Radio communication device, signal strength output method and radio communication system

There is provided a radio communication device including a radio receiving unit to receive a radio signals, a synchronization unit to detect synchronization based on a result of a correlation for received signals output from the radio receiving unit, and a signal strength output unit to output, as signal strength of the received signals that is RSSI (Received Signal Strength Indicator) value, a level of a correlation signal that is output as a result of the correlation by the synchronization unit.

CROSS REFERENCE TO RELATED APPLICATION

This application claims priority to Japanese Priority Patent Application JP 2009-233025, filed in the Japan Patent Office on Oct. 7, 2009, the entire content of which is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a radio communication device, a signal strength output method and a radio communication system.

2. Description of the Related Art

Generally, a radio communication device includes a function called RSSI (Received Signal Strength Indicator) which measures signal strength of received radio signals. The signal strength measured by such RSSI function is notified to a user through a screen of a display device, or to be used as a parameter for controlling an output level of signals from a local device.

Examples of a technological development regarding the RSSI function are Japanese Unexamined Patent Application Publication No. 2004-304627 and Japanese Unexamined Patent Application Publication No. 2006-295462. The Japanese Unexamined Patent Application Publication No. 2004-304627 provides a method which converts control level information of an auto gain control to the received signal strength to render a dedicated RSSI circuit unnecessary eliminating a number of circuit parts. Moreover, the Japanese Unexamined Patent Application Publication No. 2006-295462 provides a method which controls a window length for smoothing digital RSSI signals so as to prevent turn-on delay in the RSSI signals.

SUMMARY OF THE INVENTION

However, a short-range high-speed radio communication such as TransferJet (registered trademark) has a relatively narrow communication range, and takes a short time for data transmission. Therefore, unless a change in the received signal strength is reflected promptly on RSSI signals, its value does not make sense. Accordingly, there is a need for a mechanism for obtaining received signal strength promptly with less processing amount which does not require processes such as the conversion from control level information of the auto gain control or the smoothing of the RSSI signals.

In light of the foregoing, it is desirable to provide a novel and improved radio communication device, a signal strength output method and a radio communication system that enable obtaining received signal strength promptly with less processing amount.

According to an embodiment of the present invention, there is provided a radio communication device including a radio receiving unit to receive radio signals, a synchronization unit to detect synchronization based on a result of a correlation for received signals output from the radio receiving unit, and a signal strength output unit to output, as signal strength of the received signals, a level of a correlation signal that is output as a result of the correlation by the synchronization unit.

In such configuration, correlation calculation for received signals to detect synchronization is performed by the synchronization unit, and a level of a correlation signal that is output as a result of the correlation would be output as a signal strength of the received signals.

The signal strength output unit may continuously output the level of the correlation signal as the signal strength until a prescribed time period elapses after the synchronization unit has detected the synchronization.

The signal strength output unit may output a value equivalent to zero as the signal strength if the synchronization unit does not detect synchronization.

The radio communication device may further include a radio transmitting unit to transmit a request signal that requests another radio communication device located in close proximity thereto to transmit radio signals if the synchronization unit does not detect synchronization over a certain period of time.

The radio communication device may further include a display unit to display the signal strength in accordance with the level of the correlation signal output by the signal strength output unit.

The request signal may be transmitted only when the radio communication device requests to communicate with another radio communication device.

Further, according to another embodiment of the present invention, there is provided a signal strength output method for outputting signal strength of received signals in a radio communication device including a radio receiving unit to receive radio signals and to output the received signals, including the steps of detecting synchronization based on a result of a correlation for the received signals, and outputting, as signal strength of the received signals, a level of a correlation signal that is output as a result of the correlation.

