Phased array spread spectrum system and method

CA 02136408 1999-06-03

PHASED ARRAY SPREAD SPRECTRUM SYSTEM AND METHOD

BACKGROUND OF THE INVENTION

The present invention relates to spread-spectrum communications and more particularly to a method and apparatus for enhancing communications by using phased array principles for increasing signal-to-noise ratio for a spread spectrum signal with multipath arriving at a receiver.

DESCRIPTION OPL-THE_RELEVANT ART

Achieving sufficient signal strength when a received signal comes from two paths is a problem when communicating with spreadspectrum modulation in a multipath environment. The received signal from the two paths may have phase cancellation, yielding no reception, or reception with an unacceptable error rate.

SUMMARY OF THE INVENTION

A general object of an aspect of the invention is an improved system and method for receiving spread-spectrum signals in a multipath environment.

Another object of an aspect of the invention is to increase the received signal-to-noise ratio or reduce the probability of error of a spread-spectrum signal arriving from two or more paths.

According to the present invention, as embodied and broadly described herein, a phased array spread-spectrum system is provided comprising receiving means, delaying means, combining means, despreading means, generating means, storing means, and comparing means. The receiving means receives a spread-spectrum signal and a phased version of the spread-spectrum signal.

-1CA2136408

Typically, the spread-spectrum signal is received with a first receiver coupled to a first antenna, and the phased version of the spread-spectrum signal is received with a second receiver coupled to a second antenna. The received spread-spectrum signal and the phased version of the spread-spectrum signal are digitized. The delaying means can delay the received spread-spectrum signal with respect to the phase version of the spread-spectrum signal by a delay. The received spread-spectrum signal consequently becomes a delayed signal.

The combining means combines the delayed signal and the phased 10 version of the spread-spectrum signal as a combined signal. An in-phase component of the delayed signal is combined with an in-phase component of the phased version of the spread-spectrum signal. A quadrature-phase component of the delayed signal is combined with a quadrature-phase component of the phased version of the spread-spectrum signal.

The despreading means despreads the combined signal as a despread signal. This may be accomplished using a product detector with a chipping sequence matched to the received spread-spectrum signal, or a matched filter having an impulse function matched to the chipping sequence of the received spread-spectrum signal.

The generating means generates from the despread signal a magnitude value. The magnitude value may be an absolute value, or the square of the in-phase component and quadrature-phase component of the despread signal.

The storing means stores a previous-magnitude value

-2CA 02136408 1999-06-03 previously generated by the generating means and a presentmagnitude value presently generated by the generating means. The previous-magnitude value and the present-magnitude value are compared by the comparing means. In response to this comparison, the comparing means outputs a comparison signal. The delaying means may change the delay based on the comparison signal.

The present invention also includes a method for maximizing the signal strength of a spread-spectrum signal with multipath comprising the steps of receiving the spread-spectrum signal and a phased version of the spread-spectrum signal. The received spread-spectrum signal is delayed with respect to the phased version of the spread-spectrum signal by a delay, to generate a delayed signal. The delayed signal and the phased version of the spread-spectrum signal are combined as a combined signal, and the combined signal is despread as a despread signal.

The method includes generating a magnitude value from the despread signal, and storing a previous-magnitude value and a present-magnitude value. The previous-magnitude value and the present-magnitude value are compared, and a comparison signal is output based on this comparison. Using the comparison signal, the delay is changed. The step of generating the magnitude value is a way of locating a maximum. Other procedures for locating a maximum or equivalent may be used.

-3CA 02136408 1999-06-03

3a

In accordance with one embodiment of the invention, a phased array spread spectrum system comprises :

a first antenna for receiving a spread-spectrum signal; a first analog-to-digital converter, coupled to the first antenna, for digitizing or phase shift network the spreadspectrum signal;

a first digital-delay device, coupled to the first analog-to-digital converter, for delaying the digitized spread-spectrum signal by a first delay, thereby generating a first delayed signal;

a second antenna for receiving a phased version of the spread-spectrum signal;

a second analog-to-digital converter, coupled to the second antenna, for digitizing the phased version of the spread-spectrum signal;

a second digital-delay device coupled to the second analog-to-digital converter for delaying the digitized phased version of the spread-spectrum signal by a second delay, thereby generating a second delay signal;

a first summer, coupled to the first digital-delay device and to the second digital-delay device, for combining quadrature components of the first delayed signal and the second delayed signal, as a first combined signal;

a second summer, coupled to the first digital-delay and to the second digital-delay device, for combining in-phase components of the first delayed signal and the second delayed signal, as a second combined signal;

a despreading device, coupled to the first summer and to the second summer, for despreading the first combined signal and the second combined signal, as a despread in-phase signal and a despread quadrature -signal;

a magnitude device, coupled to the despreading device, for generating a magnitude value of the despread in-phase signal and the despread quadrature-signal;

