Spread spectrum communication apparatus

A communication apparatus for code-division multiplexing a transmission path includes a spread spectrum transmitter and receiver. The spread spectrum transmitter includes a code-division multiple signal generator, a non-linear code-division multiple signal converter, and a transmitter for transmitting the converted signal. The code-division multiple signal is converted so that the transmitted signal has a plurality of amplitude values. The spread spectrum receiver includes a code-division multiple signal receiver, a non-linear converter for converting an amplitude of the received code-division multiple signal, and a de-spreader for de-spreading the non-linearly converted code-division multiple signal.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a communication apparatus for 
code-division multiplexing a transmission path by using a spread spectrum 
communication system in order to increase the transmission capacity. 
2. Related Background Art 
Hitherto, in a spread spectrum (SS) communication system, a code-division 
multiple communication system which increases the information transmission 
speed by multiplexing codes utilizing plural spread codes having low 
mutual correlative characteristics has been considered. This system has 
been remarked as one method for conducting a high-speed information 
transmission within a limited band. 
FIGS. 3A and 3B are block diagrams showing a structural example of a 
code-division multiple communication apparatus in such a conventional 
spread spectrum communication system. FIG. 3A indicates a transmission 
unit and FIG. 3B indicates a reception unit. 
First, transmission information 31 is converted into n parallel data 33 (1 
to n) by an S/P (serial-parallel) converter 32 in the transmission unit 
shown in FIG. 3A. Each of the parallel data 33 (1 to n) is modulated by 
each of different spread codes of an SS modulators 34 (1 to n). N spectrum 
spread SS signals 35 (1 to n) are added by an adder 36 to become a 
code-division multiple signal 37. 
In the reception unit shown in FIG. 3B, a code-division multiple signal 38 
is distributed into n code-division multiple signals 40 (1 to n) by a 
power distributor 39. The distributed multiple signals 40 (1 to n) are 
inversely spread by SS demodulators 41 (1 to n) to be demodulated to n 
data 42 (1 to n). The demodulated data 42 (1 to n) are converted into 
serial data by a P/S (parallel-serial) converter 43 to obtain a 
demodulation signal 44. 
In this way, a high-speed transmission can be achieved while an occupied 
bandwidth is fixed by converting the transmission information into the 
parallel data 33 (1 to n) and utilizing a code-division multiple system. 
In the transmission unit, the spectrum spread signals 35 (1 to n) are 
synthesized by utilizing a linear adder 36 for code-division multiplexing. 
In case where the transmission information 31 is converted into the n 
parallel data 33 (1 to n), the value of the code-division multiple signal 
37 becomes available within the range of 0 to n. In a reception unit, the 
SS demodulators 41 (1 to n) which enable to execute an inverse spread and 
treat the value of 0 to n as inputs are required. 
However, the value of 0 to n which appear in a code-division multiple 
signal are not uniformly distributed and it is not efficient to maintain a 
dynamic-range for an entire area of 0 to n. Also, since useless portions 
are required on circuit arrangement, simplification or mintaturization of 
a communication apparatus is prevented. 
SUMMARY OF THE INVENTION 
An object of the present invention is to improve the code-division multiple 
communication utilizing a spread spectrum communication system. 
Another object of the present invention is to miniaturize or simplify a 
code-division multiple communication apparatus which utilizes a spread 
spectrum communication system. 
Still another object of the present invention is to improve a reliability 
in the code-division multiple communication utilizing a spread spectrum 
communication system. 
The above and other objects and features of the present invention will 
become apparent from the following description and the appended claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1A is a block diagram showing the structure of a transmission unit of 
a code-division multiple communication apparatus in a first embodiment of 
the present invention. 
First, transmission information 11 is converted into n parallel signals 13 
(1 to n) by an S/P (serial-parallel) converter 12. It should be noted 
that, if the transmission information is Originally formed by parallel 
signals, an S/P converter is not required. The signals 13 (1 to n) are 
input to SS modulators t4 (1 to n) in which signals are spread spectrum 
modulated by being multiplied by each of spread codes to become SS signals 
15 (1 to n). Those SS signals 15 (1 to n) are added by an adder 16 to 
become a code-division multiple signal 17 holding the amplitude value of 0 
to n. 
The amplitude value of the code-division multiple signal 17 is not 
uniformly distributed, and the maximum value of an amplitude which appears 
in one cycle of the code becomes, for example, such a distribution as 
shown in FIG. 2A. For the signal value having such a generating 
probability distribution, the amplitude of the code-division multiple 
signal 17 is limited by using a non-linear calculator 18 having, for 
example, such a non-linear characteristic as shown in FIG. 2B. It should 
be noted that FIG. 2B shows an input/output characteristic of the 
non-linear calculator 18. 
