1. A method of determining useable combinations of variables for transmitting a subpacket of an encoder packet in a communication system, the method comprising: setting a plurality of all combinations of transmission variables for transmitting a subpacket of an encoder packet; defining a plurality of predetermined sets of modulation order product code rates (MPR), each predetermined MPR set corresponding to each combination of a set of numbers of useable encoder packet bits and a set of numbers of useable slots; selecting an arbitrary combination of transmission variables from the plurality of all combinations based on a number of currently useable Walsh codes and channel environment; determining an arbitrary MPR based on a number of useable encoder packet bits, a number of useable slots, and the number of currently useable Walsh codes that belong to the selected arbitrary combination of transmission variables; selecting one of the plurality of predetermined MPR sets that corresponds to the number of useable encoder packet bits and the number of useable slots and determining a smallest one of MPR values of the selected MPR set that are greater or identical to the arbitrary MPR as a final MPR; calculating a final number of Walsh codes based on the number of useable encoder packet bits, the number of useable slots, and the determined final MPR; and determining useable combinations of transmission variables from the plurality of all combinations of transmission variables based on the set of numbers of useable encoder packet bits, the set of numbers of useable slots, and the final number of Walsh codes.