Nevio Benvenuto

On the comparison between OFDM and single carrier modulation with a DFE using a frequency-domain feedforward filter

Most comparisons between single carrier and multicarrier modulations assume frequency-domain linear equalization of the channel. We propose a new frequency-domain decision feedback equalizer (FD-DFE) for single carrier modulation, which makes use of a data block transmission format similar to that of the orthogonal frequency-division multiplexing with cyclic prefix (OFDM). The scheme is a nonadaptive DFE where the feedforward part is implemented in the frequency domain, while feedback signal is generated by time-domain filtering. Through simulations in a HIPERLAN-2 scenario, we show that FD-DFE yields a capacity very close to that of OFDM. This result is also confirmed by analytical derivations for a particular case. Furthermore, when no channel loading is considered, FD-DFE performs closely to OFDM for the same averaged frame error rate in a coded transmission. Design methods of the FD-DFE are investigated and a reduced complexity technique is developed, with the result that FD-DFE and OFDM have a similar computational complexity in signal processing.

Single Carrier Modulation With Nonlinear Frequency Domain Equalization: An Idea Whose Time Has Come—Again

In recent years single carrier modulation (SCM) has again become an interesting and complementary alternative to multicarrier modulations such as orthogonal frequency division multiplexing (OFDM). This has been largely due to the use of nonlinear equalizer structures implemented in part in the frequency domain by means of fast Fourier transforms, bringing the complexity close to that of OFDM. Here a nonlinear equalizer is formed with a linear filter to remove part of intersymbol interference, followed by a canceler of remaining interference by using previous detected data. Moreover, the capacity of SCM is similar to that of OFDM in highly dispersive channels only if a nonlinear equalizer is adopted at the receiver. Indeed, the study of efficient nonlinear frequency domain equalization techniques has further pushed the adoption of SCM in various standards. This tutorial paper aims at providing an overview of nonlinear equalization methods as a key ingredient in receivers of SCM for wideband transmission. We review both hybrid (with filters implemented both in time and frequency domain) and all-frequency-domain iterative structures. Application of nonlinear frequency domain equalizers to a multiple input multiple output scenario is also investigated, with a comparison of two architectures for interference reduction. We also present methods for channel estimation and alternatives for pilot insertion. The impact on SCM transmission of impairments such as phase noise, frequency offset and saturation due to high power amplifiers is also assessed. The comparison among the considered frequency domain equalization techniques is based both on complexity and performance, in terms of bit error rate or throughput.

On the complex backpropagation algorithm

A recursive algorithm for updating the coefficients of a neural network structure for complex signals is presented. Various complex activation functions are considered and a practical definition is proposed. The method, associated to a mean-square-error criterion, yields the complex form of the conventional backpropagation algorithm. >

Applications of simulated annealing for the design of special digital filters

The authors describe the salient features of using a simulated annealing (SA) algorithm in the context of designing digital filters with coefficient values expressed as the sum of power of two. A procedure for linear phase digital filter design, using this algorithm, is presented and tested, yielding results as good as those for known optimal methods. The algorithm is then applied to the design of Nyquist filters, optimizing at the same time both frequency response and intersymbol interference, and to the design of cascade form finite-impulse-response (FIR) filters. The drawback of using SA is that the computation time is on the order of 1-2 h for each filter design, on the Sun 3/60. However, this was more than compensated by the versatility of the new algorithm, which can be used to design filters with multiple constraints. >

Key technologies for next-generation terrestrial digital television standard DVB-T2

A new platform for HDTV transmissions, in the form of a new-generation terrestrial digital video broadcasting standard (DVB-T2), has been developed by the DVB Project and will be published by the European Telecommunications Standards Institute. A number of technical innovations have been included in DVB-T2 to boost throughput and ruggedness, enhance single-frequency network coverage, and ease both transmitter and receiver implementation. This article starts from the motivations that led the DVB project to create the new standard and then surveys the key technologies behind DVB-T2, including the LDPC/BCH forward error correction scheme, transmission scheduling, orthogonal frequency-division multiplexing with huge block size, multiple-antenna transmissions, and synchronization techniques. A comparison with the current DVB-T standard is also provided, showing that DVB-T2 is able to increase the payload throughput and allows HDTV transmission with current network planning.

