M. Nicolaou

University of Bristol, Bristol, ENG, United Kingdom

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Publications (11)1.47 Total impact

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    M. Nicolaou, A. Doufexi, S. Armour, Y. Sun
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    ABSTRACT: Link Adaptation (LA), adopted by many emerging standards, such as WCDMA, LTE and WiMAX is a key element for providing improved data rates and performance guarantees under specified Quality of Service (QoS) requirements. Link Adaptation can dynamically adjust a number of transmission parameters, most often modulation and coding, to reflect the characteristics of the wireless link to improve throughput and maintain link reliability. Most LA algorithms require only an estimate of the signal-to-noise ratio (SNR) to select an appropriate PHY mode, relying on the assumption of an AWGN channel. For channels experiencing frequency selective fading however, the conventional approach fails to accurately represent the stochastic variability across independent channels, resulting in inaccurate prediction of the current channel state and failure in selecting appropriate transmission parameters. A new channel quality metric is proposed in this paper that attempts to estimate the likelihood of packet errors in a fading channel and adjust Modulation and Coding Scheme (MCS) selection across each fading realisation independently. Results show improved mode selection efficiency over the conventional LA approach with added flexibility, as this approach is independent of the channel environment, providing a universal solution to LA.
    Vehicular Technology Conference (VTC Spring), 2011 IEEE 73rd; 06/2011
  • M. Nicolaou, A. Doufexi, S. Armour, Y. Sun
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    ABSTRACT: The joint use of opportunistic scheduling and orthogonal frequency division multiple access (OFDMA) provide significant gains in environments of low mobility and scatter for which channel variations are low. The downside of opportunistic scheduling in multicarrier systems such as OFDMA, lies in the substantial uplink overhead required to feed back by the mobile stations (MSs) describing users' instantaneous link conditions. This study presents a novel approach towards multicarrier opportunistic scheduling for a multi-input multi-output configuration that promises major overhead reductions with only minor degradations on downlink throughput performance. The conventional channel quality indicator, in the form of instantaneous signal-to-noise ratio (SNR) is replaced by an alternative metric, in the form of the strongest cluster indices of each MS, which imposes a significantly reduced overhead. This study examines the performance of the proposes metric for random beamforming and examines the benefits in terms of throughput and feedback overhead over existing schemes. The results show that the proposed scheduling approach maintains performance within 1 dB of a full SNR based feedback scheme, while achieving feedback overhead reduction up to a factor of 24 and also providing improved fairness among users experiencing different channel gain levels.
    IET Communications 01/2011; 5(6):853-863. · 0.72 Impact Factor
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    M. Nicolaou, A. Doufexi, S. Armour, Yong Sun
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    ABSTRACT: Fourth Generation Networks will invariably adopt MIMO-OFDMA techniques, in order to cope with increased data rate demands and improved Quality of Service (QoS) requirements. OFDMA is a popular multiple access candidate, since it facilitates multi-user diversity by enabling two dimensional multiple access, in time and frequency domains. Random Beamforming enables exploitation of spatial multi-user diversity and spatial capacity multiplexing gain. Layered Random Beamforming is able to achieve further multiple access benefits by enabling different users to be scheduled on different spatial layers of the same time-frequency resource. This paper examines the QoS performance of SU-MIMO and MU-MIMO precoding schemes for a VoIP system under a realistic traffic and channel environment. A novel Link Adaptation algorithm operating under PER constraints for delay sensitive VoIP traffic that promises increased data rates, flexibility and improved QoS is proposed and examined in this paper.
    Personal Indoor and Mobile Radio Communications (PIMRC), 2010 IEEE 21st International Symposium on; 10/2010
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    ABSTRACT: This paper presents a link level analysis of the rate and energy efficiency performance of the LTE downlink considering the unitary codebook based precoding scheme. In a multi-user environment, appropriate radio resource management strategies can be applied to the system to improve the performance gain by exploiting multi-user diversity in the time, frequency and space domains and the gains can be translated to energy reduction at the base station. Several existing and novel resource scheduling and allocation algorithms are considered for the LTE system in this paper. A detailed analysis of the performance gain of different algorithms in terms of throughput, rate fairness, and power efficiency is presented.
