Nenad Veselinovic

Base Station Controlled Load Balancing with Handovers in Mobile WiMAX

In this paper we examine base station controlled handover based load balancing in the mobile WiMAX system and study the framework that mobile WiMAX offers for conducting load balancing and directed handovers. We also propose and evaluate a BS initiated load balancing scheme for mobile WiMAX. The simulations show that the proposed scheme can balance load efficiently within the system but also avoid the so called handover ldquoping-pongrdquo effect especially harmful e.g. for VoIP connections. We also propose an addition to the mobile WiMAX specification to distinguish load balancing based directed handovers and signal quality based rescue handovers conducted by moving terminals. This way the more important rescue handovers can be prioritized in a target Base Station.

Iterative Frequency Domain Joint-over-Antenna Detection in Multiuser MIMO

Multiuser multiple-input-multiple-output (MIMO) wireless systems have great potential in improving information rate, diversity and resistance to against interference. The primary objective of this paper is to derive for broadband signaling a new iterative frequency domain (FD) multiuser MIMO signal detection technique for joint-over-antenna (JA) detection. The proposed detector is based on soft-cancellation and minimum mean square error (MMSE) filtering, followed by maximum a posteriori probability (MAP) detector to detect several of each users transmit antennas. The purpose of jointly detecting several transmit antennas is to preserve the degrees of freedom (DoF) for MMSE. Computational complexities with FD and its time domain (TD) counterpart are evaluated in this paper, and it is shown that FD requires significantly lower complexity than TD. Numerical results show that JA significantly outperforms the receiver that detects transmit antenna signals antenna-by- antenna (AA). The proposed iterative FD JA technique achieves larger performance gains compared to AA when the total number of transmit antennas is larger than the number of receiver antennas, as well as in the presence of spatial correlation.

Iterative MIMO turbo multiuser detection and equalization for STTrC-coded systems with unknown interference

Iterative multiuser detection in a single-carrier broadband multiple-input multiple-output (MIMO) system is studied in this paper. A minimum mean squared error (MMSE) low-complexity multiuser receiver is derived for space-division multiple-access (SDMA) space-time trellis-coded (STTrC) systems in frequency-selective fading channels. The receiver uses MMSE filtering to jointly detect several transmit antennas of the user of interest, while the interference from the undetected transmit antennas, cochannel interference (CCI), and intersymbol interference (ISI) are all cancelled by the soft cancellation. The performances of two extreme receiver cases are evaluated. In the first case, only one transmit antenna of the user of interest is detected at a time and the remaining ones are cancelled by soft cancellation. In the second case, all the transmit antennas are detected jointly. The comparison of the two cases shows improvement with the latter one, both in single-user and multiuser communications and in the presence of unknown cochannel interference (UCCI). It is further shown that in the multiuser case, the proposed receivers approach the corresponding single-user bounds. The number of receive antenna elements required to achieve single-user bound is thereby equal to the number of users and not to the total number of transmit antennas.

Antenna-by-antenna and joint-over-antenna MIMO signal detection techniques for turbo-coded SC/MMSE frequency domain equalization

This paper investigates iterative frequency domain techniques for the reception of spatially multiplexed single carrier signals transmitted over frequency-selective multiple input multiple output (MIMO) channels. The investigated equalizers are based on the soft-cancellation (SC) and minimum mean square error (MMSE) altering technique for turbo-coded single carrier point-to-point MIMO systems. We consider two different transmit antenna separation techniques in the frequency domain: (1) antenna-by-antenna (AA) and (2) joint over antenna techniques (JA). (1) aims to separate signals in different layers by MMSE filtering antenna-by-antenna, whereas (2) aims to detect the composite signal comprised of the signals transmitted from the multiple antennas. In (2) the composite received signal is decomposed by using the spatial maximum a posteriori probability (MAP) algorithm. We evaluate performances in terms of frame error rate (FER) and throughput in point-to-point MIMO frequency-selective fading channels. Impacts of spatial correlation on performance of the two detectors are also investigated in this paper.

Iterative Joint-Over-Antenna Detection and WNRA Decoding in Single-Carrier Multiuser MIMO Systems

In this paper, we propose a combined iterative detection and decoding technique that is capable of achieving the maximum diversity of order NTtimesLtimesNR over single-carrier multiple-input-multiple-output (MIMO) frequency-selective Rayleigh fading channels, where NT and NR denote the number of transmit and receive antennas, respectively, and L is the number of multipath components. The so-called space-time weighted-nonbinary-repeat-accumulate (ST-WNRA) codes are considered in our paper due to their ability to provide a full transmit antenna diversity and their relatively simple encoding and decoding algorithms. Multipath diversity is obtained using a joint-over-antenna turbo-equalization technique based on the minimum-mean-square-error filtering with soft interference cancellation. Computer simulations demonstrate that our proposed turbo-equalized system with ST-WNRA codes is capable of achieving the maximum diversity order with a relatively short codeword length and that the multiuser performance approaches the single-user bound so far as the number of users is smaller than or equal to the number of receive antennas in multiuser MIMO setups. We will also show that by modifying our proposed scheme to an equivalent multilevel coded system, higher bandwidth efficiency can be achieved at the expense of a performance loss while the system still retains the maximum diversity benefit

