WO2009136140A1 - Signal detection - Google Patents
Signal detection Download PDFInfo
- Publication number
- WO2009136140A1 WO2009136140A1 PCT/GB2009/001046 GB2009001046W WO2009136140A1 WO 2009136140 A1 WO2009136140 A1 WO 2009136140A1 GB 2009001046 W GB2009001046 W GB 2009001046W WO 2009136140 A1 WO2009136140 A1 WO 2009136140A1
- Authority
- WO
- WIPO (PCT)
- Prior art keywords
- spectrum
- signal
- analysing
- selected section
- receiver
- Prior art date
Links
Classifications
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04W—WIRELESS COMMUNICATION NETWORKS
- H04W24/00—Supervisory, monitoring or testing arrangements
-
- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03J—TUNING RESONANT CIRCUITS; SELECTING RESONANT CIRCUITS
- H03J1/00—Details of adjusting, driving, indicating, or mechanical control arrangements for resonant circuits in general
- H03J1/0008—Details of adjusting, driving, indicating, or mechanical control arrangements for resonant circuits in general using a central processing unit, e.g. a microprocessor
- H03J1/0058—Details of adjusting, driving, indicating, or mechanical control arrangements for resonant circuits in general using a central processing unit, e.g. a microprocessor provided with channel identification means
- H03J1/0066—Details of adjusting, driving, indicating, or mechanical control arrangements for resonant circuits in general using a central processing unit, e.g. a microprocessor provided with channel identification means with means for analysing the received signal strength
-
- 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
- H04B1/06—Receivers
- H04B1/16—Circuits
- H04B1/26—Circuits for superheterodyne receivers
- H04B1/28—Circuits for superheterodyne receivers the receiver comprising at least one semiconductor device having three or more electrodes
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04W—WIRELESS COMMUNICATION NETWORKS
- H04W88/00—Devices specially adapted for wireless communication networks, e.g. terminals, base stations or access point devices
- H04W88/02—Terminal devices
Landscapes
- Engineering & Computer Science (AREA)
- Computer Networks & Wireless Communication (AREA)
- Signal Processing (AREA)
- Computer Hardware Design (AREA)
- Microelectronics & Electronic Packaging (AREA)
- Mobile Radio Communication Systems (AREA)
Abstract
A receiver for a telecommunications system, the receiver comprising selection means (26) for selecting, from a signal received by the receiver, a section comprising components having a range of frequencies, transformation means (30) for producing a spectrum of the selected section and analysing means (32) for analysing the spectrum of the selected section to identify a signal from an element of the telecommunications system.
Description
SIGNAL DETECTION
The present invention relates to a receiver for a telecommunications system and to method of detecting a signal.
When a handset of a mobile telephone or similar mobile communications device is switched on, it searches for an active base station with which to communicate. It is to be understood that the term "base station" used herein encompasses not only base stations of a 2G GSM telecommunications system, but also elements of other telecommunications system which perform the same or a similar role, such as a Node B in a 3G system, or a wireless access point in an IEEE 802.11 system.
The first action of the handset in performing this search is typically to try to locate the base station which was serving it before it was switched off. If this base station, whose details are stored by the handset, cannot be located the handset must search for an alternative base station.
Typically, searching for an alternative base station is carried out by scanning a particular range of frequencies within a frequency band used by mobile communications systems for a signal transmitted by an active base station. Although the active base station operates at a nominal centre frequency, a signal transmitted by the active base station typically occupies a certain bandwidth centred on the nominal frequency of the active base station. Thus, a signal transmitted by the active base station can be detected within a narrow frequency range centred on the nominal frequency of the active base station.
A typical detection arrangement is illustrated schematically at 10 in Figure 1 , and comprises a mixer 12 which mixes a signal from a local oscillator 14 (which is typically derived from a crystal oscillator) operating at a frequency fuo with a signal received by the handset at a frequency (or range of frequencies) fRF. The mixer 12 down-converts the signal received by the handset to an intermediate frequency fLo - fRF (or more accurately, a range of lower frequencies), and this down-converted signal is filtered by the filter 16. An intermediate frequency of 0 Hz is commonly used. The filtered signal is passed to a signal identification
stage 18, which determines whether the down-converted received signal contains a signal from an active base station.