CA 02136408 1999-06-03

3b a shift register, coupled to the magnitude device, for storing a previous-magnitude value and a present-magnitude value;

a comparator, having at least two inputs coupled to the shift register, and responsive to comparing the previousmagnitude value with the present-magnitude value, for generating a comparison signal; and a counter, coupled to the comparator and to the first digital-delay device and to the second digital-delay device, responsive to the comparison signal, for changing the first delay of the first digital-delay device.

In accordance with another embodiment of the invention, a phased array spread-spectrum system comprises:

means for receiving a spread-spectrum signal and a phased version of the spread-spectrum signal;

means for delaying the spread-spectrum signal with respect to the phased version of the spread-spectrum signal by a delay, thereby generating a delayed signal;

means for combining the delayed signal and the phased version of the spread-spectrum signal, as a combined signal;

means for despreading the combined signal as a despread

<td> means</td><td> for</td>

<td> version of</td><td> the</td>

<td> means</td><td> for</td>

<td> signal;</td><td></td>

<td> means</td><td> for</td>

<td> signal;</td><td></td>

<td> means</td><td> for</td>

means for generating a magnitude value of the despread means for storing a previous-magnitude value and a present-magnitude value generated from the first generating means ; and means for comparing the previous-magnitude value with the present magnitude value, and for outputting a comparison signal, with the delaying means responsive to the comparison signal for changing the delay.

In accordance with another embodiment of the invention, a method for receiving a spread-spectrum signal comprising the steps of :

receiving a spread-spectrum signal;

receiving a phased version of the spread-spectrum signal;

CA 02136408 1999-06-03

3C delaying at least the spread-spectrum signal with respect to the phased version of the spread-spectrum signal by a delay, thereby generating a delayed signal;

combining the delayed signal and the phased version of the spread-spectrum signal, as a combined signal;

despreading the combined signal as a despread signal; generating a magnitude value of the despread signal; storing the magnitude value as a present-magnitude value; comparing a previous-magnitude value with the presentmagnitude value;

outputting, in response to comparing the previousmagnitude value with the present-magnitude value, a comparison signal; and changing, in response to the comparison signal, the delay.

CA 02136408 1999-06-03

BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate preferred embodiments of the invention and, together with the description, serve to explain the principles of the invention.

FIG. 1 is a block diagram illustrating the general concept of the invention;

FIG. 2 shows two multipath signals being received by a user;

and

FIG. 3 is a block diagram for adjusting a phase between two receivers.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 15 Reference now is made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals indicate like elements throughout the several views.

The present invention provides a unique phased array spread20 spectrum system comprising receiving means, delaying means, combining means, despreading means, generating means, storing means, and comparing means. The delaying means is coupled between the receiving means and the combining means. The despreading means is coupled between the combining means and the generating means. The storing means is coupled between the

-4CA2 ! 36408 generating means and the comparing means, and the comparing means is coupled to the delaying means.

The receiving means of FIG. 1 receives a spread-spectrum signal and a phased version of the spread-spectrum signal. The term phased version as used herein includes a version of the spread-spectrum signal having a phase different from the received spread-spectrum signal, and/or a version of the spread-spectrum signal having a time delay with respect to the received spread-spectrum signal. The different phase and/or time delay arises, as shown in FIG. 2, from the spread-spectrum signal 15 and the phased version of the spread-spectrum signal 16 arriving from different paths, such as bouncing off different buildings 17, 18. The phased array spread-spectrum system may be implemented at a base station or, as shown in FIG. 2, a remote subscriber unit (RSU) such as a handset 19. The phase change occurs upon each reflection, since a first ray 15 has one reflection and a second ray 16 has two reflections. As a result of the difference in time between the two rays, the multipath signals can undergo phase cancellation and cause a fade. The phased array spread-spectrum system of FIG. 1 delays or phase shifts one of the two antennas 11,12 enough to steer the beam from the two antennas to either building, or ray path having maximum signal strength.

