JP2011193528A - Method and system for receiving dsss signal - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method which communicates with a medical device (10). <P>SOLUTION: In a method, an interface (12) is provided to which either a measuring means (14) or an external device (16) is connected and through which a measured signal or data are transmitted from the measuring means (14), or the external device (16) to the medical device (10). It is necessary to use the medical device for both a measurement mode and a communication mode with only a single interface. In the interface, e.g., software updating can be performed in the medical device through the interface. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a method and receiver for receiving a direct sequence spread spectrum (DSSS) signal, and a radio system. The present invention has unique but non-limiting applications in low cost wireless systems such as low cost wireless networks.

  Conventionally, both wireless transmitters and receivers include an accurate frequency reference that is implemented using a quartz crystal. Since both the transmitter and receiver know the frequency accurately, the receiver's filter can be accurately matched to the transmitted spectrum using fine tuning blocks.

  Providing fine tuning blocks in the receiver's integrated circuit not only introduces complexity, but also requires a relatively large area chip. Inevitably, this makes the receiver chip relatively expensive and weakens the price of the receiver. For example, omitting the relatively expensive quartz crystal and reducing the number of components in a fine tuning block will adversely affect the performance of conventional receivers.

  An object of the present invention is to reduce the cost of a radio receiver.

  According to an aspect of the present invention, there is provided a method for receiving a direct sequence spread spectrum (DSSS) signal, down-converting the DSSS signal, and reducing the down-converted DSSS signal to a bandwidth smaller than the DSSS signal. A method is provided that includes filtering with a bandwidth channel filter and correlating the filtered signal to a sequence equal to that used to spread the spectrum.

  According to a second aspect of the present invention, a primary station having means for transmitting a direct sequence spread spectrum (DSSS) signal, and downconverting means for downconverting the DSSS signal, are downconverted. A channel filter that filters a filtered DSSS signal, and correlates the filtered signal to the same sequence that is used to spread the spectrum, with a channel filter that has a narrower bandwidth than that of the DSSS signal A wireless system is provided having at least one secondary station including a receiver with a correlation means.

  According to a third aspect of the present invention, there is provided a receiver for receiving a direct sequence spread spectrum (DSSS) signal, the receiver comprising downconverting means for downconverting the DSSS signal, and the downconverted DSSS. A channel filter that filters the signal and has a narrower bandwidth than that of the DSSS signal and a correlation means that correlates the filtered signal to the same sequence that is used to spread the spectrum And have.

  In the description and claims of the present invention, “narrower” bandwidth means that the 3 dB bandwidth of the channel filter is substantially the same as the 3 dB bandwidth of the DSSS signal, that is, the preface of this specification. Means three-quarters of the matched filter bandwidth in a conventional receiver of the type described, but more generally no more than one-half.

  The present invention is based on the realization that at least for DSSS signals, the transmitted signal can be received using a channel filter with a narrower bandwidth than that conventionally used. This is in contrast to conventional practice where the filter bandwidth is matched to the signal bandwidth and selection is made for good sensitivity while rejecting adjacent channel signals. If the filter is off-tune, it may allow undesired adjacent channel signals, so tuning with conventional filters is desirable. On the other hand, a narrower bandwidth channel filter will automatically reject adjacent channels, even if it is off-tuned, thus avoiding the need for tuning and provision of tuning components.

  A narrower bandwidth channel filter will keep the receiver acquiring the transmitted signal, even if a limited amount of frequency drift occurs between the center frequency of the transmitted signal and the channel filter. Make sure you are waxing. By using this approach, a receiver with integrated passive frequency determining components can be manufactured, which has a typical accuracy between 5% and 10%. In addition, it is possible to avoid the use of a relatively expensive crystal resonator.

  In an embodiment of the present invention, the channel filter bandwidth is substantially half the bandwidth of the DSSS signal.

  At least some of the loss in sensitivity caused by using a narrower bandwidth channel filter can be offset by increasing the power of the transmitted signal, for example, by 3 dB.

