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Probabilistic constellation shaping (PCS) has attracted a lot of attention for increasing the spectral efficiency (SE) in coherent fiber optic communication. Unfortunately, increasing the transmit power could increase non-linear self- and cross-phase modulations due to the Kerr effect in fiber optic. Owing to smaller peak-to-average power ratio (PAPR), higher angular distance between constellation points, and lower amplitude levels, amplitude and phase shift keying (APSK) can be more robust to phase noise than quadrature amplitude modulation (QAM). A new PCS scheme is proposed for the standard APSK 16 and 64 DVB-S2/S2X modulation formats. Simulation results in additive white gaussian noise (AWGN) channels show a close bit error rate performance for the proposed system compared to probabilistically-shaped QAM.

A real-time algorithm for spectrum balancing (SB) of digital subscriber lines (DSL) is proposed. A real-time SB algorithm is an algorithm for which the PSD and power regulatory constraints are always met after each iteration. The algorithm is based on fixed-point iterations and has a low computational complexity. Our simulations show that the proposed technique converges much faster than a recently proposed real-time SB algorithm. Moreover, the achievable bit rates by the algorithm are near-optimal. Particularly the algorithm outperforms the iterative SB (ISB) algorithm in large scenarios.

Characterizing the satellite beam footprint boundary can be used to measure the overlap of the beam coverage area of a satellite with that of other communication systems. A crucial application of this is in emerging cognitive satellite communication systems. In this paper, an analytical model based on ray tracing is proposed for specifying the boundary and the size of the satellite beam footprint and the distance of the footprint center from the beam center on Earth's surface. These characteristics are primarily a function of four parameters, namely, the coordinates of satellite, the beam cross-sectional shape, the coordinates of beam center, and the beamwidth. These parameters are reduced to only two parameters of Earth central angle between the nadir and the beam center and the beamwidth. Numerical results show that by increasing any of the two parameters the asymmetry of the footprint, its size, and the distance between the footprint center and the beam center increase.

One of the main impairments for advanced digital subscriber line (DSL) systems is crosstalk. Crosstalk can be effectively cancelled using vectoring. However, some challenges such as implementation and computational complexities associated with full users' coordination when the number of DSL lines is large or when they are not co-located at any end make full vectoring impractical in many scenarios. In this paper, we consider interference alignment (IA) as an alternative technique when full vectoring is not practical. We apply this technique to both non-coordinated and partially coordinated very fast digital subscriber line 2 (VDSL2) systems. The computational complexity of IA can be prohibitive, however, we reduce it by applying IA to subsets of tones resulting in a computational complexity much smaller than that of vectoring. We also use iterative IA algorithms for performance improvement. Simulation results show that IA increases the achievable rates of VDSL2 loops considerably without a need for signal coordination among users. Moreover, when IA and partial vectoring are applied together, the users can achieve bit rates moderately close to that of a crosstalk-free network.

In this paper, we propose a new approach for signal level estimation in binary phase shift keying (BPSK) modulation based on the empirical characteristic function (ECF), when the probability density function (PDF) of the noise is unknown. Then, we compare our proposed method with two other estimators that are suggested for systems with known noises. Numerical results show that, in the presence of Laplace noise, the ECF estimator has a better performance in low signal-to-noise ratios (SNR) in comparison with previously proposed methods. Moreover, the proposed method does not require the knowledge of noise PDF and works without any training sequence.

Previous works in spectrum leasing for centralized cognitive radio networks (CRNs) are based on selecting a secondary user as a relay to cooperatively transmit the primary data and then leasing the released spectrum due to the cooperation to the selected user. In this paper, we propose a new spectrum leasing scheme to improve the throughput and thus the spectrum efficiency of the secondary system. The proposed approach selects two secondary users for cooperation and secondary transmission independently, which are the best users towards the primary and secondary systems, respectively. We show that the outage performance of the secondary system is enhanced as a result of the independent secondary user selection. Analytical and simulation results are presented to verify the efficiency of our approach and performance improvement in the secondary system.

