Interference cancellation with diagonalized maximum likelihood decoder for space-time/space-frequency block coded OFDM

Space-time block coded OFDM (ST-OFDM) has been proposed as an attractive solution for a high bit rate data transmission in a multipath fading environment. Space-frequency block coded OFDM (SF-OFDM) has been also proposed as another solution. These two systems utilize the STBC with the 2/spl times/2 transmission matrix, using two transmit antennas. If we apply the STBC with the 4/spl times/4 transmission matrix to OFDM, using four transmit antennas, we can expect the performance improvement. In addition, applying the STBC with the 4/spl times/4 transmission matrix to OFDM, we can form block codes over the space, time, and frequency domains, that is, we can implement the space-time-frequency block coded OFDM (STF-OFDM). In this paper, we apply the STBC with the 4/spl times/4 transmission matrix to OFDM and compare the performance of the three systems (ST-OFDM, SF-OFDM, STF-OFDM). We show that the best system with respect to the error rate performance differs in the different channel conditions.

This paper investigates the joint maximum likelihood (ML) data detection and channel estimation problem for Alamouti space-time block-coded (STBC) orthogonal frequency-division multiplexing (OFDM) wireless systems. The joint ML estimation and data detection is generally considered a hard combinatorial optimization problem. We propose an efficient low-complexity algorithm based on branch-estimate-bound strategy that renders exact joint ML solution. However, the computational complexity of blind algorithm becomes critical at low signal-to-noise ratio (SNR) as the number of OFDM carriers and constellation size are increased especially in multiple-antenna systems. To overcome this problem, a semi-blind algorithm based on a new framework for reducing the complexity is proposed by relying on subcarrier reordering and decoding the carriers with different levels of confidence using a suitable reliability criterion. In addition, it is shown that by utilizing the inherent structure of Alamouti coding, the estimation performance improvement or the complexity reduction can be achieved. The proposed algorithms can reliably track the wireless Rayleigh fading channel without requiring any channel statistics. Simulation results presented against the perfect coherent detection demonstrate the effectiveness of blind and semi-blind algorithms over frequency-selective channels with different fading characteristics.

In this letter, an orthogonal space-time (ST) block coding (STBC) technique for continuous phase modulation (CPM) is proposed and combined with a frequency domain equalizer (FDE) to obtain high performance gains over frequency-selective channels. The new method maintains the constant envelope and phase continuity of the CPM waveforms perfectly by using appropriate tail symbols during the addition of cyclic prefix and interblock transitions and, therefore, has no negative impact on the spectral efficiency. Because the STcoded CPM blocks remain orthogonal, FDE is applied as in the case of single antenna transmissions without any increase in the computational load. Simulations corroborate the performance improvement offered by STBC while also preserving the desired bandwidth efficiency of CPM.

It is well known that the performance of the wireless communication systems can be enhanced by using multiple transmit and receive antennas, which is generally referred to as the MIMO technique, and has been incorporated into the IEEE 802.11n standard, i.e. one of the Wi-Fi systems. The MIMO technique has also been adopted by the 3GPP Long Term Evolution (LTE) and the 3GPP2 Ultra Mobile Broadband (UMB) standards. The space-time coding is a promising way to realize the gain in the wireless communications system using MIMO. The space time block code, and the orthogonal space time block code in particular have been proposed in research and implemented in practical wireless communication systems due to their low computational complexity in maximum likelihood decoding implementation. However, the code rates of the orthogonal space time block codes are usually lower than $3/4$ when the number of transmit antennas is greater than two, which limit their throughput. To increase the code rate and the throughput of the orthogonal space time block code for more than two transmit antennas, a family of new space-time block codes (STBC) for 3 and 4 transmit antennas in a MIMO wireless communication system have been proposed in this Ph.D. research. These new codes are orthogonal and achieve full diversity, full rate using Triple QPSK'' modulation, which is proposed in this research. We investigated the performance of these STBCs when the channel state information (CSI) is perfectly known at the receiver. The CSI is not known to the transmitter in all of the studies in this dissertation. An analytical bound of the probability of bit error is developed and the Monte Carlo simulation is used when investigating these new STBC codes. In practical wireless communication system, the CSI is generally not available at the receiver and it is usually estimated by the receiver through the pilot signals or training signals known to the receiver. No matter what channel estimation method is used, the estimate of CSI will not match the CSI exactly and is noisy. The performance of these new STBCs using theTriple QPSK'' modulation when the estimate of the CSI is employed to detect the symbol is investigated in this dissertation. An upper bound of the bit error probability is developed when the noisy CSI is used to detect the symbols. The performance of the new STBCs using the Triple QPSK'' modulation is also evaluated using Monte Carlo simulation. When the CSI is not available to either the receiver or the transmitter, the differential encoding and decoding algorithm is applied to these new STBCs, which use theTriple QPSK'' modulation. The results showed that these orthogonal STBCs achieve full diversity full rate using ""Triple-QPSK"" modulation with differential detection. This provides a means of achieving space diversity and multiplexing gain without the CSI. A new differential encoding and decoding algorithm is proposed and analyzed for STBCs with 3 transmit antennas, whose transmission matrix is not square matrix, as part of this Ph.D. research. The STBCs using this algorithm achieve full diversity the MIMO system can possibly provide.

