Single carrier transmissions with frequency domain equalization (SC-FDE) have gained widespread use in emergent broadband wireless systems becoming an attractive alternative to popular Orthogonal Frequency Division Multiplexing (OFDM) schemes, particularly at the uplink. Since coherent receivers are usually employed with SC-FDE, accurate channel estimates are required so as to avoid substantial performance degradation. Several channel estimation strategies have been proposed for SC-FDE, but a thoroughly evaluation of the degradation caused by channel estimation errors and a comparison against OFDM is still lacking. In this paper we study the impact of imperfect channel knowledge on SC transmission with focus on the linear frequency domain equalizer (FDE) and on the Iterative Block Decision Feedback Equalizer (IB-DFE). We propose a modified IB-DFE which incorporates knowledge of the channel estimation error model and show that its performance becomes more robust against the presence of strong error components in the channel estimates. We also evaluate, analytically and through simulations, the degradation caused by imperfect channel estimation in SC-FDE and compare it against OFDM schemes (Orthogonal Frequency Division Multiplexing). It is shown that the channel estimation requirements for SC-FDE are higher than for OFDM unless a channel estimation error aware receiver is employed.

The aim of this paper is to design new multi-user receivers based on the iterative block decision feedback equalization concept for MC-CDMA systems with closed-form interference alignment (IA) at the transmitted side. IA is a promising technique that allows high capacity gains in interfering channels. On the other hand, iterative frequency-domain detection receivers based on the IB-DFE concept can efficiently exploit the inherent space-frequency diversity of the MIMO MC-CDMA systems. In IA-precoded based systems the spatial streams are usually separated by using a standard linear MMSE equalizer. However, for MC-CDMA based systems, linear equalization is not the most efficient way of separating spatial streams due to the residual inter-carrier interference (ICI). Therefore, we design new non-linear iterative receiver structures to efficiently remove the aligned interference and separate the spatial streams in presence of residual ICI. Two strategies are considered: in the first one the equalizer matrices are obtained by minimizing the mean square error (MSE) of each individual data stream at each subcarrier, while in the second approach the matrices are computed by minimizing the overall MSE of all data streams at each subcarrier. We also propose an accurate analytical approach for obtaining the performance of the proposed receivers. Our schemes achieve the maximum degrees of freedom provided by the IA precoding, while allowing close-to-optimum space-diversity gain, with performance approaching the matched filter bound.

In this paper, we characterize the radio frequency spectrum opportunities available in a common global system for mobile communications (GSM) channel to support the operation of a cognitive radio network (CRN). In a first step, we describe the technical details involved to sample the channel using a software defined radio device. Adopting a simple energy-based detector, we identify the two energy regions where the GSM system is active or inactive and evaluate the spectrum sensing accuracy. Based on the output of the detector, we show that the distribution of the durations of busy and idle periods are approximated by geometric distributions. Finally, we validate a theoretical model for the distribution of the service time. The validation results indicate that the service time can be successfully represented by a discrete generalized Pareto distribution, which is confirmed by the Kolmogorov–Smirnov test. Because the throughput of the CRN is represented by the inverse of the service time, the proposed analysis provides an upper bound for the networks’ throughput, indicating the maximum throughput that can be attained when a single secondary user transmits over a GSM cellular channel. The results presented in this paper are validated with real data, confirming the accuracy of the proposed service time model.

In this paper, we consider a high-speed highway mobility scenario, where the available knowledge about the network’s topology is used to improve the routing path duration. The improvement is mainly due to the use of a topology control algorithm, which increases the path duration by decreasing the probability of path breaks. For network regions having an enough density of vehicles, the packets are preferentially routed over the oldest links created by the vehicles moving in the same direction. For smaller values of vehicles’ density, the routing preferentially uses the most recent links created in both moving directions. This choice is shown to increase the routing path duration. The topology control scheme here proposed can be easily integrated in the existing routing protocols: we describe how to integrate it in the Optimized Link-State Routing Protocol (OLSR).1 We compare the performance of our approach with other routing protocols for different values of vehicles’ density. The comparison includes end-to-end path delay, path availability and path length (in number of hops). Finally, we evaluate the path duration achieved with our approach, concluding that it exhibits a significant improvement over the most relevant topology and position-based routing protocols.

Offset signals can be designed to have reduced envelope fluctuations allowing high amplification efficiency. When combined with SC-FDE (Single-Carrier with Frequency-Domain Equalization) schemes, they become excellent candidates for broadband wireless systems with severe power constraints. However, the performance of these modulations with conventional FDE receivers is sub-par, even for receivers specifically designed for offset modulations. This is especially serious when large offset QAM (Quadrature Amplitude Modulation) constellations are used. In this paper we consider SC-FDE schemes combined with offset modulations and study the reason behind the poor performance of conventional FDE implementations. We also present pragmatic FDE receivers for offset modulations that have low complexity and excellent performance.

