Investigation of optimum pilot channel structure for VSF-OFCDM broadband wireless access in forward link

This paper clarifies the optimum pilot channel structure in the broadband forward link for variable spreading factor-orthogonal frequency and code division multiplexing (VSF-OFCDM) wireless access using two-dimensional spreading while taking into account the following factors that affect radio link performance: (1) channel estimation accuracy for coherent detection, (2) the accuracy of the signal-to-interference power ratio (SIR) measurement for adaptive modulation and channel coding and the reliability information of the received signals, (3) the configuration of simultaneously multiplexed orthogonal pilot patterns, and (4) the impact on cell search, as well as flexibility in allocating transmission power resources. We show that although the time-multiplexed and code-multiplexed structure achieves almost the same channel estimation accuracy, the code-multiplexed structure is more advantageous due to flexible multiplexing of the orthogonal pilot patterns within one sub-carrier resulting from the accommodation of numerous OFCDM symbols in a frame. Assuming the use of an adaptive antenna array beam-forming transmitter where many orthogonal pilot patterns are required, we clarify that the cod-multiplexed structure achieves more accurate SIR measurement compared to the time-multiplexed structure. Meanwhile, the time-multiplexed structure is advantageous in that it does not require an exclusive synchronization channel for cell search. Consequently, we elucidate that the code-multiplexed structure is overall slightly superior to the time-multiplexed structure because the code-multiplexed pilot channel structure is advantageous in that it can flexibly share transmission power resources among other physical channels, thereby resulting in increasing the number of orthogonal pilot patterns and increasing the number of simultaneous packet channels despite obtaining almost the identical merits of the time-multiplexed structure from the viewpoint of radio link performance.