To carry out simple linear decoding, it is usually assumed in space-frequency block coded orthogonal frequency division multiplexing (SFBC-OFDM) systems that the channel frequency responses between sub-carriers remain constant within a block length [1]. However, this assumption becomes unreliable when the system operates in a severe multipath environment. The conventional linear decoder [1] gives rise to inter-channel interference (ICI) due to variations of channel frequency response between sub-carriers. In this paper, we propose a turbo-based interference cancellation scheme to mitigate this effect. While the implementation complexity is higher than for conventional detection, the proposed scheme can effectively eliminate ICI, and exploit the increased diversity available in channels with long delay profile.

We consider a combined space-time (ST) block coding and direct-sequence code division multiple access (DS/CDMA) scheme for downlink transmissions over multipath fading channels, in which ST block coding is performed at the chip level, i.e. after code spreading. For this scheme, we develop low complexity and low decoding delay linear ST single-user detection algorithms. In addition, we demonstrate that by exploiting the signal structure imposed by the chip-level ST block coding, blind channel multipath estimation (with only a scalar ambiguity) is feasible, even with the assignment of a single code vector to each user.

This paper compares the performance of a blind relay communication system with that of a non-blind relay communication system. For a blind system, a differential space-time block code (DSTBC) or a blind Viterbi detection is used, and for a non-blind system, a space-time block code (STBC) is used. Also, this paper takes another blind detection mechanism, the Zhang's code, and compares its performance with that of the blind Viterbi detection under a Jakes' fading environment. It was found that the case of a relay closer to the source shows better performance than the case of a relay closer to the destination. It was also observed that the blind Viterbi decoder shows better performance than the Zhang's code under the Jakes' fading environment.

Time-selectivity in fast fading channels has been one of big obstacles to attain high efficiency and diversity gain for space-times codes. Orthogonal-frequency-division-multiplexing (OFDM) combined with space-time trellis codes has been introduced to combat effect of frequency-selectivity. Subsequently, space-time block codes, space-frequency codes and concatenation with diverse error-correcting codes were investigated. However, for multiple-input multiple-out (MIMO) OFDM systems, to design high-efficiency robust space-time-frequency codes becomes one of big problems. In this paper, we presented pairwise error probability analysis of space-frequency-time block codes, and then concluded one general design criterion for space-time-frequency block codes by utilizing the frequency-correlation feature of sub-channel in mobile OFDM systems. Then, we investigated performance of space-frequency block codes for MIMO-OFDM systems with 4 transmit antennas. And comparison between space-frequency block codes and space-time block codes is performed with different number of transmit antenna. Link-level simulations on COSSAP® verified analytical results. The results show that SFBC is superior to STBC in fast fading channel. And power-delay profile of fading channel, i.e., channel order and delay spread, must be included into design consideration on space-time-frequency codes.

The Channel estimation in Vehicle-to-Vehicle (V2V) communications is critical due to the time-varying characteristics of the V2V channel. Hence, the authors propose a hybrid approach towards channel estimation by combining Pilot Symbol Aided Channel Estimation (PSACE), Improved Crow Search Algorithm (ICSA), and Decision Directed Channel Estimation (DDCE) by employing Weight Regularized Convolutional Neural Network (WCNN). Hybrid channel estimation by combining the properties of PSACE and DDCE. PSACE suffers from pilot overhead issue due to the demand of huge number of pilots for channel estimation. On the other hand, DDCE suffers from error propagation in fast fading environments. The proposed system employs an ICSA and WCNN to overcome the issues faced by channel estimation methods. Further, the proposed strategy replaces the analytical modeling of the channel with the help of the neural network. Optimized pilots obtained from ICSA along with data symbols are fed to WCNN to estimate the channel in a non-stationary environment. Finally, the performance of the proposed method is evaluated in terms of Bit Error Rate, Mean Square Error, and Packet Error Rate and the simulations are carried out under various V2V scenarios for testing its sturdiness.

