GB2181024A - Spread spectrum radio transmission detection system - Google Patents
Spread spectrum radio transmission detection system Download PDFInfo
- Publication number
- GB2181024A GB2181024A GB08523953A GB8523953A GB2181024A GB 2181024 A GB2181024 A GB 2181024A GB 08523953 A GB08523953 A GB 08523953A GB 8523953 A GB8523953 A GB 8523953A GB 2181024 A GB2181024 A GB 2181024A
- Authority
- GB
- United Kingdom
- Prior art keywords
- signal
- spread spectrum
- received
- output signals
- mixers
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Withdrawn
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Classifications
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B1/00—Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
- H04B1/69—Spread spectrum techniques
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- Engineering & Computer Science (AREA)
- Computer Networks & Wireless Communication (AREA)
- Signal Processing (AREA)
- Superheterodyne Receivers (AREA)
Abstract
The system comprising two mixers 24, 26 each arranged to receive a signal dependent on the transmission signal. Each mixer is connected to a respective local oscillator 28, 30, the local oscillators having predetermined different operating frequencies to provide at the outputs of the mixers two signals having the same intermediate frequencies which have a fixed relationship with a nominally chosen centre frequency for a transmission signal which may be received, and when the transmission signal is received to provide that one of the two output signals is spectrally inverted relative to the other of said two output signals. The output signals of the mixers are mixed and the resultant signal passed through a low pass filter 14. The presence of a d.c. signal or low frequency signal from the output of the low pass filter is indicative of the centre frequency of the received transmission signal being susbtantially the same as the nominally chosen centre frequency. <IMAGE>
Description
SPECIFICATION
Improvements in or Relating to Signal Detection
The present invention relates to signal detection and more particularly to a system for detecting and a method of detecting the presence of a spread spectrum transmission signal in a received signal.
In communication systems a known method for the transmission of a signal involves the spreading of the signal over a frequency spectrum. The detection of the presence of the signal and the timing synchronization of the final detection process may give rise to difficulties as a result of the randomness of the signal over the frequency spectrum, the presence of noise, and by the fact that a discrete frequency component may be intentionally absent from the transmitted signal.
A possible method of detecting spread spectrum transmission signals is to design a system based on the theory described in Research Report No. 488 published in December, 1965 bytheApplied Research Laboratory of Sylvania Electronics
Systems, Massachusetts. We have designed such a system and it is referred to as System 1 in the accompanying drawing.
Referring to the drawing the transmission of a spread spectrum signal is schematically represented to the left of the vertical dotted line shown. The signal is transmitted from a spread spectrum transmission source 2. A source of noise 4 is illustrative of all the noise added to the signal and includes not only extraneous noise but noise which may be intentionally added to mask the spread spectrum transmission signal. The signal from the source 2 itself may typically take the waveform illustrated and has a signal centre frequency fo.
To the top right of the vertical dotted line in the drawing is a schematic representation of System 1 which is a system for detecting the presence of the spread spectrum signal. System 1 comprises a band limiting filter 6 which has a band width sufficiently wide to allow transmission therethrough of the signal to be detected. An output signal from the band limiting filter 6 is fed via an amplifier 8 to a "single ended" mixing process stage having a mixer 10 coupled to an associated local oscillator 12 operating at the signal center frequency fo. The output signal from the mixer 10 is fed through a low pass filter 14.
When a spread spectrum signal having a centre frequency fo has been received the output signal from the low pass filter 14 is a d.c. signal with an AC component proportional to the difference frequency of the local oscillator 12 and the nominal spread spectrum centre frequency fo. Detection of the presence of spread spectrum signal transmission using System 1 has not proved dependable when the power of the spread spectrum transmission is low, for example comparable with or below thermal noise levels in the received signal.
An objective of the present invention is to provide an improved system for detecting the presence of a spread spectrum transmission signal and for giving an indication of the signal centre frequency of the signal.
