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Papers by Paulo Silva
IEEE Transactions on Vehicular Technology, 2009
IEEE Transactions on Vehicular Technology, 2009
Whenever a direct sequence code division multiple access (DS-CDMA) signal is the sum of several c... more 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.
IEEE Transactions on Wireless Communications, 2010
As with other multicarrier modulations, OFDMA signals (Orthogonal Frequency Division Multiple Acc... more 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). 1
2005 IEEE 61st Vehicular Technology Conference
The MC-CDMA techniques (Multicarrier Coded Division Multiple Access) combine a CDMA scheme with a... more The MC-CDMA techniques (Multicarrier Coded Division Multiple Access) combine a CDMA scheme with an OFDM modulation (Orthogonal Frequency Division Multiplexing), so as to allow high transmission rates over severely time-dispersive channels without the need of a complex receiver implementation. In this paper we propose an iterative frequency-domain decision-feedback receiver for MC-CDMA signals. The proposed receiver has excellent performance, close to the single-code MFB (Matched Filter Bound), even for severe time-dispersive scenarios and/or in the presence of strong interfering channels.
IEEE Transactions on Vehicular Technology, 2009
IEEE Transactions on Vehicular Technology, 2009
Whenever a direct sequence code division multiple access (DS-CDMA) signal is the sum of several c... more 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.
IEEE Transactions on Wireless Communications, 2010
As with other multicarrier modulations, OFDMA signals (Orthogonal Frequency Division Multiple Acc... more 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). 1
2005 IEEE 61st Vehicular Technology Conference
The MC-CDMA techniques (Multicarrier Coded Division Multiple Access) combine a CDMA scheme with a... more The MC-CDMA techniques (Multicarrier Coded Division Multiple Access) combine a CDMA scheme with an OFDM modulation (Orthogonal Frequency Division Multiplexing), so as to allow high transmission rates over severely time-dispersive channels without the need of a complex receiver implementation. In this paper we propose an iterative frequency-domain decision-feedback receiver for MC-CDMA signals. The proposed receiver has excellent performance, close to the single-code MFB (Matched Filter Bound), even for severe time-dispersive scenarios and/or in the presence of strong interfering channels.