The filter 16 is a low pass filter for systems having an intermediate frequency of 0 Hz, or may be a bandpass filter for systems having an intermediate frequency greater than OHz, and has a relatively small pass band within which the down-converted version of a signal transmitted by an active base station falls, and thus will be passed by the filter 16. The bandwidth and cut off frequency of the filter 16 are selected based on the frequency bands in which the handset is expected to operate, as will be apparent to those skilled in the art.
In operation of the detection arrangement 10, the local oscillator 14 is first set to an initial frequency, causing the received signal to be down-converted to an initial intermediate frequency. The filter 16 selects a portion of this initial IF signal, by blocking unwanted components and passing wanted components to the signal identification stage 18, where a signal from an active base station may be identified. If such a signal is identified, the handset performs the various validation procedures required to establish communication with the base station from which the signal was transmitted.
If no signal from an active base station is identified by the signal identification stage 18, the frequency of the local oscillator 14 is altered, allowing a different range of frequencies to be selected and to be present in the portion of the down-converted signal that is passed by the filter 16 to the signal identification stage 18. If a signal from an active base station is identified in this different range of frequencies, the handset can perform the validation procedures necessary to establish communication with the base station, whilst if no such signal is identified, the frequency of the local oscillator is again altered, and the process is repeated until an active base station has been identified or the whole of the selected frequency band has been searched.
It is desirable that the crystal used to derive the local oscillator 14 is a low-cost item, to minimise the overall cost of the handset. A problem with such a detection arrangement is that such crystals have wide tolerances, meaning that the actual operating frequency of the local oscillator 14 may be significantly offset from its nominal frequency. The detection arrangement can only detect a signal with a limited range of frequency offset, and thus the
detection arrangement must compensate for this by stepping the local oscillator 14 through a number of frequencies which are offset from the nominal frequency by small amounts, to ensure that the output of the filter 16 produces a signal that can be detected. In the exemplary case of a WCDMA receiver using a reasonably high quality crystal, only five such frequency offsets may be necessary, whereas for a low quality crystal thirty frequency offsets may be required. Thus, scanning even a single channel for an active base station may take a considerable amount of time, and if the handset has to scan an entire band containing a large number of channels, or multiple bands (for example if the handset has moved from one country to another, in which a different frequency band is used), the time taken to locate an active base station can be very long.
This problem is exacerbated for handsets capable of using different standards, for example handsets which can use both the 2G and the 3 G standards, as the bands for each standard may overlap, so the handset is forced to scan each band more than once, with a first pass seeking, for example, an active 2G base station and a second pass seeking an active 3G base station. As well as being very slow, this approach is also very power-intensive. The handset must continuously receive signals until an active base station has been found. The receiver and signal processing in the handset over a prolonged period of time uses a significant amount of power.
According to a first aspect of the present invention, there is provided a receiver for a telecommunications system, the receiver comprising selection means for selecting, from a signal received by the receiver, a section comprising components having a range of frequencies, transformation means for producing a spectrum of the selected section and analysing means for analysing the spectrum of the selected section to identify a signal from an element of the telecommunications system.
By selecting a section of the wideband signal received by the receiver and analysing a spectrum of that section, a signal from an active base station can be identified without having to step through numerous frequency offsets, as a relatively wide range of frequencies are analysed at once. Thus, there is no need to use an expensive accurate crystal in the handset, and the cost of manufacturing the handset can be reduced in comparison to other handsets which require accurate crystals.
The analysing means may be configured to detect a feature of the spectrum which is characteristic of a signal transmitted by an element of a telecommunications system.
The analysing means may comprise filtering means for filtering the spectrum.
The analysing means may comprise means for detecting the power in subsections of the spectrum.
The analysing means may comprise means for detecting the bandwidth of a signal contained in the spectrum.
The selection means may select a plurality of sections of the received signal and the transformation means may produce a plurality of corresponding spectra, the spectra being combined to produce a single spectrum which may be analysed by the analysing means.
The selection means may be adjustable to increase the range of frequencies of the components of the selected section.
The selection means may comprise a filter.
The transformation means may be configured to perform a Fourier transform on the selected section.
