Mobile communication system, and a radio base station, a radio apparatus and a mobile terminal

A mobile communication system comprises a detecting unit to detect information concerning a moving speed of a mobile terminal, and a selection controlling unit to select a use frequency in a higher frequency band when the speed information detected by the detecting unit is a higher speed, while selecting the use frequency in a lower frequency band when the detected information is a lower speed, and assigning it to the mobile terminal. In a mobile communication system in which a relationship between a terminal moving speed (Doppler frequency) and transmission quality degradation is non-monotonous, the communication quality can be improved and the channel capacity can be increased.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention relates to a mobile communication system, and a radio station and a mobile terminal. Particularly, the present invention relates to a technique suitable for use in mobile communications in which communication is carried out in code division multiple access (CDMA).

(2) Description of Related Art

CDMA is widely used as a mobile communication system. Frequency division multiple access (FDMA) or time division multiplex access (TDMA) is basically a system operated under conditions without interference between subscribers since it assigns resources (frequencies, time or the like) orthogonal to one another to subscribers (mobile terminals such as portable telephones or the like). On the other hand, CDMA is operated under conditions that subscribers are interfered with each other, thus being a system the channel capacity of which is expected to be improved by the statistical multiplexing effect.

Since the CDMA communication system is operated under conditions that signals of subscribers are interfered with each other as above, transmission power control (TPC) or forward error correction (FEC) or the like is used to control a transmitting signal power of each subscriber to be the minimum requirement, minimize the interference between them, thereby to maximize the channel capacity.

It is said that the channel capacity of a CDMA communication system is generally calculated by the following equation (1) (reference 1; A. Viterbi, CDMA Principles of Speed Spectrum Communication Addison-Wesley, (1995)):

In the above equation (1), N is a channel capacity (ch/sector/RF), pg is a processing gain, Eb/No_th is a required Eb/No, fs is a self-cell interference ratio, fo is an other-cell interference ratio, d is a voice activity factor, Dt is degradation due to a transmitting power control error, Gs is a sectorization effect, Ns is the number of sectors/cells, and Lf is a loading factor. Incidentally, the channel capacity is considered using the following equation (2), omitting parameters not relating to the discussion:

In the equation (2), K represents a factor of an effect of other parameter.

The equation (2) signifies that when a signal-to-(interference+noise)power ratio (required Eb/No) required to satisfy desired communication quality (hereinafter also referred as transmission quality) increases, a transmission power of each station increases, thus interference is increases, which leads to a decrease in channel capacity N.

CDMA is a system that optimizes the communication quality and the channel capacity by TPC and FEC. However, CDMA has a disadvantage that the required Eb/No is degraded at a specific terminal moving speed because of a difference in response speed between TPC and FEC (reference 2; R. Padovani, Reverse Link Performance of IS-95 Based Cellular Systems IEEE Personal Communications, Third Quarter, (1994)).

This phenomenon occurs as follows:

In the mobile communication, there occurs fading (Reyleigh fading) that propagation loss fluctuates due to mainly interference between reflections from objects around the terminal, which fluctuates at a speed according to a moving speed of the terminal. The fluctuating speed of Reyleigh fading is characterized by a Doppler frequency fD due to movement of the terminal as shown by the following equation (3) (reference 3; Okumura, Shinshi, “Fundamentals of Mobile Communications”, The Institute of Electronics Information and Communication Engineers, (1986)):
fD≈fR×VM/C(3)
where fR is a radio frequency, VM is a terminal moving speed and C is the speed of light.

It is seen that higher the radio frequency fR and faster the terminal moving speed VM, faster is the fluctuating speed of Reyleigh fading. Since the response speed is generally slow in TPC, it is possible to follow the propagation loss fluctuation due to fading to control the transmitting power when the Reyleigh fading fluctuation is slow (that is, when the Doppler frequency fD is low), thus degradation of the communication quality can be prevented. However, when the Doppler frequency (that is, the fading frequency) fD is high, the communication quality is degraded since TPC cannot follow the fading fluctuation.

On the other hand, it is possible to disperse an effect of burst signal power reduction due to fading by combining FEC with interleaving (IL). However, when the fading speed is slow, there occurs signal power reduction for such a long time that the signal power reduction cannot be corrected since the IL cycle is finite, which leads to degradation of the communication quality.

For this, there occurs a phenomenon that noticeable reduction of the communication quality at the intermediate frequency fD between the Doppler frequency fD at which an effect of improvement of the communication quantity by TPC is obtained and the Doppler frequency fD at which an effect of improvement by FEC is obtained (refer toFIG. 13and the reference2).FIG. 13shows Eb/No required to satisfy the frame error rate (FER)=1% in a reverse link (mobile terminal to base station, 9.6 kbps) in an IS-95 system. When the radio frequency=850 MHz, fD=40 Hz corresponds to a mobile terminal moving speed=50.8 km/h, for example.

Since the terminal moving speed cannot be defined uniquely, the system has to be designed at the Doppler frequency at which the quality degradation is maximum [namely, the required Eb/No is maximum (in the example inFIG. 13, fD=47 Hz and the required Eb/No=6.1 dB)]. However, a large required Eb/No degrades the channel capacity N as expressed by the above equation (2). On the other hand, when the channel capacity N is secured, the required Eb/No cannot be satisfied, which leads to degradation of the communication quality.

In a system such as TDMA or FDMA in which the transmission quality monotonically degrades when the fading pitch (fD) increases, an attempt is made to prevent large degradation of the transmission quality of a specific terminal to equalize the communication quality in the entire system, thereby improving the overall performance, as disclosed in, for example, Japanese Patent Laid-Open Publication No. 5-259969.

In concrete, the technique disclosed in Japanese Patent Laid-Open Publication No. 5-259969 (hereinafter referred as known technique 1) accomplishes the above effect by a simple control based on only magnitude of the terminal moving speed on the assumption of monotonousness of the Doppler frequency fD and the transmission quality degradation that degradation of the transmission quality decreases when the fading pitch becomes smaller. However, such a simple control can provide only a small effect when degradation of the transmission quality to the Doppler frequency is non-monotonous as in CDMA (when there is employed a communication system having a characteristic that a required signal-to-noise power ratio of a received signal in a mobile terminal changes from a tendency to increase to a tendency to decrease according to the moving speed of the mobile terminal) which sometimes causes degradation of the communication quality in some cases.

Namely, the known technique 1 tries to assure the communication quality of a control channel by assigning a low frequency band to the control channel such that fD (∝(moving speed)×(radio frequency)) of the control channel required a high communication quality becomes as low as possible. However, when the Doppler frequency fD is lowered in a system such as CDMA in which degradation of the transmission quality to the Doppler frequency fD is non-monotonous in the similar manner, there is a case where the communication quality degrades as with the case where FD>42 Hz inFIG. 13, for example.

The known technique 1 accommodates traffic of a high-speed mobile terminal in a low radio frequency band and a low-speed moving terminal in a high radio frequency band to equalize the Doppler frequency fD (∝(moving speed)×(radio frequency)) as much as possible, thereby homogenizing the transmission quality to improve the overall performance.

