Optimum signal decision apparatus for digital signal receiver

Abstract: A received signal strength detector detects a received signal strength corresponding to an envelope of a received signal and outputs a received strength signal corresponding to the strength level. A DC component extracting filter extracts a DC component from the received strength signal and outputs an extracted DC component signal corresponding to the level of the DC component. A modulation rate component extracting filter extracts a line spectrum component corresponding to a modulation rate from the received strength signal and outputs an extracted modulation rate component signal corresponding to the level of the modulation rate component. A decision compares the extracted DC component signal and the extracted modulation rate component signal with respective threshold levels set therefor and decides, when the levels of the two signals are higher than the respective threshold levels, that the received signal is an optimum signal. The decision may otherwise be based on comparison of a ratio between the two extracted signals with a threshold level set therefor. An attenuator may be provided on the input side of the received signal strength detector such that it may be controlled, when the decision decides that the received signal is not optimum, so as to lower the gain of the receiver to prevent otherwise possible saturation of the received signal output of the receiver.

Description: This invention relates to a digital modulated signal receiver, and more particularly to a carrier detector for a linear digital modulated signal receiver. It is important to measure the signal strength of a received signal to detect a carrier particularly when signals from a plurality of base stations are received and it is required to select an optimum signal from among the received signals as with a terminal station in mobile communications. Fig. 1 is a block diagram showing the construction of a conventional carrier detector. Received signal 101 is inputted to received signal strength detector 2, from which received signal 103 and received strength signal 104 are outputted. A logarithmic amplifier or the like is used for received signal strength detector 2. Received strength signal 104 is produced by signal conversion based on a control signal (for example, a gain control signal) in the logarithmic amplifier for securing a logarithmic linearity of the amplifier and is generally called RSSI (Received Signal Strength Indicator). DC component extracting filter 3 receives received strength signal 104 and extracts DC component 105 of received strength signal 104. Decision circuit 8 compares a voltage value corresponding to DC component 105 with a predetermined level and outputs decision signal 108 which represents whether or not the strength of the received signal is sufficiently high. With the conventional method described above, however, since only the field strength of a received signal is measured simply, even if the received signal is a signal of a different modulation rate or is a signal whose demodulation is difficult due to cochannel interference or to some other cause, only if the received signal has a sufficiently high field strength, it may possibly be selected as an optimum signal in error. Accordingly, an optimum received signal is not necessarily selected. Further, even if the received signal is so strong as to put the receiver into a saturated condition in which the receiver cannot demodulate the received signal, since it is merely decided whether or not the field strength of the received signal is higher than a certain level, the received signal may possibly be selected as it is, resulting in error in selection of an optimum received signal. A countermeasure to this problem is disclosed, for example, in JP-A-04343547 wherein, when the field strength of a received signal is higher than a certain level, the output of the received signal is attenuated by an attenuator to a level with which the receiver is not saturated. A four-phase phase modulation signal inputted to a high frequency receiving part is converted into an intermediate frequency signal and the converted signal is outputted and demodulated by the demodulation part and outputted. Baud timing for extracting a digital signal from the demodulation part is applied from an amplitude information detecting part for detecting an amplitude variation component from the intermediate frequency signal and extracting timing. Simultaneously an electric field information detecting part monitors the intensity of an electric field based upon the intermediate frequency signal in each baud timing. At the time of inputting the four-phase phase modulation signal with a strong electric field, the intermediate frequency signal is saturated, but the detecting part detects the storing electric field state and reduces the gain of the receiving part, preventing the intermediate frequency signal from being saturated. Thereby the amplitude variation component can be always effectively detected. In this instance, however, the result of detection of the received signal strength is required to have a monotonous increasing characteristic with respect to the received signal level. In particular, since the monotonous increasing characteristic is lost if the receiver is saturated, it is indefinite whether the RSSI output level with respect to the input level of the received signal exhibits a condition wherein the output of the received signal is fully saturated with respect to a certain RSSI output level or another condition wherein the output of the received signal is not saturated as yet. Accordingly, the prior art method has a problem in that, since it cannot be decided whether the RSSI output level is nearly in a saturated condition or is in a saturated condition, it cannot be determined how the attenuator should be controlled. Consequently, with the prior art method, an optimum received signal is not necessarily selected. It is an object of the present invention to provide a received signal decision apparatus which, eliminating the problems of the conventional received signal decision apparatus for a digital modulated signal receiver described above, can