Method for adjustment of a transmitter carrier frequency in a subscriber station of a point-to-multipoint radio transmission system

A method for setting a transmitter carrier frequency in a subscriber station of a point-to-multipoint radio transmission system, said transmission system comprising said subscriber station, a base station and additional subscriber stations, said base station including a number of base station modems equal to a maximum number of said subscriber stations connectable with said base station and each of said subscriber stations also has a subscriber station modem, said method comprising the steps of:

• a) deriving all receiver carrier frequencies of all receiver carriers and all transmitter carrier frequencies of all transmitter carriers of the base station from a common reference frequency;
• b) comparing a transmitter carrier frequency of a transmitter carrier propagated from said subscriber station with a corresponding one of the receiver carrier frequencies of the base station;
• c) transmitting an average frequency deviation between said transmitter carrier frequency of said subscriber station and said one of said receiver carrier frequencies of said base station obtained in the comparing of step b) to said subscriber station;
• d) adjusting said transmitter carrier frequency of said subscriber station so that said frequency deviation is minimized;
• e) providing each of said modems (MOD1, DMOD1) of the base station and each of said modems (MOD1′, DMOD1′) of the subscriber station with at least two receiver channels and at least two transmitter channels;
• f) when data should be transmitted with another pair of said transmitter and receiver carrier frequencies (BSTx1, BSRx1, TMRx1, TMTx1), switching from one of said at least two receiver channels to another and from one of said at least two transmitter channels to another;
• and
• g) prior to said switching of step f), deriving new transmitter and receiver carrier frequencies (TMTx2, TMRx2) for said subscriber station from said frequency deviation between said transmitter carrier frequency (TMTx1) of the subscriber station and said receiver carrier frequency (BSRx1) of the base station, from a new transmitter carrier frequency (BSTx2) and from a new receiver carrier frequency (BSRx2) of the base station, wherein said new transmitter and receiver carrier frequencies for said subscriber station are calculated by the following formulae:
• TMTx2={1/C}[BSRx2+ZFLOBS(m′−n·C)+RFLOBS(1−R)],
• TMRx2={1/C}[BSTx2+ZFLOBS(m−n′·C)+RFLOBS(1−R)],
• wherein RFLOBS and ZFLOBS are respective reference frequencies for corresponding local oscillators for a high frequency converter and an intermediate frequency converter in the base station, C is equal to ZFLOTM/ZFLOBS, ZFLOTM and ZFLOBS being respective references frequencies for corresponding local oscillators for a high frequency converter and an intermediate frequency converter in the subscriber station, R equals RFLOTM/RFLOBS, m and n are respective multiplying factors for the transmitter converter and receiver converter of the base station and m′ and n′ are respective multiplying factors for the transmitter converter and receiver converter of the subscriber station.