In this method, when a radio frequency band in which the Doppler frequency brings about the worst value of the transmission quality (fD=42 Hz in the example inFIG. 13) is assigned, the transmission quality of the both terminals becomes the worst, thus the overall performance becomes the worst. The known technique 1 cannot prevent this.

The above problem is caused by that the known technique 1 performs a simple control based on only magnitude of the terminal moving speed on the assumption of monotonousness of the Doppler frequency fD and the transmission quality degradation.

In consideration of assignment of frequencies in a CDMA communication system, there is a technique disclosed in, for example, Japanese Patent Laid-Open Publication No. 10-23502 (hereinafter referred as known technique 2) as a known technique. An objective of the known technique 2 is to equalize the number of terminals to be accommodated in each radio frequency band (strictly, the number of radio channels to be used) in order to secure a channel for soft hand-off in the CDMA communication system.

However, the known technique 2 does not consider non-monotonousness or the like of the transmission quality to the terminal moving speed and the Doppler frequency fD, but assigns a radio frequency band without considering the terminal moving speed. For this, when the Doppler frequency fD brings about the worst value of the transmission quality at the assigned frequency (fD=42 Hz in the example in FIG.13), large degradation occurs.

SUMMARY OF THE INVENTION

In the light of the above problems, an object of the present invention is to improve communication quality and increase a channel capacity in a mobile communication system in which a relationship between a terminal moving speed (Doppler frequency) and transmission quality degradation is non-monotonous.

The present invention therefore provides a mobile communication system comprising a detecting unit to detect information concerning a moving speed of the mobile terminal (hereinafter referred as a terminal moving speed) on the basis of a received signal from the mobile terminal, and a selection controlling unit to select the use frequency in a higher radio frequency band when the information detected by the detecting unit is higher, selecting the use frequency in a lower radio frequency band when the information is lower, and assigning the selected use frequency to the communication between the mobile terminal and the radio base station.

A radio base station according to this invention for realizing the above mobile communication system comprises:(1) a radio communicating unit being able to communicate with the mobile terminal using any one of M (M being an integer not less than two) radio frequency bands;(2) a speed information detecting unit to detect information concerning a moving speed of the mobile terminal (herein after referred as speed information) on the basis of a received signal from the mobile terminal received by the radio communicating unit; and(3) a use frequency selection controlling unit to select the use frequency in a higher radio frequency band when the speed information detected by the speed information detecting unit is higher, select the use frequency in a lower radio frequency band when the speed information is lower, and assign the selected use frequency to the communication with the mobile terminal.

By providing the above units to the radio base station, it is possible to switch the use radio frequency band at specific terminal moving speed information to improve the worst value of the required signal-to-noise power ratio.

A radio apparatus according to this invention being able to use both a frequency belonging to a first frequency band and a frequency belonging to a second frequency band higher than the first frequency band for communication on forward and reverse links with a mobile terminal, the radio apparatus comprising:

(1) a transmitting unit to convert a signal obtained by error-correction-encoding and interleave transmitting data into a radio signal, and transmit the radio signal for communication on the forward link to the mobile terminal;

(2) a transmitting power controlling unit to control a transmitting power of the radio signal for communication on the forward link on the basis of a received signal from the mobile terminal; and

(3) a selection controlling unit to use a frequency belonging to the second frequency band for communication with the mobile terminal when determining that a fading cycle of the received signal from the mobile terminal or a moving speed of the mobile terminal is fast, use a frequency belonging to the first frequency band for communication with the mobile terminal when determining that the fading cycle or the moving speed of the mobile terminal is slow.

A radio apparatus according to this invention being able to use both a frequency belonging to a first frequency band and a frequency belonging to a second frequency band higher than the first frequency band for communication on forward and reverse links with a mobile terminal, the radio apparatus comprising:

(1) a transmitting unit to convert a signal obtained by encoding and interleaving transmitting data into a radio signal, and transmit the radio signal for communication on the forward link to the mobile terminal;

2) a transmitting power controlling unit to control a transmitting power of the radio signal for communication on the forward link on the basis of a received signal from the mobile terminal; and

3) a selection controlling unit to use a frequency belonging to the second frequency band in communication with the mobile terminal when determining on the basis of the received signal from the mobile terminal that a fading cycle in a received signal on the forward link received by the mobile terminal or a moving speed of the mobile terminal is fast, use a frequency belonging to the first frequency band in communication with the mobile terminal when determining that the fading cycle or the moving speed of the mobile terminal is slow.

A reference to determine whether a speed of the mobile terminal is fast or slow is determined on the basis of a required signal-to-noise power ratio of each of the above frequency bands. For example, when a communication system having a characteristic that the required signal-to-noise power ratio of a received signal in the terminal changes from a tendency to increase to a tendency to decrease according to a moving speed of the mobile terminal is employed, a terminal moving speed satisfying the required signal-to-noise power ratio of each frequency band may be the reference.

Since the above Doppler frequency fD is expressed by the above equation (3), the Doppler frequency fD changes according to the use radio frequency even at the same terminal moving speed, thus the required signal-to-noise power ratio changes. Therefore, a terminal moving speed at which the required signal-to-noise power ratio is maximum differs from use radio frequency to use radio frequency. However, the use radio frequency band is switched at a specific terminal moving speed as a boundary, the worst value of the required signal-to-noise power ratio can be improved.

Since the channel capacity of mobile communication depends on not only a required signal-to-noise power ratio but also interference power information such as a self-cell interference ratio (fs), an other-cell interference ratio (fo) or the like as shown by the equation (2), it is desirable that a selection (switching) reference (threshold value information) for the use radio frequency band is determined, adding interference power information to the communication with the mobile terminal.

In such case, if the above interference power information is determined on the basis of a signal transmission characteristic of each of the above radio frequency bands, it is possible to determine the above reference (threshold value information), considering fluctuation in radio wave propagation loss and the like, for example.

A mobile terminal according to this invention for realizing the above mobile communication system comprising:

1) a radio communicating unit being able to communicate with the radio base station using any one of M (M being an integer not less than two) radio frequency bands;

2) a selected frequency notification signal receiving unit to receive, from the radio communicating unit, a selected frequency notification signal for notifying of a use frequency selected among higher radio frequency bands in the radio base station when speed information of its own is faster or selected among lower radio frequency bands when the speed information of its own is slower; and

3) a use frequency selection controlling unit to select a radio frequency to be used in the radio communicating unit among the radio frequency bands according to the selected frequency notification signal received by the selected frequency notification signal receiving unit.

With the above structure, the mobile terminal according to this invention selects a radio frequency to be used in the communication with the radio base station in each of the above radio frequency bands according to the selected frequency notification signal for notifying of the radio frequency selected as above in the radio base station, thereby improving the worst value of the required signal-to-noise power ratio and the channel capacity in the whole system.