The signal from the element of the telecommunications system may be a signal from an active base station of the telecommunications system.
According to a second aspect of the invention there is provided a mobile communications device comprising a receiver according to the first aspect.
According to a third aspect of the invention there is provided a method of detecting a signal from an element of a telecommunications system, the method comprising receiving a signal, selecting from the received signal a section comprising components having a range
of frequencies, producing a spectrum of the selected section and analysing the spectrum of the selected section to identify the signal from the element.
Analysing the spectrum of the selected section may comprise detecting a feature of the spectrum which is characteristic of a signal transmitted by the element of the telecommunications system.
Analysing the spectrum of the selected section may comprise filtering the spectrum.
Analysing the spectrum of the selected section may comprise detecting the power in subsections of the spectrum.
Analysing the spectrum of the selected section may comprise detecting the bandwidth of a signal contained in the spectrum.
A plurality of sections of the received signal may be selected and a plurality of corresponding spectra my be produced, the spectra being combined to produce a single spectrum which is analysed.
The range of frequencies of the components of the selected section may be adjustable.
Selecting a section may comprise filtering the received signal.
Producing a spectrum may comprise performing a Fourier transform on the selected section.
The signal from the element of the telecommunications system may be a signal from an active base station of the telecommunications system.
Embodiments of the invention will now be described, strictly by way of example only, with reference to the accompanying drawings, of which:
Figure 1 is a schematic representation of a prior art arrangement for detecting an active base station with which a mobile communications device may communicate; and
Figure 2 is a schematic representation of a detector arrangement according to the present invention.
Figure 2 is a schematic representation of a possible architecture of a detector forming part of a receiver according to the present invention. It will be appreciated by those skilled in the art that the functional blocks shown in Figure 2 represent processing operations performed on a received signal, but do not necessarily correspond directly to physical units that may appear within a practical implementation of a receiver.
The detector, shown generally at 20, comprises a mixer 22 which mixes a received wideband signal (i.e. a signal having components at a wide range of frequencies) with a signal from a variable local oscillator 24, to down-convert the received signal. The resulting down-converted signal is filtered through a low pass filter 26, with the output of the low pass filter 26 being converted into digital samples by an analogue to digital converter (ADC) 28. The low pass filter 26 should ideally have a relatively large bandwidth, such that a wide frequency range of the received signal (i.e. components of the received signal with a wide range of frequencies) is selected by the low pass filter 26 and sampled by the ADC 28, although the bandwidth of the low pass filter 26 is restricted by other functions that it is required to carry out, as is explained below. In this way, a section of the radio frequency spectrum is selected and sampled by the receiver.
At processing stage 30, a Fast Fourier Transform (FFT) is performed on the digital samples of the selected section of the radio frequency spectrum produced by the ADC 28, typically using a suitably programmed digital signal processor (DSP), to obtain a power spectrum for the selected section. This power spectrum is analysed at a signal identification stage 32, which is typically implemented as a microprocessor or DSP running a suitable set of instructions, to identify a signal from an active base station in the selected section.
The bandwidth of the section of the radio spectrum that may be selected at any one time by the receiver is typically limited by the components of the receiver, such as the low pass
filter 26 which is used to suppress adjacent channel signals in normal use of the handset. For good adjacent channel suppression, the bandwidth of the low pass filter 26 should be relatively small, but if such a filter is used the bandwidth of the section of the radio spectrum that may be selected is not sufficiently large to cover an entire band of a mobile communications system. To alleviate this problem, the desired bandwidth of the radio frequency spectrum may be selected in two or more passes by the detector 20, with the power spectra for each pass being concatenated to obtain an overall power spectrum for the whole of the desired bandwidth. This is achieved by adjusting the frequency of the variable local oscillator 24, to alter the range of frequencies in the received signal which are passed to the low pass filter 26. The power spectrum produced by the FFT at step 30 can then be appended to that produced on the first pass to create a spectrum having a larger bandwidth.
This process is repeated until a power spectrum of the desired bandwidth has been obtained, and the power spectrum can then be analysed, at step 32, to detect a signal from one or more active base stations.