Accordingly, the large value of an amplitude with the low code generating 
probability is compressed to limit the amplitude value. A code-division 
multiple signal 19 is modulated to the carrier frequency and is 
transmitted. If this non-linearly calculated code-division multiple signal 
19 is demodulated by a reception apparatus which does not execute a 
previous process, the data with a large amplitude value loses the little 
information, however, since the SS communication can logically expect the 
process gain, a fault ratio of the demodulation signal is very little 
influenced. The non-linear characteristic shown in FIG. 2B can be properly 
changed in accordance with the generating probability of the amplitude 
value of a code-division multiple signal. 
FIG. 1B is a block diagram showing the structure of a reception unit of a 
code-division multiple communication apparatus in a second embodiment of 
the present invention. 
First, the amplitude of a code-division multiple signal 20 is limited by, 
for example, a non-linear calculator 21 having the non-linear 
characteristic shown in FIG. 2B. The non-linear calculator 21 outputs an 
input signal to which the coefficient shown in FIG. 2B is multiplied. A 
code-division multiple signal 22 of which amplitude is limited is 
distributed to n code-division multiple signals 24 (1 to n) by a power 
distributor 23. Each of the signals 24 is inversely spread by being 
multiplied by each of spread codes by SS demodulators 25 (1 to n) to 
become demodulation data 26 (1 to n) for each channel and further to 
become a demodulation signal 28 by a P/S (parallel-serial) converter 27. 
It should be noted that if the demodulation signal is output as a parallel 
signal, the P/S converter 27 is not required. 
Here, the non-linear characteristic shown in FIG. 2B is utilized, for 
example, if the amplitude is already limited as in a first embodiment, the 
amplitude can be developed by using a non-linear calculator having the 
non-linear characteristic shown in FIG. 2C. 
FIG. 4A is a block diagram showing the structure of a transmission unit of 
a code-division multiple communication apparatus in a third embodiment of 
the present invention. 
In FIG. 4A, the common structural devices as those in FIG. 1A are indicated 
by the same reference numerals. 
In this embodiment, for example, against a signal 17 having the generating 
probability distribution shown in FIG. 7A, an output limits the amplitude 
of the code-division multiple signal 17 in a digital calculator 118 by 
using a conversion table 10 having the non-linear characteristic for 
square rooting an input as shown in FIG. 7B. It should be noted that 
square root map is not always strictly required by executing the 
quantization, in a conversion table 10. 
In this embodiment, since the conversion table is provided, the non-linear 
characteristic can be easily varied. 
This non-linear characteristic can also select a non-linear characteristic 
(FIG. 8A) limiting the square root characteristic with a certain input or 
a raised-cosine type characteristic (FIG. 8B) under the consideration of 
the generating probability of the amplitude value of a code-division 
multiple signal, the capacity of a system or the like. 
FIG. 4B is a block diagram showing the structure of a reception unit of a 
code-division multiple communication apparatus in a fourth embodiment of 
the present invention. 
In FIG. 4B, the common structural devices as those in FIG. 1A are indicated 
by the same reference numerals. 
The amplitude of a digital code-division multiple signal 20 is developed by 
using, for example, a conversion table 29 having the non-linear square 
characteristic as shown in FIG. 9A by a digital calculator 121. A 
code-division multiple signal 22 of which amplitude is developed is 
distributed to n code-division multiple signals 24 (1 to n) by a power 
distributor 23. It should be noted that square characteristic map is not 
always strictly required by executing the quantization in the conversion 
table 29. It is effective to develop the amplitude in case where the 
amplitude is limited in a transmission unit or the code-division multiple 
signal 20 is distorted by the characteristic of a transmission path or the 
like. Also, the non-linear characteristic can be easily changed because a 
conversion table is provided. 
Here, the non-linear characteristic having the square characteristic shown 
in FIG. 9A is utilized however, it is also possible to develop the 
amplitude by using, for example, a conversion table having the non-linear 
characteristic synthesized with the square characteristic and the primary 
characteristic shown in FIG. 9B or a conversion table having the 
raised-cosine type inverse characteristic shown in FIG. 9C. 
FIG. 5A is a block diagram showing the structure of a transmission unit of 
a code-division multiple communication apparatus in a fifth embodiment of 
the present invention. 
In FIG. 5A, the common structural devices as those in FIG. 1A are indicated 
by the same reference numerals. 
In this embodiment, SS signals 15 (1 to n) in a base band becomes an analog 
code-division multiple signal 56 having the amplitude value of 0 to n, 
after being added and D/A (digital/analog) converted by an adder 55. For 
example, against the signal 56 having the generating probability 
distribution as shown in FIG. 7A, an output limits the amplitude of the 
code-division multiple signal 56 in a non-linear processor 57 having the 
non-linear characteristic for square rooting an input as shown in FIG. 7B. 
