Interference may pose a severe limitation to massive wireless multiple access unless the central receiver is endowed with strong multipacket reception capabilities. In that respect, Interference Cancellation (IC) has been extensively studied [1]–[3]. Such systems have been proposed for satellite communications, combining Direct Sequence (DS) Code Division Multiple Access (CDMA) with a SIC receiver [4]–[6] or using multipacket transmission as in Contention Resolution Diversity Slotted ALOHA (CRDSA) [7] [8]. The use of powerful encoders coupled with power imbalance at reception favours IC and has considerably improved the throughput of such random access protocols [4] [9]. In a context characterized by high user activity and spectrum/orbit congestion [10], it is of interest to develop schemes for space assets that optimize the aggregate spectral efficiency of the user population.Peer ReviewedPostprint (published version

Abstract-This paper presents a Bayesian approach to the design of transmit prefiltering matrices in closed-loop schemes robust to channel estimation errors. The algorithms are derived for a multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) system. Two different optimization criteria are analyzed: the minimization of the mean square error and the minimization of the bit error rate. In both cases, the transmitter design is based on the singular value decomposition (SVD) of the conditional mean of the channel response, given the channel estimate. The performance of the proposed algorithms is analyzed, and their relationship with existing algorithms is indicated. As with other previously proposed solutions, the minimum bit error rate algorithm converges to the open-loop transmission scheme for very poor CSI estimates.

Abstract-An expression is derived for the average Packet Error Rate (PER) of a Successive Interference Canceller (SIC) for DS-CDMA when the number of users asymptotically tends to infinity. The asymptotic probability density function of the interference power is governed by a Fokker-Planck differential equation with drift and (asymptotically vanishing) diffusion depending on the PER function of the adopted forward errorcorrecting code (FEC). In addition to the asymptotic solution for the PER, a particle-based algorithm is also developed for computing efficiently the PER in the finite user case.Index Terms-Successive interference cancellation, FokkerPlanck equation, packet error rate, CDMA, particle.

This work addresses the optimization of the network spectral efficiency (SE) under successive interference cancellation (SIC) at a given blocklength n. We adopt a proof-of-concept satellite scenario where network users can vary their transmission power and select their transmission rate from a set of encoders, for which decoding is characterized by a known packet error rate (PER) function. In the large-system limit, we apply variational calculus (VC) to obtain the user-energy distribution, the assigned per-user rate and the SIC decoding order maximizing the network SE under a sum-power constraint at the SIC input. We analyze two encoder sets: (i) an infinite set of encoders achieving information-theoretic finite blocklength PER results over a continuum of code rates, where the large-n second order expansion of the maximal channel coding rate is used; (ii) a feasible finite set of encoders. Simulations quantify the performance gap between the two schemes.

A multiple access (MA) optimization technique for massive low-rate direct-sequence spread spectrum communications is analyzed in this work. A dense network of users transmitting at the same rate to a common central node under channelaware energy allocation is evaluated. At reception, successive interference cancellation (SIC) aided by channel decoding is adopted. Our contribution focuses on wireless scenarios involving a vast number of users for which the provided user-asymptotic model holds. Variational calculus (VC) is employed to derive the energy allocation function that, via user-power imbalance, maximizes the network spectral efficiency (SE) when perfect channel state information at transmission (CSIT) is available and both average and maximum per-user energy constraints are set. Monte Carlo simulations at chip-level of a SIC receiver using a real decoder assess the proposed optimization method.

Abstract-A new Ordinary Differential Equation (ODE) governing the SNIR evolution of a Successive Interference Canceller (SIC) for DS-CDMA is derived when the number of users tends to infinity and all users share the same channel encoder. Using Variational Calculus, this ODE is applied to obtaining the energy profile that maximizes the average spectral efficiency when a constraint on the power unbalance (maximum power to minimum power ratio) of received users is enforced. The conditions for extremality of the optimum energy profile are established in terms of the common encoder's Packet Error Rate (PER) function.Index Terms-Successive interference cancellation, power unbalance, differential equation, variational calculus, packet error rate, CDMA, error propagation.

This paper studies the throughput maximization of a dense multiple access network of low-rate subscribers that share the same practical Forward Error Correction (FEC) code and modulation scheme, and transmit to a central node that implements a Successive Soft Interference Cancellation (soft SIC) strategy in order to mitigate Multiple Access Interference (MAI). In the user-asymptotic case, we make use of Variational Calculus (VC) tools to derive, in terms of the Packet Error Rate (PER) of the shared encoder and the Residual Energy (RE) from imperfect cancellation, the optimum energy profile that maximizes the network spectral efficiency, when a sum power constraint at the SIC input is enforced. Comparative performance analyses using a representative encoder are carried out. Simulation results show the benefit of the adopted soft SIC scheme in front of other SIC strategies, obtaining relevant throughput gains under high traffic loads.

In dense wireless scenarios, and particularly under\ud high traffic loads, the design of efficient random access protocols\ud is necessary. Some candidate solutions are based on Direct-\ud Sequence Spread Spectrum (DS-SS) combined with a Successive\ud Interference Cancellation (SIC) demodulator, but the perfor-\ud mance of these techniques is highly related to the distribution\ud of the users received power. In that context, this paper presents\ud a theoretical analysis to calculate the optimum user SINR profile\ud at the decoder maximizing the spectral efficiency in bps/Hz for\ud a specific modulation and practical Forward Error Correction\ud (FEC) code. This solution is achieved by means of Variational\ud Calculus operating in the asymptotic large-user case. Although\ud a constant SINR function has been typically assumed in the\ud literature (the one maximizing capacity), the theoretical results\ud evidence that the optimum SINR profile must be an increasing\ud function of the users received power. Its performance is compared\ud with that of the uniform profile for two representative scenarios\ud with different channel codes in a slightly overloaded system.\ud The numerical results show that the optimum solution regulates\ud the network load preventing the aggregate throughput from\ud collapsing when the system is overloaded. In scenarios with a\ud large number of transmitters, this optimum solution can be\ud implemented in an uncoordinated manner with the knowledge\ud of a few public system parameters.Peer ReviewedPostprint (published version