The present invention provides the following advantages and effects:

1) As a radio frequency to be used (assigned) in the communication between the radio base station (radio apparatus) and the mobile terminal, the use frequency is selected in a higher radio frequency band when the speed information concerning a moving speed of the mobile terminal is higher, while the use frequency is selected in a lower radio frequency band when the speed information is lower. It is thus possible to improve the worst value of the required signal-to-noise power ratio in a mobile communication system in which a relationship between the Doppler frequency and transmission quality degradation is non-monotonous (employing a communication system having a characteristic that the required signal-to-noise power ratio of a received signal in the mobile terminal changes from a tendency to increase to a tendency to decrease according to a moving speed of the mobile terminal), improve the communication quality, and increase the channel capacity.

2) When the threshold value information for the speed information that is a reference (boundary) between the above higher speed and lower speed is determined adding interference power information to the communication with the mobile terminal, threshold value information meeting actual communication environments can be obtained, which leads to further improvement of the communication quality and channel capacity.

3) If the above interference power information is determined on the basis of a signal transmission characteristic of each of plural radio frequency bands, it is possible to determine the above threshold value in consideration of fluctuation in radio wave propagation loss and the like. Accordingly, improvement of the worst value of the required signal-to-noise power ratio may bring more effects of improvement of the channel capacity.

4) The above interference power information may be calculated on the basis of information on the number of mobile terminals presently in communication and an actual measured value of the received signal-to-noise power. In such case, even if the interference power information fluctuates due to fluctuation in system operational environments, fluctuation in base station installation conditions or fluctuation in traffic with time, optimal threshold value information can be re-calculated following the fluctuation. It is therefore possible to appropriately switch the frequency in consideration of fluctuation in actual system operational environments, in base station installation conditions, in traffic with time and the like. Accordingly, a large improvement of the communication quality and a large increase in channel capacity are expected.

5) The above threshold value information may be prepared (2×M−1) pieces, and the use radio frequency may be selected on the basis of information on which range of the threshold value information speed information on the mobile terminal falls in, and priority information defining which radio frequency band should be used in each of the plural terminal speed ranges defined by each piece of the threshold value information. In such case, it is possible to alleviate concentration of calls in a specific radio frequency band without causing degradation of the worst value of the required signal-to-noise power ratio.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, description will be made of embodiments of this invention with reference to the drawings.

(A) Description of an Embodiment

FIG. 1is a block diagram showing a structure of a CDMA communication system (mobile communication system) according to an embodiment of this invention. The CDMA communication system shown inFIG. 1comprises at least one radio base station apparatus (radio apparatus; hereinafter referred simply as a base station)1, and at least one mobile terminal apparatus (hereinafter referred simply as a terminal)2. The base station1can make a two-way radio communication in CDMA system with a terminal (in-zone terminal) present in its own communicable area (cell).

(A1) Base Station Structure

In concrete, the base station1according to this embodiment comprises, when attention is paid to its essential parts, a multiplexing unit11, a change-over switch12, a CDMA transmitting unit13afor fR1band (first frequency band) and a CDMA transmitting unit13bfor fR2band (second frequency band) as a transmitting system, a CDMA receiving unit16afor fR1band, a CDMA receiving unit16bfor fR2band, a change-over switch17and a demultiplexing unit18as a receiving system, as shown in, for example,FIG. 1. The base station1further comprises a change-over switch19, a Doppler frequency (fD) measuring unit20A, a terminal moving speed estimating unit20B, a switching speed threshold value holding unit20C and a frequency band switching controlling unit20D as a control system20.

Although not shown inFIG. 1, the base station1has a TPC function for performing TPC (transmitting power control) on the forward link (radio signals for forward communication) on the basis of a received signal from the terminal2. InFIG. 1, reference characters14aand14bdenote hybrids (H),15aand15bdenote transmitting/receiving antennae for fR1and fR2band, respectively, which are shared by the above transmitting system and the receiving system.

Namely, a transmitting signal from the CDMA transmitting unit for fR1band (fR2band)13a(13b) is transmitted toward the terminal2from the transmitting/receiving antenna15a(15b) through the hybrid14a(14b). On the other hand, a radio signal in the fR1band (the fR2band) from the terminal2is received by the transmitting/receiving antenna15a(15b), and outputted to the CDMA receiving unit for fR1band (fR2band)16a(16b) by the hybrid14a(14b).

Namely, the CDMA transmitting units13aand13b, the hybrids14aand14b, the CDMA receiving units16aand16b, and the transmitting/receiving antennae15aand15bfunction as a radio communicating unit, which can communicate with the terminal2using either one of M=2 types of radio frequency bands (the fR1band, the fR2band).

(A1.1) Transmitting System Structure

In the above transmitting system, the multiplexing unit (control signal adding unit)11multiplexes (adds) transmitting data to the terminal2and a control signal (a selected frequency notification signal, a switching timing instruction signal or the like) from the frequency band switching controlling unit20D. The change-over switch12switches its output according to a switching controlling signal from the frequency band switching controlling unit20D to supply the output (a transmitting signal) from the multiplexing unit11to either the CDMA transmitting unit for fR1band13aor the CDMA transmitting unit for fR2band13b.

The CDMA transmitting unit13afor fR1band outputs the transmitting signal from the change-over switch12as a signal in a specific frequency band (the fR1band). The CDMA transmitting unit13bfor fR2band outputs the transmitting signal from the change-over switch12as a signal in another frequency band (the fR2band) differing from the above fR1band. Incidentally, a relationship in height between fR1(the first frequency band) and fR2(the second frequency band) is fR2>fR1in this embodiment; the fR1band=850-MHz band, and the fR2band=2-GHz band, for example.

CDMA Transmitting Unit Structure

Each of the above CDMA transmitting units13aand13bhas a common structure in view of hardware, comprising traffic channel transmission processing units131equal in number to traffic channels, a multiplexing unit (MUX)132, a scrambler (multiplier)133, a digital filter134, a DA converter (DAC: Digital-to-Analog Converter)135, an orthogonal modulator136, an RF (Radio Frequency) oscillator for forward link (base station1to terminal2)137, a filter138, a high-power amplifier (HPA)139and the like. An oscillated frequency of the above RF oscillator137is changed by each of the CDMA transmitting units13aand13b, whereby the CDMA transmitting unit13acan generate a transmitting signal in the fR1band, while the CDMA transmitting unit13bcan generate a transmitting signal in the fR2band.

In concrete, a transmitting signal [(transmitting data)+(control signal)] is undergone necessary FEC (error correction) coding and interleaving process in an FEC encoder131aand an interleaver131b, multiplied by a channelisation code such as a Walsh code or the like in a multiplier131cto be a channelisation signal, and multiplexed on a pilot channel signal and a control channel signal in the multiplexing unit132.

The multiplexed signal obtained as above is scrambled (spectrum-spread-modulated) with a scrambling code that is a PN (pseudo noise) code (chip rate=1.2288 Mcps) in the scrambler133, filtered (removed unnecessary signal components thereof) in the digital filter134, DA-converted in the DA converter135, undergone orthogonal-modulation such as QPSK or the like with an RF signal from the RF oscillator137in the orthogonal modulator136, finally outputted to the hybrid14aor14bshown inFIG. 1through the filter138and the HPA139to be transmitted toward the terminal2from the transmitting/receiving antenna15aor15b.

