IEEE COMMUNICATIONS LETTERS, VOL. 22, NO. 1, JANUARY 2018 133 Buffer-Aided Relay Selection With Equal-Weight Links in Cooperative Wireless Networks Waseem Raza, Nadeem Javaid , Hina Nasir , Nabil Alrajeh, and Nadra Guizani Abstract—In this letter, we discuss multiple links with equal weights, in buffer size based relay selection schemes in coopera- tive wireless networks. A general relay selection factor is defined, which includes the weight of the link as the first metric and the link quality, or priority, as the second metric for different cases of the same weight. The Markov chain based theoretical framework is employed to evaluate the outage probability, delay and throughput of the system. The proposed scheme is evaluated for symmetric and asymmetric channel conditions. The link quality based second selection metric achieves lower outage prob- ability, while the link priority based selection shows significant improvements in terms of delay and throughput. Theoretical results are validated through extensive Monte carlo simulations. Index Terms— Cooperative communication, markov chain, relay selection, link quality, link priority. I. I NTRODUCTION C OOPERATIVE relaying (CR) provides diversity gain by employing a set of relays between the source and the destination [1]. Relay selection harvests the benefits of CR with its applications in smart grids, device-to-device commu- nication and 5G networks [2]. Conventional (buffer-less) CR schemes select single relay among all source-to-relay (SR) and relay-to-destination (RD) links for reception and transmission, respectively [3]. Buffer-aided relay selection in CR enhances throughput and diversity gain and offers the flexibility to select a relay separately for SR and RD links [4]. The max-max scheme [4] selects a relay for reception (trans- mission) in odd (even) time-slots. Hence, a full diversity gain is not achieved because of a fixed transmission schedule. The sequential relaying paradigm is relaxed in max-link [5] which allows the selection of the best relay among all SR and RD links in each time-slot. Additionally, the direct link is also exploited in modified max-link [6] that achieves a significant performance gain in terms of diversity and delay. In max-weight [7], each link is assigned a weight; equal to the number of an available (occupied) buffer space in a corresponding relay for SR (RD) links. A link having a maximum weight is selected and the scheme achieves a full diversity gain for small buffer sizes. However, if multiple links have the same weight, the max-weight randomly selects a link Manuscript received July 26, 2017; accepted September 5, 2017. Date of publication September 28, 2017; date of current version January 8, 2018. This work was supported by the Deanship of Scientific Research, King Saud University, under Grant RG-1435-0037. The associate editor coordinating the review of this paper and approving it for publication was O. Popescu. (Corresponding author: Nadeem Javaid.) W. Raza and N. Javaid are with the COMSATS Institute of Information Technology, Islamabad 44000, Pakistan. H. Nasir is with International Islamic University, Islamabad 44000, Pakistan. N. Alrajeh is with King Saud University, Riyadh 11633, Saudi Arabia. N. Guizani is with Purdue University, West Lafayette, IN 47907 USA. Digital Object Identifier 10.1109/LCOMM.2017.2756833 for reception or transmission. The selection of a less reliable link is unavoidable in this case. In this work, we assess the equal-weight situation and pro- pose two schemes which consider a second selection metric for different cases of the equal-weights for buffer size based relay selection in cooperative wireless networks. First, we analyze the weights assigned to the links in buffer-aided relaying, and highlight the important properties of the vector containing these weights. Then, the different states of Markov chain (MC) for different values of buffer size L and number of relays K is designed. This becomes the motivation for considering the second selection metric in case of equal weights. The first contribution considers the link quality, i.e., received SNR, as the second selection metric. If the weights of multiple links are equal to the maximum weight, then a link with a maximum SNR is selected for reception or transmission. The proposed scheme is called improved max-weight with link quality (imax- weight-quality). However, for delay sensitive applications, we propose to prioritize the RD links having equal weights with the SR links. This scheme is called improved max- weight with link priority (imax-weight-priority). The problem of selecting the best link among all equal-weight links is studied as an NP hard problem [8]. A general relay selection factor is defined, which redistrib- utes the weights of equal-weight links in the aforementioned schemes. The changes in buffers are modeled as the states of the MC to calculate the outage probability, delay and throughput. Further, Monte carlo simulations are performed to validate the proposed theoretical analysis. Most of the work on relay selection assumes independent and identically distributed (i.i.d) channel conditions, i.e., all links receive the same average SNR. We consider both the i.i.d and non- identical (i.ni.d) channel conditions in our simulations. II. SYSTEM MODEL AND MOTIVATION The system model considered in this work is similar to that discussed in [7] which consists of a source node S, a des- tination node D and a set of relay nodes { R 1 , R 2 ,..., R K }. Each relay R k is equipped with a limited buffer B k of size L . There are a total of 2 K relayed links given in the set L = {l 1 , l 2 ,..., l 2 K }, and there is no direct link due to path loss and shadowing affects. At time-slot t , Q t k = ψ( B k ) is the occupied buffer space (OBS) and ¯ Q t k = L − Q t k is the available buffer space (ABS) in R k . The relayed link l i is related with SR k and R k D as, l i =  SR i , i ≤ K , R i −K D, i > K . (1) The channel gain g i oflink l i is a mutually indepen- dent zero mean complex Gaussian random variable with 1558-2558 © 2017 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.