Siu-Wai Ho

Indoor MIMO Visible Light Communications: Novel Angle Diversity Receivers for Mobile Users

This paper proposes two novel and practical designs of angle diversity receivers to achieve multiple-input-multiple-output (MIMO) capacity for indoor visible light communications (VLC). Both designs are easy to construct and suitable for small mobile devices. By using light emitting diodes for both illumination and data transmission, our receiver designs consist of multiple photodetectors (PDs), which are oriented with different inclination angles to achieve high-rank MIMO channels and can be closely packed without the requirement of spatial separation. Due to the orientations of the PDs, the proposed receiver designs are named pyramid receiver (PR) and hemispheric receiver (HR). In a PR, the normal vectors of PDs are chosen the same as the normal vectors of the triangle faces of a pyramid with equilateral N-gon base. On the other hand, the idea behind HR is to evenly distribute the PDs on a hemisphere. Through analytical investigation, simulations and experiments, the channel capacity and bit-error-rate (BER) performance under various settings are presented to show that our receiver designs are practical and promising for enabling VLC-MIMO. In comparison to induced link-blocked receiver, our designs do not require any hardware adjustment at the receiver from location to location so that they can support user mobility. Besides, their channel capacities and BER performance are quite close to that of link-blocked receiver. Meanwhile, they substantially outperform spatially-separated receiver. This study reveals that using angle diversity to build VLC-MIMO system is very promising.

On the Interplay Between Conditional Entropy and Error Probability

Fanos inequality relates the error probability of guessing a finitely-valued random variable X given another random variable Y and the conditional entropy of X given Y. It is not necessarily tight when the marginal distribution of X is fixed. This paper gives a tight upper bound on the conditional entropy of X given Y in terms of the error probability and the marginal distribution of X. A new lower bound on the conditional entropy for countably infinite alphabets is also found. The relationship between the reliability criteria of vanishing error probability and vanishing conditional entropy is also discussed. A strengthened form of the Schur-concavity of entropy which holds for finite or countably infinite random variables is given.

The Interplay Between Entropy and Variational Distance

The relation between the Shannon entropy and variational distance, two fundamental and frequently-used quantities in information theory, is studied in this paper by means of certain bounds on the entropy difference between two probability distributions in terms of the variational distance between them and their alphabet sizes. We also show how to find the distribution achieving the minimum (or maximum) entropy among those distributions within a given variational distance from any given distribution. These results are applied to solve a number of problems that are of fundamental interest. For entropy estimation, we obtain an analytic formula for the confidence interval, solving a problem that has been opened for more than 30 years. For approximation of probability distributions, we find the minimum entropy difference between two distributions in terms of their alphabet sizes and the variational distance between them. In particular, we show that the entropy difference between two distributions that are close in variational distance can be arbitrarily large if the alphabet sizes of the two distributions are unconstrained. For random number generation, we characterize the tradeoff between the amount of randomness required and the distortion in terms of variation distance. New tools for non-convex optimization have been developed to establish the results in this paper.

Private information retrieval for coded storage

Private information retrieval scheme for coded data storage is considered in this paper. We focus on the case where the size of each data record is large and hence only the download cost (but not the upload cost for transmitting retrieval queries) is of interest. We prove that the tradeoff between storage cost and retrieval/download cost depends on the number of data records in the system. We propose a class of linear storage codes and retrieval schemes, and derive conditions under which our schemes are error-free and private. Tradeoffs between the storage cost and retrieval costs are also obtained.

Privacy–Security Trade-Offs in Biometric Security Systems—Part II: Multiple Use Case

This is the second part of a two-part paper on the information theoretic study of biometric security systems. In this paper, the performance of reusable biometric security systems, in which the same biometric information is reused in multiple locations, is analyzed. The scenario in which the subsystems are jointly designed is first considered. An outer bound on the achievable trade-off between the privacy leakage of the biometric measurements and rates of keys generated at the subsystems is derived. A scheme that achieves the derived outer bound is then presented. Next, an incremental design approach is studied, in which the biometric measurements are reused while keeping the existing system intact. An achievable privacy-security trade-off region for this design approach is derived. It is shown that under certain conditions, the incremental design approach can achieve the performance of the joint design approach. Finally, examples are given to illustrate the results derived.

