Network control system for controlling relative errors between network nodes

A network control system for eliminating a relative error among control signals transmitted from nodes provided in radio zones respectively, a mobil station moving among the radio zones, each of the nodes comprising: a relative error detecting unit for detecting a relative error between an after-controlled transmitting control signal to be transmitted to adjacent nodes and a received control signal from an adjacent node; and a control unit for controlling a before-controlled transmitting control signal in such a way that the relative error becomes zero so as to output the after-controlled transmitting control signal.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a network control system, and 
particularly, to a network control system with a mobile station moving 
among a plurality of cells and communicating with a base station in a cell 
to which the mobile station belongs. 
A mobile communication network divides a service area into a plurality of 
radio zones (cells) and provides each radio zone with a base radio station 
(hereinafter also referred to as a "node", depending on the situation). A 
network control system employed to establish communication between the 
base radio station and the mobile station is usually based on a TDMA, 
FDMA, or CDMA method. According to these methods, the mobile station 
synchronizes itself with the base radio station of a radio zone in which 
the mobile station is present and communicates with the base station. 
Each of the base radio stations has its own reference clock, and according 
to which, sends a signal. When travelling among the radio zones, the 
mobile station must synchronize itself with the frequency and timing of a 
signal transmitted from the base radio station in a new zone any time the 
mobile station enters the zone. Until synchronization is established, 
communication is suspended. It is necessary, therefore, to provide means 
for eliminating relative errors or differences in controlled values such 
as the phases and frequencies of clocks among adjacent base radio 
stations. 
2. Description of the Related Art 
In a conventional network control system, a plurality of base radio 
stations serving as nodes are monitored by a central radio control 
station. These central radio control stations constitute a centralized 
control network for harmonizing and correcting the frequencies, and timing 
of signals used to communicate among the base radio stations. With this 
conventional network, a mobile station is not required to synchronize 
itself with the signals provided by the base radio stations when 
travelling among their zones. 
This conventional network system determines the size of each radio zone 
depending on the efficiency of frequency use, transmission power of the 
mobile station, etc., and each zone is made relatively large in size (for 
example, several kilometers in radius). 
In recent years, light and compact mobile stations have been developed 
having low transmission power. Recently, however, it has been required to 
improve the efficiency of frequency use, which has extremely reduced the 
size of each radio zone to, for example, 50 to 100 meters in radius. A 
network involving such miniature zones must have a large number of 
conventional base radio stations, thereby increasing the load on an upper 
apparatus such as a central radio control station and requiring a 
complicated control system. 
To solve these problems, an object of the present invention is to provide a 
network control system for controlling a plurality of radio zones with a 
mobile station moving among the zones and communicating with a base radio 
station of one of the zones where the mobile station is present, wherein 
each base radio station is capable of adjusting a controlled value of its 
own transmission signal without relying on a centralized control network 
involving a central control station thereby reducing the load on the 
central control station. In more detail, an object of the present 
invention is to provide such a network control system as above wherein 
even when the mobile station enters one radio zone from another radio 
zone, instant disabling of communication does not occur. 
SUMMARY OF THE INVENTION 
To attain the above objects, there is provided, according to the present 
invention, a network control system for eliminating a relative error among 
control signals transmitted from a plurality of nodes provided in a 
plurality of radio zones respectively, in which a mobile station is moving 
among the radio zones. Each of the nodes comprises: a relative error 
detecting unit for detecting a relative error between an after-controlled 
transmitting control signal to be transmitted to adjacent nodes and a 
received control signal from an adjacent node; and a control unit 
operatively connected to the relative error detecting unit, for 
controlling a before-controlled transmitting control signal in such a way 
that the relative error becomes zero so as to output the after-controlled 
transmitting control signal. 
In the above system, it is preferable that the relative error detecting 
unit detects a plurality of the relative errors between the 
after-controlled transmitting control signal and a plurality of received 
control signals from a plurality of adjacent nodes, and that the system 
further comprises a filtering unit, operatively connected between the 
relative error detecting unit and the control unit, for filtering noise 
from the relative errors. 
Alternatively, the system comprises a filtering unit for filtering noises 
from a plurality of received control signals from a plurality of adjacent 
nodes to output a filtered signal, and the relative error detecting unit 
is connected between the filtering unit and the control unit so as to 
detect a relative error between the filtered signal and the 
after-controlled transmitting control signal. 
According to another aspect of the present invention, there is provided a 
network control system for controlling nodes involving fixed base stations 
and a mobile station with the same frequency for reception and 
transmission as controlled values, each of the base stations comprising: a 
reception automatic frequency control unit for adjusting itself with a 
transmission frequency of an adjacent base station; a frequency error 
detection unit for detecting a frequency error between an output frequency 
of the reception automatic frequency control unit and a frequency that is 
obtained by adding a nominal frequency gap between reference stations to 
the transmission frequency of its own node; an averaging unit for 
spatially filtering the frequency errors of the adjacent base stations and 
providing a spatial mean value; and a transmission frequency control unit 
for controlling the transmission frequency of its own node in a way to 
eliminate the frequency errors. 
According to still another aspect of the present invention, there is 
provided a network control system for controlling a plurality of cells 
each having a base radio station serving as a node for transmitting a 
transmission signal at optional timing that is a controlled value, each 
base radio station communicating with a mobile station when the mobile 
station is located in the zone of the corresponding node, each of the base 
radio stations comprising: a notification unit for notifying adjacent base 
radio stations of transmission timing information of its own node; and a 
correction unit for receiving, as input signals, pieces of transmission 
timing information of the adjacent base radio stations, and correcting at 
least the timing of the transmission signal of its own node in a way to 
minimize a difference between the timing of the signal of its own node and 
the timing of the transmission signals of the adjacent base radio stations 
that have been spatially filtered.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
For better understanding the background of the present invention, a 
conventional network control system will first be described with reference 
to FIG. 58. In FIG. 58, 700 represents a plurality of base radio stations, 
and 800 represents a central control station. The conventional network 
control system is a centralized control network in which each base radio 
station 700 is synchronized with the central control station 800. The 
problem in the conventional system is that, in accordance with the 
increase of the number of the base radio stations, 700 the load on the 
central radio control station 800 is increased. 
Embodiments of the present invention will be described in the following. 
Basic concept and operation of an embodiment shown in FIG. 1A and FIG. 1B 
FIG. 1A shows a network control system according to an embodiment of the 
present invention. 
In FIG. 1A, 1 is a relative error detecting unit for detecting a relative 
error between a transmission signal A(t+1) after transmission, i.e., an 
after-controlled transmitting control signal A(t+1) to be transmitted to 
adjacent nodes and a received control signal Ai(t) from an adjacent node; 
and 3 is a control unit having an output connected to the relative error 
detecting unit, for controlling a before transmission signal (a 
before-controlled transmitting control signal) A(t) to output the 
after-controlled transmitting control signal A(t+1) in such a way that the 
relative error becomes zero. 
The relative error detecting unit 1 detects a plurality of relative errors 
between the after-controlled transmitting control signal A(t+1) and a 
plurality of received control signals Ai(t) (i=1, 2, . . . , n) from a 
plurality of adjacent nodes. In FIG. 1A, the system further comprises a 
filtering unit 2, operatively connected between the relative error 
detecting unit 1 and the control unit 3, for filtering noises from the 
relative errors. 
In the following, the before-controlled transmitting control signal A(t) is 
called a controlled value to be controlled in the node; the 
after-controlled transmitting control signal A(t+1) is called a next 
controlled value to be controlled in the next node; and the received 
control signal Ai(t) is called a controlled value already controlled in an 
adjacent node. The controlled value is a synchronizing signal such as a 
clock timing or a frequency. The controlled value Ai(t) may be referred to 
as a first signal; the controlled value A(t) may be referred to as a 
second signal; and the controlled value A(t+1) may be referred to as a 
third signal. 
A node under consideration involves a controlled value A(t) at time t and 
controls the next controlled value A(t+1) for the next time t+1 as 
follows: 
EQU A(t+1)=A(t)+C(t) (1) 
where C(t) is a value obtained by a time filtering operation Ft carried out 
at the time t and expressed as follows: 
EQU C(t)=Ft{B(t), B(t-1), . . . , B(t-m)} (2) 
where B(t) is a value obtained by a space filtering operation Fs carried 
out at the time t and expressed as follows: 
EQU B(t)=Fs{E1(t), E2(t), . . . , En(t)} (3) 
where Ei(t) (i=1, 2, . . . , n) are relative errors at the time t between 
the controlled value of the given node and controlled values Ai(t) of 
nodes that are referred to by the given node and expressed as follows: 
EQU Ei(t)=A(t)-Ai(t) (i=1, 2, . . . , n) (4) 
In this way, the node under consideration provides and receives information 
of controlled values to and from the adjacent nodes. Without a central 
control station, this method can remove relative errors of controlled 
values between a given node and adjacent nodes, thereby correctly 
controlling controlled values of the system as a whole. 
The spatial filtering carried out on error signals removes or reduces the 
influences of disturbance and abnormality occurring on some reference 
signals with respect to space. The time filtering removes or reduces the 
influences of disturbance and abnormality occurring on some of a string of 
continuous error signals with respect to time. 
Order of the spatial filtering Fs and time filtering Ft may be reversed. It 
is possible to carry out at least one of the space and time filtering 
operations. 
Further, as shown in FIG. 1B, the space and time filtering may be carried 
out on the received controlled value Ai(t) before detecting the relative 
error. 
Aspects of the embodiments of invention of FIGS. 2 to 9 
FIGS. 2 to 9 show preferred aspects of the embodiments of the invention 
based on the system of FIG. 1A and FIG. 1B. 