Detection of a signal from an active base station is achieved by identifying features present in the power spectrum which are characteristic of such signals. For example, a signal from an active base station may have a particular power profile, which can be detected by filtering the power spectrum to detect one or more features of this profile, such as rising or falling edges which are a certain distance (in frequency) apart. For example, the 3GPP standard stipulates that a WCDMA signal must have a bandwidth of 3.84MHz. Therefore a signal from a 3GPP WCDMA Node B can be detected by searching for a peak in the power spectrum whose rising and falling edges are separated by 3.84MHz. In addition, by careful measurement of the position of the edges, it is possible to obtain an accurate frequency offset measurement, allowing the local oscillator 14 derived from the crystal oscillator to be calibrated.
An advantage of using this method to detect signals from active base stations is that once the complete power spectrum of a frequency band or channel has been obtained, the entire frequency band can be scanned in a single operation. As the frequency bands of different telecommunications systems may overlap (e.g. the 3 G frequency bands used in the USA overlap with 2G frequency bands), active base stations of different communications
systems can be detected in a single pass, rather than having to scan the same frequency band twice, applying different criteria to detect signals from active base stations of the different telecommunications systems. Thus, the time taken to perform a search for an active base station with which the handset can communicate is reduced, and the power consumed by the handset in performing the search is also reduced, as the length of time for which signals must be received and processed is reduced.
As is explained above, the bandwidth of the section of the radio spectrum that may be scanned at any one time by the detector 20 is typically limited by components of the receiver, such as the low pass filter 26 which is provided in the analogue front end of the receiver to suppress adjacent channel signals in normal use of the handset. Thus, scans covering several different frequency ranges may be necessary to obtain a power spectrum for the whole of the desired frequency range. To alleviate this problem, the handset may include components such as capacitors or inductors which can be switched into the analogue front end to increase the bandwidth that can be scanned during a search for an active base station, and switched out of the analogue front end during normal operation of the handset. In this way, the scanning bandwidth may be maximised, thus reducing the number of passes required to obtain a power spectrum for the whole of a desired bandwidth without having any negative impact on the normal operation of the handset.
Where several limited-bandwidth sections of the desired frequency spectrum are created, the frequency domain processing of the resulting spectrum can be reduced, by performing a time-domain analysis of the individual sampled sections of the frequency spectrum as they are created, to determine if they are likely to contain a signal from one or more active base stations. In the event that this analysis indicates that a sampled section of the frequency spectrum is unlikely to contain such a signal, the FFT need not be performed for that section, and the section will be overlooked during the subsequent frequency domain analysis of the spectrum, thus reducing the amount of processing required. For example, this time domain analysis may comprise measuring the RMS power of the received signal to assess whether there is likely to be any signal which merits further analysis in the sampled section being analysed.
Claims
1. A receiver for a telecommunications system, the receiver comprising selection means for selecting, from a signal received by the receiver, a section comprising components having a range of frequencies, transformation means for producing a spectrum of the selected section and analysing means for analysing the spectrum of the selected section to identify a signal from an element of the telecommunications system.
2. A receiver according to claim 1 wherein the analysing means is configured to detect a feature of the spectrum which is characteristic of a signal transmitted by an element of a telecommunications system.
3. A receiver according to claim 2 wherein the analysing means comprises filtering means for filtering the spectrum.
4. A receiver according to claim 2 or claim 3 wherein the analysing means comprises means for detecting the power in subsections of the spectrum.
5. A receiver according to any one of claims 2 to 4 wherein the analysing means comprises means for detecting the bandwidth of a signal contained in the spectrum.
6. A receiver according to any one of the preceding claims wherein the selection means selects a plurality of sections of the received signal and the transformation means produces a plurality of corresponding spectra, the spectra being combined to produce a single spectrum which is analysed by the analysing means.
7. A receiver according to any one of the preceding claims wherein the selection means is adjustable to increase the range of frequencies of the components of the selected section.
8. A receiver according to any one of the preceding claims wherein the selection means comprises a filter.
9. A receiver according to any one of the preceding claims wherein the transformation means is configured to perform a Fourier transform on the selected section.
10. A receiver according to any one of the preceding claims wherein the signal from the element of the telecommunications system is a signal from an active base station of the telecommunications system.
