Timothy O'Farrell

An Enhanced Color Shift Keying Modulation Scheme for High-Speed Wireless Visible Light Communications

This paper presents a new color shift keying (CSK) modulation format for wireless visible light communication (VLC), based on four colors instead of the three colors used in the existing IEEE 802.15.7 CSK physical layer standard. The new four color system uses a novel intensity modulation and direct detection approach to realize a four-dimensional signaling scheme that uses the available color and signal spaces efficiently. The bit error rate evaluation of both the existing and proposed system shows that the new four color scheme achieves a significant 4.4-dB electrical SNR gain over the three color scheme for an additive white Gaussian noise channel. The performance of existing and proposed CSK systems is examined over a range of dispersive optical wireless channels including the channel crosstalk and insertion losses, which reveals that the four color CSK scheme is more power efficient and reliable than the three color scheme for a particular amount of delay spread that the optical wireless channel may have.

Relay Deployment in Cellular Networks: Planning and Optimization

This paper presents closed-form capacity expressions for interfere-limited relay channels. Existing theoretical analysis has primarily focused on Gaussian relay channels, and the analysis of interference-limited relay deployment has been confined to simulation based approaches. The novel contribution of this paper is to consolidate on these approaches by proposing a theoretical analysis that includes the effects of interference and capacity saturation of realistic transmission schemes. The performance and optimization results are reinforced by matching simulation results. The benefit of this approach is that given a small set of network parameters, the researcher can use the closed-form expressions to determine the capacity of the network, as well as the deployment parameters that maximize capacity without committing to protracted system simulation studies. The deployment parameters considered in this paper include the optimal location and number of relays, and resource sharing between relay and base-stations. The paper shows that the optimal deployment parameters are pre-dominantly a function of the saturation capacity, pathloss exponent and transmit powers. Furthermore, to demonstrate the wider applicability of the theoretical framework, the analysis is extended to a multi-room indoor building. The capacity improvements demonstrated in this paper show that deployment optimization can improve capacity by up to 60% for outdoor and 38% for indoor users. The proposed closed-form expressions on interference-limited relay capacity are useful as a framework to examine how key propagation and network parameters affect relay performance and can yield insight into future research directions.

The performance of optical wireless OOK, 2-PPM and spread spectrum under the effects of multipath dispersion and artificial light interference

Indoor wireless infrared transmission systems are mainly affected by ambient light-induced shot noise, multipath dispersion and the interference produced by artificial light sources. In this paper, spread spectrum techniques are proposed to combat multipath dispersion and artificial light interference. The bit error rate performance of the spread spectrum technique under the combination of these detrimental effects are studied and compared with that of OOK and 2-PPM. A novel analysis is derived to provide a simplified common analytical platform to facilitate the comparison among the three modulation schemes. Bit error rate analyses for OOK, PPM and spread spectrum taking into account the combined effects of multipath dispersion and artificial light interference are derived for the first time. Analytical, simulation and experimental results of bit error rate versus optical signal-to-noise ratio are then presented. The results show that spread spectrum techniques are very robust to multipath dispersion and artificial light interference. Copyright

Dynamic Cell Expansion with Self-Organizing Cooperation

This paper addresses the challenge of how to reduce the energy consumption of a multi-cell network under a dynamic traffic load. The body of investigation first shows that the energy reduction upper-bound for transmission improving techniques is hardware-limited, and the bound for infrastructure reduction is capacity-limited. The paper proposes a novel cell expansion technique, where the coverage area of cells can expand and contract based on the traffic load. This is accomplished by switching off low load cell-sites and compensating for the coverage loss by expanding the neighboring cells through antenna beam tilting. The multi-cell coordination is resolved by using either a centralized controller or a distributed self-organizing-network (SON) algorithm. The analysis demonstrates that the proposed distributed algorithm is able to exploit flexibility and performance uncertainty through reinforced learning and improves on the centralized solution. The combined energy saving benefit of the proposed techniques is up to 50% compared to a reference deployment and 44% compared with alternative state-of-the-art dynamic base-station techniques.

Dynamics of glacier calving at the ungrounded margin of Helheim Glacier, southeast Greenland

Abstract During summer 2013 we installed a network of 19 GPS nodes at the ungrounded margin of Helheim Glacier in southeast Greenland together with three cameras to study iceberg calving mechanisms. The network collected data at rates up to every 7 s and was designed to be robust to loss of nodes as the glacier calved. Data collection covered 55 days, and many nodes survived in locations right at the glacier front to the time of iceberg calving. The observations included a number of significant calving events, and as a consequence the glacier retreated ~1.5 km. The data provide real‐time, high‐frequency observations in unprecedented proximity to the calving front. The glacier calved by a process of buoyancy‐force‐induced crevassing in which the ice downglacier of flexion zones rotates upward because it is out of buoyant equilibrium. Calving then occurs back to the flexion zone. This calving process provides a compelling and complete explanation for the data. Tracking of oblique camera images allows identification and characterisation of the flexion zones and their propagation downglacier. Interpretation of the GPS data and camera data in combination allows us to place constraints on the height of the basal cavity that forms beneath the rotating ice downglacier of the flexion zone before calving. The flexion zones are probably formed by the exploitation of basal crevasses, and theoretical considerations suggest that their propagation is strongly enhanced when the glacier base is deeper than buoyant equilibrium. Thus, this calving mechanism is likely to dominate whenever such geometry occurs and is of increasing importance in Greenland.

