Method of assigning optimal channel in multi-station radio communications system

In a multi-station radio communications system wherein plural channels are accessible for communication between plural fixed stations and plural mobile units (viz., dynamic channel assignment) and wherein each of plural fixed stations selects a channel according to channel assignment priority data, the radio channels are grouped into a plurality of channel groups. The channels in each of the channel groups forms a numerically looped sequence of channels. The first channel of the numerically loop sequence in one channel group is different from the first channel of the numerically looped sequence in each of the other channel groups. Each of the plural fixed stations is set to select the channel having the highest priority or to select one of the first channel of the numerically looped sequence and a channel nearest to the first channel in the event that a plurality of channels have the same priority.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to a method of assigning an optimal 
speech or transmission channel in a multi-station radio communications 
system, and more specifically to such a method wherein plural fixed 
stations share in common, all the channels assigned to a system. The 
present invention is able to minimize co-channel interference and is able 
to retain optimal channel assignment information in the case of a power 
failure which cuts off the power supply to one or more of the fixed 
station(s). 
2. Description of the Prior Art 
It is known in the art to apply a multi-station radio communications system 
wherein all of the channels assigned to the system are accessible by a 
plurality of fixed stations located in the system. This kind of channel 
assignment is referred to as dynamic channel assignment, and finds 
extensive application within relatively small service areas such as within 
a building, factory etc. On the contrary, an application of fixed channel 
assignment to each of the fixed stations is practically difficult or 
impossible due to the sophisticated nature of the propagation 
characteristics. The term "fixed channel assignment" implies that each of 
fixed stations closely located is assigned a plurality of fixed carrier 
frequencies. The fixed channel assignment features a so-called frequency 
reuse. 
In order to avoid co-channel interference in either of dynamic or fixed 
channel assignment systems, it is well known in the art that a given fixed 
station selects an optimal channel according to channel assignment 
priority data which has been accumulated in the given station from the 
beginning of the system operation. More specifically, the channel 
assignment priority of a given channel at a given station is raised if it 
is accessed and used for communication, and is lowered if accessed but 
fails to establish communication. Viz., a channel which is more likely to 
provide optimal communication, is given a higher rating or priority over 
those which tends to exhibit less preferable characteristics and which are 
apt not to provide the required characteristics. 
However, when the system is initially put into use, all of the channels 
provided for the system have the same priority. That is to say, there is 
no difference in the channel priority. According to one prior art 
technique, all of the fixed stations are set to select the lowest numbered 
channel at the initial stage of operation. 
Accordingly, this prior art system has suffered from the drawbacks that as 
the fixed stations operate independently of one another and the lowest 
numbered channel is initially used at each of said mobile or stationary 
stations, in the event that relatively close stations are simultaneously 
contacted by different mobile units and the stations in question assign 
the same channel, situations wherein co-channel interference or cross-talk 
is highly apt to occur. 
A further drawback has been encountered in that the channel assignment 
priority data which is compiled over a period of time is stored in a 
volatile memory which can be subject to undesirable erasure. More 
specifically, the above-mentioned prior art system is usually connected 
with a commercially available power supply, and hence in the event that a 
power failure or similar type of power disturbance occurs, the power 
supply which maintains the memory in question is temporarily 
disturbed/cut-off. In the event of such a memory loss, an undesirably long 
period of time is required before the channel status data is 
re-accumulated and the system is restored to the level of operation 
acquired prior the power interruption. Accordingly, the system is 
temporarily reduced to the situation wherein the initial cross-talk and/or 
co-channel interference problems are undesirably encountered all over 
again. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a method of eliminating 
the co-channel interferences which are highly apt to occur at the initial 
stage of operation. 
Another object of the present invention is to provide a method by which the 
channel assignment priority data which has been compiled over a long 
period of time can be retained in the event of a power failure. 
In brief, the above object is achieved by a method wherein plural channels 
are accessible for communication between plural fixed stations and plural 
mobile units (viz., dynamic channel assignment) and wherein each of plural 
fixed stations selects a channel according to channel assignment priority 
data, the radio channels are grouped into a plurality of channel groups. 
