Research On Key Technologies Of OFDM Based Distributed Antenna Systems With Multiple Frequency Offsets

The fourth generation (4G) mobile communication systems require high-speed and high-quality data transmission. The distributed multiple input multiple output (MIMO) systems can achieve higher channel capacity than the conventional centralized MIMO systems. Orthogonal frequency division multiplexing (OFDM) can effectively combat the multipath effects with a low-complexity channel equalizer. This means that the distributed MIMO systems can efficiently avoid the inter-symbol interference (ISI) in frequency selective channels when OFDM is employed. Based on the above facts, the distributed MIMO OFDM technology has become a competitive candidate for the 4G mobile communication systems.In distributed MIMO OFDM systems, each distributed transmit antenna has its own oscillator, which leads to different frequency offsets (FOs) between the receiver and each distributed transmit antenna, and hence results in inter-carrier interference (ICI). Therefore, it is of primary importance to compensate for FOs for distributed MIMO OFDM systems. Meanwhile, in distributed MIMO OFDM systems, not only the small-scale fading between the receiver and each distributed transmit antenna are independent, but there are also differences in large-scale fading, which means that the power allocation is needed to optimize the system performance.This dissertation investigates two key technologies of distributed MIMO OFDM systems with multiple FOs. One is FOs compensation, and the other one is power allocation. The details of the investigation are as follows:Firstly, considering the case where the FOs between the receiver and each transmit antenna are different, a FOs correction algorithm is proposed for distributed spatial multiplexing MIMO OFDM systems, which maximizes the conditional average signal-to-interference-plus-noise ratio (SINR) of subcarriers on each receive antenna. By exploiting the polynomial approximation method to approximate the conditional average SINR, a computationally efficient expression of the FOs correction value is obtained. Compared with the existing FOs correction algorithm, the proposed algorithm offers slight performance improvement and achieves at least 50% complexity reduction.Secondly, the problem of average transmit power allocation in distributed space-time block coded OFDM (STBC OFDM) systems with per-antenna power constraints in the presence of multiple residual FOs is investigated. Assuming that the transmitter is equipped with two distributed antennas, the receiver exploits decision-feedback detection and the signals experience multipath Rayleigh fading, the lower bound on the bit error ratio (BER) performance is derived. Then, an average transmit power allocation method based on minimizing that bound is proposed and the optimal average transmit power for each distributed transmit antenna is also derived. Simulation results show that the proposed method can save the transmit power while enhancing the BER performance compared with the conventional full power transmit method. Finally, based on the system model which is exploited in the investigation of power allocation with per-antenna power constraints presented above, this dissertation investigates the average transmit power allocation problem of minimizing the lower bound on the BER performance in distributed STBC OFDM systems with multiple residual FOs under the sum power constraint. It is firstly proven that the sum of the optimal average transmit power for each distributed transmit antenna should be equal to the maximum total average transmit power under the sum power constraint, then the closed-form expression of the transmit power allocation factor (PAF) is derived. Simulation results show that the average transmit power allocation method exploiting the derived PAF can significantly enhance the BER performance compared with the conventional average transmit power allocation method which ignores the existence of the residual FOs.This dissertation investigates the problem of FOs compensation for distributed spatial multiplexing MIMO OFDM systems, and explores the problem of power allocation in distributed MIMO OFDM systems with multiple residual FOs. The research results can be applied to the wireless communication systems based on the distributed MIMO OFDM technology.