In this paper, we propose a soft-decoding near-ML MIMO demodulation scheme that achieves near optimal performance with fixed and low complexity. Exploiting the regular structure of bit-to-symbol mapping, the proposed scheme performs hard demodulation to find the first candidate symbol for each stream followed by selection of nearby candidate points such that at least one candidate exists for the computation of likelihood information of bit 0 and 1 without intermediate calculation of the Euclidean distance. This demodulation scheme enables an improvement in performance by guaranteeing the existence of candidates and a significant reduction in the number of distance calculations which is a major complexity burden. The performance is evaluated by computer simulation, and computational complexity is also assessed in terms of the number of complex multiplication.

This letter considers a two-way relaying network where two nodes exchange their information based on the principle of physical layer network coding (PNC). We study the amplify-and-forward (AF) relay filter design with multiple-input multiple-output (MIMO) system. In order to maximize the sum-rate for information exchange, we propose a relay filter for two-way relaying network. Simulation results show that the proposed scheme performs better than the conventional schemes for two-way relay channel.

A feed-point-selective, asymmetrically fed dipole antenna has been proposed for multiple-input multiple-output (MIMO) applications. By using PIN diode switches, an asymmetrical antenna feed is realized so as to control antenna directivities. The two basic requirements for MIMO antenna radiation patterns, namely, a decrease in overlap and control in direction, have been achieved. Additionally, to enhance directivities for the antenna with PIN diodes, a reflector has been introduced. The gain toward the reflector decreased by 2 dB, while the gain in the direction of the maximum gain increased by 2 dB. The developed antenna can correspond to a variable power angular spectrum (PAS).

The concept of regional diversity-multiplexing tradeoff (DMT) is introduced by extending the asymptotic outage probability expression for multiple-input multiple-output (MIMO) channels. It is shown that for both Rayleigh and Rician MIMO channels, the regional diversity gain is a linear function of the regional multiplexing gain and that the original DMT curve can be obtained from the set of regional DMT lines. As a result, vital information for capturing both finite and infinite signal-to-noise ratio characteristics in terms of DMT is provided.

The probability of making mistakes on the decoded signals at the relay has been used for the maximum-likelihood (ML) decision at the receiver in the decode-and-forward (DF) relay network. It is well known that deriving the probability is relatively easy for the uncoded single-antenna transmission with M-pulse amplitude modulation (PAM). However, in the multiplexing multiple-input multiple-output (MIMO) transmission, the multi-dimensional decision region is getting too complicated to derive the probability. In this paper, a high-performance near-ML decoder is devised by applying a well-known pairwise error probability (PEP) of two paired-signals at the relay in the MIMO DF relay network. It also proves that the near-ML decoder can achieve the maximum diversity of MSMD+MR min (MS,MD), where MS, MR, and MD are the number of antennas at the source, relay, and destination, respectively. The simulation results show that 1) the near-ML decoder achieves the diversity we derived and 2) the bit error probability of the near-ML decoder is almost the same as that of the ML decoder.

Recently proposed full-rate quasi-orthogonal space-time block codes (QSTBCs) with power scaling is able to achieve full-diversity through linearly combining two adequately power scaled orthogonal space-time block codes (OSTBCs). While in our initial work we numerically derived the optimal value of the power scaling factor to achieve full-diversity, our goal in this letter is to analytically derive the optimal power scaling, especially for square lattice constellations (e.g., 4-QAM, 16-QAM, etc.) by maximizing the coding gain.

In this letter, we analyze the average symbol error rate (SER) performance for multiple-input multiple-output (MIMO) wireless communication links with transmit beamforming and maximum ratio combining (MRC), known as MIMO-MRC, in the presence of multiple interferers in Rayleigh fading channels. An upper bound and an approximation of the average SER for M-ary signaling and an exact average SER for some modulation formats are evaluated. Moreover, an exact closed-form expression of the average SER in an interference-limited environment is derived. The analytical results are confirmed by numerical simulations.

The use of cross-polarized antennas for multiple-input multiple-output (MIMO) systems is receiving attention as they are able to double the number of antenna for half antenna spacing needs. This paper presents the channel correlation property of the 3rd Generation Partner Project (3GPP)/3GPP2 spatial channel model (SCM) with the polarization propagation. The statistical average of the per path polarization correlation given random cross-polarization discrimination (XPD) with co-located ideal tilted dipole antennas is derived. The impact on the random behavior of the polarization correlation due to the slant offset angle, the per path angular spread (AS), and the random XPD is analyzed. The simulation results show that the variation of polarization correlation caused by the random XPD is maximized with a 58 slant offset angle under the assumptions of all predefined scenarios in SCM. The per path AS has minor impact on the statistics of the polarization correlations. The randomness of polarization correlation is negligible for an XPD with small standard deviation.

