Multiuser Capacity Research Articles

Experimental assessment of the performance of cooperative links in visible light communications

Cooperative communication is a technique that is employed in the context of visible light communications (VLC) to improve data rate, reach, coverage area and multi-user capacity of installed systems. In this work, we assess the performance of cooperative links with an experimental setup arranged in three configurations, where the position of source, relay and destination is varied, employing an OFDM modulation scheme. Results show that the cooperation improves the bit error ratio (BER) of VLC links, thus enabling faster data rates and more robust systems. BER analysis of the transmitted OFDM signal shows that an improvement of up to one order of magnitude is achieved when comparing the direct and combined cases.

Pulse-Shaped Walsh–Hadamard Orthogonal Signaling for Multiuser DS-UWB Systems

This article presents signaling schemes for direct-sequence ultrawideband (DS-UWB) multiuser communication with pulse shaping and orthogonal Walsh–Hadamard (WH) modulation. A waveform design approach is applied to obtain novel UWB pulses with strict spectral emission compliance and improved multiaccess capability. WH codeword sequences are then combined with optimally shaped DS-UWB signals, yielding orthogonal modulation formats that can be scaled to variable order while retaining the desired spectral characteristics and achieving robust performance in multiuser, multipath-impaired environments. Interference analysis with a random spreading model is used to derive decision statistics with MRC over frequency-selective Nakagami fading channels, and analytical expressions are obtained for outage and bit error probabilities. Comparative numerical results and sensitivity analysis with respect to waveform selection, modulation order, and channel parameters are given to demonstrate the merits of the proposed orthogonal WH-based DS-UWB schemes in terms of improved outage and error performance, and increased multiuser capacity.

Auto-Tuning for Cellular Scheduling Through Bandit-Learning and Low-Dimensional Clustering

We propose an online algorithm for clustering channel-states and learning the associated achievable multiuser rates. Our motivation stems from the complexity of multiuser scheduling. For instance, MU-MIMO scheduling involves the selection of a user subset and associated rate selection each time-slot for varying channel states (the vector of quantized channels matrices for each of the users) — a complex integer optimization problem that is different for each channel state. Instead, our algorithm clusters the collection of channel states to a much lower dimension, and for each cluster provides achievable multiuser capacity trade-offs, which can be used for user and rate selection. Our algorithm uses a bandit approach, where it learns both the unknown partitions of the channel-state space (channel-state clustering) as well as the rate region for each cluster along a pre-specified set of directions, by observing the success/failure of the scheduling decisions (e.g. through packet loss). We propose an epoch-greedy learning algorithm that achieves a sub-linear regret, given access to a class of classifying functions over the channel-state space. We empirically validate our approach on a high-fidelity 5G New Radio (NR) wireless simulator developed within AT&T Labs. We show that our epoch-greedy bandit algorithm learns the channel-state clusters and the associated rate regions. Further, adaptive scheduling using this learned rate-region model (map from channel-state to the set of feasible rates) outperforms the corresponding hand-tuned static maps in multiple settings. Thus, we believe that auto-tuning cellular systems through learning-assisted scheduling algorithms can significantly improve performance in real deployments.

Exploration of Multiple Access Interference Suppression Based on Multi-user Detection.

A multi-user detection algorithm for multiple access interference suppression combined with the minimum mean squared error and an artificial fish swarm algorithm is proposed. By taking advantage of the artificial fish swarm algorithm, the proposed algorithm can quickly converge to the optimal solution. The variation in the minimum mean squared error based test results is assigned to the artificial fish swarm algorithm, which is regarded as the initial state of artificial fish. The simulation results show that the proposed algorithm has a better multi-user capacity performance and lower bit-error rate than the minimum mean squared error detector.

Closed-Form BER Expression for Fourier and Wavelet Transform-Based Pulse-Shaped Data in Downlink NOMA

The non-orthogonal multiple access (NOMA) technique is a strong candidate for 5G cellular networks that enable greater multiuser capacity and user fairness through multiplexing in the power domain. The user data are pulse-shaped using the orthogonal frequency-division multiplexing (OFDM) technique based on the fast Fourier transform (FFT) for conventional NOMA. We propose a discrete wavelet transform-based pulse shaping technique for NOMA. We present a closed-form expression of the bit error rate (BER) for FFT-NOMA as well as wavelet-based NOMA (WNOMA) systems. The theoretical and simulation BER results show that WNOMA outperforms FFT-NOMA in additive white Gaussian noise.

