Code multiplexing transmitting apparatus

Details Code multiplexing transmitting apparatus Code multiplexing transmitting apparatus

2002-10-17

2004-09-14

Phan, Man (Department: 2665)

Multiplex communications

Communication over free space

Combining or distributing information via code word channels...

C370S320000, C370S335000, C370S479000, C375S144000, C375S302000

Reexamination Certificate

active

06791965

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates to a code multiplexing transmitting apparatus and, more particularly, to a code multiplexing transmitting apparatus for spread-spectrum modulating signals of a plurality of channels by respective ones of codes that differ from one another, combining the spread-spectrum modulated signals of each of the channels and transmitting the resultant spread-spectrum modulated signal.
Wireless access using CDMA (Code Division Multiple Access) has been studied and is being put to use as the next generation of digital mobile communication. CDMA is a method of multiple access using spread-spectrum communication. Specifically, transmission information of a plurality of channels or users is multiplexed by coding and transmitted over a transmission path such as a radio link.
Spread-spectrum communication is a method of modulation that is different from ordinary narrow-band modulation. In spread-spectrum communication, the bandwidth of a signal after modulation is made very large in comparison with that of the narrow band in narrow-band modulation. With spread-spectrum communication, two-stage modulation/demodulation is performed in the transceiver.
FIG. 16
is a structural view illustrating the operating principle of a transmitter in spread-spectrum communication. Shown in
FIG. 16
are a modulator
1
such as a (phase-shift keying) PSK modulator, a spreading circuit
2
, a power amplifier
3
and an antenna
4
. The positions of the modulator
1
and spreading circuit
2
may be interchanged. The spreading circuit
2
includes a spreading code generator
2
a
for outputting a rectangular spreading code sequence (see
FIG. 17
) that randomly takes on levels of ±1 referred to as a pseudorandom noise (PN) sequence, and a multiplier
2
b
for multiplying digital transmission data, which has been modulated by the modulator
1
, by the spreading code.
As shown in
FIG. 17
, the speed at which the spreading code changes (namely duration Tc of the rectangular wave) is set so as to change over at a very high rate in comparison with symbol changeover speed (one bit interval T of the PSK-modulated signal) of the narrow-band modulated signal that is modulated by the spreading code. That is, T>>Tc holds. The duration of T is referred to as the “bit duration”, the duration of Tc is referred to as the “chip duration”, and the reciprocals of these are referred to as the “bit rate” and “chip rate”, respectively. The ratio of T to Tc (i.e. T/Tc) is referred to as the “spreading ratio”.
The spectrum distribution of a spread-spectrum modulated signal exhibits the shape of a sinc function, as shown in FIG.
18
. The bandwidth of a main lobe ML is equal to twice the chip rate (i.e. ML=2/Tc), and the bandwidth of a side lobe SL is 1/Tc. Since the PSK signal prior to spread-spectrum modulation is an ordinary PSK signal modulated at the bit rate 1/T, the occupied bandwidth is 2/T. Accordingly, if the occupied bandwidth of the spread-spectrum modulated signal is made the bandwidth (=2/Tc) of the main lobe, the bandwidth of the original PSK-modulated signal will be broadened T/Tc times by applying spread-spectrum modulation. The energy is diffused as a result.
FIG. 19
is an explanatory view illustrating the manner in which bandwidth is enlarged by spread-spectrum modulation. Shown in
FIG. 19
are a narrow bandwidth-modulated signal NM and a spread-spectrum modulated signal SM.
FIG. 20
is a structural view illustrating the operating principle of a receiver in spread-spectrum communication. Shown in
FIG. 20
are an antenna
5
, a wide-band bandpass filter
6
for passing only signals of necessary frequency bands, a de-spreading circuit
7
, a bandpass filter
8
and a detector circuit
9
such as a PSK demodulator. The de-spreading circuit
7
has a construction identical with that of the spreading circuit
2
on the transmitting side and includes a spreading code generator
