# Hybrid ARQ Scheme with Autonomous Retransmission for Multicasting in Wireless Sensor Networks
## 1 Introduction  
Multicast services have received considerable attention as a means for supporting point-to-multipoint applications such as video conferencing, mobile broadcasting, multimedia streaming services, and news distribution. These applications are achieved in mobile networks by the inclusion of an enhanced multimedia broadcast multicast service (eMBMS) and an enhanced multicast broadcast service (E-MBS) in current wireless communication standards such as the 3 rd Generation Partnership Project Long Term Evolution (3 GPP LTE) and IEEE 802.16m include an enhanced, respectively [1; 2]. The original version of the eMBMS system was mainly focused on broadcast service based on single frequency network consisting of multiple cells [3] and did not support uplink feedback channels. Therefore, the modulation and coding scheme (MCS) of eMBMS packets need to as robust as possible so as to accommodate the user in the worst channel quality [4]. Recently, there has been a growing need for communication systems that efficiently provide multicast services rather than broadcast services in various applications such as public safety services, surveillance services, smart grid services, and reconnaissance services using unmanned air-vehicles (UAVs) [5; 6; 7; 8; 9]. In multicast services, it is important to securely and efficiently share information in a wireless sensor network (WSN) which is composed of various sensors such as cameras, thermal sensors, air pollution sensors, and smart meters. For example, if an effective cooperative search service that uses multiple UAVs equipped with cameras and thermal sensors, the information about search target, search area, and surrounding obstacles must be shared with all the UAVs. In a smart grid service, real-time power cost information must be reliably transmitted to all smart meters. Therefore, unlike a broadcasting service, multicasting in WSNs requires a guarantee of quality of service (QoS). In order to improve the spectral efficiency of multicast services, 3 GPP Technical Specification Group Radio AccessNetwork Working Group 2 has studied the Single-Cell Point-to-Multipoint (SC-PtM) transmission for LTE Release 13 [10]. In case of SC-PtM transmission, the need to support uplink feedback channels in link adaptation schemes has been studied and significant gain in spectral efficiency was observed in the adaptive MCS selection and hybrid automatic repeat request (ARQ) [11]. In ARQ schemes, the receiver examines a packet error using an error detection code such as cyclic redundancy check, and sends an acknowledgement (ACK) or no acknowledgement (NACK) message to the transmitter. In order to enhance link performance and reliability, ARQ schemes are combined with channel coding. This yields hybrid ARQ (HARQ) mechanisms, in which the receiver decodes a retransmitted packet in conjunction with previously transmitted erroneous packets. Recent results concerning HARQ are presented in [12] and the references therein. The HARQ schemes used in 3 GPP LTE and IEEE 802.16m are described in [13; 14; 15], and the use of HARQ for multicasting has been studied in [5; 16; 17; 18; 19]. When the conventional HARQ method is applied to downlink multicast packets, multiple ACK channels are assigned in the uplink for multicast users. Consequently, the feedback overhead in the uplink is increased with an increasing number of multicast users. A proposal to reduce the ACK/NACK feedback overhead from user specific feedback involved a NACK based common feedback channel [18; 19]. The physical resources for feedback are shared by all mobile stations (MSs) in the multicast group and the MSs that fail to decode send NACK signals. The NACK-based common feedback channel can save feedback overhead in the high signal to interference plus noise ratio (SINR) regime if multiple antennas are used, but the probability of misdetection is quite high in realistic conditions. Moreover, there exists a risk of ACK or NACK misinterpretation in the NACK-based common feedback channel [5; 20].