Multi-carrier communication method and device

Frequencies (f1 to f7) are selected in respective time slots (T1 to T7) according to digital data to be transmitted. Digital data to be transmitted is indicated by a combination of frequencies selected in the respective time slots. Since a single time slot can select a plurality of frequencies, e.g., f2, f4 and f5, the total number of frequency patterns which can be achieved is increased. Therefore, since the number of combined patterns of substantially available frequencies is also increased, the number of bits of digital data which can be transmitted in a single data cycle (a period related to T1 to T7) is increased, and a transmission efficiency is improved.

BACKGROUND OF THE INVENTION 
a) Field of the Invention 
The invention relates to a spread spectrum communication method and a 
communication device adopting this method, and more particularly to a 
frequency hopping type spread spectrum communication method and device. 
b) Description of the Related Art 
A spread spectrum communication is a communication method to transmit a 
signal with its bandwidth spread over a bandwidth wider than the frequency 
bandwidth of data to be transmitted, and has advantages that it is 
resistive to interference, can keep signals in secrecy, and can accomplish 
high resolution distance measuring. In addition to fields of satellite 
communications, ground communications, or the like, a spread spectrum 
communication, is being applied to mobile and local communications with 
the expectation of improving frequency utilization efficiency while 
maintaining compatability with existing systems. 
Typical methods for achieving spread spectrum communication include DS 
(Direct Sequence) and FH (Frequency Hopping) methods. A DS method spreads 
an occupied frequency bandwidth by balanced modulation of a direct spread 
code pulse to data modulated by a carrier wave, while an FH method 
utilizes a broad occupied frequency bandwidth by switching (or hopping) 
the carrier frequency of the modulated data according to a spread code 
pulse. Especially, a fast FH method switches the frequency faster than the 
information rate so as to be resistive against interference and to excel 
in distance and frequency diversity effect. This fast FH method is being 
marked for mobile communications and indoor communications which are 
heavily affected by fading. 
Generally, the FH method modulates data by an FSK (Frequency Shift Keying). 
Specifically, data to be transmitted is converted into a codeword for 
every several bits, and a frequency is shifted according to the codes 
(codeword chip) forming the codeword. For example, data is converted into 
one of eight codewords for every 3 bits of input data. More specifically, 
when input data is "000", it is converted into a codeword "7-6-5-2-4-1-3". 
"0" to "1" which form the codeword are simply referred to as a code or 
codeword chip, and it is devised to array the codes in each codeword so 
that data of "000" to "111", can be classified on a receiving side. 
A different frequency is allocated to the respective codes. For example, 
frequencies f0 to f7 correspond to codes "0" to "1", respectively. To 
transmit "000", according to its corresponding codeword "7-6-5-2-4-1-3", 
the frequencies are changed into order of "f7, f6, f5, f2, f4, f1, f3" and 
outputted. Since the eight types of frequencies f0 to f7 are used, 
modulation in this case can be 8-level MFSK (Multilevel FSK) modulation. 
Data modulation (modulation regardless of the frequency hopping) will be 
referred to as primary modulation. 
The frequency of a carrier wave is hopped according to a diffusion code 
series (pulse train of pseudo noise code; hereinafter referred to as "code 
series") for frequency hopping modulation. If the number of codes 
contained in this code series is 31, 31 different frequencies are selected 
as hopping frequencies in a frequency band approved for use (the frequency 
hopping itself may also be said to be FSK in a broad sense, but the term 
FSK in this specification is used for the primary modulation only). A 
cycle in which the code series takes a round is referred to as a code 
cycle, and a cycle (1/31 of the code cycle in this case) in which the 
hopping frequency is switched is referred to as a hopping period. The 
hopping frequency is switched in synchronization with the switching of a 
frequency by the primary modulation. 
Such an FH-MFSK method is introduced as a combination having higher 
affinity than the existing methods in various publications, one example 
being "Frequency-Hopped Multilevel FSK for Mobile Radio" by D. J. Goodman, 
et. al., The Bell System Technical Journal, Vol. 59, No. 7, pp. 1257-1275, 
1980. A combined method of M-ary FSK which is an improved version of MFSK 
and FH is reported in "General Description and Basic Characteristics of 
Ground Mobile Frequency Hopping Type Communication Experimental 
Arrangement" by Eimatsu Moriyama, et. al., Quarterly Journal of Radio 
Research Laboratory, Vol. 32, No. 164, pp. 165-177, 1986. 
