Kamal Singh

Abstract: We consider a fast fading AWGN multiple-access channel (MAC) with full receiver CSI and distributed CSI at the transmitters. The objective is to evaluate the ergodic sum-capacity of this decentralized model, under identical average powers and channel statistics across users. While an optimal water-filling solution can be found for centralized MACs with full CSI at all terminals, such an explicit solution is not considered feasible in distributed CSI models. Our main contribution is an upper-bound on the ergodic sum-capacity when each transmitter is aware only of its own fading coefficients. Interestingly, our techniques also suggest an appropriate lower bound. These bounds are shown to be very close to each other, suggesting the tight nature of the results.

Abstract: We consider decentralized power control in fading multiple-access channels (MAC). When compared to a centralized MAC, solving the optimal power control for a decentralized system is difficult even for moderate number of users. Good thumb-rules for decentralized power control are available in literature, which are also asymptotically tight in the number of users. In this paper, we first derive a structural property that any optimal decentralized power control scheme must follow, and use this property to suggest a modified power control scheme. Numerical results suggest that the scheme we propose outperforms the existing ones.

Abstract: We consider a distributed MAC setting with block-wise flat fading links and full receiver CSI (channel state information). Of the L transmitters, a subset is assumed to have knowledge of the global CSI vector in each block, whereas the remaining users have access only to their respective link qualities, i.e. each one in the latter set is unaware of the quality of other links. Outage is not allowed in any communication block. We propose efficient power-allocation and rate-adaptation strategies which are sum-rate optimal when users in each subset observe identical fading distributions chosen from a class, which includes the popular Rayleigh, Ricean etc.

Abstract: In this paper, we evaluate and compare the performances of Viterbi decoding schemes based on soft decision and hard decisions, with results applicable to the Digital Video Broadcast-Terrrestial (DVB-T) standard. The standard suggests that the BER ≤ 2 × 10−4 at the decoder output is good enough for the quasi error-free condition. We assume that the channel state information (CSI) of all sub-carriers are available at the receiver at all times. The multipath fading is assumed to be Rayleigh type. Simulation results confirm the advantage of using soft-decision decoding over the hard-decision type. Approximate E b /N 0 gains in the range 3–4.5 dB are reported for the different choices of code rates and OFDM transmission modes available in the standard.

Abstract: In a coherent time-varying MIMO multiple access channel (MAC), the ergodic sum-capacity can be achieved by suitable power control, which adapts the transmit covariance matrices based on the available channel state information (CSI). We consider a MIMO-MAC model where each transmitter is only aware of its own fading realization. For this distributed MAC, we propose decentralized power control schemes. First, some key structural properties on the optimal power control laws are derived, followed by an upper bound on the sum-capacity. Numerical results show that the proposed power control schemes closely characterize the sum-capacity in several regimes of interest.

Abstract: A two-user multiple access channel is considered, in which the channels undergo slow block fading and the state of each channel is known only to its corresponding transmitter. This paper proposes a novel broadcast strategy for multiple access communication in this channel. In the broadcast approach, in principle, a transmitter with CSI uncertainty sends multiple independent superimposed information layers where the rate of each layer is adapted to a specific channel realization. In the existing broadcast approaches to multiuser communication, the transmitters often directly adopt a single-user strategy and each transmitter adapts its transmission to one unknown channel. The novel aspect of the proposed strategy is that it adapts the designed codebooks to the state of the entire network. This is motivated by the fact that the contribution of each user to the network-wide measures (e.g., capacity region) depends not only on the user’s direct channel to the receiver, but also on the qualities of other channels. Average achievable rate region and outer bounds on the capacity region are characterized. Furthermore, the expected capacity region is investigated, where most part of the capacity region boundary is characterized. Finally, an asymptotic capacity region is also characterized.

Abstract: We consider a two-user block-fading MAC with distributed channel state information (CSI), where each user has access to only its own fading coefficients. The average rate-pairs of communication while employing within-block coding is known as the adaptive capacity region, where each user adapts the rate based on its perceived link gain. We evaluate the adaptive sum-capacity of MAC channels with general fading distributions, for discrete as well as continuous valued ones.

Abstract: We consider a fast fading AWGN multiple-access channel (MAC) with full receiver CSI and distributed CSI at the transmitters. The objective is to evaluate the ergodic sum-capacity of this decentralized model, under identical average powers and channel statistics across users. While an optimal water-filling solution can be found for centralized MACs with full CSI at all terminals, such an explicit solution is not considered feasible in distributed CSI models. Our main contribution is an upper-bound on the ergodic sum-capacity when each transmitter is aware only of its own fading coefficients. Interestingly, our techniques also suggest an appropriate lower bound. These bounds are shown to be very close to each other, suggesting the tight nature of the results.

Abstract: In this dissertation we investigate the usefulness of feedback and propose new ways of exploiting rate-limited feedback for the memoryless broadcast channels (BC). In the first part of the dissertation, we consider K-reciever Gaussian BC with only common message and feedback. We show that linear-feedback schemes with a message point, in the spirit of the Schalkwijk-Kailath scheme, are strictly suboptimal for this setup. To contrast this negative result, we describe a scheme for rate-limited feedback that uses the feedback in an intermittent way, which achieves all rates R up to capacity C with an L-th order exponential decay of the probability of error if the feedback rate is no smaller than (L-1)R, for some positive integer L. In the second part, we study the two-receiver DMBC with private messages and rate-limited feedback. Two types of schemes based on block-Markov strategy and Marton's coding, have been proposed for this setup. In the first type of scheme, the transmitter simply relays the feedback messages obtained over the feedback links by encoding them into the Marton cloud center of the next-following block. With this type of scheme, we show that any positive feedback rate can strictly improve over the non-feedback capacity region for the class of strictly essentially less-noisy BCs, which we introduce in this dissertation. In our second type of scheme, the transmitter decodes all the feedback information and processes it with some local information before sending the result to the receivers. When the feedback rates are sufficiently large, then our scheme can recover all previously results of capacity region and degrees of freedom for memoryless BCs with feedback.