The non-linear characteristic can also select a non-linear characteristic 
(FIG. 8A) limiting the square root characteristic with a certain input or 
the raised-cosine type characteristic (FIG. 8B) under the consideration of 
the generating probability of the amplitude value of a code-division 
multiple signal, the capacity of a system or the like. 
FIG. 5B is a block diagram showing the structure of a reception unit of a 
code-division multiple communication apparatus in a sixth embodiment of 
the present invention. 
In FIG. 5B, the common structural devices as those in FIG. 1B are indicated 
by the same reference numerals. 
In this embodiment, the amplitude of an analog code-division multiple 
signal 60 is developed and the signal 60 is A/D (analog/digital) converted 
by, for example, a non-linear processor 61 having the nonlinear square 
characteristic as shown in FIG. 9A. A code-division multiple signal 22 of 
which amplitude is developed and which is A/D converted is distributed to 
n code-division multiple signals 24 (1 to n) by a power distributor 23. 
Here, the non-linear characteristic having the square characteristic shown 
in FIG. 9A is utilized, however, it is also possible to develop the 
amplitude by using, for example, a non-linear processor having the 
non-linear characteristic synthesized with the square characteristic and 
the primary characteristic shown in FIG. 9B or a non-linear processor 
having the raised-cosine type inverse characteristic shown in FIG. 9C. 
FIG. 6 is a block diagram showing the structure of a transmission unit and 
a reception unit of a code-division multiple communication apparatus in a 
seventh embodiment of the present invention. 
Transmission information 70 becomes a multiplexed SS signal by a 
code-division multiplexer 71 to be input to a non-linear process unit 72. 
The non-linear characteristic of a non-linear process unit 72 selects, for 
example, the square root characteristic as shown in FIG. 7B under the 
consideration of the generating probability distribution of the 
code-division multiple signal value, the dynamic range of a high-frequency 
conversion unit 73 or the band characteristic of a transmission path. The 
code-division multiple signal of which amplitude is limited in a 
non-linear process unit 72 is converted to the desired carrier frequency 
in a high-frequency conversion unit and is transmitted through a 
transmission path 74. 
In a frequency conversion unit 75, the signal transmitted through a 
transmission path 74 is converted to the demoduatable frequency which can 
be demodulated by a demodulator. A non-linear process unit 76 develops the 
code-division multiple signal of which amplitude is limited by using, for 
example, the non-linear square characteristic having the non-linear 
characteristic and the inverse characteristic of a transmission unit as 
shown in FIG. 9A. The developed code-division multiple signal is 
demodulated by a code-division multiple decoder 77 to become a 
demodulation signal 78. 
Like this, by executing the non-linear process to a code-division multiple 
signal in a transmission unit, limiting the amplitude and developing the 
code-division multiple signal with the non-linear characteristic having 
the inverse characteristic of a transmission unit in a reception unit, the 
analog portion can be operated in a linear area of the narrow dynamic 
range in a high-frequency conversion unit of a transmission unit, a 
transmission path and a frequency conversion unit of a reception unit. 
Therefore, a communication apparatus can be simplified and miniaturized 
without deteriorating the communication quality. 
FIG. 10A is a block diagram showing a transmitter of a code-division 
multiple communication apparatus in a eighth embodiment utilizing a 
non-linear operation and FIG. 11A is a block diagram showing the structure 
of a receiver in said code-division multiple communication apparatus. In 
this embodiment, a phase modulation method is utilized as a modulation 
system and a synchronous demodulation method is utilized as a demodulation 
system. 
In FIG. 10A, the high speed transmission data is converted into low speed 
parallel data 102 (1 to n) by a serial-parallel converter 101. It should 
be noted that if the input data is originally parallel data, the converter 
101 is not required. These parallel data are spectrum spread modulated by 
plural different spread codes generated from a code generator 103 and 
exclusive OR circuits 104 and are further added by an adder 105 to obtain 
the code-division multiple signal. The code-division multiple signal 
multiplexed in this way, of which amplitude is compressed by, for example, 
a non-linear calculator 106 having the non-linear input/output 
characteristic as shown in FIG. 10B, thereafter thus multiplexed signal is 
converted into an analog base band signal 108 by a digital-analog 
converter 107. This code-division multiple base band signal is carrier 
modulated by a carrier modulator 109 against the carrier and is 
transmitted to a transmission path 110. 
While, in a receiver, a reception signal 201 which is base band demodulated 
from the intermediate frequency signal in a converter 200 is firstly 
quantized to the digital signal by an analog/digital converter 202 as 
shown in FIG. 11A. 