Equalization methods in OFDM and FMT systems for broadband wireless communications

Multicarrier systems are adopted in several standards for their ability to achieve optimal performance in very dispersive channels. In particular, orthogonal-frequency division multiplexing (OFDM) and filtered multitone (FMT) systems are two examples where the modulation filter has an ideal rectangular amplitude characteristic in time and frequency domains, respectively. In this letter, we propose new equalization schemes for FMT and compare their performances with OFDM. In general, FMT has a greater spectral efficiency than OFDM, due to the absence of the cyclic prefix and a reduced number of virtual carriers. However, it exhibits a higher distortion per subchannel, due to the imperfect equalization of the transmit filters. As a performance comparison, we considered both the achievable bit rate (ABR) and the bit error rate (BER) in a multipath Rayleigh fading channel. We note that while ABR gives a theoretical bound on the system bit rate, assuming the knowledge of the channel at the transmit side, the BER refers to an uncoiled system with a fixed modulation. Although FMT requires a fixed structure with a higher computational complexity than OFDM, it turns out that FMT, even with the simplest one tap per subchannel adaptive equalizer, yields a better performance than OFDM, both in terms of ABR and BER. Hence, FMT can be a valid alternative to OFDM for broadband wireless applications, also.

Realization of Finite Impulse Response Filters Using Coefficients +1, 0, and -1

New classes of nonrecursive linear digital filters are proposed as alternatives to conventional methods for digital signal processing devices. In fact, the traditional approach, where signal samples are represented by 8-15 bit words, can be very expensive to implement due to the complexity of multibit multipliers and adders. Filter structures consisting of a transversal filter with tap coefficients restricted to - 1, 0, or + 1 and cascaded with simple recursive sections are proposed. Using a mean square error criterion, dynamic programming methods to select optimum sets of coefficient values for the taps are presented and evaluated. It is shown that these structures lend themselves to very simple hardware implementations and offer a valid alternative to the conventional filtering approach. Designs for several example filters are presented, and their implementation complexity is examined.

The Viterbi algorithm for sparse channels

This paper presents a reduced complexity Viterbi algorithm for use as sequence estimator for linear intersymbol interference channels with coarsely located coefficients. In particular, the complexity does not depend on the channel impulse response length but only on the number of nonzero coefficients. No approximations are used in the algorithm. We consider a multipath environment producing time spreads.

A least squares path-loss estimation approach to handover algorithms

In this paper, a new class of linear handover algorithms is introduced and analyzed for two conflicting performance measures, namely, the average number of outages and the average number of handovers, for a given trip of the mobile station (MS). The new algorithm is based on the least squares (LS) estimate of path-loss parameters of the various radio links, assuming the distances between the MS and surrounding base stations (BSs) are known. First, a simplified system scenario consisting of only two BSs in a lognormal fading environment is assumed. Performance measures are derived through a theoretical approach, along the trip of a mobile terminal from one BS to the other. It is seen that for an average number of 1.2 handovers (the minimum value is one, given by the ideal reference case), the LS algorithm reduces the average number of outages by more than 30% with respect to the classical averaging algorithm. Furthermore, the above performance measures improve for longer estimation windows: both the estimation of the mobile speed and the window length tradeoff of averaging algorithms do not apply to the LS method. By using an extended version of the basic algorithm, these results were also observed in simulations of a more general system scenario, consisting of several BSs and a mobile terminal moving along a generic path.

State control in networked control systems under packet drops and limited transmission bandwidth

The widespread proliferation of wireless sensor networks is revolutionizing our capabilities of monitoring and controlling the environment. The wireless connection of spatially distributed sensors, controllers and actuators poses challenges to the control system, due to packet drops, delays and measurements quantization, as well as to the wireless network resource allocator. These challenges push for a cross-layer design of communication and estimation/control systems. In this paper, assuming a TCP-like protocol between controller and actuator, we solve the problem of optimum control around a target state for a stable system in case of both packet drops and signal quantization. Generalization for unstable systems is also given in case of negligible quantization error. Moreover we propose a general framework for cross-layer optimization of signal quantization and network resource allocation. Here our main contributions are on i) quantizer design (how many bits to allocate for signal quantization), ii) network resource allocation (bit-rate for each radio link) and iii) choice of the transmission mode (constellation and channel coding rate). As application example, we consider a simple scalar, stable system and compare network resource allocation in the presence of i) low-cost sensors using a fix modulation and ii) long-term future sensors capable of rate adaptation. Interestingly, almost optimal control is achievable with small bandwidth transmissions and simple BPSK, supporting the use of low-cost sensors in applications dealing with state control around a target state trajectory.