Closed-form approximations of the peak-to-average power ratio distribution for multi-carrier modulation and their applications

International audienceThe theoretical analysis of the peak-to-average power ratio (PAPR) distribution for an orthogonal frequency division multiplexing (OFDM) system, depends on the particular waveform considered in the modulation system. In this paper, we generalize this analysis by considering the generalized waveforms for multi-carrier (GWMC) modulation system based on any family of modulation functions, and we derive a general approximate expression for the cumulative distribution function (CDF) of its PAPR, for both finite and infinite integration time. These equations allow us to directly find the expressions of the PAPR distribution for any particular functions and characterize the behaviour of the PAPR distribution associated with different transmission and observation scenarios. In addition to that, a new approach to formulating the PAPR reduction problem as an optimization problem, is presented in this study

A space communications study Final report, 15 Sep. 1966 - 15 Sep. 1967

Investigation of signal to noise ratios and signal transmission efficiency for space communication system

A Space Communications Study Final Report, Sep. 15, 1965 - Sep. 15, 1966

Reception of frequency modulated signals passed through deterministic and random time-varying channel

Ultra-wideband technology for short-range wireless communication

The ultra-wideband (UWB) radio core idea is to open large amounts of spectrum to a variety of users with little mutual interference between them. While ultra-wideband is being championed by several commercial companies, this technology has not followed the conventional path where commercial interest is preceded by years of academic research. This work attempts to fill in some of the gap by studying fundamental properties of communications with impulse-based radio UWB signals. We study jam resistance and capacity of UWB. Jam resistance is analyzed for binary pulse position modulation (PPM) with the interference being modeled as correlated Gaussian. Closed-form expressions are provided for the jam resistance of a PPM UWB system utilizing rectangular pulses. Simple approximations are obtained for special cases (narrowband interference). Such analysis is extended to other practical UWB waveforms such as Gaussian and Rayleigh monocycles. It is shown that under some conditions, the UWB jam resistance is superior to that of direct sequence spread spectrum (DS-SS). In the second part of this work, we study the capacity of the single-user UWB communication systems utilizing M-ary PPM and bi-phase as well as on-off keying modulation scheme over additive white Gaussian noise (AWGN) and multipath channels. Starting from the known capacity of M-ary modulated signals, the computation of UWB capacity over the AWGN channel takes into account UWB specific constraints. The constraints are the power spectrum density limitation under Federal Communications Commission (FCC) Part 15 rules and the spreading ratio required to achieve a specified jam resistance level. UWB capacity over AWGN channel is expressed as a function of spreading ratio and communication range. Trade-offs between capacity and range of communications and between capacity and spreading ratio are explored. We extend the study of capacity of UWB communications to the multipath channel using the modified S-V model proposed by the IEEE 802.15.3a task group. The complementary cumulative distribution function (CCDF) of the capacities, subject to the FCC power spectral density (PSD) limitation, are obtained for the all Rake (ARake) and selective Rake (SRake) receivers. In both of the cases, maximum ratio combining is employed. Finally, the capacity of multiple-access UWB communications is studied over the AWGN channel. Under certain assumptions, the multiple-access noise component at the receiver is modeled as Gaussian. An expression for the UWB capacity of the multiple-access channel is developed as a function of number of users

Thermoelectric efficiency has three Degrees of Freedom

Thermal energy can be directly converted to electrical energy as a result of thermoelectric effects. Because this conversion realises clean energy technology, such as waste heat recovery and energy harvesting, substantial efforts have been made to search for thermoelectric materials. Under the belief that the material figure of merit $zT$ represents the energy conversion efficiencies of thermoelectric devices, various high peak-$zT$ materials have been explored for half a century. However, thermoelectric properties vary greatly with temperature $T$, so the single value $zT$ does not represent device efficiency accurately. Here we show that the efficiency of thermoelectric conversion is completely determined by \emph{three} parameters $Z_{\mathrm{gen}}$, $\tau$, and $\beta$, which we call the \emph{thermoelectric degrees of freedom}. The $Z_{\mathrm{gen}}$, which is an average of material properties, is a generalisation of the traditional figure of merit. The $\tau$ and $\beta$, which reflect the gradients of the material properties, are proportional to escaped heat caused by the Thomson effect and asymmetric Joule heat, respectively. Our finding proposes new directions for achieving high thermoelectric efficiency; increasing one of the thermoelectric degrees of freedom results in higher efficiency. For example, thermoelectric efficiency can be enhanced up to 176\% by tuning the thermoelectric degrees of freedom in segmented legs, compared to the best efficiency of single-material legs.Comment: main articles with 9 pages, 4 figures, supplementary information with 35 pages, 9 figures, 6 table