Tandem Free Operation Simulator for Wireless Communications
Anne Kumar · 2001
Speech coding is used as an effective method to achieve bandwidth efficiency. The disadvantage of using speech compression is that it introduces signal distortion, which becomes more severe if a signal is passed through a series of coder/decoder operations. There are encoders and decoders (or transcoders) present in the mobile handsets as well as in the base stations. The transcoders in the base stations provide a considerable gain in system capacity but degrade the speech quality. Any call originating at a mobile is first converted into a compressed digital bit stream at the base station, then decoded to a 64 kbps digital signal for transmission over the terrestrial network. If the call destination is another mobile, another coding from 64 kbps to the compressed digital bit stream must take place, degrading the speech signal a second time. The speech quality may be improved if the compressed signal is not decoded at the terrestrial interface, but rather is transmitted as a data signal to the destination mobile, avoiding one of the coding steps. This approach has recently been standardized as the Tandem Free Operation (TFO) protocol. TFO bypasses the decoder if a call is destined to another mobile telephone. The motivation behind this is that by removing the transcoding performed in the base stations, better end-to-end speech quality is realized. This report studies the behavior of the TFO protocol standard. TFO is applicable only if the transmitting and destination mobiles use the same speech coding algorithms. The standard describes two different state machines that perform the same TFO functionality. The difference being that one state machine has additional states to handle hand-offs and the other does not. The main goal of this report is to investigate the need for the extra states. In this study, a high-level language is used to simulate the TFO protocol. Specific tests are performed, such as, for local and distant hand-offs. The results reveal that the additional hand-off information does not contribute significantly to its performance.