select an optimum received signal or can detect that a receiver is in a saturated condition and prevent the saturated condition of the receiver. The object is achieved by the features of the claims. According to a first aspect of the present invention, there is provided a received signal decision apparatus for a digital modulated signal receiver, comprising received signal strength detecting means for detecting a received signal strength corresponding to an envelope of a received signal and outputting a received strength signal, a DC component extracting filter for extracting a DC component of the received strength signal, a modulation rate component extracting filter for extracting a modulation rate component of the received strength signal, and decision means for deciding based on the DC component and the modulation rate component of the received strength signal whether or not the received signal is optimum. The decision of the decision means may be based on the relationship between the DC component and the modulation rate component of the received signal described above, and it may decide that the received signal is optimum (1) when both components are higher than certain respective threshold levels, or (2) when the (modulation rate component)/(DC component) is higher than a certain threshold level, or when the (DC component)/(modulation rate component) is lower than another certain threshold level, but in any other case, it decides that the received signal is not optimum. The received signal decision apparatus may further comprise an attenuator provided on the input side of the received signal strength detecting means. In this instance, the decision means may decide that the received signal is not optimum (3) when the DC component is higher than a threshold level set therefor and the modulation rate component is lower than another threshold level set therefor, or (4) when the (modulation rate component)/(DC component) is lower than a certain threshold level, or when the (DC component)/(modulation rate component) is higher than another certain threshold level, and control the attenuator to lower the gain of the receiver so that the output of the receiver may not be saturated, but in any other case, and the decision means decides that the received signal is optimum. Further, where a plurality of candidates are present as alternatives for a signal to be received, the decision means may calculate a ratio of the DC component to the modulation rate component of the received strength signal or a reciprocal ratio to this ratio for each of the candidates and decide that that one of the alternatives for the signal to be received whose ratio exhibits the highest value or whose reciprocal rate exhibits the lowest value is optimum as the received signal. According to a second aspect of the present invention, there is provided a received signal decision apparatus for a digital modulated signal receiver, comprising received signal strength detecting means for detecting a received signal strength corresponding to an envelope of a received signal and outputting a received strength signal, a DC component extracting filter for extracting a DC component of the received strength signal, a modulation rate component extracting filter for extracting a modulation rate component of the received strength signal, an attenuator provided on the input side of the received signal strength detecting means, and control means for controlling the attenuator in response to the DC component and the modulation rate component of the received strength signal, the control means continuously controlling the output of the attenuator in accordance with the value of a ratio of the DC component to the modulation rate component of the received strength signal so as to prevent saturation of the output of the receiver, the control means deciding based on the output of the receiver as a result of the control of the attenuator whether or not the received signal is optimum. Fig. 1 is a block diagram showing the construction of a conventional received signal decision apparatus for a digital modulated signal receiver; Fig. 2 is a block diagram illustrating the construction of an embodiment of the present invention; Fig. 3 is a block diagram illustrating the construction of another embodiment of the present invention; and Fig. 4 is a block diagram illustrating the construction of a further embodiment of the present invention. Embodiments of the present invention are described below with reference to the drawings. Fig. 2 is a block diagram showing an embodiment of a received signal decision apparatus for a digital modulated signal receiver of the present invention. Referring to Fig. 2, received signal strength detector 2 produces received strength signal 104 and received signal 103 from received signal 101 inputted thereto. Received strength signal 104 is outputted as an RSSI detected in accordance with an envelope of received signal 101 as described above to DC component extracting filter 3 and modulation rate component extracting filter 4. DC component extracting filter 3 is constituted using a low-pass filter (LPF), extracts a DC component of received strength signal 104 and outputs extracted DC component signal 105 having an output level (voltage value or current value) corresponding to the extracted DC component to decision circuit 5. Meanwhile, modulation rate component extracting filter 4 is constituted by a band-pass filter (BPF) and a detector, extracts a line spectrum component corresponding to a modulation rate from received strength signal 104 by means of the band-pass filter and outputs extracted modulation rate component signal 106 having an output level (voltage value or current value) corresponding to the modulation rate component to decision circuit 5. Decision circuit 5 compares the output level of extracted DC component signal 105 with a predetermined threshold level, and when the output level of extracted DC component signal 105 is higher than the threshold level and besides a comparison of the output level of extracted modulation rate component signal 106 with another predetermined threshold level reveals that also-the output level of extracted modulation rate component signal 106 is