Frequency-hopping arrangement for a radio communication system
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to the realization of frequency-hopping in radio transmitter systems, radio receiver systems and radio transceiver systems. 
     2. Description of the Related Art 
     The specifications of the European mobile radio system GSM (General Systeme Mobile) define a so-called frequency-hopping function in which the transmitting frequency during signal transmission and correspondingly the receiving frequency at the receiving end is varied continuously by using a suitable number of frequencies, e.g. four predetermined frequencies. Frequency-hopping has two advantageous effects, viz. frequency diversity and interference diversity. 
     Frequency diversity improves the quality of a radio link mainly in cases where the radio transmitter and the radio receiver move slowly with respect to each other. When the frequency of a radio link in a digital mobile radio system is varied sufficiently from one channel time slot to another, fading situations imposed on different channel time slots do not correlate with each other consequently the time slot interleaving utilized in the system also helps to improve the quality of transmission at the reception when fading occurs. 
     The interference diversity effect is based on the fact that the frequency-hopping sequences of base stations using the same or adjacent frequencies are different or at least at different phases with respect to each other, whereby the frequencies interfering with each other also vary from one time slot to another, and the effect of severe sources of interference is divided between several radio links, and so the effect of the interference on an individual link is reduced. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to provide a radio system realizing such a frequency-hopping function. 
     This is achieved by a radio transmitter system according to the invention, which system comprises means for transmitting at least one signal in least at two transmitting frequencies, the frequencies varying in accordance with a predetermined frequency-hopping sequence. The system comprises a fixed-frequency radio transmitter for each transmitting frequency, and means for switching the at least one signal as a baseband signal from baseband signal processing stages to fixed-frequency radio transmitters in accordance with the frequency-hopping sequence. 
     The invention is also concerned with a radio receiver system which comprises a radio receiver for receiving at least one signal in at least two receiving frequencies, which frequencies vary in accordance with a predetermined frequency-hopping sequence. The system comprises a fixed-frequency radio receiver for each receiving frequency, and means for switching the at least one received signal as a baseband signal from the fixed-frequency radio receivers to baseband signal processing stages in accordance with the frequency-hopping sequence. 
     The invention is further concerned with a radio transceiver system which includes a radio transceiver for transmitting at least one outbound signal in at least two transmitting frequencies, which frequencies vary in accordance with a predetermined frequency-hopping sequence. The radio transceiver also receives at least one inbound signal in at least two receiving frequencies, which frequencies also vary in accordance with the frequency-hopping sequence. The system comprises a fixed-frequency radio transceiver for each pair of transmitting and receiving frequencies, and means for switching the at least one outbound signal as a baseband signal from baseband signal processing stages to the fixed-frequency transceivers and for switching the at least one received inbound signal as a baseband signal from the fixed-frequency transceivers to the baseband signal processing stages in accordance with the frequency-hopping sequence. 
     The basic idea of the invention is that a separate dedicated fixed-frequency transmitter or receiver is provided for each transmitting or receiving frequency, and the frequency hopping is carried out by switching baseband signals. For instance, the frequency of an outbound signal is caused to hop in accordance with a predetermined hopping algorithm by switching a baseband signal to the different transmitters in sequence. Signals from the fixed-frequency transmitters are combined to a transmitting antenna by a combiner circuit. 
     The invention offers a solution to a serious problem associated with the realization of frequency hopping in such as a GSM system. According to a frequency-hopping algorithm the base station of the system has to change the frequency of a radio link twice within each signal frame. Because the base station transmits the TDMA frame in each time slot, it is necessary that the transmitting frequency can be changed during a guard interval, about 30 μs, between time slots. The frequency synthesizers of the transmitters and the combiner circuit combining the transmitting signals to a common antenna must also have enough time to adapt in a timed relationship with the frequency-hopping algorithm if variable-frequency transmitters are to be used. In practice, however, the realization of variable-frequency transmitting filters, particularly by ones that are sufficiently rapid and economical is very difficult. The realization of the combiner circuit without transmitting filters or by way of broadband transmitting filters results in complicated constructions and high signal attenuations. When using fixed-frequency radio transmitters according to the present invention, the transmitting filters can be fixed, thus avoiding the above-mentioned problem. Furthermore, the construction of the fixed-frequency transmitter (frequency synthesizer) is simpler and more advantageous than that of the variable-frequency transmitter. The same is true for the receiver. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described in greater detail by reference to the attached drawing, in which 
     FIG. 1 shows a block diagram of a transceiver system according to the invention; 
     FIG. 2 shows a block diagram of the principal features of the baseband units 7a to 7c shown in FIG. 1; and 
     FIG. 3 illustrates the multiplexing of time-multiplexed serial buses 8a and 9a shown in FIG. 1, into transmitting and receiving directions. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Frequency-hopping according to the present invention can be applied in any radio system comprising several radio transmitters, radio receivers or radio transceivers. It is, however, especially suitable for use in cellular digital mobile radio systems, such as the GSM mobile radio system, for realizing frequency hopping in radio communication between base stations and mobile radio stations (e.g. mobile telephones). 
     FIG. 1 shows a block diagram of a transceiver system according to the invention. The system comprises at least two, preferably four, transceiver units 6a, 6b and 6c. In a GSM system, each transceiver is full-duplex, and the duplex spacing between its receiving and transmitting RF frequencies is 45 MHz. The frequency range utilized by the system is preferably from 800 to 1,000 MHz. The transceivers 6a to 6c have mutually different fixed transmitting and receiving frequencies. Outputs 10a to 10c in the transceivers 6a to 6c are connected to a radio-frequency combiner circuit 1, which combines the transmitters of the transceivers 6a to 6c to a common transmitting antenna 2 and the receivers to a common receiving antenna 3. 