In the structure shown inFIG. 2, the transmitting signal is modulated in the RF band. However, various manners are possible. For example, the transmitting signal may be modulated in the intermediate frequency (IF) band, then frequency-converted to the RF band (up-converted).

(A1.2) Receiving System Structure

InFIG. 1, the CDMA receiving unit for fR1band16aperforms a necessary demodulating process on a received signal in the fR1band from the terminal2inputted through the hybrid14a. The CDMA receiving unit for fR2band16bperforms a necessary demodulating process on a received signal in the fR2band from the terminal2inputted through the hybrid14b.

CDMA Receiving Unit Structure

Each of the CDMA receiving units16aand16baccording to this embodiment comprises a low noise amplifier (LNA)161, a filter162, traffic channel reception processing units163equal in number to the traffic channels, and the like, as shown in, for example,FIG. 3. Each of the traffic channel receiving units163comprises an orthogonal detector163a, an RF oscillator for reverse link (terminal2to base station1)163b, an AD converter (ADC: Analog-to-Digital Converter)163c, a descrambler (multiplier)163d, a multiplier163e, an M-Array correlating detector163f, a deinterleaver163g, an error correction decoder163h, and the like. Like a relationship between the CDMA transmitting units13aand13b, an oscillated frequency of the above RF oscillator163bis changed by each of the CDMA receiving units16aand16b, whereby the CDMA receiving unit16acan process a received signal in the fR1band, while the CDMA receiving unit16bcan process a received signal in the fR2band.

In concrete, the received signal from the terminal2is amplified and filtered in the LNA161and the filter162, then inputted to each of the traffic channel reception processing units163.

In each of the traffic channel reception processing units163, the inputted signal (received signal) is orthogonal-detected with the RF signal from the RF oscillator163bin the orthogonal detector163a, AD-converted in the AD converter163c, and undergone a descrambling (despreading) process with a PN code and a channel demodulating process with a channelisation code in the descrambler163dand the multiplier163e.

Thereafter, the received signal is undergone an M-Array correlating detecting process in the M-Array correlating detector163f, undergone a deinterleaving process and an error correction decoding process in the deinterleaver163gand the error correction decoder163h, and outputted to the demultiplexing unit18through the above change-over switch17.

Here, description will be made of the above M-Array correlating detecting process.

In a land mobile communication system such as a cellular system, the amplitude or phase of a transmitting signal rapidly fluctuates due to an effect of fading, so that it is impossible to perform coherent detection to regenerate a reference phase from the received signal and demodulate it. For this reason, a CDMA communication system in conformity with IS-95transmits a pilot channel on the forward link, the receiving side (terminal2) performs coherent detection with the pilot channel as the reference phase, while adopting the M-Array correlating detection system on the reverse link.

Incidentally, although the IS-95 system uses the M-Array correlating detecting system with Walsh codes having a length of 64 bits, here is described a case in which Walsh codes having a length of 16 bits (16 to 4-bit M-Array) are used as shown in the following table 1 will be here described, for the sake of simplicity.

It is clearly seen from Table 1 that eight bits in each code differ from the others in other code (code distance=8). The transmitting side (terminal2) collects four bits of data to be transmitted (interleaved output), and selects a Walsh code corresponding to the binary digit value. For instance, when the interleaved output is “1011”, a code number =W11 (0110 0110 1001 1001) is selected (16 to 4-bit M-Array encoding), and transmitted.

On the other hand, the receiving side (base station1) collects every 16 bits of received data, and compares them with each of all the Walsh codes (W0–w15) to determine whether they coincide with each other. The receiving side determines that a Walsh code having the largest number of coinciding bits is a code actually transmitted, and outputs its number (binary number four bits) as a result of correlation detection. For instance, assuming that an error of three bits occurs while a Walsh code (0110 0110 1001 1001) transmitted from the terminal2, and the base station1receives a code (0100 0111 1001 1000).

When this code is compared with each of the code numbers W0–w15, the results are as shown in the following table 2.

As seen from Table 2, the code number W11 has the largest number (13) of coinciding bits, so that a code number W11 is selected, and “1011” is outputted as a result of the correlation detection. In this case, since a code distance of the Walsh codes is eight, it is possible to accurately detect even if an error of up to three bits occurs in 16 bits.

The actual IS-95 system uses Walsh codes having a length of 64 bits, thus an input of the correlating detector163fis 64 bits, while an output of the same is six bits (26=64) (namely, 64 to 6-bit M-Array), as shown inFIG. 3. In this case, since the code distance of a 64-bit Walsh code is 32, it is possible to decode even if an error of up to 15 bits occurs in 64 bits.

In the practical system, a process for extracting soft decision information for error correction decoding is often added. In CDMA 2000 or the like, coherent detection using a pilot channel is performed even on the reverse link.

In the change-over switch17inFIG. 1, an input thereof is switched according to a switching controlling signal from the frequency band switching controlling unit20D, whereby the change-over switch17outputs either one of outputs from the above CDMA receiving units16aand16b(outputs from the error correction decoders163h) to the demultiplexing unit18. The demultiplexing unit (confirmation signal extracting unit)18demultiplexes the output (received signal) from the change-over switch17into received data and a control signal (reception confirmation signal in response to a selected frequency notification signal, a confirmation signal in response to a switching timing instruction signal, or the like to be described later) from the terminal2. Incidentally, the control signal from the terminal2is supplied to the frequency band switching controlling unit20D (switching timing instruction signal generating unit to be described later).

(A1.3) Control System Structure

In the change-over switch19, an input thereof is switched according to a switching control signal from the frequency band switching controlling unit20D, whereby the change-over switch19supplies either one of outputs of information useful to measure (detect) the Doppler frequency fD obtained in the CDMA receiving units16aor16bto the Doppler frequency measuring unit20A. The Doppler frequency measuring unit (speed information detecting unit; detecting unit)20A measures (detects) the Doppler frequency fD (information on a moving speed of the terminal2) of a received signal from the received signal inputted from the change-over switch19.

The terminal moving speed estimating unit20B estimates a moving speed of the terminal2that has transmitted the received signal on the basis of the Doppler frequency fD obtained by the above Doppler frequency measuring unit20A. The Doppler frequency measuring unit20A and the terminal moving speed estimating unit20B can be configured, by applying a technique described in, for example, Japanese Patent Laid-Open Publication No. 6-242225 or the like.

In this case, an output of the AD converter163c(an output of the orthogonal detector163a) may be used as useful information to measure (detect) the above Doppler frequency fD. Alternatively, when AGC (Automatic Gain Control) is performed in the CDMA receiving units16aand16b, gain information thereof may be used, or an output of the M-Array correlating detector163f(a result of correlating detection) may be used, or the both may be used for improvement of accuracy of measurement of the Doppler frequency fD.

The switching speed threshold value holding unit20C holds a threshold value (hereinafter referred as a switching speed threshold value) for a terminal moving speed that becomes a reference when the change-over switches12,17and19are switched. The switching speed threshold value holding unit20C is configured with a RAM or the like, for example. The switching speed threshold value may be suitably set by a host system managing operation of the entire base station1.