    Vehicular Technology Conference Fall (VTC 2010-Fall), 2010 IEEE 72nd; 10/2010
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    ABSTRACT: Environmental issues and the need to reduce energy consumption for lowering operating costs have pushed power efficiency to become one of the major issues of current research in the field of wireless networks. This paper addresses a number of multiple input multiple output (MIMO) precoding and scheduling techniques across the PHY and MAC layers that can operate under a reduced link budget and collectively improve the transmit power efficiency of a base station, while maintaining the same levels of service. Different MIMO transmission and precoding schemes proposed for LTE, achieving varying degrees of multiuser diversity in both the time, frequency as well as the space domain, are examined. Several fairness-aware resource allocation algorithms are applied to the considered MIMO schemes and a detailed analysis of the tradeoffs between power efficiency and quality of service is presented. This paper explicitly examines the performance of a system serving real-time, VoIP traffic under different traffic loading conditions and transmit power levels. It is demonstrated that by use of efficient scheduling and resource allocation techniques significant savings in terms of consumed energy can be achieved, without compromising QoS.
    Journal of Communications and Networks 01/2010; 12(2):130-139. · 0.75 Impact Factor
  • M. Nicolaou, A. Doufexi, S. Armour, Y. Sun
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    ABSTRACT: Fourth generation networks will almost invariably adopt OFDMA (orthogonal frequency division multiple access) and MIMO (multiple input-multiple output) technologies, in order to meet high data rate and quality of service (QoS) requirements. The worldwide interoperability for microwave access (WiMAX) MAC Layer, which is based on the IEEE 802.16 standard, is designed to support a variety of applications, including voice and multimedia services. The problem of providing QoS in broadband wireless systems is one of managing radio resources effectively. Efficient scheduling algorithms that balance the QoS requirements of each application and user with the available radio resources need to be developed. This paper considers a number of scheduling policies concentrating on real-time voice over IP (VoIP) traffic. Via numerical results we show that the conventional notion of fairness fails to guarantee service for low latency applications such as VoIP for an increasing traffic load. We show better QoS is achieved via a greedy scheduling approach that manages to serve packets faster.
    Vehicular Technology Conference Fall (VTC 2009-Fall), 2009 IEEE 70th; 10/2009
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    ABSTRACT: Environmental issues and the need to reduce energy consumption for lowering operating costs have pushed power efficiency to become one of the major issues of current research in the field of wireless networks. The objective of the Green Radio research programme (Core 5) of Mobile VCE is to deliver reduced power consumption of radio access networks. This paper attempts to show that an efficient exploitation of multiple antenna techniques and multiuser diversity in both the time, frequency as well as the space domain can significantly ease the power requirements of a base station, whilst maintaining the same levels of service. Different MIMO transmission and preceding schemes proposed for LTE, achieving varying degrees of multiuser diversity are examined.
    Vehicular Technology Conference Fall (VTC 2009-Fall), 2009 IEEE 70th; 10/2009
  • Source
    M. Nicolaou, A. Doufexi, S. Armour, Y. Sun
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    ABSTRACT: Until recently, mobile communication tasks have been grouped into different layers in a hierarchical layer approach. In this conventional approach, layer-specific protocols are developed and optimised independently. As new wireless applications emerge, requiring high data rates and flexibility for supporting applications with significantly varying QoS requirements, it has become imperative that a higher level of communication between adjacent layers needs to be established. A unitary codebook based beamforming approach has been proposed for LTE that incorporates an improved cross-layer interface in order to improve service to users according to specified QoS criteria. This paper examines this beamforming approach under partial feedback conditions for different MIMO configurations. It is shown that non-obvious precoding matrix selection strategies and scheduling policies for the DLC, can achieve superior performance for LTE system, whilst meeting the QoS requirements of urgent users.