A Lattice-Partition Framework of Downlink Non-Orthogonal Multiple Access Without SIC

In this paper, a novel lattice-partition-based downlink non-orthogonal multiple access framework is proposed. This framework is motivated by recognizing the algebraic structure behind the previous scheme recently proposed by Shieh and Huang as a lattice partition in $ \mathbb {Z}$ and is in fact a generalization of the scheme to any base lattice. The schemes in the proposed framework enjoy many desirable properties such as explicit and systematic design and discrete input distributions. Moreover, the proposed method only requires a limited knowledge of channel parameters. The rates achieved by the proposed scheme with any base lattice and with single-user decoding (i.e., without successive interference cancellation) are analyzed, and a universal upper bound on the gap to the multiuser capacity is obtained as a function of the normalized second moment of the base lattice. Since the proposed framework has a substantially larger design space than that of the previous scheme of Shieh and Huang whose base lattice is a 1-D lattice, one can easily find instances in larger dimensions that can provide superior performance. Design examples with the base lattices $A_{2}$ , $D_{4}$ , $E_{8}$ , and Construction A lattices, respectively, are provided, and both theoretical and simulation results exhibit smaller gaps to the multiuser capacity as dimensions increase.

Dual PHY Layer for Non-Orthogonal Multiple Access Transceiver in 5G Networks

Non-orthogonal multiple access (NOMA) is a promising multiple access technique, proposed in literature for the fifth generation (5G) mobile networks. The NOMA system model consists of the conventional orthogonal frequency division multiplexing (OFDM), as a pulse shaping technique in conjunction with a variable power domain for various users, allocated in proportion to each user’s channel gain. OFDM technique based on wavelet filter banks, namely wavelet OFDM (WOFDM) has been utilized in digital communication to improve the system robustness to noise and adjacent channel interference, and is therefore anticipated to be adopted for the NOMA technique. WOFDM in NOMA (WNOMA) outperforms OFDM-based conventional NOMA (CNOMA) with reference to interference mitigation, bandwidth efficiency, spectral confinement, and multi-user capacity. Most of the fourth generation (4G) networks are based on OFDM and its variants. Therefore, in this paper, keeping in view the interoperability with the 4G networks and the latency requirements in 5G, a dual physical layer based on conventional OFDM and WOFDM as pulse shaping methods, is proposed for the NOMA transceiver. Performance of WNOMA and CNOMA is analyzed for bit error rate in the presence of channel impairments including additive noise and IQ imbalance and multiuser capacity is also computed. Comparison of various parameters indicates the advantage of adopting WNOMA over its conventional counterpart for relatively poor channel conditions.

3-D MIMO: How Much Does It Meet Our Expectations Observed From Channel Measurements?

By taking advantage of the elevation domain, three-dimensional (3-D) multiple input and multiple output (MIMO) with massive antenna elements is considered as a promising and practical technique for the fifth Generation mobile communication system. So far, 3-D MIMO is mostly studied by simulation and a few field trials have been launched recently. It still remains unknown how much does the 3-D MIMO meet our expectations in versatile scenarios. In this paper, we answer this based on measurements with 56 × 32 antenna elements at 3.5 GHz with 100-MHz bandwidth in three typical deployment scenarios, including outdoor to indoor (O2I), urban microcell (UMi), and urban macrocell (UMa). Each scenario contains two different site locations and 2-5 test routes under the same configuration. Based on the measured data, both elevation and azimuth angles are extracted and their stochastic behaviors are investigated. Then, we reconstruct two dimensional and 3-D MIMO channels based on the measured data, and compare the capacity and eigenvalues distribution. It is observed that 3-D MIMO channel which fully utilizes the elevation domain does improve capacity and also enhance the contributing eigenvalue number. However, this gain varies from scenario to scenario in reality, O2I is the most beneficial scenario, then followed by UMi and UMa scenarios. More results of multiuser capacity varying with the scenario, antenna number and user number can provide the experimental insights for the efficient utilization of 3-D MIMO in future.

Capacity Analysis of Opportunistic Scheduling in Nakagami-<inline-formula> <tex-math notation="LaTeX">$m$</tex-math></inline-formula> Fading Environments

The wireless medium has a time-varying feature due to fading, and individual users experience different degrees of fading. Opportunistic scheduling exploits such diversity of fading across users to improve network performance. In this paper, we explicitly analyze the capacity of opportunistic scheduling in various fading environments. To that end, we use Nakagami- ${m}$ fading as a fading channel model and analyze the multiuser diversity capacity when considering the number of users and fading parameters. Our analysis shows that the mean capacity is explicitly decomposed into three factors: the average signal-to-noise ratio (SNR), the number of users, and the capacity improvement that comes from diversity of fading across users. Our analytical results provide the precise impact of fading channels on multiuser capacity, which asymptotic analysis cannot capture. We verify the analytical results via simulations.