The frequency band switching controlling unit (use frequency selection controlling unit; selection controlling unit)20D compares the terminal moving speed obtained by the terminal moving speed estimating unit20B with the above switching speed threshold value, controls the change-over switches12,17and19according to a result of the comparison, and selects (switches) a radio frequency band (the fR1band or the fR2band) to be used in the communication with the terminal2.

Here is described a manner of determining the above switching speed threshold value.FIG. 5is a graph showing an example of required Eb/No (signal-to-(interference+noise)power ratios required to satisfy desired communication quality) to the terminal moving speed when two kinds of radio frequency bands (the fR1band and the fR2band) are 850-MHz band and 2-GHz band, respectively. The graph is obtained by calculating the Doppler frequency fD to the terminal moving speed using the above equation (3) for each of the radio frequency bands, applying a result of the calculation toFIG. 13, and determining a required Eb/No. InFIG. 5, a characteristic31represents a required Eb/No characteristic at a radio frequency in the 850-MHz band, while a characteristic32represents a required Eb/No characteristic at a radio frequency in the 2-GHz band.

As seen fromFIG. 5, when a radio frequency in the 850-MHz band is used, the required Eb/No is maximum (the worst value) (the required Eb/No changing from a tendency to increase to a tendency to decrease) at a point (at the terminal moving speed of about 60 km/h) denoted by a reference numeral31a. When a radio frequency in the 2-GHz band is used, the required Eb/No is of the worst value at a point (at the terminal moving speed of about 25 km/h) denoted by a reference numeral32a. Incidentally, a non-monotonous characteristic as this is peculiar when a CDMA system using both TPC and FEC is employed, as described above.

Accordingly, if the use radio frequency band is switched at a point corresponding to the maximum value (about 5.8 dB) of the required Eb/No that the above both characteristics31and32(that is, an intersection of the characteristics31and32) are satisfied as a boundary (namely, a switching speed threshold value), it is possible to improve the worst value of the required Eb/No.

Namely, when the terminal moving speed is below 42.5 km/h, a radio frequency belonging to the 850-MHz band is selected as a radio frequency to be used to communicate with the base station1. When the terminal moving speed exceeds it, a radio frequency belonging to the 2 GHz-band is selected. In other words, the higher the terminal moving speed, the use frequency is selected from a higher frequency band (2-GHz), and assigned for the communication with the terminal2. Conversely, the lower the terminal moving speed, the use frequency is selected from a lower frequency band (850-MHz).

As this, the switching speed threshold value is determined on the basis of the required Eb/No characteristics31and32relating to the radio frequency band and the terminal moving speed, the use radio frequency band is switched according to a result of comparison between the switching speed threshold value and the terminal moving speed. Whereby, the worst value of the required Eb/No can be improved to about 5.8 dB in this embodiment.

Assuming that the worst values of the required Eb/Nos in the 850-MHz band and 2-GHz band are about 6.1 dB, a quantity of the improvement becomes about 6.1−5.8=0.3 dB (7.2% when it is converted into a quantity of improvement of the channel capacity). Meanwhile, since there is a possibility to obtain a larger effect of the improvement depending on the terminal moving speed, it is expected that an actual quantity of improvement of the channel capacity is becomes larger. A practical switching speed threshold value or an effect of the improvement changes according to a used radio frequency, TPC system or FEC system. It is thus possible to readily determine them by making a graph corresponding toFIG. 5in simulation or the like when the characteristics are determined.

In this case, the frequency band switching controlling unit20D such controls the change-over switches12,17and19that the use radio frequency band is the 850-MHz band when the terminal moving speed is not higher than 42.5 km/h, or the 2-GHz band when the terminal moving speed exceeds it. When the base station1switches the use radio frequency band, the communication cannot be continued if the terminal2does not switch the use radio frequency band to the same radio frequency band as the base station1.

Therefore, it is necessary to synchronize the use radio frequency between the base station1and the terminal2. According to this embodiment, a radio frequency band (to be switched to) and a switching timing selected on the side of the base station1is notified to the terminal2with a control signal (selected frequency notification signal, switching timing instruction signal), whereby synchronization is established.

For this purpose, the frequency band switching controlling unit20D according to this embodiment also has a function as a notification signal generating unit for generating the above selected frequency notification signal, and a switching timing instruction signal generating unit for generating a switching timing instruction signal. The latter switching timing instruction signal is generated when a reception confirmation signal in response to the former selected frequency notification signal is received from the terminal2(when the reception confirmation signal is demultiplexed from the received signal in the demultiplexing unit18) as an opportunity, and multiplexed on transmitting data in the multiplexing unit11as well as the former selected frequency notification signal.

(A2) Moving Terminal Structure

Next description will be made of a structure of the terminal2according to this embodiment. The terminal2according to this embodiment comprises, when attention is paid to essential parts thereof, a CDMA transmitting/receiving unit23for fR1band, a CDMA transmitting/receiving unit24for fR2band, a change-over switch25, a control signal multiplexing/demultiplexing unit26and a switching controlling unit27, as shown inFIG. 1.

The CDMA transmitting/receiving unit23for fR1band makes a radio communication with the base station1in CDMA system using a radio signal in a radio frequency band in the fR1band (for example, 850-MHz or the like), whereas the CDMA transmitting/receiving unit24for fR2band makes a radio communication with the base station1in CDMA system using a radio signal in a radio frequency band in the fR2band (for example, the 2-GHz band). Namely, these CDMA transmitting/receiving units23and24function as a radio communicating unit which can communicate with the base station1using either one of two kinds (M=2) of radio frequency bands (the fR1band and the fR2band) correspondingly to a kind of the radio frequency band used by the base station1.

In concrete, the CDMA transmitting/receiving units23and24according to this embodiment have a common hardware structure, as shown in, for example,FIG. 4. Each of the CDMA transmitting/receiving units23and24comprises a low noise amplifier (LNA)281, a filter282, an orthogonal detector283, an RF oscillator for forward link283a, an AD converter (ADC)284, a descrambler (multiplier)285, a multiplier286, an integrator (filter)287, a deinterleaver288, an FEC decoder289and the like as a receiving system28, and an FEC encoder291, an interleaver292, an M-Array correlating encoder293, a multiplier294, a scrambler (multiplier)295, a digital filter296, a DA converter (DAC)297, an orthogonal modulator298, an RF oscillator for reverse link298a, a filter299, a high-power amplifier (HPA)300and the like as a transmitting system29, along with a hybrid (H)30aand a transmitting/receiving antenna30bshared by the transmitting system28and the receiving system29.

Oscillated frequencies of the RF oscillators283aand298ain the above receiving system28and transmitting system29are made different from each other between the CDMA transmitting/receiving unit23and the CDMA transmitting/receiving unit24, whereby the CDMA transmitting unit23can make a two-way radio communication with the base station1using a radio signal in the fR1band, whereas the CDMA transmitting/receiving unit24can make a two-way radio communication with the base station1using a radio signal in the fR2band.

