A case for adaptive sub-carrier level power allocation in OFDMA networks

In today's OFDMA networks, the transmission power is typically fixed and the same for all the sub-carriers that compose a channel. The sub-carriers though, experience different degrees of fading and thus, the received power is different for different sub-carriers; while some frequencies experience deep fades, others are relatively unaffected. In this paper, we make a case of redistributing the power across the sub-carriers (subject to a fixed power budget constraint) to better cope with this frequency selectivity. Specifically, we design a joint power and rate adaptation scheme (called JPRA for short) wherein power redistribution is combined with sub-carrier level rate adaptation to yield significant throughput benefits. We further consider two variants of JPRA: (a) JPRA-CR where, the power is redistributed across sub-carriers so as to support a maximum common rate (CR) across sub-carriers and (b) JPRA-MT where, the goal is to redistribute power such that the transmission time of a packet is minimized. While the first variant decreases transceiver complexity and is simpler, the second is geared towards achieving the maximum throughput possible. We implement both variants of JPRA on our WARP radio testbed. Our extensive experiments demonstrate that our scheme provides a 35% improvement in total network throughput in testbed experiments compared to FARA, a scheme where only sub-carrier level rate adaptation is used. We also perform simulations to demonstrate the efficacy of JPRA in larger scale networks. © 2012 ACM

Efficient sum-of-sinusoids-based simulation of mobile fading channels with asymmetric Doppler power spectra,” The

Abstract-In this paper, we deal with the problem of designing efficient sum-of-sinusoids (SOS) based simulators for frequency non-selective mobile fading channels under non-isotropic scattering conditions. To cope with this problem, we propose a new parameter computation method that can be applied on any given asymmetrical Doppler power spectrum (DPS). With the aim to reduce the computational costs associated with the simulation of high-quality channel waveforms, we also present an efficient simulation approach that combines the proposed parameter computation method with the principle of set partitioning. By considering a reference model for a fading channel with asymmetrical DPS, it is shown that the resulting SOSbased channel simulator satisfactorily emulates the channel's autocorrelation function (ACF). Owing to its characteristics, the proposed channel simulation procedure proves to be a helpful tool for the test and performance analysis of modern wireless communication systems under non-isotropic scattering scenarios

Spectral Efficiency and Energy Efficiency of OFDM Systems: Impact of Power Amplifiers and Countermeasures

In wireless communication systems, the nonlinear effect and inefficiency of power amplifier (PA) have posed practical challenges for system designs to achieve high spectral efficiency (SE) and energy efficiency (EE). In this paper, we analyze the impact of PA on the SE-EE tradeoff of orthogonal frequency division multiplex (OFDM) systems. An ideal PA that is always linear and incurs no additional power consumption can be shown to yield a decreasing convex function in the SE-EE tradeoff. In contrast, we show that a practical PA has an SE-EE tradeoff that has a turning point and decreases sharply after its maximum EE point. In other words, the Pareto-optimal tradeoff boundary of the SE-EE curve is very narrow. A wide range of SE-EE tradeoff, however, is desired for future wireless communications that have dynamic demand depending on the traffic loads, channel conditions, and system applications, e.g., high-SE-with-low-EE for rate-limited systems and high-EE-with-low-SE for energy-limited systems. For the SE-EE tradeoff improvement, we propose a PA switching (PAS) technique. In a PAS transmitter, one or more PAs are switched on intermittently to maximize the EE and deliver an overall required SE. As a consequence, a high EE over a wide range SE can be achieved, which is verified by numerical evaluations: with 15% SE reduction for low SE demand, the PAS between a low power PA and a high power PA can improve EE by 323%, while a single high power PA transmitter improves EE by only 68%.Comment: to be published, IEEE J. Sel. Areas Commu

Stochastic modeling and simulation of frequency-correlated wideband fading channels

