GB2342820A

GB2342820A - Employing RSSI or phase likelihood values in determining antenna diversity switching

Info

Publication number: GB2342820A
Application number: GB9918860A
Authority: GB
Inventors: Akihiro Fukui
Original assignee: NEC Corp
Current assignee: NEC Corp
Priority date: 1998-08-10
Filing date: 1999-08-10
Publication date: 2000-04-19
Also published as: JP3105869B2; CN1248827A; JP2000059277A; AU4345699A; GB9918860D0

Abstract

An antenna diversity switching method and receiver to avoid selecting an antenna providing poor reception sensitivity from among a plurality of antennas in response to a measured RSSI value equal to or lower than a predetermined value and thus preventing reception error that is caused by a noise component overlapping a received signal. A RSSI value and a phase likelihood value of a currently connected antenna are measured S1 and on basis of a comparison of RSSI value with a predetermined selection threshold value S4 , either one of the RSSI value and phase likelihood value is selected for comparison against a respective diversity switching criterion S7, S22. Antenna switching is performed if the criterion comparison S7, S22 of the selected value indicates that a switching condition is met. The selection threshold for selecting phase likelihood is a minimum value of a RSSI use range and thus appropriate switching will be carried out, even with a low level received signal, phase likelihood selectively being used when a noise component is at a high level. The criterion for the selection of antennas may be a function of an averaged, past or present value.

Description

ANTENNA DIVERSITY SWITCHING METHOD AND ANTENNA DIVERSITY RECEIVER EMPLOYING THE METHOD BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an antenna diversity switching method and to an antenna diversity receiver employing the method, in which when a predetermined condition is set in the antenna diversity from one antenna from among a plurality of antennas, switching from one antenna to another is performed. More particularly, the present invention relates to antenna diversity switching method and an antenna diversity receiver employing the method, in which antenna diversity switching is correctly performed even when a low-level RSSI value is measured.

2. Description of the Related Art In referential antenna diversity receivers, such as the one shown in FIG. 1, a control circuit 108 controls an antenna switching unit 3 to switch between two antennas 1 and 2. In FIG. 1, the antenna 1 is connected to a receiver circuit 4. An antenna switching determination is performed in accordance with an RSSI (Received Signal Strength Indication) value derived from a signal received by the receiver circuit 4.

When a communication starts in a radio coverage area, a radio signal, received by the antenna 1, is fed to the receiver circuit 4 via the antenna switching unit 3. The receiver circuit 4 converts the radio frequency signal into an intermediate frequency signal, amplifies the intermediate frequency signal, and converts the intermediate frequency signal into a low-frequency signal (in a down conversion), which can be handled by a demodulator circuit 105. The low-frequency signal is then sent to the demodulator circuit 105. The demodulator circuit 105 detects data and a data clock from the lowfrequency signal.

The receiver circuit 4 feeds an RSSI value derived from the radio signal, in the form a DC voltage, to a received power sampling circuit 6. The received power sampling circuit 6 samples the DC voltage of the received power at a reception timing output by the control circuit 108.

FIG. 10 shows three sampling timings A, B, and C in-a PHS (Personal Handy-phone System). The PHS has a frame structure of eight slots including receiving channels R1 through R4 and transmitting channels T1 through T4. The three sampling timings are a first timing point"A", an intermediate timing point"B", and a last timing point"C" in the period of the received electric field which includes the receiving channel R1 as a reception timing.

Referring to FIG. 1, a DC voltage, corresponding to the RSSI provided by the received power sampling circuit 6 through sampling, is analog-to-digital converted by an AD converter 7 and is then fed to the control circuit 108.

The control circuit 108 adopts the measurement result from the intermediate timing point"B"shown in FIG. 10, which is expected to be a relatively accurate RSSI measurement value, and performs an antenna diversity operation.

The algorithm for the antenna diversity operation executed by the control circuit 108 is now discussed, referring to both a block diagram shown in FIG. 1 and a flow diagram shown in FIG. 2.

Through the received power sampling circuit 6 and AD converter 7, the control circuit 108 receives the RSSI value of a received signal which the receiver circuit 4 measures at the reception timing discussed with reference to FIG. 10 (step S101).

