Marc Deumal

Evaluation of Performance Improvement Capabilities of PAPR-reducing Methods

One of the major drawbacks of multicarrier modulation is the large envelope fluctuations which either require an inefficient use of high power amplifiers or decrease the system performance. Peak-to-average power ratio (PAPR) is a very well known measure of the envelope fluctuations and has become the cost function used to evaluate and design multicarrier systems. Several PAPR-reducing techniques have been proposed with the aim to alleviate back-off specifications or increase the system performance. Besides the fact that these techniques have varying PAPR-reduction capabilities, power, bandwidth and complexity requirements, it is interesting to notice that the performance of a system employing these techniques has not been fully analyzed. In this paper we, first, develop a theoretical framework for both PAPR and the distortion introduced by a nonlinearity, and then simulate an OFDM system employing several well known PAPR-reducing techniques from the literature. By means of the theoretical analysis and the simulation results we will show the relation between PAPR and the performance of OFDM systems when a clipping device is present and we will evaluate the real performance improvement capabilities of the PAPR-reducing methods. The agreement between the theoretical and the simulation results demonstrate the validity of the analysis.

On Cubic Metric Reduction in OFDM Systems by Tone Reservation

One of the major drawbacks of multicarrier modulation is the large envelope fluctuations which either requires an inefficient use of high power amplifiers (HPAs) or decreases the system performance. Peak-to-average power ratio (PAPR) is the best known measure of the envelope fluctuations and is widely employed to design multicarrier systems. However, recently, another metric known as cubic metric (CM), is being considered in several multicarrier systems since it can predict HPA power de-rating more accurately. In this paper tone reservation (TR) technique, originally used for PAPR-reduction, is reformulated to reduce CM. We first simplify CM to define the objective function, denoted as objective CM (OCM). Then we demonstrate that OCM is convex and formulate TR for CM-reduction as an unconstrained convex optimization problem. The solution to this problem is found by means of an iterative algorithm which approaches the optimal solution with few steps. In this paper a low-complexity suboptimal algorithm capable of approaching the optimum with sufficient accuracy is also proposed. Simulation results show that for similar computational complexity, the performance of CM-reduction is superior to that of PAPR-reduction both in terms of bit error rate and out-of-band leakage.

Partially clipping (PC) method for the peak-to-average power ratio (PAPR) reduction in OFDM

OFDM (orthogonal frequency division multiplexing) has emerged as one of the best modulations for broadband mobile communications. Despite its robustness against multipath interference, the drawback of peak-to-average power ratio (PAPR) has to be overcome. PAPR increases the A/D and D/A complexity and reduces the high power amplifier efficiency as well as battery life. A new technique for reducing the PAPR of OFDM modulation is presented. This method does not spread the OFDM signal bandwidth, it scarcely increases the computational complexity and no additional information has to be known by the receiver.

Receiver based compensation of nonlinear distortion in MIMO-OFDM

Multiple antenna orthogonal frequency division multiplex (MIMO-OFDM) is transmission technique being employed in the future software defined/cognitive radio solutions. The main disadvantage of using MIMO-OFDM is its high sensitivity to the nonlinear amplification due to the specific characteristic of the high power amplifier (HPA) in the transmitter front-end. This phenomena results in the large overall performance degradation in terms of the bit error rate (BER). In this contribution, we present an iterative receiver scheme to reduce the BER of MIMO-OFDM systems undergoing nonlinear distortion. The technique consist in the iterative estimation and compensation of the nonlinear distortion introduced by transmitters HPA. As it will be shown by means of the numerical performance evaluation, the proposed technique reduces significantly BER of MIMO-OFDM systems.

Constrained Clipping for Peak Power Reduction of Multicarrier Systems by Tone Reservation

In this paper we introduce a new method to limit the peak factor of a multicarrier signal based on tone reservation. A peak-reducing signal is computed by clipping the original OFDM symbol. The peak-reducing signal is forced to have the constraints of tone reservation, so that when added to the original signal, the peak factor is reduced without increasing the BER and the out-of-band radiation. The proposed method has low computational complexity requirements and is fast-converging. Moreover, simulation results show that it has higher PAPR-reduction capabilities than other PAPR-reducing methods based on TR in the literature.

