Browsing by Author "Dinis, Rui"
Now showing 1 - 10 of 32
Results Per Page
Sort Options
- Adaptive transceiver - antenna techniques for mobile broadband communications at millimetre - wave frequenciesPublication . Silva, Paulo; Dinis, Rui; Gusmão, AntónioThis paper is concerned with the implementation of a Mobile Broadband System, currently under research in Europe. We present a low-complexity, adaptive transceiver/antenna approach where simple, linear, phased arrays are adjusted under a transmission quality measurement provided by a decision-feedback equalizer. Several simulation results are presented and discussed.
- An iterative detection technique for DS-CDMA signals with strong nonlinear distortion effectsPublication . Dinis, Rui; Silva, PauloWhenever a DS-CDMA signal (direct sequence code division multiple access) is the sum of several components associated to different spreading codes (e.g., the DS-CDMA signal to be transmitted by the BS (Base Station) in the downlink, or any multicode DS- CDMA signal), it has high envelope fluctuations, and a high PMEPR (Peak-to-Mean Envelope Power Ratio) setting strong linearity requirements for the power amplifiers. For this reason, it is desirable to reduce the envelope fluctuations of the transmitted signals. A promising class of nonlinear signal processing schemes, combining a nonlinear operation in the time- domain with a linear, frequency-domain filtering operation, was recently proposed for reducing the envelope fluctuations (and, inherently, the PMEPR) of DS-CDMA signals, while maintaining the spectral occupation of the corresponding conventional DS-CDMA signals. However, the performance degradation due to the nonlinear distortion effects on the transmitted signals can be relatively high, especially when a low PMEPR is intended. This can be especially serious if different powers are assigned to different spreading codes. To avoid significant performance degradation in these situations, we consider 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.
- An iterative frequency-domain decision feedback receiver for CDMA systemsPublication . Silva, Paulo; Dinis, RuiUnless high-complexity receiver structures are employed, conventional DS-CDMA (direct sequence code division multiple access) schemes can have a poor performance in severe time-dispersive channels, since the orthogonality between the users is lost. In this paper, we propose an iterative frequency-domain decision feedback equalizer for the downlink transmission within DS-CDMA systems employing block-transmission techniques, with an appropriate cyclic extension appended to each block. Our performance results show that the proposed receiver structure has excellent performance, close to the single-user MFB (matched filter bound), even for severe time-dispersive scenarios and/or in the presence of strong interfering channels.
- An iterative frequency-domain decision-feedback receiver for MC-CDMA schemesPublication . Dinis, Rui; Silva, Paulo; Gusmão, AntónioThe MC-CDMA techniques (multicarrier code division multiple access) combine a CDMA scheme with 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. 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.
- Analytical evaluation of nonlinear effects in MC-CDMA signalsPublication . Dinis, Rui; Silva, PauloThe MC-CDMA signals (multicarrier coded division multiple access) have strong envelope fluctuations which make them very prone to nonlinear effects. These effects can be both intentional (such as the ones inherent to a nonlinear signal processing for reducing the envelope fluctuations) or not (such as the ones inherent to a nonlinear power amplification). In this paper we present an analytical tool for the performance evaluation of nonlinear effects in MC-CDMA signals which takes advantage of the Gaussian-like behavior of MC-CDMA signals with a large number of subcarriers and employs results on memoryless nonlinear devices with Gaussian inputs so as to characterize statistically the signals at the output of the nonlinear device. This characterization is then used for an analytical computation of the SIR levels (signal-to-interference ratio) and the BER performance (bit error rate). A set of numerical results is presented and discussed, showing the accuracy of the analytical BER performance analysis in the presence of nonlinear effects.
- Analytical evaluation of nonlinear effects in MC-CDMA signalsPublication . Dinis, Rui; Silva, PauloIn this paper we present an analytical tool for the per formance evaluation of nonlinear effects in MC-CDMA signals. This tool takes advantage of the Gaussian-like behavior of MC-CDMA signals with a large number of subcarriers and employs results on memoryless nonlinear devices with Gaussian inputs so as to characterize statis tically the signals at the output of the nonlinear device. This characterization is then used for an analytical com putation of the SIR levels (Signal-to-Interference Ratio) and the BER performance (Bit Error Rate). A set of numerical results is presented and discussed, showing the accuracy of the proposed analytical BER per formance analysis.
