Heterogeneous deployment comprising overlays macrocell and small cells is a promising solution to enhance both system capacity and coverage. Nevertheless, the performance of heterogeneous networks uplink is severely degraded due by cross-layer interference, especially the interference that small cell suffers from the proximate macro UEs (MUEs). In this paper, we propose a Cross-Tier Interference-Aware (CTIA) scheduling scheme which exploits scheduling coordination and small cell spacial degree of freedom to mitigate the uplink interference. The system level simulations show that substantial throughput gains can be achieved with the proposed CTIA scheme and practical receivers. It is also suggested to properly balance between the capacity improvement achieved by the proposed method and the backhaul overhead consumption, which can be implemented by adapting the cooperating set size with a system-defined parameter.

The existing finite-rate feedback methods for multiuser MIMO systems suffer from significant performance degradation as well as multiuser diversity loss when users are in moderate mobility. To achieve multiuser diversity for users even in fast mobility, a lower bound for the ergodic signal to interference and noise ratio (SINR) is derived, which serves as a near optimal prediction of the future channel condition. Numerical results show that the proposed scheme far surpasses conventional methods and what is more important, multiuser diversity can be maintained.

In this paper, we investigate efficient beamforming schemes for downlink broadcast channels of MIMO systems. Optimal solution to this problem is known to be infeasible due to the prohibitive complexity and recent research focuses on suboptimal approaches with reduced complexity. We first present a progressive beamforming method that maximizes the net gain in sum throughput for newly added users and achieves high sum throughput in MU-MIMO channels. Nevertheless, there is severe asymmetry in the performance between data streams: data streams scheduled at an earlier stage are better protected compared to their counterparts configured in later stages. We then show that inter-stream asymmetry can be substantially alleviated by updating beamforming vectors over iterations. An adaptive algorithm is developed to enhance the efficiency of iterative processing and is shown to substantially improve the system performance with manageable complexity.

In this paper, we consider user selection algorithms for Multi-user MIMO systems with perfect or partial channel state information at the transmitter (CSIT). A novel user selection algorithm based on chordal distance is proposed. The basic principle is to serve the user subset in which one is approximately orthogonal to the others. Since the chordal distance relies only on the channel direction information, the proposed algorithm has a low computational complexity, and can be extended straightly to the limited feedback system. Analysis and simulations verify the effectiveness of the proposed method.

Multicell collaboration is considered for power allocation in OFDMA downlink to mitigate the intercell interference. We propose a coordination approach to maximize the sum utility of a cluster of co-channel users by properly configuring user transmit power in local cells. In particular, we employ the well-known α-PF metric as the utility function to accounts for the fairness between users in different cells. It is arguably shown that optimization of fairness-oriented utilities in the multicell scenario through intercell coordination effectively mitigates the interference and improves signal transmission. Our work sheds new lights on intercell interference coordination in multicarrier downlink, and leads to a distributed framework for multicell resource coordination.

The multiple input multiple output (MIMO) technology and the distributed antenna system (DAS) have emerged as the promising candidates for the future mobile communications. In this paper, we observe the downlink (DL) transmission in a single-cell multi-user MIMO (MU-MIMO) system based on the distributed antennas for 3GPP Long Term Evolution-Advanced (LTE-A). It is found that the existing methods presented in 3GPP LTE discussion may deteriorate system throughput sharply due to the inaccurate channel information feedback from user equipment (UE). We propose a novel strategy including several methods for the MU-MIMO DL transmission in DAS. First, a UE-specific channel vector quantization method is proposed to enable the BS to acquire more accurate channel direction information (CDI). In addition, we develop a new method for the channel quality information (CQI) computation at the side of UE. The extensive simulation performed in the various scenarios discloses that the proposed strategy achieves the performance gain in terms of system throughput by 600% at most and 20% at least.

This letter analyzes the bit error probability (BEP) performance of multiple-input/multiple-output (MIMO) systems employing maximal ratio transmission/maximal ratio combining (MRT/MRC) diversity in Rayleigh fading channels, where the channels are known at the both transmitter and receiver. An exact closed-from expression for the BEP performances of square and rectangular quadrature amplitude modulation (QAM) is derived for systems with arbitrary number of transmit antennas and two receive antennas. Moreover, by approximating the probability density function of output signal-to-noise ratio (SNR) and using the close approximation of BEP performance with QAM in additive white Gaussian noise (AWGN), we obtain a simple expression for arbitrary number of transmitter antennas and receive antennas. Simulations show that our approximation is very close to the exact one.

