On the design of LAS spreading codes

A family of large area synchronised (LAS) codes is studied, which exhibits a so-called interference free window (IFW), where both the intersymbol interference (ISI) and the multiple user interference (MAI) are suppressed, provided that the relative time offset of the codes is within the IFW. Hence, LAS codes have the potential of increasing the capacity of CDMA networks. However, a specific drawback of this family of sequences is their relatively low duty ratio. A modified algorithm is proposed for determining the constituent LA code´s pulse positions for the sake of improving the duty ratio of the corresponding LAS codes. It is shown that the modified LAS codes exhibit an 18.6% higher duty ratio, than that of the original LAS codes, when considering a minimum LA code pulse interval of 38. The LAS code design examples provided revealed that the system employing the proposed approach improved the data throughput by 40% for the scenario, when the required IFW is ±4-chips and the minimum pulse interval is 19. The availability of the LAS codes is also investigated in conjunction with for the various LS and LA code lengths, employing an extended LS code construction scheme for polyphase complementary sequences and orthogonal LA pulse codes. It is shown that these modified spreading code construction schemes can be combined for generating a family of LAS codes in order to meet various design requirements.