To implement 4G wireless that should deliver a much higher data-rate than that possible in the current 3G systems, development of innovative air-link technologies is the key. This article reviews our on-going research activities in the design of 'isotropic air-interface technologies' capable of substantially improving air-link performance of a CDMA wireless system. To enable bidirectional high-speed data transmission in the future 4G systems, the problem associated with incompatible up- and down-link performance in 2-3G systems has to be addressed. The importance of the issue becomes even more evident due to the fact that the future wireless needs to support 'mobile server' applications, where a mobile terminal may act as a data source such that up-link traffic can possibly exceed that in down-link. In this article, two alternative methods enabling isotropic air-interface will be discussed, either using 'isotropic spreading techniques' that ensure a homogeneous link-performance regardless of its operation mode (synchronous or asynchronous), or relying on a precision up-link synchronization control techniques to pave a way for successful application of orthogonal codes in up-link channels, which otherwise are asynchronous and destroy the 'orthogonality' amongst the codes. The isotropic spreading techniques can minimize multiple access interference (MAI) and multipath-interference (MI), the prevalent impairing effects in the current 2-3G systems. This article will introduce a promising complete complementary CC/DS-CDMA scheme based on isotropic spreading techniques, which carries several extremely attractive properties such as MAI-free, Mi-free, near-far effect resistance and low hardware complexity. The performance of the proposed CC/DS-CDMA scheme is limited only by noise. To implement up-link synchronization control, smart antenna and GPS-based techniques play an important role and a working example based on TD-SCDMA standard will be illustrated. Finally, we will also discuss the TDD techniques and its significance to implement isotropic air-interface technologies for the future 4G systems.
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