Multi-Carrier and Spread Spectrum Systems: From OFDM and MC ...

Future Mobile Communications Concepts and Field Trials 277Table 5-43OFDM-based experiments and field trials performed by NTT DOCOMOMaximumratedata100 Mbit/s 1 Gbit/s 2.5 Gbit/s 4.92 Gbit/sAccess scheme VSF-OFCDM OFDM OFDM OFDMBandwidth 101.5 MHz 101.5 MHz 101.4 MHz 101.4 MHzSymbol mapping QPSK 16-QAM 64-QAM 64-QAMChannel coding Rate 3/4Turbo codeAntenna scheme1 spatialstreamRate 8/9Turbo codeRate 8/9Turbo codeRate 8/9Turbo code4x4 MIMO 6x6 MIMO 12x12MIMODate of field trial 07-2003 05-2005 12-2005 12-2006Reference [27] [32] [31] [30]The experiments with variable spreading factor orthogonal frequency and code divisionmultiplexing (VSF-OFCDM) [10–12] were performed in cellular environments andapplied spreading codes of variable length. For isolated cell scenarios without inter-cellinterference, a spreading factor of one can be chosen, resulting in an OFDM transmissionwithout spreading. In scenarios with high inter-cell interference the spreading factorcan be adapted accordingly. While the first experiments targeted scenarios with cellularinterference, the later ones focused on maximum data rates in isolated cells exploitingOFDM in combination with multiple antenna transmission (MIMO). The VSF-OFCDMaccess scheme is explained in detail in the following section.5.4.4 VSF-OFCDM Access SchemeFigure 5-44 illustrates the generic architecture of a broadband packet based air interfacewith VSF-OFCDM with two-dimensional spreading in the downlink and MC-DS-CDMAfor the uplink proposed by NTT DOCOMO [10–12]. The use of a two-dimensional variablespreading code together with adaptive channel coding and M-QAM modulation inan MC-CDMA system allows an automatic adaptation of the radio link parameters todifferent traffic, channel, and cellular environment conditions. Furthermore, by appropriateselection of the transmission parameters (FEC, constellation, frame length, FFT size,duplex, i.e. TDD/FDD, etc.), this concept can support different multi-carrier or spreadspectrum based transmission schemes. For instance, by choosing a spreading factor ofone in both time and frequency directions, one obtains a pure OFDM transmission system.If the spreading factor in frequency direction and the number of sub-carriers areset to one, we can configure the system to a classical DS-CDMA scheme. Hence, sucha flexible architecture can be seen as a basic platform for the integration of existingtechnologies as well.The proposal mainly focuses on FDD in order to avoid the necessity of inter-cellsynchronization in multi-cell environments and to accommodate an independent trafficassignment in the up- and downlinks according to the respective traffic. An application