Remote sensing and skywave digital communication from antarctica.

This paper presents an overview of the research activities undertaken by La Salle and the Ebro Observatory in the field of remote sensing. On 2003 we started a research project with two main objectives: implement a long-haul oblique ionospheric sounder and transmit the data from remote sensors located at the Spanish Antarctic station Juan Carlos I to Spain. The paper focuses on a study of feasibility of two possible physical layer candidates for the skywave link between both points. A DS-SS based solution and an OFDM based solution are considered to achieve a reliable low-power low-rate communication system between Antarctica and Spain.

Multi-user Detection of Nonlinearly Distorted MC-CDMA Symbols by Microstatistic Filtering

Multi-carrier code division multiple access is a powerful modulation technique that is being considered in many emerging broadband communication systems. In a downlink scenario orthogonal spreading sequences are used since they reduce multiple access interference compared to non-orthogonal. However, the nonlinear amplification of the transmitted signal destroys the orthogonality and, thus, reduces the system performance. In order to avoid performance degradation without requiring large back-offs in the transmitter amplifier, it becomes necessary to use multi-user detection techniques at the receiver side. Conventional multi-user detectors (MUD) are designed for linear environments and, as a result, might not exhibit enough performance improvement. In this paper a new MUD based on microstatistic filtering is proposed. The presented MUD uses piece-wise linear filtering in conjunction with threshold decomposition of the input signal, which introduces a nonlinear effect, to improve performance when a nonlinearity is present. Maximum performance improvement compared to conventional MUD is achieved for low spreading factors and user loads no greater than 50%.

Iterative algorithm for nonlinear noise cancellation and channel re-estimation in nonlinearly distorted OFDM system

This paper presets a new algorithm for nonlinear distortion cancellation and channel re-estimation in an iterative manner. Whereas in practical OFDM systems, in which nonlinear amplifiers are used, data as well as pilot symbols are nonlinearly distorted. As perfect channel state information is often assumed in many works and papers, the effect of nonlinear distortion on pilot symbols is often not accounted for, and only data symbols are considered. However, as pilot symbols are used to obtain channel state information, distortion inflicted to pilot symbols by nonlinear amplification is of great importance, and for this reason should be accounted for. To address this issue, simple algorithm will be introduced for joint data recovery and channel re-estimation in an iterative manner. This algorithm iteratively estimates nonlinear distortion and then uses it to compensate this nonlinear distortion present at data and pilot symbols, which are then used for subsequent and more precise channel re-estimation. As it is shown in this paper, proposed algorithm can be used to reduce nonlinear distortions even in a situation, when highly nonlinear amplifiers are applied.

Peak Reduction of Multi-Carrier Systems by Controlled Spectral Outgrowth

One of the major drawbacks with multi-carrier (MC) modulations is the large peak-to-average power ratio (PAPR) of the transmitted signal which significantly reduces the power amplifier efficiency and the system performance. Several PAPR reducing methods which do not decrease the system performance have been proposed. In this paper, we present a new low complexity PAPR reduction technique based on the controlled spectral outgrowth (CSO) of the nearby subcarriers which neither requires the transmission of additional information nor reduces the system performance and is independent of the mapping scheme. The proposed method can be easily combined with most of the methods in the literature to provide a further peak reduction in any MC or MC spread spectrum (MC-SS) system. An analog PAPR reduction about 2.5 dB at 10-4 symbol clip probability and a power increase about 0.12 dB are obtained for a 256-subcarrier of OFDM transmission

A Low Complexity Multicarrier Proposal for Medium Rate Demanding Automatic Meter Reading Systems

In automatic meter reading (AMR) technology, electrical utilities (EUs) have been exploiting their own infrastructure to bill their customers in an efficient and economical way using power line communications (PLC) technologies. Since the amount of data that has to be send is quite low related to the available time to perform this task, AMR applications have been demanding low bit rates. At this moment, EUs are exploring and demanding other services as load and alarm management, remote monitoring and disconnections, etc. In this context, the low voltage PLC modems should provide more throughput while keeping the cost of the hardware low. In this paper, a low complexity multicarrier modulation is proposed in order to exploit the CENELEC A band.