Reordering soft handoff frames to minimise delay in CDMA cellular networks

This paper focuses on delay and jitter for voice traffic in an IP based infrastructure for CDMA cellular networks. During soft handoff, a key aspect of CDMA networks, a mobile station (MS) may maintain connections with multiple base station transceivers (BTS). Each connection is called a soft handoff connection. The MS transmits copies of the voice frame to each BTS. The multiple copies are sent by the BTSs to a selector distributor unit (SDU) that selects the 'best' frame based on frame error conditions. The SDU has to wait for all copies to arrive prior to selection and then forwards the voice frame to the public switched telephone network (PSTN) through a circuit gateway. This causes additional delay and jitter for the voice traffic. We propose multiple schemes to reduce voice delays between the BTS and SDU and subsequently the circuit gateway. These schemes are based on reordering the voice frames based on two identifiers available on each voice packet: (1) their network identifier derived from IP addresses and UDP port numbers; (2) the number of soft handoff connections maintained by the MS. For each scheme, we derive analytical results for the delay distribution and use the results for a numerical comparison of the schemes. We also conduct a simulation study to validate the assumptions made in the analytical work.