Upon completion of the receive slot discussed with reference to FIG. 10, the control circuit 108 shifts a measured RSSI value stored in from a current measured value register 111 to a past measured value buffer 112. And then, reading a current measured RSSI value from the AD converter 7, the current measured RSSI value is stored onto the current measured value register 111 (step S102).

The control circuit 108 sets a predetermined RSSI criterion (step S103). In this embodiment, an averaged value of a predetermined number of measured RSSI values stored in the past measured value buffer 112 is set to the predetermined RSSI criterion.

After setting the RSSI criterion, the control circuit 108 reads the current measured RSSI value from the current measured value register 111, and compares the current measured RSSI value with the RSSI criterion (step S104).

When the comparison in step S104 reveals that the current measured RSSI value is even smaller than a value that is obtained by subtracting a predetermined value from the criterion, the control circuit 108 controls the antenna switching unit 3 to switch to the antenna 2 from the currently connected antenna 1. When the current measured RSSI value is equal to or greater than the value that is obtained by subtracting the predetermined value from the criterion, the control circuit 108 maintains the connection to the antenna 1. The control circuit 108 thus switches between the antennas 1 and 2 as necessary (step S105).

Referring to FIG. 11, an antenna switching timing is set to be within a guard time"G"immediately prior to a next receive slot.

Upon completion of the antenna selection and switching operation in step S105, the control circuit 108 accumulates the RSSI values and the phase likelihood data for the antenna 1 currently connected, stored in the current measured value register 111 and the past measured value register buffer 112 (step S106). The control circuit 108 performs an averaging process (step S107) to determine a criterion to be set in step S103 in connection with the antenna through which the current measured RSSI value is obtained, and ends the antenna diversity operation.

In the above referential antenna diversity switching method and the antenna diversity receiver employing the method, inappropriate selection of an antenna can cause poor reception sensitivity.

The antenna selection is performed referring to the RSSI value only, and a measured RSSI value can contain a large noise component relative to a received signal level.

When a circuit receiving the signal is poor in linearity with a signal input to the antenna at a low level, the receiver can continue to use the antenna providing poor reception sensitivity without switching to the other antenna.

To resolve this problem, phase likelihood is used instead of the RSSI value, as disclosed in Japanese Unexamined Patent Publication No. Hei 5-183476, for example.

According to the disclosure, a diversity unit receives phase likelihood data from a demodulator circuit. When selecting antennas, the diversity unit uses the phase likelihood instead of the RSSI value, referring to the status of the RSSI value received from a receiver circuit.

According to the disclosure, when a difference between a maximum input RSSI value and an RSSI value closest to the maximum input RSSI value is equal to or greater than a predetermined value, the antenna selection is performed based on the comparison result of the RSSI values. When the difference is within the predetermined value, the antenna selection is performed based on the comparison result of phase likelihood values.

Referring to FIG. 3, the diversity switching method for switching between two antennas la and lb in the abovereferenced disclosure is discussed. Referring to FIG. 3, there are shown receiver circuits 4a and 4b, demodulator circuits 5a and 5b, and a diversity unit 110.

The diversity unit 110 receives RSSI-a and RSSI-b, respectively, from the receiver circuits 4a and 4b, and phase likelihood values'a'and'b', respectively from the demodulator circuits 5a and 5b. The diversity unit 110 first compares levels of RSSI-a and RSSI-b, and selects an antenna based on the comparison result of the phase likelihood values when the difference in level is smaller than 3 dB referenced to the predetermined value. When the difference in level is equal to or greater than 3 dB with reference to the predetermined value, the antenna selection is performed based on the RSSI value comparison.

The phase likelihood and the RSSI are measured in each of the antennas, and in the comparison of the antennas, a condition, under which an antenna is selected, is set beforehand. The antenna selection is performed based on the comparison result of the RSSI values, when the condition is met. When the condition is not met, the antenna selection is carried out based on the comparison of the phase likelihood values.

According to the disclosure, the referential antenna diversity switching method and the antenna diversity receiver employing the method employ a receiver circuit and a demodulator circuit on a per antenna basis, requiring a bulky and costly facility. This arrangement is not very suitable for a simple and compact telephone terminal such as a PHS.