According to the embodiment shown in FIG. 2, a network control system 
involves a plurality of base radio stations serving as nodes for 
transmitting signals. A mobile station communicates with any one of the 
base radio stations while the mobile station is present in the radio zone 
of the base radio station in question. Each of the nodes (base radio 
stations) comprises a first operation unit 11, a first error detection 
unit 12, and a control unit 13. The first operation unit 11 spatially 
filters reference signals (which are the controlled values in FIG. 1A or 
1B) received from adjacent nodes. The first error detection unit 12 
provides an error signal indicating a difference between an output signal 
of the first operation unit 11 and an output signal A(t+1) of its own 
node. The control unit 13 controls a synchronization object input signal 
A(t) in a way to minimize the error signal, and provides the adjacent 
nodes with a reference signal A(t+1) of its own node. 
In FIG. 3, the first error detection unit 12 in FIG. 2 is replaced with a 
plurality of second error detection units 12.sub.1 to 12.sub.k each 
providing an error signal indicating a difference between a corresponding 
reference signal Ai(t) (i=1,2, . . . , n) and an output signal A(t+1) of 
its own node. The first operation unit 11 in FIG. 2 is replaced with a 
second operation unit 14 for spatially filtering output signals from the 
second error detection units 12.sub.1 to 12.sub.k and providing a control 
unit 13 with a control signal. 
In FIG. 4, a first storage unit 15 stores a reference signal Ai(t) from an 
adjacent node at a plurality of instants. A third operation unit 16 
filters, with respect to time, at least the reference signals of a 
plurality of instants provided by the first storage unit 15 and provides a 
first error detection unit 12 with a filtered signal filtered with respect 
to time. Other arrangements in FIG. 4 are the same as those in FIG. 2. 
In FIG. 5, a third error detection unit 17 prepares an error signal 
indicating a difference between an output signal A(t+1) of its own node 
and a reference signal Ai(t). A second storage unit 18 stores the error 
signal at a plurality of instants. A fourth operation unit 19 filters, 
with respect to time, at least the error signal of a plurality of instants 
provided by the second storage unit 18 and provides a control unit 13 with 
the signal, filtered with respect to time, as a control signal. Other 
arrangements of FIG. 5 are the same as those of FIG. 3. 
In addition to the arrangement of FIG. 2, an arrangement of FIG. 6 has a 
third storage unit 21 and a fifth operation unit 22. The third storage 
unit 21 stores an output signal of the first operation unit 11 at a 
plurality of instants. The fifth operation unit 22 filters, with respect 
to time, at least an output signal of plurality of instants provided by 
the third storage unit 21 and provides a first error detection unit 12 
with the signal filtered with respect to time. 
In addition to the arrangement of FIG. 2, an arrangement of FIG. 7 has a 
plurality of first storage units 15.sub.1 to 15.sub.k, a plurality of 
third operation units 16.sub.1 to 16.sub.k, and a sixth operation unit 24 
for spatially filtering temporally filtered reference signals provided by 
the third operation units 16.sub.1 to 16.sub.k. The term spatially 
filtering refers to a filtering with respect to space, and the term 
temporally filtering refers to a filtering with respect to time. 
In addition to the arrangement of FIG. 3, an arrangement of FIG. 8 has a 
fourth storage unit 26 and a seventh operation unit 27. An error signal 
spatially filtered by the second operation unit 14 is temporally filtered 
by the fourth storage unit 26 and seventh operation unit 27 and provided 
as a control signal to a control unit 13. 
FIG. 9 shows a combination of the arrangements of a modification of FIG. 
1B, wherein the reference signals Ai(t), illustrated as A.sub.1 (t) . . . 
A.sub.n (t), are spatially filtered by the first operation unit 11. The 
first error detection unit 12 prepares an error signal indicating a 
difference between an output signal A(t+1) of its own node and the output 
signal of the first operation unit 11. The second storage unit 18 stores 
the error signal at a plurality of instants. The forth operation unit 19 
temporally filters at least the error signal of a plurality of instants 
provided by the second storage unit 18 and provides the control unit 13 
with the temporally filtered signal as a control signal. 
Aspects of the embodiments of the invention of FIGS. 10 to 12 
FIGS. 10 to 12 show preferred examples of apparatuses directly employable 
for the basic system of FIG. 1A. 
Each node usually periodically updates the timing and parameters (which are 
the controlled values or the controlled transmitting signals in FIGS. 1A 
and 1B) of its own signal in order to synchronize the controlled values 
with those of adjacent nodes. Shortening signal updating intervals will 
reduce a convergent time of the system. This, however, raises a problem 
that the system is easily affected and destabilized by disturbance. 
Elongating the time width of a temporal filtering operation will improve 
an effect of suppressing the influence of disturbance. This, however, 
raises a problem of extending a convergent time of the system. Improving 
the accuracy of quantization will minimize a residual error, thereby 
achieving accurate synchronous control. This, however, causes a problem of 
elongating a convergent time of the system. 
In FIG. 10 of the invention, a detection unit 2-1 detects a differential 
signal E between an output signal Q (corresponding to the controlled value 
of FIG. 1A) of its own node and a signal R from another node. A signal 
generation unit 2-2 controls the signal Q in a way to minimize the 
differential signal E according to a timing signal T from a control unit 
2-3. The control unit 2-3 changes intervals of activating the signal 
generation unit 2-2 when the system is initialized or depending on the 
magnitude of the differential signal E. When the system is initialized 
according to, for example, a power ON reset signal, or when the 
differential signal E is greater than a predetermined value T.sub.H, the 
control unit 2-3 shortens the intervals of activation to promote 
convergence to the signal R, and in other cases, elongates the intervals 
of activation to avoid an influence of disturbance. 
In FIG. 11, a detection unit 2-1 detects a differential signal E between a 
signal Q of its own node and a signal R from another node. A signal 
generation unit 2-2 controls the signal Q in a way to minimize the 
differential signal E. A filter 2-4 spatially or temporally filters the 
signal R provided by another node, the differential signal E provided by 
the detection unit 2-1, or a control signal C prepared for the signal 
generation unit 2-2 from the differential signal E. A control unit 2-5 
changes the filtering characteristics of the filter 2-4 when the system is 
initialized or depending on the magnitude of the differential signal E. 
When the system is initialized or when the differential signal E is 
greater than a predetermined value T.sub.H, response of the filter 2-4 is 
quickened to speed up convergence to the signal R. In other cases, the 
response is slowed down to avoid disturbance. 
In FIG. 12, a detection unit 2-1 detects a differential signal E between a 
signal Q of its own node and a signal R of another node. A signal 
generation unit 2-2 controls the signal Q in a way to minimize the 
differential signal E. An input/output level conversion unit 2-6 converts 
input and output levels of the signal R from another node, of the 
differential signal E, or of a control signal C prepared for the signal 
generation unit 2-2 from the differential signal E. A control unit 2-7 
changes the level converting characteristics of the input/output level 
conversion unit 2-6 when the system is initialized or depending on the 
magnitude of the differential signal E. When the system is initialized or 
when the differential signal E is greater than a predetermined value 
T.sub.H, the level conversion characteristics are made to be coarse for 
quantization or linearization, to expedite convergence to the signal R. In 
other cases, quantization accuracy around an input level of 0 is improved 
to increase the accuracy of convergence. 
Aspect of the embodiments of the invention of FIG. 13 
FIG. 13 shows a preferred embodiment of the invention for controlling 
frequencies as controlled values. This embodiment is directly applicably 
for the basic system of FIG. 1. 
Generally, radio channel frequencies f.sub.A and f.sub.B (which are the 
controlled values of FIG. 1A) of base stations of adjacent nodes have a 
nominal frequency gap f.sub.S to minimize an interference between the 
frequencies. When a mobile station travels among zones from one base 
station to an adjacent base station, the mobile station changes its 
frequency from f.sub.A to f.sub.B for communicating with a corresponding 
base station. 
The oscillation frequency of a local oscillator that determines a radio 
channel frequency has a finite stability. For this reason, a frequency 
allocated for one base station always involves a finite error. In a 
conventional mobile radio communication system, this frequency error is 
absorbed by an automatic frequency control (hereinafter referred to as 
AFC) circuit provided for the mobile station. 
Accordingly, when the mobile station travels from one zone to another 
(hereinafter, the travelling of the mobile station among zones or cells 
will be referred to as a hand-over), the mobile station receives 
information related to the radio channel frequency f.sub.B of the base 
station of the zone into which the mobile station has entered. Even if 
such information is provided, the mobile station must absorb, through the 
AFC circuit, an error frequency (f.sub.B -f.sub.B ') between a nominal 
frequency f.sub.B and an actual frequency f.sub.B ' of the base station in 
question. In practice, a signal provided by a base station with which the 
mobile station was communicating before a hand-over involves a frequency 
error, so that a maximum frequency error that must be absorbed may be 
twice the frequency error allowed for the base station. 
Accordingly, the AFC circuit extensively operates during a transient period 
for every hand-over, and until the operation of the AFC circuit is 
completed, a call or data transmission is suspended, thereby causing 
inconvenience for a user. It is pointed out that effective use of 
frequencies is achievable by reducing the size of each zone (cell) and by 
spatially repeatedly using the frequencies. This causes many hand-overs to 
occur, thereby causing many momentary stoppages of communication and 
deteriorating system serviceability. 
To solve this problem, the invention intends to eliminate relative 
frequency errors between adjacent base stations where hand-overs occur. 
This requires a mobile station to only change its frequency for a nominal 
radio channel frequency gap f.sub.S whenever a hand-over occurs. 
To achieve this, each base station detects a difference between the signal 
frequency of an adjacent base station and the signal frequency of its own 
node and zeroes the difference by controlling the transmission frequency 
of its own node. 