According to another embodiment of the present invention, there is provided a radio communication system including a radio transmitting device including a radio transmitting unit to transmit radio signals, and a radio receiving device including, a radio receiving unit to receive the radio signals transmitted from the radio transmitting device, a synchronization unit to detect synchronization based on a result of a correlation for received signals output from the radio receiving unit, and a signal strength output unit to output, as signal strength of the received signals, a level of a correlation signal that is output as a result of the correlation by the synchronization unit.

As described above, a radio communication device, a signal strength output method, and a radio communication system according to the present invention enable obtaining received signal strength promptly with less processing amount.

DETAILED DESCRIPTION OF THE EMBODIMENT(S)

Preferred embodiments of the present invention will be described hereinafter in the following order:

1. Overview of a radio communication system

2. An exemplary configuration of a radio communication device

3. Timing chart

<1. Overview of a Radio Communication System>

Firstly, the overview of a radio communication system according to an embodiment of the present invention is described with reference toFIG. 1.FIG. 1is a schematic view showing the overview of a radio communication system according to an embodiment. Referring toFIG. 1, the radio communication system1includes radio communication devices100aand100b.

The radio communication devices100aand100bare typically communication devices capable of transmitting and receiving data at high speed using radio signals. The example ofFIG. 1shows a PC (personal computer) as an example of the radio communication device100a, and a digital camera as an example of the radio communication device100b. However, the radio communication devices100aand100bare not limited to these examples, and may be other kinds of terminal device (such as a cellular phone terminal or a game machine, for example), a digital appliance (such as a television set or an audio player, for example), or a network device (such as a router or a radio interface card, for example).

The radio communication device100acan transmit and receive various data including image data, audio data or the like, for example, with the radio communication device100blocated inside a communication area102.

Communication protocol used for the communication between the radio communication devices100aand100bmay be TransferJet (registered trademark), Bluetooth (registered trademark), Zigbee (registered trademark), wireless LAN (Local Area Network) or UWB (Ultra Wide Band), for example. A later-described signal strength output method according to an embodiment of the present invention is effective especially when to use a short-range high-speed radio communication method which is highly needed for obtaining the received signal strength promptly. However, it will be understood that by reading the later explanation that the signal strength output method can also be effective for a radio communication method for non-short-range communication such as 3G, 3.5G or 4G mobile communications.

One or both of the radio communication devices100aand100bhas typically an indicator104to notify a user of the received signal strength. The indicator104may be implemented as an image to be displayed on a display of each device, or may be implemented as hardware using light-emitting elements such as LED. The indicator104is capable of displaying signal strength of the received signals in 5 levels from 0 to 4 (from Low to High), for example. According to the embodiment of the present invention later described, the signal strength of the received signals that the indicator104displays can be obtained promptly with less processing amount.

In the following description, when there is no particular need to distinguish between the radio communication devices100aand100b, they are referred to collectively as the radio communication device100by eliminating the alphabetical letter affixed to the reference numeral.

<2. An Exemplary Configuration of a Radio Communication Device>

FIG. 2is a block diagram exemplifying a configuration of a radio communication device according to an embodiment. Referring toFIG. 2, the radio communication device100includes an antenna108, a radio receiving unit110, analog/digital (A/D) converter120, a synchronization unit130, a signal strength output unit140, a display unit148, a demodulation unit150, an upper layer160, a modulation unit170, digital/analog (D/A) converter180and a radio transmitting unit190.

The antenna108is connected to the radio receiving unit110and the radio transmitting unit190, and is used to receive and transmits radio signals. The radio receiving unit110is typically implemented as a RF (radio frequency) circuit. The radio receiving unit110receives the radio signals via the antenna108, and outputs the received signals to the A/D converter120as described later.