The Alamouti Space Time Block Code (STBC) based Orthogonal Frequency Division Multiplexing (OFDM) is very attractive in frequency selective channel as the channels remain constant over Alamouti code period (two consecutive OFDM symbol block). But when the channel is fast fading, this assumption does not holds good and causes co-channel interference (CCI). Hence, the simple Alamouti detection method is not sufficient to recover the original transmitted signal from the mixed transmitted signals at the receiver side. To cancel the effects of CCI, several detection techniques are addressed namely SIC, DMLD, DZFD, DF and ML. The ML method has the optimum performance but the computational complexity is very high. Therefore, we propose a suboptimal MDF detection which provides comparable performance to that of ML with very less computational complexity. Finally, the performances of the above detection methods are compared in terms of symbol error rate (SER) and computational complexity for STBC OFDM system over doubly selective channel.

Analytical expressions concerning the capacity and bit error rate (BER) of multiple-input multiple-output systems with space-time block coding (STBC) are derived. Two fading environments are examined, log-normal and Rician channels. A tight closed-form upper bound is presented for the BER of systems operating in log-normal fading environments in addition to an upper bound for the capacity of this type of systems. The latter bound applies to systems that operate under Rician fading as well. The analytical results were validated against ample numerical simulations for three STBC schemes and three phase-shift-keying modulations. The proposed bounds proved to be a tractable way to evaluate the system performance when no closed-form expression for the probability density function or the moment generating function is known.

MC-CDMA, which is based on OFDM and CDMA, is a major candidate for systems beyond IMT-2000. A time-domain spreading MC-CDMA system that has consecutive pilot symbols can achieve high quality channel estimation in fast fading environments. Furthermore, it is known that time-domain spreading MC-CDMA is suited to the use of AMC (adaptive modulation and channel coding). MC-CDMA based on OFDM generally has a guard interval (GI), which is a cyclic prefix to avoid the intersymbol interference (ISI) and interchannel interference (ICI) caused by delay paths. The receiving characteristics of MC-CDMA are significantly degraded by ISI and ICI when there are delayed paths that exceed the GI. To accurately estimate the transmission characteristics, we need an accurate delay profile model that well reflects actual delay profiles. The existing model is, however, very complicated, so it cannot be used in computer simulations. We propose an extremely simple equivalent path model for time-domain code spreading MC-CDMA; it consists of just two paths but its transmission characteristics are reasonable if the delay profile has paths that exceed GI. Computer simulation results validate the proposed model. Moreover, we consider a parameter to characterize the model with which we transform an arbitrary path model into the equivalent path model.

Orthogonal Frequency Division Multiplexing (OFDM) has made tremendous improvement possible in wireless technology. Wavelet analysis is used as an alternative to Fourier analysis in OFDM system to transform and map data onto orthogonal subcarriers, as it is a time-frequency domain operation thus it offers the advantages of optimal flexibility with less complexity. This paper presents performance analysis in terms of Bit Error rate (BER) of different wavelet families in OFDM System under the joint effect of path loss, multipath fading and noisy environment for Mobile WiMAX. The results obtained show that the performance of the mobile-multipath environments for 16QAM modulation schemes is enhanced as Daubechies and Biorthogonal wavelet families offers lower BER with different Path loss model. The performance measurements and analysis was done in simulation developed in MATLAB.

Large MIMO systems with tens of antennas in each communication terminal using full-rate non-orthogonal space- time block codes (STBC) from cyclic division algebras (CDA) can achieve the benefits of both transmit diversity as well as high spectral efficiencies. Maximum-likelihood (ML) or near-ML decoding of these large-sized STBCs at low complexities, however, has been a challenge. In this paper, we establish that near-ML decoding of these large STBCs is possible at practically affordable low complexities. We show that the likelihood ascent search (LAS) detector, reported earlier by us for V-BLAST, is able to achieve near-ML uncoded BER performance in decoding a 32 times 32 STBC from CDA, which employs 32 transmit antennas and sends 32 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> = 1024 complex data symbols in 32 time slots in one STBC matrix (i.e., 32 data symbols sent per channel use). In terms of coded BER, with a 16 times 16 STBC, rate-3/4 turbo code and 4-QAM (i.e., 24 bps/Hz), the LAS detector performs close to within just about 4 dB from the theoretical MIMO capacity. Our results further show that, with LAS detection, information lossless (ILL) STBCs perform almost as good as full-diversity ILL (FD-ILL) STBCs. Such low-complexity detectors can potentially enable implementation of high spectral efficiency large MIMO systems that could be considered in wireless standards.

Two transmitter diversity techniques for turbo coded orthogonal frequency division multiplexing (OFDM) are presented: space-time block coding (STBC) and space-frequency block coding (SFBC). With different channel environment and block size, STBC and SFBC have different performance. Also, with the increase of interleaver size (equal to block size for BPSK), turbo has better performance. So the performance of STBC and SFBC with different transmission environment and block size is simulated and discussed. Some important conclusions about the transmitter diversity for turbo coded OFDM systems are achieved.