IFDMA for Uplink Mobile Radio Communication Systems

Kurzfassung

The first objective of this thesis is to develop the interleaved frequency-division multiple-access (IFDMA) receiver with frequency domain equalization, low complexity and possibility to separate users in the frequency domain. Note that at the time the work on this thesis was started, the efficient IFDMA receiver was not developed and relationship between OFDMA and IFDMA systems were not yet understood. The IFDMA technique is introduced as a special case of the orthogonal frequency-division multipleaccess code division multiplexing (OFDMA-CDM). The proposed IFDMA transmitter does not have the computationally intensive discrete Fourier transform (DFT) operation and, thus, is more preferable than the conventional OFDMA and OFDMA-CDM transmitters. The concept of minimum shift keying (MSK) and Gaussian-shaped offset QPSK (GOQPSK) IFDMA is proposed. As a result of GOQPSK and MSK combination with IFDMA, the MSK/GOQPSKIFDMA transmit signal is generated without phase transitions and has better spectral properties than the conventional OFDMA transmit signal. The peak-to-average power ratio (PAPR) of the OFDMA, IFDMA and newly introduced GOQPSK- and MSK-IFDMA are compared. The cumulative distribution function of PAPR distribution is analyzed for the GSMK/MSK-IFDMA. As a reference, the conventional OFDMA and OFDMA-CDM systems are used. Finally, the circumstances under which PAPR distribution of GOQPSK-IFDMA outperforms the PAPR distribution of the conventional IFDMA are identified. The most valuable results is that IFDMA based systems can achieve the PAPR of single-carrier systems providing any required data rate for data transmission. The effect of frequency offsets on the performance of IFDMA uplink system is investigated using raised cosine (RC) window. It is shown that MAI caused by the frequency offsets can be reduced significantly if the RC window is applied instead of conventional rectangular window. Generally, frequency offset estimation in the uplink of multi-carrier systems is a complicated task, since the received signal at the BS comprises of the received signals from many users and each user can have its own frequency offset. Moreover, received signals from different users are distorted by different transmission channels which makes the problem of channel estimation complicated. A frequency domain algorithm for frequency offset estimation is proposed and its performance is investigated in the mobile radio channel. This algorithm utilizes pilot symbols and provides joint frequency offset and channel estimation. A special construction of pilot symbols with additional spreading in the time domain is proposed which allows reduction of the frequency offset estimation error. A practical time domain (TD) algorithm is evaluated which uses repetitive structure of the IFDMA transmit signal in the time domain. The TD algorithm is independent of the transmission channel and modulation alphabet. Statistical properties of the estimate are analyzed and proven analytically and by Monte-Carlo simulations. As a result, the obtained estimate is unbiased and is able to provide a reliable result at the SNR of practical interest. The proposed algorithm is compared with existing techniques and its superiority is proven. The optimum MMSE equalization technique for OFDM and IFDMA with windowing at the receiver is presented. As shown in literature, the application of the window in the receiver changes the received spectrum of each individual subcarrier. Varying the roll-off factor of the DFT window function and, therefore, changing the length of the prefix and postfix, MAI caused by the frequency offset can be reduced. The minimum mean square error algorithm which utilizes part of the energy invested into prefix and postfix for the equalization is proposed which allows to improve the BER of OFDM system with windowing. For simplicity, the proposed algorithm is presented for OFDM but can also be applied for IFDMA. Additionally, The SI of IFDMA systems is investigated in an independent Rayleigh channel and it is shown that IFDMA has significantly less SI than the conventional OFDMA-CDM. The IFDMA uplink system with non-linear amplifier is investigated and compared with OFDMA and OFDMA-CDM techniques in terms of passband interference, out-of-band radiation and BER performance. Finally, the performance of IFDMA system is simulated in the mobile radio channel.