This letter analyzes the outage probability of limited feedback beamforming systems with receive antenna selection. Tight analytical closed-form expressions of outage performance are derived for both cases, with and without spatial fading correlation, which allow for evaluation of the performance as a function of the codebook size, the level of fading correlation, and the number of transmit and receive antennas. Simulation results are also provided to verify the analysis.

MIMO (Multiple Input Multiple Output) communications systems equipped with array antennas at both the transmitter and receiver sides are a promising scheme to realize higher rate and/or reliable data transmission. In this paper, capacity analysis of MIMO Rayleigh channel with spatial correlation at the receiver of multipath taken into account is presented. In general, a model configuration of local scattering around a mobile station in MIMO environment is carried out by simulation to examine spatial correlation coefficients. Based on statistical properties of the eigenvalues of correlated complex random Wishart matrices, the exact closed-form expressions of distribution of the eigenvalues are investigated. Then, the general closed-form evaluation of integral form is proposed based on Meijer's G-function. The results demonstrate that the ergodic capacities are improved by increasing the number of the antennas and the SNR's. Compared with i.i.d. (independent identically distributed) Rayleigh channel, the incremental improvement of correlated Rayleigh channel is reduced by spatial fading correlation. The analytical results validated by Monte-Carlo simulations show a good agreement.

In a codebook based precoding MIMO system, the precoding codebook significantly determines the system performance. Consequently, it is crucial to design the precoding codebook, which is related to the channel fading, antenna number, spatial correlation etc. So specific channel conditions correspond to respective optimum codebooks. In this paper, in order to obtain the optimum codebooks, a universal unitary space vector quantization (USVQ) codebook design criterion is provided, which can design the optimum codebooks for various fading and spatial correlated channels with arbitrary antenna configurations. Furthermore, the unitary space K-mean (USK) algorithm is also proposed to generate the USVQ codebook, which is iterative and convergent. Simulations show that the capacities of the precoding MIMO schemes using the USVQ codebooks are very close to those of the ideal precoding cases and outperform those of the schemes using the traditional Grassmannian codebooks and the 3GPP LTE DFT (discrete Fourier transform) codebooks.

Sum power iterative water-filling (SPIWF) algorithm provides sum-rate-optimal transmission scheme for wireless multiple-input multiple-output (MIMO) broadcast channels (BC), whereas it suffers from its high complexity. In this paper, we propose a new transmission scheme based on a novel block zero-forcing dirty paper coding (Block ZF-DPC) strategy and multiuser-diversity-achieving user selection procedure. The Block ZF-DPC can be considered as an extension of existing ZF-DPC into MIMO BCs. Two user selection algorithms having linear increasing complexity with the number of users have been proposed. One aims at maximizing the achievable sum rate directly and the other is based on Gram-Schmidt Orthogonalization (GSO) and Frobenius norm. The proposed scheme is shown to achieve a sum rate close to the sum capacity of MIMO BC and obtain optimal multiplexing and multiuser diversity gain. In addition, we also show that both selection algorithms achieve a significant part of the sum rate of the optimal greedy selection algorithm at low computation expenditure.

In this letter, we analyze the performance of limited feedback beamforming in a distributed antenna system. We propose a novel codebook design scheme to maximize a lower bound of the averaged effective signal-to-noise ratio (SNR), which is a function of the power of the signal and noise, the number of antennas, and the number of total feedback bits for characterizing the quantized channel vector. Simulations verify that the proposed scheme can provide effective capacity improvement.

In this letter, we study the performance of the multiple-input multiple-output macrodiversity transmission with limited feedback. We modify the model of the quantized channel by Jindal [9] such that the phase ambiguity in the vector quantization procedure can be characterized. Using the modified model, we show that the conventional limited feedback methods cannot obtain the macrodiversity gain even with asymptotically large codebook size, and that the macrodiversity gain can be attained by adding only one bit of phase feedback.