A conventional MFSK and FH combined method will be described. FIG. 4 shows 
a conceptual diagram illustrating the allocation of carrier frequencies by 
the conventional MFSK+FH method. 
FIG. 4 shows that 6-bit digital data is first converted into a combined 
pattern of seven frequencies by primary modulation. These seven 
frequencies have frequency patterns selected according to the 6-bit 
digital data, and a single carrier frequency is selected in respective 
time slots (T1, T2, . . . , T7). A table showing which frequency (f1, f2, 
. . . , f8) is selected in the respective time slots (T1 to T7) is 
referred to as a primary modulation matrix through this specification. 
In the case of the primary modulation matrix indicated by (a) in FIG. 4, 
the frequency f2 is selected in time slot T1, and frequency f6 is selected 
in the time slot T2. Thus, the 6-bit digital data is, so to speak, coded 
into a combined pattern of seven frequencies by the primary modulation 
matrix. 
Then, frequency conversion, namely spread modulation, is performed for 
frequency hopping (FH). This frequency hopping is performed by further 
spreading the frequencies f1 to f8 which were selected by the primary 
modulation matrix to 127 types of carrier frequencies. In this 
specification, a table showing diffusion is referred to as a hopping 
matrix. In the hopping matrix shown in FIG. 4, the horizontal axis 
indicates the time slots T1 to T7, while the vertical axis indicates 127 
types of frequency carrier waves. These frequencies F1 to F127 are 
indicated with an uppercase "F" added to distinguish them from the 
intermediate frequencies f1 to f8. 
For example, in the case shown in FIG. 4, frequency f2 is selected in time 
slot T1 by primary modulation. This frequency f2 is converted into 
frequency F4 by the hopping matrix shown in FIG. 4. The frequencies f1 to 
f8 undergone the primary modulation are converted into the frequencies F3 
to F10 in time slot T1 as shown in the hopping matrix. Accordingly, eight 
boxes indicated by a heavy line in the hopping matrix correspond to the 
eight frequencies f1 to f8 in a single time slot of the primary modulation 
matrix. 
Diffusion of frequencies by the hopping matrix is determined by which 
hopping matrix frequency (F1 to F127) the frequency f1 in a predetermined 
time slot is to be converted. For example, by the frequency hopping 
indicated by the hopping matrix shown in FIG. 4, the frequency f1 is 
converted into the frequency F3 in the time slot T1, F100 in the time slot 
T2, and F55 in the time slot T3. 
As described above, the communication method, in that the primary 
modulation is performed by the MFSK modulation using a plurality of 
frequencies and the frequency hopping (diffusion modulation) is performed 
by the diffusion code series, uses a signal with only a single frequency 
in a single time slot. Therefore, improvement of the transmission 
efficiency is limited. 
The transmission efficiency will be described with reference to the primary 
modulation matrix shown in FIG. 4. 
By the conventional method shown in FIG. 4, the eight types of frequencies 
f1 to f8 can be selected in each time slot. Since these eight frequencies 
are selected over seven time slots, the total number of possible patterns 
is 8.sup.7 (=2,097,152). Meanwhile, since 6-bit digital data is converted 
into a pattern formed of these seven frequencies, the number of actually 
used patterns is 2.sup.6 (=64). Therefore, the total number of patterns is 
related to the number of used patterns and expressed by the following 
expression. 
EQU Total number of patterns/Number of used patterns=8.sup.7 /2.sup.6 =2.sup.21 
/2.sup.6 =2.sup.15 (1) 
Therefore, the number of patterns to be used is only 1/2.sup.15 the total 
number of patterns. In other words, the pattern of selectable frequencies 
is given an allowance of 15 bits. 
Since the conventional communication method adopting MFSK and FH outputs 
only one frequency in each time slot, improvement of the transmission 
efficiency is limited. 
SUMMARY OF THE INVENTION 
In view of the above disadvantages, it is an object of the present 
invention to provide a communication method which can increase the number 
of transmission bits per time slot and improve a transmission efficiency 
by using a frequency hopping communication method to render the frequency 
of a carrier wave which can be outputted in a single time slot multiple. 
