WO1990010982A1

WO1990010982A1 - Spread spectrum communications system with randomizing signal

Info

Publication number: WO1990010982A1
Application number: PCT/US1990/000783
Authority: WO
Inventors: Alan Pate
Original assignee: Hillier Technologies Limited Partnership
Priority date: 1989-03-06
Filing date: 1990-02-12
Publication date: 1990-09-20
Also published as: AU5197490A

Abstract

A direct sequence spread spectrum system employs a random signal generator (22). The output of the random signal generator (22) is added (26) to a signal denoting chip values (16), and a carrier signal is modulated (30) with this combined signal. The random signal provides enhanced dispersion of carrier signal power over a frequency range. The receiver (40) incorporates a filter (46) for stripping out the random signal component from the received signal.

Description

SPREAD SPECTRUM COMMUNICATIONS SYSTEM WITH RANDOMIZING

SIGNAL TECHNICAL FIELD

The present invention relates to methods and apparatus for spread-spectrum information transmission. BACKGROUND ART

Spread-spectrum signal transmission methods have been utilized heretofore in military, space, and other applications where cost is relatively unimportant. In an ordinary or non-spread signal transmission scheme, a transmission signal such as a radio frequency carrier signal is simply modulated with the information to be transmitted. This modulation causes some spreading of the power in the carrier signal over a range of frequencies. However, the major portion of the power remains concentrated within relatively narrow bands of frequencies.

In spread-spectrum techniques, the power of the transmission signal is spread over a bandwidth greater than that required to carry the information. In a "direct sequence^*" spread-spectrum transmission scheme, the information to be transmitted is encoded according to one or more predetermined spreading or "chipping" codes so that each bit value in the series of information bit value to be sent is represented by a plurality of bit values, commonly referred to as "chip" values in a chip value signal. Where the original information bits are provided in a bit value signal at a predetermined bit rate or frequency, the chip values are provided at a substantially higher chip frequency. The transmission signal is modulated with the chip value signal, thereby impressing both the original information bits and the spreading code on the transmission signal. This causes substantial spreading or dispersion of the power in the transmission or carrier signal over an entire band of frequencies. At the receiver, the transmission signal is demodulated to recover a base band signal corresponding substantially to the chip value signal. The chip value signal is then decoded by applying a despreading scheme inverse to the spreading scheme used in the transmitter, thereby recovering the original information bit signal. Spread spectrum communication systems provide several significant benefits. They are distinctly less sensitive to interference such as that caused by random noise signals or by willful jamming. Further, such systems have substantial resistance to loss of information caused by multipath interference. Additionally, a receiver arranged to decode information based upon one code will provide substantial immunity against unwanted reception of information encoded according to a different code. Because the power of the transmitted signal is effectively dispersed over a significant range of frequencies, the effective power at any single frequency within the range is minimal. Thus, spread-spectrum systems do not tend to cause substantial interference with other spread-spectrum systems or with conventional communication systems. Regulatory authorities therefore permit unlicensed operation of spread-spectrum communication systems at greater levels of total radiated power than comparable non-spread systems. For example, United States Federal Communications Commission regulations limit conventional, non-spread radio frequency systems to operation at minimal radiated power levels unless an individual license is obtained for each system, but permit operation of spread-spectrum systems at substantial levels of total radiated power without individual licenses.

As disclosed in commonly assigned International Patent Publication W0-88/06365, published 28 August 1988, the disclosure of which is hereby incorporated by reference herein, these characteristics of spread spectrum transmission can be exploited in a low-cost communications and control system for applications such as lighting and appliance control within buildings. For example, the '365 publication discloses one system in which wall-mounted light switches are not connected directly to the building wiring system. Instead, a small battery powered spread-spectrum transmitter is provided at each wall switch, and a corresponding receiver is provided at each lamp or other appliance to be controlled. Control signals are sent from the wall switch to the appliance via free space radio frequency communication. This markedly reduces the cost of system installation. In a system of this nature, the cost of the receivers and transmitters is significant. The '365 publication therefore discloses particularly effective low-cost spreading and despreading devices, which, when coupled with radio frequency modulation and demodulation components, provide effective, low-cost spread-spectrum transmitters and receivers.

