D.K.C. So

The University of Manchester, Manchester, England, United Kingdom

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Publications (45)39.97 Total impact

  • IEEE Journal on Selected Areas in Communications 01/2015; DOI:10.1109/JSAC.2015.2435351 · 4.14 Impact Factor
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    ABSTRACT: Characterizing the fundamental energy efficiency (EE) performance of multiple-input multiple-output interfering broadcast channels (MIMO-IFBC) is important for the design of green wireless system. In this paper, we propose a new network architecture proposition based on EE maximization for multicell MIMO-IFBC within the context of interference alignment (IA). Particularly, EE is maximized subject to maximum power and minimum throughput constraints. We propose two schemes to optimize EE for different signal-to-noise ratio (SNR) regions. For high SNR operating regions, we employ a grouping-based IA scheme to jointly cancel intra- and inter-cell interferences and thus transform the MIMO-IFBC to a single-cell MIMO scenario. A gradient-based power adaptation scheme is proposed based on water-filling power adaptation and singular value decomposition (SVD) to maximize EE for each cell. For moderate SNR cases, we propose an approach using dirty paper coding (DPC) with the principle of multiple access channel (MAC) - broadcast channel (BC) duality to perform IA whilst maximizing EE in each cell. The algorithm in its dual form is solved using a sub-gradient method and bisection searching scheme. Simulation results demonstrate the superior performance of the proposed schemes over several existing approaches. It also shows that interference nulling based IA approaches outperform hybrid DPC-IA approach in high SNR region and the opposite occurs in low SNR region.
    IEEE Transactions on Communications 01/2015; DOI:10.1109/TCOMM.2015.2442246 · 1.98 Impact Factor
  • Fahimeh Jasbi, Daniel Ka Chun So
    IEEE Transactions on Vehicular Technology 01/2015; DOI:10.1109/TVT.2015.2417195 · 2.64 Impact Factor
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    ABSTRACT: As 4G wireless networks are vastly and rapidly deployed worldwide, 5G with its advanced vision of all connected world and zero distance communications is already at the corner. Along with the super quality of user experience brought by these new networks, the shockingly increasing energy consumption of wireless networks has become a worrying economic issue for operators and a big challenge for sustainable development. Green Transmission Technologies (GTT) is a project focusing on the energy-efficient design of physical-layer transmission technologies and MAC-layer radio resource management in wireless networks. In particular, fundamental tradeoffs between spectrum efficiency and energy efficiency have been identified and explored for energy-efficiency-oriented design and optimization. In this article, four selected GTT solutions are introduced, focusing on how they utilize the degrees of freedom in different resource domains, as well as how they balance the tradeoff between energy and spectrum efficiency. On top of the elaboration of separated solutions, the GTT toolbox is introduced as a systematic tool and unified simulation platform to integrate the proposed GTT solutions together.
    IEEE Communications Magazine 11/2014; 52(11):112-120. DOI:10.1109/MCOM.2014.6957151 · 4.46 Impact Factor
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    ABSTRACT: Spectral efficiency (SE) and energy efficiency (EE) are the main metrics for designing wireless networks. Rather than focusing on either SE or EE separately, recent works have focused on the relationship between EE and SE and provided good insight into the joint EE–SE tradeoff. However, such works have assumed that the bandwidth was fully occupied regardless of the transmission requirements and therefore are only valid for this type of scenario. In this paper, we propose a new paradigm for EE–SE tradeoff, namely the resource efficiency (RE) for orthogonal frequency division multiple access (OFDMA) cellular network in which we take into consideration different transmission-bandwidth requirements. We analyse the properties of the proposed RE and prove that it is capable of exploiting the tradeoff between EE and SE by balancing consumption power and occupied bandwidth; hence simultaneously optimizing both EE and SE. We then formulate the generalized RE optimization problem with guaranteed quality of service (QoS) and provide a gradient based optimal power adaptation scheme to solve it. We also provide an upper bound near optimal method to jointly solve the optimization problem. Furthermore, a low-complexity suboptimal algorithm based on a uniform power allocation scheme is proposed to reduce the complexity. Numerical results confirm the analytical findings and demonstrate the effectiveness of the proposed resource allocation schemes for efficient resource usage.