SUMMARY OF THE INVENTION The present invention is thus directed to the first method in which the RSSI and the phase likelihood are measured in one antenna selected from among a plurality of antennas and the one antenna is switched to another antenna when a predetermined condition is not met.

It is an object of the present invention to provide an antenna diversity switching method and an antenna diversity receiver employing the method, which avoids selecting an antenna providing poor reception sensitivity from among a plurality of antennas, in response to a measured RSSI value equal to or lower than a predetermined value, thereby precluding a reception error that is caused by a noise component overlapping a received signal.

An antenna diversity switching method of the present invention includes the steps of selecting one of an RSSI value and a phase likelihood concerning one antenna selected from among a plurality of antennas according to a predetermined selection threshold value, based on the measured RSSI value and the measured phase likelihood of the one antenna. And the selected antenna is switched to the other antenna if the selected one of the measured RSSI value and the measured phase likelihood value satisfies a predetermined antenna switching condition when the measured RSSI value and the measured phase likelihood value are compared to respective antenna diversity switching criteria.

Specifically, the determination of antenna diversity switching is first performed in connection with the RSSI value. When the RSSI value is equal to or lower than a predetermined selection threshold value, the phase likelihood is preferably selected for the determination of the antenna diversity switching.

An antenna diversity receiver of the present invention switches to the other antenna when a predetermined condition is met in the determination of the antenna diversity switching resulting from one antenna selected from among a plurality of antennas. The antenna diversity receiver includes a control circuit, which selects one of the RSSI value and the phase likelihood value measured through one connected antenna according to a selection threshold value as an antenna switching condition. The selection threshold value is a minimum value of the RSSI value used in the antenna diversity. When a current measured value is smaller than a value predetermined against the minimum RSSI value, the phase likelihood value is used for the antenna diversity.

The control circuit selects a current measured phase likelihood value instead of the RSSI value when a current measured RSSI value is equal to or smaller than the selection threshold value. The control circuit compares the current measured phase likelihood value with a predetermined criterion, and switches from the currently connected antenna to the other antenna when the measured phase likelihood value is smaller than a value predetermined against the criterion.

In accordance with the above-described method and arrangement, the antenna diversity receiver connects a single receiver circuit and a single demodulator circuit to a single antenna only. Both the RSSI value and the phase likelihood are used in the selection of the antenna in the single selected antenna diversity and antenna switching.

In the antenna selection based on the phase likelihood, the effect of noise overlapping the received signal is avoided.

Particularly, when the received signal is at a low level, the effect of noise is excluded in the selection process of the antennas, by selecting the antenna in accordance with the phase likelihood of the received signal, even when noise overlaps the received signal.

The criterion, serving as the antenna switching condition, is an average value of a predetermined number of measured values, if the number of measured values reaches the predetermined number, or a past measured value (if no average value is available), or a current measured value (if no past measured value is available). When the current measured value is lower than the criterion by a predetermined value, the antennas are switched.

The control circuit includes a switching counter for decrement of a predetermined value to be set as its initial value, by one each time when a current measurement is input.

The control circuit disables the antenna switching for at least the predetermined count of measurements consecutively input in succession to a last antenna switching. With this arrangement, reliable and accurate reception performance is obtained under a relatively slow fading environment.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a functional block diagram of a referential diversity receiver; FIG. 2 is a flow diagram of the operational procedure of the referential diversity receiver shown in FIG. 1; FIG. 3 is a functional block diagram of another referential diversity receiver different from the one shown in FIG. 1; FIG. 4 is a functional block diagram of one embodiment of the present invention; FIG. 5 is a flow diagram of the embodiment of the present invention; FIG. 6 is a flow diagram of an RSSI criterion setting procedure shown in FIG. 5 ; FIG. 7 is a flow diagram of a phase likelihood criterion setting procedure shown in FIG. 5; FIG. 8 is a flow diagram of an antenna switching procedure in FIG. 5 ; FIG. 9 is a flow diagram of an antenna switching procedure shown in FIG. 5, which differs from the one shown in FIG. 8; FIG. 10 shows how an RSSI value is measured in the PHS system; FIG. 11 is an explanatory view showing an antenna switching timing in the PHS system; FIGs. 12A through 12C show an IQ plane in D-QPSK modulation; and FIG. 13 shows one example of a reception format and a phase likelihood measurement segment in the PHS system.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The embodiments of the present invention are now described, referring to the drawings.

