Repeater for 5G Wireless: a Complementary Contender for Spectrum Sensing Intelligence
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Repeater for 5G Wireless: A Complementary Contender for Spectrum Sensing Intelligence Shree Krishna Sharma∗, Mohammad Patwary†, Symeon Chatzinotas∗, Bjorn¨ Ottersten∗, Mohamed Abdel-Maguid‡ ∗SnT - securityandtrust.lu, University of Luxembourg, Luxembourg Email: shree.sharma, symeon.chatzinotas, bjorn.ottersten @uni.lu { } † FCES, Staffordshire University, United Kingdom, Email: [email protected] ‡ University Campus Suffolk, United Kingdom, Email: [email protected] Abstract—Exploring innovative cellular architectures to the network capacity [4]. In 4G and beyond 4G networks, achieve enhanced system capacity and good coverage has become the data service is the main priority and the system capacity a critical issue towards realizing the fifth generation (5G) of is mainly measured as the aggregate sector throughput which wireless communications. In this context, this paper proposes can be contributed from every parts of the network regardless a novel concept of an intelligent Amplify and Forward (AF) of their locations. In addition to the capacity enhancement 5G repeater for enabling the densification of future cellular due to increased Modulation and Coding Scheme (MCS) and networks. The proposed repeater features a Spectrum Sensing (SS) intelligence capability and utilizes such intelligence in a improved service time allocation, repeaters can enhance the complementary fashion in comparison to its existing counterpart Multiple Input Multiple Output (MIMO) gain under Line of (e.g., Cognitive Radio) by detecting the active channels within the Sight (LoS) conditions. In this context, the Third Generation assigned spectrum. This intelligence allows the proposed repeater Partnership Project (3GPP) has already identified relaying as to carry out selective amplification of the active channels in a potential technique in order to increase the coverage as well contrast to the full amplification in conventional AF repeaters. as to enhance the system throughput [7]. Furthermore, the performance of a Frequency Division Multiple Access (FDMA) based two hop cellular network utilizing the Related Literature: In the existing multihop cellular liter- proposed repeater is evaluated in terms of the system throughput. ature, there exist the following two categories of devices for Simulation results demonstrate up to 13 % increase when enhancing the signal levels [8]: (i) Layer 1 repeater, and (ii) compared with the conventional repeaters. Moreover, the effect Layer 2 relay. Layer 2 relays basically operate in a Decode of SS errors on the system capacity is analyzed. and Forward (DF) mode whereas layer 1 repeaters work in an Amplify and forward (AF) mode. The DF relays can I. INTRODUCTION provide better noise immunity and better intercell interference The fifth generation (5G) of wireless communications is avoidance/mtigation than AF repeaters. However, the DF relay targeting enhanced data rates and better quality of service requires a complicated transceiver and can further introduce with proper fairness among multiple users [1]. Although cur- the transmission delay. One of the main aims of 5G networks rent Long Term Evolution-Advanced (LTE-A) technology can is to reduce the data transmission delay present in the current provide high data rate access on the air interface, providing wireless networks. In this context, it is crucial to explore the higher capacity to the areas having high user density with applications of AF repeaters in comparison to DF relays for de- diverse traffic types and data rate demand is still challenging. lay sensitive services. For example, the additional delay caused Further, in urban areas, the call dropping problem becomes by the layer 2 relay may be unacceptable for uplink signalling more annoying to the customers than the call blocking problem in IEEE 802.16 based systems [9]. Herein, the AF repeater is [2]. One of the cost-effective solutions to address the afore- of practical interest and hence investigated in this paper since mentioned issue is to deploy repeaters in specific urban areas it simply applies a linear signal processing operation on the where severe call dropping has been reported based on network signals and has low implementation complexity [10]. utilization reports. Additionally, in rural and sub-urban areas, Several contributions in the literature have studied various the deployment of repeaters significantly helps in extending spectrum awareness techniques such as Spectrum Sensing the coverage area of a cellular system [3]. (SS), Signal to Noise Ratio (SNR) and sparsity order esti- In today’s wireless networks, the statement that “repeaters mation in the context of a Cognitive Radio (CR) [11]–[14]. are for