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  • 1.
    Wang, Liping
    KTH, School of Electrical Engineering (EES), Communication Networks.
    Cooperative and Cognitive Communication in Wireless Networks2013Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    In the last decade, significant efforts and progress have been made by both the industry and academia to meet the rapidly growing demand for wireless applications and services. To achieve more flexible, dynamic and intelligent use of the limited wireless spectrum, cooperative transmission and cognitive networking are proposed as two of the key technologies for the next generation wireless communication systems, such as Long-Term Evolution Advanced. Cooperative transmission techniques, such as cooperative relaying and Multiple-Input and Multiple-Output (MIMO) can increase spectrum efficiency by utilizing the diversity of wireless channels, while cognitive transmitters tune their transmission parameters according to the environment to optimize network level performance. In this thesis, we provide performance modeling and analysis of different cooperative and cognitive communication techniques to exploit their potential.

    In the first part of the thesis, we investigate the performance of hop-by-hop cooperative communication on a multihop transmission path applying spatial reuse time division multiplexing, where interference from simultaneous transmissions exists. Based on the models, we compare the performance of hop-by-hop cooperation with the performance of traditional simple multihopping schemes, and give the regimes where hop-by-hop cooperation achieves significant gain. Considering random networks, we propose cooperative geographic routing, the integration of hop-by-hop cooperation with traditional geographic routing, and evaluate the effects of the topology knowledge range and the network density.

    In the second part of the thesis, we discuss how cooperative transmission techniques can be utilized in cognitive and hierarchical spectrum sharing networks, where the primary users have transmission guarantees, and the coexisting secondary users need to be cognitive and adjust their transmissions in the shared spectrum bands to conform constraints from the primary users. We consider large-scale coexisting primary and secondary networks, where concurrent primary and secondary transmissions are allowed, and the secondary users provide cooperative relaying for the primary ones and control the interference at the primary receivers by tuning the probability of transmitting and by forming a primary exclusive region around each primary receiver within which all secondary users have to be silent. We define a unified analytic framework to model the performance of cooperative spectrum sharing and cognitive transmission control, characterize their achievable gains, and show that both of the networks have strong incentives to participate in the collaboration.

    Finally, we investigate spectrum sharing networks where both primary and secondary users have stochastic packet arrival. Under the constraint that the performance of primary users does not degrade, we find the dilemma for the secondary users. That is, if a secondary user chooses to cooperate, it can transmit immediately even if the primary queue is not empty, but has additional costs for relaying primary packets, such as increased power consumption. We propose a dynamic cooperation scheme for the secondary user so that it can make sequential decision on whether to cooperate or not in each time slot based on the state of the network. We show that optimal sequential decision is necessary to efficiently trade off the cooperation cost and the packet delay of the secondary user.

  • 2.
    Wang, Liping
    et al.
    KTH, School of Electrical Engineering (EES), Communication Networks. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Fodor, Viktoria
    KTH, School of Electrical Engineering (EES), Communication Networks. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Cooperate or not: The secondary user's dilemma in hierarchical spectrum sharing networks2013In: 2013 IEEE International Conference on Communications (ICC), IEEE conference proceedings, 2013, p. 2650-2655Conference paper (Refereed)
    Abstract [en]

    We consider a spectrum sharing network consisting of a primary and a cognitive secondary transmitter-receiver pair, where the secondary transmitter can cooperatively relay primary traffic. If the secondary user chooses not to cooperate, it can transmit only when the channel is sensed idle. Otherwise, it relays the primary packet and transmits its own packet in the same time slot while guaranteeing the performance of the primary transmission. Choosing cooperation, the secondary user can transmit a packet immediately even if the primary queue is not empty, but it has to bear the additional cost of relaying. We consider a cognitive system, where, to solve this dilemma, the secondary user decides dynamically on when to cooperate. We derive the bounds of the stable-throughput region of the system, and formulate the problem as a Markov decision process (MDP). We prove the existence of a stationary policy that is average cost optimal. Numerical results show that the optimal dynamic secondary access can trade off between the gain and the cost of cooperation, and the average cost can be decreased significantly.