7
a
for outputting a rectangular spreading code sequence the same as that on the transmitting side, and a multiplier
7
b
for multiplying the output signal of the bandpass filter
6
by the spreading code.
The wide-band reception signal sent to the receiver is restored to the original narrow-band modulated signal via the de-spreading circuit
7
similar to the spreading circuit on the transmitting side. This is followed by the generation of a baseband waveform via the detector circuit
9
, which is of the ordinary type. The reason why the narrow-band modulated signal is obtained by the de-spreading circuit
7
is as set forth below.
As shown in
FIG. 21
, let a(t) represent the modulated wave on the transmitting side, c(t) the spreading code sequence (spreading code) and x(t) the transmitted waveform. These are related as follows:
x
(
t
)=
a
(
t
)·
c
(
t
)
If attenuation and the effects of noise during transmission are neglected, the transmitted waveform x(t) arrives on the receiving side intact. The spreading code sequence used by the de-spreading circuit
7
has a waveform exactly the same as that of the spreading code used in spread-spectrum modulation on the transmitting side, as mentioned above. Accordingly, the output y(t) of the de-spreading circuit
7
is given by the following equation:
y
(
t
)=
x
(
t
)·
c
(
t
)=
a
(
t
)·
c
2
(
t
)
The output signal y(t) enters the bandpass filter
8
. Passing this signal through the bandpass filter is the same as integrating the signal. Thus the output of the bandpass filter is given by the following equation:
 ∫
y
(
t
)
dt=a
(
t
)·∫
c
2
(
t
)
dt
The integral on the right side of this equation is an autocorrelation value obtained when the shift in time is made zero. The autocorrelation value is unity. Accordingly, the output of the bandpass filter is a(t) and the modulating information signal is obtained.
Code division multiple access (CDMA) is a method of communication using a different spreading code for each channel or user, whereby the information transmitted on the respective channels is multiplexed by the codes.
FIG. 22
is a diagram for describing the principle of CDMA on two channels. Shown in
FIG. 22
are a transmitter TR in which CH
1
is a first channel, CH
2
a second channel and CMP a combining unit, and first and second receivers RV
1
, RV
2
, respectively.
An important point in CDMA is the “similarity” of the spreading codes used by each of the channels. When almost identical spreading codes are used by each of the channels, the channels interfere with each other severely. A so-called “correlation value” is a measure of the degree to which interference between channels occurs. The correlation value is defined by the following equation with respect to two waveforms a(t) and b(t):
R=∫a
(
t
)·
b
(
t
)
dt
  T:period
The integration is carried out over one period T of a(t), b(t). We have R=1 when a(t) and b(t) are exactly identical waveforms and R=−1 when the waveforms are of opposite signs. On the average, looking at one period, the value of R obtained is zero when there is no relationship between the value of a(t) at a certain time and the value of b(t) at the same time.
Consider the first receiver RV
1
in a situation where CDMA is performed using, as the spreading code, two waveforms c
1
(t) and c
2
(t) of such a combination that the correlation value R is zero. The signals from the first and second channels CH
1
and CH
2
arrive at the first receiver RV
1
. When the first receiver RV
1
de-spreads the received signals using the code Co.(t), a bandpass filter
8
1
outputs a signal represented by the following equation:
∫{a
1
(t)c
1
(t)c
1
(t)+a
2
(t)c
2
(t)c
1
(t)}dt
The ∫{a
2
(t)c
2
(t)c
1
(t)}dt part of this is zero because the correlation value between c
2
(t) and c
1
(t) is zero. Further, ∫c
1
(t)c
1
(t)dt is unity since this is an autocorrelation value in which the displacement in time is zero. Accordingly, the output of the bandpass filter
8
1
of the first receiver RV
1
is a
1
(t

Affiliated with

Also associated with

Rating

Code multiplexing transmitting apparatus does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Code multiplexing transmitting apparatus, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Code multiplexing transmitting apparatus will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-3241284