The softest handoff design using iterative decoding (Turbo coding)

Communication systems, including cell-based mobile communication systems, multiple satellite communication systems or multi-beam satellite systems, require reliable handoff methods between cell-to-cell, satellite-to-satellite or beam-to-beam, respectively. Recent measurement of a CDMA cellular system indicates that the system is in handoff at about 35% to 70% of an average call period. Therefore, system reliability during handoff is one of the major system performance parameters and eventually becomes a factor in the overall system capacity. This paper presents novel and improved techniques for handoff in cellular communications, multi-beam and multi-satellite systems that require handoff during a session. This new handoff system combines the soft handoff mechanism currently implemented in the IS-95 CDMA with code and packet diversity combining techniques and an iterative decoding algorithm (Turbo Coding). The Turbo code introduced by Berrou et al. has been demonstrated its remarkable performance achieving the near Shannon channel capacity [1]. Recently, Turbo codes have been adapted as the coding scheme for the data transmission of the third generation international cellular communication standards: UTRA and CDMA 2000. Our proposed encoder and decoder schemes modified from the original Turbo code is suitable for the code and packet diversity combining techniques. This proposed system provides not only an unprecedented coding gain from the Turbo code and its iterative decoding, but also gain induced by the code and packet diversity combining technique which is similar to the hybrid Type IIARQ. We demonstrate performance improvements in AWGN channel and Rayleigh fading channel with perfect channel state information (CSI) through simulations for at low signal to noise ratio and analyses using exact upper bounding techniques for medium to high signal to noise ratio.