FIG. 3is a block diagram exemplifying the detailed configuration of a radio receiving unit110according to an embodiment. Referring toFIG. 3, the radio receiving unit110includes a band pass filter (BPF)112, a low noise amplifier (LNA)114, frequency converters116aand116b, low pass filters (LPF)117aand117b, and variable gain amplifiers (VGA)118aand118b. The BPF112extracts signals that include specific frequency components from the received signals input from the antenna108. The LNA114amplifies the received signals that have been extracted including specific frequency components by the BPF112. The frequency converter116aconverts frequency of I-channel components in the received signals amplified by the LNA114into intermediate frequency. Moreover, the frequency converter116bconverts frequency of Q-channel components in the received signals amplified by the LNA114into intermediate frequency. The LPH117areduces high-frequency components included in I-channel components of the received signals at intermediate frequency input from the frequency converters116a. Moreover, the LPH117breduces high-frequency components included in Q-channel components of the received signals at intermediate frequency input from the frequency converters116b. The VGA118aamplifies I-channel components of the received signals input from the LPF117a. Moreover, the VGA118bamplifies Q-channel components of the received signals input from the LPF117b. Then, I-channel components of the received signals amplified by the VGA118a, and Q-channel components of the received signals amplified by the VGA118bare to be output to the A/D converter120respectively.

The A/D converter120samples the received signals (I-channel components and Q-channel components) in analog form, which is input from the radio receiving unit110, and converts them into digital form. The A/D converter120outputs the received signals, being converted into digital form, to the synchronization unit130.

As described later, the synchronization unit130performs correlation calculation for the received signals that are output from the radio receiving unit110and converted into digital form by the A/D converter120, and detects synchronization based on a result of the correlation.

FIG. 4is a block diagram exemplifying the more detailed configuration of the synchronization unit130. Referring toFIG. 4, the synchronization unit130includes a correlation unit132, a square absolute calculation134, and a synchronization detection unit136.

The correlation unit132correlate I-channel components and Q-channel components of the received signals being input from the A/D converter120with known signals stored in an inner memory.

The square absolute calculation134calculates squares sum of the absolute value of a correlation of I-channel components and of a correlation of Q-channel components, being calculated by the correlation unit132. Correlation signals to be used for detecting synchronization are thereby generated. The square absolute calculation134outputs the correlation signals generated in such manner to the synchronization detection unit136and the signal strength output unit140.

The synchronization detection unit136compares a level of the correlation signal input from the square absolute calculation134with a prescribed threshold. Then, the synchronization detection unit136outputs a synchronization detection signal (Sync=True) to the demodulation unit150and the signal strength output unit140, if the level of the correlation signal exceeds the threshold.

Referring back toFIG. 2, the explanation will be continuously given on an example of the configuration of the radio communication device100.

The signal strength output unit140outputs the above-described level of the correlation signal being output as a result of the correlation by the synchronization unit130to the display unit148and the upper layer160as the received signal strength (or instead, it may write it into a specific address of a register). Further, the signal strength output unit140continuously outputs the level of the correlation signal as a signal strength of the received signals until a prescribed time period elapses after the synchronization unit130has detected synchronization. The signal strength output unit140may output a value equivalent to zero (which means “outside the range”, “outside of communication range” or the like”) as the signal strength of the received signals if the synchronization unit130does not detect the synchronization.

Here, an explanation will be given on a model indicating a relationship between transmitted signals and received signals in the radio communication system1. Firstly, given that a complex baseband signal at the transmission side is SBB(t), the transmitted signal STX(t) as a real-valued signal modulated from the complex baseband signal at center frequency FC [Hz] is represented by the following equation.

The transmitted signal STX(t) is received by the radio receiving unit110. Given that an impulse response of a propagation channel and a receiving circuit between transmitting and receiving apparatuses is h(t), the received signal SRX(t) is represented by the convolution between the transmitted signal STX(t) and the impulse response h(t) as the following equation.

Through processing such as frequency conversion or the like by the radio receiving unit110, the complex baseband signal SBBRX(t) at the receiving side is generated from the receiving signal SRX(t). The complex baseband signal SBBRX(t) at the receiving side is represented by the following equation. Note that Tcrepresents a transmit symbol period.