This quantized data is returned to the linear code-division multiplexed 
value by the non-linear calculator 106 in a modulator and a secondary 
non-linear calculator 203 having inverse map. If the non-linear map in a 
modulator has the characteristic described above as shown in FIG. 10B, the 
secondary non-linear calculator in a demodulator has the input/output 
characteristic as shown in FIG. 11B. It should be noted that a vertical 
line and a lateral line in FIG. 11B are graduated at the same rate. 
When spread codes generated from a code generator 206 are correlatively 
calculated with thus code-division multiplexed data by digital correlators 
205 (1 to n), the code-division multiple signal is spread inversely 
processed and is demodulated to low speed parallel data 207 (1 to n). Thus 
parallel data are finally converted to the high speed reception data by a 
parallel/serial converter 208. It should be noted that if the data is 
output as the parallel data, the converter 208 is not required. 
FIG. 12A is a block diagram showing the structure of a reception unit of a 
code-division multiple communication apparatus in a ninth embodiment of 
the present invention. In FIG. 12A, the common structural devices as those 
in FIG. 11A and 11B are indicated by the same reference numerals. Although 
a transmission unit is same as that shown in FIG. 10A, a transmitter 
transmits the pilot signal which is spectrum spread by the single spread 
code as the preamble for a constant period immediately previous to the 
transmission of multiplexed information and transmits the signal which is 
compressed against the code-division multiple value by the non-linear map 
when the multiplexed information signal is transmitted. A demodulator 
firstly quantizes a base band signal 201 being a preamble by an 
analog/digital converter 202 and executes the cumulative adding 
calculation during the period corresponding to the code cycle by a 
cumulative adder 209 which serves as the integration means. An output of 
said cumulative adder 209 becomes zero when the preamble is depended on 
the single spread code and the spread code completely has the equilibrate 
characteristic. However, in case where the DC component is appeared in a 
transmission system and the off-set is observed in the analog/digital 
conversion value, an output of said cumulative adder 209 does not become 
zero but has a certain value. The result obtained to divide thus value by 
the code cycle becomes the DC component value in this transmission system. 
Therefore, a process circuit 209 holds the value of said DC component 
utilizing a latch circuit 210 which serves as the hold means matching with 
a timing for switching the preamble to the multiplexed information 
transmission and the off-set value can always be referred during the 
receiving of the multiplexed information signal. 
Next, an operation during the receiving of the multiplexed information 
signal will be described. The reception signal as the information signal 
is similarly converted to the digital code-division multiplexed value by 
an analog/digital converter 202. Since this converted value is appeared as 
the value including the DC component of a transmission system, against 
this value, by subtracting the corresponded off-set in a transmission 
system being an output from a latch circuit by a subtracter 211, the 
corresponded off-set in a transmission system can be compensated. 
In this way, the code-division multiplexed value of which off-set is 
compensated is correlatively demodulated by the replica of spread codes 
generated from a code generator 206 and a digital correlator 205 so that 
the information symbol for each channel is demodulated. Thus obtained 
parallel data 207 (1 to n) are obtained as the high-speed reception data 
by a parallel/serial converter 208. 
The structural example of a demodulator depending on a digital circuit is 
described in said ninth embodiment, however, the present invention can 
also be similarly adopted to an analog demodulator. 
FIG. 12B is a block diagram showing the structure of a reception apparatus 
in a tenth embodiment of the present invention. In a demodulator, the 
reception base band is firstly divided into two paths. The one of them is 
input to a low-pass filter 212 and the corresponded off-set of the 
transmission system is appeared as the voltage signal in its output by 
executing the integration process during the receiving of the preamble. 
Therefore, a process circuit 209' holds this offset value utilizing a 
sample/hold circuit 213 matching with a timing for switching the preamble 
to the multiplexed information transmission and this off-set value can 
always be referred during the receiving of the multiplexed information 
signal. 
Next, an analog subtracter 214 structured by an ope-amplifier and the like 
subtracts the corresponded voltage off-set in a base band signal in 
accordance with the outputs from another path of the reception base band 
signal and the sample/hold circuit 213. The amplitude of this subtracter 
output is expanded by an analog non-linear element having the similar 
characteristic as that of a non-linear calculator 203, and thus output is 
further correlatively demodulated by the replica of spread codes generated 
from a code generator 206 and a correlator 216 to demodulate the 
information symbol of each channel. Thus obtained parallel data 207 (1 to 
n) are output as the high-speed reception data by a parallel/serial 
converter 208. 
In the above ninth and tenth embodiments, as examples, the description is 
given as to a digital and an analog demodulator respectively, however, a 
digital/analog mixed demodulator can be also adopted to the present 
invention. 
As above, the description was Given based on the preferable embodiments, 
however, the present invention is not limited to the foregoing embodiments 
but many modifications and variations are possible within the spirit and 
scope of the appended claims of the invention.