The channels in each of the channel groups forms a numerically looped 
sequence of channels. The first channel of the numerically loop sequence 
in one channel group is different from the first channel of the 
numerically looped sequence in each of the other channel groups. Each of 
the plural fixed stations is set to select the channel having the highest 
priority or to select one of the first channel of the numerically looped 
sequence and a channel nearest to the first channel in the event that a 
plurality of channels have the same priority. 
More specifically an aspect of the present invention comes in a method of 
assigning radio channels in a multi-station radio communication system 
wherein plural channels are accessible for communication between plural 
fixed stations and plural mobile units and wherein each of the plural 
fixed stations selects a channel according to channel assignment priority, 
the method comprising steps of: (a) grouping the radio channels which are 
numbered in ascending or descending order, into a plurality of channel 
groups each of which includes the same number of the radio channels, the 
number of the channel groups corresponding to the number of the fixed 
stations, the radio channels in each of the channel groups forming a 
numerically looped sequence of channels wherein each numerically looped 
sequence finishes at a channel which is numerically adjacent the first 
channel of the sequence, the first channel of a numerically looped 
sequence in one channel group being different from the first channel of 
the numerically looped sequences in each of the other channel groups; (b) 
assigning the channel groups to the plural fixed stations on a one-to-one 
basis; and (c) setting each of the plural fixed stations to select one of 
the channels assigned thereto which has a highest channel assignment 
priority, or to select one of the first channel of a numerically looped 
sequence and a channel nearest to the first channel in the event that 
plural channels have the same channel assignment priority.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 is a sketch schematically illustrating an overall arrangement of a 
multi-station radio communications system to which the present invention 
is directed. 
A plurality of fixed stations 10(1)-10(10) are provided for communication 
with a plurality of mobile units 12(1)-12(n). Merely for the convenience 
of description it is assumed that the number of the fixed stations is ten 
(10). A mobile communications switching controller (MCSC) 14 is 
interconnected to the fixed stations 10(1)-10(10) via lines 16(1)-16(10) 
respectively, and also is coupled to a telephone network 18 via a 
plurality of lines (no numerals). 
As best shown in FIG. 2 each of the mobile units 12(1)-12(n) is able to 
communicate with each of the fixed stations 10(1)-10(10) through a 
selected one of plural radio speech channels 1-50. A control channel 51 is 
provided for managing radio communications between the fixed stations 
10(1)-10(10) and the mobile units 12(1)-12(n). The above-mentioned number 
of the radio speech channels (viz., 50) is merely exemplary and of course 
is not limited thereto. Further, more than two control channels can be 
provided if necessary. 
Reference is made to FIG. 3, wherein the fixed station 10(1) is shown in 
detail in block diagram form together with a single mobile unit denoted by 
12(m). This mobile unit 12(m) is one of the above-mentioned plural mobile 
units 12(1)-12(n). It should be noted that each of the other fixed 
stations 10(2)-10(10) has the same configuration as the station 10(1). 
Before discussing the first preferred embodiment in detail, the background 
thereof will first be described. 
Each of a receiver 20 and a transmitter 36 is previously tuned, under 
control of a microprocessor 26, to a frequency of the control channel 51 
(FIG. 2) before a speech channel is established between the station 10 and 
the mobile unit 12(m). When the mobile unit 12(m) wishes to initiate a 
call, the call request is transmitted via the control channel 51 to the 
receiver 20 via an antenna 22 and a duplexer 24. A signal indicative of 
the call request is demodulated at the receiver 20 and is applied to the 
microprocessor 26 via an interface 28. The microprocessor 26 supervises 
the operation of the fixed unit 10(1) using an operation program 
pre-stored in a ROM (read-only-memory) 30. A RAM (random-access-memory) 32 
is used as a work space of the microprocessor 26. A non-volatile memory 34 
is provided for storing channel assignment priority data for all of the 
radio speech channels 1-50 (FIG. 2) in the system in question. The memory 
34 takes the form of an EEPROM (electrically erasable programmable ROM) 
merely by way of example, and is directly concerned with a second 
preferred embodiment of the present invention as referred to below. 