A compact built-in handset antenna for multiple-input multiple-output (MIMO) system at 2 GHz, comprising two elements array of newly proposed L-shaped folded monopole antenna (LFMA), is evaluated under the multipath radio wave propagation environments. By analyzing the fundamental characteristics, mean effective gain (MEG), correlation, and channel capacity, the significant enhancement in the capability, as a handset MIMO antenna under practical use conditions, was confirmed. The performances were also compared to those of an array antenna comprising two planar inversed-F antenna (PIFA) elements in order to verify the effectiveness of the proposed antenna. The results show that the equivalent or improved performances can be realized, by using the proposed LFMA array with a compact size, taking only the volume of 44% of a PIFA array. The LFMA array provides almost the same bandwidth and enhanced isolation compared with a PIFA array, and the sufficiently low correlation and acceptable effective gain are obtained under the multipath radio wave propagation environments. In addition, a greater channel capacity than a PIFA array is achieved especially when the proposed LFMA array is inclined for the display-viewing mode, and moreover, an almost doubled increase in the channel capacity is obtained by using MIMO transmission compared with single-input single-output (SISO). This study also show that the MEG has much effects on the channel capacity, rather than the correlations, for the proposed antenna.

A novel low-complexity iterative receiver for coded multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) systems is proposed in this letter. The iterative receiver uses the parallel interference cancellation (PIC)-maximum ratio combining (MRC) detector for MIMO-OFDM detection, which is a popular alternative to the minimum mean square error (MMSE) detector due to its lower computational complexity. However, we have found that the conventional PIC-MRC detector tends to underestimate the magnitude of its output log likelihood ratios (LLRs). Based on this discovery, we propose to multiply these LLRs by a constant factor, which is optimized according to the extrinsic information transfer (EXIT) chart of the soft-in soft-out (SISO) detector. Simulation results show that the proposed scheme significantly improves the performance of the PIC-MRC-based receiver with little additional cost in computational complexity, allowing it to closely approach the performance of receiver using the much more complex MMSE detector.

In this letter, we discuss the problem of receive antenna selection in the downlink of multiuser multiple-input multiple-output (MIMO) systems with Tomlinson-Harashima precoding (THP), where the number of receivers is assumed equal to that of transmit antennas. Based on the criterion of maximum system sum-capacity, a per-layer receive antenna selection scheme is proposed. This scheme, which selects one receive antenna for each receiver, can well exploit the nonlinear and successive characteristics of THP. Two models are established for the proposed per-layer scheme and the conventional per-user scheme. Both the theoretical analysis and simulation results indicate that the proposed scheme can greatly improve the equivalent channel power gains and the system sum-capacity.

Multilink MIMO technique is a promising technology for cellular networks with a guaranteed quality-of-service. It will provide high capacity and wide coverage. We evaluated the downlink performance of the multilink MIMO system from the perspective of quality-of-service. The presence of Rayleigh fading, shadowing, and path loss was assumed. To evaluate the proposed system, we developed a performance measure for MIMO cellular system. The measure is ergodic capacity taking into account area coverage. Our numerical results show that the area coverage of proposed multilink MIMO system is greatly improved compared with that of the conventional singlelink MIMO system. Using the proposed measure, we also found that the multilink MIMO system could achieve high capacity with guaranteed QoS for a wide coverage.

In this letter, we investigate the application of Tomlinson-Harashima precoding (THP) in the downlink of multiuser multiple-input multiple-output (MIMO) systems, where multiple antennas are located at all the transceivers. Based on the criterion of maximum system sum-capacity, a per-layer optimization scheme is proposed, in which the subchannel ordering and transceiver filters design are generated. In the proposed scheme, the successive character of THP can be fully exploited, so that both the minimum cost of interference suppression and the maximum power and diversity gains can be implemented, and hence, the system sum-capacity can be improved effectively.

This paper presents a novel threshold-based selection scheme to combine adaptive transmit antenna selection with an adaptive quadrature amplitude modulation (AQAM) for a spatial multiplexing (SM) multiple-input multiple-output (MIMO) system with linear receivers in practical uncorrelated and correlated channel conditions. The proposed scheme aims to maximize the average spectral efficiency (ASE) for a given bit error rate (BER) constraint and also to lower the hardware complexity. Our simulations are run on a general MIMO channel model, under the assumption that the channel state information (CSI) is known at the receiver and the adaptive control signaling can be perfectly fed back to the transmitter. We deploy the low rank-revealing QR (LRRQR) algorithm in transmit antenna subset selection. LRRQR is computationally less expensive than a singular value decomposition (SVD) based algorithm while the two algorithms achieve similar error rate performances. We show that both the conventional AQAM scheme (i.e., without adaptive transmit antenna selection) and the SM scheme perform poorly in a highly correlated channel environment. We demonstrate that our proposed scheme provides a well-behaved trade-off between the ASE and BER under various channel environments. The ASE (i.e., throughput) can be maximized with a proper choice of the channel quality threshold and AQAM mode switching threshold levels for a target BER.