  There is a trade-off between avoiding the need for tuning and having to accept a loss in sensitivity. By using a channel filter with a narrower bandwidth than the transmitted signal, the tuning block, if used, is relatively coarse with fewer parts and less complexity than the fine tuning block It can be a tuning block.

  A tuning procedure using a coarse tuning block can be implemented each time a desired or active base station in a wireless network contacts a new slave station. Also, the tuning procedure can be repeated at intervals by subordinate stations already registered in the wireless network to ensure that the base station transmitter remains at least coarsely tuned.

  In an improvement of the invention, the output frequency of the reference signal generator used to frequency downconvert the received signal is adjustable, and in operation, the reference frequency is generated locally with the received DSSS signal. It can be adjusted until an acceptable correlation with the direct sequence is achieved.

  In another refinement, the channel filter bandwidth is incrementally changed to improve the reception of DSSS signals.

1 is a block schematic diagram of an embodiment of a low cost wireless network made in accordance with the present invention. It is a figure which shows the spectrum of the DSSS signal transmitted schematically. It is a figure which shows the upper limit and lower limit of a channel filter with the bandwidth of the order of 75% in the transmitted DSSS signal schematically. It is a figure which shows the upper limit and lower limit of a channel filter with the bandwidth of the order of 75% in the transmitted DSSS signal schematically. It is a figure which shows typically the upper limit and lower limit of a channel filter with a bandwidth of the order of 50% in the transmitted DSSS signal. It is a figure which shows typically the upper limit and lower limit of a channel filter with a bandwidth of the order of 50% in the transmitted DSSS signal. It is a figure which shows the example of the spectrum of a DSSS signal. FIG. 4 is a graph of bit error rate (BER) for different channel filter bandwidths. FIG. It is a graph which shows the effect of the frequency offset regarding BER. 4 is a flowchart of an embodiment of a method according to the invention. FIG. 6 shows a variant of the method according to the invention with diagrams A, B, C and D, where the channel filter bandwidth is shifted with respect to the center frequency of the transmitted DSSS signal. FIG. 5 is a flowchart of a process for aligning a secondary station receiver with a primary station transmitter.

  The invention will now be described by way of example with reference to the corresponding drawings.

  In the drawings, the same reference numerals are used to indicate corresponding features.

  Referring to FIG. 1, a low-cost wireless network has a primary (or basic) station 10 and a plurality of secondary (or mobile) stations 12, although only one is depicted for clarity. The wireless network is intended for use in the 2.4 GHz ISM band with an order width of 85 MHz and a fractional bandwidth of about 3.5%. The primary station 10 has a transceiver that includes a transmitter section TX10 and a receiver section RX10, and the secondary station also has a transceiver that includes a transmitter section TX12 and a receiver section RX12. Both types of stations have other parts, which are not shown because they are not necessary for an understanding of the present invention. The signal transmitted by transmitter section TX10 is spread over the entire band using an antipodal (± 1) 11-bit Barker sequence. In the following description, such a direct sequence spread spectrum signal is referred to as a DSSS signal.

  The transmitter section TX 10 has a data source 14 that is coupled to the DSSS signal generator 16. A code store 18 that stores an 11-bit Barker sequence and a reference frequency source 20 whose output frequency is fixed using a crystal 22 are coupled to the signal generator 16. The DSSS signal output from the signal generator 16 is coupled to the first input of the modulator 24. The output of the reference frequency source 20 is coupled to the second input of the modulator 24. Antenna 26 is coupled to the output of modulator 24.

  The receiver section RX 10 has a frequency downconverter 28 having a first input coupled to the antenna 26 and a second input coupled to the reference frequency source 20. The signal received by the antenna 26 of the primary station 10 will be a DSSS signal with a bandwidth that may be narrower than the bandwidth of the signal transmitted by the primary station 10, as will be described later. The wideband channel filter 30 will be coupled to the output of the frequency downconverter 28 and will pass any narrower bandwidth signal that falls within its passband. Despreading and correlation stage 32 is coupled to the output of wideband channel filter 30 and the output of code store 18. Baseband output stage 34 is coupled to the output of stage 32 to provide a data signal output.