  • 3.
    Wang, Liping
    et al.
    KTH, School of Electrical Engineering (EES), Communication Networks. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Fodor, Viktoria
    KTH, School of Electrical Engineering (EES), Communication Networks. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Cooperative geographic routing in wireless mesh networks2010Conference paper (Refereed)
    Abstract [en]

    We propose cooperative geographic routing (cGeo-routing) for wireless mesh networks by combining cooperative transmission with traditional geographic routing. We model and evaluate two cGeo-routing schemes including Cooperative-Random Progress Forwarding (C-RPF) and Cooperative-Nearest with Forward Progress (C-NFP). We show that cGeo-routing significantly increases the average transport capacity for a single hop in well connected mesh networks, and the gain increases with the transmitted signal-to noise ratio (SNR). Moreover, there exists an optimal topology knowledge range in C-RPF, whereas an optimal node density in C-NFP. Our results also suggest that hop-by-hop cooperation can increase transport capacity in high-connectivity and high-SNR regimes, however, it does not change the transport capacity scaling law of the mesh network.

  • 4.
    Wang, Liping
    et al.
    Ericsson Corp R&D, Sweden.
    Fodor, Viktoria
    KTH, School of Electrical Engineering (EES), Communication Networks. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Dynamic Cooperative Secondary Access in Hierarchical Spectrum Sharing Networks2014In: IEEE Transactions on Wireless Communications, ISSN 1536-1276, E-ISSN 1558-2248, Vol. 13, no 11, p. 6068-6080Article in journal (Refereed)
    Abstract [en]

    We address the challenge of energy efficiency in hierarchical spectrum sharing networks with dynamic traffic. We consider a primary and a cognitive secondary transmitter-receiver pair, where the secondary transmitter can utilize cooperative transmission to relay primary traffic while superimposing its own information. The secondary user meets a dilemma in this scenario. By choosing cooperation, it can transmit a packet immediately, but it has to bear the additional cost of relaying. Otherwise, it can wait for the primary user to become idle, which increases the queuing delay that secondary packets experience. To solve this dilemma and trade off delay and energy consumption, we propose dynamic cooperative secondary access control that takes the state of the spectrum sharing network into account. We formulate the problem as a Markov decision process and prove the existence of a stationary policy that is average cost optimal. We evaluate reinforcement learning to find optimal transmission strategy when the traffic and link statistics are not known. We demonstrate that dynamic cooperation is necessary for the secondary system to be able to adapt to changing network conditions and show that optimal sequential decision can significantly improve the tradeoff of the energy consumption and the delay.

  • 5.
    Wang, Liping
    et al.
    KTH, School of Electrical Engineering (EES), Communication Networks.
    Fodor, Viktoria
    KTH, School of Electrical Engineering (EES), Communication Networks.
    On the Gain of Primary Exclusion Region and Vertical Cooperation in Spectrum Sharing Wireless Networks2012In: IEEE Transactions on Vehicular Technology, ISSN 0018-9545, E-ISSN 1939-9359, Vol. 61, no 8, p. 3746-3758Article in journal (Refereed)
    Abstract [en]

    The emerging cognitive radio (CR) technology enables the introduction of hierarchical spectrum sharing in wireless networks, where the primary users (PUs) have transmission guarantees, but the coexisting secondary users (SUs) need to be cognitive toward primary activities and adjust their transmissions to conform to the primary constraints. We consider large-scale coexisting primary and secondary networks, where concurrent primary and secondary transmissions are allowed and where the SUs control the interference at the primary receivers by tuning the probability of transmitting and by forming a primary exclusive region (PER) around each primary receiver within which all SUs have to be silent. Moreover, the primary source-destination pairs utilize vertical cooperation by selecting a nearby SU to act as a cooperative relay. We define a unified analytic framework to model cognition and cooperative transmission in large-scale networks. We characterize the achievable gains considering the transmission density region and show that both of the networks have strong incentives to participate in the collaboration.