A first aspect of the invention relates to a multi-carrier communication 
method which comprises a selecting step for selecting, on the basis of 
data to be transmitted, a frequency pattern consisting of a plurality of 
frequencies which correspond to the data, and an outputting step for 
outputting carrier waves of the plurality of frequencies. 
Thus, since the plurality of frequencies are transmitted in correspondence 
with the data, a utilization efficiency of frequencies can be improved, 
and a transmission efficiency can be improved. 
A second aspect of the invention relates to a multi-carrier frequency 
hopping communication method for transmitting by sequentially switching 
the frequency of a carrier wave at a given time period, which comprises a 
primary modulation step for selecting, based on data to be transmitted, N 
frequencies corresponding to the data and outputting information about the 
selected frequencies, and a diffusion modulation step for outputting 
carrier frequencies of N types of frequencies based on information about 
the N frequencies selected in the primary modulation step and information 
about diffusion code series for frequency hopping, N being an integer 
greater than or equal to 2. 
Especially, since a plurality of frequencies to be hopped are selected in 
the frequency hopping communication, the number of bits which can be 
transmitted in a single data period is increased by virtue of a 
combination of frequency patterns. 
A third aspect of the invention relates to the multi-carrier frequency 
hopping communication method according to the second aspect of the 
invention, wherein the diffusion modulation step has an N carrier 
diffusion modulation step for outputting carrier frequencies of N types of 
frequencies by modulating a signal with the selected N frequencies based 
on information about diffusion code series for the frequency hopping. 
In the third aspect of the invention, the diffusion modulation step 
performs a conventional diffusion modulation based on the diffusion code 
series. Therefore, the conventional diffusion modulation step can be used 
as it is. 
In order to remedy the above-described disadvantages, a fourth aspect of 
the invention relates to the multi-carrier frequency hopping communication 
method according to the second aspect of the invention, wherein the 
diffusion modulation step includes a waveform reading step for reading 
waveform information about N types of frequencies from waveform 
information about carrier waves of prerecorded multiple frequencies based 
on a signal with the selected N frequencies and information about 
diffusion code series for the frequency hopping, and a synthesized 
diffusion modulation step for outputting a signal with a synthesized 
waveform by synthesizing the waveform read in the waveform reading step. 
In the fourth aspect of the invention, a waveform is stored for every 
carrier wave, and the waveforms are merely synthesized, so that a number 
of frequencies can be readily produced. 
A fifth aspect of the invention relates to the multi-carrier frequency 
hopping communication method according to the second aspect of the 
invention, wherein the diffusion modulation step includes a synthesized 
diffusion modulation step for reading pertinent synthesized waveform 
information from information about prerecorded multiple synthesized 
waveforms based on a signal with the selected N frequencies and 
information about diffusion code series for the frequency hopping, and 
outputting a signal with the synthesized waveform. 
In the fifth aspect of the invention, the synthesized waveforms of the 
respective carrier waves are stored, and the waveforms are simply read, so 
that a plurality of frequencies can be readily produced. 
A sixth aspect of the invention relates to a multi-carrier communication 
device which comprises a selection unit for selecting a frequency pattern 
consisting of a plurality of frequencies corresponding to data to be 
transmitted, on the basis of the data, and an output unit for outputting 
carrier waves of the plurality of frequencies. 
The sixth aspect of the invention is a device producing the effects of the 
first aspect of the invention. 
A seventh aspect of the invention relates to a multi-carrier frequency 
hopping communication device for communicating by sequentially switching 
carrier frequencies at a given time period, which comprises a primary 
modulation unit for selecting, based on data to be transmitted, N 
frequencies corresponding to the data and outputting information about the 
selected frequencies, and a diffusion modulation device for outputting 
carrier frequencies of N types of frequencies based on information about 
the N frequencies selected by the primary modulation unit and information 
about diffusion code series for frequency hopping, N being an integer 
greater than or equal to 2. 
The device of the seventh aspect of the invention produces the effects of 
the second aspect of the invention. 
An eighth aspect of the invention relates to a multi-carrier frequency 
hopping communication device according to the seventh aspect of the 
invention, wherein the diffusion modulation unit has an N carrier 
diffusion modulation unit for outputting carrier frequencies of N types of 
frequencies by modulating a signal with the selected N frequencies based 
on information about diffusion code series for the frequency hopping. 