Typically, the receiving means, as shown in FIG. 1, includes a first antenna 11 and a second antenna 12. The spread-spectrum signal d(t)g{t) οοεωθΐ is received with a first receiver coupled to a first antenna 11, and the phased version of the spread-spectrum signal d(t-r)g(t-r)cosw_o(t-r) is received with a second

-5receiver coupled to a second antenna 12. The first receiver and the second receiver include radio frequency (RF) and intermediate frequency (IF) amplifiers and filters, as appropriate. The received spread-spectrum signal and the phased version of the spread-spectrum signal may be digitized.

The delaying means, shown in FIG. 1 as a delay device 13, can delay the received spread-spectrum signal with respect to the phase version of the spread-spectrum signal by a delay. The received spread-spectrum signal consequently becomes a delayed signal, with the delay approximately equal to a delay of the phased version of the spread-spectrum signal. A preferred embodiment employs digital signal processing. Accordingly, the delaying means would include a digital delay device such as a shift register. Alternatively, analog circuitry would employ an analog delay device, or a phase shifter.

Although illustrated with two antennas, the receiving means may include additional antennas for enhanced performance. The delaying means would have appropriate delaying circuits to accommodate the multiple antennas.

The combining means, shown in FIG. 1 as a combiner 14, combines the delayed signal and the phased version of the spread-spectrum signal as a combined signal. The delayed signal and the phased version of the spreadspectrum signal have approximately the same phase or time delay. Thus, an in-phase component of the delayed signal combines with an in-phase component of the phased version of the spread-spectrum signal, and a quadrature-phase component of the delayed signal combines with a quadrature-phase

-6CA2136408 component of the phased version of the spread-spectrum signal.

The despreading means despreads the combined signal as a despread signal. This may be accomplished using a product detector with a chipping sequence matched to the received spread-spectrum signal, or a matched filter such as a surface acoustic wave (SAW) device having an impulse function matched to the chipping sequence of the received spread-spectrum signal. Product detectors, digital signal processors and SAW devices for despreading spread-spectrum signals are well known in the art.

The generating means generates a magnitude value from the despread signal. The magnitude value may be an absolute value, the square of the inphase component and quadrature-phase component of the despread signal, or other metric of the despread signal for determining a relative signal strength value. A magnitude value currently being generated by the generating means is referred to herein as a present-magnitude value. A

5 magnitude value previously generated by the generating means is referred to herein as a previous-magnitude value. The invention is taught with the previous-magnitude value being generated just before the present-magnitude value, although a previous-magnitude value may be separated in time and other magnitude values from the present magnitude value. Also, more than one previous-magnitude value may be used. The concept of the present invention is taught with one previous-magnitude value.

The storing means stores the previous-magnitude value previously generated by the generating means and the present-magnitude value presently generated by the generating means. In

-7CA2 ί 36408 a digital implementation, the storing means might be embodied as a shift register or, equivalently, as gates for performing the storing function. In an analog implementation, the storing means might be embodied as two or more capacitors for storing the previous-magnitude value and the presentmagnitude value.

The previous-magnitude value and the present-magnitude value are compared by the comparing means. In response to this comparison, the comparing means outputs a comparison signal. The comparing means, for example, may output a comparison signal to increase the delay of the delaying means, if the present-magnitude value were greater than the previous-magnitude value; conversely, the comparing means may output a comparison signal to decrease the delay of delaying means, if the presentmagnitude value were less than the previous-magnitude value. The delaying means changes the first delay based on the comparison signal. If a plurality of previous-magnitude values were used, then a scheme may be implemented with the comparing means to weight the plurality of previousmagnitude values.

In the exemplary arrangement shown in FIG. 3, the receiving means is embodied as the first antenna 11, a first RF/IF section 21, a first analog-todigital converter 23, the second antenna 12, a second RF/IF section 22, and a second analog-to-digital converter 24. The first RF/IF section 21 is coupled between the first antenna 11 and the first analog-to-digital converter 23. The second RF/IF section 22 is coupled between the second antenna 12 and the second analog-to-digital converter 24. Typically, the first RF/IF section 11 generates an in-phase component and a

-8CA2136408 quadrature-phase component of the received spread-spectrum signal. The second RF/IF section 12 generates an in-phase component and quadraturephase component of the phased-version of the spread-spectrum signal.

