Researches On Prototype Filter Design For OQAM/FBMC Systems

In contrast to the orthogonal frequency division multiplexing(OFDM)system,the offset quadrature amplitude modulation based filter bank multicarrier(OQAM/FBMC)system shows advantages in various aspects,such as the significantly reduced out-of-band(OOB)emission,the support of asynchronous transmission,and no requirement on cyclic prefix.Thus,it is a promising technique for the Fifth Generation(5G)and future wireless communication systems,and it has been attracting significant attention in recent years.As the key component in the OQAM/FBMC system,the prototype filter significantly affects the system performances,including the OOB emission,the implementation complexity and the performance in transmission channel.Thus,employing prototype filters with different properties,the OQAM/FBMC system could achieve different performances.This thesis thoroughly studies the influence of prototype filter on the channel estimation performance,the implementation complexity and the robustness to timing offset of the system,and proposes three filter design approaches accordingly.The main contents of this thesis are listed as follows.Firstly,a prototype filter design to minimize the stopband energy of the filter with guaranteed time-domain channel estimation performance for OQAM/FBMC systems is proposed.When the impact of intrinsic interference from data symbols is taken into account,the total mean squared error(MSE)of estimated channel impulse response is analytically derived with two channel estimation schemes,i.e.,the linear minimum mean square error and weighted least square.Then,an optimization problem of filter coefficients to minimize the stopband energy with constraint on the total MSE is formulated.By exploiting the Heisenberg-Gabor uncertainty principle,the original optimization problem is transformed into an equality-constrained one.Finally,the Lagrange multiplier method and the Newton’s method are employed to solve the transformed problem and obtain the filter coefficients.Simulation results demonstrate that the proposed filter outperforms the isotropic orthogonal transform algorithm and the extended Gaussian function filters considering both the MSE and stopband energy performances.Secondly,a novel design of short prototype filter for OQAM/FBMC systems is proposed,where the filter satisfies nearly perfect reconstruction with constrained frequency spreading.In the design,the filter is generated from an inverse discrete fourier transform(IDFT)of the filter frequency response,where the number of non-zero frequency coefficients is limited to constrain the frequency spreading of filter.In addition,requirements on the short filter to guarantee perfect reconstruction(PR)property of OQAM/FBMC are analyzed.Based on the PR condition and the requirement of continuity,a system of equations is formulated and solved in the sense of least squares,resulting in the Constrained Frequency Spreading(CFS)filter.Numerical results reveal that the proposed CFS filter is able to achieve good NPR performance.Moreover,it shows robustness against multipath channels,carrier frequency offset and timing offset.Further,due to the less frequency spreading,the CFS filter with less frequency coefficients could lead to lower implementation complexity if the frequency spreading implementation is employed.Thirdly,the design of a prototype filter that increases the robustness of the OQAM/FBMC system to timing offset is proposed.Based on the frequency spreading structure of OQAM/FBMC system,the performance of signal-to-interference ratio(SIR)with the single-tap equalizer in the presence of timing offset is analyzed.In the case of short prototype filter,an optimization problem of filter frequency coefficients is formulated,in order to maximize the SIR with constraint on the nearly perfect reconstruction performance and stopband energy.Finally,the sequential quadratic programming method is employed to solve the problem and obtain the filter frequency coefficients.Simulation results show that the proposed filter outperforms the NPR1 filter with respect to timing offset sensitivity,and achieves good stopband energy performance.In conclusion,this thesis mainly focuses on the design of prototype filter for the OQAM/FBMC system,in order to meet with different performance requirements.