Abstract
In this thesis, a ultra-wideband orthogonal frequency division multiplexing (UWB-OFDM) system with space-time coding is proposed. In this system, the total available bandwidth is divided into N smaller sub-bands. The OFDM modulation is adopted on each sub-band. All the sub-bands transmit data simultaneously. The transmitter encodes the sub-bands with the space-time codes to gain the frequency diversity and the time diversity. We consider a low-cost receiver with lower-sampling-rate, one N-th of the Nyquist rate. We will show that this system can be regarded as multi-input multi-output (MIMO) systems with N transmit antennas and one receive antenna. The problem of recovering the aliased spectrum due to downsampling is similar to that of recovering the signals received simultaneously from different transmit antennas in MIMO systems. The pairwise error probability (PEP) of the proposed system for quasi-static frequency selective channels is derived and two design criteria of the space-time codes are obtained: the distance criterion and the product criterion. The optimum space-time trellis codes (STTC) based on these criteria are found by computer search. We can see that the STTC can easily achieve the maximum diversity gain and large coding gain. Simulation results show that the proposed STTC have better performance than other codes proposed before.