Channel state information at the transmitter side is an important issue for wireless communications systems, namely when precoding techniques are employed. Recent works explored random vector quantization (RVQ) as a solution for limited feedback for multi-user systems equipped with multiple antennas. Despite of being a good option for narrowband channels, this method requires large complexity and is not efficient for sparse channels. To overcome these drawbacks we consider a strategy based on uniform quantization, denoted partial uniform quantization (P-UQ), where just part of channel frequency response is quantized. This allows an efficient feedback of channel frequency response from the receivers to the transmitter, by using a reduced number of quantization bits. The comparison between the proposed P-UQ-based method and RVQ performed in this paper leads to the conclusion that the most advantageous method for sparse channels is the P-UQ.

Radio Spectrum sensing has been a topic of strong research in the last years due to its importance to Cognitive Radio (CR) systems. However, in Cognitive Radio Networks (CRNs) with multiple Primary Users (PUs), the Secondary Users (SUs) can often detect PUs that are located outside the sensing range, due to the level of the aggregated interference caused by that PUs. This effect, known as Spatial False Alarm (SFA), degrades the performance of CRNs, because it decreases the SUs’ medium access probability. This work characterizes the SFA effect in a CRN, identifying possible actions to attenuate it. Adopting Energy-based sensing (EBS) in each SU, this work starts to characterize the interference caused by multiple PUs located outside a desired sensing region. The interference formulation is then used to write the probabilities of detection and false alarm, and closed form expressions are presented and validated through simulation. The first remark to be made is that the SFA can be neglected, depending on the path loss factor and the number of samples collected by the energy detector to decide the spectrum’s occupancy state. However, it is shown that by increasing the number of samples needed to increase the sensing accuracy, the SUs may degrade their throughput, namely if SUs are equipped with a single radio that is sequentially used for sensing and transmission. Assuming this scenario, this paper ends by providing a bound for the maximum throughput achieved in a CRN with multiple active PUs and for a given level of PUs’ detection inside the SUs’ sensing region. The results presented in the paper show the impact of path loss and EBS parameterization on SUs’ throughput and are particularly useful to guide the design and parametrization of multi-hop CRNs, including future ad hoc cognitive radio networks considering multiple PUs.

Whenever a direct-sequence code-division multipleaccess (DS-CDMA) signal is the sum of several components associated with different spreading codes [e.g., the DS-CDMA signal to be transmitted by the base station (BS) in the downlink or any multicode DS-CDMA signal], it has high envelope fluctuations and a high peak-to-mean envelope power ratio (PMEPR), setting strong linearity requirements for the power amplifiers. For this reason, it is desirable to reduce the envelope fluctuations of the transmitted signals. The use of clipping techniques combined with frequency-domain filtering was shown to be an effective way of reducing the envelope fluctuations (and, inherently, the PMEPR) of DS-CDMA signals, while maintaining the spectral occupation of the corresponding conventional DS-CDMA signals. To avoid PMEPR regrowth effects, the clipping and filtering operations can be repeated several times. However, the performance degradation due to nonlinear distortion effects on the transmitted signals can be relatively high, particularly when a very low PMEPR is intended (e.g., when a low clipping level and several iterations are adopted). This can particularly be serious if different powers are assigned to different spreading codes. To avoid significant performance degradation in these situations, we consider an improved receiver where there is an iterative estimation and cancellation 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.

In this manuscript, we consider the uplink of a single carrier frequency division multiple access (SC-FDMA) based system, where a set of small-cells coexist with a macro-cell using the same spectrum. To deal with both the inter-carrier interference (ICI) and inter-system interference we combine interference alignment (IA) precoding at the small-cell transmitters with iterative decision feedback equalization at the macroreceiver. The transmitter and receiver design is performed jointly, by considering both full-coordination and limited inter-system information exchange between macro and small-cells. At the small cells side, we assume that the access points are connected through a limited capacity backhaul network to a central unit, where the separation of a quantized version of the small-cell signals is performed. For this case the iterative feedback equalizer is designed by explicitly taking into account the impact of signal quantization. Moreover, a semi-analytical approach for the performance of the proposed schemes is provided. The results show that the proposed schemes are robust to both inter-system and inter-carrier interferences and thus are able to efficiently separate the macro and small cells spatial streams under limited information exchange.

Prefix-assisted direct-sequence code-division multiple access (DS-CDMA) is a viable transmission technique for high-data-rate wireless broadband systems, capable of coping with highly dispersive channels at the uplink. Packet combining (PC) ARQ with an iterative block decision feedback equalization (IB-DFE) technique can be used to cope with errors. However, extracting the IB-DFE's packet error rate (PER) for MAC-layer simulations can be time consuming. Most works that model the DS-CDMA behavior with PC ARQ use simplified asymptotic PER models in the study of the system performance, which are not precise for low signal-to-noise ratios and for a low number of mobile terminals (MTs). In this paper, the MAC performance is modeled using a discrete-time Markov chain that relies on a PER model. As an example, this paper describes a PER analytical model for an uncoded prefix-assisted DS-CDMA system with PC ARQ that considers the following inputs: the number of MTs accessing the channel, the MTs' number of transmissions, and the channel realizations for a given bit energy over noise ratio. The results show that the model portrays with better accuracy the simulations' results, compared with an alternative asymptotic model.