Resource Allocation for OFDMA Two-Hope Cooperative Cellular Networks : Considering QoS and Fairness Constraints

Content Provider	Semantic Scholar
Author	Banizaman, Hamed
Copyright Year	2014
Abstract	Joint bit allocation, relay selection and subcarrier assignment are critical for achieving full benefits of OFDM-based cooperative relay networks. In this paper, first such a problem is studied in a dual hop OFDMA cooperative network consisting in multi source nodes, multiple decode-and-forward (DF) relays and a single destination node. The aim is to minimize overall transmission power under the bit-errorrate (BER) and data rate constraints. However, the optimal solution to the optimization problem is computationally complex to obtain and may be unfair. Assuming knowledge of the instantaneous channel gains for all links in the entire network, an iterative three-step resource allocation algorithm with low complexity is proposed. It performs the privileged user selection based on fairness criterion first, and then allocates subcarrier-relay with the given constraints. Finally, power and bit are assigned to the selected subcarriers based on the water-filling algorithm. In order to guarantee the fairness of users, several fairness criteria are also proposed to provide attractive trade-offs between network performance (i.e. overall transmission power, average network lifetime and average outage probability) and fairness to all users. Numerical studies are conducted to evaluate the performance of the proposed algorithm in two practical scenarios. Simulation results show that the proposed allocation algorithm achieves an efficient trade-off between network performance and fairness among users. Index Terms: Dynamic resource allocation, multi-user orthogonal frequency division multiplexing, cooperative relaying, fairness, quality of service. 1Introduction Cooperative communication, utilizes the broadcast nature of the wireless medium to provide spatial diversity through node cooperation when limited amounts of antennas are deployed at each node [1-3]. It can be used in extending system coverage and enhancing spectrum efficiency, and has recently attracted research and industry interest. Different relaying strategies, such as amplify-and-forward (AF), compress-and-forward (CF), and decode-andforward (DF), have also been proposed in the literature [3-5]. Meanwhile, orthogonal frequencydivision multiple-access (OFDMA) is a promising candidate for high-speed wireless communication networks, such as IEEE802.16e (WiMAX) [6] and Third-Generation Partnership Long-Term-Evolution (3GPP-LTE) [7], due to its high spectral efficiency and resistance to multi-path fading. Recently, there has been a growing interest in combining OFDMA with relaying to enhance wireless system performance [8-11]. For example, this OFDM-based relay architecture has been accepted by the current wireless standard IEEE 802.16j [12] to provide ubiquitous high-data-rate coverage by dividing one long path into several shorter links and by offering alternative paths to users located in shadow areas. However, for an OFDM system with relay, identifying a proper way to allocate resources to the source and the relay is the main bottleneck for achieving good performance. Moreover, in a practical relay network such as wireless sensor and cellular networks in which each user is powered by batteries, certain fairness constraint need to be considered in terms of the utilization of the resources. Unbalanced use of resources leads to unbalanced node battery usage, which results in a shortened network lifetime. In this paper we consider an uplink of a dual-hop relay-assisted OFDMA system with multiple DF half-duplex relay nodes. Each message is transmitted in two stages each occupying one time slot. A message transmitted by the source on one subcarrier in the first time slot is, if successfully decoded by the relay, forwarded by the relay to the destination on one (not necessarily the same) subcarrier in the second time slot. With the assumption that the channel state information (CSI) is known at the central controller, much work has been done for multiuser wireless networks to make resource utilization of this system more efficient. In [13], an adaptive resource allocation algorithm is employed in relay-to-destination links in an OFDMA cooperative network to improve the endto-end performance. In [14, 15], the power Journal of Intelligent Procedures in Electrical Technology – Vol.5 –No.17Spring 2014 2 allocation problem for non-regenerative OFDM relay links is investigated; in this work, the instantaneous rate is maximized for a given source and relay power constraint. Authors in [10] and [11] have also attempted to solve the subcarrier-to-relay assignment problem in single user multi-relay OFDM systems with DF and AF strategies, respectively. Furthermore, the subcarrier selection in multi-hop OFDM systems has been discussed in [16], and the selective OFDMA relaying strategy has been proposed in [9] based on the total transmission power minimization which performed relay selection on a per-subcarrier