This paper proposes adaptive transmit window control based on both location of mobile stations and traffic load for channel state based packet transmissions in CDMA cellular downlink communications. The proposed scheme constrains downlink packet transmissions by employing a transmit window individually given to each mobile station. The transmit window size is adjusted by using the optimum threshold value, which is selected with regard to both the mobile locations and the traffic load. The simulation results show that the proposed scheme improved the transmission delay and fairness of service compared with the conventional scheme.

Mutual interference among users can abruptly increase othercell interference and cause overload situation in coexisting WCDMA and HSDPA systems. Traffic overloading can degrade the performance of the systems. This letter proposes a new dynamic downlink load control (DDLC) algorithm to reduce performance degradation due to overload in the coexistence of WCDMA and HSDPA systems. With the proposed algorithm, the downlink load is controlled according to load states classified by two load-control thresholds, and traffic overloading is alleviated by dynamically adjusting the CQI values reported by users, based on the downlink load as well as channel variations. The proposed algorithm is simulated and results show that the DDLC scheme improves the performance of both WCDMA and HSDPA systems in terms of outage probability, total system throughput, and radio resource utilization.

This letter focuses on the simplified capacity evaluation for the downlink of a distributed antenna system (DAS) with random antenna layout. Based on system scale-up, we derive a good approximation of the downlink capacity by developing the results from random matrix theory. We also propose an iterative method to calculate the unknown parameters in the approximated expression of the downlink capacity. The approximation is illustrated to be quite accurate and the iterative method is shown to be quite efficient by Monte Carlo simulations.

As the demand for reliable high speed data transmission increases, the capacity of downlink cellular multiple-input multiple-output (MIMO) systems is of much interest. Unfortunately, the capacity analysis regarding the frequency reuse factor (FRF) is rarely reported. In this paper, theoretical analyses for both ergodic and outage capacities for cellular MIMO systems are presented. The FRF is considered and a hybrid frequency reuse scheme is proposed. It is shown by the numerical results that the proposed scheme can greatly alleviate the coverage problem of single-frequency-reuse cellular systems.

This paper investigates the influence of channel estimation error on the precoding matrix selection and signal detection for MIMO multiplexing using precoding in the downlink OFDM radio access. In a simulation, we assume codebook-based unitary precoding and signal detection that employs a minimum mean squared error (MMSE) based interference suppression filter. The simulation results clarify the effect of the channel estimation error with respect to the precoding matrix selection and signal detection from the viewpoints of the number of streams, i.e., rank order, the number of transmit antennas, the modulation scheme and channel coding rate, and codebook size, i.e., quantization for precoding matrix feedback information.

This paper presents experimental results in real propagation channel environments of real-time 1-Gbps packet transmission using antenna-dependent adaptive modulation and channel coding (AMC) with 4-by-4 MIMO multiplexing in the downlink Orthogonal Frequency Division Multiplexing (OFDM) radio access. In the experiment, Maximum Likelihood Detection employing QR decomposition and the M-algorithm (QRM-MLD) with adaptive selection of the surviving symbol replica candidates (ASESS) is employed to achieve such a high data rate at a lower received signal-to-interference plus background noise power ratio (SINR). The field experiments, which are conducted at the average moving speed of 30 km/h, show that real-time packet transmission of greater than 1 Gbps in a 100-MHz channel bandwidth (i.e., 10 bits/second/Hz) is achieved at the average received SINR of approximately 13.5 dB using 16QAM modulation and turbo coding with the coding rate of 8/9. Furthermore, we show that the measured throughput of greater than 1 Gbps is achieved at the probability of approximately 98% in a measurement course, where the maximum distance from the cell site was approximately 300 m with the respective transmitter and receiver antenna separation of 1.5 m and 40 cm with the total transmission power of 10 W. The results also clarify that the minimum required receiver antenna spacing is approximately 10 cm (1.5 carrier wave length) to suppress the loss in the required received SINR at 1-Gbps throughput to within 1 dB compared to that assuming the fading correlation between antennas of zero both under non-line-of-sight (NLOS) and line-of-sight (LOS) conditions.

