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- 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.
- 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.