In the preferred network control system of FIG. 13, a reception automatic 
frequency control unit 3-3 of a base station 3-1 carries out an AFC 
operation according to the transmission frequency of an adjacent base 
station. A frequency error detection unit 3-4 compares a frequency f.sub.R 
provided by the control unit 3-3 with a frequency "f.sub.A +f.sub.S " 
obtained by adding a prescribed frequency gap f.sub.S between base 
stations to a transmission frequency f.sub.A of the base station 3-1, and 
provides a frequency error .DELTA.f. 
Such frequency error .DELTA.f is detected between the base station in 
question and every adjacent base station. An averaging unit 3-5 averages 
the detected errors by spatially filtering the errors as shown in FIG. 1. 
When an "i"th base station has a nominal frequency f.sub.i and an actual 
frequency f.sub.i '(t) at time t, a relative frequency error 
.DELTA.f.sub.i (t) with respect to any adjacent base station j (j=1, 2, . 
. . , N) with which a hand-over may occur will be expressed as follows: 
##EQU1## 
where f.sub.sij is expressed as follows: 
EQU f.sub.sij =f.sub.j -f.sub.i (6) 
Namely, f.sub.sij is a difference between the nominal frequencies of the 
base stations i and j and is an integer multiple of the nominal frequency 
gap f.sub.S. An accumulation of .sub.j=1 .SIGMA..sup.N is carried out for 
j=1, 2, . . . , N. 
The averaging unit 3-5 provides a transmission frequency control unit 3-6 
with the error information .DELTA.f.sub.i (t), and the control unit 3-6 
controls a transmission frequency f.sub.A of its own station at time t+1 
as follows: 
EQU f.sub.i (t+1)=f.sub.i (t)-.alpha..DELTA.f.sub.i (t) (7) 
where .alpha. is a numerical coefficient of 0&lt;.alpha.1. 
When averaging relative frequency errors between a given base station and 
adjacent base stations, it is possible to use a weight of C/N of each 
signal to calculate a weighted mean, or exclude extremely large errors. 
These averaging techniques may improve the stability of the control 
system. 
The averaging unit 3-5, according to the invention, can also average the 
frequency errors .DELTA.f.sub.i (t) by temporally filtering the frequency 
errors. This may eliminate adverse effects caused by noise, etc., in 
communication paths and smooth momentary large errors, thereby further 
stabilizing the system. This is particularly needed when the C/N of a 
received signal is low. 
In this way, relative frequency errors between a given base station and 
adjacent base stations are eliminated. This effect, however, is only 
local. When an absolute frequency value must be secured, the above 
relative frequency control technique carried out between adjacent base 
stations is insufficient because it never guarantees an absolute frequency 
value. 
FIG. 14 shows a method of globally eliminating frequency errors from a 
communication system, according to the invention. In the figure, one of 
base stations 3-1 is selected as a reference station R that transmits an 
absolute transmission frequency. Being pulled by this absolute 
transmission frequency, each of the other base stations absorbs the 
frequency error .DELTA.f mentioned above, thereby guaranteeing an absolute 
frequency for the whole system and improving reliability of the system. 
This invention is applicable not only for a communication system employing 
the same frequency for transmission and reception but also for a 
communication system employing different frequencies for transmission and 
reception with each base station and mobile station always controlling a 
frequency difference between a reception frequency and a transmission 
frequency. 
Aspect of the embodiment of the invention of FIG. 15 
FIG. 15 shows an aspect of the invention based on the system of FIG. 1A or 
1B. The controlled value of this aspect is timing. 
A network control system of this aspect involves base radio stations 101 
serving as nodes. Each of the base radio stations 101 comprises a 
notification unit 103 for notifying adjacent base radio stations 101 of 
transmission timing of its own node, and a correction unit 104 for 
receiving transmission timing information as an input signal and 
correcting at least the phase of a transmission signal of its own node in 
a way to minimize a difference between the timing of its own node and the 
timing of a transmission signal that has been received from an adjacent 
base station 101 and spatially filtered. 
The base radio stations 101 communicate individual transmission timing 
information with one another through the notification unit 103. The 
correction unit 104 of each of the base radio stations 101 corrects, 
according to the transmission timing information, at least the phase of 
the transmission signal of its own node in a way to minimize a difference 
between the transmission timing of its own node and those of signals of 
the adjacent base stations. As a result, the phase difference becomes zero 
in the end. This embodiment forms a distributed control network system 
with the base radio stations 101 serving as nodes, similar to the model of 
FIG. 16. 
The correction mentioned above will be explained in more detail. The level 
(corresponding to a frame phase and clock frequency in the TDMA method, 
and a clock phase and clock frequency in the FDMA or CDMA method) of a 
given node (base radio station 101) of the distributed control network is 
expressed as B(i, t), where i is a node number and t is time. Each node 
compares the level with those of adjacent nodes and corrects a difference 
between them. 
With node numbers 1 to N (for the sake of convenience, i&gt;N) to be compared 
with one another, the difference of level between a given node and 
adjacent nodes after a spatial filtering operation is expressed as 
follows: 
EQU .DELTA.B(i, t-1)=B(i, t-1)-.sup.N .SIGMA..sub.n-1 B(n, t-1)/N (8) 
Accordingly, the level B(i, t) of the node i is corrected as follows: 
EQU B(i, t)=B(i, t-1)-[.alpha..DELTA.B(i, t-1)].sub.AVE (9) 
where .alpha. is a coefficient (0&lt;.alpha..ltoreq.1) and [].sub.AVE is 
averaging. 
For any of the TDMA, FDMA, and CDMA methods, the invention equalizes the 
levels of all nodes in the end and synchronizes all base radio stations 
with one another. 
According to the invention, each node (base radio station 101) is connected 
to adjacent nodes, so that, even if any one of the adjacent nodes fails, 
the remaining nodes will not be affected by the failure because they can 
control themselves according to timing information provided by the normal 
adjacent nodes. Even when the number of nodes (base radio stations 101) is 
increased, an increase in the number of adjacent nodes around a given node 
will be small so that load on the node will be substantially unchanged. 
According to the invention, each node may have a pull-in range to only 
control controlled values that exist in the pull-in range. This technique 
excludes uncontrollable values from mutual synchronous control, thereby 
reducing a destabilized period of the system. 
According to the invention, base stations that provide controlled values 
may be selected according to a certain rule. This technique prevents a 
fluctuation in the controlled values from spreading to adjacent nodes at 
random, thereby reducing a destabilized period of the network. 
According to the invention, the spatial filtering operation mentioned above 
may provide a weighted mean of relative errors of controlled values 
according to a reception electric field strength selected by diversity 
reception, to omit a special S/N detection circuit. 
FIG. 17 shows a more practical embodiment of the network control system of 
FIG. 2. In the figure, the same parts as those of FIG. 2 are represented 
with like numerals and their explanations will not be repeated. In FIG. 
17, a read only memory (ROM) 11a receives, through address lines, 4-bit 
reference signals (controlled values such as frequency signals, timing 
signals, and reception power signals) from four adjacent nodes. The ROM 
11a stores a table for providing an average of the address inputs of 4 
bits.times.4 through four bits of an 8-bit data line. 
A comparator 12a is, for example, a 4-bit digital subtracter. This 
comparator forms an error detection unit 12. The comparator 12a, which is 
the 4-bit digital subtracter, subtracts a 4-bit input value from another 
4-bit input value and provides a 4-bit result. 
A coefficient multiplier 13a, an adder 13b, and a controller 13c form a 
control unit 13. The coefficient multiplier 13a is, for example, a 4-bit 
digital multiplier, which multiplies a 4-bit input value by a 4-bit 
coefficient .alpha. (0&lt;.alpha.&lt;1) and provides a 4-bit result. 
The adder 13b is, for example, a 4-bit digital adder, which receives a 
4-bit digital input, i.e., a synchronization object signal (a controlled 
value such as clock information and transmission power information) of its 
own node as well as a 4-bit output of the coefficient multiplier 13a, adds 
them to each other, and provides a 4-bit result. This adder 13b provides 
an initial value or an offset. If such initial value or offset is not 
needed, the adder 13b may be omitted. 
The controller 13c comprises, for example, a 4-bit digital accumulator 
(ACC), which adds a 4-bit input value to a stored last output value and 
provides a 4-bit output. The output of the controller 13c is provided to 
the comparator 12a, used as a synchronization object signal for its own 
node, and transmitted as a reference signal to adjacent nodes through 
output means. 
Operations of this embodiment will be explained. The ROM 11a receives 
reference signals from four adjacent nodes and provides a mean value of 
the four reference signals. The comparator 12a receives the mean value and 
subtracts it from an output value of the controller 13c. When a given node 
i and an adjacent node j have signal quantities Si(n) and Sj(n), 
respectively, at time n, an output error signal of the comparator 12a of 
the given node is expressed as follows: 
EQU .DELTA.Si(n)={.sub.j=1 .SIGMA..sup.N Sj(n)}/N-Si(n) (10) 
where N is the total number of the input reference signals, which is equal 
to four in this embodiment. The first term of the right side of this 
equation indicates an output mean value of the ROM 11a. 
The output error signal .DELTA.Si(n) from the comparator 12a is multiplied 
by the coefficient .alpha. in the coefficient multiplier 13a, added to a 
synchronization object input signal A in the adder 13b, and added to the 
last signal in the controller 13c. As a result, the controller (ACC) 13c 
provides a signal Si(n+1), which is expressed as follows: 
EQU Si(n+1)=Si(n)+.alpha...DELTA.Si(n) (n.gtoreq.) 
EQU Si(n+1)=Si(n)+.alpha...DELTA.Si(n)+A (n=0) (11) 
Under a steady state, the signal value of the given node agrees with the 
mean value of the reference signals, so that .DELTA.Si(n)=0. Namely, the 
signal of the given node is unchanged. If the signal value of the given 
node is changed by some reason, or if the reference signals are changed, 
the signal value Si(n+1) is controlled to zero the .DELTA.Si(n). In this 
way, each node synchronizes itself with adjacent nodes, and all nodes in 
the network are synchronized with one another. 