Although the direct sequence devices disclosed as preferred embodiments in the aforementioned publication can operate with spreading code sequences of substantially any length or number of chips per bit, the cost of the spreading and despreading components varies directly with the length of the spreading code sequence. The most preferred embodiments disclosed in the this publication utilize a spreading code sequence of about 15 chips per bit. Such a spreading code sequence is markedly shorter than the typical spreading codes of 1000 or more chips per bit utilized in sophisticated, high-cost military and aerospace applications. Even with a relatively short, 15-bit code, the systems disclosed in the '365 publication provide sufficient immunity from interference and accidental reception of unwanted signals to permit their use in applications such as building electric power wiring controls and the like.

However, these systems do suffer from one drawback in that the relatively short spreading codes do not randomize the distribution of transmitted signal power over the frequency spectrum to the same degree as do the longer codes used in more sophisticated systems. Thus, in a system employing relatively short spreading code sequences, there are concentrations of radiated signal power within particular relatively narrow frequency bands. Although these concentrations or "peaks" are far less pronounced than with a comparable, conventional, non-spread system, they are nonetheless appreciable. Depending on the particular circumstances, may consider a short code system as having an appreciable potential for interference with other systems and may therefore subject a short code system to the same relatively stringent licensing regulation and/or power limitations as a non-spread system, thus materially impairing the value of the system and adding considerable costs. The conventional solution to this problem would be to use a somewhat longer spreading code so as to increase the degree of power dispersion and effectively eliminate the power concentrations or peaks. However, this approach adds cost and thus partially defeats the purpose of the system.

Accordingly, there have been needs for further improvements in spread spectrum transmitters, receivers and transmission methods. DISCLOSURE OF THE INVENTION

The present invention addresses these needs. One aspect of the present invention provides a transmitter for use in a spread spectrum communication system. A transmitter according to this aspect of the present invention includes spreading code means for providing a spreading code and transmission means for providing a transmission signal and impressing both the information to be transmitted and the spreading code on the transmitted signal so that the information is carried by one or more active components of the transmission signal. The transmitter further includes means for providing a randomizing signal varying in a substantially random manner with time within a randomizing signal range of frequencies substantially different from the frequency of any active, information- bearing component in the transmission signal. The transmission means is arranged to impress the randomizing signal on the transmission signal along with the spreading code and the information.

In the preferred direct sequence arrangement, the spreading code means includes means for encoding a series of information bits to be transmitted according to a predetermined spreading code to form a series of chip values wherein each information bit is represented by a plurality of chip values and providing a chip value signal including this series of chip values at a predetermined chip frequency. Thus, the chip frequency component carries the information. The randomizing signal range of frequencies is substantially different from the chip frequency. The transmission means desirably includes means for modulating the transmission signal with the chip value signal and with the randomizing signal. The transmission signal may be a radio frequency signal and the randomizing signal range of frequencies may be a range of frequencies substantially lower than the chip frequency, typically below about one-tenth of the chip frequency and preferably lower than about one fiftieth of the chip frequency. For example, the chip frequency may be between about 500 KHz and about 2 MHz, whereas the randomizing signal range of frequencies may be a range of frequencies below about 20 KHz. The spreading code desirably is a relatively short sequence code, such that each information bit is represented by about 50 or fewer, more preferably about 30 or fewer and most preferably about 15 chips.

The added modulation introduced by the randomizing signal causes additional dispersion of the power in the transmitted signal over the frequency spectrum. Thus, introduction of the randomizing signal provides a more uniform power distribution and effectively eliminates any remaining power concentrations or peaks within particular narrow frequency bands. However, the randomizing signal can be introduced at the transmitter by very simple circuit elements which do not add appreciably to the cost of the transmitter. Moreover, because the randomizing signal consists of frequencies substantially different from the frequencies carrying information in the transmission signal, and preferably differing from any active frequency by one or more orders of magnitude, the randomizing signal can be separated from the desired signal at the receiver by circuits which are also relatively simple and economical.