This paper presents comparisons between common and dedicated reference signals (RSs) for channel estimation in MIMO multiplexing using codebook-based precoding for orthogonal frequency division multiplexing (OFDM) radio access in the Evolved UTRA downlink with frequency division duplexing (FDD). We clarify the best RS structure for precoding-based MIMO multiplexing based on comparisons of the structures in terms of the achievable throughput taking into account the overhead of the common and dedicated RSs and the precoding matrix indication (PMI) signal. Based on extensive simulations on the throughput in 2-by-2 and 4-by-4 MIMO multiplexing with precoding, we clarify that channel estimation based on common RSs multiplied with the precoding matrix indicated by the PMI signal achieves higher throughput compared to that using dedicated RSs irrespective of the number of spatial multiplexing streams when the number of available precoding matrices, i.e., the codebook size, is less than approximately 16 and 32 for 2-by-2 and 4-by-4 MIMO multiplexing, respectively.

This paper presents the best transmit diversity schemes for three types of common/shared control signals from the viewpoint of the block error rate (BLER) performance in the Evolved UTRA downlink employing OFDM radio access. This paper also presents the coverage performance of the common/shared control signals using transmit diversity with respect to the outage probability that satisfies the required BLER performance, which is a major factor determining the cell configuration. Simulation results clarify that Space-Frequency Block Code (SFBC) and the combination of SFBC and Frequency Switched Transmit Diversity (FSTD) are the best transmit diversity schemes among the open-loop type transmit diversity candidates for two-antenna and four-antenna transmission cases, respectively. Furthermore, we show through system-level simulations that SFBC is very effective in reducing the outage probability at the required BLER for the physical broadcast channel (PBCH), for the common control signal with resource block (RB)-level assignment such as the dynamic broadcast channel (D-BCH) and paging channel (PCH), and in increasing the number of accommodated L1/L2 control signals over one transmission time interval duration, using mini-control channel element (CCE)-level assignment.

This paper experimentally investigates the effect of frequency error compensation provided by demodulation automatic frequency control (AFC) using the Synchronization Channel (SCH) in downlink OFDM radio access. The implemented OFDM receiver compensates for the frequency error caused by the difference in frequency between a base station (BS) and a user equipment (UE) using a time-division multiplexed SCH signal and that caused by the Doppler shift generated by the mobility of a user using reference signals with staggered multiplexing. Experimental results show that even when the standard oscillator frequency of the UE cannot be made to track the more accurate frequency of a BS, demodulation AFC can suppress the residual frequency error to a sufficiently low level, i.e., within 0.3 ppm, using the SCH so that the degradation in the block error rate of the physical broadcast channel control signals is slight, i.e., within approximately 0.1 dB, with respect to the case without frequency error for speeds greater than 350 km/h.

In this letter, a new joint precoding and decoding design scheme for multiuser MIMO downlink is proposed which dispenses with iterative operations and can achieve better performance. This scheme introduces zero-force processing into minimum mean square error (MMSE) design scheme to avoid iterative operations. We derived closed-form precoders and decoders and transmit power allocation strategy of proposed design scheme, validated performance of proposed design scheme by computer simulation. The simulation results show that the proposed design scheme can achieve better bit error rate (BER) and sum capacity performance compared to an existing non-iterative design scheme.

This study investigates subcarrier and power allocation schemes in an OFDMA downlink system. To consider client demands, a goal programming approach is proposed. The proposed algorithm minimizes the weighted sum of each client's dissatisfaction index. Simulations show that the sum of dissatisfaction indices can be reduced significantly.

Multiple Input Multiple Output (MIMO) represents a highly promising technique for 4G communication networks as it uses multiple antennas at the transmitter and receiver to improve the reliability of transmissions and to provide a high data rate. This paper introduces an adjustable scheduling algorithm for multi-user MIMO systems that can provide an advantageous trade-off solution between throughput maximization and fair resource allocation among users. Specifically, our algorithm is proposed as a solution to system requirement issues through the flexible control of fairness factors.

The downlink beamforming weights which can suppress interfering signals toward out-of-cell mobile stations for a TDD-based OFDMA system are introduced. First, the downlink beamforming weights are optimally designed based on iteration. Then, the downlink beamforming weights are designed in a sub-optimal way. It is shown that the sub-optimally designed downlink beamforming weights have the same structure as that of the uplink beamforming weights which are derived based on MMSE. The performance of these schemes is compared based on the average receive SINR. The performance of a heuristic scheme which exploits uplink beamforming weights for downlink beamforming weights is also investigated.

