The present invention relates to a frequency-hopping communication system arranged so that a radio communication device (station) can communicate in synchronous to communication frames generated by a base station. The communication device, and the base station can be quickly synchronized according to the communication frames transmitted in the frequency-hopping communication system.
A frequency mobile communication system needs to use an unstable transmission channel for controlling access to a mobile station. To control access to the mobile station, therefore, it is more preferable to use "centralized control" rather than "distributed control". The "centralized control" is arranged so that the base station may control plural mobile stations within a predetermined area in a concentrated manner. As an arrangement of such a centralized control, the base station generates some frames and the stations communicate with each other in synchronous to the generated frames. As one technique for improving transmission quality of unstable wireless channels, the "spread-spectrum" system may be referred to.
As the "spread-spectrum" system there are currently known two systems, "direct-sequence spread-spectrum" and "frequency-hopping spread-spectrum". The "frequency-hopping spread-spectrum" system is arranged so that the frequency of a local oscillating signal, that is, a carrier of a transmitted signal, may be hopped according to a predetermined hopping pattern during communication. This system is more robust to address a near-far problem because it is easier to arrange frequency-diversity in this system. Thus, this system is more applicable to land communications.
This kind of spread-spectrum system is discussed in the publication: "The Fundamentals and Applications of Spread-Spectrum Communication Technologies", Triceps 1987. In pages of 51 to 68 of this publication, frequency-hopping is discussed in more detail.
In general, the wireless channel is lower in quality than the wired channel. On the wireless channel, it is likely that errors occur in a burst manner. Applications using random-error-correcting codes such as BCH (Base-Chaudhuri-Hocqenghem) codes are not so effective in controlling errors. In principle, using both of the frequency-hopping system provided with a frequency-diversity effect and the error-correcting codes such as BCH codes may be effective in taking measure against the burst errors. However, for obtaining a frequency-diversity effect in the frequency-hopping system, it is necessary to make a hopping interval shorter than an allowable length of burst errors.
Based on this cause, the conventional "frequency-hopping" system, in general, means a fast frequency-hopping system in which plural hops take place within one symbol period or a slow frequency-hopping system in which about one hop takes place within some symbol periods. In these systems, basically, the duration of one frequency is so short as a few times of a symbol period.
The frequency hopping, in general, does not guarantee the continuity of phases in switching the frequency. In particular, when the continuity of the phases is only guaranteed within some symbols, it is difficult to take phase synchronization. Hence, the modulating method in which information is placed on the phase is not practical.
In general, the conventional frequency-hopping system employs an MFSK (M-ary Frequency Shift Keying) or a DPSK (Differential Phase Shift Keying) in which the phases are differentiated between the same frequencies separated from each other by one period in the hopping pattern.
One of the disadvantages a receiver arranged to use a frequency-hopping system suffers from is how the synchronization with a hopping pattern of a transmitter is quickly taken. The synchronization of the frequency-hopping system is discussed in the foregoing publication: "the Fundamentals and Applications of Spread-Spectrum Communication Technologies", pages 79 to 82 and 93.
The "frequency modulation" is inferior in frequency utilization to the phase modulation and is disadvantageous in the S/N to BER characteristic. The DPSK modulation in which the phases are differentiated between the same frequencies separated from each other by one period in the hopping pattern needs means for storing phase information the number of which is corresponding to the number of hopping frequencies for differentiating phases. Further, over the period of one cycle of a hopping pattern, the transmitter and receiver is required to hold carrier-phase synchronization at frequency. For this purpose, highly accurate clocks are maintained.
To realize for practical use of the fast frequency hopping or slow frequency hopping arranged so that one hop occurs within some symbols, a fast switchable frequency synthesizer is indispensable. In a mobile communication system arranged to realize a transmission rate of about several 100 K bps to several M bps, 1 symbol period is about 1 .mu.s. The current technology makes it difficult to realize a small-sized, low-power and inexpensive synthesizer to be switched for such a short time.
To realize the fast synchronization, on the other hand, there are prepared the same number of components of a demodulator such as a detector and a filter as the number of hopping frequencies, which result in making the demodulator relatively redundant in arrangement. Hence, the receiver has a complicated arrangement and is very costly.