During these years we have been focusing on developing ultra high-data-rate wireless access systems. One of such kind of systems is called DPC-OF/TDMA [2]-[4] (dynamic parameter controlled orthogonal frequency and time division multiple access) which targets at data rates beyond 100 Mbps. In order to support higher data rates, e.g., several hundreds of mega bps or even giga bps, it is necessary to evolve DPC-OF/TDMA on MIMO-OFDM (multiple-input multiple-output orthogonal frequency division multiplexing) platform. In this paper, we propose two MIMO-OFDM evolution schemes for DPC-OF/TDMA: M1 scheme and M2 scheme. M1 scheme is based on the combination of V-BLAST (vertical Bell laboratories layered space-time architecture) and OFDM. It invests all transmit antennas on multiplexing while exploits no diversity in the transmitter. M2 scheme is based on multi-layer space-time block coded OFDM (multi-layer STBC OFDM). This scheme achieves a good compromise between multiplexing and diversity in the transmitter. We conduct exhaustive simulations for 44, 46, 48, 66, 68, and 88 systems. We are assured that both evolution schemes are very promising in supporting several hundreds of mega bps data rates. Moreover, we find that each evolution scheme has its own prevailing area. When the receive diversity order is limited, M2 scheme has better performance since it embeds transmit diversity; as the receive diversity order increases, the performance gap between the two schemes shrinks and finally M1 scheme prevails in performance. Therefore, the proper choice depends on the system configuration, i.e., how many transmit and receive antennas are used.

Quasi-orthogonal Space Time Block Code (STBC) was designed to provide full transmission rate when using more than two antennas. However, it cannot have a full diversity gain because of the interference resulted from its quasi orthogonality property. In order to achieve full diversity from quasi-orthogonal STBC, the interference can be removed by a Simple Correlation Canceling (SCC) algorithm which improves a system performance significantly. However, the SCC algorithm has a disadvantage that produces noise enhancement, thereby resulting in performance degradation. Accordingly, without increasing an extra system complexity, an unequal power allocation scheme in a transmitter is proposed to achieve a better system performance than the conventional STBC system using the SCC algorithm. Also, the unequal power allocation scheme enables a receiver to use a simple decoding procedure that does not produce noise enhancement.

To achieve a very high data rate within a limited frequency band in orthogonal frequency division multiplexing (OFDM) systems, multi-input multi-output (MIMO) techniques are very promising. Moreover, if a transmitter has the channel state information (CSI), the achievable spectrum efficiency can be maximized using the eigenbeam-space division multiplexing (E-SDM). However, this scheme demands accurate channel estimation. Therefore, in a closed-loop transmission scheme, an increase in the amount of feedback is absolutely necessary for the E-SDM. This paper describes a downlink beamforming method that significantly reduces the amount of feedback needed by using the common transmission weight vectors in all sub-carriers, compared to the amount required for E-SDM. The proposed method also applies transmission diversity to compensate for the quality. The effectiveness of the proposed method was confirmed using computer simulations in both Ricean and Rayleigh fading environments.

This letter investigates the effects of the Space-Time Block Code (STBC) transmitter diversity and Time Switched Transmitter Diversity (TSTD) schemes while simultaneously considering the time diversity effect derived from Chase combining and the frequency diversity effect for downlink Spread OFDM packet wireless access in a broadband multipath fading channel. The simulation results show that when pilot channel based channel estimation is performed, TSTD improves the required average received signal energy per symbol-to-noise power spectrum density ratio by approximately 1.0 and 0.5 dB compared to one-branch transmission, without and with antenna diversity reception, respectively, while the benefit of STBC is small due to channel estimation error, when the maximum number of retransmissions in Chase combining is three in a 12-path Rayleigh fading channel with the root mean squared delay spread of 0.3 µsec and the maximum Doppler frequency of 20 Hz.

In this letter, we propose a combined scheme of minimum mean square error (MMSE) detection and successive interference cancellation (SIC) for multiuser space-time block coded orthogonal frequency division multiplexing (STBC-OFDM) systems. With the same complexity order, the proposed scheme provides significant bit error rate (BER) performance improvement over the linear MMSE multiuser detector.

The application of Orthogonal Space-Time Block Codes (O-STBC) as the encoding scheme in the presence of "non-quasi-static" fading was considered. A simple and efficient adaptive method of channel estimation based on the interpolation of estimates acquired at the pre-amble and post-amble of framed blocks of information is developed. Moreover, the proposed method is proven, both theoretically and by simulations, to outperform the alternative of channel tracking, despite its significant low complexity.

This paper presents a simple method to determine weights of single carrier multiple input multiple output (MIMO) broadband communication systems adopting tapped delay line (TDL) structure in receiver side for the effective communication under frequency selective fading (FSF) environment. First, assuming the perfect knowledge of the channel matrix in both arrays, an iterative design method of transmitter and receiver weights is proposed. In this approach, both weights are determined alternately to maximize signal to noise plus interference ratio (SINR) by fixing the weight of one side while optimizing the other, and this operation is repeated until SINR converges. Next, considering the case of uninformed transmitter, maximum SINR design method of MIMO system is extended for space time block coding (STBC) scheme working under FSF. Through computer simulations, it is demonstrated that the proposed schemes achieves higher SINR than conventional method with delay-less structure, particularly for the fading with long duration.

