Advanced transceiver design for future OFDM systems

Abstract

Orthogonal frequency division multiplexing (OFDM) has become the foundation technique in modern broadband wireless communications, such as wireless local area network (WLAN, IEEE 802.11a,g,n), wireless metropolitan area network (WiMAX,IEEE 802.16) and cellular network (long term evolution (LTE) and 4G). However, future OFDM systems see challenging problems, associated with increased data rate and signal bandwidth requirement. Signal with larger bandwidth not only leads to severer frequency-domain channel fading, but also imposes more constraints and challenges on hardware. For example, in multi-band OFDM UWB systems, where the bandwidth of signal is more than 500MHz, analog to digital converter (ADC) becomes one of the bottlenecks of system development. Spectrum efficiency becomes more important due to the scarce spectrum resource and increased bandwidth requirement. OFDM improves spectrum efficiency by removing the guard band required in conventional frequency division systems, however, the temporal guarding interval used in OFDM is questioned as a factor that reduces spectrum efficiency. Some alternative approaches have been investigated to reduce the overhead, for example, the pseudo-random prefix (PRP). One better way of improving spectrum efficiency is to apply resource optimization techniques, which assign system resources to users adaptively according to users' link requirements and channel conditions. Spatial division multiple access (SDMA), as an extension of the optimization technique to multi-access systems, exploits spacial redundancy and significantly increases system capacity. This thesis investigates several solutions for these challenging problems, with focus on addressing the difference between using zero padding and cyclic padding, ADC solution for signals with extremely large bandwidth, and joint design of transmit and receive beamforming for SDMA systems. It is highlighted that timing error in zero-padded OFDM system causes intercarrier interference, which is very different to OFDM system with cyclic prefix where only phase rotation is arisen; Frequency-channelized ADC is proposed for OFDM systems with very large bandwidth, and receiver baseband implementation including synchronization and equalization is studied. The proposed receiver does not require a signal reconstruction module which was commonly needed in such systems; Some novel joint beamforming design schemes are proposed within the framework of multiuser multiple-input multiple-output systems following minimum mean square error (MMSE) criterion. In a SDMA system with codebook based channel feedback, the proposed schemes can efficiently mitigate the multiuser interference due to the channel quantization error, and speed up the searching process significantly. Show more