    IEEE Transactions on Wireless Communications 08/2014; 13(8):4656-4669. DOI:10.1109/TWC.2014.2316791 · 2.76 Impact Factor
  • Warit Prawatmuang, Daniel K.C. So, Emad Alsusa
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    ABSTRACT: Cognitive radio opportunistically accesses the spectrum while the licensed user is idle. A spectrum sensing procedure to monitor primary users' existence is therefore vital to cognitive radios. In this paper, we investigate the energy detection based sequential cooperative spectrum sensing technique in time varying channels. By utilizing past local observations from previous sensing slots, cognitive radio nodes can aggregate the current and previously received energy values to improve the detection performance. We propose the weighted sequential energy detector (SED) in which a fixed number of past observations are taken for decision making. In addition, we propose two adaptive schemes, namely the Two-Stage SED and the Differential SED, where a cognitive radio user uses previously received energy values until it detects a change in primary user's activity. The probability of false alarm and detection for the Weighted SED and Two-Stage SED schemes are derived. The results show that the proposed schemes achieve better detection performance over the conventional cooperative energy detection technique. In particular, the Two-Stage SED and Differential SED schemes can satisfy the IEEE 802.22 standard's requirement of 10% false alarm and 90% detection in scenarios that conventional techniques cannot accommodate.
    IEEE Transactions on Wireless Communications 06/2014; 13(6):3394-3405. DOI:10.1109/TWC.2014.051314.131404 · 2.76 Impact Factor
  • Fahimeh Jasbi, Daniel K.C. So
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    ABSTRACT: This paper proposes the use of an overloaded hybrid MC-CDMA system to improve the spectral efficiency of Cognitive Radio (CR) network. The overlay transmission in the unused parts of the spectrum is full-loaded MC-CDMA signals with scrambling. The underlay signal, using a separate scrambling code from the overlay signals, utilizes the whole spectrum taking into account of the interference threshold of the Primary User (PU). On the receiver side, chip-level MMSE is used for overlay detection, interference reconstruction and cancellation. Symbollevel MMSE is then used to detect the underlay. Since overlay is performing chip-level equalization and data detection, there is no need for other users' signatures. Therefore, the proposed system does not add complexity to the overlay system in comparison with the conventional MMSE receivers. Simulation results show that the underlay transmission does not affect the overlay performance while achieving good performance for different PU occupancy levels. It is also shown that it is less sensitive to PU interference level and significantly outperforms previous methods.
    ICC 2014 - 2014 IEEE International Conference on Communications; 06/2014
  • W. Pramudito, E. Alsusa, D.K.C. So
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    ABSTRACT: This paper proposes a new pairing cooperative technique to achieve spatial diversity and constructive interference exploitation in uplink orthogonal frequency-division multiple access (OFDMA) systems. The underlying principle is to pair users such that their symbols are combined into a single constellation, according to a predefined rule, so that the same symbol is transmitted by each pair of users so their signals can be coherently summed at the receiver, hence maximizing the received signal power. To achieve such constructive combination of both signal copies, single-dimensional OFDMA is utilized in the inphase and quadrature dimensions. It will be shown that such transmission method not only maximizes the received power but also greatly reduces sensitivity to timing misalignment between the cooperating users. Both time synchronous and asynchronous transmission scenarios will be considered. In the asynchronous case, interference reduction will be applied at the base station to reinstate the pairs' orthogonality and hence minimize co-channel interference. The performance of this technique is evaluated through simulations and mathematical analysis. It will be shown that the results from both models closely match and that the proposed technique provides significant bit-error-rate reductions, particularly at low signal-to-noise ratio, and throughput improvements relative to existing cooperative transmission techniques.