Thus, a further aspect of the present invention provides a receiver. The receiver includes demodulation and despreading means for receiving and demodulating a transmitted signal carrying information and a predetermined spreading code and stripping out the spreading code. The receiver further includes means for rejecting signals within a predetermined randomizing signal range of frequencies different from any active frequency carrying information in the transmitted signal. Preferably, the demodulation and despreading means includes means for providing a base band signal and the rejection means includes filter means for processing the base band signal so as to remove from the base band signal frequencies within a predetermined filtering range and thereby provide a processed base band signal, substantially devoid of components at frequencies within the filtering range. In a direct sequence arrangement, the despreading means may include means for sampling the processed base band signal at a chip frequency substantially different from the frequencies in the filtering range, selecting sets of the samples and applying predetermined despreading codes to each such set to thereby derive information bit values from the sets. Typically, the chip frequency applied in the receiver corresponds to the chip frequency used by the transmitter, whereas the filtering range used at the receiver includes the randomizing signal range used by the transmitter. Because of the very substantial difference between the frequencies of the randomizing signal and the chip frequency the filter means used in the receiver need not be particularly precise or have a particularly sharp cutoff. Thus, simple, inexpensive filtering circuitry can be employed.

Further aspects of the present invention include a spread spectrum transmission signal system incorporating transmitters and receivers as discussed above, and methods of spread spectrum information transmission including the addition of a randomizing signal to a chip value signal at the transmitter and removal of the randomizing signal at the receiver preferably by filtering.

Other objects, features and advantages of the present invention would be more readily apparent from the detailed description of the preferred embodiment set forth below taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram of apparatus in accordance with one embodiment of the invention. BEST MODE FOR CARRYING OUT THE INVENTION

A transmitter 10 in accordance with one embodiment of the invention includes a source 12 of information to be transmitted. Source 12 is arranged to provide the information to be transmitted as a series of binary values expressed as high and low voltages in the conventional manner. Information source 12 may be arranged to accept information from an external source such as a data communications device, to generate information internally or both. For example, in a power control application the information source 12 may include a switch, means for providing a set of one or more predetermined information bits to signify a "on" actuation of the switch and means for providing another set of one or more predetermined information bits to signify an "off" actuation of the switch. Further, information source 12 may provide information bits as an address intended to designate a particular receiver. Information source 12 is arranged to provide the high and low values designating the information bits in a predetermined sequence and at a predetermined bit rate.

Information source 12 is linked to a spreading code generator 14. The spreading code generator is arranged to provide a series of digital binary chip values, represented by high and low voltages on an output line 16. Code generator 14 is arranged to produce one predetermined series of fifteen chip values in response to a binary "one" bit value from information source 12, and to produce a different series of fifteen chip values responsive to a binary "zero" bit value. The spreading code generator 14 is arranged to provide a series of high and low voltages representing the successive chip values on line 16, which voltages constitute a chip value signal. The spreading code generator is arranged to provide the successive chip values at the rate of approximately 1 MHz, i.e., 106 chip values per second. The output of code generator 18 is connected to a low-pass shaping filter 18, which filter is arranged to suppress components at frequencies substantially above the 1 MHz chip frequency and thereby eliminate undesired harmonics of the chip frequency from the chip value signal.

A randomizing signal source 20 includes a random noise generator 22 and a low-pass filter 24 connected to the output of the random noise generator. Filter 24 is arranged to eliminate those components of the signal from generator 22 leaving frequencies of about 20 KHz. Thus, the components passed through filter 24 will consist essentially of random signals in a predetermined frequency band from DC to about 20 KHz. The outputs of filters 24 and 18 are connected to a summing amplifier 26 and the output of this summing amplifier is connected to an RF signal generation and modulation unit 30. Unit 30 is arranged to generate a carrier signal of about 915 MHZ. Further, unit 30 is arranged to frequency modulate the carrier signal in accordance with the signal appearing at the output of the summing amplifier 26. Unit 30 may include substantially conventional RF signal generation units and a substantially conventional modulation circuit such as a varactor controlled oscillator. The output of unit 30 is connected to an RF amplification unit 32, which in turn is connected to an antenna 34.

A receiver 40 for use with transmitter 10 incorporates an antenna 42 connected to an amplification and demodulation unit 44. Unit 44 may incorporate substantially conventional RF amplification, intermediate frequency generation, mixing and detection stages and the like, all arranged to demodulate an FM signal using a carrier frequency of 915 MHz and to provide a base band signal replicating the received signal but without the carrier signal component. The output of amplification and demodulation unit 44 is connected to a high pass filter 46. Filter 46 is arranged to substantially suppress all signal components at frequencies below about 50 KHz and to pass the remaining higher frequency components.

