Code allocation in connection with soft handover in CDMA system

1. A method for allocating a code in connection with soft handover in a CDMA system in downlink transmissions from a base transceiver station (BTS) to a mobile station (MS), the method comprising:

allocating codes in the base transceiver station (BTS) according to a specific code tree, wherein a specific first spreading factor (SF=2) corresponds to each code (C 2, 1 ) and it is possible to derive from a code that has a specific spreading factor, two codes (C 4, 1 , C 4,2 ) that have a higher second spreading factor (SF=4), characterized in that the method comprises:

allowing the base transceiver station to use said first spreading factor (SF=2) in code allocation in all downlink transmissions;

allowing the base transceiver station to use both codes simultaneously (C 4, 1 , C 4,2 ) that have said second spreading factor (SF=4) in code allocation in downlink transmissions when the receiving mobile station is in connection with only one base transceiver station; and

limiting the use of said second spreading factor (SF=4) in code allocation in downlink transmissions when the receiving mobile station is in connection with at least two base transceiver stations (BS A, BS B) simultaneously so as to allow the base transceiver station to use only one (C 4, 1 or C 4,2 ) of said two codes (C 4, 1 , C 4,2 ) that have said second spreading factor (SF=4).

2. A method according to claim 1 , characterized in that, in connection with soft handover, the method comprises:

receiving from the mobile station a timing offset (T offset ); and

carrying out the control of the base transceiver station's timing on the basis of the received timing offset and said first spreading factor (SF=2).

3. A method according to claim 1 , characterized in that said first spreading factor is 256 and said second spreading factor is 512.

4. A base transceiver station in a CDMA system, which comprises a transmitter ( 8 ) for carrying out downlink transmissions from the base transceiver station towards a mobile station (MS), in which method for allocating a code in connection with soft handover, the base transceiver station comprises:

allocation means ( 10 ) for allocating codes according to a specific code tree, wherein a specific first spreading factor (N) corresponds to each code (C 2, 1 ) and it is possible to derive from a code that has a specific spreading factor, two codes (C 4, 1 , C 4, 2 ) that have a higher second spreading factor (SF=4), characterized in that the base transceiver station further comprises:

control means ( 10 , 19 ) for allowing the use of the first spreading factor (N) in code allocation in all downlink transmissions, and allowing the use of both codes simultaneously that have said second spreading factor (SF=4) in code allocation in downlink transmissions when the receiving mobile station is in connection with only one base transceiver station; and

limitation means ( 10 , 19 ) for limiting the use of said second spreading factor (SF=4) code in allocation in downlink transmissions when the receiving mobile station is in connection with at least two base transceiver stations (BS A, BS B) simultaneously so as to allow said allocation means ( 10 ) to use only one (C 4,1 or C 4,2 ) of said two codes (C 4, 1 , C 4,2 ) that have said second spreading factor (SF=4).

5. A base transceiver station according to claim 4 , characterized in that it comprises, in connection with soft handover,

means ( 8 , 10 , 19 ) for receiving a timing offset (T offset ) from a mobile station; and

means ( 10 , 19 ) for controlling the base transceiver station's timing on the basis of the received timing offset and said first spreading factor (SF=2).

6. A base transceiver station according to claim 4 , characterized in that said first spreading factor is 256 and said second spreading factor is 512.