  • 6.
    Wang, Liping
    et al.
    KTH, School of Electrical Engineering (EES), Communication Networks. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Fodor, Viktoria
    KTH, School of Electrical Engineering (EES), Communication Networks. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    On the Gain of Vertical Cooperation in Cognitive Radio Networks2011In: 2011 IEEE INTERNATIONAL CONFERENCE ON COMMUNICATIONS (ICC), New York: IEEE , 2011Conference paper (Refereed)
    Abstract [en]

    We consider a cognitive radio network where primary users (PUs) and secondary users (SUs) coexist and share the same spectrum. Secondary users access the spectrum randomly and limit the outage probability experienced by the primaries by controlling their transmission probability and by obeying a primary exclusion region (PER), within which all SUs have to be inactive. We propose a vertical cooperative transmission scheme where the primary source-destination pairs select a neighboring idle SU, and use the selected SU as a cooperative relay. We consider three geographic relay selection rules and derive analytic models on the primary outage probability considering the effects of the secondary random access and the PER. Our results indicate that the proposed vertical cooperation increases significantly the allowed transmission probability of the SUs both without and with PER.

  • 7.
    Wang, Liping
    et al.
    KTH, School of Electrical Engineering (EES), Communication Networks. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Fodor, Viktoria
    KTH, School of Electrical Engineering (EES), Communication Networks. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Skoglund, Mikael
    KTH, School of Electrical Engineering (EES), Communication Theory. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Cooperative Communication for Spatial Frequency Reuse Multihop Wireless Networks under Slow Rayleigh Fading2011In: 2011 IEEE INTERNATIONAL CONFERENCE ON COMMUNICATIONS (ICC), New York: IEEE , 2011Conference paper (Refereed)
    Abstract [en]

    Cooperative communication has been proposed as a means to increase the capacity of a wireless link by mitigating the path-loss, fading and shadowing effects of radio propagation. In this paper, we evaluate the efficiency of cooperative communication in large scale wireless networks under interference from simultaneous transmissions. Specifically, we consider tunable spatial reuse time division multiplexing and half-duplex decode-and-forward cooperative relaying on a hop-by-hop basis. We show that hop-by-hop cooperation improves the reliability of the transmissions particularly in the low-SINR or in the low-coding-rate regimes. Moreover, hop-by-hop cooperative relaying gains 15 - 20% more throughput compared to simple multihopping in the interference-limited regime, if the relay location and the reuse distance are jointly optimized.

  • 8.
    Wang, Liping
    et al.
    KTH, School of Electrical Engineering (EES), Communication Networks. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Fodor, Viktoria
    KTH, School of Electrical Engineering (EES), Communication Networks. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Skoglund, Mikael
    KTH, School of Electrical Engineering (EES), Communication Theory. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Using cooperative transmission in wireless multihop networks2009In: IEEE International Symposium on Personal, Indoor and Mobile Radio Communications, 2009, Vol. PIMRCConference paper (Refereed)
    Abstract [en]

    This paper investigates the efficiency of cooperative transmission when it is applied in wireless multihop networks. We consider regular linear networks and derive the achievable rate-delay tradeoff when selective relaying through a single relay node is used in each hop. We show that relaying achieves significant gain particularly in the high throughput - high delay regime.

