Survey of Spectrum Sharing for Inter-Technology Coexistence

Increasing capacity demands in emerging wireless technologies are expected to be met by network densification and spectrum bands open to multiple technologies. These will, in turn, increase the level of interference and also result in more complex inter-technology interactions, which will need to be managed through spectrum sharing mechanisms. Consequently, novel spectrum sharing mechanisms should be designed to allow spectrum access for multiple technologies, while efficiently utilizing the spectrum resources overall. Importantly, it is not trivial to design such efficient mechanisms, not only due to technical aspects, but also due to regulatory and business model constraints. In this survey we address spectrum sharing mechanisms for wireless inter-technology coexistence by means of a technology circle that incorporates in a unified, system-level view the technical and non-technical aspects. We thus systematically explore the spectrum sharing design space consisting of parameters at different layers. Using this framework, we present a literature review on inter-technology coexistence with a focus on wireless technologies with equal spectrum access rights, i.e. (i) primary/primary, (ii) secondary/secondary, and (iii) technologies operating in a spectrum commons. Moreover, we reflect on our literature review to identify possible spectrum sharing design solutions and performance evaluation approaches useful for future coexistence cases. Finally, we discuss spectrum sharing design challenges and suggest future research directions

Multi-cell Coordination Techniques for DL OFDMA Multi-hop Cellular Networks

The main objective of this project is to design coordinated spectrum sharing and reuse techniques among cells with the goal of mitigating interference at the cell edge and enhance the overall system capacity. The performance of the developed algorithm will be evaluated in an 802.16m (WiMAX) environment. In conventional cellular networks, frequency planning is usually considered to keep an acceptable signal-to-interference-plus noise ratio (SINR) level, especially at cell boundaries. Frequency assignations are done under a cell-by-cell basis, without any coordination between them to manage interference. Particularly this approach, however, hampers the system spectral efficiency at low reuse rates. For a specific reuse factor, the system throughput depends highly on the mobile station (MS) distribution and the channel conditions of the users to be served. If users served from different base stations (BS) experience a low level of interference, radio resources may be reused, applying a high reuse factor and thus, increasing the system spectral efficiency. On the other side, if the served users experience large interference, orthogonal transmissions are better and therefore a lower frequency reuse factor should be used. As a consequence, a dynamic reuse factor is preferable over a fixed one. This work addresses the design of joint multi-cell resource allocation and scheduling with coordination among neighbouring base stations (outer coordination) or sectors belonging to the same one (inner coordination) as a way to achieve flexible reuse factors. We propose a convex optimization framework to address the problem of coordinating bandwidth allocation in BS coordination problems. The proposed framework allows for different scheduling policies, which have an impact on the suitability of the reuse factor, since they determine which users have to be served. Therefore, it makes sense to consider the reuse factor as a result of the scheduling decision. To support the proposed techniques the BSs shall be capable of exchanging information with each other (decentralized approach) or with some control element in the back-haul network as an ASN gateway or some self-organization control entity (centralized approach)