FIG. 4 is a functional block diagram showing one embodiment of the present invention. An antenna diversity receiver shown in FIG. 4 includes two antennas 1 and 2, an antenna switcher 3, a receiver circuit 4, a demodulator circuit 5, a received power sampling circuit 6, an AD converter 7, a control circuit 8, an RSSI selection threshold value memory 10, a current measured value register 11, a past measured value register buffer 12, and an averaged information memory 13.

The difference between the receiver shown in FIG. 4 and the referential one lies in the demodulator circuit 5 and control circuit 8. The receiver shown in FIG. 4 is also different from the referential one in that the RSSI selection threshold value memory 10 is added. The demodulator circuit 5 extracts deviation data in a phase trajectory (phase likelihood data) of a received signal and sends it to the control circuit 8, and the control circuit 8 employs the phase likelihood data along with the RSSI value for antenna diversity control. A minimum value of an RSSI selection range is selected as the threshold value between the use range of phase likelihood and the use range of RSSI and serves as an RSSI selection threshold value.

The RSSI selection threshold value is then stored in the RSSI selection threshold value memory 10. Specifically, when a current RSSI value in the received signal, measured by the receiver circuit 4, is lower than the RSSI selection threshold value, the phase likelihood, rather than the RSSI value, is used for antenna diversity control.

Referring to FIG. 10, the PHS system includes an eight-slot frame structure composed of four slots for receiving channels R1-R4 and four slots for transmitting channels T1-T4. Available as the sampling timings for the measurement of the RSSI values are three timing points of a first timing point"A", an intermediate timing point"B", and a last timing point"C"during the received electric field segment of a receiving channel R1 set at the reception timing provided by the control circuit 8. The measurement result from the intermediate timing point"B", which is expected to be a relatively accurate RSSI measurement value, is used for antenna diversity control.

Referring to FIG. 11, the antenna switching timing is set to be within a guard time"G"immediately prior to a next receiving slot.

Referring to FIGS. 12A, 12B, and 12C, the s/4-shifted DQPSK (differential quadrature phase shift keying) technique employed in the PHS system will be described now.

As shown in FIG. 12A, a DQPSK modulation wave is shifted by n/4 in four directions every 192 kHz from a certain reference point, thereby transmitting two-bit data. If the wave is ideally phase-shifted along the eight equally spaced points around the circle as shown in FIG. 12B, one of the trajectories that is formed by sequentially connecting every three points, namely, points spaced at 135 degrees is thus formed. And the other trajectory that is formed by connecting every adjacent point is thus formed.

When fading occurs, however, a range of +s/4 with respect to an ideal direction is treated as an examination area as shown in FIG. 12C, and the wave can be shifted to any point within the examination area.

When the trajectory deviates from the center at each predetermined position, the demodulator circuit 5 is expected to reproduce received data more accurately as the trajectory is closer to the center of the examination area.

When the trajectory substantially deviates from the center of the examination area, an error is probably to occur in the received data. The demodulator circuit 5 sends such a phase offset to be a phase likelihood in the form of twobit data to the control circuit 8 shown in FIG. 1, with the two-bit data ranging from"11"to"00". Specifically, bit data"11"is assigned to the central position with bit data "00"assigned to x/4 phase offsets.

Referring to FIG. 13, the received data organized in a single format is constructed of 240 bits, and of the received data, an information (I) data portion occupies 160 bits (80 symbols). The control circuit 8 counts these phase likelihood for the I portion of 160-bit data, and performs antenna diversity control based on the phase likelihood value or the already-discussed RSSI value.

The elements of the receiver shown in FIG. 4 are now described.

Under the control of the control circuit 8, the antenna switcher 3 connects one of the antennas 1 and 2 to the receiver circuit 4. The antenna 1 is here connected to the receiver circuit 4 by the antenna switcher 3.

