A. Gusmao

On frequency-domain equalization and diversity combining for broadband wireless communications

This paper is concerned with the use of frequency-domain equalization (FDE) and space diversity within block transmission schemes for broadband wireless communications. The expected performance with both multicarrier (MC) and single-carrier (SC) modulations is emphasized, when a cyclic prefix, long enough to cope with the maximum relative channel delay, is appended to each transmitted block. A set of numerical results is presented and discussed, with the help of appropriate, analytical performance bounds which are conditional on a given channel realization. These bounds are used to explain the performance advantage of the SC/FDE option, the benefits of space diversity, and the impact of the criterion for computing the FDE parameters.

A class of nonlinear signal-processing schemes for bandwidth-efficient OFDM transmission with low envelope fluctuation

This work presents a wide class of digital signal-processing schemes for orthogonal frequency-division multiplexing (OFDM) transmission which combine a nonlinear operation in the time domain and a linear filtering operation in the frequency domain. The ultimate goal of these schemes is to reduce the envelope fluctuation of ordinary OFDM, while keeping its high spectral efficiency and allowing a low-cost, power-efficient implementation. An appropriate statistical model concerning the transmitted frequency-domain blocks is developed, which is derived from well-established results on Gaussian stochastic processes distorted by memoryless nonlinearities. This model can be employed for performance evaluation by analytical means, with highly accurate results whenever the corresponding conventional OFDM signals exhibit quasi-Gaussian characteristics. Cases where the signal-processing scheme is repeatedly used, in an iterative way, are treated through an extension of the proposed statistical modeling. A set of numerical results is presented and discussed so as to show the practical interest of both the proposed schemes and the analytical methods for evaluation of their performance. For the sake of comparisons, this paper includes numerical results concerning the partial transmit sequence technique, which is an alternative peak-to-mean envelope power ratio-reducing technique of higher complexity, often recommended due to its distortionless nature. The superior performance/complexity tradeoffs through the proposed class of nonlinear signal-processing schemes is emphasized.

A Turbo FDE Technique for Reduced-CP SC-Based Block Transmission Systems

For conventional cyclic-prefix (CP)-assisted block transmission systems, the CP length is selected on the basis of the expected maximum delay spread. With regard to single-carrier (SC)-based block transmission implementations, a full-length CP is recommendable, since it allows good performances through the use of simple frequency-domain equalization (FDE) techniques. In this letter, a soft-decision-directed correction (SDDC)-aided turbo FDE technique is presented for reduced-CP SC-based block transmission systems using conventional frame structures. The relations with some already known iterative FDE techniques are established, and a set of performance results is reported and discussed. The advantages of the proposed approach are emphasized, namely, the possibility of approximately achieving (besides the obvious bandwidth efficiency gain) the maximum power efficiency gain that a strong CP reduction allows

On the performance evaluation of OFDM transmission using clipping techniques

This paper considers the use of digital clipping techniques for OFDM transmission, so as to reduce the envelope fluctuation of the transmitted signals. The impact of oversampling and filtering issues is taken into account, together with the type of clipping and the choice of the clipping level, when calculating power spectral densities and BER performances. Our results show that an appropriate envelope clipping, together with a small oversampling factor and a mild filtering, leads to the best performance and is strongly recommendable for OFDM applications.

IB-DFE receivers with space diversity for CP-assisted DS-CDMA and MC-CDMA systems

Multi-Carrier Code Division Multiple Access (MC-CDMA), currently regarded as a promising multiple access scheme for broadband communications, is known to combine the advantages of an Orthogonal Frequency Division Multiplexing (OFDM)-based, Cyclic Prefix (CP)-assisted block transmission with those of CDMA systems. Recently, it was recognised that DS-CDMA (Direct Sequence) implementations can also take advantage of the benefits of the CP-assisted block transmission approach, therefore enabling an efficient use of Fast Fourier Transform (FFT)-based, chip level Frequency-Domain Equalisation (FDE) techniques. When employing a linear FDE with both MC-CDMA and DS-CDMA, the FDE coefficients can be optimised under the Minimum Mean Squared Error (MMSE) criterion, so as to avoid significant noise enhancement. The residual interference levels can be very high, especially for fully loaded scenarios, since the FDE/MMSE does not perform a perfect channel inversion. This paper deals with CP-assisted DS-CDMA systems and MC-CDMA systems with frequency-domain spreading. We consider the use of Iterative Block Decision Feedback Equalisation (IB-DFE) FDE techniques as an alternative to conventional, linear FDE techniques, and derive the appropriate IB-DFE parameters in a receiver diversity context. Our performance results show that IB-DFE techniques with moderate complexity allow significant performance gains in both systems, with bit error rate (BER) that can be close to the single-code matched filter bound (MFB) (especially for the CP-assisted DS-CDMA alternative), even with full code usage. Copyright