Furthermore, given that the known signal used for the correlation by the correlation unit132of the synchronization unit130is c(t), that complex conjugate transpose thereof is c*(t), and that a code length of the known signal c(t) is K·Tc, the correlation signal Scor(t) obtained as the result of the correlation is represented by the following equation.

Thus, when the known signal c(t) conforms with the complex baseband signal SBBRX(t), the correlation signal Scor(t) shows its peak. Generally, the peak value of the correlation signal becomes higher as a SNR (signal to noise ratio) is higher, that is, a receiving environment is better. Here, given that a noise level is not significantly changed during transmission in the radio communication system1in general, if the peak value of the correlation signal is proportional to the SNR, it can be assumed that the peak value is proportional to the level of the received signal SBBRX(t). Therefore, in such receiving environment, the peak value of the correlation signal can be used as a scale rRSSI(t) for indicating the signal strength of the received signal as the following equation.

Note that in equation 5, RSSI indicates a level of received signals actually received by the radio receiving unit110.

FIG. 5is a block diagram showing an example of a more specific configuration of the signal strength output unit140. Referring toFIG. 5, the signal strength output unit140includes an output control unit142, a timer144, and a switch (SW)146.

The output control unit142controls an output value of received signal strength by the signal strength output unit140. More specifically, for example, when a synchronization detection signal (Sync=True) is input from the synchronization detection unit136of the synchronization unit130, the output control unit142starts-up the timer144and starts a measurement of time. Then the output control unit142retains the value of the correlation signal input from the square absolute calculation134of the synchronization unit130at a time when the synchronization detection signal has been input, until a prescribed time period elapses, or until a new synchronization detection signal is input. It is defined herein that the prescribed time period that the output control unit142retains the value of the correlation signal is a RSSI survival period. The output control unit142continuously outputs the retained correlation signal value to the switch146as a value of the received signal strength, that is, the RSSI value. Note that the output control unit142may output the value after quantized to a numeric value in five levels as shown inFIG. 1, for example, instead of outputting the retained correlation signal value as a RSSI value as it is. Moreover, the time when the measurement of the RSSI survival period is started by the output control unit142is not limited to the time when the synchronization detection signal is input (generally it is a time when receiving a preamble of packets ends), but it may be a time when receiving the whole packets including payload ends, for example.

Further, the output control unit142may generate a request signal that requests another radio communication device located in close proximity thereto to transmit radio signals if the synchronization unit130does not detect synchronization over a certain period of time. In this case, the output control unit142controls the radio transmitting unit190to transmit the generated request signal via the antenna108. Thus, if there is another radio communication device located in close proximity thereto, this another radio communication device receives the request signal and send back a response signal. This enables the output control unit142to update the RSSI value dynamically.

The timer144is started-up by the output control unit142and measures time. For example, when the RSSI survival period mentioned above has elapsed since the time started-up, the timer144outputs a reset signal to the output control unit142to reset the correlation signal value retained by the output control unit142to zero.

The switch146outputs the RSSI (received signal strength indication) signal whose signal value is RSSI value being input from the output control unit142, to the display unit148and the upper layer160. Note that the switch146replaces the RSSI signal value with a value equivalent to zero to output if a signal (Sync=False) indicating a failure in synchronization detection is input from the synchronization detection unit136of the synchronization unit130.

Referring back toFIG. 2again, the explanation will be continuously given on an example of the configuration of the radio communication device100.

The display unit148displays the received signal strength using the indicator104shown inFIG. 1, for example, in accordance with the RSSI value input from the signal strength output unit140, that is, the level of the correlation signal.

FIG. 6is an explanatory diagram showing a relationship between received signal strength and correlation level that are displayed by a display unit.