The microprocessor 26 accesses the non-volatile memory 34 in response to 
the above-mentioned call request and determines an optimal speech channel 
on the basis of the channel priority information stored in the memory 34. 
A signal indicative of the selected channel is applied to the transmitter 
36 via the interface 28 and a switch 38. The transmitter 36 modulates the 
selected channel signal with the frequency of the control channel 51 and 
applies the same to the mobile unit 12(m) via the control channel 51. 
Thereafter, the microprocessor 26 tunes the receiver 20 and the 
transmitter 36 to the frequency of the selected channel. The mobile unit 
12(m), in response to the selected channel signal received, tunes the 
transmitter thereof (not shown) to the selected speech channel and sends 
back a reply to the fixed unit 10(1) via the selected speech channel. 
In the event that the microprocessor 26 receives the reply from the mobile 
unit 12(m) through the selected or assigned channel and detects no 
co-channel or interchannel interference in connection with the selected 
speech channel, the microprocessor 26 applies a ready signal to the mobile 
communications switching controller 14 (FIG. 1) via interfaces 40, 44 and 
a switch 42. In this event, the microprocessor 26 accesses the 
non-volatile memory 34 and increases the priority assigned to the radio 
speech channel selected. Following this, the microprocessor 26 changes the 
switch positions of the switches 38, 42 to the other positions indicated 
by broken lines via control lines 29, 41, respectively. Thereafter, the 
controller 14 issues a dial tone signal which is applied to the mobile 
unit 12(m) via the interface 44, the switch 38, the transmitter 36, etc., 
after which the mobile unit 12(m) issues a dialing signal. The following 
steps will be apparent to those skilled in the art and hence further 
description will be omitted for brevity. 
Contrary to the above case, viz., if the fixed station 10(1) is incapable 
of receiving the reply from the mobile unit 12(m) via the channel 
selected, or if the station 10(1) detects channel interference to an 
undesirable extent in connection with the channel chosen, then the 
microprocessor 26 lowers the priority assigned to the channel selected. 
In the case where a subscriber connected to the telephone network initiates 
a call to the mobile unit 12(m), the steps similar to the aboves are 
carried out. 
The manner in which the priority assigned to each of the speech channels is 
renewed is not directly concerned with the present invention and, 
therefore will not be referred to in detail. For further details 
concerning the manner in which the just mentioned channel assignment 
priority renewal is carried out, reference should be made to U.S. Pat. No. 
4,747,101 assigned to the same entity as the assignee of this invention, 
for example. 
As will be apparent, when the system shown in FIG. 1 is initially put into 
use, the priorities assigned to the radio speech channels 1-50 (FIG. 2) 
are all the same. As mentioned in the opening paragraphs of the instant 
specification, according to the prior art, during initial start-up each of 
the fixed stations 10(1)-10(10) is set to use the lowest numbered one of 
the channels which exhibit the same priority. Accordingly, if the fixed 
stations 10(1)-10(10) are permitted to independently select speech 
channels, the prior art has encountered the problem of co-channel 
interference. 
In order to overcome such a problem, in accordance with the present 
invention, the stations 10(1)-10(10) are respectively assigned channel 
groups #1-#10 as shown in FIG. 4. More specifically, the channels Nos. 
1-50 are grouped into ten as follows: 
(a) Group #1: channels Nos. 1-50; 
(b) Group #2: channels Nos. 6-50 and 1-5; 
(c) Group #3: channels Nos. 11-50 and 1-10; 
(d) Group #4: channels Nos. 16-50 and 1-15; 
(e) Group #5: channels Nos. 21-50 and 1-20; 
(f) Group #6: channels Nos. 26-50 and 1-25 
(g) Group #7: channels Nos. 31-50 and 1-30; 
(h) Group #8: channels Nos. 36-50 and 1-35; 
(i) Group #9: channels Nos. 41-50 and 1-40; and 
(j) Group #10: channels Nos. 46-50 and 1-45. 