Receiver 40 further includes a chip value recovery unit 48 arranged to convert an analog signal into a series of digital chip values by timing a series of one microsecond intervals and providing a binary one or zero value for each such interval depending upon whether the value of the analog signal is above or below a predetermined threshold during the interval. Thus, chip recovery unit 48 operates at a frequency of about 1 MHz and provides a series of digital chip values at that frequency. The receiver further includes a despreading unit 50 arranged to receive the chip values from chip recovery unit 48. The despreading unit 50 desirably includes a 15 position shift register, means for clocking successive chip values into the register and means for testing the set of values contained in the register on any given cycle of the clocking means against the sets of chip values representing one and zero bit values. The despreader 50 thus is arranged to take successive sets of chip values from recovery unit 48 and derive one and zero bit values from those sets which match the recovered one and zero bit values to an information output unit 52. The information output 52 may incorporate devices for testing the received information and/or for actuating other devices based upon the received information. For example, where receiver 40 is employed in an addressable system incorporating a plurality of different receivers, unit 52 may be arranged to examine some of the received information bits against a predetermined address sequence and to take further action only if that address sequence appears and otherwise to reject the signal. Unit 52 may be arranged to treat the remaining bits as indicating some action to be taken, such as turning on or turning off a light linked to the receiver. Alternatively, output unit 52 may be arranged to pass some or all of the received information bits to other devices. Components such as information source 12, code generator 14, chip recovery unit 48 and despreader 50 may be of the types disclosed in the aforementioned International Patent Publication W0-88/06365. Also, it is preferred that analog components of the transmitter such as shaping filter 18, modulator 30, amplification unit 32 and antenna 34 be fabricated on a common printed circuit board. In particular, it is preferred to employ elements of the printed circuit board rather than discrete components as the necessary capacitors, inductors and transmission lines in transmitter 10 to the greatest possible extent. Likewise, the antenna 42, amplification and demodulation unit 44 and high pass filter 46 of the receiver desirably are all fabricated -li¬ as elements of a unitary printed circuit board, using similar techniques.

In a transmission method according to one embodiment of the invention, information source 12 is operated to supply information bits and at a predetermined bit rate, and these bits are encoded by code generator 14 so as to provide a chip value signal incorporating the successive chip values in sequence. Random signal generator 22 and low pass filter 24 are actuated to provide narrow band noise, incorporating frequencies varying in a substantially random manner with time within the range of about DC to about 20 KHz as a randomizing signal, thereby impressing the information, code and randomizing signals on the carrier. The randomizing and chip value signals are combined in summing amplifier 26. Unit 30 forms a carrier signal and frequency modulates the carrier signal with this combined signal. The combined signal after amplification in unit 32 passes from antenna 34 through free space radiation to antenna 42. The received signal is amplified and demodulated. The resulting base band signal pass through the input of filter 46 incorporates the chip value signal formed by spreading code unit 14 and also the narrow band random noise signal from the random noise generator 22 and filter 24 of the transmitter. Filter 46 substantially strips out the randomizing signal, leaving a processed base band signal which essentially corresponds to the chip value signal alone. The chip recovery unit 48 provides a series of digital chip values, and the despreader 50 reconverts this series of digital values back into the original information supplied by unit 12.

Numerous variations and combinations of the features described above can be utilized without departing from the present invention as defined by the claims. Merely by way of example, carrier signal generator and modulator 30 need not be a frequency modulation unit, but instead may be arranged to provide a plitude modulation, phase modulation or the like, provided that the modulation unit somehow alters a characteristic of the carrier signal in accordance with the signal to be transmitted. Also, it is not essential to combine the randomizing signal with the chip value signal prior to modulation. Instead, the carrier signal can be separately modulated with these two signals.

The enhanced power dispersion provided by the randomizing signal is particularly useful with relatively short sequence codes, such as those wherein each information bit is represented by about 50 chips or less and more preferably about 30 chips or less. Also, the greatest possible difference between the chip frequency and the randomizing signal frequency range is desirable to permit effective suppression of the randomizing signal component in the received signal without appreciable loss of chip frequency signal components, even where a relatively inexpensive filter having imprecise cutoff characteristics is employed. Desirably, the ratio between the chip frequency and the maximum frequency in the randomizing signal range is about 10:1 or more and more preferably about 50:1 or more.

As these and other variations and combinations of the features described above can be employed, the foregoing description of the preferred embodiment should be taken by way of illustration rather than by way of limitation of the invention as defined by the claims. INDUSTRIAL APPLICABILITY The present invention may be applied in information transmission by radio and other signals.

