New article published in SPIE newsroom can be found here
The University of Edinburgh
The King's Buildings
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New article published in SPIE newsroom can be found here
I am currently in the process of filling two Research Fellow posts in the areas of VLC/OWC. The ideal candidates can demonstrate research in VLC and hold a strong publication record. Interested candidates are encouraged to submit their full application online at the following link (-> academics vacancies).
Here is a summary of the posts:
Research Fellow in Optical Wireless Communications Vacancy Ref : 010886 Department : Engineering, School Of Grade/Pay Scale : Ue07 (£30,424 - £36,298) Position Type : Full Time Date Posted : 22-Feb-2013 Closing Date : 27-Mar-2013 Employment Category : Fixed Term Research Fellow in Ultra-Parallel Visible Light Communications Vacancy Ref : 010887 Department : Engineering, School Of Grade/Pay Scale : Ue07 (£30,424 - £36,298) Position Type : Full Time Date Posted : 21-Feb-2013 Closing Date : 26-Mar-2013 Employment Category : Fixed Term
The 1st edition of Make Create Innovate airs tonight – 27.09.2012 – as part of Quest Means Business on CNN International at 19.00 – 20.00 BST.
from this evening. The exciting subject for this launch episode is Li-Fi – the future of communication, using light waves from LED lightbulbs.
W. Popoola, E. Poves, and H. Haas, “Spatial Pulse Position Modulation for Optical
Communications”, Journal of Lightwave Technology, Vol. PP (99) , pp. (to appear – pre-print)
Abstract: This paper proposes a low complexity spatial modulation (SM) scheme that combines spatial shift keying (SSK) with pulse position modulation (PPM) for optical wireless communication systems. SM is a multi-transmitter technique for achieving increased data rate over the traditional on-off keying (OOK) and PPM signalling methods. Analysis of the error performance of the system in the presence of noise is presented and validated via simulations. The error performance of SPPM is dictated by both the individual channel gains of the multiple transmitters and the difference between these channel gains or path losses. Hence, distinct channel gains are a prerequisite in spatial modulation. An experimental set up to measure and show the dependence of the channel gains on the relative position of the transmitter to the receiver is also presented. These measured channel parameters are then used to evaluate the system error performance. The performance of the SPPM is also compared, in terms of energy and spectral efficiencies, with the classical SSK and repetition coded (RC) schemes in which the multiple transmitters are used to transmit the same data simultaneously. The results show the SPPM as a multi-transmitter signalling scheme that combines the energy efficiency of the PPM with the high spectral efficiency of the SSK.
Brief description of SPPM: Assume a spectral efficiency of M = 4 bits/symbol, a number of LEDs, N_LED = 4, and 4-PPM. For example, symbol ‘5’ with binary representation ‘0101’ is transmitted as follows: the first log2(N_LED) most significant bits (2 in this case) determine the LED that is transmitting while the remaining bits determine the pulse position. Therefore, symbol ‘5’ is transmitted by LED 2 with a pulse in the second slot of the 4-PPM pulse pattern.
Mella, S.; Ghimire, B.; Reed C. M.; Haas, H.; “Energy Efficient Resource Allocation in OFDMA Networks using Ant-Colony Optimization” in Proc. International Symposium on Communications and Information Technologies (ISCIT), (Gold Coast, Australia), 2 -5 October (pre-print)
A novel technique for jointly allocating sub-carriers, modulation and coding scheme (MCS) and transmit power in an orthogonal frequency division multiple access (OFDMA) cellular network using ant-colony optimization technique is proposed. Different combinations of user indices, MCS indices and subcarrier indices form the nodes in the graph. Each possible assignment of the above resources is a path in the graph. Resource allocation is carried out by mimicking the behavior of ants, that the ants are likely to choose the path identified as a shortest path, which is indicated by a relatively higher density of pheromone left on the path compared to other possible paths. Likewise, the combination of resource allocation that satisfies the requested data rate with least energy consumption is preferentially allocated and therefore attains a high ‘pheromone’ level. As such, the aforesaid allocation is preferentially selected by an evolved nodeB (eNB). Also, the simulation results show 6 and 1.4 times increase in throughput, 15% increase in satisfied users and 17% decrease in transmit power when the proposed approach is compared against full chunk reuse system with link adaptation, chosen as a benchmark.