     Alternatively, there may be provided two receiving antennas if diversity reception is used. The radio-frequency combiner circuit 1 contains a bandpass-type transmitting filter for the transmitter of each transceiver unit 6, the filter being tuned automatically or manually to the transmitting frequency of the particular transmitter. Similarly, the combiner circuit 1 contains a receiving filter for the receiver of each transceiver unit 6, which filter separates from the antenna signal a signal in the band of receiving frequencies and then amplifies it and divides it to all the receivers. 
     The system of FIG. 1 further comprises several baseband signal processing units 7a, 7b and 7c, which generate baseband modulating signals for the transceiver units 6a to 6c and which process the signals received and converted to the baseband frequency by the transceivers. A cross-point type switching matrix 5 is provided between the transceiver units 6a to 6c and the baseband signal processing units 7a to 7c. This matrix switches the baseband transmitting signals generated by the baseband signal processing units 7 to the different transceivers 6a to 6c in accordance with a predetermined frequency-hopping sequence in such a way that the transmitting frequency of a signal generated by a certain baseband signal processing unit 7 hops from one transmitting frequency to another. Correspondingly, the switching matrix 5 switches the signals received and converted to the baseband frequency by the transceivers to the baseband signal processing units 7a to 7c in accordance with said predetermined frequency-hopping sequence. 
     In the preferred embodiment of the invention, the signal to be transmitted is a serial time-division multiplexed digital signal comprising several, preferably eight, channel time slots. FIG. 2 shows the principal functional blocks of the signal processing units 7a to 7c of FIG. 1 for the digital signal described above. The baseband processing of the signals takes place entirely in the units 7a to 7c. Generally speaking, data to be transmitted is coded and placed in a frame structure of the signal in the baseband signal processing units 7a to 7c. At the reception, the data is demodulated, the frame structure is disassembled, and decoding is carried out. More precisely, data (e.g. speech) from other parts of the radio system, such as the base station controller in the GSM system, is processed by a channel coding unit 71 before transmission. 
     The channel coding unit 71 may perform e.g. the following functions: adaptation of transmission rate, channel coding, interleaving, encryption, assembly of TDMA frame and adjustment of transmitter power control. Within each time slot, the unit 71 transmits a burst containing the data to be transmitted in this particular time slot to an interface unit 74. 
     The interface 74 converts the parallel data received from the unit 71 into serial form and transmits it in serial form through the switching matrix 5 to the particular transceiver 6a to 6b, the matrix 5 switching the serial base band signal in a corresponding time slot to one of the transceivers in accordance with the frequency-hopping algorithm. Similarly, the interface 74 receives a serial signal from a bus 8 within each receiving time slot, the serial signal containing baseband samples derived from the signal received by a transceiver 6a to 6c selected by the switching matrix 5 in accordance with the frequency-hopping algorithm. 
     The interface 74 applies these samples in parallel form to a demodulator 73, which processes the samples. The demodulator 73 contains e.g. the following functions: sample buffering, decryption, GMSK demodulation. 
     The demodulator 73 applies the demodulated data to a channel decoding unit 72, which performs e.g. the following functions: combining signals from different antennas when antenna diversity is used (two receiving antennas), disassembling the frame structure, breaking up interleaving, convolution decoding, block decoding, checking CRC and error correction, adaptation of transmission rate. 
     The channel decoding unit 72 transmits the decoded data further to the other units, and, for instance, to the base station controller of the GSM system when the transceiver system is located at the base station. 
     In the preferred embodiment of the present invention, a modulator corresponding to the demodulator 73 is provided in each transceiver 6a to 6c, in which the data to be transmitted is modulated and converted up to an appropriate radio frequency. Similarly, the data received at the reception is converted by the transceiver down to a baseband frequency and preprocessed for demodulation. Demodulation does not take place until in the units 7a to 7c. In addition, each transceiver 6a to 6b comprises a unit corresponding to the serial interface unit 74 for receiving the serial signal through the switching matrix 5 from the baseband unit 7a to 7c coupled to it at each particular moment and, correspondingly, for transmiting a serial signal to the same baseband unit. 
     As already mentioned above, each baseband signal processing unit 7a to 7c has a corresponding bidirectional serial bus 8a to 8c, which connects it to a respective port in the switching matrix. Similarly, each transceiver 6a to 6c is connected by a separate bidirectional serial bus 9a to 9c to a respective port in the switching matrix 5. Thus a bidirectional time-division digital connection is established between each signal processing unit 7 and the particular transceiver coupled to it in accordance with the frequency-hopping algorithm through the switching matrix 5. Preferably, the baseband signal processing units 7a to 7c are coupled to a new transceiver after each transmission or reception of a channel time slot during the guard intervals between the channel time slots. Bidirectional means that the same bus is used at different times for both transmission-direction and reception-direction time slots. A time slot complying with the GSM specification may be divided into several shorter time windows for the management of this serial bus, the windows being allocated to transmission (Tx), reception (Rx) and testing (test), as shown in FIG. 3. The illustrated time division of this serial connection is possible due to the fact that the transmitting and receiving moments of the GSM system are not simultaneous. Such serial bidirectional transmission between the units 7a to 7c and 6a to 6c is simpler and more advantageous as compared with a parallel bus, in addition to which it reduces the need for cable laying as the same bus is utilized in both transmission and reception direction and simplifies the realization of the switching matrix 5. 
     The attached drawings and the description related to them are only intended to illustrate the present invention. In their details, the systems according to the invention may vary within the scope of the accompanying claims.