Claims

WHAT IS CLAIMED IS

1. A transmitter having:

(a) spreading code means for providing a spreading code; characterized by:

(b) means for providing a randomizing signal varying in a substantially random manner with time within a randomizing signal range of frequencies; and (c) transmission means for providing a transmission signal and impressing information to be transmitted, said spreading code and said randomizing signal on said transmission signal so that the information is carried by one or more active components in the transmission signal, said randomizing signal range of frequencies being substantially different than the frequency of any said active components.

2. A transmitter as claimed in claim 1 further characterized in that said spreading code means includes means for encoding a series of information bits according to a predetermined spreading code to form a series of chip values further characterized in that each information bit is represented by a plurality of chip values in said series and providing a chip value signal including said series of chip values at a chip frequency, said transmission means includes means for modulating said transmission signal with said chip value signal and said randomizing signal and said chip frequency is substantially different from said randomizing signal range.

3. A transmitter as claimed in claim 2 further characterized in that said randomizing signal range of frequencies is a range of frequencies substantially lower than said chip frequency.

4. A transmitter as claimed in claim 3 further characterized in that said randomizing signal range of frequencies is a range of frequencies below about 1/lOth of said chip frequency.

5. A transmitter as claimed in claim 4 further characterized in that said randomizing signal range of frequencies is a range of frequencies lower than about l/50th of said chip frequency.

6. A transmitter as claimed in claim 3 further characterized in that said chip frequency is about 500 KHz to about 2 MHz and said randomizing signal range of frequencies is a range of frequencies below about 20 KHz.

7. A transmitter as claimed in claim 3 further characterized in that said spreading code means includes means for encoding said information bits so that each said information bit is represented by about 50 or fewer chip values in said series of chip values.

8. A transmitters as claimed in claim 7 further characterized in that said spreading code means includes means for encoding said information bits so that each information bit is represented by about 30 or fewer chip values in said series of chip values.

9. A transmitter as claimed in claim 3 further characterized in that said means for providing a transmission signal includes means for providing a radio frequency transmission signal.

10. A transmitter as claimed in claim 3 further characterized in that said means for modulating said transmission signal includes means for adding said randomizing signal and said chip value signal to provide a combined signal and means for modulating said transmission signal with said combined signal.

11. A transmitter as claimed in claim 2 further characterized in that said randomizing signal means includes a random signal generator providing a substantially random signal and means for filtering said substantially random signal so as to exclude frequencies outside of said randomizing signal range of frequencies.

12. A receiver having:

(a) demodulation means for receiving a transmitted signal and demodulating said transmitted signal to recover a base band signal therefrom; characterized by:

(b) filter means for processing said base band signal so as to remove from said base band signal frequencies within a predetermined filtering range and provide a processed base band signal; and

(c) despreading means for decoding said processed base band signal by determining a value of said processed base band signal for each interval in a series of intervals at a chip interval frequency substantially different from said predetermined range of frequencies, selecting sets of chip values so determined and determining whether the so selected chip values match a preselected sequence of chip values denoting a particular bit value.

13. A receiver as claimed in claim 12 further characterized in that said chip interval frequency is substantially higher than said filtering range of frequencies.

14. A receiver as claimed in claim 13 further characterized in that said chip interval frequency is at least about ten times higher than the highest frequency in said filtering range.

15. A receiver as claimed in claim 14 further characterized in that each said preselected set of chip values includes less than about 50 chip values.

16. A method of spread spectrum signal transmission including the step of: (a) encoding information bits according to a predetermined chipping code to form a series of chip values further characterized in that each information bit is represented by a plurality of chip values in said series and providing a chip value signal including said series of chip frequency; characterized by the steps of: (b) providing a randomizing signal including one or more frequencies varying in a substantially random manner with time within a randomizing signal range of frequencies substantially different from said chip frequency;

(c) modulating a transmission signal with said chip value signal and said randomizing signal and sending said transmission signal; (d) receiving and demodulating said modulated transmitted signal to recover a baseband signal therefrom;

(e) filtering said baseband signal so as to substantially remove frequencies within said randomizing signal range and thereby provide a processed baseband signal including said chip value signal but excluding said randomizing signal; and

(f) despreading said processed baseband signal by applying a despreading code substantially inverse to said spreading code.

17. A method as claimed in claim 16 further characterized in that said randomizing signal range of frequencies is a range of frequencies substantially lower than said chip frequency.