Overlapping Multiplexing OFDM System And Its Application In The Study Of Turbo Code Technology

The overlapped multiplexing theory and technology is introduced and analyzed, so as to improve the data signaling rate, spectrum utilization and power efficiency for the wireless communication system. The overlapped multiplexing theory and technology includes several implementation models, such as OVTDM technology and OVCDM technology. Several innovative solutions, including improvement of the structure for the system, the optimal selection of the waveforms, the optimal receiver, and combination with the multi-carrier and multi-antenna techniques are proposed. Besides, the channel estimation approach using zero correlation window continuous pilot scheme are presented, and the approach is compared with reference signal approach in LTE/LTE-A system. Through theoretical analysis and computer simulation, it’s turned out that the proposed technologies and solutions, can improve the system performance and spectrum efficiency significantly.For the OVTDM systems, it’s found that, using waveform segmentation approach, the technology introduces the waveform overlapped multiplexing idea by narrowing the time interval of waveform signals, thereby it can obtain encoding constraints relationship between adjacent waveform samples, and improve the spectrum efficiency of system effectively. Compared with the traditional high-order modulation technology, OVTDM shows better error performance, less output levels, and higher spectrum efficiency. With high system spectral efficiency, the normalized SNR threshold of OVTDM technology turns out to approximately linear with the spectrum efficiency, under the observed BER. The performance of coded OVTDM system is preferable, than traditional binary field coding scheme together with higher order modulation scheme. In particular, the performance gain increases with the increase of code rate for the FEC code. The Turbo-OVTDM technology, using the parallel concatenated structure of Turbo code, is turned out that, it can enhance the communication reliability of system than OVTDM technology with single branch for data transmission.In the system design for realizing high spectral efficiency, the pure OVTDM (POVTDM) with no shift superimposition is introduced (overlapped multiplicity is K1), and a serial concatenated structure is used between POVTDM and OVTDM (overlapped multiplicity is K2). The data symbols are transmitted and multiplexed in the in-phase branch and orthogonal branch, whose spectrum efficiency is2K1K2. In the receiver, data in the parallel branches are separated firstly, and then the two branches are detected respectively. Compared to the coded modulation scheme with the same spectral efficiency, the performance gain can be further improved, while the decoding complexity is reduced from22k1k2to2×2k1k2. When the OVTDM module in the sending side is replaced with Turbo-OVTDM scheme, the iterative decoding is introduced in the receiver. With typical parameters, the serial concatenated structure between POVTDM and Turbo-OVTDM is turned out to have lower SNR threshold under the observed BER level, and the iterative performance is converged after4iterations.The overlapped multiplexing technology is researched and analyzed in theoretical significance so far. For its further application in the practical systems, OVTDM is supposed to work effectively under band-limited conditions, i.e. the output signal of OVTDM is filtered before sending through the channel. Several multiplexing waveforms for OVTDM is analyzed, such as the rectangular wave, root raised cosine (RRC) spectrum waveform (it is commonly used in engineering known as RRC filter), and time domain raised cosine waveform. The theoretical performances in AWGN channel, as well as time-varying channel performance against random fading are studied intensively. Through theoretical derivation. OVTDM technology uses waveform segmentation idea to obtain the encoding constraint, and its capacity outperforms Shannon capacity under the Nyquist criterion. The traditional diversity techniques to against fading, needs additional transceiver, or extra time-frequency resources. The OVTDM technology can employ the hidden diversity gain in time-varying channel, and get good diversity performance.For band-limited OVTDM system, it can realize good band-limiting performance with the raised cosine multiplexing waveforms. Through the observation of power spectrum and phase spectrum of the waveforms, the power spectrum of the first side lobe attenuates approximately-35dB, compared with the spectrum of the main lobe. And the spectrum of the second side lobe attenuated about-45dB. The time interval of impulse response for raised cosine waveform is [0,2T], and the frequency range of the main lobe of power spectrum is [-1/T,1/T], the corresponding phase spectrum is distributed linearly in the frequency range. It’s also found that the noise power spectral density of the filter is non-white, whose power spectrum envelope is consistent with that of waveforms substantially. The optimal receiver structures of the system are discussed. The received symbol sequences are firstly sent through matched filter, and then they are processed with the maximum likelihood sequence detection using Ungerboeck metric. The structure is verified to be reasonable by computer simulation.As the multi-carrier technique, such as OFDM/OFDMA is widely used in wireless communication systems, the combination of overlapped multiplexing technology with multi-carrier technique is considered in the paper. When the OVTDM technology is used in multi-carrier systems, it would cause inter carrier interference, as the signal envelop of OVTDM output symbols is not flat. A multi-carrier OVTDM system is designed with joint detection algorithm, which is turned out that it can obtain certain performance gain when the number of subcarrier is increased. When the Turbo-OVTDM scheme is combined with multi-carrier technique, the SNR threshold can be decreased to a certain extent. For the OVCDM technology, the coding matrix can be designed flexibly, and it’s easily to apply OFDM and multi-antenna techniques in the OVCDM system. The system performance is compared with LTE PHY layer coded modulation scheme. The performance of OVCDM-OFDM systems, can hold the line, even outperform with LTE scheme in the ITU channel model, such as ITU-Vehicle A, and ITU-Urban Macro scenario.Channel estimation is an important aspect for system design, as the signal characteristics would change when it is send through the radio channel, when certain time, frequency, spatial diffusion phenomena occurs. In the paper, a novel ZCW continuous pilot scheme for OFDM-MIMO system is proposed, and the pilot sequence is generated on the base of complementary code group with good correlation characteristics. With the use of four-color principle for networking, it only needs4complementary code groups under the19cell networking environment, where the interference from the same color cell can be avoided with certain design. The pilot scheme can be applied in LTE/LTE-A system, by adjusting the length of pilot sequence, and the pilot insertion density in the resource blocks. Compared with the reference signal program in LTE/LTE-A standard, the proposed scheme is able to obtain higher estimation accuracy, and work effectively against the interference from the same color cells, and in the high SNR region, the estimation performance is substantially independent of the number of antennas, as well as the number of cells.For further study of the theoretical performance of the continuous pilot, the performance comparison with typical frequency domain schemes are analyzed. The proposed scheme is turned out to have relatively high estimate accuracy with low calculation complexity, and the estimate performance is unaffected with number of transmit antennas. Compared with LS estimator, the present scheme can obtain large gains with specific system parameters, with simple linear correlation under the same mean square estimation level. With certain parameters, the performance gains can be13.8dB. Compared with LMMSE estimator in FD, the scheme approaches LMMSE’s favorable performance in high signal to noise ratio region, with knowledge of channel taps length only and no more a priori information in the estimate process. The trade-off between complexity and estimation performance can be obtained for the continuous pilot scheme.