Two-Stage Precoding Method for the Finitely Large-Scale Antenna Systems

A simple matched-filter, which known as a promising precoder in a massive multiple-input multiple-output (MIMO) system, would not achieve the multi-user MIMO downlink capacity when base station equipped with finitely large number of antennas, due to multi-user interference. In this paper, a two-stage precoder design method has been proposed to maximize the sum-rate of cell-edge users when base station equipped with finitely large number of antennas. In the first stage, a matched-filter precoder is adopted to exploit both beamforming gain and the dimensional reduction of effective channels. Then, we derive the second stage precoder that maximizes the sum-rate by minimizing the weighted mean-squared-error. Simulation results and analysis verify the effectiveness of the proposed scheme. A modified transceiver design method is also presented which provides desired robustness in the presence of channel uncertainty.

Maximizing capacity with trellis exploration aided limited feedback precoder design for multiuser MIMO-MAC

In this paper, a linear trellis based precoding technique is proposed for maximizing the multiuser MIMO capacity in a multiple access channel with inter symbol interference (ISI). We use the trellis exploration algorithm to design a precoding matrix that minimizes the interference from other users by allocating power in the appropriate eigenmodes. Employing a finite number of phases, the precoder matrix for each user is first custom designed at the receiver. Then a bit sequence indicating the phase indices of the optimized precoding matrix elements for the corresponding user is fed back to the transmitter. We show that under this approach, the sum rate capacity achieved is comparable to the optimal sum-rate capacity employing the well-known water filling solution with complete channel knowledge at the transmitter for spatio-temporal vector coding (STVC). Our simulations for various multiuser MIMO cases, show that the per user precoding with limited feedback and equal power allocation strategy achieves desirable capacity gains relative to the eigenbeamforming and Grassmannian precoding.

MMSE-BLE Algorithm of Single-Carrier Multi-User Channel Capacity

A kind of minimum mean square error linear equalizer (MMSE-BLE) algorithm is presented for the solution on channel capacity of single-carrier code division multiple accesses (SC-CDMA) system, this algorithm received data symbol vector multiplied by covariance matrix by using Wiener estimator, and gets the decorrelation of multiple access interference (MAI), inter-symbol interference (ISI) and noise. MMSE-BLE algorithm calculates the channel capacity with MMSE criterion algorithm, so that it can analyze the multi-user uplink channel capacity of space-time coding SC-CDMA system under the multipath conditions effectively. The simulation results show that the number of user, path, transmitting and receiving antennas, multipath diversity gain, etc. All of them have an impact on the capacity of multi-user system, it seems that multiple antennas should be used for reception to get single-path diversity gain in frequency selective channels.

Interleave‐division multiple access (IDMA) using low‐rate layered LDGM codes

ABSTRACTWe propose the use of low‐rate layered serially concatenated low‐density generator matrix (SCLDGM) codes in interleave‐division multiple access (IDMA) systems to approach the multiuser capacity. We study the behavior of the soft interference cancellation (SIC) detector employed in IDMA systems and design the channel codes using EXtrinsic Information Transfer (EXIT) evolution, aiming at optimizing the system performance. Simulation results show that the designed codes approach the theoretical limits and outperform previous IDMA schemes based on Turbo‐Hadamard codes. Copyright © 2010 John Wiley & Sons, Ltd.

Measured Multi-User MIMO Capacity in Aircraft

The multipath richness typical of aircraft channels represents a potentially well-suited environment for multi-user multiple-input multiple-output technology (MU-MIMO). This paper presents results from measurements of the achievable MU-MIMO data rates in a Rockwell T-39 Sabreliner, using an open-source software-defined radio (SDR) test bed. We also compared the achievable capacity of dirty-paper coding (OPC) against time-division multiple access (TDMA) to illustrate the value of advanced MU-MIMO techniques in aircraft environments. Measured data was then compared against values obtained from a three-dimensional ray-tracing simulation. For transmitters located near the ends of the aircraft, the average error between simulated and measured capacity was on the order of 2% or less. For the more-centralized transmitter location, simulations predicted an average of 6% less capacity than what was actually measured.

