Naqueeb Ahmad Warsi

Fundamental bound on the reliability of quantum information transmission.

Information theory tells us that if the rate of sending information across a noisy channel were above the capacity of that channel, then the transmission would necessarily be unreliable. For classical information sent over classical or quantum channels, one could, under certain conditions, make a stronger statement that the reliability of the transmission shall decay exponentially to zero with the number of channel uses, and the proof of this statement typically relies on a certain fundamental bound on the reliability of the transmission. Such a statement or the bound has never been given for sending quantum information. We give this bound and then use it to give the first example where the reliability of sending quantum information at rates above the capacity decays exponentially to zero. We also show that our framework can be used for proving generalized bounds on the reliability.

One-Shot Marton Inner Bound for Classical-Quantum Broadcast Channel

We consider the problem of communication over a classical-quantum broadcast channel with one sender and two receivers. Generalizing the classical inner bounds shown by Marton and the recent quantum asymptotic version shown by Savov and Wilde, we obtain one-shot inner bounds in the quantum setting. Our bounds are stated in terms of hypothesis testing and one-shot max divergences. These results give a full justification of the claims of Savov and Wilde in the classical-quantum asymptotic iid setting; the techniques also yield similar bounds in the information spectrum setting. We obtain these results using a different analysis of the random codebook argument; our method yields a classical one-shot Marton bound with a common message and a classical one-shot mutual covering lemma based on rejection sampling.

Capacity and Power Scaling Laws for Finite Antenna MIMO Amplify-and-Forward Relay Networks

In this paper, we present a novel framework that can be used to study the capacity and power scaling properties of linear multiple-input multiple-output d×d antenna amplify-and-forward relay networks. In particular, we model these networks as random dynamical systems and calculate their d Lyapunov exponents. Our analysis can be applied to systems with any perhop channel fading distribution; although in this contribution, we focus on Rayleigh fading. Our main results are twofold: 1) the total transmit power at the nth node will follow a deterministic trajectory through the network governed by the networks maximum Lyapunov exponent and 2) the capacity of the ith eigenchannel at the nth node will follow a deterministic trajectory through the network governed by the networks ith Lyapunov exponent. Before concluding, we concentrate on some applications of our results. In particular, we show how the Lyapunov exponents are intimately related to the rate at which the eigenchannel capacities diverge from each other, and how this relates to the amplification strategy and the number of antennas at each relay. We also use them to determine the extra cost in power associated with each extra multiplexed data stream.

One-shot bounds for various information theoretic problems using smooth min and max Rényi divergences

One-shot analogues for various information theory results known in the asymptotic case are proven using smooth min and max Rényi divergences. In particular, we prove that smooth min Rényi divergence can be used to prove one-shot analogue of the Steins lemma. Using smooth min Rényi divergence we prove a special case of packing lemma in the one-shot setting. Furthermore, we prove a one-shot analogue of covering lemma using smooth max Rényi divergence. We also propose one-shot achievable rate for source coding under maximum distortion criterion. This achievable rate is quantified in terms of smooth max Rényi divergence.

One-shot source coding with coded side information available at the decoder

One-shot achievable rate region for source coding when coded side information is available at the decoder (source coding with a helper) is proposed. The achievable region proposed is in terms of conditional smooth max Rényi entropy and smooth max Rényi divergence. Asymptotically (in the limit of large block lengths) this region is quantified in terms of spectral-sup conditional entropy rate and spectral-sup mutual information rate. In particular, it coincides with the rate region derived in the limit of unlimited arbitrarily distributed copies of the sources.

Non-asymptotic information theoretic bound for some multi-party scenarios

In the last few years, there has been a great interest in extending the information-theoretic scenario for the non-asymptotic or one-shot case, i.e., where the channel is used only once. We provide the one-shot rate region for the distributed source-coding (Slepian-Wolf) and the multiple-access channel. Our results are based on defining a novel one-shot typical set based on smooth entropies that yields the one-shot achievable rate regions while leveraging the results from the asymptotic analysis. Our results are asymptotically optimal, i.e., for the distributed source coding they yield the same rate region as the Slepian-Wolf in the limit of unlimited independent and identically distributed (i.i.d.) copies. Similarly for the multiple-access channel the asymptotic analysis of our approach yields the rate region which is equal to the rate region of the memoryless multiple-access channel in the limit of large number of channel uses.

Simple one-shot bounds for various source coding problems using smooth Rényi quantities

We consider the problem of source compression under three different scenarios in the one-shot (nonasymptotic) regime. To be specific, we prove one-shot achievability and converse bounds on the coding rates for distributed source coding, source coding with coded side information available at the decoder, and source coding under maximum distortion criterion. The one-shot bounds obtained are in terms of smooth max Rényi entropy and smooth max Rényi divergence. Our results are powerful enough to yield the results that are known for these problems in the asymptotic regime both in the i.i.d. (independent and identically distributed) and non-i.i.d. settings.

Communicating under channel phase uncertainty

For a multiple antenna equipped wireless system, a new signal constellation design and decoding strategy is proposed to combat errors in the phase estimate of the channel coefficients when the channel coefficient magnitudes can be estimated accurately. For the single antenna case, the proposed constellation is a superposition of phase-shift-keying (PSK) and pulse-amplitude-modulation (PAM) constellations, where at the receiver, the PSK symbols are decoded first followed by the PAM symbols. PSK is used to provide protection against phase errors, while PAM is used to increase the transmission rate using the knowledge of the magnitude of the channel coefficient. The performance of the proposed constellation is shown to be significantly better than the widely used QAM constellation in terms of the probability of error. Using similar ideas, for multiple antenna case, the signal design is a superposition of the codebook developed for non-coherent communication and multi-dimensional PAM.