The device of the eighth aspect of the invention produces the effects of 
the third aspect of the invention. 
A ninth aspect of the invention relates to a multi-carrier frequency 
hopping communication device according to the seventh aspect of the 
invention, wherein the diffusion modulation unit includes a waveform 
reading unit for reading waveform information about N types of frequencies 
from waveform information about carrier waves of prerecorded multiple 
frequencies based on a signal with the selected N frequencies and 
information about diffusion code series for the frequency hopping, and a 
synthesized diffusion modulation unit for outputting a signal with a 
synthesized waveform by synthesizing the waveform read by the waveform 
reading unit. 
The device of the ninth aspect of the invention produces the effects as the 
fourth aspect of the invention. 
In order to remedy the above-described disadvantages, a tenth aspect of the 
invention relates to the multi-carrier frequency hopping communication 
device according to the seventh aspect of the invention, wherein the 
diffusion modulation unit includes a synthesized diffusion modulation unit 
for reading pertinent synthesized waveform information from waveform 
information about prerecorded multiple synthesized waveforms based on a 
signal with the selected N frequencies and information about diffusion 
code series for the frequency hopping, and outputting a signal with the 
synthesized waveform. 
The device of the tenth aspect of the invention produces the effects of the 
fifth aspect of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Preferred embodiments of the invention will be described with reference to 
the accompanying drawings. 
FIG. 1 is an explanatory diagram showing the principle of a multi-carrier 
frequency hopping communication method according to the invention. As 
shown in FIG. 1, digital data to be transmitted is converted into a 
pattern consisting of a plurality of frequencies by primary modulation by 
the multi-carrier frequency hopping communication method according to this 
embodiment. The primary modulation is shown by a primary modulation matrix 
as indicated by (a) in FIG. 1. 
It is apparent from this primary modulation matrix and the primary 
modulation matrix indicated by (a) in FIG. 4 that the multi-carrier 
frequency hopping communication method of the invention is a communication 
method to output a plurality of frequencies in a single time slot. 
The embodiment has a feature to select a plurality of frequencies in a 
single time slot. Since it is configured to select a plurality of 
frequencies, the number of patterns of frequencies to be adopted can be 
increased. 
With the primary modulation matrix indicated by (a) in FIG. 1, three types 
of frequencies are selected in respective time slots (T1 to T7). Thus, 
since a plurality of frequencies are outputted in a single time slot by 
the multi-carrier frequency hopping communication method of this 
embodiment, the number of selectable frequency patterns can be increased 
substantially as compared with a conventional communication method. 
Therefore, the number of bits of digital data which can be transmitted by 
a single frequency pattern can be increased, and a transmission efficiency 
can be improved. 
After converting into signals with a plurality of frequencies by the 
primary modulation matrix indicated by (a) in FIG. 1, the plurality of 
frequencies undergo a frequency hopping treatment (diffusion treatment) 
according to a predetermined code system in the same way as a conventional 
communication method. This treatment is performed on the basis of the 
hopping matrix indicated by (b) in FIG. 1. Diffusion by the hopping matrix 
is the same as the conventional hopping matrix as indicated by (b) in FIG. 
4. 
This multi-carrier frequency hopping communication method selects the 
multiple frequencies with respect to the single time slot by the primary 
modulation matrix, so that finally outputted hopping frequencies are 
present in multiple numbers in a single time slot as indicated by (b) in 
FIG. 1. For example, three types of carrier waves F4, F6 and F7 are 
outputted in time slot T1 as indicated by (b) in FIG. 1. On the other 
hand, the conventional frequency hopping communication method shown in 
FIG. 4 outputs a carrier wave with the single frequency F4 in the time 
slot T1. 
Thus, in order to output a plurality of carrier frequencies in a single 
time slot, this embodiment is configured to select a plurality of 
frequencies by a single time slot at the time of the primary modulation of 
digital data. Therefore, since the number of available frequency patterns 
is increased and the number of frequency patterns selectable by the time 
slots T1 to T7 is increased, the number of bits of transmittable digital 
data is increased. 