    IEEE Transactions on Communications 01/2014; 62(12):4255-4268. DOI:10.1109/TCOMM.2014.2364595 · 1.98 Impact Factor
  • Fahimeh Jasbi, Daniel K.C. So, Emad Alsusa
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    ABSTRACT: Cognitive Radio (CR) aims to access the spectrum opportunistically mainly by two access strategies, overlay and underlay. Overlay can utilize the spectrum whenever the primary system is off. On the other hand, underlay can utilize the spectrum at any time with considering the interference limit of the primary system. However, by using hybrid methods and performing proper equalization technique, spectrum can be utilized more efficiently. In this paper, we propose a hybrid MC-CDMA system which utilizes the whole spectrum for underlay transmission rather than solely the occupied parts. Due to the interference rejection capability, MC-CDMA can minimize the interference from the Primary User (PU) and exploit more diversity. Furthermore, overlay utilizes the spectrum holes while maintaining orthogonality to the underlay by Orthogonal Variable Spreading Factor (OVSF) codes. Chip and symbol- level MMSE-based modified equalizers are proposed for the system and the underlay performance of the system is evaluated. Simulation results show that BER performance improves significantly with the modified MMSE equalizers and for all PU occupancy levels.
    GLOBECOM 2013 - 2013 IEEE Global Communications Conference; 12/2013
  • Sarmad Sohaib, D.K.C. So
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    ABSTRACT: Cooperative diversity exploits the broadcast nature of wireless channels and uses relays to improve link reliability. Most of the cooperative communication protocols are assumed to be synchronous in nature, which is not always possible in vehicle-to-vehicle (V2V) communication due to fast moving nature of the nodes. Also the relay nodes are assumed to be half duplex which in turn reduces the spectral efficiency. In this paper, we propose an asynchronous cooperative communication protocol exploiting polarization diversity, which does not require synchronization at the relay node. Dual polarized antennas are employed at the relay node to achieve full duplex amplify-and-forward (ANF) communication. Hence the transmission duration is reduced which results into an increased throughput rate. Capacity analysis of the proposed scheme ascertains the high data rate as compared to conventional ANF. Bit error rate (BER) simulation also shows that the proposed scheme significantly outperforms both the non-cooperative single-input single-output and the conventional ANF schemes. Considering channel path loss, the proposed scheme consumes less total transmission energy as compared to the conventional ANF and non-cooperative scheme. Thus the proposed scheme is suitable for high rate and energy efficient relay-enabled communication.
    IEEE Transactions on Communications 05/2013; 61(5):1732-1738. DOI:10.1109/TCOMM.2013.031213.120008 · 1.98 Impact Factor
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    ABSTRACT: Data traffic demand in cellular networks continues to increase exponentially leading to high capital expenditure and operational costs such as electricity cost. Femtocells have been proposed as a solution to enhance network capacity without significantly increasing energy consumption and associated network deployment costs. In this paper, we analyze the network capacity and energy consumption aspects of a joint macrocell and femtocell network based on the LTE standard. Femtocells and macrocells are operated in the same frequency band to enhance spectral efficiency. Femtocell users are secondary users who only access the channel opportunistically when macrocell users are idle. We propose the use of cooperative spectrum sensing to manage adverse interference emanating from the co-channel operation of both macrocells and femtocells. Simulation results show that cooperative sensing is very effective at managing this cross-tier interference to maximize the throughput of both the macrocell and femtocell layers. This throughput maximization allows the lowest energy consumption ratio of such a heterogeneous network.
    Personal Indoor and Mobile Radio Communications (PIMRC), 2013 IEEE 24th International Symposium on; 01/2013
  • Warit Prawatmuang, Daniel K.C. So
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    ABSTRACT: Cognitive Radio (CR) is proposed to maximize spectrum utilization. When the licensed user is not using the spectrum, CR will then use it to communicate among each other. Hence, spectrum sensing, a procedure to observe primary user's existence, is important to CR. In this paper, we investigate the energy detection based cooperative spectrum sensing technique as it offers low complexity and significantly improves the sensing performance for CR. Conventional soft decision combining (SDC) scheme assumes that CR's local observation is perfectly forwarded to the fusion center. This forwarding procedure requires bandwidth, and increasingly so with more sensing nodes. By carefully applying quantization to this local observation, we could significantly reduce the number of bits required for this communication and thus reduce the overall system overhead. Uniform and non-uniform quantization approaches for SDC are investigated. We further propose a low complexity scheme based on an approximation of the cumulative density function. Simulation results show that these schemes can achieve a good detection performance comparable to the conventional SDC scheme, with reduced communication overhead.