Referring toFIG. 6, the level of correlation signals along a time axis is illustrated in a line chart. Regarding the correlation signals, when the level of the correlation signal exceeds the threshold Th, for example, the synchronization unit130output the synchronization detection signal to the signal strength output unit140. In the example ofFIG. 6, a peak P1of the correlation signal exceeds the threshold Th at a time T1, and the synchronization detection signal is output to the signal strength output unit140at the time T1. The signal strength output unit140outputs the peak value of the correlation signal to the display unit148as a value of RSSI signal during the RSSI survival period Tholdafter the synchronization detection signal is input from the synchronization unit130. During the rest of the time, the signal strength output unit140outputs a value equivalent to zero to the display unit148as a value of RSSI signal. As the result, the display unit148displays the received signal strength zero before the synchronization is detected (T<T1). From the time the synchronization is detected until the RSSI survival period elapses (T1≦T<T1+Thold), the display unit148displays the received signal strength corresponding to the RSSI value (strength3in the example ofFIG. 6). Then, when the RSSI survival period has elapsed (without any new synchronization detected) (T1+Thold≦T), the display unit148displays the received signal strength zero again.

Referring back toFIG. 2again, the explanation will be continuously given on an example of the configuration of the radio communication device100.

When the synchronization unit130detects synchronization, the demodulation unit150demodulates the received signal according to a certain modulation method. And the demodulation unit150outputs the demodulated received signal to a MAC (media access control) layer included in the upper layer160.

The upper layer160includes layers upper than a physical layer in the communication protocol stack. For example, the upper layer160may include only layers located in relatively lower position such as a MAC layer. Moreover, the upper layer160may include layers located in relatively upper position such as an application layer. For example, a part of the upper layer160may execute the quantization of the RSSI value or generation of the request signal or the like, explained above relating to the output control unit142, instead of the output control unit142.

The modulation unit170modulates the transmitted signal input from the upper layer160according to a specific modulation method. Then the modulation unit170outputs the modulated transmitted signal to the D/A converter180.

The D/A converter180converts the transmitted signal in digital form that is input from the modulation unit170or the above-described request signal that is generated by the signal strength output unit140into analog form. Then the D/A converter180outputs the signal converted into analog form to the radio transmitting unit190.

The radio transmitting unit190is typically implemented as a RF circuit, same as the above-described radio receiving unit110. The radio transmitting unit190amplifies and executes frequency-conversion the analog signal input from the D/A converter180, and then transmits it as a radio signal via the antenna108. The radio signal to be transmitted from the radio transmitting unit190may include the above-described request signal that is generated by the signal strength output unit140, for example, in addition to data signals generated in the upper layer160.

FIG. 7andFIG. 8are timing charts exemplifying a RSSI value output by the radio communication device100.FIG. 7shows a case (a first scenario) where the radio communication device100continuously receives packets in a relatively short interval. On the other hand,FIG. 8shows a case (a second scenario) where the next packet is not received for relatively long period of time more than the RSSI survival period after one packet has been received.

Referring toFIG. 7, in the upper part of the drawing, three packets P1, P2and P3received in order by the radio communication device100are illustrated. Moreover, in the lower part of the drawing, time change in the RSSI value output from the signal strength output unit140of the radio communication device100is illustrated.

Firstly, at time T1, preamble of the packet P1is received, and if the synchronization unit130detects synchronization, the RSSI value is set to L1. This RSSI value is retained until the RSSI survival period Tholdwill elapse, that is, until the time T1+Thold. Then when the RSSI survival period Tholdelapses, the RSSI value becomes zero. When a preamble of packet P2is received at time T2and the synchronization unit130detects synchronization, the RSSI value is set to L2. This RSSI value is also to be retained until the RSSI survival period Tholdelapses. Then when the RSSI survival period Tholdelapses, the RSSI value becomes zero again. Further, when a preamble of packet P3is received at time T3and the synchronization unit130detects synchronization, the RSSI value is set to L3.

