Multiple-Input Multiple-Output (MIMO) systems that realize high-speed data transmission with multiple antennas at both transmitter and receiver are drawing much attention. In line-of sight (LOS) environments, the performance of MIMO systems depends largely on the difference of the phase difference of direct paths from transmit antennas to each receive antenna. When the phase difference of direct paths are close to each other, the spatial division multiplexing (SDM) channels are not orthogonal to each other so signal detection becomes difficult. In this paper, we propose a MIMO system with relative phase difference time-shift modulation (RPDTM) in Rician fading environments. The proposed scheme transmits independent signals from each antenna at each time slot where the relative phase difference between signal constellations used by transmit antennas varies in a pre-determined pattern. This transmission virtually changes the phase difference of direct paths from transmit antennas to each receive antenna without lowering data rate and without knowledge of the channels. In addition, forward error correction coding (ECC) is applied to exploit the time slots where the receiver can detect the signals easily to improve the detection performance. If there are time slots where the receiver can separate the received signal, the receiver can decode the data by using the time slots and the correlation between data. From the results of computer simulation, we show that MIMO system with RPDTM can achieve the better bit error rate (BER) than the conventional MIMO system. We also show that the MIMO system with RPDTM is effective by about Rician factor K = 10 dB.

A method is presented that can substantially reduce the memory requirements of non-binary turbo decoders by efficient representation of the extrinsic information. In the case of the duo-binary turbo decoder employed by the IEEE 802.16e standard, the extrinsic information memory can be reduced by about 43%, which decreases the total decoder complexity by 18%. We also show that the proposed algorithm can be implemented by simple hardware architecture.

In this paper, an efficient cyclic prefix reconstruction (CPR) technique with turbo equalization is developed for multi-antenna single-carrier frequency-domain equalization (SC-FDE) systems, which are for multi-input multi-output (MIMO), space-time block code (STBC), and space-frequency block code (SFBC) applications. The proposed method includes pre-processing estimation (PPE), weighted interblock interference cancellation (WIBIC), or residual intercarrier interference suppression (RICIS). PPE is employed to compute initial values of MIMO turbo equalization and the WIBIC is developed to cancel interblock interference (IBI) at the initial iteration of the CPR for STBC SC-FDE. RICIS is used to mitigate residual intercarrier interference (ICI) after each iteration of the CPR. By applying the proposed method to the multi-antenna SC-FDE system with insufficient cyclic prefix (CP), we can significantly improve its error performance, obtaining the benefits of spectral efficiency gain and multiplexing/diversity gain in MIMO/STBC/SFBC.

In this paper, we employ time-reversal space-time block coding (TR-STBC) in single-carrier direct sequence code-division multiple access (DS-CDMA) block transmission in the presence of multiple access interference (MAI) as well as intersymbol interference (ISI), which is subject to fairly long delay spread. We introduce the transmission rate improvement by capitalizing on the assignment of additional spreading codes to each user so as to expand the cardinality of space-time code matrix with no sacrifice of diversity order. Given perfect channel state information at the receiver, a simple linear frequency-domain interference suppression scheme on a basis of symbol-by-symbol processing is developed under certain circumstances. A "turbo principle" receiver is facilitated by exploiting the serially concatenated structure at the transmitter to further enhance system performance. Simulation results justify the efficacy of our proposed system and also present performance comparisons with some existing systems in terms of bit error rate (BER).

In space division multiplexing with orthogonal frequency division multiplexing (SDM-OFDM) systems, since co-channel interference (CCI) degrades the demodulation performance, CCI suppression is essential. For CCI suppression, the turbo detector using the soft-input soft-output maximum likelihood detector (SISO-MLD) is proposed. Although SISO-MLD can deal with soft information, SISO-MLD updates only a priori information of code bits at each iteration. This prevents the performance to be improved. Meanwhile, a turbo detector with soft cancellation (SC) followed by minimum mean square error (MMSE) filter, in which the variance of the residual interference components as well as a priori information can be updated at each iteration, is well known. However, the performance of SC/MMSE detector is also limited because it is proposed to reduce the computational complexity at the cost of performance degradation. In this paper, we propose an SC-type turbo detector which uses SISO-MLD for SDM-OFDM systems. In our proposed detector, the soft cancellation unit of the SC-type turbo detector is modified to cope with the SISO-MLD. From the simulation results, the proposed SC/MLD provides a better BER performance than the turbo detector using SISO-MLD. Furthermore, the proposed SC/MLD can attain the bit error rate (BER) equivalent to that of the SC/MMSE detector with a smaller computational complexity.