When one communication starts in a radio coverage area, the receiver circuit 4 converts a radio frequency signal, received through the antenna 1 and the antenna switcher 3, into an intermediate frequency signal and amplifies the intermediate frequency signal. The receiver circuit 4 converts the intermediate frequency signal into a lowfrequency signal that can be handled by the demodulator circuit 5, and sends it to the demodulator circuit 5.

Meanwhile, the receiver circuit 4 converts the RSSI into-a DC voltage signal and sends it to the received power sampling circuit 6.

The demodulator circuit 5 demodulates the downconverted received signal, reproducing and outputting data and a data clock. Furthermore, as already described with reference to FIGs. 12A through 12C, the demodulator circuit 5 sends the phase likelihood data to the control circuit 8.

In response to the reception timing sent by the control circuit 8, the received power sampling circuit 6 samples the DC voltages signal corresponding to the RSSI value received from the receiver circuit 4, at the three sampling points"A","B", and"C", as already described with reference to FIG. 10, and sends the result to the AD converter 7.

The AD converter 7 analog-to-digital converts the DC voltage signal, received from the received power sampling circuit 6, into a binary code, and sends the binary code to the control circuit 8.

The control circuit 8 stores, in the current measured value register 11, the latest current RSSI value at the intermediate timing point of the three points, measured by the receiver circuit 4 and received from the AD converter 7.

The control circuit 8 stores, in the current measured value register 11, the latest accumulated phase likelihood data for I (information) portion of the 160-bit data shown in FIG. 13, received from the demodulator circuit 5.

Furthermore, the control circuit 8 selects either the RSSI value or the phase likelihood value, according to the selection threshold value stored in the RSSI selection threshold value memory 10, and sets the antenna selection criterion, and decides whether to switch the antenna in accordance with the antenna selection criterion. The detailed operation of these steps will be described later referring to a flow diagram.

The RSSI selection threshold value memory 10 beforehand stores the selection threshold value that determines whether the antenna diversity control by the control circuit 8 is performed based on the RSSI value or the phase likelihood value. Under the control of the control circuit 8, the current measured value register 11 stores the latest RSSI value and the latest phase likelihood value for each of the antennas 1 and 2. Under the control of the control circuit 8, the past measured value register buffer 12 stores a predetermined number of consecutive RSSI values and consecutive phase likelihood values, prior to the data stored in the current measured value register 11, for each of the antennas 1 and 2. The averaged information memory 13 stores the averaging results of a specified number of RSSI values and the phase likelihood values for each of the antennas 1 and 2.

The function and the antenna diversity algorithm of the control circuit 8 are described in detail, referring to both the block diagram shown in FIG. 4 and the flow diagram shown in FIG. 5.

The control circuit 8 receives the three RSSI values from the receiver circuit 4 via the received power sampling circuit 6 and the AD converter 7, at the reception timing, while receiving the phase likelihood value of the received signal from the demodulator circuit 5 (step S1). The control circuit 8 selects the intermediate one of the three received RSSI values, thereby determining the intermediate RSSI value as the current measured value. Furthermore, the control circuit 8 selects and accumulates the phase likelihood values for the I portion of 160-bit data, received from the demodulator circuit 5, thereby determining the result as the current measured value of phase likelihood (step S2).

Upon completion of the receive slot described with reference to FIG. 10, the control circuit 8 shifts the past RSSI value and the past phase likelihood value from the current measured value register 11 to the past measured value register buffer 12. And then, the control circuit 8 stores the two types of measurements determined in step S2 in the current measured value register 11 in its area reserved for the antenna 1 (step S3).

Under the control of the control circuit 8, the past measured value register buffer 12 stores a predetermined number of measurements for the RSSI and phase likelihood of the received signal in its areas respectively reserved for the antennas 1 and 2.

Having stored the measurements in step S3, the control circuit 8 compares, in magnitude, the current measured RSSI value for the antenna 1, stored in the current measured value register 11, to the RSSI selection threshold valuestored in the RSSI selection threshold value memory 10 (step S4).

When it is determined in the above comparison that the current measured RSSI value, because of a high received signal strength, is greater than the RSSI selection threshold value (i. e., YES in step S5), the control circuit 8 sets a predetermined RSSI criterion (step S6). The criterion setting procedure will be described later referring to FIG. 6.