11. A receiver substantially as hereinbefore described with reference to the accompanying drawings.
12. A mobile communications device comprising a receiver according to any of the preceding claims.
13. A method of detecting a signal from an element of a telecommunications system, the method comprising receiving a signal, selecting from the received signal a section comprising components having a range of frequencies, producing a spectrum of the selected section and analysing the spectrum of the selected section to identify the signal from the element.
14. A method according to claim 13 wherein analysing the spectrum of the selected section comprises detecting a feature of the spectrum which is characteristic of a signal transmitted by the element of the telecommunications system.
15. A method according to claim 14 wherein analysing the spectrum of the selected section comprises filtering the spectrum.
16. A method according to claim 13 or claim 14 wherein analysing the spectrum of the selected section comprises detecting the power in subsections of the spectrum.
17. A method according to any one of claims 13 to 15 wherein analysing the spectrum of the selected section comprises detecting the bandwidth of a signal contained in the spectrum.
18. A method according to any one of claims 13 to 17 wherein a plurality of sections of the received signal are selected and a plurality of corresponding spectra are produced, the spectra being combined to produce a single spectrum which is analysed.
19. A method according to any one of claims 13 to 18 wherein the range of frequencies of the components of the selected section is adjustable.
20. A method according to any one of claims 13 to 19 wherein selecting a section comprises filtering the received signal.
21. A method according to any one of claims 13 to 20 wherein producing a spectrum comprises performing a Fourier transform on the selected section.
22. A method according to any one of claims 13 to 21 wherein the signal from the element of the telecommunications system is a signal from an active base station of the telecommunications system.
23. A method substantially as hereinbefore described with reference to the accompanying drawings.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB0808496A GB0808496D0 (en) | 2008-05-09 | 2008-05-09 | Signal detection |
| GB0808496.4 | 2008-05-09 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| WO2009136140A1 true WO2009136140A1 (en) | 2009-11-12 |
Family
ID=39571115
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| PCT/GB2009/001046 WO2009136140A1 (en) | 2008-05-09 | 2009-04-24 | Signal detection |
Country Status (2)
| Country | Link |
|---|---|
| GB (1) | GB0808496D0 (en) |
| WO (1) | WO2009136140A1 (en) |
Citations (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO2000031998A1 (en) * | 1998-11-24 | 2000-06-02 | Ericsson, Inc. | Accelarated scanning of cellular channels by cellular radiotelephones |
-
2008
- 2008-05-09 GB GB0808496A patent/GB0808496D0/en not_active Ceased
-
2009
- 2009-04-24 WO PCT/GB2009/001046 patent/WO2009136140A1/en active Application Filing
Patent Citations (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO2000031998A1 (en) * | 1998-11-24 | 2000-06-02 | Ericsson, Inc. | Accelarated scanning of cellular channels by cellular radiotelephones |
Non-Patent Citations (2)
| Title |
|---|
| BOONE, P. BARBEAU, M. KRANAKIS, E.: "Strategies for fast scanning and handovers in WiMAX/802.16", ACCESS NETWORKS & WORKSHOPS, 2007. ACCESSNETS '07. SECOND INTERNATIONAL CONFERENCE, 22 August 2007 (2007-08-22) - 24 August 2007 (2007-08-24), Ottawa, XP002547341, ISBN: 978-1-4244-1150-4, Retrieved from the Internet <URL:http://people.scs.carleton.ca/~kranakis/Papers/wimax_529.pdf> [retrieved on 20090924] * |
| TEVFIK YÜCEK, HÜSEYIN ARSLAN: "Spectrum Characterization for Opportunistic Cognitive Radio Systems", MILITARY COMMUNICATIONS CONFERENCE, 2006. MILCOM 2006. IEEE, 23 October 2006 (2006-10-23) - 25 October 2006 (2006-10-25), University of South Florida, pages 1 - 6, XP002547253, ISBN: 1-4244-0617-X, Retrieved from the Internet <URL:http://www.eng.usf.edu/~yucek/papers/yucek_milcom06.pdf> [retrieved on 20090924] * |
Also Published As
| Publication number | Publication date |
|---|---|
| GB0808496D0 (en) | 2008-06-18 |
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