higher than the threshold level, the decision circuit 5 decides that received signal 101 is an optimum received signal and outputs decision signal 108 representing a result of the decision. It is to be noted that the decision of decision circuit 5 may be based either on the method described above wherein the output levels of extracted component signals 105 and 106 are individually compared with respective suitable levels (threshold levels) or on another method wherein a received signal is selected based on the relationship between a modulation rate component and a DC component such that the received signal is decided to be optimum when (a) the (modulation rate component)/(DC component) is higher than a certain threshold level, or (b) the (DC component)/(modulation rate component) is lower than another certain threshold level, but it is decided in any other case that the received signal is not optimum. In particular, while, according to the prior art, only the field strength is detected through an RSSI (output level of extracted DC component signal 105), according to the present invention, the ratio of the output level of extracted modulation rate component signal 106 to the output level of extracted DC component signal 105 or the reciprocal ratio-to it is detected, and the characteristic that the ratio exhibits a low value when the output level of extracted modulation rate component signal 106 is in a fully saturated condition is utilized. Thus, the ratio of the output level of extracted modulation rate component signal 106 to the output level of extracted DC component signal 105 or the reciprocal ratio to it is observed and compared with a respective predetermined threshold level. As a result of the comparison, it can be decided appropriately whether a certain RSSI output level exhibits a condition wherein the output of the received signal is in a fully saturated condition or another condition wherein the output of the received signal is not in a fully saturated condition. Another embodiment of the present embodiment is described below with reference to Fig. 3. Received signal 101 is inputted to received signal strength detector 2 by way of attenuator 1. Further, decision circuit 6 outputs decision signal 108 and further outputs control signal 107 for controlling attenuator 1 in accordance with a result of decision of decision circuit 6 to attenuator 1. The other construction and operation of the present embodiment are similar to those described hereinabove with reference to Fig. 2. In an instance, decision circuit 6 selects a received signal in the following manner. In particular, when extracted DC component signal 105 is higher than a predetermined threshold level and extracted modulation rate component signal 106 is lower than another predetermined threshold level, decision circuit 6 decides that the received signal is not optimum, and outputs control signal 107 to increase the attenuation amount of attenuator 1 to lower the gain of the receiver thereby to eliminate an otherwise possible saturated condition of the output of the receiver. Using this result, decision of whether or not the received signal is optimum is performed similarly as in the first embodiment described above. Further in another instance, decision circuit 6 decides that the received signal is not optimum when (c) the (modulation rate component)/(DC component) is lower than a certain threshold level, or (d) the (DC component)/(modulation rate component) is higher than another certain threshold level, and eliminates the saturated condition of the output of the receiver in a similar manner. In any other case, decision circuit 6 decides that the received signal is optimum. The basis of the operation of decision circuit 6 in those cases is similar to that in the first embodiment. In the second embodiment, since control signal 107 takes both of a modulation rate component and a DC component into consideration, the monotonous increasing characteristic of the amplifier in received signal strength detector 2 need not be taken into consideration, and consequently, a stable optimum received signal can be obtained. Further, it can be understood readily that, as a modification to the embodiments described above, where a plurality of candidates are present as alternatives for a signal to be received, it is possible to employ a method wherein the ratio of extracted DC component signal 105 to extracted modulation rate component signal 106 of a received strength signal or the reciprocal ratio to it is calculated for each of the alternatives for the signal to be received and that one of the alternatives as the signal to be received which exhibits the highest value or the lowest value, respectively, is determined as an optimum signal for received signal 101. While the decision operations of the decision circuits in the embodiments described above are all based on on-off control between upper and lower ranges with respect to a certain threshold level, another decision method may alternatively be employed. In particular, controller 7 is employed as shown in Fig. 4. In this instance, the controller controls the attenuator so that, for example, the attenuation amount of the attenuator has such a continuous variation that it increases in proportion to or in reciprocal proportion to the ratio of the DC component to the modulation rate component, and based on a result of the control, it is decided by controller 7 whether or not the received signal is optimum. As described above, according to the present invention, a received signal which has a sufficiently high signal strength but is difficult to demodulate due to a difference in modulation rate or due to cochannel interference can be selected. Further, according to the present invention, even if the receiver is in a saturated condition and cannot perform demodulation, the received signal can be attenuated without the necessity for a monotonous increasing characteristic of the received signal strength detection result with respect to the received signal level. Accordingly, it is possible to receive signals from a plurality of base stations and select an optimum signal from among the received signals as with a terminal station for mobile communications.