As illustratively shown in FIG. 3, the outputs of the first analog-to5 digital converter 23 and the second analog-to-digital converter 24 may go to other sections for processing different channels of the spread-spectrum signal 25, 26.

The delaying means is embodied as a first digital delay 27. The delaying means additionally may include a second digital delay 28. The first digital delay 27 is coupled to the first analog-to-digital converter 23. If a second digital delay 28 were employed, then the second digital delay 28 is coupled to the second analog-to-digital converter 24.

The combining means is embodied as first summer 29 and second summer 30. The first summer 29 is coupled to the first digital-delay device

5 27 and to the second digital-delay device 28. The second summer 30 is coupled to the first digital-delay device 27 and to the second digital-delay device 28. If the second digital delay device 28 were not employed, then the first summer 29 is coupled to the first digital-delay device 27 and to the second analog-to-digital converter 24, and the second summer 30 is coupled to the first digital-delay device 27 and to the second analog-to-digital converter 24.

The despreading means is embodied as a despreader 31. The despreader may be embodied as a product device coupled to an appropriate chipping-sequence generator and synchronization

-9ΓΛ2 ï J6408 circuitry for despreading the received spread spectrum signal. Alternatively, the despreader 31 may be a digital signal processor which includes the appropriate product devices, or a matched filter having an impulse response matched to the chipping sequence of the received spread spectrum signal.

As is well known in the art, a surface acoustic wave (SAW) device may be employed having an impulse response matched to the chipping sequence.

The generating means is embodied as a magnitude device 32. The magnitude device 32 is coupled to the despreader 31. Normally, the despreader 31 is coupled to additional circuitry for demodulating data embedded in the received spread spectrum signal.

The storing means is embodied as a shift register 33. The shift register 33 is coupled to the magnitude device 32. The storing means alternatively may be embodied as a plurality of gates, registers, or other circuitry for storing magnitude values.

The comparing means may be embodied as a comparator 34 and an up/down counter 35. The comparator 34 typically has two inputs coupled to the shift register 33. The up/down counter 35 is coupled to the output of the comparator 34 and to the first digital-delay device 27 and/or the second digital-delay device 28.

The first antenna 11 receives the spread-spectrum signal which is amplified by the first RF/IF section 21. The first RF/IF section 21 outputs an in-phase component and a quadrature-10CA2136408 phase component to the first analog-to-digital converter 23. The first analog-to-digital converter 23 converts the in-phase component and the quadrature-phase component to a digitized in-phase component and a digitized quadrature-phase component. These components may be processed by modules similar to the phase compensation circuitry 40, by coupling to the outputs of first analog-to-digital converter 23 at the outputs

25.

Similarly, a phased version of the spread-spectrum signal is received by the second antenna 12 and amplified and filtered by the second RF/IF section 22. The second RF/IF section 22 has outputs for an in-phase component and a quadrature-phase component which are fed to the second analog-to-digital converter 24. The outputs 26 of the second analog-todigital converter can go to modules similar to the phase compensation circuitry 40 for processing different chipping sequences. For example, a spread spectrum may have a plurality of spread-spectrum channels, with each spread-spectrum channel defined by a different chipping sequence. Accordingly, each module 40 would be used for a corresponding spreadspectrum channel, for processing with that particular chipping sequence.

The first digital-delay device 27 delays the digitized spread-spectrum signal by a first delay. The output of the first digital-delay device 27 is the first delayed signal. The second digital-delay device 28 delays the digitized phased version of the spread-spectrum signal by a second delay. The output of the second digital-delay device 28 is a second delayed signal. The second digital-delay device 28 is optional, and is not required

-11CA2136408 for use of the present invention. If the second digital-delay device 28 were not employed, then the term second delayed signal refers to the digitized phased version of the spread-spectrum signal, outputted from the second analog-to-digital converter 24.

The first summer 29 combines the quadrature-phase components of the first delayed signal from the first digital-delay device 27, with the second delayed signal from the second digital-delay device 28. The output of the first summer 29 is a first combined signal.

The second summer 30 combines an in-phase component from the 10 first digital-delay device 27, with an in-phase component from the second digital-delay device 28. Accordingly, the in-phase components of the first delayed signal and the second delayed signal are combined as a second combined signal.