This letter analyzes the performance of statistical cooperative diversity based on space-time block codes (STBC) (Statistical STBC cooperative diversity) considering the effects of quantization and observation noise. Binary quantization is used. The bit error rate (BER) and average mutual information of the statistical STBC cooperative diversity with Alamouti's STBC and two active nodes are derived in the presence of general observation noise. It is shown that the performance of the statistical STBC cooperative diversity depends on the effects of observation noise and the number of cooperating nodes largely. It is also shown how much the communication between sensor nodes or feedback from the fusion center improves the performance of STBC cooperative diversity.

Space-frequency transmit diversity (SFTD) and space-code transmit diversity (SCTD), which are both based on space-time block codes (STBC), were applied to time-direction spreading and two-dimensional spreading orthogonal frequency-division multiplexing code-division multiplexing (OFDM-CDM) systems, and the transmission performances were compared by computer simulation. SFTD is designed for space and two adjacent subcarriers whereas SCTD is designed for space and two distinct Walsh-Hadamard (WH) codes. The simulation results show that SCTD applied to time-direction spreading OFDM-CDM was superior to SFTD because frequency selectivity distorted STBC's orthogonality between sub-carriers in SFTD. In contrast, when they were applied to two-dimensional spreading OFDM-CDM, SFTD was superior to SCTD when the number of WH codes belonging to the same mother-code group is small because the frequency diversity provided by SFTD surpassed that provided by SCTD. In addition, both SFTD and SCTD provide high tolerance to large Doppler spread. It can be therefore concluded that both SCTD and SFTD can be used in the same frame by code-multiplexing according to their suitability to physical channels. SCTD is suitable for transmitting high-rate data via time-direction spreading, whereas SFTD is suitable for transmitting control data via two-dimensional spreading.

A multi-user space-time block coding (STBCa) system is a multi-access system where co-channel users employ space-time block codes (STBC). In this paper, we aimed at the design of efficient zero-forcing (ZF) receivers, especially ZF iterative interference cancellation (IC) receivers, for multi-user {G2, G3, G4} STBC systems with an arbitrary number of users, based on the identification of algebraic properties existing in the systems. First, we identify some algebraic properties for {G2, G3, G4} STBC systems. Then, utilizing these algebraic properties, we further expose two significative properties, called "ZF output uncorrelated property" and "ZF output equal Post-detection SNR property" respectively, for least-squares (LS) ZF receivers in multi-user {G2, G3, G4} STBC systems by detailed proofs. Based on the two properties, a novel LS ZF user-ordered successive interference cancellation (ZF UOSIC) detection algorithm is proposed subsequently. Finally, simulation results show that ZF UOSIC is superior to the conventional ZF IC and maximum-likelihood (ML) algorithms and the non-ordered ZF user-based SIC (ZF USIC) algorithm due to adopting iterative IC and optimal ordering among users, and has very close performance to the ZF symbol-ordered SIC but with lower complexity due to the fewer iterative times.

Performance of the minimum mean square error (MMSE) detection is far below that of the maximum likelihood (ML) detection in a multiuser environment and decreases significantly as the number of co-channel users increases. In this paper, we propose a combined MMSE and ML multiuser detection scheme for space-time block coded (STBC) orthogonal frequency division multiplexing (STBC-OFDM) which has improved performance but with low complexity. In particular, we propose a reduced complexity ML post-detection (ML-PDP) scheme which can correct erroneously estimated bits from the outputs of MMSE multiuser detection. The proposed ML-PDP scheme performs sequential search to detect a predefined number of bits with higher probability of error and then uses ML detection to correct them. Upon controlling the number of corrected bits it is possible to balance the system performance with complexity associated with the ML-PDP. We show that significant improvement can be achieved at the cost of only small additional complexity compared with the MMSE multiuser detection.

In this manuscript, a layered macro/micro diversity scheme is introduced at the receiver side of a MIMO STBC wireless control system under fading and shadowing environment. The combination of the outputs of micro diversity is based on soft-decision values, while the macro diversity branches are combined based on hard decision values. As a measure of the reliability of the system, the outage probability of frame-error rate is employed. The performance of the proposed system is analytically and numerically evaluated and the impact of the macro diversity in the outage probability is clarified.

In this paper, we evaluate the effect of space-time coded cooperative relaying technique in multihop inter-vehicle communication (IVC) networks. The IVC systems have an issue that communication links are often blocked by obstacles such as heavy vehicles. The breakage of a radio link in multihop connections may significantly decrease the system throughput in multihop IVC networks. It is demonstrated through system-level evaluations that the cooperative relaying can offer remarkable capacity enhancement by exploiting multi-route diversity and overcoming accidental link breakage resulting from frequent topological changes.

