Adaptive Modulation and Multiple Access for the OFDM Transmission Technique

For the transmission of flexible and high data rates in future mobile communication systems, the multicarrier transmission technique (Orthogonal Frequency Division Multiplexing, OFDM) can be regarded as a possible alternative to single-carrier transmission that is used in today's systems. One of the advantages of OFDM is the flexibility concerning modulation and multiple access techniques. Since in OFDM the total bandwidth is divided into a large number of subcarriers, it can be flexibly shared among all the users. In addition, the modulation scheme can be individually chosen for each subcarrier, thus granting the possibility of an adaptation to the radio channel characteristics. In this paper, the combination of two different multiple access schemes, OFDM-FDMA and OFDM-TDMA, and an appropriate adaptive modulation technique is considered for the downlink of a communication system. Different degrees of adaptivity are analysed and compared to show the benefit of an ``intelligent'' multiple access and modulation strategy. For the given parameters and assumptions, OFDM-TDMA with adaptive modulation yields an improvement of 6 dB in signal-to-noise ratio (SNR) at a bit error rate (BER) of 10-2. When OFDM-FDMA with adaptive subcarrier allocation is applied, the SNR gain amounts to 12.5 dB (BER of 10-2).