1. Field of the Invention
The present invention relates to a multicarrier CDMA(Code Division Multiple Access) base station system and a multi-code wave forming method thereof, and in particular to an improved multicarrier CDMA base station system and a multi-code wave forming method thereof, which are capable of preventing a decrease of a signal processing gain due to a correlation between interferences received in different frequencies by assigning random phases to multiple carriers modulated by a user data.
2. Description of the Conventional Art
In the CDMA mobile communication system, a technique for generating a multi-code signal is disclosed in the U.S. Pat. No. 5,103,459 entitled "System and method for generating signal wave forms in a CDMA cellular telephone system" and in the U.S. Pat. No. 5,467,367 entitled "Spread spectrum communication apparatus and telephone exchange system".
In the above-described two patents, a method for generating a multi-code signal at the base station of a single carrier CDMA mobile communication system is disclosed. In the above-described two patents, a spectrum is spread by multiplying the data of all users and the identical pseudo noise (PN). In addition, an orthogonal characteristic is provided between other user signals by multiplying spectrum spread signals of different users and a Walsh function sequences of different indexes.
Namely, the U.S. Pat. No. 5,103,459 is directed to transmitting other user signals based on the CDMA using a multi-code signal. In addition, the U.S. Pat. No. 5,467,367 is directed to converting the data of a user into a serial/parallel data, spreading the spectrum using other codes and then transmitting the data in parallel, and in the receiver, the data are demodulated in parallel and then a parallel/serial conversion is implemented. Namely, when multiple data bits are transmitted in parallel for transmitting the data at a high speed of the device, the multi-code signal is used. In the above-described two patents, the identical carrier is modulated by the sum of all code signals.
In addition, as another conventional multicarrier CDMA method, there is the U.S. Pat. No. 5,521,937 entitled "Multicarrier direct sequence spread system and method". In this patent, the direct sequence spread spectrum signal in which the user data is spread using the pseudo noise code is transmitted to various other frequency channels. In the receiver, the signals received to each frequency channel are combined and then demodulated. In this method, it is possible to obtain both symbol combining gain and signal process gain from the diversity of channel characteristics at each frequency.
In the above-described multicarrier CDMA system, the number of code chips per data bit is reduced by the number of the carriers compared with the single carrier CDMA system. Therefore, in the multicarrier CDMA system, the signal processing gain obtained by processing the spectrum spreading signal of one carrier is reduced by the number of the carrier compared with the signal processing gain of the single carrier system as well. However, when combining the signals received from different frequencies, since the signal components are added with the identical phase but the interference components are added with the ramdom phase, the signal-to-noise ratio after the signal combining is increased by the number of carriers compared with the signal-to-noise ratio received by each carrier receiver. Therefore, the signal processing gain becomes a value obtained by multiplying the signal process gain of each carrier receiver and the number of the carriers. Namely, it is identical with the signal processing gain of the single carrier system. The above-described signal processing gain is obtained when the carrier phases of user signals are random as in the link from the terminal to the base station in the cellular mobile telephone system, and the different carrier experience different phase transition in frequency selective channels.
When all users use the identical carriers as in the link from the base station to the terminal of the cellular mobile telephone system, the signal processing gain of the multicarrier system may be smaller than the single carrier system due to correlation between the interferences received in different frequencies. The following problem may occur.
Since the number of the orthogonal codes used for distinguishing the user is the same as the number of the code chips per data bit, the number of the orthogonal codes in the multicarrier CDMA system is reduced by the number of the carriers compared with the single carrier system using the same bandwidth. In order to overcome the above-described problems, the quasi-orthogonal functions or the random sequences may be used for distinguishing user signals. However, in the quasi-orthogonal function or the random sequence, cross-correlation values between different codes exist. Therefore, the interference from other user signals may occur in the same cell. When all carriers are modulated to the identical signal, the multicarrier CDMA is used, and all user signal modulate the carrier having the identical phase at the same frequency, the phases of the interference components after demodulating in the same cell received in different carrier frequencies are identical. Therefore, in this case, it is possible to obtain a diversity combining gain with respect to the fading of each frequency channel, but it is impossible to obtain a signal processing gain due to the combining of the signals received at each frequency with respect to the above-described components.