If one of the four reference signals is abnormally large or small, the 
averaging operation, i.e., the spatial filtering operation of the ROM 11a, 
reduces the influence of the abnormality to one fourth so that the signal 
quantity to be synchronized as a controlled value may be stabilized. 
The spatial filtering operation by the ROM 11a is not limited to the simple 
averaging operation. For example, a mean value may be calculated after 
excluding values that are greater (or smaller) than a threshold value. 
FIGS. 18 and 19 show results of simulations of the first embodiment of FIG. 
17. FIG. 18 shows a result of simulation of characteristics of signal 
quantity Si(n) versus, time with a simple averaging operation such as one 
shown in the embodiment being used as a spatial filtering operation. FIG. 
19 shows characteristics of signal quantity versus time with a spatial 
filtering operation being carried out by averaging values after excluding 
those exceeding a threshold. In each figure, changes in signal quantity of 
all nodes (36 nodes) are plotted on the same graph. 
In FIG. 18, if one reference signal changes to an abnormally great value at 
a certain time point, as indicated by "a", then a signal quantity as a 
synchronization object of each node gradually converges to a certain 
value. In FIG. 19, if one reference signal exceeds the threshold and is 
fixed at an abnormally great value, as indicated with by "a," this 
reference signal is excluded. As a result, this abnormality does not 
influence the other nodes. 
The spatial filtering operation may be achieved by obtaining most frequent 
input values (medians), or by averaging values after excluding maximum and 
minimum values. These techniques also remove the influences of extreme 
values. 
The above embodiment compares a spatially filtered result of input 
reference signals with a signal of its own node. It is also possible, as 
shown in FIG. 3, to compare input reference signals with a signal quantity 
of its own node to provide a plurality of error signals, and then 
spatially filter the error signals to provide the control unit 13 with a 
result of the spatial filtering. 
In this case, the following spatially filtered result .DELTA.Si(n) is 
obtained according to the equation (10): 
EQU .DELTA.Si(n)={.sub.j=1 .SIGMA..sup.N .DELTA.Sij(n)}/N (12) 
EQU .DELTA.Sij(n)=Sj(n)-Si(n) (13) 
FIG. 20 is a schematic view showing an embodiment of the network control 
system of FIG. 4 according to the invention. In the figure, the same parts 
as those of FIGS. 4 and 17 are represented with like numerals and their 
explanations will not be repeated. In FIG. 20, a 3-stage shift register 
15a of 4-bit width forms a first storage unit 15. The shift register 
receives a 4-bit reference signal, shifts the signal to the right at each 
clock pulse, and stores the reference signal for the last three clock 
periods. 
A ROM 16a forms a third operation unit 16. The ROM 16a stores a table, 
which provides a mean value of address inputs of 4 bits.times.4 through 
four bits of an 8-bit data line. 
According to this embodiment, the shift register 15a shifts a reference 
signal provided by an adjacent node to the right at every clock pulse, and 
the reference signal is applied to a 4-bit address terminal of the ROM 
16a. The other three 4-bit address terminals of the ROM 16a receive the 
reference signal sampled at the last three time points (the first, second, 
and third latest clock pulses) from the shift register 15a in parallel. 
The ROM 16a calculates a mean value (a moving average value) of the 
reference signal sampled at the four consecutive instants from the past to 
the present, and provides a comparator 12a with a result of the averaging. 
In this way, the ROM 16a temporally filters the reference signal and 
provides the comparator 12a with a result of the temporal filtering. The 
comparator 12a compares the result with an output signal Si(n) of a 
controller 13c and provides an error signal .DELTA.Si(n) expressed as 
follows: 
EQU .DELTA.Si(n)={.sub.m=1 .SIGMA..sup.M Sj(n-m+1)}/M-Si(n) (14) 
where M is a period (the number of instants) for which the moving average 
is calculated. In this embodiment, M is 4. 
The error signal .DELTA.Si(n) at time n is supplied as a control signal to 
a control unit 13 having the same arrangement as the control unit 13 of 
FIG. 17. The control unit 13 provides a signal Si(n+1) for the next time 
n+1 as follows: 
EQU Si(n+1)=Si(n)+.alpha..DELTA.Si(n)+A (15) 
This output signal is supplied to the comparator 12a, used as, for example, 
a clock for its own node, and transmitted as a reference signal to 
adjacent nodes. In the above equation (15), A is a synchronization object 
input signal. 
Under a steady state of this embodiment, the signal value of a given node 
agrees with a moving average of reference signals of the node. In this 
case, .DELTA.Si(n)=0, so that the signal of the given node is unchanged. 
If the signal value of the given node is changed due to some reason, or if 
the reference signals are changed, the signal value Si(n+1) of the given 
node is controlled to zero the error signal .DELTA.Si(n). In this way, 
each node operates to synchronize itself with adjacent nodes, so that all 
nodes of the network synchronize with one another. 
If reference signal values become abnormally large or small, the moving 
average operation suppresses the influence of the abnormality to one 
fourth, so that the signal quantity of a given node may gradually change. 
This will be explained in more detail with reference to FIGS. 21 and 22 
involving 36 nodes and 3 reference signals. FIG. 21 shows characteristics 
of signal quantity versus time without temporal filtering (without the 
shift register 15a in the embodiment of FIG. 12). FIG. 22 shows 
characteristics of signal versus time with the arrangement of FIG. 20. 
Similar to FIGS. 18 and 19, each of FIGS. 21 and 22 plots changes in 
signal quantity of all nodes on the same graph. 
In FIG. 21, the value of one reference signal abnormally increases as 
indicated by "d." Then, signal quantities of the other nodes slightly but 
steeply change in magnitude as indicated by "e" and then gradually 
converge to a fixed value. In FIG. 22, the signal quantities of the other 
nodes slightly and gradually change as indicated by "g" when the value of 
one reference signal abnormally increases as indicated by "f." In this 
way, the invention can achieve stable network synchronization control that 
is not affected by abnormal changes and disturbance in reference signals. 
The above embodiment achieves a temporal filtering operation through a 
simple moving average. It is also possible to employ an interval averaging 
for a certain period, a weighted mean with a weight as a value 
corresponding to a time counted from the present moment, most frequent 
input values (medians), or averaging after excluding maximum and minimum 
values. When the weighted mean technique with a weight corresponding to a 
time counted from the present moment is employed, an inverse number of the 
time from the present moment, for example, is used as a weight. In this 
case, the older the value, the lesser the value influences a present 
controlled value, so that it may shorten a convergence time from an 
initial state to a steady (stabilized) state compared with using the 
simple moving average technique. When the medians or the averaging after 
excluding maximum and minimum values is used, sudden disturbance can be 
removed. 
The above embodiment compares, in a given node, a temporally filtered 
result of input reference signal values stored at several time points from 
the present moment to the past with a signal of the node itself. As shown 
in FIG. 5, signal values stored at several instants may be compared with a 
signal quantity of the given node to provide a plurality of error signals, 
which are temporally filtered to provide a control input. 
In this case, a result of the temporal filtering is expressed as follows 
according to the equation (14): 
EQU .DELTA.Si(n)={.sub.m=1 .SIGMA..sup.M .DELTA.Sij(n-m+1)}/M (16) 
EQU .DELTA.Sij(n)=Sj(n)-Si(n) (17) 
FIG. 23 is a more practical embodiment of the system of FIG. 6 according to 
the invention. In the figure, the same parts as those shown in FIGS. 6 and 
17 are represented with like reference marks and their explanations will 
not be repeated. In FIG. 23, a 3-stage shift register 21a of 4-bit width 
forms a third storage unit 21. The structure of the shift register 21a is 
the same as that of the shift register 15a. A ROM 22a forms a fifth 
operation unit 22 and has the same structure as the ROM 16a. The ROM 22a 
stores data in advance to convert inputs of 4 bits.times.4 provided 
through address lines into an output signal through four bits of an 8-bit 
data line. 
An operation of this embodiment will be explained. A ROM 11a provides the 
shift register 21a with a mean value of four reference signals. The mean 
value is shifted to the right at each clock pulse in the shift register 
21a and supplied to one of 4-bit address terminals of the ROM 22a. Each of 
the other three 4-bit address terminals of the ROM 22 receives a 
corresponding one of the three past reference signal mean values provided 
by the shift register 21a in parallel. The ROM 22a calculates a moving 
average of the mean values at consecutive four instants of the reference 
signal from the present moment to the past, and supplies the moving 
average to the comparator 12a. Namely, the ROM 22a temporally filters the 
spatially filtered reference signals and provides the comparator 12a with 
a result thereof. 
The comparator 12a provides an error signal .DELTA.Si(n) according to a 
difference between the operation result provided by the ROM 22a and an 
output signal of its own node. The error signal .DELTA.Si(n) is expressed 
as follows: 
EQU .DELTA.Si(n)=[.sub.m=1 .SIGMA..sup.M {.sub.j=1 .SIGMA..sup.N 
Sj(n-m+1)}/N]/M-Si(n) (18) 
This error signal .DELTA.Si(n) is supplied as a control signal to the 
control unit 13, which provides the signal Si(n+1) according to the 
equations (11) and (15). 
Similar to the previous embodiment, this embodiment makes the error signal 
.DELTA.Si(n) to be zero when the reference signals are changed, to thereby 
synchronize the nodes of the network with one another. When an abnormally 
large or small reference signal is provided, the spatial filtering 
operation of the embodiment suppresses a total change due to the abnormal 
signal to one fourth, and the temporal filtering operation suppresses a 
changing ratio due to the influence of the abnormal signal to one fourth. 