Different precoder and equalizer structures have been proposed in literature for vectored digital subscriber lines (DSLs). In this paper, we assess the relative merit of advanced complex precoders and equalizers in practical DSL scenarios. Using extensive computer simulations, the achievable bit rates are investigated in the presence of alien crosstalkers, assuming a beta distribution model for crosstalk couplings. Vectoring in conventional as well as phantom mode (PM) VDSL2 transmission is considered. Vectoring techniques at the access-node (AN) are studied such as the zero-forcing equalizer (ZFE), ZF generalized decision feedback equalizer (ZF-GDFE), linear minimum mean squared error (MMSE), and MMSE-GDFE for upstream (US) scenarios and zero-forcing precoder (ZFP) for downstream (DS) scenarios. Vectoring is also considered at the customer premises (CP) in PM transmission. The results show that when there are no alien crosstalkers, the low-complexity ZFE allows reaching crosstalk-free rates in US direction. However using an MMSE-GDFE and if the noise spatial covariance information is assumed static, considerably higher bit rates can be achieved for long loops in the presence of alien crosstalkers. This rate increase comes at the expense of increased delay. In DS direction, the ZFP can achieve crosstalk-free rates in the absence of alien crosstalkers. The results also show that the use of vectoring techniques at the CP in PM transmission does not increase the achievable bit rates in US or DS direction when vectoring is already employed at the AN. © 2012 IEEE. ispartof: pages:3086-3091 ispartof: Proc. of the IEEE Global Telecommunications conference pages:3086-3091 ispartof: GLOBECOM 2012 location:Anaheim, California, USA date:Dec - Dec 2012 status: published

In this paper, we consider optimal dynamic spectrum management (DSM) for a downstream (DS) DSL scenario in which users are divided into a few separate groups, where vector encoding based signal coordination can be applied in each group and spectrum coordination is possible for all users. This can be seen as a mixed interference/broadcast channel (IF/BC) scenario. We investigate several candidates for vector encoding the signals inside the groups, including the linear zero-forcing (ZF) pre-compensator, ZF Tomlinson-Harashima pre-coder (THP), optimal linear pre-compensator (OLP), and THP with optimal transmit filters. The calculation of the optimal transmit filters (for both the linear pre-compensator and THP) is a non-convex problem. To resolve this problem, we develop a generalized duality between the IF/BC and IF/multiple-access channel (MAC). In order to achieve the highest data rates, optimal spectrum balancing (OSB) is applied to all users on top of the vector encoding inside the groups. Simulation results show that the grouped THP with optimal transmit filters achieves considerably higher bit rates than the other schemes. The resulting algorithm is referred to as the IF/BC-OSB algorithm, and encompasses the OSB (i.e., IF-OSB) and the earlier developed BC-OSB algorithm as special cases.

Lagrange dual optimization technique (LDO) is a powerful tool for solving constrained optimization problems in multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing systems (OFDM) and is generally considered to be optimal in the literature. LDO relaxes a constrained problem into an unconstrained dual problem using Lagrange multipliers. To solve the dual problem, the optimal value of the Lagrange multipliers should be found. The Lagrange multipliers are usually determined in an iterative process and reducing the number of iterations is of crucial importance to obtain systems with manageable computational complexity. In this paper, we show that for the LDO to be optimal in optimal spectrum balancing of DSL, the joint rate and power region (JRPR) should be strictly convex. Moreover, we propose a new LDO based algorithm with two basic advantages. Firstly, the computational complexity of the algorithm is logarithmic in the desired precision. Secondly, the algorithm can be used to find the optimal solution even for the cases with non-strictly convex JRPR. Our results can potentially be generalized to a wider range of optimization problems in the context of MIMO-OFDM and other complex separable systems.

The minimum mean squared error (MMSE)-generalized decision feedback equalizer (GDFE) provides a high multi-user crosstalk cancellation performance in upstream digital subscriber lines (DSL) scenarios. The weighted sum rate maximization approach can be used to find the points located on the boundary of the corresponding rate region (RR). The performance of the MMSE-GDFE depends on the decoding order of the users. Unfortunately, unlike in ideal multiple access channels (MAC), the optimal decoding order is not determined by the order of weight factors in practical DSL MACs. To reach the boundary of the RR, a tonal exhaustive search (TES) with computational complexity O(N2N!) is required for each tone, where N is the number of users. In this paper, we investigate the main deviations of the DSL receiver from the ideal MAC receiver and model their effects as a set of update factors on the weight factors. The order of the updated weight factors is then used to determine the decoding order of the users in an iterative fashion. The computational complexity of our updated weight factor ordering (UWFO) algorithm is O(N4) which is significantly smaller than that of TES. Moreover, our simulation results show that the algorithm achieves bit rates very close to the boundary of the RR leading to approximately the same performance as TES. UWFO can be applied to wireless MACs and zero-forcing GDFEs as well.