Impact of open-loop power controlled feedback on multiuser capacity in Rayleigh fading channels

This letter investigates the impact of open-loop power control (OLPC) employed in uplink (UL) feedback channel on the multiuser downlink (DL) capacity. OLPC is used to reduce the capacity loss caused by the feedback outage. Assuming flat Rayleigh channels correlated between DL and UL, we provide probability distributions on the DL SNR that are successfully fed back through UL managed by OLPC. Numerical results are used to verify the accuracy of the analysis and to show the impact of relevant parameters of OLPC on the capacity as a function of DL-UL correlation.

Fundamentals of Wireless Communication (Tse, D. and Viswanath, P.) [Book review

This book provides a thorough coverage of wireless communication fundamentals, with emphasis on information-theoretic concepts. Some of the topics covered include: mathematical models for the physical channel; digital communication and the concept of diversity transmission for fading channels; cellular systems; capacity of wireless channels; multi-user capacity and opportunistic communication; and an extensive treatment of MIMO systems. The list of references is extensive and the index has been prepared with considerable care. The book would make an ideal resource for graduate students seeking a first exposure to the field.

Frequency and Time Hopping PPM UWB Multiple Access Communication Scheme

In this paper we propose frequency and time hopping pulse position modulation (FTH-PPM) ultra wideband (UWB) for multiple access communications. We have derived and investigated the bit error probability for the multi-user synchronous transmitter case in multipath channels with Additive White Gaussian Noise (AWGN). Simulation results show that bit error probability performance of FTH-PPM UWB out performs the time hopping pulse position modulated (TH-PPM) UWB system. It also show that multiuser capacity of FTH-PPM UWB system is much better than TH-PPM UWB system.

On the Performance of Spectrally-Encoded Spread-Time Ultrawideband CDMA Communication Systems

In this paper we study the performance of spectrally-encoded spread-time (SE/ST) technique when applied to ultrawideband (UWB) signals in the context of wireless code-division multiple-access (CDMA) communication. In this technique UWB pulses are spectrally encoded with pseudorandom codes that are assigned uniquely to each user. We shall show that the SE/ST ultrawideband system provides an appropriate multi-user capacity in realistic UWB channels, along with the ability of narrowband interference (NBI) suppression. To this end, after obtaining a suitable statistical model for spectrally encoded spread-time signals, we investigate the performance of the system in the presence of multiple-access and narrowband interferers, and simple and accurate approximations will also be developed which are in good agreement with simulation results. Furthermore, the effect of variations of the main system parameters and some of the relevant trade-offs will be numerically demonstrated.

Time- and Frequency-Domain-Spread Generalized Multicarrier DS-CDMA Using Subspace-Based Blind and Group-Blind Space–Time Multiuser Detection

Subspace-based blind and group-blind space-time (ST) multiuser detection (MUD) is invoked for a smart-antenna-aided generalized multicarrier direct-sequence code-division multiple-access (MC DS-CDMA) system communicating over a dispersive ST Rayleigh fading channel, where the channel estimates are attained by using subspace-based blind techniques. Smart antennas are employed to provide increased degrees of freedom and, hence, to improve both the attainable single-user performance and the multiuser capacity. Furthermore, two adaptive subspace-tracking algorithms, i.e., the projection approximation subspace-tracking deflation (PASTd) algorithm and the noise-averaged Hermitian-Jacobi fast subspace tracking (NAHJ-FST) algorithm, are employed for the sake of reducing the computational complexity imposed by eigenvalue decomposition. When employing the NAHJ-FST algorithm, the group-blind MUD employing an antenna array having M = 2 elements exposed to uncorrelated fading achieved a 4.2-dB signal-to-noise-ratio (SNR) gain at a bit error rate (BER) of 10-4 compared with that employing a single antenna, which is an explicit benefit of its increased computational complexity.

Multiuser Capacity and Fairness Evaluation of Channel/QoS-Aware Multiplexing Algorithms

In this work multiplexing of variable-rate sources over shared generic multiple fading channels is addressed. In general, the performance of existing multiplexing algorithms has been compared in very specific environments, making it difficult to draw general conclusions. In this article we propose a generic variable-rate multiuser and multichannel system model that can be applied to any orthogonal channel multiplexing mechanism (e.g., TDM, OFDM, CDM, or SDM) through parametric modeling. Based on the model, the performance of different multiplexing algorithms proposed in the literature is evaluated over moderately fast fading snared channels. Physical channel conditions, random traffic arrival, and QoS requirements are taken into consideration in the study. Multiuser capacity and fairness are assessed and evaluated for different channel-aware and/or QoS-aware user multiplexing algorithms. Results show the joint diversity gain and QoS in terms of the throughput and delay experienced by different user information flows