We derive optimal and suboptimal multiuser transmit-optimization methods for a multicarrier broadcast channel with intersymbol interference under the frequency-division multiple-access (FDMA) restriction. The general FDMA-based multicarrier broadcast problem is formulated as a maximum weighted rate-sum problem. Given each user's subchannel assignment, the optimal transmit strategy is achieved by multilevel waterfilling. Unfortunately, the problem of finding the optimal subchannel assignments is combinatorial. However, by relaxing the FDMA restriction, we obtain a convex reformulation that allows for efficient computation of the optimal solution, and therefore, a characterization of the FDMA capacity region for a broadcast channel. If all users share the same transmission medium, we prove that the optimal frequency partitioning among the users has an ordered structure that can be exploited to significantly reduce the computational complexity. To make multiuser transmit-optimization schemes practical for applications with relatively fast time-varying user data-rate requirements or priorities, further reduction in computational complexity is necessary. This is achieved by restricting the energy distribution to be constant across the used subchannels. Simulations indicate the low-complexity constant-energy methods presented are very robust, and suffer from negligible performance loss.

This paper proposes a new method for decoding multicarrier symbols with severe nonlinear distortion. The first part evaluates mutual information expressions for practical nonlinear models and shows the performance bounds for commonly used receiver structures. Then, we derive the maximum-likelihood (ML) sequence estimator, which unfortunately has an exponential complexity due to the nonlinear distortion. This extremely large complexity can be reduced with a simple algorithm that iteratively estimates the nonlinear distortion, thereby reducing the exponential ML to the standard ML without nonlinear distortion. The proposed method can be used to reduce the peak-to-average power ratio of multicarrier signals by clipping the transmit sequence. It can also be used to correct any nonlinear distortion present in transmitter/receiver amplifiers that are operating close to saturation.

We consider the problem of optimal power allocation for a broadcast channel with ISI under the FDMA restriction. Due to the FDMA restriction, the resulting problem is combinatorial in nature and hard to solve. However, by relaxing the FDMA restriction, we obtain a convex reformulation that allows for efficient computation of the optimal solution and therefore, a characterization of the capacity region. It is also shown that under certain conditions on the channel responses, the optimal frequency partition has an ordered structure that can be exploited to further reduce the computational complexity.

This paper presents two novel and efficient water-filling algorithms for a two-user Gaussian multiaccess channel with intersymbol interference. These algorithms efficiently compute the optimal transmit power spectral density (PSD) for each user and obtain the capacity region of the channel. One algorithm is developed for the special case where two users have the same priorities and is more efficient than prior work. Another algorithm is designed for the first time to compute the general case where two users have different priorities. Both algorithms use a binary search technique and each has a computational complexity of O(NlogN) where N is the number of subcarriers.

This paper formulates the problem of embedded modulation applied to a multiuser multicarrier system. These embedded multiuser optimization problems are shown to be convex, for which globally optimal solutions can be computed efficiently. We also show that these embedded problems are convex relaxations of the combinatorial FDMA multiuser problems and thus provide performance bounds to the optimal and asymptotically optimal solutions proposed by Hoo, Tellado and Cioffi (see IEEE Global Telecommunications Conf. (Globecom), p.25-30, 1998 and IEEE PIMRC, p.278-82, 1998).

We analyze the general problem of multiuser transmit optimization for a Gaussian broadcast channel with ISI under the FDMA restriction. Due to the FDMA restriction, the resulting problem is combinatorial in nature and hard to solve. However, by relaxing the FDMA restriction, we obtain a convex reformulation that allows for efficient computation of the optimal solution. By enforcing a constant-energy restriction, further reduction in computational complexity is achieved. We present the optimal and one suboptimal constant-energy method with bandwidth optimization. Simulation results show that these methods suffer from negligible performance loss even at low SNRs where the constant-energy approximation is not valid.

We derive the optimal bit and energy allocation for a discrete multitone (DMT) modulation system simultaneously providing two services with different quality of services (QoS), and show that it is NP-hard. Thus, we propose three novel, fast, discrete dual loading algorithms. These methods apply Campello's (see IEEE International Symposium on Info. Theory, MIT, p.193, 1998) discrete loading algorithms and are based on a novel tone selection procedure that offers significant reduction in complexity but still achieves near optimal performance. Simulation results support our tone selection procedure by indicating the convergence in performance of these loading algorithms to the optimum with increasing N, N being the number of subchannels or tones.

We propose a novel loading algorithm for discrete multitone (DMT) modulation or orthogonal frequency division multiplex (OFDM) systems simultaneously providing two constant bit rate (CBR) services. Furthermore, these two services have different quality of services (QoS) characterized respectively by their target bit error rates (BER). We derive the optimal solution and show that it is NP-hard. Thus, we propose a method namely, disjoint bandwidth (DBW) which is O(N log N) that still gives near optimal performance. The reduction in complexity is enabled by the set selection procedure we propose. Simulation results for frequency selective Rayleigh fading channels with additive white Gaussian noise show that the DBW method is within 0.1% of the optimum for a total number of carriers N>12 and that it performs on the average 15% better than any previously reported method in the literature.