A novel and unified framework is presented to evaluate the exact bit error probability (BEP) ofM-ary pulse amplitude modulation (PAM) and quadrature amplitude modulation (QAM) over additive white Gaussian noise (AWGN) channel for any bit-mapping techniques. Tight approximations are also provided for both of them. Using these results, we obtain the exact and tight approximate BEP expressions for maximal-ratio combining (MRC) and selection combining (SC) in Rayleigh fading.

In this paper, we study reliable multicast at the MAC layer for IEEE 802.11 wireless LANs. In IEEE 802.11, multicast protocol is based on the basic access procedure of carrier sense multiple access with collision avoidance (CSMA/CA). This protocol does not provide any recovery mechanism for multicast frames. As a result, transmitted multicast frames may be lost due to collisions or errors. Recently, several reliable multicast protocols at MAC layer have been proposed for 802.11, and they can be classified into two categories: one is based on negative feedback (NFB-based) and the other is based on positive feedback (PFB-based). We first analyze the problems with existing reliable multicast MAC protocols and then propose a novel PFB-based scheme: "Extended Implicit MAC Acknowledgment (EIA)". With EIA, ACK packets are eliminated and collisions of CTS frames are avoided. Since ACK packets account for nearly one-third of the total control overhead, we argue that such a scheme can be beneficial. Simulation results, using the OPNET, confirm that the improvements are encouraging.

We propose a modified layered structure for multiple-input multiple-output systems, where the layer detection order is fixed and the data rate for each layer is allocated based on the detection order and channel statistics. Using a Gaussian approximation of the layer capacities, we derive an asymptotic optimum data-rate-allocation approach. For optimum data-rate allocation, the amount of backoff from the mean layer capacity is proportional to the standard deviation of the layer capacity. With statistical data-rate allocation, only limited channel feedback is needed to update channel statistics at the transmitter. Simulation results show significant performance improvement with the proposed algorithm. We also find that the performance gap between the layered structure and the channel capacity diminishes with increasing ergodicity within each codeword. Numerical results show a singal-to-noise ratio improvement of 6.3 and 3.6 dB for TGn channels B and D, respectively, for 1% outage probability and 9 b/s/Hz spectral efficiency.

IEEE 802.11 wireless local area networks (WLANs) legacy standards can provide up to 54 Mbps, and the next generation WLAN will offer a physical-layer data rate at least an-order-of-magnitude higher than the existing standards. However, when the current medium access control (MAC) is applied directly to the higher-data-rate WLAN, it leads to high protocol overhead and significant throughput degradation because of its design for lower data rates. Designing efficient MAC schemes becomes critical and crucial. In this paper, a novel reverse direction (RD) protocol is proposed to transfer data in both directions, to overcome the fundamental overhead and to improve performance. The aim is to render the MAC efficient and robust not only for current IEEE 802.11a/b/g standards, but also for the next generation WLAN with higher data rate and higher throughput, i.e, upcoming IEEE 802.11n. Simulations with OPNET are performed and the comprehensive simulation results show that the novel RD protocol improves performance significantly.

IEEE 802.15.4-2003 (2003) was introduced to address the market need for connecting low-rate devices in a wireless personal area network (WPAN). A slotted CSMA/CA medium access control (MAC) protocol is defined in the standard to coordinate the channel access of a large number of wireless devices. In this paper, we propose a novel Markov chain for IEEE 802.15.4 MAC, which faithfully captures all the essential features of the protocol, and thus can provide valuable insight into the strengths and weaknesses of this multiple access scheme. The evaluation reveals that the double carrier sensing mechanism specified in 802.15.4 MAC is not an optimal design, and a slight modification in the protocol can result in further performance improvement in terms of throughput, delay and energy efficiency

Current standardization activities for wireless local area networks (WLAN) have focused the effort on providing high throughput and supporting applications with QoS requirements. In order to achieve such goals, the MAC mechanisms of legacy IEEE 802.11 and .11e protocols need to be improved. Multipolling mechanisms are often employed to improve the protocol efficiency, however, current available multipolling mechanisms suffer from hidden terminal and/or inefficient channel usage problems. In this paper, we propose a multi-polling mechanism that utilizes novel metric for channel access and thus is capable to utilize channel resource efficiently and overcome the hidden terminal problem. Analysis shows that the proposed mechanism improves the throughput from 5% to 40% compared to legacy 802.11e.

A two-dimensional post normalization scheme is proposed to improve the performance of conventional min-sum (MS) and normalized MS decoding of irregular low density parity check codes. An iterative procedure based on parallel differential optimization algorithm is presented to obtain the optimal two-dimensional normalization factors. Both density evolution analysis and specific code simulation show that the proposed method provides a comparable performance as belief propagation decoding while requiring less complexity. Interestingly, the new method exhibits a lower error floor than that of belief propagation decoding in the high SNR region. With respect to standard MS and one-dimensional normalized MS decodings, the two-dimensional normalized MS offers a considerably better performance.