According to the present invention there is provided a system for detecting the presence of a spread spectrum transmission signal, the system comprising two mixers each arranged to receive a signal dependent on the transmission signal, the two mixers being coupled to oscillator means for providing simultaneously a respective signal to each mixer, the respective signals having predetermined different frequencies to provide at the outputs of the mixers two output signals having the same intermediate frequencies which have a fixed relationship to a nominally chosen centre frequency for a transmission signal which may be received, and when the transmission signal is received to provide that one of the said two output signals is spectrally inverted relative to the other of said two output signals, the two output signals from the mixers being coupled to a further mixer for mixing the two output signals, the output from the mixer being coupled to a low pass filter whereby the presence of a d.c. signal or low frequency signal from the output of the low pass filter is indicative of the centre frequency of the received transmission signal being substantially the same as the nominally chosen centre frequency.
In one embodiment of the present invention each of said two mixers is coupled to a respective local oscillator, the local oscillators having different operating frequencies to provide said respective signals.
In one embodiment of the present invention a band pass filter is coupled to the inputs of the said two mixers, the centre frequency of the band of frequencies which is allowed to pass through the band pass filter being substantially the same as the nominally chosen centre frequency. Conveniently a signal amplification stage may be provided between the band pass filter and said two mixers.
The present invention also provides a method of detecting the presence of a spread spectrum signal in a received signal comprising the steps of transforming the received signal into two output signals having the same intermediate frequencies each of which has a fixed relationship to a nominally chosen centre frequency for a spread spectrum signal which may be present in the received signal, and one of which output signals is spectrally inverted relative to the other output signal when the received signal has the nominally chosen centre frequency, mixing the two output signals having the same intermediate frequencies and passing the resultant signal through a low pass filter whereby the presence of a d.c. signal or low frequency signal from the output of the low pass filter is indicative of the presence of the spread spectrum signal in the received signal.
The present invention also provides signal receiving equipment wherein the signal receiving equipment incorporates the system defined above in accordance with the present invention for detecting the presence of a spread spectrum transmission signal. The present invention further provides a communication system, such as a digital data communication system, incorporating said signal receiving equipment.
An embodiment of the present invention will now be described with reference to System 2 in the accompanying drawing.
System 2 employs in the same manner as System 1 the band limiting filter 6 and the amplified signal is split along two paths 20,22 to two mixers 24,26.
Each of the mixers 24,26 is coupled to a respective local oscillator 28,30 having different respective predetermined operating frequencies fx and fy. The output signals from the mixers 24,26 have the same intermediate frequencies which have a fixed relationship with a nominally chosen centre frequency fz for a transmission signal which may be received. The frequency translation occurs to the nominally chosen centre frequency fz, the mixer 26 transmitting a signal on the high side of the frequency fz and the mixer 24 transmitting a signal on the low side of the frequency fz. The output signals having the two identical intermediate frequencies from the mixers 24 and 26 are fed to a mixer 38 in which the signals are mixed.The output of the mixer 38 is fed through a low pass filter 14 and it is found that the presence of a d.c. signal or low frequency signal from the output of the low pass filter 14 is indicative of the centre frequency of the received transmission signal being substantially the same as the nominally chosen centre frequency fo.
For example, if the nominally chosen centre frequencyfo is equal to 100 MHz then fx and fy may conveniently be chosen as 110 MHz and 90 MHz respectively. The intermediate frequencies of the output signals from the mixers 24 and 26 are both 10
MHz but the output signal from the mixer 24 will be spectrally inverted relative to the output signal from the mixer 26. The presence of a d.c. signal or low frequency signal from the output of the low pass filter 14 is indicative of the centre frequency of the received transmission signal being substantially equal to 100 MHz.
In one embodiment of the present invention means are provided for systematically sweeping across a frequency band for a set of nominally chosen centre frequencies. This is accomplished by selectively providing different values for fx and fy, the nominally chosen centre frequency selectively chosen for detection being equal to one half the sum offxand fy.
The performance of System 2, both in terms of signal detection and operation below thermal noise, is far superior to that of System 1.
Claims (10)
1. A system for detecting the presence of a spread spectrum transmission signal, the system comprising two mixers each arranged to receive a signal dependent on the transmission signal, the two mixers being coupled to oscillator means for providing simultaneously a respective signal to each mixer, the respective signals having predetermined different frequencies to provide at the outputs of the mixers two output signals having the same intermediate frequencies which have a fixed relationship with a nominally chosen centre frequency for a transmission signal which may be received, and when the transmission signal is received to provide that one of said two output signals is spectrally inverted relative to the other of said two output signals, the two output signals from the mixers being coupled to a further mixer for mixing the two output signals having the same intermediate frequencies, the output from the further mixer being coupled to a low pass filter whereby the presence of a d.c. signal or low frequency signal from the output of the low pass filter is indicative of the centre frequency of the received transmission signal being substantially the same as the nominally chosen centre frequency.