We propose a robust iterative multiuser receiver for decoding convolutional coded code-division multiple access (CDMA) signals in both Gaussian and non-Gaussian channel noise. The receiver is derived from a modified maximum a-posteriori (MAP) algorithm called the max-log-MAP algorithm for robustness against erroneous channel variance estimation. Furthermore, the effect of destructive outliers arising from impulsive noise is mitigated in the proposed receiver by incorporating the robust Huber penalty function into the multiuser detector. The proposed receiver is shown to perform satisfactorily over Gaussian and non-Gaussian impulsive channels. In every iteration, cumulative improvement in the quality of the a-posteriori probabilities is also demonstrated.

This paper proposes a co-channel interference (CCI) suppression scheme employing a frequency-domain nulling filter and turbo equalizer for single-carrier uplink time division multiple access (TDMA) systems. In the proposed scheme, after the received signal is transformed into a frequency-domain signal via fast Fourier transform (FFT), CCI from an adjacent cell is suppressed by the nulling filter. Moreover, the proposed scheme employs a soft canceller and minimum mean square error (SC/MMSE) based turbo equalizer to suppress the performance degradation due to inter-symbol interference (ISI) caused by the nulling filter as well as the ISI induced by fading channel. Computer simulation confirms that the proposed scheme is effective in suppression of CCI compared to the conventional linear frequency-domain equalizer.

An improved Max-Log-Map (MLM) turbo equalization algorithm called Scaled Max-Log-Map (SMLM) iterative equalization is presented. Simulations show that the scheme can dramatically outperform the MLM besides it is insensitive to SNR mismatch. Unfortunately, its performance is still much worse than that of Log-Map (LM) with exact SNR over high-loss channels. Accordingly, we also propose a new SNR estimation algorithm based on the reliability values of soft output extrinsic information of SMLM decoder. Using the new scheme, we obtain good performance close to that of LM with ideal knowledge of SNR.

Soft-decision decoding techniques are applied to asynchronous frequency-hop/spread-spectrum multiple-access (FH/SSMA) networks, where M-ary frequency shift keying (MFSK) is employed to transmit one modulated symbol per hop. Coding schemes using soft-decision decoded binary convolutional codes or turbo codes are considered, both with or without bit-interleaving. Performances of several soft metrics are examined for each coding scheme. It is shown that when multiple access interference is the main source of errors, the product metric offers the best performance among the soft metrics considered for all coding schemes. Furthermore, the application of soft-decision decoded convolutional codes or turbo codes without bit-interleaving is shown to allow for a much larger number of simultaneously transmitting users than hard-decision decoded Reed-Solomon codes. Finally, it is observed that when soft-decision decoding techniques are employed, synchronous networks attain better performance than asynchronous networks.

We propose an incremental redundancy (IR)-hybrid ARQ (HARQ) scheme which uses double binary turbo codes for error correction. The proposed HARQ scheme provides a higher throughput at all Es/N0 than the binary turbo IR-HARQ scheme. An extra coding gain is also attained by using the proposed HARQ scheme over turbo codes only.

In this paper, it is shown that the bit erasure probability of turbo codes with iterative decoding in the waterfall region is nonlinearly scaled by the information blocklength. This result can be used to predict efficiently the bit erasure probability of the finite-length turbo codes over the binary erasure channel.

We propose reallocating the power resource among the code symbols in such a way to minimize the post decoding error probability of turbo code. We consider several power reallocation policies and investigate their performance in slowly-varying Rayleigh flat fading channel. We show that the proposed scheme can reduce the post decoding error probability by two orders of magnitude and provide a power gain of 0.86 dB at BER=10-6 over the traditional equal power allocation among all code symbols. We also propose applying different power levels and cut-off thresholds on systematic and parity check bits depending on the channel gain, and investigate the effect of channel gain estimation error.

Recently, a versatile user cooperation method called coded cooperation diversity has been introduced, in which the codewords of each mobile are partitioned and transmitted through independent fading channels instead of simple repetition relay, to achieve remarkable gains over a conventional (non-cooperation) system, while maintaining the same information rate and transmission power. In this paper we present an adaptive space-time (AST) coded cooperation scheme based on the decoding operation on the first partition of the codeword at the base station and enables practical adaptive arrangement of resources to adopt the channel condition. Performance analysis and simulation results have proved that the proposed scheme greatly improves error rate performance and system throughput, compared with the previous framework.

We propose a new fixed-rate error correction system with a feedback channel. In our system, the receiver transmits a list of positions of unreliable information bits based on the log a-posteriori probability ratios by outputs of a soft-output decoder to the transmitter. This method is just like that of the reliability-based hybrid ARQ scheme. To dynamically select an appropriate interleaving function with feedback information is a key feature of our system. By computer simulations, we show that the performance of a system with a feedback channel is improved by dynamically selecting an appropriate interleaving function.