The spatial filtering operation may be realized not only by the simple 
averaging but also by the various techniques mentioned before. Also, the 
temporal filtering operation may be realized not only by the simple moving 
averaging but also by the various techniques mentioned before. Order of 
the spatial filtering and temporal filtering is not limited to that of the 
third embodiment. As shown in FIG. 7, reference signals may be temporally 
filtered at first and then spatially filtered. 
As shown in FIG. 8, reference signal values may be compared with the signal 
quantity of a given node to provide a plurality of error signals, which 
are then spatially and temporally filtered to provide a control input 
.DELTA.Si(n). In this case, .DELTA.Si(n) is expressed as follows according 
to the equation (18): 
EQU .DELTA.Si(n)=[.sub.m=1 .SIGMA..sup.M {.sub.j=1 .SIGMA..sup.N 
Sij(n-m+1)}/N]/M (19) 
where, 
EQU .DELTA.Sij(n)=Sj(n)-Si(n) (20) 
As shown in FIG. 9, the error signals may be spatially filtered at first 
and then temporally filtered. 
The embodiments mentioned above are applicable for each base radio station 
of a mobile communication system, to synchronize clock, frequency, and 
transmission power of the mobile communication network as a whole. These 
embodiments are particularly effective for a mobile communication network 
which involves very narrow radio zones and a large number of base radio 
stations. 
FIG. 24 is a schematic view showing a more practical embodiment of the 
network control system of FIG. 10 according to the invention. In the 
figure, numeral 2-10 denotes a phase detection unit (corresponding to 2-1 
of FIG. 10), 2-20 a signal generation unit (corresponding to 2-2 of FIG. 
10), 2-21 a multiplier, 2-22 an adder/subtracter, 2-23 a register, 2-24 an 
oscillator, 2-25 a variable stage shift register that can change the 
number of stages between an input and an output according to a control 
signal, 2-30 a control unit (corresponding to 2-3 of FIG. 10), 2-31 a 
comparator, 2-32 an OR gate circuit (O), and 2-33 a variable frequency 
divider. 
The phase detection unit 2-10 detects a phase difference between a clock 
signal Q serving as a controlled value of its own node and a clock signal 
R serving as a controlled value from another node, and provides an 
absolute signal value .vertline.E.vertline. and a sign S of the phase 
difference. The signal generation unit 2-20 is activated (in response to a 
timing signal T.sub.3) by the control unit 2-30, to control the phase of 
the clock signal Q according to the phase differential signal 
.vertline.E.vertline. and sign S in a way to minimize the phase difference 
signal .vertline.E.vertline.. This will be explained in more detail. The 
multiplier 2-21 multiplies the phase difference signal 
.vertline.E.vertline. and sign S by a predetermined value .alpha. (0 to 1) 
to maintain a loop gain of the system to be less than 1. The 
adder/subtracter 2-22 adds the phase difference 
.alpha..vertline.E.vertline. and sign S to an accumulated phase difference 
(a control signal .vertline.C.vertline. and sign S) of the register 2-23, 
thereby updating the control signal .vertline.C.vertline. with sign S. The 
oscillator 2-24 generates a clock signal Q' having the same frequency as 
that of the clock signal R. The variable stage shift register 2-25 
receives the clock signal Q' and advances or delays the phase of the 
output clock signal Q according to the control signal 
.vertline.C.vertline. with sign S. 
Under this state, the control unit 2-30 changes intervals of activation of 
the signal generation unit 2-20 when the system is initialized or 
depending on the magnitude of the phase difference signal 
.vertline.E.vertline. with sign S. For example, when the system is 
initialized or when the comparator 2-31 determines that the phase 
difference signal .vertline.E.vertline. is greater than a predetermined 
value T.sub.H, the control unit 2-30 reduces the frequency dividing ratio 
of the variable frequency divider 2-33, to thereby shorten intervals of 
the timing signal T.sub.3 and promote convergence to the signal R. In 
other cases, the control unit 2-30 increases the frequency dividing ratio 
of the variable frequency divider 2-33, to elongate intervals of the 
timing signal T.sub.3, thereby removing the influence of disturbance, etc. 
FIG. 25 is a block diagram of the phase detection unit of the embodiment 
mentioned above. In the figure, numeral 2-10 denotes the phase detection 
unit (corresponding to 2-1 of FIGS. 7 to 9), 2-11 a D-type flip-flop (FF), 
2-12 a counter, 2-13 an adder/subtracter, 2-14 a latch, 2-15 a set/reset 
flip-flop (FF), 2-16 an up/down counter (U/D counter), 2-17 a set/reset 
flip-flop (FF), D a delay circuit, I an inverter circuit, and N a NAND 
circuit. 
FIG. 26 is a timing chart showing operations of the phase detection unit. 
The operations of the phase detection unit will be explained with 
reference to FIGS. 25 and 26. A clock signal R is provided by another 
node. At each rise of the clock signal R, a series of timing signals 
T.sub.1 and T.sub.2 are generated. A gate signal G from an output of the 
flip-flop 2-11 is reset in response to the timing signal T.sub.1 and set 
in response to a rise of the last delay circuit D. While the gate signal G 
is being at level HIGH, the counter 2-12 counts up a high-frequency clock 
signal CLK. This state is indicated as a count signal A. 
Set and reset terminals of the flip-flop 2-15 receive the signals R and Q, 
respectively. Signals Q.sub.1 to Q.sub.3 show typical three phase states 
of the clock signal Q relative to the clock signal R. The signal Q.sub.1 
shows a phase delay, the signal Q.sub.2 an intermediate state, and the 
signal Q.sub.3 an advance in a phase b. The signal Q.sub.1 is controlled 
to be pulled in the direction of an arrow mark a, and the signal Q.sub.3 
is controlled to be pulled in the direction of an arrow mark b. The signal 
Q.sub.2 may be pulled in any direction. 
With respect to the signals Q.sub.1 to Q.sub.3, the flip-flop 2-15 provides 
the up/down counter 2-16 with up/down control signals U/D.sub.1 to 
U/D.sub.3. The up/down counter 2-16 is activated in response to the gate 
signal G. The up/down counter 2-16 counts up the clock signal CLK while 
the up/down control signal is at level HIGH, and counts down the clock 
signal CLK while the up/down control signal is at level LOW. Accordingly, 
a count signal B corresponding to the up/down control signals U/D.sub.1 to 
U/D.sub.3 for the up/down counter 2-16 passes through routes (1) to (3). 
In the routes (1) and (2), the count signal B may change from 0 to -1 at a 
certain point. At this time, the up/down counter 2-16 provides a borrow 
signal BO, which is stored in the flip-flop 2-17 to provide a sign S for a 
phase difference signal .vertline.E.vertline.. 
For the routes (1) and (2), the adder/subtracter 2-13 calculates (A+B) and 
sets a phase difference signal 
.vertline.a.vertline.=.vertline.A+B.vertline. and a sign S=HIGH (delayed 
phase) in the latch 2-14. For the route (3), the adder/subtracter 2-13 
calculates (A-B) and sets a differential signal 
.vertline.b.vertline.=.vertline.A-B.vertline. and a sign S=LOW (advanced 
phase) in the latch 2-14. 
FIG. 27 is a schematic view showing an embodiment of the network control 
system of FIG. 11 according to the invention. In the figure, numeral 2-40 
denotes a temporal filter (corresponding to 2-4 of FIG. 11), 2-41 to 2-43 
shift registers (Z.sup.-1) for sequentially shifting a phase difference 
signal .vertline.E.vertline., 2-44 a selector for selecting outputs of the 
shift registers 2-42 and 2-43 or data "0," 2-45 a ROM for providing an 
additive mean of address inputs 0 to n, 2-51 a D-type flip-flop (FF), and 
2-40' an analog temporal filter. 
The phase detection unit 2-10 detects a phase difference between a clock 
signal Q of its own node and a clock signal R from another node and 
provides an absolute phase difference signal .vertline.E.vertline. with a 
sign S. A signal generation unit 2-20 controls the phase of the control 
signal Q according to the phase difference signal .vertline.E.vertline. 
with sign S in a way to minimize the phase difference signal 
.vertline.E.vertline.. 
At this time, the temporal filter 2-40' or 2-40 temporally filters (finds 
out a moving average of) the signal R provided by another node, the phase 
difference signal .vertline.E.vertline. with sign S, or a control signal 
.vertline.C.vertline. with sign S prepared for the signal generation unit 
2-20 from the phase difference signal. Under this state, the control unit 
2-50 changes the filtering time width of the temporal filter 2-40' or 2-40 
when the system is initialized or depending on the magnitude of the phase 
difference signal .vertline.E.vertline.. 
More precisely, when the system is initialized or when the comparator 2-31 
determines that the phase difference signal .vertline.E.vertline. is 
greater than a predetermined value T.sub.H, the flip-flop 2-51 is set, and 
an output signal FTW of the flip-flop 2-51 connects input terminals of the 
selector 2-44 to the B side. As a result, a data width for providing a 
moving average is narrowed to quicken response of the temporal filter 
2-40, thereby speeding up convergence to the signal R. In other cases, the 
flip-flop 2-51 is reset, and the output signal FTW thereof connects the 
input terminals of the selector 2-44 to the A side. As a result, the data 
width for providing a moving average is widened to slow the response of 
the temporal filter 2-40, thereby eliminating the influence of 
disturbance, etc. 
FIG. 28 is a schematic view showing a more practical embodiment of the 
network control system of FIG. 12 according to the invention. In the 
figure, numeral 2-60 denotes an input/output level conversion unit 
(corresponding to 2-6 of FIG. 12), 2-61 a ROM for storing a plurality of 
level conversion characteristics, 2-70 a control unit, 2-71 a D-type 
flip-flop (FF), and 2-60' an analog input/output level conversion unit. 
