946 publications from this institution
This paper investigates the performance of a two-user downlink non-orthogonal multiple access (NOMA) system using hybrid automatic repeat request with chase combining (HARQ-CC) in finite blocklength. First, an analytical framework is developed by deriving closed-form approximations for the individual average block error rate (BLER) of the near and the far user. Based upon that, the performance of NOMA is discussed in comparison to orthogonal multiple access (OMA), which draws the conclusion that NOMA outperforms OMA in terms of user fairness. Further, an algorithm is devised to determine the required blocklength and power allocation coefficients for NOMA that satisfies reliability targets for the users. The required blocklength for NOMA is compared to OMA, which shows NOMA has a lower blocklength requirement in high transmit signal-to-noise ratio (SNR) conditions, leading to lower latency than OMA when reliability requirements in terms of BLER for the two users are in the order of 10^(-5).
The parallel interference cancellation (PIC) multiuser receiver is among the most interesting multiuser demodulators for practical mobile communication systems. Thus, the performance of the PIC multiuser receivers operating in frequency selective fading channels is evaluated in this paper. A decision-directed (DD) multiuser complex channel gain estimation based both on prediction and smoothing is applied. The impact of delay estimation errors on the bit error rate (BER) of the receiver in both channel coded and uncoded CDMA systems is evaluated. It is observed that a rather low standard deviation of delay estimation errors (/spl les/0.1) is required to obtain a robust receiver performance. If that is the case the performance of the PIC receiver is close to the known delay case, and superior performance can be obtained.
Improved capacity and reliability of wireless networks can be achieved by users onboard in a high speed train with the use of moving relay nodes (MRN) cooperating with each other. Further improvement in performance is achieved with codebook based precoders adopted by third generation partnership project (3GPP) for spatial multiplexed transmission mode. Open loop and closed loop precoded spatial multiplexing for fixed and adaptive rank transmissions were simulated and analyzed on the backhaul link of a cooperative MRN system in a high speed train. To obtain improved performance, channel information in the form of precoder matrix indicator (PMI) and rank indicator (RI) are required for the closed loop operation while the RI is only needed for the open loop operation. The results show that throughput gain on the backhaul link of the cooperative MRN system can be achieved with the precoded transmission when compared to unprecoded spatial multiplexing transmission. Further improvement in throughput was achieved when the transmission rank changed dynamically according to the channel condition.
A multi-cell wireless network with full spectrum reuse is considered. The energy efficiency of the entire network is evaluated based on a predefined cost function. The energy consumption per bit (J/bit) is used as the evaluation metric. A method is proposed to improve the energy efficiency (EE) of the network employing cell coordination. The main idea is to get an insight on the EE of wireless networks and how to improve that of the network by using a binary power control mechanism. A SINR threshold is calculated based on the number of cells and channel gains of the network. Using binary power control, the users who do not obtain the required SINR are kept silent for a period of time until they get good channel conditions. It is then shown that this criterion outperforms the orthogonal resource allocation scheme and random cell switch off scheme. Finally, it is observed that the number of coordinating cells in a cluster is EE limited. Hence it can be used to determine the number of cells that should be operated inside a single cluster when binary power control is employed. The results obtained can be used as upper bounds for initial calibration of a practically implemented system.
We consider the problem of cross-layer utility maximization subject to stability constraints for a multi-commodity wireless network where all links are sharing a number of orthogonal channels. We assume a time slotted network, where the channel gains are changing randomly from slot to slot. The optimal cross-layer network control policy can be decomposed into three subproblems: 1) flow control at the transport and network layers, 2) routing and scheduling at the network layer, and 3) resource allocation (RA) at the medium access control and physical layers. Every time slot, a network controller decides the amount of each commodity data admitted to the network layer, schedules different commodities over networks' links and controls the power and rate allocated to every link in every channel. To fully exploit the available multichannel diversity, we consider the general case, where multiple links can be activated in the same channel during the same time slot, and the interference is controlled solely via power and rate control. The links' achievable rates became coupled due to interference, and this makes the RA subproblem a difficult to solve nonconvex optimization problem. The main contribution of this paper is a computationally efficient power and rate control algorithm, derived via signomial programming. In contrast to other previously proposed solutions, our method is applicable to the whole range of signal-to-interference-and-noise-ratio (SINR) values of practical interest. Even though the global optimality of the solution can not be guaranteed due to the nonconvexity of the problem, the numerical results show that the proposed algorithm is able to exploit efficiently the available multi-channel diversity and it can provide significant gains at the network layer in terms of end-to-end rates and network congestion.
User association is a key factor that affects the performance of wireless cellular small cell networks. In this paper, the problem of decoupled uplink (UL) and downlink (DL) in time division duplexing (TDD)-based small cell networks is studied. In particular, a dynamic user association scheme is proposed to dynamically balance the UL and DL loads in different small cell base stations (SCBSs). Using the proposed scheme, users can connect to different SCBSs in the UL and the DL depending on the traffic load and signal strength. The proposed user association scheme allows for different UL and DL cell coverage areas that shrink and expand depending on traffic load in both directions. Simulations results show that the proposed scheme achieves significant gains in terms of packet throughput, cell-edge throughput and service delay as compared to static and dynamic duplexing schemes with coupled UL/DL user association.
Non-orthogonal multiple access (NOMA) has been identified as a promising technology for future wireless systems due to its performance gains in spectral efficiency when compared to conventional orthogonal schemes (OMA). This gain can be easily translated to an increasing number of served users, but imposes a challenge in the system reliability which is of vital importance for new services and applications of coming cellular systems. To cope with these issues we propose a NOMA rate control strategy that makes use only of topological characteristics of the scenario and the reliability constraint. We attain the necessary conditions so that NOMA overcomes the OMA alternative, while we discuss the optimum allocation strategies for the 2-user NOMA setup when operating with equal rate or maximum sum-rate goals. In such scenario we show that the user with the largest target error probability times the ratio between the average receive signal power and the average interference power, should be scheduled to be decoded first for optimum performance. We compare numerically the performance of our allocation scheme with its ideal counterpart requiring full CSI at the BSs and infinitely long blocklength, and show how the gap increases as the reliability constraint becomes more stringent. Results also evidence the benefits of NOMA when the co-interference can be efficiently canceled, specially when the goal is to maximize the sum-rate.
Multicarrier code-division multiple-access (MC-CDMA) is a transmission scheme that combines orthogonal frequency division multiplexing (OFDM) signaling with the CDMA technique allowing efficient, utilization of the bandwidth in broadband mobile radio channels. In this paper, a new practical technique based on alternative Gaussian approximation (AGA) for determining the bit error rate (BER) of maximal-ratio combining (MRC) receivers for MC-CDMA systems with multiple active users in frequency selective Rayleigh fading channels is presented. The analysis assumes that different subcarriers experience independent fading channels, which are not necessarily identically distributed. Unlike other existing analysis methods, this new procedure takes advantage of an alternate integral representation of the conditional BER, leading to the final average BER in the form of a single finite range integral and an integrand composed of an elementary function which can be easily computed numerically. The accuracy of the proposed approach based on AGA is demonstrated with computer simulations results in a Rayleigh fading channel.