basis at each hop so that the error probability of the whole OFDM symbol is greatly reduced. The work on resource allocation mentioned above mainly focused on the performance and operations of single source-destination pair in OFDMA relay network. However, resource allocation in multi-user OFDMA-based cooperative relaying networks has not been thoroughly studied so far. Moreover, in the multi-user context, maximizing the sum of user rates or minimizing the total transmission power imposes that each subcarrier is assigned to the user with the best channel quality. Such an allocation rule may penalize the users with poor or even moderate channel conditions, thus a fairness issue is raised [17]. Hence, it is necessary to develop a scheme, which considers both the fairness of resource allocation and the system efficiency [5]. In addition, the scheme must be able to take into account that users might have different quality-of-service (QoS) requirements (i.e. target data rate and bit-error-rate (BER)). Several resource allocation algorithms are explored to achieve a good tradeoff between throughput and fairness for cooperative OFDM system [18-20]. In [18], aiming at maximizing the achievable sum rate from all the sources to the destination, a source, relay, and subcarrier allocation problem for an OFDMA relay network is studied with fairness constraint on the relay nodes. However, the fairness was focused on the relay nodes and they did not consider the power distribution. In [19], a centralized utility maximization framework for cooperative OFDMA cellular networks is proposed. The proposed solution not only allocates power and bandwidth, but also selects relaying strategies for each user. In [20], based on cooperative OFDMA relay network with single source and multiple destination nodes, the throughput maximization problem is proposed while guaranteeing fairness on subcarrier occupation by multiple destination nodes. However, the sourcedestination link is not considered when the relay is used. In this paper, the resource allocation problem for multi-user OFDMA-based DF cooperative relaying systems is analyzed by focusing the attention on the fair relay, subcarrier, power and bit allocation jointly with the users power and battery energy level constraints. These constraints are motivated by the fact that in some networks, where long-term total power consumption is a major concern, restricting the total transmit power is usually a convenient and effective approach to satisfy the long-term power constraint and consequently extend the network lifetime [5, 21]. Moreover, it is important to highlight that the allocation can guarantee different QoSs demanded by users. Considering the complexity of the optimal solution, we are motivated to investigate a low-complexity three-step iterative centralized algorithm that achieves high degree of user fairness and meanwhile meets network performance. In each iteration, the privileged user is first selected based on some proposed fairness criteria. Then proper relays and subcarriers are assigned to the selected user in order to minimize its transmission power under the BER and data rate constraint. Finally, power and bits are allocated to the subcarriers assigned to the selected user based on the known water-filling algorithm. Determining the best assignment of relays and subcarriers, it will provides an efficient trade-off between network performance (i.e. overall transmission power, average network lifetime and average outage probability) and fairness to all users. The paper is organized as follows. The system model and problem formulation are described in Section 2. In Section 3, a dynamic fair multi-user resource allocation algorithm with low complexity for cooperative OFDMA relay network is proposed. Section 4 analyzes the complexity of optimal solution and proposed allocation algorithm. Simulation parameters and results are given in Section 5. Finally, in Section 6 the paper is summarized and the conclusion is provided. 2System Model and Problem Formulation We consider an uplink selective OFDMA cooperative relaying network with source nodes ( ), relay nodes ( ) and a single destination node ( ) sharing a total number of subcarriers in the cell as shown in Fig. 1 (In this paper, the terms ‘source’ and ‘user’ are often used interchangeably). Let 1,... , , ... , , , ... , , ... , and , ... , , ... , be the set of orthogonal subcarriers, relays and sources, respectively. Here, the destination can be viewed as a base station, the relay nodes are regarded as the relay station as defined in 802.16j and source nodes are considered as user terminals [12]. Therefore, the system model Journal of Intelligent Procedures in Electrical Technology – Vol.5 –No.17Spring 2014 3 described here models an 802.16-based relay network in uplink operation. The OFDMA selective relaying scheme performs relay selection on a persubcarrier basis at each hop. In this network model, each subcarrier of source node can be used in direct link and/or relay link . Here, we adop
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