  Referring now to the secondary station 12, the receiver RX12 has a frequency downconverter 42 with a signal input coupled to the antenna 40. The reference frequency generator 44 provides the local oscillator signal fLO to the frequency down converter 42. The reference frequency generator 44 is a low cost device with passive and integral frequency determining components. The tolerance and stability of the generated frequency is governed by the characteristics of the process used to make the integrated circuit of the receiver RX12. As a cost saving means, a frequency stabilizing element such as a crystal resonator is not provided. However, the structure of the reference frequency generator 44 is not important for implementing the method according to the invention.

  The downconverted DSSS signal from the frequency downconverter 42 is filtered by a channel filter 46 having a narrower bandwidth than the signal transmitted by the primary station 10. A sliding correlator 48, which can be implemented by a known method using a series of flip-flops, is coupled to the channel filter 46 for receiving the filtered DSSS signal. The sliding correlator 48 also has an input for the timing signal obtained from the reference frequency generator 44 and an input for a replica of the 11-bit Barker code used to spread the signal at the transmitter TX10. The code is held in the code storage unit 50. The output stage 52 is coupled to the output of the sliding correlator 48 to provide a signal output, for example an indication regarding the data signal or the presence of the signal.

  A correlation scoring stage 54 is coupled to the output of the sliding correlator 48. The output of correlation score stage 54 is coupled to the input of microcontroller 56. A sliding correlator 48 generates an indication as to the relative degree of correlation achieved with the current input signal from the channel filter 46. If the indication is considered to be acceptable based on the predetermined criteria when compared to the reference value or value measure by the correlation score stage 54, the receiver RX12 is deemed to have acquired the transmitted signal. The microcontroller 56 then controls the receiver RX12 to remain excited to provide an output signal at the output stage 52. However, if instead it is deemed unacceptable, the receiver RX12 returns to sleep mode or goes into a non-excited state.

  The transmitter TX 12 of the secondary station 12 has a data input stage 60 that is coupled to the DSSS stage 62. The outputs from the reference frequency generator 44 and the code storage 50 that supplies the 11-bit Barker code are also connected to the stage 62. The DSSS signal from stage 62 is then modulated by modulator 64 and the result is provided to antenna 40 for propagation to primary station 10.

  The signal transmitted by the secondary station 12 can have a narrower bandwidth than the DSSS signal transmitted by the primary station 10. The receiver RX10 will be included in the passband range of its channel filter 30, even if the center frequency of the transmitter TX12 is not perfectly aligned with the local oscillator frequency of the receiver RX10. Therefore, it will not be difficult to process such a narrower bandwidth DSSS signal.

  FIG. 2 schematically illustrates the bandwidth of the DSSS signal transmitted by the primary station 10. The illustrated band has a center frequency fc, and upper and lower frequency limits are fu and fL, respectively. According to the present invention, the bandwidth of the channel filter 46 is narrower than the bandwidth of the transmitted signal, and ideally is substantially centered on the center frequency fc of the transmitted signal. However, for various reasons including component tolerance and temperature effects, this ideal placement may not be successful and the center of the channel filter bandwidth may not be the same as fc. Bandwidth may also drift. However, if the DSSS signal is received and results in an acceptable correlation score or has an acceptable bit error rate (BER), the secondary station has acquired the transmitted signal. Can be considered.

  3A and 3B illustrate the situation of a channel filter with a 3 dB bandwidth corresponding to three quarters (ie 75%) of the transmitted signal, respectively. In FIG. 3A, the lower edge of the filter bandwidth has a frequency corresponding to fL, and in FIG. 3B, the upper edge of the filter bandwidth has a frequency corresponding to fU. Thus, the center frequency of the channel filter 46 is drifted or shifted by ８ of the transmitted bandwidth and still receives the transmitted signal without receiving the signal from the adjacent channel. It becomes possible to be in a state where it can be.

  As shown in FIGS. 4A and 4B, FIG. 4A shows the lower frequency limit and FIG. 4B shows the higher frequency limit, but the channel filter bandwidth is reduced to half the transmitter bandwidth (ie 50 Narrowing to (%) will cause the center frequency of the filter 46 to drift or shift by a quarter of the transmitted bandwidth and still be transmitted without receiving signals from adjacent channels. It is possible to be ready to receive a signal.