As shown above, each group forms a loop, in terms of channels numbers, 
whose direction corresponds to ascending channel number in this case. By 
way of example, the loop of the group #4 begins at the channel No. 16 and 
ends at the channel No. 15 after passing through channel Nos. 50 and 1 in 
this order. It is assumed that the lowest numbered channel in a given loop 
is the first channel in the loop and the highest numbered channel in the 
given loop is the last channel in the loop. In the event that there exists 
a plurality of channels having the same priority in a given loop 
especially during the initial stage of the system operation, then a given 
fixed station selects the lowest numbered one among the channels each of 
which has the same priority and is not used by any of the stations. 
This procedure continues until each of the channels exhibits a different 
priority, whereafter a known technique can be utilized to perform an 
optimal channel selection according to the priority data stored. 
As mentioned previously in connection with FIG. 3, the non-volatile memory 
(EEPROM) 34 is provided for storing the channel assignment priority data 
which has been compiled over a long period of time. A multi-station radio 
communications system employing the dynamic channel assignment, to which 
the present invention is applicable, is commonly coupled with a 
commercially available power supply and, therefore a volatile memory is 
subject to undersirable erasure in the event of a power failure. However, 
this problem can effectively be minimized through the use of the 
non-volatile memory 34 as shown in FIG. 3 (second preferred embodiment). 
A third preferred embodiment of the present invention will be discussed 
with reference to FIG. 5, wherein the mobile communications controller 14 
is shown in detail and arranged between the telephone network 18 and a 
plurality of fixed stations 10(1)'-10(10)'. The controller 14 comprises a 
speech path switching circuit 50, a plurality of interfaces 52(1)-52(10) 
provided at the side of the telephone network 18, a plurality of 
interfaces 54(1)-54(10) arranged between the circuit 50 and the stations 
10(1)'-10(10)', a microprocessor 56, a main memory 58 and a channel 
assignment data memory 60, all of which are coupled as shown. 
Although not illustrated in FIG. 5, the fixed stations 10(1)'-10(10)' 
respectively differ from the counterparts 10(1)-10(10) in that each of the 
former stations is not provided with a non-volatile memory and stores the 
channel assignment data in a volatile memory such as a RAM. The memory 60 
is a non-volatile memory and is provided with a plurality of memory 
sections which are respectively allocated to store the channel assignment 
data of the fixed stations 10(1)'-10(10)'. 
The microprocessor 56 controls, using an operation program pre-stored in 
the main memory 58, the overall operation of the switching controller 14, 
which includes speech path selection by which each of the fixed stations 
10(1)'-10(10)' is selectively coupled to a plurality of transmission lines 
of the telephone network 18. The microprocessor 56, in addition to the 
above operation control, instructs each of the fixed stations 
10(1)'-10(10)' to apply, periodically or non-periodically, the channel 
assignment data stored in the corresponding volatile memory (not shown) to 
the controller 14. The data thus applied to the controller 14 are 
respectively stored in the predetermined memory sections of the data 
memory 60. Therefore, if a power failure occurs at the fixed stations 
10(1)'-10(10)' and/or the controller 14, the channel assignment data up to 
the power failure can be retained in the non-volatile memory 60. After the 
recovery of power supply, the microprocessor 56 applies a plurality of 
data stored in the channel assignment data memory 60 to the corresponding 
fixed stations 10(1)'-10(10)'. Thus, the worst case can be prevented 
wherein the whole entire channel assignment data of each of the fixed 
stations 10(1)'-10(10)' is totally erased. 
As an alternative to the above, each of the fixed stations 10(1)'-10(10)' 
may apply the channel assignment data stored therein to the controller 14 
at a predetermined time interval or non-periodically. 
While the foregoing description described preferred embodiments according 
to the present invention, the various alternatives and modifications 
possible without departing from the scope of the present invention, which 
is limited only by the appended claims, will be apparent to those skilled 
in the art.