We address the issue of sum-capacity in an overloaded, uplink, synchronous CDMA system in a quasi-static fading environment. Part of each fading block is reserved for channel training and the remainder of the block is used for data transmission. We evaluate bounds on sum-capacity using the estimates from channel estimation, and consider the effects on the lower bound caused by the spreading-coding-training tradeoff.

The focus of this paper is on training and signature sequence design in an overloaded synchronous CDMA system. The channel fading gains are assumed to be unknown and are estimated using training sequences. We derive a maximum-likelihood (ML) estimator and design sequences to minimize the mean-squared error (MSE) of the estimate. Two design scenarios are considered. One case assumes that the spreading sequences are fixed due to existing system constraints and optimal training sequences are designed using an iterative algorithm in order to minimize the MSE of the estimate. Performance of the iterative algorithm is examined using Welch-bound equality (WBE) sequences as the pre-designed spreading sequences. In the other scenario, spreading sequences and training sequences are designed jointly to minimize the MSE of the estimate. The jointly designed training and spreading sequences achieve optimum performance in the sense of minimizing the MSE. It is observed that when WBE sequences are used as spreading sequences the performance of the iterative algorithm is close to optimum, or even optimum, in certain situations

A framework is presented that allows a number of known results relating feedback equalization, linear prediction, and mutual information to be easily understood. A lossless, additive decomposition of mutual information in a general class of Gaussian channels is introduced and shown to produce an information-preserving canonical decision-feedback receiver. The approach is applied to intersymbol interference (ISI) channels to derive the well-known minimum mean-square error (MMSE) decision-feedback equalizer (DFE). When applied to the synchronous code-division multiple-access (CDMA) channel, the result is the MMSE (or signal-to-interference ratio (SIR) maximizing) decision-feedback detector, which is shown to achieve the channel sum-capacity at the vertices of the capacity region. Finally, in the case of the asynchronous CDMA channel we are able to give new connections between information theory, decision-feedback receivers, and structured factorizations of multivariate spectra.

This work addresses the issue of sum-capacity in an overloaded, uplink, synchronous code-division multiple access (CDMA) system in a quasi-static Rayleigh-fading environment when the receiver does not have a priori knowledge of the fading gains. Part of each fading block is reserved for channel training and the remainder of the block is used for data transmission. Two modes of operation are considered: one in which the users are allowed to update their spreading sequences based on the channel estimates, and one in which their sequences cannot be modified. For both schemes we derive the optimal training scheme and then evaluate a lower bound on sum-capacity. When all the users have the same average received power, we consider the effects on this bound caused by the training-spreading-coding tradeoff.

We explore a symbol-synchronous CDMA system in which each user is guaranteed a certain quality of service (QoS). The QoS requirements need not be the same for all users so that multiple classes of users can be supported. In the context of root-mean-square (RMS) bandwidth, we show how to optimally design the users' signature waveforms so that the required time-bandwidth product is minimized while meeting the QoS constraints. This is accomplished for two receiver architectures, namely the linear and decision-feedback receivers. For both scenarios, we reformulate the problem so that efficient interior-point methods may be applied. The optimality of the resulting design is practically verified by checking the Kuhn-Tucker conditions. Our approach is also found to yield an efficient technique for solving the problem of signal design for capacity-maximization that was posed and solved in [1].

We consider the symbol-synchronous code-division multiple-access (CDMA) channel equipped with either a multiuser linear receiver or a multiuser decision-feedback receiver. The network, or user, capacity is defined to be the number of users that can be supported with available resources, such that every user achieves a certain quality of service (QoS). In this paper, the QoS threshold is given as a signal-to-interference ratio and the bandwidth is given as the root mean squared bandwidth of the received power spectral density of the users' transmitted waveforms. Given the QoS threshold and constraints on bandwidth and the sum of the users' received powers, we maximize user capacity for both the linear and decision-feedback receivers by optimally and jointly designing the users' signature waveforms and power-control polices.