Improvement of transmission efficiency: 
As shown by the primary modulation matrix indicated by (a) in FIG. 1, three 
types of frequencies can be selected in a single time slot by the primary 
modulation matrix according to this embodiment. As a result, the total 
number of frequency patterns selectable in one data cycle (a period of 
time slots T1 to T7) is increased. Consequently, the number of available 
frequency patterns is also increased, and the number of transmission bits 
per time slot is expected to increase. 
Descriptions will be made of the number of bits of transmittable digital 
data when three frequencies are selected in a single time slot as shown by 
the primary modulation matrix as indicated by (a) in FIG. 1. 
Since three types of frequencies are first selected from eight types of 
frequencies in a single time slot, the number of possible combinations 
thereof is (.sub.8 C.sub.3).sup.7 =56.sup.7. The number of available 
patterns is calculated from this total pattern number of 56.sup.7 with an 
allowance of 15 bits provided in the same way as the conventional 
communication method shown in FIG. 4. In other words, patterns of 
1/2.sup.15 of the total number of patterns were actually used by the 
conventional communication method shown in FIG. 4. In this embodiment, it 
is appropriate to adopt the same allowance of bits in view of making a 
comparison with the conventional communication method. 
Then, since the total number of patterns is (56.sup.7) in this embodiment, 
the number of available patterns becomes (56.sup.7)/(2.sup.15)=2.sup.5 
.times.7.sup.7. And, 2.sup.5 .times.7.sup.7 is equal to or more than 
2.sup.25, then digital data of about 25 bits or more can be transmitted in 
one cycle under the conditions with the same allowance of bits as the 
conventional communication method. 
As described above, this embodiment is configured in a different way from 
the conventional MFSK so that a plurality of frequencies can be selected 
in a single time slot. Therefore, the total number of frequency patterns 
in a single data cycle (a period of time slots T1 to T7) can be increased. 
As a result, the number of available patterns can be increased. Also, the 
number of bits of digital data transmittable in a single data cycle can be 
increased, and a transmission efficiency can be improved extensively. 
Since three types of frequencies are selected in a single time slot in the 
embodiment shown in FIG. 1, the number of bits transmittable in a single 
data cycle (a period of time slots T1 to T7) is increased to about 25 
bits, more than 4 times the 6 bits of the conventional method as described 
above. However, the number of frequencies selectable in a single time slot 
can itself be another numerical value. For example, when it is assumed 
that the number of frequencies selectable in a single time slot is four, 
the total number of patterns can be increased further. Since, it can be 
confirmed by the same calculation as described above that when the number 
of frequencies selected in a single time slot is determined to be two, the 
number of bits of transmittable digital data grows by a factor of three. 
As shown in FIG. 1, this embodiment selects multiple frequencies in a 
single time slot by the primary modulation in order to output carrier 
waves for the multiple frequencies in the single time slot and uses the 
conventional communication method as it is "in principle" to perform 
diffusion modulation (frequency hopping). But, it is thought that the same 
effects can be obtained by selecting only one frequency in a single time 
slot by the primary modulation in the same way as the conventional 
communication method and dispersing to a plurality of frequencies by the 
diffusion modulation. However, it is thought that the structure configured 
as described above is somewhat hard to establish synchronization by the 
frequency hopping communication method. Therefore, for convenience in 
configuring the device, it is preferably configured to select previously a 
plurality of frequencies in a single time slot by the primary modulation 
as shown by the primary modulation matrix indicated by (a) in FIG. 1. 
Configuration of transmitter and receiver: 
Descriptions will be made of the structures of a transmitter and a receiver 
which adopt the principle of the communication method of the 
above-described embodiment with reference to the drawings. 
FIG. 2 is a block diagram showing the structure of a transmitter which 
adopts the communication method of the invention. It is seen from FIG. 2 
that digital data to be transmitted is converted by an encoding circuit 
into the frequency pattern shown by the primary modulation matrix as 
indicated by (a) in FIG. 1. This frequency pattern is converted into a 
single frequency pattern on a 25-bit basis. This encoding circuit 10 
outputs three types of modulation frequency data for every time slot T1 . 