In this letter, we propose a novel singular value decomposition zero-forcing beamforming (SVD-ZFBF) relaying scheme in the multiuser downlink MIMO broadcasting channel with fixed relays. Based on the processing scheme, we apply SUS [5] to select users at the relay station (RS) and develop a joint power allocation strategy at the base station (BS) and RS. By increasing the power at RS or selecting active users to obtain more multiuser diversity, SVD-ZFBF can approach an upper bound and outperform SVD-ZFDPC [1] with much lower complexity. Moreover, we show that the noise power ratio of RS to users significantly impacts the performance.

This letter addresses the problem of robust transceiver design for the multiuser multiple-input-multiple-output (MIMO) downlink where the channel state information at the base station (BS) is imperfect. A stochastic approach which minimizes the expectation of the total mean square error (MSE) of the downlink conditioned on the channel estimates under a total transmit power constraint is adopted. The iterative algorithm reported in [2] is improved to handle the proposed robust optimization problem. Simulation results show that our proposed robust scheme effectively reduces the performance loss due to channel uncertainties and outperforms existing methods, especially when the channel errors of the users are different.

In this paper we investigate designing optimal linear transmit/receive processing filters for multiuser MIMO downlinks with imperfect channel state information (CSI) and spatial fading correlation between antenna array at BS. A robust scheme is proposed to obtain the optimal linear transmit/receive filters in the sense of minimizing the average sum mean square error (SMSE) conditional on noisy channel estimates under a per-user transmit power constraint. Using an iterative procedure, the proposed scheme extends the existing optimization algorithm for uncorrelated single-user MIMO systems with perfect CSI to solve the problem of minimizing SMSE in spatially correlated MIMO downlinks with imperfect CSI. Comparing with non-robust scheme, we show that robust scheme efficiently mitigates the BER loss induced by imperfect CSI. In addition, the impact of fading correlation at BS on the performance of the proposed robust scheme is analyzed.

In time-varying channels, the channel state information available at the transmitter (CSIT) is outdated due to inherent time delay between the uplink channel estimation and the downlink data transmission in TDD systems. In this letter, we propose an iterative precoding method and a linear decoding method which are both based on minimum mean-squared error (MMSE) criteria to mitigate the interference among data streams and users created by outdated CSIT for multiuser MIMO downlink systems. Analysis and simulation results show that the proposed method can effectively reduce the impairment of the outdated CSIT and improve the system capacity.

In multiple-input multiple-output (MIMO) cellular networks, certain schedulers have two independent phases: the first selects a group of users based on the scheduler criterion, and the second assigns the selected users to the transmit antennas by using an assignment scheme taking into consideration capacity maximization. Other schedulers directly select among the available channels between users and the base station in a centralized way. The schedulers of the first category can be implemented with lower complexity compared with the schedulers of the second category. For the first category, we propose three near-optimal assignment schemes with low complexities. We conducted a simulation in which the mobility of users was considered that demonstrated the superior performance of our assignment schemes. Furthermore, we analytically demonstrate their efficiency.

In this letter, a Tomlinson-Harashima precoding (THP) scheme is proposed for the downlink of multiuser MIMO systems with multiple antennas at each receiver. Assuming single data stream communication for each user, joint transmitter and receiver design is done to maximize the signal to noise ratio (SNR) for each user. Furthermore, a heuristic user ordering algorithm is proposed to optimize the encoding order and improve the bit error rate (BER) performance. Simulation results have shown that the proposed approach is superior to some existing precoding schemes.

Multiple input multiple output (MIMO) systems represent a very attractive candidate for future high data rate wireless access systems to increase the channel capacity. To obtain a further increase in the channel capacity, a distributed wireless communication system that has multiple access points (APs) and an access controller (AC), was proposed. This system increases not only the peak transmission rate but also the overall transmission rate in the service area. In this paper, we consider a cooperative multiple AP system where APs are synchronized in terms of time, but not in terms of phase. We propose a transmission method for the cooperative multiple AP system that enables the mobile station to obtain a high achievable bit rate regardless of decoding algorithms. The performance of the multiple AP systems is evaluated in comparison to the conventional single AP system. To evaluate the effectiveness of the proposed method in the cooperative multiple AP system, computer simulations are performed using the i.i.d. channel in Rayleigh and Ricean fading environments. Furthermore, we evaluate the performance of the cooperative multiple AP system using the channel measured in an actual indoor environment.