Thus, by outputting, as the RSSI value, the level of the correlation signal output as a result of the correlation by the synchronization unit130, and outputting continuously this RSSI value during a prescribed period of time after synchronization has been detected, it is possible to reflect the change of receiving signals status in the RSSI value to notify the user promptly.

Referring toFIG. 8, packet to be received by a radio communication device100bis illustrated in the upper part of the drawing, and packets to be received by the radio communication device100ais illustrated in the middle part of the drawing. Further, in the lower part of the drawing, time change in the RSSI value output by the signal strength output unit140of the radio communication device100a.

Firstly, at time T4, preamble of the packet P1is received by the radio receiving unit110of the radio communication device100a, and if the synchronization unit130detects synchronization, the RSSI value is set to L4. This RSSI value is retained until the RSSI survival period Tholdwill elapse, that is, until the time T4+Thold. Then when the RSSI survival period Tholdelapses, the RSSI value becomes zero.

Assuming that no packet is received by the radio communication device100aover a certain period of time Thold*α (α is real number), the radio transmitting unit190of the radio communication device100atransmits a packet for probe P5illustrated inFIG. 8, for example, as a request signal for requesting another radio communication device located in close proximity thereto to transmit radio signals.

Such packet for probe P5is to be received, for example, by the radio communication device100b. Then the radio communication device100btransmits an acknowledgment packet (Ack) P6as a response signal to the packet for probe P5. Then the radio communication device100areceives the acknowledgment packet P6, and synchronization is detected at time T6, for example. As the result, the RSSI value is updated to L6in the radio communication device100a.

As described above, the radio communication device100can dynamically update the RSSI value by transmitting a request signal that requests another radio communication device located in close proximity thereto to transmit radio signals if synchronization is not detected over a certain period of time. Note that if a time period for providing a trigger to transmit a request signal is set to the integral multiple of the RSSI survival period (that is, setting the above-mentioned α to integral number, it enables to reuse the time setting on the timer144for the measurement of the RSSI survival period. And this can simplify the implementation of the signal strength output unit140.

Moreover, the request signal may be transmitted only when the radio communication device100requests to communicate with another radio communication device. For example, in the example ofFIG. 1, in a case where the radio communication device100arequests the radio communication device100bto transmit an image data, the radio communication device100ais on the side of requesting communication. According to this configuration, if the radio communication device100aalone transmits the request signal, the RSSI value can be updated in both of the radio communication device100aand the radio communication device100b. This enables to prevent transmission of redundant radio signals in the whole system, and to decrease power consumption. Note that determination whether the local device requests for communication can be executed by an application or the like that is included in the upper layer160shown inFIG. 2, for example.

Up here, referring toFIG. 1toFIG. 8, explanations are given on the overview of the radio communication system1and the specific configuration of the radio communication device100according to an embodiment of the present invention. According to the embodiment, in the radio communication device100, the level of the correlation signal calculated by the correlation performed in order to detect synchronization can be output as received signal strength. In such a configuration, it is unnecessary to execute additional processing such as conversion from control level information of the auto gain control or smoothing in order to obtain received signal strength, therefore, it is possible to obtain the received signal strength promptly with less processing amount. This enables a short-range high-speed radio communication to display received signal strength following promptly the change in receiving status of radio signals. Moreover, according to the present embodiment, the level of the correlation signal at a time when detected synchronization is continuously output as received signal strength until a prescribed time period elapses after synchronization has been detected. This enables a user to surely stay in visual contact with the received signal strength corresponding to the peak value of the correlation signal at a time when detected synchronization. Moreover, if synchronization is not detected, a value equivalent to zero is output instead of the level of the correlation signal as the received signal strength, therefore, it is possible to notify the user promptly that communication is not to be performed due to conditions such as “outside the range” or “outside of communication range”. Further, if synchronization is not detected over a certain period of time, a request signal that requests another radio communication device located in close proximity to transmit radio signals, therefore it is possible to continuously display or output the received signal strength while control communication or data communication is not performed.