In concrete, a received signal from the base station1is inputted to the receiving system28through the transmitting/receiving antenna30band the hybrid30a, amplified and filtered by the LNA281and the filter282, and undergone orthogonal detection with an RF signal (an RF signal in the fR1band in the case of the CMDA transmitting/receiving unit23, or an RF signal in the fR2band in the case of the transmitting/receiving unit24) from the RF oscillator283ain the orthogonal detector283. The received signal undergone the quadrature detection is converted into a digital signal in the AD converter284, then despread and channel-decoded.

Namely, an output of the AD converter284is multiplied by a scrambling code (PN code) to be descrambled (despread) in the descrambler285, multiplied by a channelisation code to be undergone the channel decoding process in the multiplier286, and integrated in the integrator287. The integrated received signal is deinterleaved in the deinterleaver288, decoded in the FEC decoder289, and outputted to the change-over switch25.

On the other hand, a transmitting signal to the base station1is FEC-encoded in the FEC encoder291in the transmitting system29, interleaved in the interleaver292, and M-Array encoded in the M-Array encoder293, as described above. The M-Array-encoded transmitting signal is multiplied by a channelisation code in the multiplier294to be a channel signal, scrambled (spread) by a PN code in the scrambler297, filtered in the digital filter296, and converted into an analog signal in the DA converter297.

This analog signal is orthogonal-modulated in OQPSK (Offset QPSK) or the like with an RF signal from the FR oscillator298ain the quadrature modulator298, filtered in the filter299, amplified to a necessary transmitting power in the high-power amplifier300, and transmitted toward the base station1through the hybrid30aand the transmitting/receiving antenna30b.

In the terminal structure shown inFIG. 1, the change-over switch25switches to (selects) the CDMA transmitting/receiving unit23or24to be used (namely, a use radio frequency band) under control of the switching controlling unit27. The control signal multiplexing/demultiplexing unit26has a function of demultiplexing a control signal (selected frequency notification signal, switching timing instruction signal or the like) contained (multiplexed) in a received signal from the base station1, and outputting it to the switching controlling unit27, on the other hand, multiplexing a control signal (reception confirmation signal in response to the selected frequency notification signal, confirmation signal in response to the switching timing instruction signal or the like) to the base station1on a transmitting signal to the base station1.

1) function as a selected frequency notification signal receiving unit which receives a selected frequency notification signal for notifying of a radio frequency band selected according to a result of comparison between an own terminal moving speed and the above switching speed threshold value in the base station1from the CDMA transmitting/receiving unit23or24configuring the radio communicating unit;

2) function as a confirmation signal transmitting unit which transmits a confirmation signal in response to the above selected frequency notification signal to the base station1; and

3) function as a switching timing instruction signal receiving unit which receives a switching timing instruction signal in response to the above confirmation signal from the base station1.

The switching controlling unit (use frequency selection controlling unit; selection controlling unit)27controls the change-over switch25according to a control signal (selected frequency notification signal, switching timing instruction signal) from the base station1demultiplexed from a received signal by the above control signal multiplexing/demultiplexing unit26to switch a radio frequency band to be used in the communication with the base station1.

InFIG. 1, the CDMA transmitting units16aand16b, the CDMA receiving unit16aand16b, and the CDMA transmitting/receiving units23and24are provided exclusively for the respective two frequency bands fR1and fR2. However, it is alternatively possible to provide only parts or circuits (for example, oscillator, synthesizer, filter and the like) required for each of the frequency bands fR1and fR2, and share parts or circuits other than these, thereby reducing the size of the apparatus.

(A3) Description of Operation

Now, description will be made of an operation of the CDMA communication system in the above structure according to this embodiment with reference toFIG. 6.

As shown inFIG. 6, the base station1measures the Doppler frequency fD on the basis of a received signal by the Doppler frequency measuring unit20A, and determines a moving speed of the terminal2on the basis of the Doppler frequency fD by the terminal moving speed estimating unit20B (step S1). At this time, either the fR1band or the fR2band may be used for the communication between the base station1and the terminal2. However, in the initial state, it is desirable to use a lower frequency band (the fR1band in this embodiment).

In the base station1, the frequency band switching controlling unit20D compares the terminal moving speed determined by the terminal moving speed estimating unit20B with the switching speed threshold value (42.5 km/h in the above case) at predetermined cycles. When the terminal moving speed is not less than the switching speed threshold value, the base station1selects the fR2band (the 2-GHz band, for example) as the use radio frequency band. When the terminal moving speed is less than the switching speed threshold value, the base station1selects the fR1band (the 850-MHz band) as the use radio frequency band (step S2). When the radio frequency bands before and after the selection are the same, selection of the radio frequency band used up to that time is kept.

The frequency band switching controlling unit20D confirms whether there is an idle channel in the selected frequency band fR1or fR2. When there is an idle channel, the frequency band switching controlling unit20D notifies the terminal2of the selected frequency band fR1or fR2with a control signal (selected frequency notification signal). When there is no idle channel, a frequency band fR1or fR2presently in use is continuously used (steps S3and S4). In this case, notification of the selected frequency with the selected frequency notification signal to the terminal2may be performed, or not.

When the terminal2receives the selected frequency notification signal from the base station1, the switching controlling unit27generates a reception confirmation signal, and sends it back to the base station1(step S5). When receiving the reception confirmation signal, the base station1instructs a switching timing of the use radio frequency band to the terminal2with a switching timing instruction signal (step S6). When the terminal2receives the switching timing instruction signal, the switching controlling unit27generates a confirmation signal, and sends it back to the base station1(step S7).

After that, the base station1and the terminal2together wait for a timing defined by the above switching timing instruction signal, and simultaneously switch the use radio frequency band thereof to a selected radio frequency band (steps S8and S9). When radio frequency bands before and after the switching are the same, a radio frequency band used up to that time is kept, as a result. Synchronization of the switching timing may be established in another manner.

The use radio frequency band to be used between the base station1and the terminal2is switched to an optimum frequency band according a terminal moving speed using a communication quality characteristic (frequency dependency) to the Doppler frequency fD in the CDMA communication system to improve the worst value of the required Eb/No in the CDMA communication system in which a relationship between the Doppler frequency fD and the transmission quality degradation is non-monotonous, as described above with reference toFIG. 5. This allows improvement of the communication quality and increases the channel capacity.

(B) First Modification

In a radio propagation environment of a mobile communication system such as a CDMA communication system, there generates fluctuation (local median value fluctuation) in radio wave propagation loss due to shadowing or the like caused by a building or the like (refer to, for example, the above reference 3). Since the local median value fluctuation is log-normal fluctuation, the larger the local median value fluctuation standard deviation (σ stm) of an interference wave from other cell, the more the other-cell interference ratio (fo) increases and the channel capacity decreases (for example, refer to references 4 and 5 below).Reference 4: A. J. Viterbi and A. M. Viterbi, “Other-Cell Interference in Cellular Power-Controlled CDMA”, IEEE Trans. On Commun., Vol. COM-42, No. 2/3/4, pp. 1501–1504,(1994).Reference 5: A. J. Viterbi, et al., “Soft Handoff Extends CDMA Cell Coverage and Increases Reverse Link Capacity”, IEEE J, Selected Areas in Commun., 12(8), pp. 1281–1288.