    Personal Indoor and Mobile Radio Communications (PIMRC), 2013 IEEE 24th International Symposium on; 01/2013
  • Juei-chin Shen, Emad Alsusa, Daniel K.C. So
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    ABSTRACT: Cyclostationary feature detection enables a cognitive radio system to reliably sensing the existence of licensed users. A statistical test based on the second-order cyclostationary features has been widely used. This paper evaluates the detection performance of this 2nd-order feature based technique subject to Nakagami fading. The analytical upper and lower bounds on the detection probability are derived by exploiting some available series expansion and exponential-type bound of the generalized Marcum Q-function. As a result, the preliminary detection performance can be acquired without relying on Monte Carlo simulations.
    Wireless Communications and Networking Conference (WCNC), 2013 IEEE; 01/2013
  • Daniel K. C. So, Yang Lan
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    ABSTRACT: Layered space time processing in spatial multiplexing systems requires the number of receive antennas to be equal to or larger than the number of transmit antennas. However, it is impractical for small-sized mobile units to accommodate a large number of antennas. To loosen this stringent requirement, a novel concept of virtual receive antennas (VRA) for overloaded MIMO system is presented. The VRA system architecture consists of two major parts: fractional timing offset in the transmitter and oversampling at the receive matched filter. This procedure expands the received signal dimension and thus creates virtual receive antennas. Due to these created virtual receive antennas, the minimum number of physical receive antennas could be reduced. In order to explore the potential of VRA, this paper evaluates its system capacity and performance bound. Results show that the VRA system can achieve higher ergodic capacity and outage rate than conventional overloaded MIMO system. Performance analysis also suggests a potential performance gain for the VRA system. To eliminate the created inter-symbol interference in the VRA, time domain and frequency domain equalization are utilized and evaluated.
    IEEE Transactions on Communications 06/2012; 60(6):1610-1620. DOI:10.1109/TCOMM.2012.040212.100377 · 1.98 Impact Factor
  • W. Prawatmuang, D.K.C. So
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    ABSTRACT: Cognitive radio is proposed to opportunistically access the spectrum while the licensed user is idle. As a result, spectrum sensing procedure to observe primary user's existence is vital to cognitive radio. In this paper, we propose two adaptive sequential cooperative spectrum sensing techniques in time varying channel, namely the Two Stage Sequential Energy Detector and Differential Sequential Energy Detector. By utilizing past local observations from previous sensing slots, cognitive radio nodes can aggregate the current and previously received energy values to improve the detection performance. We explore the adaptive schemes, where cognitive radio user takes in previously received energy values when it thinks that the primary user's activity remains unchanged. Simulation results show that both proposed schemes provide better detection over conventional energy detection based cooperative technique. Additionally, both proposed schemes can satisfy the IEEE 802.22 standard's requirement of 10% false alarm and 90% detection in scenario that conventional techniques can not accommodate.
    Personal Indoor and Mobile Radio Communications (PIMRC), 2012 IEEE 23rd International Symposium on; 01/2012
  • W. Prawatmuang, D.K.C. So
    [Show abstract] [Hide abstract]
    ABSTRACT: Cognitive radio is proposed to opportunistically access the spectrum while the licensed user is idle. As a result, spectrum sensing procedure to observe primary user's existence is vital to cognitive radio. In this paper, we investigate the energy detection based sequential cooperative spectrum sensing technique in time varying channel. By utilizing past local observations from previous sensing slots, cognitive radio nodes can aggregate the current and previously received energy values to improve the detection performance. We explored the moving-average technique using equal and exponential weighting. Simulation results show that this technique provides better detection performance compared to conventional energy detection based cooperative technique. Equal weighting provides the best performance when primary user's activity is not varying. If the primary users often change states, the proposed exponential weighting approach provides better detection performance.
    Vehicular Technology Conference (VTC Spring), 2012 IEEE 75th; 01/2012
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    ABSTRACT: Energy efficiency has become increasingly important in wireless communications nowadays. Saving energy will not only reduce operating cost but also reduces greenhouse gas emissions, which is important for combating climate change. The authors propose energy-efficient user grouping algorithms to provide power minimisation of grouped multi-carrier code division multiple access (MC-CDMA) and space-time block coding MC-CDMA systems in a cellular environment. Depending on the channel fading conditions, power control is utilised to minimise the total transmitted power under a bit error rate constraint. When the allocation is performed without a fair data rate requirement, the authors provide the optimal solution to the minimisation problem. However, when some fairness is considered, the optimal solution requires high computational complexity. Thus, the authors solve the problem by proposing two suboptimal algorithms. Simulation results illustrate a significantly reduced power consumption in comparison with other techniques.