2. A system for detecting the presence of a spread spectrum transmission signal as claimed in claim 1 wherein each of said two mixers is coupled to a respective local oscillator, the local oscillators having different operating frequencies to provide the respective signals having the predetermined different frequencies.
3. A system for detecting the presence of a spread spectrum transmission signal as claimed in claim 1 or 2 wherein a band pass filter is coupled to the inputs of the said two mixers, the centre frequency of the band of frequencies which is allowed to pass through the band pass filter being substantially the same as the nominally chosen centre frequency.
4. A system for detecting the presence of a spread spectrum transmission signal as claimed in claim 3, wherein a signal amplification stage is provided to amplify the output signal from the band pass filter.
5. Signal receiving equipment incorporating the system for detecting the presence of a spread spectrum transmission as claimed in any one of claims 1 to 4.
6. A communication system incorporating the signal receiving equipment claimed in claim 5.
7. A digital data communication system incorporating the signal receiving equipment claimed in claim 5.
8. A method of detecting the presence of a spread spectrum signal in a received signal comprising the steps of transforming the received signal into two output signals having the same intermediate frequencies each of which bears a fixed relationship to a nominally chosen centre frequency for a spread spectrum signal which may be present in the received signal, and one of which output signals is spectrally inverted relative to the other output signal when the received signal has the nominally chosen centre frequency, mixing the two output signals having the same intermediate frequencies and passing the resultant signal through a low pass filter whereby the presence of a d.c. signal or low frequency signal from the output of the low pass filter is indicative of the presence of the spread spectrum signal in the received signal.
9. A system for detecting the presence of a spread spectrum transmission signal, the system being substantially as hereinbefore described with reference to, and as illustrated in, System 2 of the accompanying drawing.
10. A method of detecting the presence of a spread spectrum signal in a received signal, the method being substantially as hereinbefore described with reference to, and as illustrated in,
System 2 of the accompanying drawing.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB08523953A GB2181024A (en) | 1985-09-28 | 1985-09-28 | Spread spectrum radio transmission detection system |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB08523953A GB2181024A (en) | 1985-09-28 | 1985-09-28 | Spread spectrum radio transmission detection system |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| GB2181024A true GB2181024A (en) | 1987-04-08 |
Family
ID=10585875
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| GB08523953A Withdrawn GB2181024A (en) | 1985-09-28 | 1985-09-28 | Spread spectrum radio transmission detection system |
Country Status (1)
| Country | Link |
|---|---|
| GB (1) | GB2181024A (en) |
Citations (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB981791A (en) * | 1961-04-22 | 1965-01-27 | Marconi Co Ltd | Improvements in or relating to heterodyne receivers |
| US4221005A (en) * | 1979-05-21 | 1980-09-02 | Nasa | Pseudonoise code tracking loop |
| EP0072403A2 (en) * | 1981-08-17 | 1983-02-23 | ANT Nachrichtentechnik GmbH | Demodulator for spread spectrum frequency-modulated signals |
| EP0138485A2 (en) * | 1983-09-29 | 1985-04-24 | Nippon Telegraph And Telephone Corporation | Radio reception system for a phase modulation signal |
-
1985
- 1985-09-28 GB GB08523953A patent/GB2181024A/en not_active Withdrawn
Patent Citations (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB981791A (en) * | 1961-04-22 | 1965-01-27 | Marconi Co Ltd | Improvements in or relating to heterodyne receivers |
| US4221005A (en) * | 1979-05-21 | 1980-09-02 | Nasa | Pseudonoise code tracking loop |
| EP0072403A2 (en) * | 1981-08-17 | 1983-02-23 | ANT Nachrichtentechnik GmbH | Demodulator for spread spectrum frequency-modulated signals |
| EP0138485A2 (en) * | 1983-09-29 | 1985-04-24 | Nippon Telegraph And Telephone Corporation | Radio reception system for a phase modulation signal |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| WAP | Application withdrawn, taken to be withdrawn or refused ** after publication under section 16(1) |