1 On OFDMA FDD mode in 3G-LTE Rainer Schoenen, Josef Eichinger, and Bernhard H. Walke Abstract— New generations of cellular radio systems are currently being developed based on OFDM transmission with OFDMA as the multiple access scheme. The demand for high data rates in reasonably large areas is omnipresent, but the conven- tional cellular architecture offers does not only a maximum rate depending on the distance. Close to the base station, the higher received SINR valueallows the highest Modulation&Coding scheme (PhyMode), which offers the highest data rate. In this paper we user the mutual information approach to calculate the maximum data rate based on the SINR at all positions in an interference-limited radio cell. Near the cell border the offered data rate is one order of magnitude lower. Relaying or Multihop operation is an option to massively improve the coverage as well as the capacity issue at low cost, without the need of a cable or fibre access. In this method, the base station coordinates the partitioning of radio resources within the relay enhanced cell (REC). Frequency division duplex (FDD) is preferred for large area coverage and is the preferred mode for the beyond-3GPP project LTE [1]. The OFDMA multiple access scheme allows the base station (BS) to transmit to several user terminals (UT) at the same time, in distinct subcarriers. The principle coordination tasks of OFDMA resources in the singlehop and multihop case are discussed. Using an analytic framework in Matlab, we obtain performance results to show the radio coverage in a REC in terms of maximum data rate over the area. 1 I. I NTRODUCTION M ULTIHOP capable air interfaces are becoming more and more interesting due to their improvements of coverage and capacity. For that reason they they are proposed for the next generation cellular systems like 3G-LTE [1]. The maximum data rate offered by a base station depends on the distance of the mobile to the base station. Close to the base station, the higher received SINR value 2 allows the highest Modulation&Coding scheme (PhyMode 64QAM - 5/6), which offers the highest raw data rate of approximately 100M bit/s. At the cell border and in significant fractions of the cell area, the offered data rate is one order of magnitude lower. What makes this even worse is that some terminals operating at the lowest PhyMode occupy a ten times bigger part of the spectrum than the same number of terminals operating at the highest PhyMode. That means the average cell capacity is overproportionally determined by the maximum possible rate at the outer regions. Figure 1 gives an idea of this problem. By using relays (relay nodes, RN) positioned near the cell border the coverage can be extended significantly, assuming – Josef Eichinger is with Siemens, Munich, Germany – The other authors are with the Chair of Communication Networks at RWTH Aachen University, Faculty 6, Germany 1 The presented work was funded by the BMBF in Germany in the ScaleNet project 2 SINR = signal/(interf erence + noise), a measure for the quality of the transmission in [dB]. Fig. 1. The single hop problem: With a constant user density the number of users increases with d, so the cell capacity offered per area element differs from the capacity requested by users Fig. 2. Left: Conventional cellular geometry, Middle: Best geometry to increase capacity, Right: Best geometry to increase coverage that the stationary link between RN and BS has a low pathloss due to close to line-of-sight propagation or higher antenna gains. By positioning a relay within the classical cell boundaries the capacity of an area around the RN is also increased. Figure 2 shows the two ways from a conventional cellular layout to multihop-augmented cells for both goals. The paper is organized as follows. The first section dis- cusses the FDD OFDMA topic. Then, the analytical model is explained. The last section deals with performance results. II. THE FDD MODE WITH OFDMA OFDM has evolved as an efficient multiplex scheme of typically 1024 small orthogonal subcarriers within the system bandwidth. Small subcarriers mean long symbols in time, so the problem of inter symbol interference (ISI) is relieved. The big advantage of OFDM is that each subcarrier can be modulated differently, so a robust BPSK can be used on