H. Burchardt, Z. Bharucha, G. Auer, and H. Haas, “ Uplink Interference Protection and Scheduling for Energy Efficient OFDMA Networks” in EURASIP Journal on Wireless Communications and Networking, to appear.
One of the key challenges for future orthogonal frequency division multiple access (OFDMA)-based networks is inter-cell interference coordination (ICIC). With full frequency reuse and small inter-site distances, coping with co-channel interference (CCI) in such networks has become increasingly important. Here, an uplink interference protection (ULIP) technique to combat CCI is introduced and investigated. The level of uplink interference originating from neighbouring cells (affecting co-channel mobile stations (MSs) in the cell of interest) can be effectively controlled by reducing the transmit power of the interfering MSs. This is done based on the target signal-to-noise-plus-interference ratio (SINR) and tolerable interference of the vulnerable link. Bands are prioritised in order to differentiate those (vulnerable/victim) MSs that are to be protected from interference and those (aggressor/interfering MSs) that are required to sacrifice transmission power to facilitate the protection. Furthermore, MSs are scheduled such that those users with poorer transmission conditions receive the highest interference protection, thus balancing the areal SINR distribution and creating a fairer allocation of the available resources.
In addition to interference protection, the individual power reductions serve to decrease the total system uplink power, resulting in a greener system. This is of great significance for future networks, in which energy efficiency (especially at the mobile-side, where battery lifetimes are struggling to keep up with enhanced data applications) will play a huge role in the development of wireless technologies. It is shown through analytic derivation that the introduction of ULIP guarantees an increase in energy efficiency for all MSs in the network, and hence also a substantial improvement for the system as a whole. Furthermore, it is shown that the system capacity may be augmented depending on the locations and channel-states of individual users. Extensive system level simulations validate these findings.
a pre-print of the paper can be found here.
In this article, self-organising interference management for optical wireless networks deployed inside an aircraft cabin is investigated. A user that has received data in a given frame and intends to continue receiving data in the next frame broadcasts a busy burst (BB) in a time-multiplexed BB slot. The tagged access point (AP) intending to reuse a resource reserved in a neighbouring cell must listen to the BB slot. Provided that channel reciprocity holds, the tagged AP infers (prior to transmission) the amount of co-channel interference (CCI) potentially caused towards the victim user in neighbouring cell. This is a vital information for an AP to decide without any central supervision whether to transmit or defer the transmission to another time or frequency slot so as to limit CCI caused to the active link to a threshold value. Simulation results demonstrate that the BB approach significantly improves both fairness and spectral efficiency in the system compared to a static resource partitioning approach.
The paper can be downloaded here
Dimitrov, S.; Sinanovic, S.; and Haas, H., “Signal Shaping and Modulation for Optical Wireless Communication”, IEEE Journal of Lightwave Technology, (to appear)
The data transmission in optical wireless communication (OWC) systems is achieved through intensity modulation and direct detection (IM/DD). The information-carrying signal modulates the intensity of a light emitting diode (LED) at the transmitter, and it is detected by a photodiode (PD) at the receiver. For this purpose, the signal needs to be real-valued and positive. Suitable candidates for data modulation are the single-carrier pulse modulation schemes such as multi-level pulse position modulation (M-PPM) and multi-level pulse amplitude modulation (M-PAM) [1,2]. In addition, multi-carrier modulation such as multi-level quadrature amplitude modulation with orthogonal frequency division multiplexing (M-QAM OFDM) can be used to achieve high data rates. In order to ensure that the time domain signal is real-valued, Hermitian symmetry within the OFDM frame is applied. Unipolarity is generally achieved through direct-current (DC) biasing, e.g. DC-biased optical OFDM (DCO-OFDM) . An alternative approach employs a structure on the OFDM frame which allows the zero-level time domain signal clipping without any loss of information, yet reducing the spectral efficiency by 50%, e.g. asymmetrically clipped optical OFDM (ACO-OFDM) .