We propose a modified layered structure for multiple-input multiple-output (MIMO) systems, where the layer detection order is fixed and the data rate for each layer is allocated based on the detection order and channel statistics. With Gaussian approximation of layer capacities, we derive the optimum data rate allocation and the amount of backoff from mean layer capacity is proportional to the standard deviation of the layer capacity. The minimum overall outage probability of a layered system is uniquely determined by the normalized capacity margin. We then investigate how to select the total information rate to maximize effective throughput. Simulation results show significant performance improvement with the proposed algorithm, and the performance gap between layered structure and the channel capacity diminishes with increasing ergodicity within each codeword.

Basic service set (BSS) is the fundamental building block of an IEEE 802.11 wireless local area network (WLAN). The overlapping BSS problem refers to situations that two or more systems, unrelated to each other, are in close enough proximity to hear each other physically. Overlapping BSS problem may degrade the network performance severely. In this paper, a two-level carrier sensing mechanism is proposed to solve the overlapping BSS problem. In the proposed solution, a new network allocation vector is introduced to solve the overlapping BSS problem with minimal hardware requirement. Moreover, the proposed mechanism introduces very little complexity - when there is no overlapping BSS problem exists, the proposed mechanism works the same as legacy 802.11 MAC mechanism.

With fast deployment of wireless local area networks (WLANs), the ability of WLANs to support real time services with stringent quality of service (QoS) requirements has come to the fore. We evaluate the capability of QoS support in the IEEE 802.11e standard, which provides medium access control (MAC) enhancements for QoS support in 802.11. Both the enhanced distributed channel access (EDCA) and the polling based channel access modes are evaluated, and their performance under real time audio and video traffic is shown through simulations. We find that EDCA provides satisfactory service differentiation among its four access categories. However, in the presence of heavy load traffic, such as a high definition television (HDTV) signal transmission, it is more desirable to place such load under the HCF (hybrid coordination function) polling mode to avoid the adverse impact of other traffic on this class of traffic. With a hybrid polling and EDCA protocol, network capacity is effectively increased for better support of real-time audio and video transmissions in future home networks.

The next-generation wireless technologies, e.g., 802.11n and 802.15.3a, offer a physical-layer speed at least an-order-of-magnitude higher than the current standards. However, direct application of current MACs leads to high protocol overhead and significant throughput degradation. We propose ADCA, a high-performance MAC that works with high-capacity physical layer. ADCA exploits two ideas of adaptive batch transmission and opportunistic selection of high-rate hosts to simultaneously reduce the overhead and improve the aggregate throughput. It opportunistically favors high-rate hosts by providing higher access probability and more access time, while ensuring each low-rate host certain minimum amount of channel access time. Simulations show that the ADCA design increases the throughput by 112% and reduces the average delay by 55% compared with the legacy DCF. It delivers more than 100 Mbps MAC-layer throughput as compared with 35 Mbps offered by the legacy MAC.

The upcoming IEEE 802.11e standard supports the applications with QoS requirements by using differentiated medium access mechanism for different traffic categories. In order to protect the high priority data flows and improve network performance in a heavy-loaded IEEE 802.11e network, a new measurement-based distributed call admission control method is introduced in this paper. The proposed method is based on the measurement of the existing traffic load over IEEE 802.11e network. Depending on the amount of the existing traffic load, the admission controller decides whether or not to allow a data unit to have the right to access the wireless medium. The simulation results show that the proposed mechanism works well.

In this paper, a trellis based blind channel estimation and equalization technique is presented. First, blind channel estimation is accomplished by incorporating a list parallel Viterbi algorithm with the least mean square (LMS) updating approach. In this operation, multiple trellis mappings are preserved and ranked in terms of path metrics. Equivalently, multiple channel estimates are maintained and updated once a single symbol is received. Second, at a certain point, which is determined by the evolution of path metrics and the linear constraint possessed in the trellis mapping, only the best channel estimate is maintained. Third, this channel estimate is adopted to construct the whole trellis that is used by a conventional adaptive Viterbi algorithm. Signal detection and further channel updating are then conducted alternatively. To alleviate the noise impact, a small delay is introduced before the feedback of detected symbols to further update the channel estimate. Simulation has shown the overall good performance of the proposed scheme in terms of mean square error (MSE) convergence of the channel estimation, stability to the initial channel guess, and computational complexity etc.