Abstract—For the simulation of practical frequency-diversity wireless communication systems, such as frequency-hopping systems, multicarrier code-division multiple-access systems, and orthogonal frequency-division multiplexing systems, it is often desirable to produce multiple Rayleigh fading processes with given frequency correlation properties. In this paper, a novel stochastic wide-sense stationary sum-of-sinusoids channel simulator is proposed to emulate frequency-correlated wideband fading channels, where the frequency correlation properties are controlled by only adjusting the constant phases. Closed-form expressions are provided for all the parameters of the simulation model. This enables us to investigate analytically the overall correlation properties (not only the correlation coefficients) of the simulated processes with respect to both time separation and frequency separation. It is shown that the wideband channel simulator will be reduced to a narrowband Rayleigh fading-channel simulator by removing the frequency selectivity. Furthermore, the COST 207 typical-urban and rural-area channels are applied to evaluate the performance of the resulting wideband and narrowband channel simulators, respectively. The correlation properties of the simulation models approach the desired ones of the underlying reference models as the number of exponential functions tends to infinity, while very good approximations are achieved with the chosen limited number of exponential functions. Index Terms—Frequency correlation, Rayleigh fading channels, statistics, stochastic sum-of-sinusoids channel simulators, wideband fading channels. I

Design of Mobile Radio Channel Simulators Using the Iterative Nonlinear Least Square Approximation Method with Applications in Vehicle-to-X Communications

Doktorgradsavhandling i informasjons- og kommunikasjonsteknologi, Universitetet i Agder, 2015Vehicle-to-X (V2X) communication systems are expected to provide tremendous benefits associated with the safety and traffic efficiency on roads. The successful deployment of emerging technologies like V2X requires channel models accurately representing fading statistics in environments where those technologies are used. The accuracy is, of course, a major concern when adapting or developing a suitable channel model for test and evaluation purposes. However, it is also important to take into account the simplicity of a channel model, which is crucial for efficient numerical computations and computer simulations. Reconciling simplicity and accuracy is a rather complex task to accomplish, which requires sophisticated parameter computation methods. To the best of our knowledge, only a limited number of investigations address the channel modelling and parametrization problems for vehicular propagation scenarios in the literature. In order to fill this gap, we concentrate on the development of new sophisticated channel modelling approaches and efficient parameter computation methods for the design of V2X communication systems in this dissertation. In general, there are two main applications of channel models: (1) for the design and test of wireless communication systems and (2) for the optimization of existing communication systems. For the design and test purposes, more general statistical models such as Rice and Rayleigh channel models are preferred. Those channel models provide a fundamental insight into propagation phenomena and at the same time they greatly simplify the theoretical and numerical computations to assess the performance of wireless communication systems. For the optimization purposes, however, measurement-based channel models are commonly used. The main advantage of such channel models is that they always accurately reflect the physical reality. In this dissertation, we will focus on the channel models designed for both of those application purposes. A significant part of this dissertation will be devoted to the thorough analysis and design of Rayleigh and Rice fading channel models. We investigate the correlation properties of those channels assuming asymmetrical shapes of Doppler power spectral densities (PSDs). In fact, this is what we often observe in real-world propagation scenarios. In this regard, we will present an analytical expression for the autocorrelation function (ACF) of Rice processes that captures such realistic scenarios. Another important contribution to this topic is the novel iterative nonlinear least square approximation method for the design of Rice and Rayleigh channel simulators based on sum-of-sinusoids (SOS), as well as sum-of-cisoids (SOC) approaches. The idea behind the proposed method is very simple. The parameters of the simulation model are extracted from the reference model, such as the stochastic Rice and Rayleigh channel models, by fitting the statistical properties of interest, e.g. the ACF and the probability density function (PDF). We show that the proposed method outperforms several other methods in designing channel simulators with desired distribution and correlation properties. We also show that the proposed method provides a subtle balance between channel model’s simplicity and accuracy in designing Rayleigh and Rice channel simulators. The parametrization is a process of determining the key parameters specifying the channel model. This process has a great influence on the reliability of the developed channel model. It is therefore highly desirable if those parameters are extracted from measurements. In fact, this idea constitutes the fundamental concept behind measurement-based channel modelling approach. The measurement-based models are important in the sense that they can be used for the optimizations of the wireless communication system. Hence, the problem of computing the channel model parameters from the measurements is of special interest. In this regard, we propose iterative nonlinear least square approximation method for the design of measurementbased channel simulators. Through detailed investigations and comparative studies, we demonstrate that the proposed method is highly flexible and outperforms several other conventional methods in terms of reproducing the correlation characteristics obtained from several measurements. In addition, we introduce a new approach for the design of channel models for V2X communications in tunnel environments, where the number of scatterers contributing to the total received power is relatively small