  • 9.
    Xu, Yuzhe
    et al.
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Liping, Wang
    KTH, School of Electrical Engineering (EES), Communication Networks.
    Fischione, Carlo
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Fodor, Viktoria
    KTH, School of Electrical Engineering (EES), Communication Networks.
    Distributed spectrum leasing via vertical cooperation in spectrum sharing networks2015In: Cognitive Radio Oriented Wireless Networks and Communications (CROWNCOM), 2014 9th International Conference on, IEEE conference proceedings, 2015, p. 185-190Conference paper (Refereed)
    Abstract [en]

    In hierarchical cognitive radio networks, unlicensedsecondary users can increase their achievable rates by assistinglicensed primary user transmissions via cooperation. In thispaper, a novel approach to maximize the transmission rates inthe secondary network by optimizing the relay selection, thesecondary transmit powers, and the cooperative relaying powersplitting parameters is proposed. The resulting optimizationproblem is mixed integer and non-convex, which makes it NPhard to find the optimal solutions. Therefore, centralized anddistributed solution methods to find near-to-optimal solutions ofthis challenging problem are proposed. The methods are basedon iteratively solving the secondary relay selection by a greedyapproach, and the optimal power allocation problem by a fixed-point approach together with alternating direction method ofmultipliers. It is established that both centralized and distributedsolution methods always converge. The numerical results illus-trate the performance of the proposed solution methods, andshow that they give a near-to-optimal solution. Moreover, theperformance margins of the primary transmitters that permitthe accommodation of relaying secondary users, still having highachievable transmit rates, are characterized.

  • 10.
    Xu, Yuzhe
    et al.
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Liping, Wang
    KTH, School of Electrical Engineering (EES), Communication Networks.
    Fodor, Viktoria
    KTH, School of Electrical Engineering (EES), Communication Networks.
    Fischione, Carlo
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Distributed spectrum leasing via vertical cooperation in cognitive radio networks2015In: IEEE Transactions on Wireless Communications, ISSN 1536-1276, E-ISSN 1558-2248, p. 185-190Article in journal (Refereed)
    Abstract [en]

    In hierarchical cognitive radio networks, unlicensed secondary users can increase their achievable rates by assisting licensed primary user transmissions via cooperation. In this paper, a novel approach to maximize the transmission rates in the secondary network by optimizing the relay selection, the secondary transmit powers, and the cooperative relaying power splitting parameters is proposed. The resulting optimization problem is mixed integer and non-convex, which makes it NP hard to find the optimal solutions. Therefore, centralized and distributed solution methods to find near-to-optimal solutions of this challenging problem are proposed. The methods are based on iteratively solving the secondary relay selection by a greedy approach, and the optimal power allocation problem by a fixed-point approach together with alternating direction method of multipliers. It is established that both centralized and distributed solution methods always converge. The numerical results illustrate how the performance of the proposed solution methods depend on the primary performance margins, and show that they give a near-to-optimal solution in few iterations.

  • 11. Zu, L.
    et al.
    Ji, Y.
    Wang, Liping
    KTH, School of Electrical Engineering (EES), Communication Networks.
    Zhong, L.
    Liu, F.
    Wang, P.
    Xu, J.
    Joint optimization in multi-user MIMO-OFDMA relay-enhanced cellular networks2011In: 2011 IEEE Wireless Communications and Networking Conferenc, 2011, p. 13-18Conference paper (Refereed)
    Abstract [en]

    MIMO, OFDMA and cooperative relaying are the key technologies in future wireless communication systems. However, under the usage of these technologies, resource allocation becomes a more crucial and challenging task. In multi-user MIMO-OFDMA relay-enhanced cellular networks, we formulate the optimal instantaneous resource allocation problem including user group selection, path selection, power allocation, and subchannel scheduling to maximize system capacity. We first propose a low-complex resource allocation algorithm named CP-CP under constant uniform power allocation and then use a water-filling method named CP-AP to allocate power among transmitting antennas. Moreover, we solve the original optimization problem efficiently by using the Jensen's inequality and propose a modified iterative water-filling algorithm named AP-CP. Based on AP-CP, the AP-AP algorithm is proposed to allocate power adaptively not only among subchannels but also among multiple transmitting. Finally, we compare the performance of the four schemes. Our results show that allocating power among subchannels is more effective than among transmitting antennas if the average signal-to-noise radio of users is low, and vice versa. Furthermore, the AP-AP algorithm achieves the highest throughout especially for users near the cell edge.

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