  If the filter bandwidth is at least partially outside the frequency fU or fL, the quality of the received signal will be degraded. The bit error rate will then increase to the point where it would not be considered to have acquired the desired channel. If there is an adjacent channel signal, it will not correlate with the receiver code and will appear as noise.

  Referring to FIG. 5, the spread spectrum of the DSSS signal transmitted by the primary station 10 has a lobe sequence centered at 55 MHz, which is a simulated carrier frequency. In practical applications, the outer lobe of the transmitted spectrum is filtered-out by post modulation filtering. However, this greatly affects the demodulation of the central main lobe. More specifically, the transmitted signal has a BPSK signal with a data rate of 1 MHz, provided that it is spread by an 11-bit Barker sequence. The chip rate is thus 11 MHz and the system sampling rate is 275 MHz, in other words 25 times oversampling. The simulated channel filter 46 (FIG. 1) is a 20 tap Butterworth filter, and the 3 dB-3 dB bandwidth used for the simulation is (a) the first null of the transmitted spectrum. (null) sets the 3 dB point of the channel filter at 22 MHz, (b) sets the 3 dB point of the channel filter at the 3 dB point of the transmitted spectrum, 9.75 MHz, (c) is half of the preceding value 4.87 MHz. The channel filter is initially set to the center frequency of the desired signal.

  FIG. 6 shows the bit error rate (BER) results measured at the receiver. As the channel filter bandwidth is reduced from 22 MHz (curve X) to 9.75 MHz (curve Y), the system performance decreases by approximately 0.6 dB. Interpretation of this result indicates that most of the information carried by the system is still within the 9.75 MHz bandwidth. This is the 3 dB bandwidth in a conventional receiver.

  As the channel filter bandwidth is reduced below the conventional 3 dB bandwidth of 9.75 MHz to 4.87 MHz (curve Z), system performance degrades by approximately 3.3 dB. However, it has been found that it can operate with such reduced performance. For example, such reduced performance can be compensated by increasing the power of the transmitter.

  FIG. 7 shows the effect of frequency offset on the bit error rate. When the filter is not centered on the transmitted spectrum, the signal is degraded due to the fact that the transmitted power received by the filter is smaller and the distribution of energy across the spectrum is altered. However, the system can withstand up to about 3.5 MHz (4.87 MHz filter) for fairly large offsets, eg, doubling of the bit error rate.

  When the offset is 11 MHz (that is, the channel filter is on the null of the spectrum being transmitted), the fact that the signal amplitude is very small will significantly worsen the BER, but theoretically getting some information Is possible.

  FIG. 8 is a flowchart summarizing the operations described. Block 70 relates to switching on or activation of secondary station 12 (FIG. 1). Block 72 relates to the receiver RX12 (FIG. 1) that receives the DSSS signal. Block 76 relates to the DSSS signal being frequency downconverted and filtered with channel filter 46 (FIG. 1) to form a narrower bandwidth signal for the mixing result. Block 78 relates to using a sliding correlator 48 (FIG. 1) that attempts to correlate with this narrower bandwidth signal. Block 80 relates to a correlation score stage 54 (FIG. 1) that determines the degree of correlation and provides a score or other suitable indication. At block 82, a check is made to see if the score is acceptable. If (Y), the receiver RX 12 is deemed to have acquired the transmitted signal and at block 84 data is recovered. If the score is not acceptable (N), the flowchart returns to block 70 and either the receiver is de-energized or placed in sleep mode.

  In a variation of the embodiment of the invention described with reference to FIG. 1, the channel filter is coarsely tuned for the frequency domain that may exceed the frequency domain of the transmitted bandwidth. )be able to. In this variation, the reference frequency generator 44 is tunable under the control of the output 58 by the microcontroller 56. The reference frequency generator 44 includes at least one integrable frequency determining component such as a varactor.