A phase detection unit 2-10 detects a phase difference between a clock 
signal Q of its own node and a clock signal R from another node and 
provides an absolute signal value .vertline.E.vertline. with sign S of the 
phase difference. A signal generation unit 2-20 controls the clock signal 
Q according to the phase difference signal .vertline.E.vertline. with sign 
S in a way to reduce the phase difference signal .vertline.E.vertline.. 
At this moment, the input/output level conversion unit 2-60' or 2-60 
converts the input/output level of the signal R from another node, of the 
phase difference signal .vertline.E.vertline. with sign S, or of a control 
signal .vertline.C.vertline. with sign S prepared for the signal 
generation unit 2-20 from the phase difference signal. The control unit 
2-70 changes the level converting characteristics of the input/output 
level conversion unit 2-60 or 2-60 when the system is initialized or 
depending on the magnitude of the phase difference signal 
.vertline.E.vertline.. 
More precisely, when the system is initialized or when a comparator 2-31 
determines that the phase difference signal .vertline.E.vertline. is 
greater than a predetermined value T.sub.H, the flip-flop 2-71 is set, and 
an output signal LC thereof selects one of the characteristics (for 
example, coarse quantization characteristics, or linear quantization 
characteristics, etc.,) stored in the ROM 2-61. As a result, an 
input/output dynamic quantization range is widened or linearized, to 
promote convergence to the signal R. In other cases, the flip-flop 2-71 is 
reset, and the output signal LC thereof selects another characteristics 
(for example, nonlinear quantization characteristics that improves 
quantization accuracy around an input level of 0) stored in the ROM 2-61, 
thereby improving the quantization accuracy around an input level of 0 and 
increasing convergence accuracy. 
This embodiment is applicable not only for phase synchronization of a clock 
signal but also for the magnitude synchronization of various signal 
parameters, control parameters, etc. 
FIG. 29 is a block diagram showing an error detection unit of the 
embodiment mentioned above. In the figure, numeral 2-80 denotes the error 
detection unit (corresponding to 2-1 of FIGS. 10 to 12), 2-81 a reception 
unit, 2-82 a demodulation unit, 2-83 an A/D conversion unit, and 2-84 a 
subtracter. 
A signal R provided by another node may be an amplitude modulated signal 
with predetermined signal parameters. The reception unit 2-81 receives the 
modulated signal, and the demodulation unit 2-82 demodulates an amplitude 
among the signal parameters. The A/D converter 2-83 converts the 
demodulated signal into a digital signal. The subtracter 2-84 provides a 
differential signal indicating an amplitude difference between the output 
of the A/D converter 2-83 and a signal parameter P of its own node, 
thereby synchronizing the signal parameter, similar to the previous case. 
In this case, the signal generation unit 2-20 may control the amplitude of 
the signal parameter P of its own node according to the error signal 
.vertline.E.vertline. with sign S in a way to reduce the error signal 
.vertline.E.vertline.. 
In the above embodiment, the stage variable shift register 2-25 shifts the 
phase of the clock signal Q'. It is possible to directly control the 
oscillation frequency of the oscillator 2-24 according to the control 
signal .vertline.C.vertline. with sign S. 
In the above embodiment, the comparator 2-31 determines whether or not the 
differential signal .vertline.E.vertline. is greater than a predetermined 
value T.sub.H. It is possible to arrange a plurality of threshold values 
T.sub.Hi to determine the magnitude of the differential signal 
.vertline.E.vertline. in multiple steps, to thereby control a controlled 
value in multiple steps. 
In the above embodiment, only one signal R is provided by another node. It 
is possible to mutually and simultaneously synchronize a plurality of 
nodes with one another. In this case, a plurality of signals R are 
averaged, and the averaged signal is provided to the detection unit. 
Alternatively, differential signals E.sub.i between signals R.sub.i from a 
plurality of nodes and a signal Q of a given node are prepared, and the 
differential signals are averaged to provide a differential signal E. 
The loop gain .alpha. may be changed when the system is initialized or 
depending on the magnitude of the differential signal E. 
The above embodiment uses the temporal filter 2-40. It is possible to 
prepare a high pass filter, a notch filter, a band-pass filter, etc., to 
change filter characteristics from one to another. 
Embodiments of averaging in frequency control (FIGS. 30A to 38) 
An embodiment of the spatial averaging (filtering) carried out by the 
averaging unit 3-5 of FIG. 13 will be explained. This embodiment employs 
g.sub.1i to g.sub.6i for substituting for .DELTA.f.sub.i (t) in the second 
term of the right side of the equation (7), to control f.sub.i (t+1). 
(1) The simplest averaging is arithmetic averaging, which will be expressed 
as follows according to the equation (5): 
##EQU2## 
where .DELTA.f.sub.ij (t) is a relative frequency error between a 
frequency f.sub.i '(t) of an "i"th base station and a frequency f.sub.j 
'(t) of a "j"th base station adjacent to the "i"th base station and 
expressed as follows: 
EQU .DELTA.f.sub.ij (t)=(f.sub.i '(t)+f.sub.sij)-f.sub.j '(t) (22) 
(2) In the arithmetic averaging of the equation (21), the maximum and 
minimum values of .DELTA.f.sub.ij (t) may be excluded from the equation 
(21) to achieve the following averaging: 
EQU g.sub.2i (t)=[.sub.k=1 .SIGMA..sup.N-2 .DELTA.f.sub.ik (t)]/(N-2) (23) 
where N must be equal to or larger than 3, and k indicates the number "N-2" 
of base stations that is obtained after excluding the maximum and minimum 
values from j=1,2, . . . , N. 
(3) In addition to detecting the .DELTA.f.sub.ij (t), a unit (not shown) 
for detecting C/N (carrier power/noise power) or S/N (signal/noise) may be 
arranged to detect the C/N of each adjacent base station. In this case, 
each frequency error .DELTA.f.sub.ij (t) is weighted while 
C/N=.sub..gamma.j, to obtain a weighted mean value: 
EQU g.sub.3i (t)=[{.sub.j=1 .SIGMA..sup.N.sub..gamma.j (t)..DELTA.f.sub.ij 
(t)}/{.sub.j=1 .SIGMA..sup.N.sub..gamma.j (t)}]/N (24) 
(4) A combination of the mean values g.sub.2i and g.sub.3i will provide the 
following mean value: 
EQU g.sub.4i (t)=[{.sub.k=1 .SIGMA..sup.N-2.sub..gamma.k (t)..DELTA.f.sub.ik 
(t)}/{.sub.k=1 .SIGMA..sup.N-2.sub..gamma.k (t)}]/(N-2) (25) 
(5) The ratio C/N=.sub..gamma.j may have a threshold .sub.65 th to exclude 
.DELTA.f.sub.ij (t) from averaging when .sub..gamma.j .sub..gamma.th : 
EQU g.sub.5i (t)={.sub.l=1 .SIGMA..sup.N' .DELTA.f.sub.i1 (t)}/N'(26) 
This is used as a mean value. In this case, .sub.t=1 .SIGMA..sup.N' means 
to accumulate N' pieces of .DELTA.f.sub.i1 (t) that satisfy .sub..gamma.1 
&gt;.sub..gamma.th. 
(6) A combination of the mean values g.sub.3i and g.sub.5i provides the 
following mean value: 
EQU g.sub.6i (t)=[{.sub.1=1 .SIGMA..sup.N'.sub..gamma.1 (t)..DELTA.f.sub.i1 
(t)}/{.sub.1-1 .SIGMA..sup.N'.sub..gamma.1 (t)}]/N' (27) 
FIGS. 30A to 30C show an embodiment of a temporal averaging (filtering) 
operation that is carried out as and when required, after a spatial 
averaging operation by the averaging unit 2-5 mentioned above. This 
embodiment employs G.sup.a.sub.ki (t) (k=1 to 3, a=1 to 6) for 
substituting .alpha..DELTA.f.sub.i (t) of the second term of the right 
side of the equation (7), to control f.sub.1 (t+1). 
FIG. 30A employs a so-called transversal filter. M pieces of delay elements 
T receive spatial mean values .DELTA.f.sub.i (t) of frequency errors in 
series. These delay elements T, M pieces of tap coefficients 
.alpha..sub.n, and an adder ADD1 provide the following frequency control 
signal: 
EQU G.sup.a.sub.1i (t)=.sub.n=0 .SIGMA..sup.M .alpha..sub.n g.sub.ai (t-n) (28) 
where .alpha..sub.n &lt;1, and a=1 to 6 for indicating the kind of spatial 
averaging. 
FIG. 30B shows a complete integration filter. In this example, multipliers 
M1 and M2 having multiplication coefficients .alpha. and .beta., 
respectively, adders ADD2 and ADD3, and a delay element T provide the 
following frequency control signal: 
EQU G.sup.a.sub.2i (t)=.alpha.g.sub.ai (t)+.beta..h.sub.ai (t) (29) 
EQU h.sub.ai (t)=h.sub.ai (t-1)+g.sub.ai (t) (30) 
FIG. 30C shows an incomplete integration filter. According to this example, 
multipliers M3 and M4 having multiplication coefficients .alpha. and 
.beta., respectively an adder ADD4, and a delay element T provide the 
following filtered mean value: 
EQU G.sup.a.sub.3i (t)=.alpha.g.sub.ai (t)+.beta.. G.sup.a.sub.3i (t-1) (31) 
This filter corresponds to that of FIG. 30A with the following conditions: 
EQU .alpha..sub.n =.alpha. 
EQU .alpha..sub.n =.beta..sup.n, n=1, 2, . . ., M 
EQU M.fwdarw..infin. (32) 
FIGS. 31 to 38 show results of simulations of frequency errors, i.e., 
controlled value errors. These simulations have been made with the 
averaging unit 2-5 with the filters of FIGS. 30A to 30C carrying out 
averaging operations and with the transmission frequency control unit 2-6 
of FIG. 13 controlling transmission frequencies. In these examples, a base 
station A is located adjacent to six base stations, as shown in FIG. 14. 
