inputs thereto and to generate the second spreading signal as logic output therefrom, and a signal mixer for multiplying the second spreading signal with the first spread carrier signal to substantially delete the first spreading signal from the content of the second spread carrier signal.
7. A spread-spectrum signal generator according to Claim 4 wherein the spreading signal cancellation means includes a signal mixer for multiplying the first spreading signal with the second spread carrier signal to substantially delete the first spreading signal from the content of the second spread carrier signal.
8. A spread-spectrum signal generator according to any preceding claim wherein the first spreading signal and the second spreading signal are binary sequences and the first spreading signal has a chip rate which is lower than that of the second spreading signal.
9. A spread-spectrum signal generator according to Claim 8 wherein the first spreading signal oscillates between the binary states substantially equal to +1 (plus one) and -1 (minus one).
10. A signal transmitter comprising a spread-spectrum signal generator according to Claim 8 or Claim 9 and including a signal transmission control means arranged to control signal transmissions such that transitions between the binary states of the first spreading signal do not occur during periods of signal transmission.
11. A signal transmitter comprising a spread-spectrum signal generator according to any preceding claim.
12. A method of spread-spectrum signal generation including: generating a carrier signal) generating a first spreading signal and a second spreading signal; and, mixing the first spreading signal with a carrier signal generated by the carrier signal generator means thereby to produce a first spread carrier signal, and mixing the second spreading signal with the first spread carrier signal thereby to produce a second spread carrier signal having a spectrum being spread according to the first spreading signal and the second spreading signal.
13. A method of spread-spectrum signal generation according to Claim 12 including substantially cancelling the first spreading signal from the content of the second spread carrier signal.
14. A method of spread-spectrum signal generation according to Claim 13 including substantially cancelling the first spreading signal from the content of the second spread carrier signal when mixing the second spreading signal with the first spread carrier signal.
15. A method of spread-spectrum signal generation according to Claim 13 including substantially cancelling the first spreading signal from the content of the second spread carrier signal after mixing the second spreading S signal with the first spread carrier signal.
16. A method of spread-spectrum signal generation according to Claim 14 including generating a third spreading signal and generating the second spreading signal according to the first spreading signal and the third spreading signal.
17. A method of spread-spectrum signal generation according to Claim 16 including applying Exclusive OR logic or Exclusive NOR logic to the first spreading signal and the third spreading signal as logic inputs to generate the second spreading signal as logic output, and multiplying the second spreading signal with the first spread carrier signal to substantially delete the first spreading signal from the content of the second spread carrier signal.
18. A method of spread-spectrum signal generation according to Claim 15 including multiplying the first spreading signal with the second spread carrier signal to substantially delete the first spreading signal from the content of the second spread carrier signal.
19. A method of spread-spectrum signal generation S according to any of preceding claims 12 to 18 wherein the first spreading signal and the second spreading signal are binary sequences and the first spreading signal has a chip rate which is lower than that of the second spreading signal.
20. A method of spread-spectrum signal generation according to Claim 19 wherein the first spreading signal oscillates between the binary states substantially equal to +1 (plus one) and -1 (minus one).
21. A method of signal transmission comprising a method of spreadspectrum signal generation according to Claim 19 or Claim 20 and including controlling signal transmissions such that transitions between the binary states of the first spreading signal do not occur during periods of signal transmission.
22. A method of signal transmission comprising a method of spreadspectrum signal generation according to any preceding claim.
23. A positioning system comprising a signal transmitter according to Claim 10 or Claim 11.
24. A communications system comprising a signal transmitter according to Claim 10 or Claim 11.
25. A spread-spectrum signal generator substantially as described in any embodiment herein with reference to Figures 4 and 5 the accompanying drawings.
26. A method of spread-spectrum signal generation substantially as described in any embodiment herein with reference to the Figures 4 and 5 of accompanying drawings.