Muliple-access techniques for adaptive wireless systems

Licentiate thesis, 2006

With the growing demand for mobile content, the roles of mobile handsets have started shifting from merely communication tools to multi-media channels. This trend will push the current 3G/UMTS networks towards higher throughput, mobility and connectivity. To achieve the goals of the future wireless communication systems, an air interface with spectrum efficient technologies is definitely necessary. The integration of mobile communications and the Internet will also require an IP-based packet-switched network to handle multiple services on a single channel. In this thesis, an adaptive OFDM-based wireless communication system is proposed and evaluated. To facilitate a comparison with the current WCDMA system, the proposed system works at 1900 MHz with a 5 MHz bandwidth. Frequency Division Duplex (FDD) is assumed. A slotted OFDM radio interface is used, for both uplink and downlink, which exploits both frequency and time diversity of the channel. The user who has the best predicted channel in each time-frequency bin transmits. Consequently, the spectral efficiency increases with the number of active users in the system, which is denoted as multiuser diversity. An OFDM uplink faces more challenges than a downlink. A model is provided to quantify the effects of multiuser carrier frequency synchronization errors. This effect is taken into account when calculating the spectral efficiency of the OFDM uplink. Simulation results indicate that an adaptive OFDM-based system with scheduling is a strong candidate for future wireless systems. An adaptive single-carrier TDMA uplink with a frequency-domain equalizer is suggested as an alternative to the OFDM uplink. It is superior to an OFDM uplink in that good synchronization performance and lower peak-to-average power ratio are easier to obtain. However, the spectral efficiency of such a design is not satisfactory over wideband channels due to the loss of frequency-domain variability.