The higher the radio frequency, the larger the local median value fluctuation standard deviation (σ stm); the higher the radio frequency, the larger is the other-cell interference ratio fo. Since the channel capacity is inversely proportional to Eb/No_th×(fs+fo), as shown by the equation (2), it is possible to more increase the effect of the channel capacity improvement by considering a difference in the other-cell interference ratio fo between the radio frequency bands.

For instance, table 3 shows results of determination of the other-cell interference ratios on forward link in computer simulation where the local median value fluctuation standard deviations in the 850-MHz band and the 2-GHz band are 6 dB and 8 Db, respectively (refer to the above reference 3). Wherein, conditions of the computation are a distance attenuation parameter=3.5, shadowing correlation=0.5, a soft hand-off ratio=100% and a site ratio=90%. The self-cell interference ratio is 1 because of forward link.

“Distance attenuation parameter” is an index representing how much a power of the radio wave attenuates according to a propagation distance. “Distance attenuation parameter=3.5” means that a power of the radio wave attenuates of the order of the negative 3.5-th power. “Shadowing correlation” signifies correlation of interference due to shadowing of the radio wave. The larger the correlation value, the larger is an increase or decrease in power of the radio wave due to the interference of shadowing. The smaller this value, the smaller is a relationship between an increase or decrease in power of the radio wave and the interference of shadowing. “Soft handoff ratio” represents how many terminals2perform soft handoff. For example, when the number of the links (channels) used for soft handoff is two and one terminal2uses these channels, the soft handoff ratio is 100%. “Site ratio” represents a rate of a site in which the terminal2can actually communicate inside a cell.

TABLE 3SIMULATION RESULTS OF f o to σ stmσ astmf o6 d B0.888 d B1.26

FIG. 7shows results of calculation of Eb/No_th×(fs+fo) to a terminal moving speed based on the above results. As shown inFIG. 7, when a change in the other-cell interference ratio fo is considered, improvement of about 0.7 dB to the worst value (approximately 17.5% when converted into the channel capacity) is expected by switching between the 850-MHz band (the fR1band) and the 2-GHz band (the fR2band) at a terminal moving speed 60–80 km/h as the switching speed threshold value, similarly to the above embodiment.

As above, the switching speed threshold value is determined on the basis of a characteristic of “a product of Eb/No_th and an interference power information (fs+fo)” to the radio frequency band and the terminal moving speed, so that a change in the other-cell interference ratio fo according to a radio frequency band is considered, which leads to an effect of further improvement of the communication quality and channel capacity.

Meanwhile, structures and operations of the base station1and the terminal2are similar to those of the above embodiment, excepting that the switching speed threshold value is determined on the basis of Eb/No_th×(fs+fo) to the terminal moving speed as shown inFIG. 7as above, differently from the above embodiment.

In the above example, the interference power information (fs+fo) is calculated using a local median value fluctuation standard deviation to the radio frequency band. It is alternatively possible to calculate the interference power information (fs+fo) on each radio frequency band adding a change in other wave propagation parameters (for example, refer to the reference 3) such as a distance attenuation parameter to the radio frequency band and the like to increase the effect of the improvement.

(C) Second Modification

Local median value fluctuation standard deviation or multi-path interference changes according to operational environments of the system, installation conditions of the base station and the like. For this, values of the self(same)-cell interference ratio fs and the other-cell interference ratio fo may fluctuate according to operational conditions of the system, installation conditions of the base station or fluctuation in traffic with time.

This embodiment deals with fluctuation in (fs+fo) as this. When the above equation (2) is modified, the following equation (4) is given.

In the equation (4), N is the number of accommodated channels (ch/sector/FR), pg is a processing gain, Eb/No is an actual Eb/No, and K is a coefficient representing an effect of other parameters. Since K and pg are values determined when operational conditions of the system and the like are determined, it is possible to know (fs+fo) when the number of channels K and the actual signal quality Eb/No are determined.

The number of channels N is generally a quantity monitored at all times for charging or the like. Eb/No can be readily determined by measuring an error rate or a received power of received data. Accordingly, it is possible to determine an optimum switching speed threshold value under conditions of the actual system operation by beforehand measuring the number of channels and Eb/No in the operational environments of the system or under installation conditions of the base station, calculating (fs+fo), and making a graph corresponding toFIG. 7using the value.

FIG. 8shows a structure of a base station1A in this case. A structure of the terminal2is similar to that shown inFIGS. 1 and 4. Dissimilarly to the base station1shown inFIG. 1, the base station1A according to this modification comprises, in the controlling system20, a switching speed threshold value processing unit20E including a subscriber monitoring unit20E-1, an Eb/No estimating unit20E-2, an (fs+fo) calculating unit20E-3, a switching speed threshold value determining unit20E-4and a switching speed information update controlling unit20E-5. Incidentally, other structural elements have similar functions shown inFIG. 1when not specifically mentioned.

The subscriber monitoring unit20E-1monitors the number of channels (the number of subscribers) during communication, which can be realized using a channel monitoring function for charging or the like as described above, for example. The Eb/No estimating unit20E-2estimates an actual Eb/No in each radio frequency band on the basis of error rate information on a received signal and received power information on a received signal in each radio frequency band (the fR1band, the fR2band).

The (fs+fo) calculating unit (interference power ratio information calculating unit)20E-3carries out calculation for each radio frequency band using the above equation (4) on the basis of the number of channels N monitored by the subscriber monitoring unit20E-1and an actual measured value of Eb/No obtained by the Eb/No estimating unit20E-2to calculate interference power ratio information (fs+fo) on each radio frequency band. The switching speed threshold value determining unit20E-4makes a graph corresponding toFIG. 7on the basis of a result (fs+fo) of the calculation by the (fs+fo) calculating unit20E-3to determine an optimum switching speed threshold value under actual operational conditions of the system.

The switching speed information update controlling unit20E-5applies a start trigger to the (fs+fo) calculating unit20E-3at predetermined cycles to make the (fs+fo) calculating unit20E-3calculate (fs+fo) at predetermined cycles.

With such the structure of the base station, the system according to this modification can again calculate an optimum switching speed threshold value, following fluctuation, even when a value of the self(same)-cell interference ratio fs or the other-cell interference ratio fo fluctuates due to the fluctuation in operational environments of the system, in installation conditions of the base station or in traffic with time, thereby realizing a suitable frequency band switching in consideration of fluctuation or the like in actual operational environments of the system, in installation conditions of the base station or in traffic with time. Accordingly, it is possible to largely improve the communication quality and increase the channel capacity.

(D) Third Modification

According to the above embodiment and modifications, the use radio frequency band is switched at one switching speed threshold value (hereinafter simply referred as “threshold value”). For this, when there is deviation in the terminal moving speed, there is a possibility that a phenomenon that one radio frequency band (the fR1band or the fR2band) is more frequently selected is generate. According to this modification, there are prepared a plurality of threshold values to alleviate concentration of calls in a specific frequency band.