We consider the symbol-synchronous code-division multiple-access (CDMA) channel in which every user is assigned a rate at which arbitrarily reliable transmission in the Shannon sense is to be guaranteed. For an overloaded system in which the number of active users exceeds the available processing gain, we optimally design the users' signature sequences and a power-control policy to minimize the required sum-power (i.e., sum of the users' powers) while meeting the rate-tuple constraint with a (joint) maximum-likelihood receiver. This result is extended to find the power-constrained capacity region of the system; this is the set of all achievable rate-tuples over all signature sequences and power-control policies whose sum-power is constrained. Furthermore, it is shown that this capacity region may be substantially and maximally expanded in those regions where there are oversized users whose rate requirements are relatively large compared to those of the other users; this is accomplished by allowing for the flexibility of multidimensional signaling in the sense of a user simultaneously transmitting several different scalar symbols, each modulated by its own signature sequence. From the viewpoint of resource efficiency, this means that a multicarrier approach is essential in systems that support multiple classes of users. Finally, we also address the dual problem of determining the region of valid power-control policies subject to a sum-capacity constraint on the system.

The paper considers the multiple-input multiple-output (MIMO) channel with quasi-static Rayleigh fading. Lower bounds are derived for the outage capacity of a recently introduced approach to horizontally encoded BLAST (Bell Labs layered space time) in which the rates of the layers are optimally designed. The outage capacity of this scheme, rate-tailored BLAST (RT-BLAST), is shown to behave in the same manner as the true outage capacity of the MIMO channel (e.g., using a maximum-likelihood, ML, receiver) in the high signal-to-noise ratio (SNR) regime. That is, the asymptotical growth rates of the ML and RT-BLAST outage capacities as a function of SNR are equal to each other.

This paper is concerned with multiple-input multiple-output (MIMO) channel that experience quasi-static flat fading and Gaussian noise. The information-theoretic quantity of outage capacity is precisely defined as it relates to the successive-decoding approach known as the BLAST (or Bell labs layered space-time) architecture. It is shown that finding the outage capacity of a BLAST system is equivalent to the solution of a K-parameter optimization problem, where K is the numbers of transmit antennas. This optimization is solved and the resulting outage capacity of BLAST is found to sustain a loss relative to the unconstrained (i.e., optimal) system, though it still provides for very high-rate data transmission.

We consider a symbol-synchronous code-division multiple-access (CDMA) system that is equipped with a multiuser decision-feedback receiver and for which power control is available. The users are each assigned a quality-of-service (QoS) threshold to be guaranteed by the system, and to cover scenarios for which there are multiple classes of users, these are not required to be equal to each other. For an ideal decision-feedback receiver, it is known that with enough power the system can always meet the users' QoS thresholds, so we instead minimize the sum of the users' received powers over system designs (i.e., signature sequences, power-control policy, and decision-feedback receiver) which guarantee the QoS requirements. It is found that the optimal design produces two classes of users, those whose sequences and powers satisfy with equality the generalized Welch bound inequality and those oversized users that are mutually orthogonal to each other and the rest of the users. In terms of power and bandwidth savings, the optimal sequences for the decision-feedback receiver are found to compare very favorably to optimal designs for linear receivers and to random sequences for the decision-feedback receiver.

There have been several papers in the literature that deal with the design of signature waveforms for use by the transmitters in uplink, single-cell, multiple-access communications. In particular, we consider the approach introduced by Guess and Varanasi (1996, 1997), where the signature waveforms are specifically designed for the centralized multiuser receiver at the base so that each transmitter can be guaranteed a preassigned quality-of-service (QoS) requirement in terms of the received signal-to-interference ratio (SIR). The resulting strategy is called bandwidth-efficient multiple access (BEMA). When all users employ pulse amplitude modulation (PAM) and a common signaling rate, the key question in BEMA is how the waveforms must be designed to occupy as little bandwidth as possible and still meet the QoS objectives. For a strict measure of bandwidth, and for a given set of received powers, this question was addressed by the authors for the maximum SIR decision-feedback (MSIR-DF) receiver of Varanasi and Guess (1998). A similar question was addressed by Viswanath, Anantharam and Tse (see ibid., vol.45, p.1968-1983, Sept. 1999) where, for a sum constraint on the received powers, optimal signature signals and transmit powers were obtained for the linear MSIR receiver (without decision feedback). A somewhat different but related non-QoS approach proposes the design of signature signals that maximize the total capacity of the multiple-access channel under a spreading-gain constraint. This article undertakes a comparison of the minimum bandwidth required (to achieve the QoS requirements) for the signals designed for the MSIR-DF receiver, for the linear MSIR receiver, and for sum-capacity maximization as shown by Viswanath and Anantharam (see ibid., vol. 45, p.1984-1991, Sept. 1999). We show that the bandwidth required for multiuser receivers with decision feedback can be significantly less than that required for linear receivers or for sum-capacity maximization when the MSIR-DF receiver is used.

