1. Field of the Invention
The present invention relates to a spread spectrum communication method in which a plurality of items of data are spread by using a plurality of sets of spreading codes.
2. Description of the Related Art
In connection with spread spectrum communication, a code division multiplex communication system has been proposed, according to which the low cross-correlation characteristic of spreading codes is utilized to multiplex a plurality of communication paths in the same band, thereby attaining an increase in information transmitting speed (U.S. Ser. No. 08/233,244 filed on Apr. 26, 1994).
Apart from this, orthogonal modulation/demodulation techniques utilizing the orthogonality of the I and Q-components of a carrier, for example, m-phase shift modulation (m-PSK) and orthogonal amplitude modulation (QAM), have also been proposed as a means for attaining an increase in information transmitting speed within a limited band.
FIG. 4 is a circuit diagram showing an example of the construction of a code division multiplexing modulation circuit using orthogonal modulation when the multiplexing number is 16, as shown in U.S. Ser. No. 08/233,244.
First, high-speed transmission data is converted to 16 items of parallel data by a serial/parallel converter 101. These items of parallel data are divided into a group which is to be modulated to the I-channel of a carrier and a group which is to be modulated to the Q-channel of the carrier, each group consisting of 8 items. When the transmission data is input as parallel data beforehand, the serial/parallel converter 101 is not needed.
These items of parallel data are respectively subjected to spread spectrum modulation by a plurality of different spreading codes, generated by a spreading code generator 110, and exclusive "OR" circuits 102, and added by adders 103I and 103Q, whereby multiplexing signals of I and Q-components are obtained.
In code division multiplexing, the spreading codes used should be, in principle, all different from each other. However, in this example, in which orthogonal modulation is effected, the orthogonality of both phases at the time of carrier modulation is ensured, so that, unless the same spreading code is used in a set, it is possible to use the same spreading code for both in-phase and orthogonal components.
In the present example, eight spreading codes are prepared, each of which is used for I and Q components. Since eight signals of the same weight are multiplexed in each set, the code division multiplexing signal assumes the nine values of 0 to 8.
The code division multiplexing signals thus multiplexed are converted to analog base band signals by digital/analog converters 105I and 105Q, respectively, and, further, are carrier-modulated to in-phase and orthogonal components by balanced modulators 106I and 106Q and a .pi./2 phase shifter 107 with respect to a carrier generated by a local oscillator 108. The signals are then synthesized by a power synthesizer 109, whereby a code division multiplexing orthogonal modulation output is obtained.
At this time, the signal points in the phase arrangement diagram of this modulation output assume a phase arrangement similar to a QAM modulation, which assumes 81 values, as shown in FIG. 6.
Next, FIG. 5 is a circuit diagram showing the construction of a demodulation circuit as disclosed in U.S. Ser. No. 08/233,244.
First, a reception signal is transferred by way of analog multipliers 201I and 201Q, low-pass filters 202I and 202Q, and a .pi./2 phase shifter 203, and is subjected to synchronous detection by a reproduction carrier reproduced in the receiver, whereby base band signals of I-channel and Q-channel components are obtained. When a correlation computation is performed on these detection outputs by using an analog multiplier 205 and a low-pass filter 206 with respect to spreading codes generated by a code generator 204, the code division multiplexing signals undergo reverse spread processing to be thereby demodulated into 16 items of parallel data.
These items of parallel data are converted to a digital level by a comparator 207, and finally converted to high-speed reception data by a parallel/serial converter 208.
In this way, a communication path is multiplexed by using a code division multiplexing communication system and an orthogonal modulation/demodulation technique, thereby attaining an increase in information transmitting speed.
However, as the use of an equalizer is a prerequisite in narrow-band QAM transmission, the above-described conventional technique has a problem in that a deterioration in error rate characteristics is involved due to the small inter-signal distance in the phase arrangement diagram of the orthogonal modulator output, although a processing gain can be expected in spread spectrum communication.