FIG. 9shows a structure of a base station1B according to a third modification. Incidentally, a structure of the terminal2according to this modification is similar to that shown inFIGS. 1 and 4. Unlike the base station1shown inFIG. 1, the base station1B according to this modification is provided with a frequency band switching controlling unit20F which accepts a plurality (2×M−1) of threshold values, in lieu of the frequency band switching controlling unit20D.

When M=2, that is, when two frequency bands fR1and fR2(fR1<fR2) are prepared, three threshold values V1, V2and V3are set as the above threshold value, as shown inFIG. 10. V1is a terminal moving speed at which Eb/No is at the worst value in the above embodiment and modifications, V2is a terminal moving speed at which Eb/No_th is at the same value as when the terminal moving speed is V1in the case where the fR2band is used, and V3is a terminal moving speed at which Eb/No th is at the same value as when the terminal moving speed is V2in the case where the fR1band is used.

In this case, the frequency band switching controlling unit20F such controls the change-over switches12,17and19to use the fR1band or the fR2band (using fR1in preference) when VM≦V1where the terminal moving speed is VM, use the fR1band when V1<VM≦2V, use the fR2band when V2<VM≦V3, and use the fR2band or the fR1band (using the fR2band in preference) when V3<VM.

Namely, the frequency band switching controlling unit20F has a function as a determining unit which compares a terminal moving speed detected by the terminal moving speed estimating unit20A with each of (2×M−1) threshold values, and determines which range of threshold value the terminal moving speed falls in. The radio frequency band is selected on the basis of a result of determination by the determining unit and priority information defining which radio frequency band should be used for each of plural terminal speed ranges defined by the threshold values.

A sequence for switching the use radio frequency band between the base station1B and the terminal2is as shown inFIG. 12. In the base station1B, the Doppler frequency measuring unit20A measures the Doppler frequency fD on the basis of a received signal, and the terminal moving speed estimating unit20B determines a moving speed of the terminal2on the basis of the Doppler frequency fD (step S1). Incidentally, either the fR1and fR2bands may be used in the communication between the base station1and the terminal2at this time. However, it is desirable to use the lower frequency band (the fR1band in this embodiment) in the initial state.

In the base station1B, the frequency band switching controlling unit20F compares the terminal moving speed determined by the terminal moving speed estimating unit20B with the threshold value at predetermined cycles. And, the frequency band switching controlling unit20F selects a use radio frequency band under the above conditions (which range of the threshold value V1, V2or V3VM falls in) shown inFIG. 10. At this time, when there are a plurality of selectable radio frequency bands, the frequency band switching controlling unit20F selects one having a higher priority (step S2′). When radio frequency bands before and after the selection are the same, selection of a radio frequency band used up to that time is kept, as well.

The frequency band switching controlling unit20F confirms whether there is an idle channel in the selected frequency band. When there is an idle channel, the frequency band switching controlling unit20F notifies the terminal2of the selected frequency band with a control signal (selected frequency notification signal). When there is no idle channel, the frequency band switching controlling unit20F selects a frequency band having a lower priority, confirms presence of an idle channel in the similar manner, and notifies the terminal2of the selected frequency band with a selected frequency notification signal. When there is no idle channel in any selectable frequency band, a frequency band presently in use is continuously used (steps S3′ and S4). In this case, a frequency band presently in use may be newly notified to the terminal2with a selected frequency notification signal, or this notification may be omitted.

After that, the base station1B and the terminal2exchange a reception confirmation signal, a switching timing instruction signal and a confirmation signal in the similar manner shown inFIG. 6, together wait for a timing defined by the switching timing instruction signal, and simultaneously switch the use radio frequency band to a selected radio frequency band (steps S5to S9). When the radio frequency bands before and after the switching are the same, a radio frequency band used up to that time is kept as a result, as well. Synchronization of the switching timing may be established in another manner.

With a plurality of threshold values, the use radio frequency band is selected and given a priority according to which range of threshold value the terminal moving speed falls in, as above. It is thereby possible to alleviate concentration of calls in a specific frequency band without degrading the worst value of Eb/No_th, similarly to the above embodiment.

When M=3, that is, when three frequency bands fR1, fR2and fR3are prepared, five threshold values V1–5are set as shown in, for example, shown inFIG. 11. When VM≦V1, the fR1band, the fR2band or the fR3band is used (preferentially in order of the fR1band and the fR2band). When V1<VM≦V2, the fR1band or the fR2band is used (fR1being preferentially used). When V2<VM≦V3, the fR1band is used. When V3<VM≦V4, the fR3band is used. When V4<VM≦V5, the fR3band or the fR2band is used (preferentially the fR3band). When V5<VM, the fR1band, the fR2band or the fR3band is used (preferentially in order of the fR3band and the fR2band). This is the same when M>3.

Even when a plurality of threshold values are prepared, as this modification, it is possible to determine each threshold value on the basis of Eb/No_th×(fs+fo) to a terminal moving speed as described with reference toFIG. 7, calculate an other-cell interference ratio fo adding a change in another wave propagation parameter (for example, refer to the reference 3) such as a distance attenuation parameter to the radio frequency band or the like, or sequentially update values of the self-cell interference ratio fs and the other-cell interference ratio fo according to fluctuation in operational environments of the system, in installation conditions of the base station or in traffic with time, similarly to the first and second modifications. In any case, a larger effect of improvement of the communication quality and channel capacity is expected while concentration of calls in a specific frequency band is prevented.

In the above embodiment and modifications, a terminal moving speed itself is detected as information about a terminal moving speed from the Doppler frequency fD. However, the Doppler frequency fD (fading pitch (cycle) information) itself may be used to set a threshold value for the information. In such case, the similar functions and effects can be provided, of course.

In the above embodiment and modifications, the base station1estimates a state of reception (terminal moving speed or fading cycle) of a signal on forward link in the terminal2on the basis of a received signal (signal on reverse link) from the terminal2, and switches the use frequency band. Alternatively, the terminal2may notify the base station1of an actual state of reception (terminal moving speed or fading cycle) of the terminal2, whereby the base station1determines whether the fading cycle or the terminal moving speed in the received signal on forward link received by the terminal2is fast or slow to switch the frequency in the above manner.

In the above embodiment and modifications, the switching speed threshold value is set to a speed corresponding to an intersection of the required Eb/No characteristics of the radio frequency bands. Alternatively, it is possible to select, as the use frequency, a frequency belonging to at least the 2-GHz band when the terminal moving speed is below a speed at which the tendency of the required Eb/No characteristic31in the 850-MHz band changes in, for example,FIG. 5, or select, as the use frequency, a frequency belonging to the 850-MHz band when the terminal moving speed is not slower than a speed at which the tendency of the required Eb/No characteristic32in the 2-GHz band changes.

Note that the present invention is not limited to the above examples, but may be modified in various ways without departing from the scope of the invention.