    IET Communications 12/2011; 5(18):2640-2647. DOI:10.1049/iet-com.2011.0116 · 0.74 Impact Factor
  • A. Phasouliotis, D.K.C. So
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    ABSTRACT: Multi-carrier code division multiple access (MC-CDMA) allows multiuser communication with frequency diversity. To increase the system data rates, spatial multiplexing for multiple-input multiple-output (MIMO) MC-CDMA has been investigated. This study proposes a chip level layered space-time (LST) receiver architecture for coded downlink MIMO MC-CDMA systems. As the conventional chip level ordered successive interference cancellation (OSIC) receiver is unable to overcome multiple access interference and performs poorly in multiuser scenarios, the proposed receiver cancels both spatial and multiuser interference in an ordered LST detection process by requiring only the knowledge of the desired user-s spreading sequence. Simulation results show that the proposed receiver not only performs better than the existing linear detectors but also outperforms both the chip and symbol level OSIC receivers. In this study the authors also compare the error rate performance between the proposed system and MIMO orthogonal frequency division multiple access (MIMO OFDMA) system and they justify the comparisons by deriving and analysing the pairwise error probability (PEP) for both systems. MIMO MC-CDMA demonstrates a better performance over MIMO OFDMA under low system load. If all users- spreading sequences are known, multiuser interference can be reduced and MIMO MC-CDMA performs better than MIMO OFDMA at all system loads.
    IET Communications 10/2011; 5(13-5):1907 - 1917. DOI:10.1049/iet-com.2010.0367 · 0.74 Impact Factor
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    S. Sohaib, D.K.C. So
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    ABSTRACT: Nodes in most cooperative networks are powered by batteries and some of which are even non-rechargeable. Therefore, power allocation schemes must be developed to save the transmit power and improve the life-time of the system. In this context, we present a novel power allocation scheme for multiple relay nodes that results in efficient cooperative communication. Considering channel path loss, the total transmission energy is distributed between the source and the relay nodes. The energy distribution ratio between the relay and direct link is optimized such that the quality of received signal is maintained with minimum total transmission energy consumption. We calculate the energy distribution ratio analytically and verified it through computer simulation. With the new power allocation scheme, the system also obtains an increased channel capacity as compared to cooperative scheme with conventional equal power allocation. Optimal relay positioning with proposed energy allocation scheme is also explored to maximize the capacity.
    Communications (ICC), 2011 IEEE International Conference on; 07/2011
  • Sarmad Sohaib, D.K.C. So
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    ABSTRACT: Cooperative communication exploits the broadcast nature of wireless channel and uses relay nodes to provide better reliability and higher data rates without increasing power and bandwidth. In this paper, the energy analysis of the asynchronous polarized cooperative (APC) scheme is performed. APC employs multiple antennas at the relay and destination nodes to achieve full duplex amplify-and-forward (ANF) communication. Hence the transmission duration is reduced which results into an increased spectral efficiency. Considering channel path loss, the APC scheme consumes less total transmission energy as compared to ANF and non-cooperative scheme over more practical distance between the nodes. Thus the APC scheme is both spectral and energy efficient, and is suitable for the cooperative communication systems.
    Personal Indoor and Mobile Radio Communications (PIMRC), 2010 IEEE 21st International Symposium on; 10/2010

Publication Stats

143 Citations
39.97 Total Impact Points

Institutions

  • 2006–2014
    • The University of Manchester
      • School of Electrical and Electronic Engineering
      Manchester, England, United Kingdom
  • 2011
    • University of Engineering and Technology, Taxila
      • Department of Electrical Engineering
      Islāmābād, Islamabad Capital Territory, Pakistan
  • 2002–2004
    • The Hong Kong University of Science and Technology
      Chiu-lung, Kowloon City, Hong Kong