coverage enhancement only” has become a miscon- Further, cognitive relay networks have been investigated in ception [4]. With the advancement of technology, the system several contributions mainly focusing on the DF relay scenario capacity gain that can be obtained from the repeaters has [6], [15], [16]. Recently, the contribution in [17] investigates become more significant [2]. In this context, several contribu- the performance of cognitive AF repeaters in a dense urban tions have investigated the application of repeaters in different environment. It has been demonstrated that the AF repeater environments [2], [5], [6]. One of the aims of deploying can be used in LTE-A based systems in order to offload mobile repeaters is to enhance the Signal to Interference plus Noise traffic within unused frequency bands effectively. Further, au- Ratio (SINR) of cell-edge users, which is equivalent to bring thors in [18] have studied the deployment of a cognitive relay users from the cell-edge much closer to the cell center. This for indoor scenarios as an underlay system. In addition, authors increased SINR directly translates into an increased data rate, in [19] propose to deploy several relay stations equipped with thus the deployment of a repeater can significantly enhance a CR in order to mitigate intercell interference in cellular systems. However, the aspect of introducing intelligence in the The conventional belief about the repeaters is that they AF repeater has received limited attention despite its several are responsible for amplifying the unnecessary spectrum of important aspects and is the main focus of this paper. information, injecting additional interference to the received signal from the post amplification stage. In order to address Motivation and Contributions: It is well understood in the aforementioned problems, we propose an intelligent AF the literature that repeaters can significantly boost the system repeater which employs a dynamic SS technique. The em- capacity and increase the coverage area [2], [5], [20]. Despite ployed SS technique can vary from a simple Energy Detection several advantages of AF repeaters, there exist some disadvan- (ED) to the eigenvalue based detection depending on the level tages which limit their practical applicability. The biggest chal- of allowable complexity and performance level. For the sake lenge with the AF repeaters is that it will amplify interference of simplicity, we consider an energy detector in this work. as well as noise in unnecessary bands. To address this issue, By comparing the received energy levels with a predefined we propose to have SS intelligence at the repeater which is threshold, the employed energy detector finds the dynamic widely investigated in the context of CR communications [12], spectrum energy peaks (active channels) as well as regular non- [21]. In the CR paradigm, the main aim of SS is to find the zero energy occupancy within the spectrum of interest. Based spectral holes dynamically in different domains in such a way on the knowledge of active channels, the proposed repeater that they can be allocated to potential Secondary Users (SUs) performs the selective amplification of the channels instead of in order to maximize the spectral efficiency. Motivated by this the whole assigned bandwidth. dynamic SS concept, if we can embed SS intelligence into the repeaters, only necessary bands will be amplified and the Some of the fundamental assumptions of the proposed 5G interference plus noise amplification problem can be drastically repeater are provided below. minimized. Hence, there exists a scope of utilization of SS intelligence within the repeater. In contrast to the CR context, 1) The repeaters are expected to be used to implement the proposed repeaters employ SS intelligence in order to find the small (micro/pico/nano) cell vision of future gen- the spectral peaks (above a certain energy threshold) instead of eration of cellular wireless communication networks. the spectral holes. Thus the proposed repeater is herein treated 2) We consider that the proposed repeater is a passive as a “complementary contender” for finding out active channels network entity although being a part of the network. within the assigned spectrum. In future 5G wireless systems, All radio-resource management functions such as the proposed repeaters will perform selective amplification mobility, retransmission mechanism and scheduling of the active channels and hence do not necessarily have to are handled by the BS. Although the network can amplify all the other bands, hence saving resources and also provide with a list of active users within the cell reducing the effect of interference and noise. as well as within the network, the repeater with the In this paper, first, we describe the important aspects and proposed intelligence