. . T7. For example, the encoding circuit 10 outputs signals indicating 
three frequencies f4, f6 and f7 in the time slot T2. Since eight types of 
frequencies f1 to f8 can be outputted, the modulation frequency data 
outputted by the encoding circuit 10 includes three 3-bit digital data 
indicating any frequency from f1 to f8. This encoding circuit 10 can have 
various structures if it can convert 25-bit digital data into patterns 
each having three frequencies in the time slots T1 to T7. For example, a 
predetermined encoding circuit having a feedback shift register is 
preferably used, and it may also be configured to convert 25-bit digital 
data into a frequency pattern by simply storing a conversion table into a 
memory and referring to this conversion table stored in the memory. This 
conversion is nothing but encoding of digital data, but a Reed-Solomon 
code or the like is suitably used as an actual method for encoding. 
The transmitter shown in FIG. 2 is provided with a diffusion code 
controller 12 to produce diffusion codes for frequency hopping. The 
diffusion code controller 12 outputs hopping frequency data for 
controlling the frequency hopping, and in the embodiment shown in FIG. 1, 
127 types of frequency data F1 to F127 are outputted in every time slot. 
Specifically, this hopping frequency data is 7-bit digital data which 
represents one of the frequencies F1 to F127. 
Modulation frequency data outputted by the encoding circuit 10 and hopping 
frequency data outputted by the diffusion code controller 12 are supplied 
to a frequency synthesizer 14. This frequency synthesizer 14 is a 
synthesizer which outputs three types of final carrier frequencies on the 
basis of the hopping frequency data and three modulation frequency data. 
In the functional view, the frequency synthesizer adds each of three 
modulation frequency data and hopping frequency data to output three types 
of carrier frequencies. For example, in the embodiment shown in FIG. 1, 
the encoding circuit 10 outputs three types of modulation frequency data 
f2, f4 and f5 in time slot T1. And, the diffusion code controller 12 
outputs hopping frequency data F3 in the time slot T1. The frequency 
synthesizer 14 adds the hopping frequency data F3 and each of f2, f4 and 
f5 in order to output carrier frequencies F4, F6 and F7. In practice, it 
is suitable to use a DDS (Direct Digital Synthesizer) as the frequency 
synthesizer 14. This DDS stores the waveforms of the frequencies F1 to 
F127 into a memory (wave memory) and reads from the memory waveforms 
corresponding to the three types of frequencies (e.g., F4, F6 and F7) 
calculated. The waveform data read from the memory is synthesized to 
output analog signals containing three types of carrier waves. The DDS 
keeps the waveforms of synthesized frequencies corresponding to required 
combinations in a memory (wave memory) at frequencies F1 to F127, reads 
from the memory the synthesized carrier waveforms corresponding to the 
calculated three types of frequencies (e.g., F4, F6 and F7), and outputs 
the selected values. The carrier waves containing the three types of 
frequencies outputted by the frequency synthesizer 14 have their 
frequencies converted by an up-converter 16 into the frequency band of a 
predetermined wave to correspond to the frequency band which can be used 
finally by the transmitter. 
The output signal from the up-converter 16 is power-amplified to a signal 
having a desired intensity by a transmission circuit 18 and emitted into 
the air from an antenna 20. 
Thus, since the DDS is used as the frequency synthesizer 14 in this 
embodiment, the multi-carrier frequency hopping communication method shown 
in FIG. 1 can be achieved quite easily. 
Now, the structure of a receiver which adopts the multi-carrier frequency 
hopping communication method according to this embodiment will be 
described with reference to the drawings. 
FIG. 3 is a block diagram showing the structure of a receiver which 
demodulates after receiving a plurality of carrier frequencies by the 
multi-carrier frequency hopping communication method described with 
reference to FIG. 1. 
First, an antenna 30 receives a radio wave containing a plurality of 
frequencies, and a receiving circuit 32 detects carrier waves for such 
multiple frequencies. Prior to demodulation, a down-converter 34 converts 
the detected multiple frequencies into frequencies at which demodulation 
can be readily performed. The receiving circuit 32 and the down-converter 
34 operate contrary to the transmission circuit 18 and the up-converter 
16. The output signal from the down-converter 34 is a signal with any one 
of the frequencies F1 to F127 shown by the hopping matrix indicated by (b) 
in FIG. 1. Reverse-diffusion modulation (reverse-frequency hopping) is 
performed on the frequencies F1 to F127 and multiplied with the output 
signal from a frequency synthesizer 38 by a multiplier 36 in order to 
convert them into the frequencies f1 to f8 shown by the primary modulation 
matrix indicated by (a) in FIG. 1. This frequency synthesizer 38 is also a 
DDS or the like as in the case of the frequency synthesizer 14 shown in 
FIG. 2 and outputs a hopping frequency for every time slot on the basis of 
the hopping frequency data outputted from a diffusion code controller 40. 