  In operation, if the degree of correlation is considered low, it means that the channel filter 46 is not within the bandwidth range of the received DSSS signal or is not close enough to the center of that bandwidth. As shown, correlation score stage 54 produces an appropriate output that is provided to microcontroller 56. Microcontroller 56 transmits an appropriate tuning signal for its output 58. That signal causes the reference frequency generator 44 to change the local oscillator frequency fLO. The cycle of operation is repeated for different local oscillator frequencies until an acceptable output is obtained from the correlation score stage 54. A sequence of cycles can also be provided so that the local oscillator frequency fLO is changed to scan the entire bandwidth of the DSSS signal. The score obtained by the correlation score stage 54 is examined by the microcontroller 56 which selects the local oscillator frequency fLO that gives the best or acceptable score. In either case, receiver RX12 will be considered tuned to transmitter TX10.

  As a precaution against signal loss due to excessive drift at the local oscillator frequency fLO when the primary station 10 is in contact with a particular secondary station 12 for a relatively long time, the microcontroller 56 Another scan can be started.

  When the secondary station 12 joins a wireless local area network (WLAN) or becomes active, what has been dormant is tuned for its receiver RX12.

  The frequency shift in the channel filter 46 is described below with reference to FIG. 9 having diagrams A, B, C, and D.

  Diagram A is similar to FIG. 2 but shows a DSSS signal transmitted by the primary station 10. Diagram B shows the position of the channel filter 46 (FIG. 1) relative to the first value, the local oscillator frequency fLO1. As is apparent by comparing diagram A and diagram B, there is no overlap between the channel filter 46 and the transmitted signal, and the sliding correlator 48 (FIG. 1) will not detect any correlation. . Correlation score stage 54 (FIG. 1) will then give a proper low output that is only an absolute value or simply a low / unacceptable indication. Any adjacent channel interference will also have a low correlation score.

  Diagram C shows the situation for the local oscillator frequency fLO2 which places the channel filter 46 in the preferred range of the bandwidth of the DSSS signal, thereby giving a high degree of correlation. Consequently, a high or acceptable indication will be given by the correlation score stage 54.

  Diagram D shows the situation for the local oscillator frequency fLO3 that partially overlaps the channel filter 46 to the high end of the bandwidth of the DSSS signal, thereby giving a low degree of correlation. Consequently, a low or unacceptable indication will be given by correlation score stage 54.

  Once the local oscillator frequency scan is complete, the microcontroller 56 selects the local oscillator frequency fLO2 as the best frequency that gives the best correlation score or best BER. After acquisition is achieved, various improvements, not shown, can be used to process or enhance the processing of the received DSSS signal.

  FIG. 10 is a flow chart summarizing the operations described with reference to FIG. 9, along with variations of the process described below. Block 70 relates to switching at secondary station 12 (FIG. 1). Block 72 relates to the receiver RX12 (FIG. 1) that receives the DSSS signal. Block 74 relates to the receiver RX12 that sets the local oscillator frequency fLO. Block 76 relates to the DSSS signal being frequency downconverted with the set local oscillator frequency and filtered with channel filter 46 to form a narrower bandwidth signal for the mixing result. Block 78 relates to using a sliding correlator 48 (FIG. 1) that attempts to correlate with this narrower bandwidth signal. Block 80 relates to a correlation score stage 54 (FIG. 1) that determines the degree of correlation and provides a score or other suitable indication. At block 82, a check is made to see if the score is acceptable. If (Y), the receiver is deemed to have acquired the transmitted signal and at block 84 the data is recovered. If the score is not acceptable (N), the flowchart returns to block 74 and another local oscillator frequency fLO is set and the cycle is repeated.

  A variation of the process is described and illustrated below by blocks 86, 88, 90 and 92 in FIG. Block 86 is inserted between blocks 80 and 82 and functions to check whether the local oscillator frequency fLO is to be scanned. If the answer is no (N), the flowchart proceeds to block 82 and beyond. If the answer is yes (Y), then in block 88, the microcontroller 56 provides a scan to find the best local oscillator frequency by storing the correlation score for each local oscillator frequency.

