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- Turbo multiuser detection for MC-CDMA signals with strongly nonlinear transmittersPublication . Silva, Paulo; Dinis, RuiIn this paper we consider the uplink transmission in MC-CDMA (multicarrier -coded division multiple access) systems. Since MC-CDMA signals are OFDM-like multicarrier signals, they have high envelope fluctuations and a high PMEPR (Peak-to-Mean Envelope Power Ratio) which leads to amplification difficulties. To reduce the envelope fluctuations of the transmitted signals, while maintaining the spectral efficiency, the MC-CDMA signal associated to each MT (mobile terminal) is submitted to a clipping device, followed by a frequency-domain filtering operation. However, the nonlinear distortion effects can be high when an MC-CDMA transmitter with reduced envelope fluctuations is intended. In this paper, we define an iterative receiver that jointly performs a turbo-MUD (Multiuser Detection) and the estimation and cancellation of the nonlinear distortion effects. The set of simulation results presented shows that the proposed receiver structure allows good performances, very close to the linear receiver ones, even for high system load and/or when a low-PMEPR is intended for each MT.
- A technique for reducing the PMEPR of MC-CDMA signalsPublication . Silva, Paulo; Dinis, RuiIn this paper we consider the use of MCCDMA (Multi-Carrier Code Division Multiple Access) schemes with a frequency-domain orthogonal spreading. The transmitter structure is especially modified for reducing the envelope fluctuations of the transmitted signals. An analytical approach for the performance evaluation is also included which relies on the Gaussian characteristics of MC signals with a high number of suhcarriers. A set of performance results is presented. Our performance results show that the proposed transmitter structure is adequate for power-efficient MC-CDMA transmission since we can reduce the envelope fluctuations of the transmitted signals, while keeping essentially the same spectral efficiency of the conventional MC-CDMA transmission.
- 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.
- Iterative detection of multicode DS-CDMA signals with strongly nonlinear transmittersPublication . Dinis, Rui; Silva, PauloWhenever a DS-CDMA signal is the sum of several components associated to different spreading codes it has high envelope fluctuations and a high peak-to-mean envelope power ratio (PMEPR) setting strong linearity requirements for the power amplifiers. The use of clipping and filtering techniques (which can be repeated several times) was shown to be an effective way of reducing the PMEPR of DS-CDMA signals. We consider an improved receiver where there is an iterative estimation and cancelation of nonlinear distortion effects. The proposed receiver allows significant performance improvements after just a few iterations, even when we have strong nonlinear distortion effects.
- A spectrally efficient transmission scheme for signals with large bandwidthPublication . Silva, Paulo; Dinis, RuiIn this paper we consider single-carrier with frequency-domain equalization (SC-FDE) schemes where the transmission bandwidth is above the symbol rate. To allow high spectral efficiencies, several channels share the same bandwidth. Since the co-channel interference (CCI) levels can be very high, we propose iterative FDE receivers where we jointly detect all users sharing the same channel. Our performance results show that we can have excellent performances, even with several users sharing the same channel. In fact, we can have the maximum theoretical spectral efficiency even with signals that have bandwidth substantially above the symbol rate.
- 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.
- Turbo detection for the uplink of CP-Assisted DS-CDMA systemsPublication . Silva, Paulo; Dinis, RuiIn this paper we consider the use of SDMA (Space Division Multiple Access) techniques for the uplink of MC-CDMA systems (MultiCarrier -Coded Division Multiple Access) where the transmitted signals face strong nonlinear distortion effects. We define an iterative receiver that jointly performs a turbo-MUD (Multiuser Detection) as well as estimation and cancellation of the nonlinear distortion effects.
- A turbo SDMA receiver for strongly nonlinearly distorted MC-CDMA signalsPublication . Silva, Paulo; Dinis, RuiIn this paper we consider the use of SDMA (Space Division Multiple Access) techniques for the uplink of MC-CDMA systems (MultiCarrier -Coded Division Multiple Access) where the transmitted signals face strong nonlinear distortion effects. We define an iterative receiver that jointly performs a turbo-MUD (Multiuser Detection) as well as estimation and cancellation of the nonlinear distortion effects.