Multiple-input multiple-output (MIMO) multiplexing has recently been attracting considerable attention for increasing the transmission rate in a limited bandwidth. In MIMO multiplexing, the signals transmitted simultaneously from different transmit antennas must be separated and detected at a receiver. Maximum likelihood detection with QR-decomposition and M-algorithm (QRM-MLD) can achieve good performance while keeping computational complexity low. However, when the number of surviving symbol replica candidates in the M-algorithm is set to be small, the performance of QRM-MLD degrades compared to that of MLD because of wrong selection of surviving symbol replica candidates. Furthermore, when channel estimation is inaccurate, accurate signal ranking and QR-decomposition cannot be carried out. In this paper, we propose an iterative QRM-MLD with decision directed channel estimation to improve the packet error rate (PER) performance. In the proposed QRM-MLD, decision feedback data symbols are also used for channel estimation in addition to pilot symbols in order to improve the channel estimation accuracy. Signal detection/channel estimation are then carried out in an iterative fashion. Computer simulation results show that the proposed QRM-MLD reduces the required average received Eb/N0 for PER of 10-2 by about 1.2 dB compared to the conventional method using orthogonal pilot symbols only.

This paper proposes a multiple-input multiple-output (MIMO) eigenmode transmission technique which transmits different data streams on eigenmodes of different multi-path components while suppressing intra and inter-eigenmode interferences by means of a turbo equalization technique. This paper also evaluates the effectiveness of the proposed system in frequency selective fading conditions. Computer simulation results confirms the proposed technique is effective even in high spatial correlation cases.

Wireless multi-hop networks have drawn much attention for the future generation mobile communication systems. These networks can establish multiple routes from a source node to a destination node because of flexible construction of network topology. Transmissions by multiple routes have enough capability to achieve reliable communication because we can expect to obtain diversity gain by multiple routes. In this paper, we propose the multi-route coding scheme. At first, we discuss a channel model in multi-hop networks employing regenerative relay, which we named the virtual channel model. By using the virtual channel model, a packet is encoded on multiple routes as follows; a bit sequence of a packet is encoded and divided into subpackets, and each subpacket is transmitted on each route. We evaluate its packet error rate performance, and clarify effectiveness of the proposed scheme. In general, we should face degradation of a route condition such as the case when a subpacket does not reach a destination node. Hence, we have to consider the influence of subpacket loss. We also investigate it, and show tolerance of the proposed scheme over that.

In this paper, we define a stopping set of turbo codes with the iterative decoding in the binary erasure channel. Based on the stopping set analysis, we study the block and bit erasure probabilities of turbo codes and the performance degradation of the iterative decoding against the maximum-likelihood decoding. The error floor performance of turbo codes with the iterative decoding is dominated by the small stopping sets. The performance degradation of the iterative decoding is negligible in the error floor region, so the error floor performance is asymptotically dominated by the low weight codewords.

As turbo decoding is a highly memory-intensive algorithm consuming large power, a major issue to be solved in practical implementation is to reduce power consumption. This paper presents an efficient reverse calculation method to lower the power consumption by reducing the number of memory accesses required in turbo decoding. The reverse calculation method is proposed for the Max-log-MAP algorithm, and it is combined with a scaling technique to achieve a new decoding algorithm, called hybrid log-MAP, that results in a similar BER performance to the log-MAP algorithm. For the W-CDMA standard, experimental results show that 80% of memory accesses are reduced through the proposed reverse calculation method. A hybrid log-MAP turbo decoder based on the proposed reverse calculation reduces power consumption and memory size by 34.4% and 39.2%, respectively.

With the growing demand for mobile communications, multicarrier (MC) schemes are receiving an increasing amount of attention, primarily because they handle frequency selective channels better than ordinary single-carrier schemes. However, despite offering several advantages, MC systems have certain weak points. One is their high sensitivity to interchannel interference (ICI). The influence of Doppler shift and ICI are the focus of this paper. Newly proposed B3G/4G systems are developed for data transmission rates higher than those of the IEEE 801.11. It is then necessary that the bandwidth of the subcarrier be small. Moreover, for a higher carrier frequency and mobile speed, the influence of the Doppler shift will be large; therefore, the influence of ICI becomes severer. Using a Markov chain approach, we synthesized a turbo equalizer (TE) that minimizes ICI when interference affects the arbitrary number M of adjacent subchannels. This approach shows the complexity of the proposed algorithm exhibits linear growth with respect to M and independence with respect to the total number of subchannels in the multicarrier system. The proposed ICI cancellation scheme can also be effective in the case of multiple Doppler frequency offsets. This makes the proposed approach attractive for practical implementations.