  At block 90, a check is made to see if the last local oscillator frequency fLO has been used. If the answer is No (N), then the flowchart returns to block 74. If the answer is yes (Y), the flowchart proceeds to block 92 to determine which local oscillator frequency gives the best score and the appropriate output for block 84 from which data is to be recovered.

  When selecting the bandwidth of the channel filter 46, the accuracy and stability of the reference frequency generator 44 and the maximum search time allowed should be considered. In the simulation, it was found that 50% of the bandwidth of the transmitted DSSS signal is acceptable with the upper limit of 75%.

  In another variant of the method according to the invention, the bandwidth of the channel filter 46 is changed and reduced incrementally at each step, for example to avoid adjacent channel interference. The sequence of operations is similar to that described with reference to FIG. 10, except that the operation represented by block 74 is replaced by an operation that changes the bandwidth of the filter, requiring blocks 86, 88, and 90. The point that disappears is different.

  In the present specification and claims, the word “a” or “an” preceding a component does not exclude the presence of a plurality of such components. Further, the word “comprising” does not exclude the presence of elements or steps other than those listed.

  It will be apparent to those skilled in the art from reading the present disclosure that other variations exist. Such variations are already known in the design, manufacture and use of low cost wireless communicators and components therefor and can be used in place of or in addition to the features already described herein. The features of can be incorporated.

Claims (13)

  In a method of receiving a direct sequence spread spectrum signal, the direct sequence spread spectrum signal is downconverted, and the downconverted direct sequence spread spectrum signal is narrower than the bandwidth of the direct sequence spread spectrum signal. Filtering in a channel filter having and correlating the filtered signal to a sequence equal to that used to spread the spectrum.   The method of claim 1, comprising determining whether the degree of correlation is acceptable based on a predetermined criterion and providing an output signal if acceptable.   3. The method of claim 2, comprising changing a center frequency of the channel filter, monitoring the degree of correlation for each center frequency, and selecting a center frequency that provides an acceptable degree of correlation.   4. The method of claim 1, 2 or 3, wherein the bandwidth of the channel filter is substantially half of the bandwidth of the direct sequence spread spectrum signal.   A primary station having means for transmitting a direct sequence / spread spectrum signal; down-converting means for down-converting the direct sequence / spread spectrum signal; and a channel filter for filtering the down-converted direct sequence / spread spectrum signal. A channel filter having a narrower bandwidth than the bandwidth of the direct sequence spread spectrum signal, and a correlating means for correlating the filtered signal with a sequence equal to that used to spread the spectrum. A wireless system having at least one secondary station including a receiver having   6. The system of claim 5, wherein the receiver comprises means for determining whether the degree of correlation is acceptable based on a predetermined criterion and means for providing an output signal if the degree of correlation is acceptable.   The receiver further includes a reference frequency generator including frequency adjusting means for adjusting the output frequency, and control means for causing the frequency reference generator to adjust the output frequency to provide an acceptable degree of correlation. The system according to claim 6.   The system according to claim 5, 6 or 7, wherein the bandwidth of the channel filter is substantially half of the bandwidth of the direct sequence spread spectrum signal.   In a receiver for receiving a direct sequence / spread spectrum signal, the receiver includes down-converting means for down-converting the direct sequence / spread spectrum signal, and a channel filter for filtering the down-converted direct sequence / spread spectrum signal. And a correlation means for correlating the filtered signal with a reference sequence, wherein the bandwidth of the channel filter is narrower than the bandwidth of the direct sequence spread spectrum signal.   The receiver of claim 9, comprising: means for determining whether the degree of correlation is acceptable based on a predetermined criterion; and means for providing an output signal if the degree of correlation is acceptable.   11. The method of claim 10, further comprising: a reference frequency generator including frequency adjustment means for adjusting the output frequency; and control means for causing the reference frequency generator to adjust the output frequency to provide an acceptable degree of correlation. The listed receiver.   The receiver of claim 11, wherein the reference frequency generator has an integrated frequency determination component.   13. A receiver as claimed in any of claims 9 to 12, wherein the bandwidth of the channel filter is substantially half of the bandwidth of the direct sequence spread spectrum signal.

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