Spectrally encoded (SE) CDMA systems, a. k. a. spread time (ST) CDMA systems, have several advantages over their frequency-time dual, direct-sequence CDMA such as the abilities to shape the transmitted spectrum and to avoid narrow-band interference. However, no orthogonal spreading code has been proposed to efficiently avoid multiple-access interference (MAI). In this paper, we propose a novel orthogonal spreading code for SE-CDMA. We study undesirable channel conditions under which the orthogonality property of the code could be lost. We propose a chip normalizing scheme (CNS) to restore the orthogonality property of the code in these cases. Our simulation results show that the encoded system can achieve the performance of interference-free channels. Furthermore, using our CNS the users can achieve a higher bit rate at the same bit error rate as the unnormalized case.

Line selection (LS), tone selection (TS), and joint tone-line selection (JTLS) partial crosstalk cancellers have been proposed to reduce the online computational complexity of far-end crosstalk (FEXT) cancellers in DMT DSL. However, in contrast to perfect crosstalk cancellers, partial crosstalk cancellers should also provide a low pre-processing complexity when crosstalk profile changes rapidly in time. In this paper, we propose two novel fast classes of JTLS algorithms, namely, a family of sub-sort JTLS algorithms and the Lagrange JTLS. The computational complexity of our algorithms is much lower than that of the optimal greedy algorithm. Simulation results show that the performances of the proposed sub-sort algorithms are very close to the optimal greedy algorithm. More importantly, our Lagrange JTLS algorithm is optimal for the DSL channels we consider.

Loop loss and crosstalk are the main impairments in DSL. Recently, novel crosstalk rejection and reduction techniques have been proposed to achieve the maximum capacity of the DSL channel. The performance of these techniques depends strongly on the degree of coordination among the users. In this paper, we consider maximizing the throughput of DSL when the users are coordinated at different levels. Moreover, we propose a novel centralized power allocation algorithm by generalizing a recently proposed iterative spectrum balancing (ISB) algorithm. Our generalized ISB (GISB) algorithm provides a superior performance to the ISB with polynomial complexity in the number of users. This complexity is much lower than that of the optimal spectral balancing (OSB) algorithm. Moreover, it can be selected adaptively based on the available processing power

Iterative median filtering for restoration of images corrupted by impulsive noise is considered. A modified version of median filtering that can be also applied iteratively is also proposed. The methods are compared with an iterative method suggested by Marvasti for images with high pixel loss. The results show that applying the median filter iteratively results in well sighted resulting images. Moreover, despite the implementation simplicity, the proposed modified median filtering scheme provides a considerably higher convergence speed which intends a lower numerical complexity.

In a previous paper by R.A. Scholtz (see Proc. IEEE MILCOM '93, p.447-50, Oct. 1993), an ultrawide bandwidth time-hopping spread-spectrum code division multiple access system employing a binary PPM signaling was introduced, and its performance was obtained based on a Gaussian distribution assumption for the multiple access interference. A coded scheme of this system was proposed by the authors (see Proc. PIMRC '00, vol.2, p.1555-8, Sept. 2000), which showed much better performance in comparison to the uncoded scheme under the above Gaussian assumption. In this paper, we present a more precise performance analysis of the system for both coded and uncoded schemes. Our analysis shows that the Gaussian assumption is not accurate for predicting bit error rates at high data transmission rates.

We propose to use a practical low rate error-correcting code in a time-hopping spread spectrum multiple access system employing a binary PPM signaling. To this end, superorthogonal codes, which are near optimal and have relatively low implementation complexity, are considered. The performance analysis shows that the proposed coded scheme outperforms the uncoded scheme significantly, or more importantly, at a given bit error rate the proposed scheme increases the number of users by a factor which is logarithmic in the number of pulses used in a time-hopping spread spectrum system.