Claims: A received signal decision apparatus for a digital modulated signal receiver, comprising received signal strength detecting means for detecting a received signal strength corresponding to an envelope of a received signal and outputting a received strength signal, a DC component extracting filter for extracting the level of a DC component signal of the received strength signal, a modulation rate component extracting filter for extracting the level of a modulation rate component signal of the received strength signal, and decision means for deciding based on the respective levels of the DC component and the modulation rate component signals of the received strength signal whether or not the received signal is optimum. A received signal decision apparatus as claimed in claim 1, wherein said decision means compares the respective levels of the DC component and the modulation rate component signals of the received strength signal with respective threshold levels and decides that the received signal is optimum when both the levels of the DC component and the modulation rate component signals are higher than the respective threshold levels. A received signal decision apparatus as claimed in claim 1, wherein said decision means compares a ratio of the level of the modulation rate component signal to the level of the DC component signal of the received strength signal with a predetermined threshold level or compares a ratio of the level of the DC component signal to the level of the modulation rate component signal of the received strength signal with another predetermined threshold level different from the first-mentioned threshold level and decides that the received signal is optimum when the first-mentioned ratio is higher than the first-mentioned threshold level or when the second-mentioned ratio is lower than the second-mentioned threshold level. A received signal decision apparatus as claimed in claim 1, 2, or 3, further comprising an attenuator provided on the input side of said received signal strength detecting means and wherein said decision means decides, when the level of the DC component signal of the received strength signal is higher than a threshold level set therefor and the level of the modulation rate component signal is lower than another threshold level set therefor, that the received signal is not optimum and controls said attenuator to lower the gain of said receiver so that the output of said receiver may not be saturated, but decides, in any other case, that the received signal is optimum. A received signal decision apparatus as claimed in claim 1, 2, or 3, further comprising an attenuator provided on the input side of said received signal strength detecting means, and wherein said decision means compares a ratio of the level of the modulation rate component signal to the level of the DC component signal of the received strength signal with a predetermined threshold level or compares a ratio of the level of the DC component signal to the level of the modulation rate component signal of the received strength signal with another predetermined threshold level different from the first-mentioned threshold level and controls, when the first-mentioned ratio is lower than the first-mentioned threshold level or when the second-mentioned ratio is higher than the second-mentioned threshold level, said attenuator to lower the gain of said receiver so that the output of said receiver may not be saturated and then decides from the output of said receiver as a result of the control whether or not the received signal is optimum. A received signal decision apparatus as claimed in any of claims 1 to 5, wherein said decision means calculates, where a plurality of candidates are present as alternatives for a signal to be received, a ratio of the level of the DC component signal to the level of the modulation rate component signal of the received strength signal or a reciprocal ratio to this ratio for each of the candidates and decides that one of the alternatives for the signal to be received whose ratio exhibits the highest value or whose reciprocal ratio exhibits the lowest value is optimum as the received signal. A received signal decision apparatus for a digital modulated signal receiver, comprising received signal strength detecting means for detecting a received signal strength corresponding to an envelope of a received signal and outputting a received strength signal, a DC component extracting filter for extracting the level of a DC component signal of the received strength signal, a modulation rate component extracting filter for extracting the level of a modulation rate component signal of the received strength signal, an attenuator provided on the input side of said received signal strength detecting means, and control means for controlling said attenuator in response to the respective levels of the DC component and the modulation rate component signals of the received strength signal, said control means continuously controlling the output of said attenuator in accordance with the value of a ratio of the level of the DC component signal to the level of the modulation rate component signal of the received strength signal so as to prevent saturation of the output of said receiver, said control means deciding based on the output of said receiver as a result of the control of said attenuator whether or not the received signal is optimum.

Inventors: TSUMURA SOICHI, TSUMURA, SOICHI
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1995
Main classification: H04L25
IPC codes: H04L25, H04L27