    Communications Workshops, 2009. ICC Workshops 2009. IEEE International Conference on; 07/2009
  • M. Nicolaou, A. Doufexi, S. Armour
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    ABSTRACT: OFDM is an attractive solution for the design of future wireless communications due to its robustness to dispersion in multipath environments. Additional diversity gains can be realised by OFDMA by exploiting not only the temporal fading, but also spectral fading, that can result in higher rates due to increased diversity. A scheduler exploiting multiuser diversity imposes strict feedback requirements as channel quality information is required for every user's feedback unit with a feedback rate at least equal to the coherence time of the channel. The instantaneous signal to noise ratio is the most commonly used metric to quantify users' channel conditions. In OFDMA, the overhead due to feedback transmission increases significantly due to the fact that signal to noise ratio is required in the frequency domain, for every subcarrier of the OFDM symbol, which can potentially mitigate any multiuser diversity gains. This paper proposes a reduced feedback OFDMA design that requires very limited feedback information on a clustered subcarrier basis, whilst maintaining high downlink rates via opportunistic beamforming with multiple weighting vector transmission from the base station. This scheme is shown to be quite robust, as it can be easily adapted to variable channel conditions without any significant increase in complexity.
    Vehicular Technology Conference, 2008. VTC 2008-Fall. IEEE 68th; 10/2008
  • Source
    Marios Nicolaou, Angela Doufexi, Simon Armour
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    ABSTRACT: Opportunistic beamforming (OB) relies on the transmission of Channel State Information (CSI) in the form of instantaneous Signal to Noise Ratio (SNR) from Mobile Stations (MSs) back to the Base Station (BS) for scheduling purposes that increase throughput and/or maintain resource allocation fairness. OB is employed in environments of low mobility and low scatter, artificially inducing channel fluctuations that can better exploit Multiuser diversity (MUD). Multiple antennas at the BS are randomly alter the channel's response and generate peaks in gain where users can be scheduled on, maximising system throughput. Additional gains are achieved by transmitting multiple weighting vectors from the BS, but their use can significantly increase the load of the feedback channel and mitigate MUD gains. Selective Multiuser diversity (SMUD) has been proposed for the original beamforming scheme as a technique that reduces feedback requirements substantially without any significant throughput degradations. This paper considers the joined use of multiple weights and SMUD in opportunistic beamforming, aiming to increase capacity while reducing feedback overhead. Results show that with an appropriate use of threshold levels, not only average throughput can be increased through the use of multiple weighting vectors but also a notable decrease in feedback load requirements compared to the conventional OB with SMUD design is achieved. The terms Mobile Station (MS) and User are used interchangeably in this paper, with the same meaning.
    Proceedings of the 68th IEEE Vehicular Technology Conference, VTC Fall 2008, 21-24 September 2008, Calgary, Alberta, Canada; 01/2008
  • C Han, M Nicolaou, A Doufexi, S Armour
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    ABSTRACT: This paper considers a number of techniques employed in opportunistic and layered random beamforming OFDMA schemes that aim to reduce the feedback required to perform scheduling whilst also minimising the throughput degradations resulting from this reduced throughput (due to the lack of full CSI at the Base Station (BS)). Frequency response correlation among subcarriers can be exploited to group adjacent subcarriers into clusters which can then be treated as a single unit for feedback purposes to reduce the total amount of feedback. Given that these grouped subcarriers are highly correlated, good throughput performance can be preserved. In addition, it is possible to schedule clusters of size greater that 1 with a single feedback bit per cluster, achieving the same rate growth as a full CSI aware scheme. The idea of transmitting multiple weights to increase diversity has also been developed for an OFDM/OFDMA scenario where different clusters pick different weighting vectors for transmission, resulting in an overall throughput increase at the expense of only a minor increase in feedback information.