Comparison of Adaptive MIMO OFDM Schemes for 3G LTE

Multiple-input multiple-output (MIMO) concept has been recognized as one of the key approaches for providing required bandwidth efficiency in the evolution of 3G wireless systems. However, the choice of the particular MIMO transceiver scheme is still an open issue and it is under intensive discussion in standardization bodies. In this paper we tackle the topic of the choice of the appropriate MIMO scheme for the long term evolution of 3G systems. We show that, among the considered schemes, quasiorthogonal matrix modulation approach is the most appealing one in 2 times 2 and 4 times 2 configurations

Frequency Domain Joint-over-Antenna MIMO Turbo Equalization

This paper proposes a novel iterative frequency domain multiple input multiple output (MIMO) signal detection technique for the reception of overloaded spatially multiplexed MIMO transmission for single carrier signalling in frequency-selective channels. In the presence of spatial correlation as well as the case where there are more transmit antennae than receive antennae (referred to as overloaded transmission in this paper) the performance of antenna-by-antenna (AA)-based MIMO minimum mean square error (MMSE) receivers are, in general, degraded. Therefore, we propose joint-over-antenna (JA) detection-based iterative frequency domain technique to improve the performance of receiver in the presence of overloaded antenna transmission and spatial correlation. The proposed receiver is based on the combination of maximum a posteriori probability (MAP) algorithm, and soft-cancellation (SC) and MMSE filtering for turbo-coded single carrier point-to-point MIMO systems. In the proposed receiver MIMO signal detection is comprised of two stages. At the first stage, inter-symbol interference (ISI) is suppressed with MMSE filtering. At the second stage, co-antenna interference (CAI) is suppressed by the MAP algorithm and transmitter antennae are decomposed each other. Simulation results show that performance gain of JA compared to AA technique is significant in the presence of spatial correlation as well as in overloaded cases. The performance comparison is made in terms of frame error rate (FER) with different antenna configurations and different spatial correlations in point-to-point MIMO frequency-selective fading channels

Space-time weighted nonbinary repeat-accumulate codes in frequency-selective MIMO channels

The so-called space-time weighted nonbinary repeat-accumulate (ST-WNRA) codes are capable of achieving full antenna diversity in single-user flat fading channels. In this paper, the performance of these codes is evaluated over a frequency-selective Rayleigh fading multiple-input multiple-output channel in a multiuser environment. Under this channel condition, the full antenna diversity supported by the ST-WNRA codes can no longer be achieved due to the presence of intersymbol interference and multiple access interference. Hence, in order to suppress these interferences, a turbo equalization technique based on the minimum mean square-error filtering with soft interference cancellation (SC-MMSE) is employed. The SC-MMSE detector is designed such that the diversity that can be gleaned from the multiple antennas as well as from the multipath components can be fully exploited. Computer simulations demonstrate that our proposed turbo-equalized system with ST-WNRA codes is capable of achieving the maximum diversity order and hence achieving an excellent bit error rate performance with a reasonable spectral efficiency at a low complexity cost.

Iterative receivers for STTrC-coded MIMO turbo equalization

The problem of iterative multiuser detection in single-carrier broadband multiple-input multiple-output (MIMO) system is studied in this paper. Two minimum mean squared error (MMSE) multiuser receivers are proposed for space-time trellis coded systems in frequency selective channels. The first receiver uses the MMSE criterion both for multiuser detection and equalization. The second one uses the MMSE criterion only for multiuser detection, while the equalization part is an optimal maximum a posteriori (MAP) equalizer. The second receiver significantly outperforms the first one both in the presence and in the absence of the unknown co-channel interference, at the expense of increased complexity.

Space-time coded turbo equalization and multiuser detection-asymptotic performance analysis in the presence of unknown interference

Turbo MIMO equalization in multiuser space-time-trellis-coded (STTrC) system is considered in this paper. The low complexity MMSE receiver with soft cancellation (SC) is proposed for joint inter-symbol-interference (ISI), known co-channel interference (CCI) and unknown co-channel interference (UCCI) suppression. It is shown that in the multiuser scenario without UCCI the proposed iterative receiver achieves maximum-likelihood (ML) bound on performance of the single user. The number of receive antennas is thereby equal to the number of users and not to the number of transmit antennas. Furthermore, the upper bound on pairwise error probability (PEP) is derived in the presence of UCCI in the asymptotic case of ideal SC. The result shows that the effect of the interference on the diversity and coding gain is very similar to that of the channel correlation at the receiver side. Namely the diversity and coding gain depend on the rank of the inverse of the matrix R of the UCCI.