The despreading device 31 despreads the first combined signal and 15 the second combined signal as a despread in-phase signal and a despread quadrature-phase signal. The despread in-phase signal and the despread quadrature-phase signal can be processed by further processing devices, not shown, for demodulating data embedded in the received spread-spectrum signal.

The magnitude device 32 generates a magnitude value from the despread in-phase signal and the despread quadrature-phase signal. The magnitude value may be an absolute value determined from the despread inphase signal and the despread quadrature-phase signal, or a square of the despread in-phase signal plus a

-12CA2'ί 36408 square of the despread quadrature-phase signal. Other metrics may be used for accomplishing the same function of determining a relative signal strength value. The function of the magnitude value is to compare the signal strength of a present-magnitude value with a previous-magnitude value.

The shift register 33 stores the previous-magnitude value and the present-magnitude value in order that a comparison may be made by the comparator 34. The comparator 34, when comparing the previousmagnitude value with the present-magnitude value, outputs a comparison signal. The comparison signal can control the up/down counter 35 to increase or decrease a delay of the first digital-delay device 27. Optionally, the up/down counter 35 may increase or decrease a second delay of the second digital-delay device 28.

The present invention also includes a method for maximizing signal strength of a spread-spectrum signal with multipath comprising the steps of receiving the spread-spectrum signal and a phased version of the spreadspectrum signal. The in-phase and quadrature-phase components of the received spread-spectrum signal are delayed with respect to the in-phase and quadrature-phase components of the phased version of the spread-spectrum signal by a delay, to generate a delayed signal. The in-phase component and the quadrature-phase component of the delayed signal and the phased version of the spread-spectrum signal are combined, respectively, as the inphase component and quadrature-phase component of a combined signal, and the combined signal is

-13CA21364Ü8 despread as in-phase and quadrature-phase components of a despread signal.

The method includes generating a magnitude value from the in-phase component and the quadrature-phase component of the despread signal, and storing a previous-magnitude value and a present-magnitude value. The previous-magnitude value and the present-magnitude value are compared, and a comparison signal is output based on this comparison. Using the comparison signal, the delay is changed.

In use, the phased array spread-spectrum system and method may be 10 used at a base station or a remote unit. A spread-spectrum signal being received by the phased array spread spectrum system and method is received by the first antenna 11 and the second antenna 12, processed by the first and second RF/IF sections 21, 22, and converted to a digital form by first analog-to-digital converter 23 and second analog-to-digital converter

24. Preferably, digital signal processing is used and may be embodied in an application specific integrated circuit (ASIC). The digitized spread-spectrum signal from the first analog-to-digital converter 23 is preferably delayed with respect to the digitized phased version of the spread-spectrum signal from the second analog-to-digital converter 24. The first digital-delay device 27 is adjusted by up/down counter 35 until the phased and/or time delay between the digitized spread-spectrum signal, and the digitized phased version of the spread-spectrum signal, are more closely aligned. The alignment accrues due to the variations of the up/down counter 35 in response to

-14CA2136408 comparisons by comparator 34 of a previous-magnitude value and a presentmagnitude value stored in register 33.

Thus, the spread-spectrum signal and a phased version of the spreadspectrum signal are received, processed to an intermediate frequency or base band and digitized. In-phase and quadrature-phase components are used and delayed and added. The resulting in-phase component and quadrature-phase component are then despread. The magnitude of the despread spread-spectrum signal is then taken and this represents the power or signal strength of the desired signal. The present-magnitude value and the previous-magnitude value are inputted to the shift register 33 and compared by comparator 34. The comparator 34 tells the up/down counter 35 to count as an increase or decrease, i.e., up or down, thereby controlling the delay. Thus, an increase in count might increase the delay, whereas a decrease in count would decrease the delay. Various control algorithms may be used with the up/down counter 35, for more efficiency.

The phased array spread-spectrum system steers an antenna beam formed by the first antenna 11 and a second antenna 1 2 in the direction of the strongest multipath. This function can be performed continually, thus always looking for the optimal multipath. This beam steering can be done at a base station and at an handset, i.e., a remote subscriber unit.

It will be apparent to those skilled in the art that various modifications can be made to the phased array spread spectrum system and method of the instant invention without departing from the scope or spirit of the invention, and it is intended that the

-15f 56408 present invention cover modifications and variations of the phased array spread spectrum system and method provided they come within the scope of the appended claims and their equivalents.