The STBC-OFDM symbols are transmitted through the frequency selective fading channel after performing Inverse Fast Fourier Transform (IFFT) and adding cyclic prefix (CP) to eliminate ISI.
The channel estimation for STBC-OFDM is performed by transmitting pilot symbols from different antennas at the same subcarrier simultaneously.
The STBC-OFDM system assumes that [H.sub.i](n, [k.sub.p]) = [H.sub.i](n + 1, [k.sub.p]) = [H.sub.i]([k.sub.p]), n = 0, 2, 4, ..., (N - 1), and i = 1, 2 which means the channel is quasi-static for two consecutive times.
In considered STBC-OFDM model, this parameter can have two different values for each subchannel, since the symbols and their complex conjugate values are transmitted in two consecutive times.
However, using higher order modulation schemes not only increase the data rate of the system but also increase the complexity of the frequency domain EM-based channel estimation for STBC-OFDM systems significantly.
In this section, we evaluate the performance of the proposed low complexity frequency domain EM-based channel estimation algorithm for Alamouti STBC-OFDM with higher order constellations.
1 shows the proposed STBC-OFDM systems with two transmit antennas and one receive antenna.
A two-stage channel estimation method is used to realize a successful STBC-OFDM system in outdoor mobile channels.
Since the STBC-OFDM system has two transmit antennas and one receive antenna, two received OFDM symbols in a time slot must be ready at the same time for channel estimation and STBC decoding.
A robust channel estimator for high-mobility STBC-OFDM systems.
Table I: Major Parameters of The Proposed Stbc-Ofdm System Parameters Values RF frequency 2.5 GHz System channel bandwidth (BW) 10 MHz Sampling frequency ([F.sub.s]) 11.2 MHz Sampling factor 28/25 FFT size (N) 1024 Subcarrier spacing ([DELTA]f) 10.9 kHz Useful symbol time ([T.sub.b]) 91.4 [micro]s Guard time ([T.sub.g]) 11.4 [micro]s OFDM symbol duration ([T.sub.s]) 102.9 [micro]s Number of OFDM data symbols in a DL sub-frame 40 DL Number of pilot subcarriers ([N.sub.Pilot]) 120 PUSC Number of data subcarriers ([N.sub.Data]) 720 Table II: Word Lengths Of Several Key Signals In The Proposed Receiver.