The control circuit 8 compares the current measured RSSI value to the RSSI criterion set in step S6 (step S7).

When it is determined in the comparison that the current measured value is even smaller than a value that is obtained by subtracting a predetermined value from the RSSI criterion, the control circuit 8 selects the antenna 2, deselecting the currently connected antenna 1. Accordingly, the control circuit 8 controls the antenna switcher 3 to replace the connection of the receiver circuit 4 from the antenna 1 to the antenna 2. When this antenna switching condition is not met, no antenna switching is performed, and a predetermined antenna selection and switching step (step S8) is carried out. The specific operation for this step will be described later, referring to FIG. 8.

In the antenna diversity control, as described above, the selection and switching operation is carried out to switch from the antenna 1 to the other antenna 2, when the antenna 1 connected to the receiver circuit 4 provides a received signal having an amplitude smaller than the predetermined required value.

To switch the antennas based on the RSSI value, the level of the RSSI value needs to be determined. The determination of the level of the RSSI of the received signal is subjected to an error, because the phase shift varies in every receive slot. To preclude such an error, the RSSI values are accumulated and averaged throughout a predetermined number of frames, and the averaged RSSI value is used as the criterion. The same is true of the phase likelihood. In this embodiment, the averaged values are used as the criteria.

Upon completion of the antenna selection and switching operation in step S8, the control circuit 8 accumulates the RSSI values and the phase likelihood data for the antenna 1 currently measured, stored in the current measured value register 11 and the past measured value register buffer 12 (step S9). When the number of measurements is smaller than a predetermined number (i. e., NO in step S10), the process ends. If the number of measurements reaches the predetermined number, i. e., if YES in step S10, the control circuit 8 performs an averaging process on the predetermined number of measurements (step S11). Then, the control circuit 8 stores the averaged information of the measurements in the averaged information memory 13 and sets an average end flag (step S12). The process here ends.

When the answer in step S5 is NO, i. e., when it is determined in step S5 that the current measured RSSI value is not greater than the RSSI selection threshold value, the control circuit 8 employs the phase likelihood instead of the RSSI value for antenna diversity control. The control circuit 8 sets a predetermined phase likelihood criterion (step S21). The criterion setting procedure will be discussed later, referring to FIG. 7.

In succession, the control circuit 8 compares the current measured phase likelihood value to the phase likelihood criterion set in step S21 (step S22). When it is determined in the comparison that the current measured value is smaller than a value that is obtained by subtracting a predetermined value from the phase likelihood criterion, the control circuit 8 selects the antenna 2, deselecting the antenna 1. Accordingly, the control circuit 8 controls the antenna switcher 3 to replace the connection of the receiver circuit 4 from the antenna 1 to the antenna 2. When this antenna switching condition is not met, a predetermined antenna selection and switching operation not to switch the antenna (step S23) is carried out. The specific operation for this step will be discussed later, referring to FIG. 8.

After the step S23, the control circuit 8 goes to step S9. Accordingly, the control circuit 8 accumulates the current measured values information memory 13, and sets the average to the criterion (step S33). When no average end flag is present with NO in step S32, the control circuit 8 searches for the past measured RSSI value last stored in the past measured value register buffer 12 (step S34). When the past measured RSSI value is present (i. e., YES in step S35), the control circuit 8 reads the past measured RSSI value and sets the past measured RSSI value to the criterion (step S36). When no past measured RSSI value is present with NO in step S35, the control circuit 8 reads the current measured RSSI value from the current measured value register 11, and sets the current measured RSSI value to the criterion (step S37).

The RSSI criterion thus set is compared to the current measured RSSI value in the above step S7.

The phase likelihood criterion setting procedure in the above step S21 is now described, referring to a flow diagram shown in FIG. 7, and FIGS. 4 and 5.

When the phase likelihood is employed for diversity control with NO in the above step S5, the control circuit 8 checks the averaged information memory 13 for the phase likelihood average end flag for the antenna 1 connected to the receiver circuit 4 (step S41).

When the average end flag is present (i. e., YES in step S42), the control circuit 8 reads the average for the currently connected antenna 1 from the averaged information memory 13, and sets the average to the criterion (step S43).