- Employing the Block Fourier algorithm for solving the LMMSE receiver equation under variable channel conditionsPublication . Silva, J. C.; Dinis, Rui; Rodrigues, A.; Cercas, F.; Souto, N.; Jesus, S. M.The LMMSE (Linear Minimum Mean Square Error) algorithm are one of the best linear receivers for DS-CDMA (Direct Sequence-Code Division Multiple Access). However, for the case of MIMO/BLAST (Multiple Input, Multiple Output / Bell Laboratories Layered Space Time) with high loading, the perceived complexity of the LMMSE receiver is taken as too big, and thus other types of receivers are employed, yielding worse results. In this paper, we investigate the complexity of the solution to the MIMO LMMSE receiver's equations using Block-Fourier algorithms, for both steady and unsteady channel situations.
- Frequency- domain multiuser detection for highly overloaded DS-CDMA systemsPublication . Silva, Paulo; Dinis, RuiA DS-CDMA (Direct Sequence-Coded Division Multiple Access) system has maximum spectral efficiency if the system is fully loaded (i.e., the number of users is equal to the spreading factor) and we employ signals with bandwidth equal to the chip rate. However, due to implementation constraints we need to employ signals with higher bandwidth, decreasing the system’s spectral efficiency. In this paper we consider prefixassisted DS-CDMA systems with bandwidth that can be significantly above the chip rate. To allow high spectral efficiency we consider highly overloaded systems where the number of users can be twice the spreading factor or even more. To cope with the strong interference levels we present an iterative frequencydomain receiver that takes full advantage of the total bandwidth of the transmitted signals. Our performance results show that the proposed receiver can have excellent performance, even for highly overloaded systems. Moreover, the overall system performance can be close to the maximum theoretical spectral efficiency, even with transmitted signals that have bandwidth significantly above the chip rate.
- Frequency-domain multiuser detection for CDMA systemsPublication . Silva, Paulo; Dinis, RuiThe design of future broadband wireless systems presents a big challenge, since these systems should be able to cope with severely time-dispersive channels and are expected to provide a wide range of services (which may involve data rates of several tens of Mbit/s) and to have high spectral and power efficiencies. The use of equalization techniques to deal with the channel time-dispersion effects associated to the multipath signal propagation between transmitter and receiver becomes inevitable to compensate the inherent signal distortion levels and ensure good performance. However, implementation complexity and power consumption cannot be too high, especially at the mobile terminals (MT), since low-cost and relatively long live batteries are desirable at these terminals. The optimum receiver structure for time-dispersive channels corresponds to the well-known Viterbi equalizer [1], whose complexity grows exponentially with the channel impulse response length, making it recommendable only for channels whose impulse response spans over just a few symbols. Time-domain equalization techniques are traditionally employed to mitigate channel frequency selectivity effects, leading to a much lower implementation complexity than Viterbi equalizers. Nonlinear equalizers such as decisions feedback equalizers (DFE) [2] can significantly outperform linear equalizers and have a good complexity/performance tradeoff. However, for conventional single carrier (SC) modulations the signal processing complexity (number of arithmetic operations per data symbol) required to mitigate the strong intersymbol interference (ISI) levels inherent to digital transmission over severely time-dispersive channels rapidly becomes prohibitive, especially when conventional time-domain equalization is employed at the receiver side.
- Frequency-domain multiuser detection for CP-assisted DS-CDMA signalsPublication . Silva, Paulo; Dinis, RuiIn this paper we consider the use of CP-assisted (cyclic prefix) DS-CDMA schemes (direct sequence code division multiple access) in broadband wireless systems. We present an iterative, frequency-domain MUD (multiuser detection) receiver for the uplink transmission that combines IB-DFE (iterative block decision feedback equalization) principles with serial interference cancellation. It is shown that the performance proposed receiver can be close to the single-user MFB (matched filter bound), even for fully loaded systems, in severely time-dispersive channel and/or in the presence of strong interfering signals.