Multiplex communications – Communication over free space – Having a plurality of contiguous regions served by...
Patent
1996-01-31
1998-02-17
Chin, Wellington
Multiplex communications
Communication over free space
Having a plurality of contiguous regions served by...
370339, 375202, 375267, 455101, H04J 312
Patent
active
057198578
DESCRIPTION:
BRIEF SUMMARY
This application claims benefit of international application PCT/FI95/00142 filed Mar. 16, 1995.
BACKGROUND OF THE INVENTION
The present invention relates to a method for implementing frequency-hopping in a base station of a digital cellular radio system. In the base station, a first signal modulated by base band data signal is transmitted on a certain carrier frequency. The signal is divided in the time domain into several time slots of which one time slot transmits the common control channel of the base station equipment and the other time slots transmit information regarding different connections.
In cellular radio systems, the quality of a radio channel varies as a function of time and place. On many occasions, a transmitting antenna and a receiving antenna are not within sight of each other due to obstacles formed by terrain or buildings in the line of sight, and thus the signal detected at the receiver is a sum of beams that have travelled different paths and have been reflected from obstacles and that are, further, with different phases. The sum of several signals with different phases follows the Rayleigh distribution in cases of randomly distributed phases.
Signal fading is, furthermore, dependent on the frequency of the signal. Thus, if a frequency difference between two signals is great enough, their fadings do not correlate. A difference of 1 MHz, for example, is large enough for signal fadings to be independent of each other.
The frequency selective fading of a signal described above is one reason for the introduction of frequency-hopping technology in digital cellular radio networks. Frequency-hopping means that the transmission frequency used in a connection is changed at predetermined intervals. Due to frequency-hopping, the transmission quality can be improved especially in situations in which the terminal equipment moves very slowly, or not at all, as is often the case with, for example, hand-held phones.
Apart from the frequency diversity achieved by frequency-hopping, the method is also advantageous in distributing the interferece caused by the radio connection onto several frequencies, in which case momentary interference on any one frequency will remain small.
In the prior art solutions, if frequency-hopping has been desired in a cell served by a base station, the base station equipment must have been provided with at least two complete transceiver units which contain base band sections and radio frequency sections. Each unit, as such can form an independent base station, but to implement frequency-hopping several complete transceiver units have been required in the same cell. This has resulted in that the base station equipment capable of frequency-hopping has been disproportionately large in small cells in comparison with the capacity needed, and the implementation of frequency-hopping has thus required heavy investments.
SUMMARY OF THE INVENTION
It is thus an object of the present invention to implement frequency-hopping in a base station equipment economically and without creating excessive capacity.
This is achieved with the method of the type set forth in the introduction, which is characterized in that the base station further transmits a second signal modulated by the same base band data signal on a certain carrier frequency, which second signal is divided in the time domain into several time slots, each of which transmits the same information as the first signal, except the time slot which transmits the common control channel of the base band equipment, and which carrier frequency is changed time-slot-specifically so that the frequency is either the same, as, or different from, the carrier frequency.
The invention also relates to a base station equipment in a digital cellular radio system, this equipment comprising means for generating a base band data signal, a first means for transmitting a signal modulated by a certain carrier frequency, and this signal being divided in the time domain into several time slots of which one transmits the common control channel (CCCH) o
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patent: 5381443 (1995-01-01), Borth et al.
patent: 5455962 (1995-10-01), Kotzin
patent: 5504783 (1996-04-01), Tomisato
patent: 5542107 (1996-07-01), Kay
patent: 5570352 (1996-10-01), Poyhonen
Chin Wellington
Nokia Telecommunications Oy
Vu Huy D.
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