Indoor Position Tracking Using Multiple Optical Receivers

An indoor positioning system is a key component in enabling location-based services in future wireless networks. The need for a highly accurate indoor positioning system is rapidly increasing. In the past couple of years, several positioning systems based on visible light communications that achieve good positioning accuracy have been proposed. Some of these systems are based on assumptions such as complete knowledge of the height of the receiver, and exact alignment of the transmitter and receiver normals to the normal of the ceiling. Some other systems do not support user mobility because they require a user to vary the receiver orientation at a fixed location. Another common assumption is that the transmitters are at the same height from the ground. In order to support user mobility, this paper proposes a novel positioning system using multiple optical receivers that provides coordinates and an orientation of the mobile receiver. The remarkable features of the system are as follows: 1) the receiver can be mobile; 2) the positioning is done within 2.5 ms in our experimental setup; 3) the heights of the transmitters need not be the same; 4) the receivers height need not be known; and 5) the receivers normal need not be aligned with those of the transmitters. Experimental results show that mean position errors of less than 0.06 m is achievable even when the average receiver speed is 1.3 m/s.

Evolving small-cell communications towards mobile-over-FTTx networks

Small cell techniques are recognized as the best way to deliver high capacity for broadband cellular communications. Femtocell and distributed antenna systems (DASs) are important components in the overall small cell story, but are not the complete solution. They have major disadvantages of very limited cooperation capability and expensive deployment cost, respectively. In this article, we propose a novel mobile-over-FTTx (MoF) network architecture, where an FTTx network is enhanced as an integrated rather than a simple backhauling component of a new mobile network delivering low-cost and powerful small cell solutions. In part, the MoF architecture combines the advantages of femtocells and DASs, while overcoming their disadvantages. Implementation challenges and potential solutions are discussed. Simulation results are presented and demonstrate the strong potential of the MoF in boosting the capacity of mobile networks.

Position Modulating OFDM for optical wireless communications

Due to only the intensity of a signal is used to carry information, optical wireless systems are constrained to send real and positive values at the optical transmitter. Therefore, the conventional OFDM cannot be directly applied in optical systems. To combat multipath distortion, several modified OFDM systems have been studied, such as DC-biased optical OFDM (DCO-OFDM), asymmetrically clipped optical OFDM (ACO-OFDM), Flip-OFDM and Unipolar OFDM (U-OFDM), etc. In order to transmit real signal in optical environments, there is a Hermitian symmetric constraint with discrete Fourier transform (DFT), or discrete Hartley transform (DHT) without the Hermitian requirement. In this paper, we propose a novel OFDM system, namely Position Modulating OFDM (PM-OFDM), which utilizes DFT but removes the Hermitian constraint. Two receiver structures will be presented with different complexities and performance. The proposed PM-OFDM outperforms the existing systems in terms of bit-error-rate (BER) and complexity, with the same spectral efficiency.

Error-free perfect-secrecy systems

Shannons fundamental bound for perfect secrecy says that the entropy of the secret message U cannot be larger than the entropy of the secret key R shared by the sender and the legitimate receiver. Massey gave an information theoretic proof of this result and the proof does not require U and R to be independent. By adding an extra assumption that I(U; R) = 0, we show a tighter lower bound on H(R) by proving that the logarithm of the message sample size cannot be larger than the entropy of the secret key. Then we consider that a perfect secrecy system is used multiple times. A new parameter, namely effective key consumption, is defined and justified. This paper shows the existence of a fundamental tradeoff between the effective key consumption and the number of channel uses for transmitting a ciphertext.

On Information Divergence Measures and a Unified Typicality

Strong typicality, which is more powerful for theorem proving than weak typicality, can be applied to finite alphabets only, while weak typicality can be applied to countable alphabets. In this paper, the relation between typicality and information divergence measures is discussed. The new definition of information divergence measure in this paper leads to the definition of a unified typicality for finite or countably infinite alphabets which is stronger than both weak typicality and strong typicality. Unified typicality retains the asymptotic equipartition property and the structural properties of strong typicality, and it can potentially be used to generalize those theorems which are previously established by strong typicality to countable alphabets. The applications in rate-distortion theory and multisource network coding problems are discussed.