The diffusion code controller 40 shown in FIG. 3 outputs the hopping 
frequency data based on the same code series as in the case of the 
diffusion code controller 12 shown in FIG. 2. 
The output signal from the multiplier 36 becomes a signal so that any three 
frequencies among the eight frequencies f1 to f8 shown by the primary 
modulation matrix indicated by (a) in FIG. 1 are outputted in every time 
slot. A demodulation circuit 42 detects which three frequency signals are 
contained in the output signal from the multiplier 36. These three 
detected frequency signals are outputted as converted three 3-bit digital 
signals. These three 3-bit digital signals (9 bits in total) indicate 
three carrier frequencies as in the case of the output signal from the 
encoding circuit 10 shown in FIG. 2. 
A decoding circuit 44 operates contrary to the encoding circuit 10 shown in 
FIG. 2. The decoding circuit 44 collects the three 3-bit digital signals 
in time slots T1 to T7 and forms a frequency pattern in the data cycle 
(time slots T1 to T7), while 25-bit digital data is decoded from the 
frequency pattern. 
Specifically, various methods can be adopted to decode the 25-bit digital 
data from the frequency pattern as described with reference to FIG. 2. For 
example, a conversion table for converting from the frequency pattern to 
the 25-bit digital data may be stored in a memory. 
As shown in FIG. 3, this receiver is characterized by having a decoding 
circuit 44 which converts a predetermined frequency pattern into 25-bit 
digital data and a demodulation circuit 42 which can demodulate respective 
carrier waves even when a single time slot includes a plurality of 
frequency carrier waves. Thus, the receiver according to the multi-carrier 
frequency hopping communication method of the invention has a 
characteristic structure. 
A synchronizing circuit 46 is disposed for synchronization in the frequency 
hopping as shown in FIG. 3. This synchronizing circuit 46 has the same 
structure as the synchronizing circuit of the receiver according to a 
conventional frequency hopping communication method. And, as in the case 
of the receiver according to the conventional frequency hopping 
communication method, the synchronizing circuit 46 and the diffusion code 
controller 40 operate in the same way as the conventional method, 
according to whether synchronization is being taken or the established 
synchronization is being maintained in response to a mode switching signal 
from outside. Such synchronizing operation is not be described because it 
is not directly related with the present invention. 
As described above, according to the first aspect of the invention, a 
plurality of frequency signals corresponding to data are outputted as 
carrier wave in order to transmit the data, so that the number of 
frequency patterns is increased as compared with the conventional MFSK 
method which outputs only a single frequency signal at a time. Thus, it 
can provide a communication method having an improved transmission 
efficiency. 
The second aspect of the invention can achieve a frequency hopping 
communication method which adopts a plurality of carrier waves and can 
provide a frequency hopping communication method having an improved 
transmission efficiency. 
The third aspect of the invention performs diffusion modulation of N 
frequency signals according to data to be transmitted by the hopping 
frequency, so that it can provide a frequency hopping communication method 
which can use a conventional diffusion modulation method with no 
modification. 
The fourth aspect of the invention reads a signal waveform from those 
previously stored according to N frequency signals and information about 
diffusion code series and synthesizes the signal waveform. Therefore, it 
provides a frequency hopping communication method which can form a precise 
signal waveform even when the number of frequencies forming a frequency 
pattern is increased. 
The fifth aspect of the invention reads and outputs a synthesized carrier 
waveform from those previously stored according to N frequency signals and 
information about diffusion code series. Therefore, a plurality of carrier 
frequencies can be outputted simultaneously by a single waveform reading 
circuit. 
The sixth to tenth aspects of the invention are devices which have 
substantially the same effects as the first to fifth aspects of the 
invention. 
While there have been described that what are at present considered to be 
preferred embodiments of the invention, it is to be understood that 
various modifications may be made thereto, and it is intended that the 
appended claims cover all such modifications as fall within the true 
spirit and scope of the invention.