An FDD Multihop Cellular Network for 3GPP-LTE Rainer Schoenen, Ruediger Halfmann, and Bernhard H. Walke Abstract—The future cellular radio networks like 3G-LTE [1] are based on an OFDMA physical layer. The duplex scheme is preferably frequency division (FDD) because of its advantages in long range. Huge area coverage in a cost-efficient way is important problem for the early deployment. Time after time the demand in densely populated areas will grow, so a higher cell capacity over the area is needed. The requirements are expensive to solve with a traditional cellular architecture, because a fibre line access will be needed at any base station location, which are sometimes only a few 100 meters apart. This paper deals with Multihop operation as an option to improve the coverage as well as the capacity issue at low cost. Homogeneous Relays act like Base Stations, but without the need of a cable or fibre access. They are simply fed by the same radio technology in their first hop. The contribution of this paper is especially the multihop operation in the FDD mode and performance results for the throughput on the MAC-layer. The results were obtained with an analytic model, numerically evaluated with Matlab. Index Terms— FDD, Multihop, Relaying, OFDMA, LTE I. I NTRODUCTION M ULTIHOP FDD systems have rarely been studied. Relaying has been studied mostly for TDD systems [2]. But multihop capable air interfaces are feasible for both duplex schemes and provide their benefit because of the improved coverage and capacity in a cell. While there is always demand for high data rates almost ho- mogeneously over the area, conventional cellular architectures cannot offer this exactly. Due to the limited transmit power (EIRP limited), the higher transmission rates lead to a lower energy per bit. The radio propagation at higher frequencies is vulnerable to bad non-line-of-sight conditions. In effect, the path loss is higher between base station (BS) and user terminal (UT). Also the offered rate is not homogeneous over the area. The maximum data rate offered by a BS depends on the distance of the mobile to the base station. Close to the BS, the higher received SINR value allows the highest Modulation&Coding scheme (PhyMode), which results in the highest data rate. At the cell border the offered data rate is one order of magnitude lower (QPSK 1 3 compared to QAM 64 5 6 for LTE [1]). What makes this even worse is that a transmission operating with the lowest PhyMode occupies a ten times higher part of the spectrum than a transmission using the highest PhyMode. That means the average cell capacity is overproportionally determined by the maximum possible rate at the outer regions. More base stations per area is one approach, but this comes with much higher deployment costs, since every base station – Ruediger Halfmann is with Siemens, Munich, Germany – The other authors are with the Chair of Communication Networks at RWTH Aachen University, Faculty 6, Germany – This work has been funded by the BMBF in Germany in the ScaleNet project Fig. 1. Left: Singlehop cellular geometry; Right: Multihop cell with increased capacity. Here a cluster order of 3 is shown. needs its own access to the fiber backbone network. Alterna- tively we can deploy Fixed Relay Stations, also called Relay Nodes (RN), that are fed over the same wireless technology. The goal of this is the almost ubiquitous provision of very high data rates for any user terminal within the cell. Figure 1 shows the two ways from a conventional cellular layout to multihop-augmented cells for both goals. The OFDM transmission scheme allows the optimum use of radio resources, especially if multiple access is possible within a frame (OFDMA). OFDMA scheduling determines the parameters to use for each resource unit [3], i.e. choosing the best subcarrier for a user terminal, setting the PhyMode to use and controlling the RF power per subcarrier. In this paper we concentrate on FDD (preferred in huge area cellular networks) and address the MAC frame organization, relay enhanced extensions and performance results for the upcoming long term evolution (LTE) technology [1]. The paper is organized as follows. The first section defines the relaying operation and their properties. Next, the addi- tional opportunities provided by orthogonal frequency division multiple access (OFDMA) are explained. The principles of the FDD mode operation is explained in section IV. The last section presents performance results for capacity and coverage increase scenarios. II. MULTIHOP OPERATION We consider here decode-and-forward or layer-2 schemes, where relays act as repeaters, bridges or routers. In these sys- tems, received PDUs are fully error-corrected, automatically repeated (ARQ), stored and scheduled for transmission, and if necessary even segmented and reassembled prior to their next-hop transmission. Multihop systems are transparent for the UT. The RN acts like a BS towards the UT on the second (last) hop. Only for the first hop(s), where the RN acts like a UT towards the BS, 978-1-4244-1645-5/08/$25.00 ©2008 IEEE 1990