Calculation of Intermodulation from a Single Carrier Amplitude Characteristic

IEEE Trans. Wireless Commun.

2010

Abstract-As with other multicarrier modulations, OFDMA signals (Orthogonal Frequency Division Multiple Access) have large envelope fluctuations making them very prone to nonlinear distortion effects.In this paper we study the impact of nonlinear devices on OFDMA signals. We present an analytical characterization of the signals at the output of a nonlinear device that can be used for obtaining the spectral characterization of the transmitted signals, as well as the computation of the nonlinear interference levels on the received signals.It is shown that the power allocated to each user has a key impact on the nonlinear distortion effects. For this reason, we should avoid having low-power and high-power users in adjacent subcarriers. It is also shown that nonlinear distortion levels are lower when just a small fraction of the subcarriers is used (i.e., when the system load is small).

Section: Appendixmentioning

confidence: 99%

Analytical Evaluation of Nonlinear Effects on OFDMA Signals

Araujo

IEEE Trans. Wireless Commun.

2010

“…where the coefficient P 2γ+1 denotes the total power associated to the inter-modulation product (IMP) of order 2γ + 1, which can be calculated as described in [9], following [28], [29].…”

Section: A Basic Signal Processing Schemementioning

confidence: 99%

Iterative Detection of Multicode DS-CDMA Signals With Strong Nonlinear Distortion Effects

IEEE Trans. Veh. Technol.

Silva

2009

Abstract-Whenever a direct sequence code division multiple access (DS-CDMA) signal is the sum of several components associated to different spreading codes (e.g., the DS-CDMA signal to be transmitted by the base station (BS) in the downlink or any multicode DS-CDMA signal) it has high envelope fluctuations and a high peak-to-mean envelope power ratio (PMEPR) setting strong linearity requirements for the power amplifiers. For this reason, it is desirable to reduce the envelope fluctuations of the transmitted signals.The use of clipping techniques combined with frequencydomain filtering was shown to be an effective way of reducing the envelope fluctuations (and, inherently, the PMEPR) of DS-CDMA signals, while maintaining the spectral occupation of the corresponding conventional DS-CDMA signals. To avoid PMEPR regrowth effects the clipping and filtering operations can be repeated several times. However, the performance degradation due to nonlinear distortion effects on the transmitted signals can be relatively high, especially when a very low PMEPR is intended (e.g., when a low clipping level and several iterations are adopted). This can be especially serious if different powers are assigned to different spreading codes.To avoid significant performance degradation in these situations, we consider an improved receiver where there is an iterative estimation and cancelation of nonlinear distortion effects. Our performance results show that the proposed receiver allows significant performance improvements after just a few iterations, even when we have strong nonlinear distortion effects.

“…in the right-hand side of Equations (19) or (20), for Cartesian or polar nonlinearities, respectively. The total power of the self-interference term is I = +∞ γ=1 P 2γ+1 = P out − S. In Appendix B, we present formulas for the computation of α, P out and the power coefficients P 2γ+1 , when 'clipping'-type nonlinearities (polar or Cartesian) are employed.…”

Section: Model For Conventional Transmitter Structuresmentioning

confidence: 99%

“…This method for statistical characterisation of the transmitted blocks is quite appropriate whenever the power series in Equations (19) and (20) can be reasonably truncated while ensuring an accurate computation. However, for strongly nonlinear conditions, the required number of terms become very high.…”

Section: Model For Conventional Transmitter Structuresmentioning

confidence: 99%

Nonlinear signal processing schemes for OFDM modulations within conventional or LINC transmitter structures

Trans Emerging Tel Tech

Gusmão²

2008

SUMMARYThis paper considers two classes of digital signal processing schemes, recently proposed by the authors, both combining a nonlinear operation in the time domain with a linear operation in the frequency domain, so as to reduce the envelope fluctuations and allow good power/bandwidth tradeoffs with Orthogonal Frequency Division Multiplexing (OFDM) transmission. These classes can be adopted within both conventional and two-branch, LInear amplification with Nonlinear Components (LINC) transmitter structures . The paper addresses the statistical characterisation of the transmitted frequency-domain blocks, so as to evaluate performances by analytical means, in a unified manner. A set of numerical results, which have been obtained with the help of this analytical method, is presented and discussed. These results allow a comparison between the two classes of signal processing schemes, namely for nonlinear operations of the 'clipping' type, when a conventional transmitter structure is employed. They also allow an accurate evaluation of the phase and gain imbalance effects in the two-branch power amplification case.