Chung Shue Chen

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.

Shift-Invariant Protocol Sequences for the Collision Channel Without Feedback

The authors consider collision channel without feedback in which collided packets are considered unrecoverable. For each user, the transmission of packets follows a specific periodical pattern, called the protocol sequence. Due to the lack of feedback, the beginning of the protocol sequences cannot be synchronized and nonzero relative offsets are inevitable. It results in variation of throughput. In this paper, we investigate optimal protocol sequence sets, in the sense that the throughput variance is zero. Such protocol sequences are said to be shift-invariant (SI). The characterizing properties of SI protocol sequences are presented. We also prove that SI sequences are identifiable, meaning that the receiver is able to determine the sender of each successfully received packet without any packet header. A general construction of SI sequences that meets the lower bound on sequence length is given. Besides, we study the least periods of SI sequences, and show that the least periods must be distinct in some cases. The throughput performance is compared numerically with other protocol sequences.

On Small Cell Network Deployment: A Comparative Study of Random and Grid Topologies

Small cell network is designed to provide mobile services to hot spots by deploying a large number of small access points (APs). As traditional network deployment requires costly AP location acquisition, cost-effective network deployment is necessary for small cell networks. We investigate this question by studying the network performance in terms of spatial outage and throughput of a completely random topology in comparison to that of a perfectly regular topology. Using a stochastic geometry model of user SINR in a random topology, our results show that the performance gap in terms of user SINR guarantee becomes narrow when the network density increases during the network densification. By a massive deployment, the loss is about 1 dB. Besides, it is at about 18% loss in user average throughput. These comparative results would provide helpful information to choose an appropriate deployment. In particular, as far as this relatively small performance loss can be compensated by other network control algorithms, the massive random deployment of a small cell network becomes attractive considering the cost reduction by the given deployment freedom.

Optimal power allocation for two-cell sum rate maximization under minimum rate constraints

In this paper, we analyze the sum rate maximization problem in a two-cell wireless system under peak power and minimum service rate constraints. The optimal power allocation is found to be binary, in the sense that each base station power will have one out of only two possible discrete levels, and the result can greatly reduce the optimization and power control complexity required. A closed-form solution of the optimal assignment is reported. The new result provides a generalization of the binary power control (BPC) proposed in. It is possible to apply the new result under the two-cell structure when clustering a larger network into groups of two cells. In a comparison with the full power transmission, it is shown that the proposed scheme outperforms in both the outage probability and sum rate performance generally.

Design and construction of protocol sequences: Shift invariance and user irrepressibility

Protocol sequences are used for channel access in the collision channel without feedback. Each user is assigned a deterministic zero-one pattern, called protocol sequence. The zeros and ones in a protocol sequence are read out periodically, and a packet is sent if and only if it is one. A collision occurs if two or more users transmit at the same time. Due to the lack of feedback from the receiver and cooperation among users, the beginning of the protocol sequences cannot be synchronized and relative delay offsets are incurred.We study the design of protocol sequences from two different perspectives. Under the first one, called shift invariance, we aim at minimizing the fluctuation of throughput due to relative delay offsets. As for the second one, called user irrepressibility, we want to guarantee that each user can send at least one packet successfully in each period. For both design criteria, we derive a lower bound on sequence period and give an optimal construction that achieves this lower bound.

Distributed power control for time varying systems: performance and convergence analysis

This paper deals with a class of power control problems where the system link gains are assumed to be time varying and the signal-to-interference ratio (SIR) estimates are allowed to be corrupted with bounded noises. A simple control algorithm is devised by applying a distributed, fixed step approach. It is a feedback algorithm that requires only local information. By modifying the distributed, fixed step power control algorithm proposed by Sung and Wong, we obtain here a more robust version that can handle time varying link gains and measurement noises. The convergence property of the new algorithm is established and simulation studies were carried out to show that it is effective.

Bandwidth allocation for wireless multimedia systems with most regular sequences

In integrated wireless multimedia service, isochronous traffic of different connections can be scheduled by using a most regular binary sequence (MRBS). Such a sequence schedules traffic in an evenly spaced manner to achieve any arbitrary rate asymptotically and while avoiding excessive delay or buffering requirement. Flexible slot assignment that can match requests exactly improves bandwidth efficiency in multirate operations. The most regular binary sequence provides a distributed solution for multiaccess control that is based on limited information exchange. As a generalization, the concept of a most regular code sequence (MRCS) is proposed to support variable rate transmission in wideband code division multiple access (CDMA) systems and to provide spreading factor (SF) optimization. This scheme improves channel utilization efficiency in supporting traffics of various classes and hence results in an overall capacity gain.

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.

Constructions of Robust Protocol Sequences for Wireless Sensor and Ad hoc Networks

A class of periodic unipolar binary sequences is investigated for their potential applications in defining new protocols for distributed multiple accessing. Based on linear congruence sequences, one can show that, for any finite subset of these sequences, with the total proportional rate not exceeding a specific threshold, there cannot be enough collisions to completely block any particular sequence, no matter how they are shifted with respect to one another. This property can be exploited in certain applications, such as wireless sensor and ad hoc networks. A further investigation into how to enhance the allowable rate sum is conducted. New protocol sequences with interesting and useful properties are accordingly designed.

Data sharing coordination and blind interference alignment for cellular networks

We consider coordination in a multi-user multiple input single output cellular system. In contrast with existing base station cooperation methods that rely on sharing CSI with or without user data to manage interference, we propose to share user data only. We consider a system where blind interference alignment (BIA) is applied to serve multiple users in each cell. We apply interference coordination through data sharing to mitigate other-cell interference at the cell-edge users. While BIA mitigates intra-cell interference in MU-MISO systems, it does not address the problem of inter-cell interference. We apply interference coordination through data sharing to mitigate inter-cell interference at the cell-edge users. We propose a new cooperative BIA scheme that takes into account the users whose data is being shared between adjacent base stations. We derive the achievable sum rate with interference mitigation and we compare it to achievable rates with the original BIA strategy. Numerical results show that the achievable sum rate of the cell-edge users with data sharing decreases with increasing number of served users in each cell and increasing number of antennas at the base stations.