Base stations B, C, and D, shown in FIG. 14, carry out control according 
to frequency errors detected with respect to closest two, three, and four 
stations, respectively. Under an initial state with t=0, a frequency error 
of each base station occurs at random and is substantially uniformly 
distributed in a range of -1 to +1. 
(1) FIG. 31 corresponds to transmission frequency control carried out by 
the simple spatial arithmetic averaging of g.sub.li (t) of the equation 
(21). As shown in the figure, frequency errors with respect to the 
adjacent base stations approach 0 as time elapses. An abscissa indicates 
time, and an ordinate indicates normalized frequency errors. 
(2) FIG. 32 shows frequency errors according to control based on 
G.sup.a.sub.1i (t) obtained by a combination of the simple spatial 
arithmetic averaging of g.sub.1i (t) of the equation (21) and the temporal 
averaging operation of the transversal filter of FIG. 30A. Here, 
.alpha..sub.n =0.5 and M=8. Compared with those of FIG. 31, frequency 
errors of FIG. 29 more quickly converge (pull in) and involve a smaller 
time constant. The smaller time constant, however, is vulnerable to noise. 
(3) FIG. 33 shows frequency errors obtained by a combination of the simple 
spatial arithmetic averaging of g.sub.1i (t) of the equation (21) and the 
temporal averaging of the complete integration filter of FIG. 30B. 
Compared with those of FIG. 32, frequency errors of FIG. 33 more speedily 
converge. In this example, .alpha.=0.8 and .beta.=0.6 in the equation 
(29). 
(4) FIG. 34 shows frequency errors obtained by a combination of the simple 
spatial arithmetic averaging of g.sub.1i (t) of the equation (21) and the 
temporal averaging of the incomplete integration filter of FIG. 30C. This 
example achieves substantially the same convergence of frequency errors as 
that of FIG. 33. In this example, .alpha.=0.8 and .beta.=0.6 in the 
equation (32). 
(5) FIG. 35 shows frequency errors obtained by the simple spatial 
arithmetic averaging operation of g.sub.1i (t) of the equation (21) with a 
reference station R being set among base stations as shown in FIG. 14. 
FIG. 35 shows gradual convergence to a transmission frequency of the 
reference station R. In this example, the frequency error of the reference 
station R is set to 0.75 to clearly show the effect of the reference 
station. 
(6) FIG. 36 shows frequency errors obtained with use of the reference 
station R and a combination of the simple arithmetic averaging of g.sub.1i 
(t) of the equation (21) and the temporal averaging of the transversal 
filter of FIG. 30A. Compared with FIG. 35, it is understood that the speed 
of convergence to the transmission frequency of the reference station R is 
faster in FIG. 36. 
(7) FIG. 37 shows frequency errors obtained with use of the reference 
station R and a combination of the simple arithmetic averaging of g.sub.1i 
(t) of the equation (21) and the temporal averaging of the complete 
integration filter of FIG. 30B. It is understood that convergence to the 
transmission frequency of the reference station R is oscillating. 
(8) FIG. 38 shows frequency errors obtained with use of the reference 
station R and a combination of the simple arithmetic averaging of the 
above item (1) and the temporal averaging of the incomplete integration 
filter of FIG. 30C. It is understood that a waveform of gradual 
convergence to the transmission frequency of the reference station R 
resembles to that of the transversal filter. 
Embodiments of timing (phase) control (FIGS. 39 to 48) 
FIG. 39 is a schematic view showing essential part of a more practical 
embodiment of the network control system of FIG. 15 according to the 
invention. According to this embodiment, a base radio station BS1 is 
connected to adjacent base radio stations BS2 to BS5 to form a network, as 
shown in FIG. 40. FIG. 39 shows essential parts of the base radio station 
BS1. Each of the other base radio stations BS2 to BS5 has the same 
configuration as the base radio station BS1. A mobile station communicates 
with any one of the base radio stations BS1 to BS5 according to, for 
example, the TDMA method. At the same time, each of the base radio 
stations BS1 to BS5 transmits a signal having an optional frame clock 
frequency and an optional frame phase (controlled values). The embodiment 
explained below employs the TDMA method. The invention is also applicable 
for FDMA or CDMA method, if each of the base radio stations BS1 to BS5 
transmits a signal having an optional clock frequency and an optional 
clock phase with no regard to frames. 
In FIG. 39, comparators 201 to 204 have terminals 206 to 209, respectively. 
These terminals receive at time t-1, by wire or by radio, information 
pieces I(f.sub.2, .THETA..sub.2, t-1) to I(f.sub.5, .THETA..sub.5, t-1) 
for frame clock frequencies f.sub.2 to f.sub.5 and frame phases 
.THETA..sub.2 to .THETA..sub.5 from the base radio stations BS2 to BS5, 
respectively, as well as an information piece I(f1, .THETA..sub.1, t-1) of 
the base radio station BS1 from a voltage control oscillator (VCO) 210. 
The comparators 201 to 204 compare these signals. 
The comparators 201 to 204 provide errors between the frame clock frequency 
and frame phase at time t-1 of the base radio station BS1 and the frame 
clock frequencies and frame phases at time t-1 of the adjacent base radio 
stations BS2 to BS5. In this case, the comparators 201 to 204 each 
compares the numbers of clocks, i.e., clock frequencies that provide 
frames for determining time slots of TDMA with each other and the clock 
phases indicating frame heads with each other, and provides the error 
information. An adder 211 adds the output signals of the comparators 201 
to 204 to one another, and a divider 212 spatially filters, i.e., divides 
a result of the addition by the number of the adjacent radio stations 
(four in total, i.e., BS2 to BS5). A multiplier 213 multiplies a result of 
the spatial filtering by a coefficient .alpha. provided by a coefficient 
generator 214 and provides a first control signal indicating an error 
.DELTA.B(i, t-1) of the frame clock frequency and frame phase between the 
base radio station BS1 and the adjacent base radio stations BS2 to BS5, as 
shown in the equation (8). 
An averaging circuit 216 averages the first control signals and supplies 
the averaged result to a signal processing circuit 215, which converts the 
signal into a second control signal appropriate for controlling the VCO 
210. As a result, the VCO 210 is variably controlled to minimize the 
output oscillation frequency and phase error .DELTA.B(i, t-1). Then, the 
VCO 210 provides the signal B(i, t) of the equation (9) indicating the 
level of the node. The output signal of the VCO 210 is transmitted as 
transmission timing information through wire or radio to each of the 
adjacent base radio stations BS2 to BS5. 
According to this embodiment based on the TDMA method, the frame clock 
frequency and frame phase of the base radio station BS1 are corrected to 
match those of the adjacent base radio stations BS2 to BS5. According to 
the TDMA method, the clock phase and frame clock frequency are both 
controlled by the VCO 210. In the FDMA and CDMA methods, the VCO 210 
controls a clock frequency and a clock phase because these methods are 
irrelevant to frames. 
Next, essential parts of another embodiment of the network control system 
according to the invention will be explained. This embodiment involves 
base radio stations that communicate with a mobile station according to 
the TDMA method. In FIG. 39, base radio stations 402 to 404 controlled by 
a radio circuit central control station 401 transmit signals with the same 
frame phase and clock frequency as those of other base radio stations. A 
controlled value of this embodiment is only a frame phase. 
In FIG. 41, comparators 301 to 304 receive clock phase information 
I(.THETA..sub.1, t) of their own base radio station at time t from a phase 
control circuit 309 to be explained later. At the same time, terminals 305 
to 308 of the comparators 301 to 304 receive clock phase information 
i(.THETA..sub.2, t) to I(.THETA..sub.5, t) at time t of adjacent base 
radio stations (there are four adjacent base radio stations in this 
embodiment) by wire or radio. The comparators 301 to 304 detect 
differences between the information of their own station and the 
information of the adjacent stations. At this time, the phase control 
circuit 309 receives reference clock phase information I(.THETA..sub.0, t) 
from the central control station 401 of FIG. 42. 
An adder 310 adds output signals of the comparators 301 to 304 to one 
another, and a divider 311 divides a result of the addition by the number 
of the adjacent base radio stations (four in this embodiment) and provides 
a mean value. A multiplier 312 multiplies the mean value by a coefficient 
.alpha. provided by a coefficient generator 313 and provides a first 
control signal indicating a clock phase error between the base radio 
station in question and the adjacent base radio stations. 
An averaging circuit 315 averages the first control signals and supplies 
the averaged result to a signal processing circuit 314. The signal 
processing circuit 314 converts the received signal into a second control 
signal having a proper signal configuration for controlling the phase 
control circuit 309. The phase control circuit 309 controls the reference 
clock phase I(.THETA..sub.0, t) to zero a difference between the reference 
clock phase I(.THETA..sub.0, t) and the clock phases of the adjacent base 
radio stations. Namely, the phase control circuit 309 receives the 
reference clock phase I(.THETA..sub.0, t), adjusts it according to the 
second control signal, and provides a clock phase signal of its own 
station. Output signals of the comparators 301 to 304 provide information 
related to errors between the clock phase of its own station and the clock 
phases of the adjacent base radio stations. When the FDMA and CDMA methods 
are employed, the phase control circuit 309 controls clock phases. 
FIG. 43 shows a network based on the above embodiment having 80 nodes. 
FIGS. 44A to 44C and FIGS. 45A and 45B show results of simulations carried 
out on this network with initial values given at random and a coefficient 
.alpha. of 0.5. In each of the figures, an ordinate indicates the level 
B(i, t) of a node, and an abscissa time. FIGS. 44B and 44C show pull-in 
characteristics of different two nodes. FIG. 44A shows pull-in 
characteristics of all of the 80 nodes. As is apparent in FIG. 44A, the 
levels of all nodes become equal to one another after a predetermined 
period. 
