Mobile and Satellite Communications Research Centre, University of Bradford, UK BD7 1DP
T. S. Ghazaany
Mobile and Satellite Communications Research Centre, University of Bradford, UK BD7 1DP
J. Rodrigues
Mobile and Satellite Communications Research Centre, University of Bradford, UK BD7 1DP
K. N. Voudouris
Mobile and Satellite Communications Research Centre, University of Bradford, UK BD7 1DP
Preferred Abstract (Original):
ABSTRACT
The quasi-orthogonal space
time block coding (QO-STBC) over orthogonal frequency division
multiplexing (OFDM) is investigated. Traditionally, QO-STBC does not
achieve full diversity since the detection matrix of QO-STBC scheme is
not a diagonal matrix. In STBC, the decoding matrix is a diagonal matrix
which enables linear decoding whereas the decoding matrix in
traditional QO-STBC does not enable linear decoding. In this paper it is
shown that there are some interfering terms in terms of non-diagonal
elements that result from the decoding process which limit the linear
decoding. As a result, interference from the application of the QO-STBC
decoding matrix depletes the performance of the scheme such that full
diversity is not attained. A method of eliminating this interference in
QO-STBC is investigated by nulling the interfering terms towards full
diversity for an OFDM system. It was found that the interference
reduction technique permits circa 2dB BER performance gain in QO-STBC.
The theoretical and simulation results are presented, for both
traditional QOSTBC and interference-free QO-STBC applying OFDM.
In OFDM systems, time-varying channels destroy orthogonality between subcarriers causing Inter-Carrier Interference. To reduce this effect, a Kalman filter, as a benchmark, is used for channel estimation, based on comb-type pilot arrangements of the OFDM system. An advantage of comb-type pilot arrangements in channel estimation is the ability to track the variation of the channel caused by Doppler frequency. Kalman method has been proposed to estimate the channel frequency response (CFR) at the pilot locations, then CFR, at data subchannels, is obtained by means of interpolation between estimates at pilot locations. The low-complexity Kalman method is introduced to reduce the complexity of the system while achieving the same BER/SNR. Different types of interpolations have been also compared such as Low-pass, Spline-cubic and Linear interpolation methods. The BER/SNR performance of BPSK modulation schemes are considered for time varying Rayleigh fading channels. Our results has shown that the low-complexity Kalman estimation, used with the pilot arrangement and a suitable interpolation method, gives almost the same performance as that of the Kalman method specially for low SNR values and hence the effect of Doppler shift effect is controlled.
Orthogonal Frequency Division Multiplexing (OFDM) has been recently applied widely in wireless communication systems, due to its high data rate, transmission capability with high bandwidth, efficiency and its robustness to multipath delay .Channel estimation is an essential problem in OFDM system. Pilot-aided channel estimation has been used; a good choice of the pilot pattern should match the channel behavior both in time and frequency domains. We explored comb pilot arrangements. The advantage for comb type pilots arrangement in channel estimation is the ability to track the variation of the channel caused by doppler frequency, it is observed that the doppler effect can be reduced, and so this will increase the system mobility. Kalman and Least Square (LS) estimators have been proposed to estimate the Channel Frequency Response (CFR) at the pilots location, then CFR at data sub channels are obtained by mean of interpolation between estimates at pilot locations. Different types of interpolations have been used such as; low pass interpolation; spline cubic interpolation and linear interpolation. Kalman estimation has better performance than LS estimation. The estimators perform about the same for SNR lower than 10 dB. The performances of all schemes have been compared by finding Bit Error Rate (BER), where BPSK modulation scheme was used.