We consider the design of code-division multiple-access (CDMA) systems in which the users are each guaranteed some quality-of-service (QoS) threshold that is given in terms of a bit error rate (BER). In long-code CDMA systems for which the load (defined as the ratio of the number of users in the system to the processing gain) is less than or equal to unity, we show that a multiuser linear receiver with appropriately designed powers does well at meeting the BER requirements with reasonable power consumption. In fact, the design significantly surpasses a recent approach from the literature both in terms of increased user capacity and reduced power requirements. For short-code CDMA systems, we investigate the effects of erroneous feedback in multiuser decision-feedback detectors in overloaded systems (i.e., more users than processing gain). An exact analysis of systems designed for a load ratio of two indicates that feedback errors are deleterious at lower signal-to-noise ratios (SNR). In this regime significant increases in SNR have almost no effect on the error rate. However, above a certain SNR threshold the error rate is found to decrease as it would in a single-user channel, but with a SNR penalty of approximately 4.6 dB.

This paper considers the design of signature waveforms for successive-decoding-type multiuser receivers (including the optimum successive decoder (OSD)) in a correlated-waveform multiple-access channel. The problem is to obtain signature waveforms that require as little bandwidth as possible while allowing the receiver to meet a given set of quality-of-service (QoS) objectives. The QoS objectives are specified for each user in terms of capacity, or equivalently, the signal-to-interference ratio. A (generally unachievable) lower bound is obtained on the minimum bandwidth required to achieve these QoS constraints. Moreover, a simple algorithm is proposed for obtaining signal sets that meet the QoS constraints when used with the OSD, and which, while not optimal, require a bandwidth that can be very close to the minimum required bandwidth. It is also shown that such signal sets allow for a significantly more efficient use of bandwidth than do orthogonal signals used in time- or frequency-division multiple access (TDMA/FDMA). Based on our signal design approach, we propose a new multiple-access strategy that we refer to as bandwidth-efficient multiple access (BEMA). While BEMA is more bandwidth efficient than TDMA or FDMA, it retains their desirable feature of needing only single-user coding (and decoding) for each user.

Random-coding error exponents are derived for the Gaussian code-division multiple-access (CDMA) channel for the maximum-likelihood and optimum successive decoders. Error exponents not only specify the capacity region of the channel, which is known, but also give lower bounds on the rate of exponential decay of the average probability of error as a function of the block length of random codes. A comparison of the two decoders in terms of their error exponents is included.

Bandwidth-efficient multiple access (BEMA) is a strategy where transmitter pulses are continually designed at the base station and are dynamically allocated to the transmitters via a feedback channel. Such pulses (or "signature waveforms") are designed to conserve bandwidth while simultaneously enabling the receiver at the base station to meet a quality-of-service (QoS) specification for each transmitter. The key technical problem in BEMA communication is therefore the design of the transmitter pulses for the base station receiver. In an earlier paper, we presented solutions to this problem that were shown to be superior (in terms of strict bandwidth) to common signaling schemes such as time-, frequency-, and code-division multiple access (TDMA, FDMA, and CDMA). This paper uses the framework developed earlier, but considers strictly time-limited transmitter pulses and the root-mean squared (RMS) bandwidth measure. As in the earlier paper, significant bandwidth savings over the traditional multiple-access strategies are obtained. However, in contrast to the rank-conserving approach, the bandwidth gains of this paper are realized by tailoring the signature waveform design to conserve RMS bandwidth via eigenvalue optimization problems.

We have developed, prototyped, and demonstrated the feasibility of a novel robotic technique for rapid fabrication of composite structures. Its chief innovation is that, unlike all other available fabrication methods, it does not require a mold. Instead, the structure is built patch by patch, using a rapidly reconfigurable forming surface, and a robot to position the evolving part. Both of these components are programmable, so only the control software needs to be changed to produce a new shape. Hence it should be possible to automatically program the system to produce a shape directly from an electronic model of it. It is therefore likely that the method will enable faster and less expensive fabrication of composites.