In FIG. 43, if a node 81 is added to the network, pull-in characteristics 
of each node of the network will be slightly disturbed just after the 
addition of the node 81 as indicated with "I" in FIG. 45A. This 
disturbance, however, is absorbed to a synchronized state at once. If a 
node 73 is removed from the network as shown in FIG. 42, pull-in 
characteristics of each node of the network will be disturbed just after 
the removal of the node 73 as indicated with "II" in FIG. 45A. This 
disturbance is also absorbed to a synchronized state at once. 
In FIG. 45B, "a" indicates pull-in characteristics of the added node 80 
with an initial value of 0, and "b" represents characteristics that the 
value B(i, t) of the node 73 is forcibly zeroed and then removed from the 
network. Even if a node is added to or removed from the network, each node 
in the network is maintained in a stable synchronized state. 
With the TDMA, FDMA, or CDMA method for mobile communication, this 
embodiment can synchronize timing of base radio stations with one another 
in order to realize a hand-over with no momentary stoppage. In this case, 
when each node corresponds to a base radio station and when the magnitude 
of the node is given as frequency information or phase information, 
results of simulations show that the nodes are synchronized with one 
another. According to the embodiment, it is understood that frame 
synchronization is not greatly disturbed by addition of base radio 
stations, or failure of base radio stations. 
The invention is not limited to the network arrangement of FIG. 43. For 
example, the invention is applicable for network arrangements of FIGS. 46 
to 48. In FIGS. 46 and 47, adjacent nodes are basically connected to one 
another. In FIG. 48, some nodes (base radio stations) are locally 
synchronized with one another, and four nodes are disposed around and 
connected to each of the locally connected nodes. 
Embodiments with pull-in range (FIGS. 49 to 52) 
When a network is in a mutually synchronized state with a timing phase 
serving as a controlled value, and when one node (base station) in the 
network becomes uncontrollable for the mutual synchronization, the network 
will converge to a mutual mean value of all nodes because the network has 
no absolute reference. The effect is that, when one node in the network 
becomes uncontrollable, the other nodes follow the value of the 
uncontrollable node, and it will take a long time to restore the mutually 
synchronized state. This phenomenon is shown in FIG. 49, which is a 
simulation graph showing characteristics of time versus signal quantity 
Si(n). As shown in the figure, the mutually synchronized state is restored 
anyway but after a long time. 
If several nodes become uncontrollable, it will take quite a long time or 
be impossible to restore the mutually synchronized state. This phenomenon 
is shown in a simulation graph of FIG. 50. Since synchronization never 
progresses as time elapses, the signal quantity Si(n) never decreases. 
FIG. 51 shows means for solving these problems, according to the invention. 
In the figure, a signal received by a hybrid 501 is demodulated by a 
demodulator 502 and separated by a separator 503 into signals of 
individual adjacent nodes. The separated signals are compared with a 
reference value in differential detectors 504-1 to 504-n, which provide a 
pull-in range detector 505 with differences. The received data are 
received by a terminal circuit 509 through the separator 503. 
When the pull-in range detector 505 detects that any one of the differences 
is exceeding a predetermined threshold value, an indicator 506 displays, 
as alarm information, the node number, etc., of the exceeding difference, 
and a reset signal generation unit 507 generates a reset signal for the 
node number and provides the reset signal to a synthesis unit 508. 
The synthesis unit 508 adds the reset signal to transmission data from the 
terminal circuit 509 and provides it to a delay adjustment circuit 510. 
The delay adjustment circuit 510 delays the transmission data by a 
predetermined delay time in response to a control signal from the pull-in 
range detector 505. Thereafter, the transmission data is passed through a 
modulator 504 and transmitted from the hybrid 501. 
When the node that caused the exceeding difference receives the reset 
signal, the separator 503 and reset signal separator 511 thereof separate 
the reset signal and reset the pull-in range detector 505 thereof. At the 
same time, the delay adjustment circuit 510 is controlled to adjust 
pull-in timing into a pull-in range. 
As a result, a pull-in time is greatly shortened, as shown in a simulation 
graph of FIG. 52. 
Embodiments of node selection (FIGS. 53 to 55) 
As explained above, a time for stabilizing the mutual synchronization 
control is shortened by setting a pull-in range. When each node selects 
its comparison objective nodes at random, a sudden timing difference 
between adjacent nodes may spread to adjacent nodes, thereby taking a very 
long time (t=140) to stabilize the whole network (FIG. 53). 
To solve this problem, FIG. 54 shows a rule for selecting comparison 
objective nodes. The rule is set such that fluctuations in a network do 
not spread over nodes in the network. Nodes existing in the directions of 
arrow marks serve as central nodes of the network, and a given node is 
synchronized only with nodes that are closer to the central node, thereby 
establishing directionality. In practice, each node is provided with a 
directional antenna with which nodes to be selected are predetermined, 
thereby establishing directionality of the whole network as shown in FIG. 
51. Control information of each node is provided through a broadcast 
channel such as radio BCCH, and each node can optionally receive the 
information and correct an error of its own node. 
This technique can shorten (t-27) a convergence time needed for stabilizing 
the network, as shown in a simulation result of FIG. 55. 
Embodiments of synchronous control based on diversity reception (FIGS. 56 
and 57) 
As explained above, in the network control system of, for example, FIG. 13, 
the averaging unit 3-5 calculates a spatial mean of a frequency error 
.DELTA.f provided by the frequency error detection unit 3-4. At this time, 
the .DELTA.f is usually weighted by multiplying it by a received C/N 
provided by an S/N detection unit (not shown). This is also achieved in 
the timing control of FIG. 12. 
In these cases, it is necessary to prepare the S/N detection unit. Instead, 
this embodiment of the invention employs a diversity reception method and 
uses, in place of the S/N detection unit, a value measured by a received 
electric field strength measuring device that is usually arranged for each 
system. 
FIG. 56 shows a combination of the network control system of FIG. 13 based 
on frequencies and the diversity reception method. Reception frequency 
automatic control units 3-31 and 3-32 are arranged for signals received by 
two reception antennas RA1 and RA2, respectively. Measuring devices (E1, 
E2) 3-7 and 3-8 measure the received electric field strengths of 
intermediate frequency signals of the reception frequency automatic 
control units 3-31 and 3-32. A comparison/selection unit 3-9 compares 
values measured by the measuring devices 3-7 and 3-8 with each other and 
provides an averaging unit 3-5 with a larger one of the measured values. 
The selected result is also sent as a selection control signal for 
received data to a selector 3-10. 
FIG. 57 shows a combination of the network control system based on timing 
control and the diversity reception. What is different from FIG. 56 is 
that a transmission control unit 3-6 involves a timing control unit 3-11 
for receiving an output signal of an averaging unit 3-5 and controlling 
the timing of a modulator, and a timing error detection unit 3-12 for 
receiving a reception timing signal from one reception control unit 3-31 
as well as a transmission timing signal from the timing control unit 3-11 
and providing the averaging unit 3-5 with a timing error signal .DELTA.T. 
In this way, in either of the frequency control and timing control, one 
having a larger received electric field strength is added to a frequency 
error or a timing error in calculating a weighted mean value, thereby 
omitting the S/N detection unit. 
As explained above, the network control system according to the invention 
arranges communication means between adjacent nodes. Each node transmits 
its own controlled value. When receiving a controlled value from an 
adjacent station, a given node spatially or temporally filters the 
received controlled value and controls its own controlled value for the 
next time by minimizing a relative error between the controlled value that 
has been spatially or temporally filtered and its own controlled value. In 
this way, the given node communicates data related to the controlled 
values with adjacent nodes and finds a relative error between them. It is 
possible, therefore, to eliminate the relative errors of the controlled 
values at least around the given node. Consequently, the controlled values 
are surely corrected in the end over the whole system. 
Even if some of spatially spread reference signals are disturbed or cause 
abnormality, the spatial filtering operation carried out on error signals 
can remove or relax the influence of the disturbance and abnormality. Even 
if a string of temporally continuous error signals are partly disturbed or 
cause abnormality, the temporal filtering operation carried out on the 
error signals can remove or relax the influence of the disturbance and 
abnormality. 
According to the invention, a given node controls its own signal in a way 
to minimize a differential signal between the signal of its own and 
signals from other nodes according to the differential signal. When the 
system is initialized or depending on the magnitude of the differential 
signal, the invention changes control intervals, filtering 
characteristics, or level conversion characteristics, thereby most 
preferably maintains the speed of convergence of network synchronization 
and the stability of the network. 
According to the invention, each base station compares its own transmission 
frequency with transmission frequencies of adjacent base stations, and 
controls its own transmission frequency in a way to bring a relative 
frequency error close to a nominal frequency gap with respect to a 
reference station. Accordingly, under a steady state, a transient 
operation of an AFC for absorbing the frequency error of a mobile station 
is carried out only after a power source is turned ON. Similarly, it is 
not necessary for a base station to transiently operate the AFC when a 
mobile station enters into the zone of the base station in question or 
newly transmits a signal. This may greatly reduce momentary communication 
stoppage during a hand-over. 
The invention correctly controls a frequency gap between radio channels of 
adjacent base stations. Accordingly, it is not necessary to provide a 
margin in the frequency gap between the radio channels of the adjacent 
base stations in consideration of a frequency error, thereby improving the 
efficiency of frequency use in the communication system as a whole. 
The invention can form a distributed control network according to any one 
of TDMA, FDMA, and CDMA methods, with base radio stations serving as 
nodes. Unlike a conventional centralized control network, the invention 
can increase the number of nodes without substantially changing load on 
each node. According to the invention, each base radio station is 
connected to a plurality of adjacent base radio stations, so that, even if 
a given base radio station is disconnected from any one of its adjacent 
base radio stations, the given base radio station is able to stably carry 
out synchronous control according to the remaining normal base radio 
stations, thereby providing a backup function.