In a realistic communication scenario, the signal is passed through an LED with a non-linear transfer characteristic. Furthermore, a linear optical wireless channel with a finite frequency response attenuates the signal, and additive white Gaussian noise (AWGN) is added at the receiver. The LED characteristic can be linearized through predistortion between positive minimum and maximum levels of radiated optical power. A consistent framework for optical-to-electrical conversion enables the comparison between the single-carrier and multi-carrier modulation formats in terms of spectral efficiency for a given signal-to-noise ratio (SNR). While the uniform single-carrier pulsed signals fit within the dynamic range of the transmitter, the Gaussian OFDM signals require optimum pre-clipping. The systems apply different approaches to minimize the channel effect on the transmitted signal. In practical implementations, single-carrier transmission employs a linear feed-forward equalizer (FFE) or a non-linear decision feedback equalizer (DFE) with zero forcing (ZF) or minimum mean squared error (MMSE) criteria. OFDM benefits from a cyclic prefix (CP) extension of the time domain signal, and through bit and power leading at the transmitter side the computational effort is reduced to single-tap linear FFE with ZF or MMSE criteria.
For a practical dynamic range of the transmitter front-end, the systems are compared in a novel fashion in terms of electrical SNR requirement and spectral efficiency in the dispersive optical wireless channel. In visible light communication (VLC) systems, the additional DC bias power required to create a non-negative signal can serve a complementary functionality, such as illumination. Therefore, it can be excluded in the calculation of the electrical SNR. In infrared (IR) communication, however, the DC power is generally constrained by eye-safety regulations, and it is included in the calculation of the electrical SNR. When the additional DC bias power is neglected, DCO-OFDM and PAM show the greatest spectral efficiency for a flat fading channel in the SNR region above 6.8 dB. However, since optical OFDM with bit and power loading suffers a lower SNR penalty than PAM with DFE as the signal bandwidth exceeds the coherence bandwidth of the dispersive optical wireless channel, DCO-OFDM demonstrates a superior spectral efficiency. When the DC bias power is counted towards the electrical signal power, DCO-OFDM and ACO-OFDM suffer a greater SNR penalty due to the DC bias as compared to PAM and PPM, respectively. However, the presented optimum signal shaping framework enables O-OFDM to greatly reduce this penalty and minimize the gap to single-carrier transmission within 2 dB in the flat fading channel. When the signal bandwidth exceeds the channel coherence bandwidth, DCO-OFDM outperforms PAM with FFE, and it approaches the spectral efficiency of the more computationally intensive PAM with DFE, while ACO-OFDM outperforms PPM with FFE and DFE.
 J. M. Kahn and J. R. Barry, “Wireless Infrared Communications”, Proceedings of the IEEE, vol. 85, no. 2, pp. 265–298, Feb. 1997.
 J. G. Proakis, Digital Communications, 4th ed., ser. McGraw-Hill Series in Electrical and Computer Engineering, S. W. Director, Ed. McGraw-Hill Higher Education, December 2000.
 J. B. Carruthers and J. M. Kahn, “Multiple-subcarrier Modulation for Nondirected Wireless Infrared Communication”, IEEE Journal on Selected Areas in Communications, vol. 14, no. 3, pp. 538–546, Apr. 1996.
 J. Armstrong and A. Lowery, “Power Efﬁcient Optical OFDM”, Electronics Letters, vol. 42, no. 6, pp. 370–372, Mar. 16, 2006.