TABLE OF CONTENTS Recently, there have been several sessions on Beyond 3G or 4G Systems at major international conferences on wireless communications, where different speakers have given different images and concepts for 4G mobile communications systems. We often asked prominent researchers at these conference venues the question, "What are 4G systems?" but even their answers have been very different. For instance, some systems are based on multiple access techniques in multiple cell environments, while others are based on random access techniques in isolated cell environments. In addition, some services are provided in microwave frequency bands, while others are provided in millimetric frequency bands. This can be very confusing. We still might not know the answer to our question, or perhaps all the answers are correct. Such ambiguity is why it is worthwhile for academic and company researchers to continue to conduct research on 4G systems. Instead of providing a conclusion for the book, this chapter offers some interesting research topics related to multicarrier techniques for future research directions.
Section 10.2 discusses two possible paths toward the realization of OFDM-based 4G systems, such as a migration from cellular phone systems and a migration from wireless LANs. Section 10.3 introduces two variants based on the MC-CDMA scheme, including orthogonal frequency and code division multiplexing (OFCDM). Section 10.4 shows the application of an adaptive array antenna for the OFDM scheme and discusses four different kinds of array configurations. Section 10.5 introduces a multiple input and multiple output (MIMO) system and discusses the suitability of...