Low-PMEPR OFDM transmission with an iterative receiver technique for cancellation of nonlinear distortion

When a low-PMEPR (Peak-to-Mean Enve- lope Power Ratio) is intended for OFDM (Orthogonal Frequency Division Multiplexing) transmission, a well- known solution is to resort to signal processing schemes that involve nonlinear operations (e.g., an appropriate clipping) in the time domain. However, with a conventional OFDM receiver, decreased clipping levels for PMEPR reduction purposes always lead to increasingly poor detec- tion performances. This paper considers advanced OFDM- type receivers where an iterative cancellation of deliberate nonlinear distortion effects is carried out, and evaluates their performance. The potentials and limitations of this approach are explained with the help of an analytical performance bound. Moreover, we also consider, in de- tail, the impact of channel coding/decoding. A set of numerical results is presented and discussed, namely to show the feasibility of a low-PMEPR, spectrally efficient, coded OFDM-type transmission where the cancellation of nonlinear distortion effects is carried out through a modified iterative decision-feedback procedure, involving the decoder outputs. 1

Iterative block-DFE techniques for single-carrier-based broadband communications with transmit/receive space diversity

Conventional SC (single-carrier) modulations have been shown to be suitable for block transmission schemes with cyclic prefix. When selected for broadband wireless communication systems, they allow better performances than the corresponding OFDM-based (orthogonal frequency division multiplexing) block transmission schemes, while also offering a low complexity of implementation. In this paper, we propose an extension of the recently developed IB-DFE concept (iterative block decision feedback equalization), for SC-based block transmission, so that it can be adopted within transmit/receive space diversity implementations. The particular case where only one iteration is employed corresponds to a conventional, linear FDE scheme (frequency domain equalization); the subsequent iterations with the IB-DFE scheme are shown to provide increasingly improved performances.

Adaptive HARQ schemes using punctured RS codes for ATM-compatible broadband wireless communications

This paper considers the use of an adaptive error control, based on punctured Reed-Solomon codes, for an ATM-compatible mobile broadband system. Several alternative HARQ schemes are considered and their performance is evaluated through a semi-analytical method which resorts to an appropriate signal flow graph technique. A set of performance results is presented and discussed.

A class of signal processing algorithms for good power/bandwidth tradeoffs with OFDM transmission

This paper presents a wide class of signal processing algorithms which employs a nonlinear operation in the time domain and is capable of providing good power/bandwidth tradeoffs with OFDM transmission. A suitable analytical approach is proposed for efficiently evaluating the performance within this class of algorithms, and several performance results are shown and discussed in detail.

Performance evaluation of a multicarrier modulation technique allowing strongly nonlinear amplification

OFDM techniques, known to be well-suited for high rate transmission in time-dispersive channels, are currently being considered for broadband wireless LAN systems. However, the strong envelope fluctuation of conventional OFDM signals is hardly compatible with an efficient, somewhat nonlinear, power amplification. A new multicarrier modulation technique, called CEPB-OFDM (constant envelope paired burst OFDM), was proposed to allow high transmit power efficiencies. This paper is concerned with the performance evaluation of CEPB-OFDM and includes a performance comparison, when a grossly nonlinear power amplifier is employed, with other previously proposed OFDM techniques which are able to reduce the envelope fluctuation. The performance results clearly indicate that the CEPB-OFDM technique is recommendable for the above-mentioned wireless systems.