When no average end flag is present with NO in step S42, the control circuit 8 searches for the past measured phase likelihood value last stored in the past measured value register buffer 12 (step S44). When the past measured phase likelihood value is present (i. e., YES in step S45), the control circuit 8 reads the past measured phase likelihood value and sets the past measured phase likelihood value to the criterion (step S46). When no past measured phase likelihood value is present with NO in step S45, the control circuit 8 reads the current measured phase likelihood value from the current measured value register 11 and sets the current measured phase likelihood value to the criterion (step S47).

The phase likelihood criterion thus set is compared to the current measured phase likelihood value in the above step S22.

The antenna selection and switching procedure in the above steps S8 and S23 is now described in more detail, referring to a flow diagram shown in FIG. 8, along both with FIG. 4 and FIG. 5.

The RSSI value and the phase likelihood value are respectively compared to their respective criteria in the above step S7 or S22. When the current measured value is smaller than the criterion (i. e., YES in step S51), the control circuit 8 determines the difference between the criterion and the current measured value (step S52), and compares the difference to a value predetermined for the RSSI or the phase likelihood (step S53). When it is determined in the comparison that the difference is greater than the predetermined value (i. e., YES in step S54), in other words, when a value of interest is smaller than a required value, the connection of the receiver circuit 4 is changed to the antenna 2, from the currently connected antenna 1 being measured (step S55).

The receiver thus receives a signal under a favorable condition with one antenna switched to another antenna presenting a measured value, which is at least better than the required value.

When the current measured value is equal to or greater than the criterion with NO in step S51, or when the difference is equal to or smaller than the predetermined value with NO in step S54, the control circuit 8 selects the currently connected antenna 1 with no antenna reconnection being involved. The predetermined value is set for each of the RSSI and the phase likelihood.

In the above description, the antenna switching condition is checked at each receiving frame. When the radio wave reception status is poor, causing all antennas to meet the antenna switching condition, the antennas are switched at each receiving frame.

To prevent the antenna switching at each receiving frame, the control circuit 8 includes a switching counter for antenna switching as shown in FIG. 4. The switching counter defaults to its initial value of a small count when the antenna is switched, and is subtracted by one at each receiving frame reception. The antenna switching is not performed at least for a duration of time corresponding to the initial count times of the receiving frame.

An embodiment having the function of precluding the antenna switching at each receiving frame is now described, referring to FIG. 9.

The RSSI value and the phase likelihood value are respectively compared to their respective criteria in the above steps S7 and S22. When the current measured value is smaller than the criterion (i. e., YES in step S61), the control circuit 8 determines a difference between the criterion and the current measured value (step S62), and compares the difference to a value predetermined for the RSSI or the phase likelihood (step S63). When it is determined in the comparison that the difference is greater than the predetermined value (i. e., YES in step 64), in other words, when a value of interest is smaller than a required value, the control circuit 8 checks the switching counter (step S65).

When the count of the switching counter is zero (i. e., YES in step S66), the control circuit 8 selects the antenna 2, deselecting the currently connected antenna 1 being measured, and reconnects to the antenna 2 (step S67), while defaulting the switching counter to the initial value (step S68).

When the current measured value is equal to or greater than the criterion with NO in step S61, or when the difference is equal to or smaller than the predetermined value with NO in step S64, or when the count of the switching counter is not zero with NO in step S66, the control circuit 8 selects the currently connected antenna 1 with no antenna reconnection involved. The control circuit 8 subtracts the count of the switching counter by one and ends the process (step S69).

With this arrangement, the receiver stays on the same antenna, with no antenna switching performed, at least for the initial count times of the receiving frame. A reliable and excellent reception performance is expected even under a slow fading environment.

The functional blocks and operational procedure of the antenna diversity receiver have been discussed. Optionally, a change is possible in the arrangement of the functional blocks, for example, a function block may be split into several blocks or a plurality of blocks may be integrated into one block, as long as the function of each block is performed. A change is also possible in the operation procedure, for example, the order of several steps may be reversed or several steps may be concurrently performed in parallel. The above description is not intended to limit the scope of the present invention.