The combination of OFDM and multiple antennas in either the transmitter or receiver is attractive to increase a diversity gain. However, multiple antennas system requires an antenna separation of 5-10 λ to keep the correlation coefficient below 0.7 for the space diversity, so this may be difficult to implement in a mobile station with high mobility. Recently, the polarization transmit diversity is considered in a mobile station. However, polarization transmit diversity requires twice transmit powers to compare with the conventional transmit diversity, since only vertically polar antenna cannot receive the horizontal signal components. In this paper, we express the cross correlation of each polarization antenna and the cross polarization discrimination (XPD) of multiple polarization antennas with simple model, and we propose an wideband OFDM using Alamouti coded heterogeneous polarization antennas for reducing the previous problem. From the simulated results, the proposed system shows better BER performance than that of the conventional STBC/OFDM.

Recently, Space-Time Block Coded OFDM (ST-OFDM) that applies Space-Time Block Code (STBC) to OFDM has been proposed. Space-Frequency Block Coded OFDM (SF-OFDM) has been also proposed where the block codes are formed over the space and frequency domain. ST-OFDM and SF-OFDM are known as the schemes that achieve good performance over the multipath fading environments and the fast fading environments, respectively. For the systems with two transmit antennas, the orthogonal conditions required to separate the received signals are that in ST-OFDM, the frequency responses of the consecutive two OFDM symbols are almost identical and that in SF-OFDM, the frequency responses of the adjacent two subcarriers are almost identical. In practical fading environments, however, these conditions of the orthogonality sometimes cannot be satisfied. In those environments, the received signals cannot be well separated and the performances are degraded. Recently, the diagonalized maximum likelihood decoder (DMLD) of new zero-forcing (ZF) type was proposed for the space-time block coded single carrier QPSK system to maintain the orthogonality of STBC under the fast fading environments and the flat fading environments, where the channel separation in DMLD is performed by the ZF algorithm using two receive signals at time index 2n, 2n+1 (Space Time Code: STC) or two subcarriers (Space Frequency Code: SFC). Note that the matrix generated after the channel separation is not an identity matrix but the matrix proportional to an identity matrix. We show that ST/SF-OFDM with DMLD outperform ST/SF-OFDM in terms of Bit Error Rate (BER).

Space-time block coded orthogonal frequency division multiplexing (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 STBC with a 22 transmission matrix, using two transmit antennas. In ST-OFDM the block codes are formed over the space-time domains. In SF-OFDM the block codes are formed over the space-frequency domains. If we apply STBC with a 44 transmission matrix to OFDM, using four transmit antennas, we can expect the performance improvement. However, when the block codes are formed over space-time (frequency) domains with four transmit antennas, the conditions of the orthogonality become more strict. We can expect that if the block codes are formed over space-time-frequency domains with four transmit antennas, that is, if we implement space-time-frequency block coded OFDM (STF-OFDM), the condition of the orthogonality is more relaxed. In this paper, we apply STBC with a 44 transmission matrix to OFDM and propose STF-OFDM. We evaluate the performance of the three types of 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. When the effect of the Doppler spread is large and the effect of the delay spread is small, SF-OFDM has the best error rate performance, and STF-OFDM and ST-OFDM follow in order. When the effect of the delay spread is large and the effect of the Doppler spread is small, ST-OFDM has the best error rate performance, and STF-OFDM and SF-OFDM follow in order. We also show that STF-OFDM is attractive in wireless communications. STF-OFDM is more tolerant than ST-OFDM with respect to the Doppler spread and SF-OFDM with respect to the delay spread, respectively.

In this letter, we investigate a decimated selective mapping (SLM) method for the peak-to-mean envelope power ratio (PMEPR) reduction in an OFDM system. Under the condition of the same side information (SI) bits, the SLM can be implemented by decimating OFDM samples, which is less complex compared to the ordinary SLM incurring a slight degradation of the PMEPR performance. The decimated SLM (DSLM) approach can be generalized to a multiple-antenna OFDM system employing a space-time block coding (STBC).

Rapid time variations of the mobile communication channel have a dramatic impact on the performance of multicarrier modulation. This letter analyzes the effect of the Doppler-induced interchannel interference (ICI) on a space-time block coded (STBC) OFDM-CDMA system in a time-varying Rayleigh fading channel. At the same time, we compute the effect of the ICI on the BER performance of the STBC OFDM-CDMA system using the maximal ratio combining (MRC) and equal gain combining (EGC) schemes.

In this letter, a receive frequency diversity technique is proposed to improve the performance of a multiplexed STBC OFDM system. Frequency diversity in the multiplexed STBC OFDM system is obtained by introducing frequency shifter in the successive STBC symbols and applying MRRC technique to regenerated and subtracted signals of the predecoded data from multiplexed STBC decoder. It is shown by computer simulation that the performance of the proposed multiplexed STBC OFDM systems with frequency diversity is improved by 5 dB at the BER of 10-3 over the existing multiplexed STBC OFDM systems with the same data rate.

In this letter, we address the performance evaluation of a space-time block coded (STBC) orthogonal frequency and code division multiplexing (OFCDM) system encountered with time-variant channels. For the performance evaluation, the average bit error rate (BER) impairment due to imperfect channel information is investigated taking into account the effect of time-varying channels. Derived results show that the loss of orthogonality due to the time variation of the channel incurs the channel estimation error, which causes performance degradation.