Practical Network Coding for Two Way Relay Channels in LTE Networks Hassan Hamdoun, Pavel Loskot, Timothy O’Farrell and Jianhua He School of Engineering Swansea University, Swansea,United Kingdom, SA2 8PP Email: {476506,p.loskot,t.ofarrell,j.he}@swansea.ac.uk Abstract— In this paper, the implementation aspects and con- straints of the simplest network coding (NC) schemes for a two-way relay channel (TWRC) composed of a user equipment (mobile terminal), an LTE relay station (RS) and an LTE base station (eNB) are considered in order to assess the usefulness of the NC in more realistic scenarios. The information exchange rate gain (IERG), the energy reduction gain (ERG) and the resource utilization gain (RUG) of the NC schemes with and without subcarrier division duplexing (SDD) are obtained by computer simulations. The usefulness of the NC schemes are evaluated for varying traffic load levels, the geographical distances between the nodes, the RS transmit powers, and the maximum numbers of retransmissions. Simulation results show that the NC schemes with and without SDD, have the throughput gains 0.5% and 25%, the ERGs 7 - 12% and 16 - 25%, and the RUGs 0.5 - 3.2%, respectively. It is found that the NC can provide performance gains also for the users at the cell edge. Furthermore, the ERGs of the NC increase with the transmit power of the relay while the ERGs of the NC remain the same even when the maximum number of retransmissions is reduced. I. I NTRODUCTION The ever increasing energy costs and the stringent govern- ment climate policies are forcing the radio access network (RAN) operators to search for innovative techniques that can achieve significant energy and power savings. At the same time, the energy savings must be provided without significantly sacrificing the required quality-of-service (QoS). Recently, network coding (NC) protocols have been introduced to enhance the throughput of wireless networks [1]. The results on how to employ NC in orthogonal frequency division multiple access (OFDMA) based wireless networks are scarce [2], [3], and more importantly, practical implementation con- straints of NC are rarely considered. Since the hybrid ARQ (HARQ) retransmission unit is a transport block (TB), in the authors’ opinion, the most suitable implementation of the NC is a binary exclusive-or (XOR) of the TBs at the MAC sublayer of the link layer [4]. The NC of the TBs at the MAC sublayer is followed by the packet scheduling, the HARQ retransmissions, and link adaptation. The LTE standard defines physical resource blocks (PRBs) of 180kHz bandwidth over a subframe of 1ms to carry the TBs. The LTE downlink is based on OFDMA, however, the LTE uplink employs a single carrier frequency division multiple access (SC-FDMA). In both the uplink and the downlink, the spectral resources allocation can be changed every subframe for each of the PRBs. The results in literature on network coding (NC) indicate that the usefulness of NC strongly depends on the conceptual system model adopted. In this paper, we investigate NC schemes for TWRCs in LTE networks assuming the time division duplex (TDD) option as well as the subcarrier division duplexing (SDD) option. The TWRC consists of a base station (eNB), a relay station (RS) and the user equipment (UE). We note that we use the term ‘network coding’ in the sense of a distributed encoding among the network nodes. In the distributed encoding, the packets from several source nodes are encoded in one or more relay nodes. For example, the NC schemes for TWRCs considered in this paper generate a single parity bit at the RS for each of the two source bits transmitted from the eNB and the UE, respectively. We quantify the energy savings due to saved resource blocks in an LTE cell due to the NC used at the RS relaying a bidirectional traffic between the eNB and the UE. We also evaluate the usefulness of the NC in terms of the utilization of the radio resources and the cell throughput. We adopt several realistic implementation assumptions that most significantly influence the performance of the NC schemes in LTE networks. The first assumption is a non-zero packet error rate (PER) of communication links which, in our case, corresponds to the transport block error rate (TBLER). The second realistic assumption is the link adaptation that determines the Modulation and Coding Scheme (MCS), and hence, the number of bits transmitted by a given PRB. In addition, the link adaption implies that the network coded TB should be broadcasted from the RS using the smaller of the signal-to-noise ratios (SNRs) of the two incoming links. Lastly, we also consider the the impact of the maximum number of retransmissions of the HARQ, the impact of the geographical distance between the RS, the eNB and the UE, and the impact of the transmission power of the RS. The remainder of this paper is organized as follows. Section II provides details on how to employ NC at the LTE link layer. The system model and simulation methodology are discussed in Section III. Simulation results are presented in Section IV. Conclusions are given in Section V. II. PRACTICAL NC FOR A TWRC IN LTE CELL The 3GPP standard requires orthogonality of all transmis- sions within the single LTE cell. Hence, we only consider XOR based NC schemes. Such NC schemes reduce the number of transmissions, or equivalently, reduce the traffic 978-1-4244-8331-0/11/$26.00 ©2011 IEEE