Spread spectrum techniques for indoor wireless IR communications

Multipath dispersion and fluorescent light interference are two major problems in indoor wireless infrared communications systems. Multipath dispersion introduces intersymbol interference at data rates above 10 Mb/s, while fluorescent light induces severe narrowband interference to baseband modulation schemes commonly used such as OOK and PPM. This article reviews the research into the application of direct sequence spread spectrum techniques to ameliorate these key channel impairments without having to resort to complex signal Processing techniques. The inherent properties of a spreading sequence are exploited in order to combat the ISI and narrowband interference. In addition, to reducing the impact of these impairments, the DSSS modulation schemes have strived to be bandwidth-efficient and simple to implement. Three main DSSS waveform techniques have been developed and investigated. These are sequence inverse keying, complementary sequence inverse keying, and M-ary biorthogonal keying (MBOK). The operations of the three systems are explained; their performances were evaluated through simulations and experiments for a number of system parameters, including spreading sequence type and length. By comparison with OOK, our results show that SIK, CSIK, and MBOK are effective against multipath dispersion and fluorescent light interference because the penalties incurred on the DSSS schemes are between 0-7 dB, while the penalty on OOK in the same environment is more than 17 dB. The DSSS solution for IR wireless transmission demonstrates that a transmission waveform can be designed to remove the key channel impairments in a wireless IR system.

Throughput and Delay Analysis for p-Persistent CSMA with Heterogeneous Traffic

CSMA/CA is a common access mechanism for contention based wireless channels. One of the ways in which quality of service in contention based wireless networks is achieved is by service differentiation. This can be realised by prioritising packets based on their importance while taking into account the network load. p-persistent CSMA is a form of CSMA/CA and in this paper an analytical model for the throughput and delay of a p-persistent CSMA system is developed where the system contains a finite number of non-saturated stations with both heterogeneous loads and priorities. Results based on this new model show how the throughput and delay for the traffic flows on the network are affected by the relative p-persistence and offered load values for all of the stations in the network.

Performance evaluation of IEEE 802.15.7 CSK physical layer

Colour Shift Keying (CSK) modulates the intensity of the multi-colour light emitting diodes (LEDs) for information transmission in visible light communication (VLC). Currently, no work has been published which details the performance of this standardised modulation scheme that can be used by the CSK developers as a baseline. In this work, the various colour band combinations (CBCs) of multilevel CSK are studied and their performance over an additive white Gaussian noise (AWGN) and the dispersive optical wireless channel is detailed. The analysis of the performance evaluation suggests that CSK has a spread of BER performance, i.e. all the colour band combinations do not have the same BER performance. For the AWGN channel, CBC-2 (CBC no. 2) has an electrical SNR gain of up to 2.5 dB over the other eight CBCs of CSK. The performance evaluation suggests that the CBC-2 is also the most robust CBC of CSK to be used over the dispersive optical channel.

Capacity-Energy-Cost Tradeoff in Small Cell Networks

Wireless communications has been recognized as a key enabler to the growth of the future economy. There is an unprecedented growth in data volume and the associated energy consumption. The challenge addressed in this paper is how to meet the growth in data traffic, whilst reducing both the cost and energy consumed. The paper shows that small cell deployments can significantly reduce energy consumption (30%), but increase the network cost (14%). The novel characterization of the tradeoff between Capacity, Energy and Cost (CEC) is of importance to researchers and operators.

A High-Resolution Sensor Network for Monitoring Glacier Dynamics

This paper provides an overview of a wide area wireless sensor network that was deployed on the calving front of the Helheim Glacier in Greenland during the summer of 2013. The purpose of the network was to measure the flow rate of the glacier using accurate satellite positioning data. The challenge in this extreme environment was to collect data in real time at the calving edge of the glacier. This was achieved using a solar powered 2.4-GHz Zigbee wireless sensor network operated in a novel hybrid cellular/mesh access architecture consisting of ice nodes communicating with base stations placed on the rock adjacent to the glacier. This highly challenging transmission environment created substantial signal outage conditions, which were successfully mitigated by a radio network diversity scheme. The network development and measurement campaign were highly successful yielding significant results on glacial dynamics associated with climate change.