In accordance with the present invention, a reception error due to a noise component overlapping a received signal is avoided. The reason for this is that a diversity gain is improved by accounting for the phase likelihood, free from the noise component overlapping the received signal, besides the referential RSSI, in the determination of the antenna selection.

The present invention is particularly advantageous when a low-level signal is received. The reason is that the switching threshold value between the use of the RSSI and the use of the phase likelihood is set to be the minimum value of the RSSI range in use.

The reception performance is expected to be good and reliable with a low-level received signal under a slow fading environment. The reason is that the use of the switching counter precludes frequent antenna switching actions.

Claims

1. An antenna diversity switching method of switching a plurality of antennas from one to another, comprising: measuring an RSSI value and a phase likelihood value of a current one of the antenna which is currently being selected and used; selecting either one of the measured RSSI and the measured phase likelihood on the basis of a predetermined selection threshold value; and switching from the current antenna to another antenna if the selected one value satisfies a predetermined antenna switching condition different from respective antenna diversity switching criteria when the measured selected value is compared with the above respective antenna diversity switching criteria.

2. An antenna diversity switching method according to Claim 1, wherein the RSSI value is first selected in the determination of antenna diversity switching and the phase likelihood value is selected when the RSSI value is lower than a level based on the predetermined selection threshold value.

3. An antenna diversity switching method according to Claim 1, wherein the antenna switching condition for switching from the first selected antenna to another antenna is satisfied when a current measured value is lower than the antenna diversity switching criterion, and wherein the antenna diversity switching criterion is an average of measured values when a predetermined number of measured values are present, or a past measured value when no average value is present, or a current measured value when no past measured value is present.

4. An antenna diversity switching method according to Claim 1, wherein once antenna switching is performed, a next antenna switching is suspended until obtainment of a predetermined number of measured values, subsequent to the antenna switching.

5. An antenna diversity receiver for switching from a first antenna, selected from among a plurality of antennas, to another antenna when a predetermined condition is met in the determination of antenna diversity switching, said antenna diversity receiver comprising: an antenna switcher for selecting and connecting a selected one antenna from among a plurality of antennas ; a receiver circuit for converting a received signal from said connected antenna through said antenna switcher in low frequency to send out a frequency converted signal, together with an RSSI; a demodulator circuit for demodulating the frequency converted signal to send out a demodulated signal, together with a phase likelihood; a control circuit for switching a currently connected antenna to another antenna under control of said antenna switcher when a predetermined condition is met in either one of the RSSI and the phase likelihood obtained from said currently connected antenna; wherein said control circuit receives both an RSSI value and a phase likelihood value measured through said currently connected antenna, selects either one of the measured RSSI and the measured likelihood on the basis of a predetermined selection threshold value, and switches from the current antenna to another antenna if the selected one value satisfies a predetermined antenna switching condition when the measured value is compared with respective antenna diversity switching criteria.

6. An antenna diversity receiver according to Claim 5, wherein said control circuit sets a minimum value of RSSI within the selection range of the RSSI to be used for antenna diversity, to the selection threshold value for the first selection of said antenna, and performs antenna selection based on the phase likelihood value when a current measured RSSI value is smaller than the minimum value.

7. An antenna diversity receiver according to Claim 5, wherein said control circuit sets a minimum value of RSSI within the selection range of the RSSI to be used for antenna diversity, to the selection threshold value for said currently connected antenna, selects a current measured phase likelihood value when a current measured RSSI value is not more than the selection threshold value, compares the current measured phase likelihood value to a predetermined criterion, and switches from the currently connected antenna to another antenna when the current measured likelihood value is smaller than a predetermined value that is set to be smaller than the criterion.

8. An antenna diversity receiver according to Claim 7, wherein said control circuit adopts, as the criterion to be compared to the current measured value, an average of measured values when a predetermined number of measured values are present, or a past measured value when no average value is present, or a current measured value when no past measured value is present.

9. An antenna diversity receiver according to Claim 5, wherein said control circuit comprises a switching counter which defaults to an initial value as a count and is subtracted by one each time current measured value is input, wherein once an antenna switching is made, the switching counter suspends a next antenna switching until obtainment of a predetermined number of measured values, subsequent to the antenna switching.