Self-organising network management for heterogeneous LTE-advanced networks

This thesis was submitted for the award of Doctor of Philosophy and awarded by Brunel University LondonSince 2004, when the Long Term Evolution (LTE) was first proposed to be publicly available in the year 2009, a plethora of new characteristics, techniques and applications have been constantly enhancing it since its first release, over the past decade. As a result, the research aims for LTE-Advanced (LTE-A) have been released to create a ubiquitous and supportive network for mobile users. The incorporation of heterogeneous networks (HetNets) has been proposed as one of the main enhancements of LTE-A systems over the existing LTE releases, by proposing the deployment of small-cell applications, such as femtocells, to provide more coverage and quality of service (QoS) within the network, whilst also reducing capital expenditure. These principal advantages can be obtained at the cost of new challenges such as inter-cell interference, which occurs when different network applications share the same frequency channel in the network. In this thesis, the main challenges of HetNets in LTE-A platform have been addressed and novel solutions are proposed by using self-organising network (SON) management approaches, which allows the cooperative cellular systems to observe, decide and amend their ongoing operation based on network conditions. The novel SON algorithms are modelled and simulated in OPNET modeler simulation software for the three processes of resource allocation, mobility management and interference coordination in multi-tier macro-femto networks. Different channel allocation methods based on cooperative transmission, frequency reuse and dynamic spectrum access are investigated and a novel SON sub-channel allocation method is proposed based on hybrid fractional frequency reuse (HFFR) scheme to provide dynamic resource allocation between macrocells and femtocells, while avoiding co-tier and cross-tier interference. Mobility management is also addressed as another important issue in HetNets, especially in hand-ins from macrocell to femtocell base stations. The existing research considers a limited number of methods for handover optimisation, such as signal strength and call admission control (CAC) to avoid unnecessary handovers, while our novel SON handover management method implements a comprehensive algorithm that performs sensing process, as well as resource availability and user residence checks to initiate the handover process at the optimal time. In addition to this, the novel femto over macro priority (FoMP) check in this process also gives the femtocell target nodes priority over the congested macrocells in order to improve the QoS at both the network tiers. Inter-cell interference, as the key challenge of HetNets, is also investigated by research on the existing time-domain, frequency-domain and power control methods. A novel SON interference mitigation algorithm is proposed, which is based on enhanced inter-cell interference coordination (eICIC) with power control process. The 3-phase power control algorithm contains signal to interference plus noise ratio (SINR) measurements, channel quality indicator (CQI) mapping and transmission power amendments to avoid the occurrence of interference due to the effects of high transmission power. The results of this research confirm that if heterogeneous systems are backed-up with SON management strategies, not only can improve the network capacity and QoS, but also the new network challenges such as inter-cell interference can also be mitigated in new releases of LTE-A network

Busy burst technology applied to OFDMA–TDD systems

The most significant bottleneck in wireless communication systems is an ever-increasing disproportion between the bandwidth demand and the available spectrum. A major challenge in the field of wireless communications is to maximise the spatial reuse of resources whilst avoiding detrimental co-channel interference (CCI). To this end, frequency planning and centralised coordination approaches are widely used in wireless networks. However, the networks for the next generation of wireless communications are often envisioned to be decentralised, randomly distributed in space, hierarchical and support heterogeneous traffic and service types. Fixed frequency allocation would not cater for the heterogeneous demands and centralised resource allocation would be cumbersome and require a lot of signalling. Decentralised radio resource allocation based on locally available information is considered the key. In this context, the busy burst (BB) signalling concept is identified as a potential mechanism for decentralised interference management in future generation networks. Interference aware allocation of time-frequency slots (chunks) is accomplished by letting receivers transmit a BB in a time-multiplexed mini-slot, upon successful reception of data. Exploiting channel reciprocity of the time division duplex (TDD) mode, the transmitters avoid reusing the chunks where the received BB power is above a pre-determined threshold so as to limit the CCI caused towards the reserved chunks to a threshold value. In this thesis, the performance of BB signalling mechanism in orthogonal frequency division multiple access - time division duplexing (OFDMA-TDD) systems is evaluated by means of system level simulations in networks operating in ad hoc and cellular scenarios. Comparisons are made against the state-of-the-art centralised CCI avoidance and mitigation methods, viz. frequency planning, fractional frequency reuse, and antenna array with switched grid of beams, as well as decentralised methods such as the carrier sense multiple access method that attempt to avoid CCI by avoiding transmission on chunks deemed busy. The results demonstrate that with an appropriate choice of threshold parameter, BB-based techniques outperform all of the above state-of-the-art methods. Moreover, it is demonstrated that by adjusting the BB-specific threshold parameter, the system throughput can be traded off for improving throughput for links with worse channel condition, both in the ad hoc and cellular scenario. Moreover, by utilising a variable BB power that allows a receiver to signal the maximum CCI it can tolerate, it is shown that a more favourable trade-off between total system throughput and link throughput can be made. Furthermore, by performing link adaptation, it is demonstrated that the spatial reuse and the energy efficiency can be traded off by adjusting the threshold parameter. Although the BB signalling mechanism is shown to be effective in avoiding detrimental CCI, it cannot mitigate CCI by itself. On the other hand, multiple antenna techniques such as adaptive beamforming or switched beam approaches allow CCI to be mitigated but suffer from hidden node problems. The final contribution of this thesis is that by combining the BB signalling mechanism with multiple antenna techniques, it is demonstrated that the hybrid approach enhances spatial reusability of resources whilst avoiding detrimental CCI. In summary, this thesis has demonstrated that BB provides a flexible radio resource mechanism that is suitable for future generation networks