Method and apparatus for allocating time slots within a frame of a TDMA frequency channel
A satellite communications system provides a communications service to a mobile terminal on which different communications applications may be run. Calls are set up between any of the applications via a satellite to a network management center which provides different service adaptors which adapt the calls to different types of service provided over terrestrial networks such as telephony, facsimile, internet or ATM services. The bandwidth allocated to each call over the satellite link may be varied during the call according to demand either from the relevant application or from the network management center. Multiple calls may be connected concurrently to or from different applications running on the mobile terminal. A maximum bandwidth is allocated to each call. Efficient use is thereby made of the limited bandwidth available over the satellite, according to the instantaneous bandwidth requirements of different applications. For real-time calls requiring multiple slots per frame in a TDMA channel, the slots are mutually spaced apart to reduce delay.
1. Apparatus for assigning time slots within a TDMA frame of a frequency channel to a plurality of calls between a base station and one or more mobile terminals, said calls being either real-time time calls or non real-time time calls which have less sensitivity to delay then real-time time calls and comprising at least one real-time call requiring a plurality of time slots per frame; the apparatus comprising:
means for determining which of said plurality of calls are real-time calls requiring allocation of a plurality of time slots in said TDMA frame; and
means for allocating said time slots in said frame to said real-time calls such that the plurality of time slots allocated to each of said real-time time calls are mutually spaced apart in said TDMA frame in said frequency channel.
2. Apparatus as claimed in claim 1 , wherein said means for allocating are further arranged to allocate one or more time-slots to each of said non real-time time calls from the time slots in said frame not allocated to each of said real-time calls, a number of said time slots allocated to said non real-time call being variable during said non real-time call according to a current bandwidth allocation for said non real-time call.
3. Apparatus as claimed in claim 1 or 2 , further comprising means for transmitting call signals in said time slots in accordance with the allocation of said time slots.
4. The apparatus as claimed in claimed 1 , wherein at least one of the real-time calls comprises a voice call.
5. A method of assigning time slots within a TDMA frame of a frequency channel to a plurality of calls between a base station and one or more mobile terminals, said calls being either real-time calls or non real-time calls and comprising at least one real-time call requiring a plurality of time slots per frame; the method comprising:
determining which of said plurality of calls are real-time calls requiring allocation of a plurality of time slots in said TDMA frame; and
allocating said time slots in said frame to said real-time calls such that the plurality of time slots allocated to each of said real-time call are mutually spaced apart in said TMDA frame in said frequency channel.
6. A method as claimed in claim 5 , wherein said TDMA frequency channel is a return channel for communication from said one or more mobile terminals to said base station, the method further comprising:
transmitting to said one or more mobile terminals information relating to the allocation of said time slots in the return channel, such that call signals are transmitted by said one or more mobile terminals in said allocated slots of the return channel.
7. A method as claimed in claim 5 , wherein said TDMA frequency channel is a forward channel for communication from said base station to said one or more mobile terminals, the method further comprising transmitting call signals in said time slots in accordance with the allocation of said time slots.
8. A method as claimed in any preceding claim, further comprising allocating one or more time slots to each of said non real-time calls from the time slots in said frame not allocated to each of said real-time calls, a number of said time slots allocated to said non real-time call being variable during said non real-time call according to a current bandwidth allocation determined for said non real-time call.
9. The method as claimed in claim 5 , wherein at least one of the real-time calls comprises a voice call.