Practical Introduction to LTE Radio Planning

8 J. Salo et alCell edge throughput [Megabits/sec]24222018161412108642Cell edge throughput versus cell range, for fixed network loadγ = 0, 0.1, ..., 0.9, 100 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2Cell range [km]4. MULTIPLE-INPUT MULTIPLE-OUTPUT(MIMO) CONFIGURATIONS4.1. MIMO Transmission Schemes in LTEWhile in common engineer talk there are appearsto be several interpretations for the term,in this paper MIMO is taken to mean any radiointerface that has at least two antennas at bothends, i.e., with this definition smallest MIMOantenna configuration would then be 2Tx×2Rx.Note that in LTE all UEs are required to havetwo receive antennas, while the number of basestation antennas can be 2 1, 2, or 4. In principle,there are three ways to utilize MIMO antennas:Figure 6. Cell range versus cell edge throughputfor fixed network load.3.3.3. Cell range versus cell edge throughput,fixed network loadIn Fig. 6, the relation of cell range and celledge throughput is shown for various valuesof network load. As the cell range increases,the cell edge throughput approaches the samelimit regardless of cell load. Even the low networkload of 10% limits throughput to less than14Mbps by creating a noise floor from othercellinterference. It should be noted that downlinkcommon channels and signals alone causeat least > 10% network load over the ≈ 1 MHzbandwidth over which they are transmitted. Ontop of this, reference signals are transmitted overthe entire system bandwidth. Sites in early FDDnetwork implementations are not expected to beframe-synchronized, so that it is not possible toavoid inter-site interference by cleverly coordinatingtransmission in time and frequency.• Combined transmit/receive diversity: Normaltwo-branch diversity reception or transmissionhas the benefit of producing twocopies of the same signal for reception,which with a suitable signal combiningtechnique reduces fading variation. Similarly,a 2Tx×2Rx antenna transmissionresults in reception of four signal replicas,with the corresponding additional reductionin fading. This version of MIMOis hence simply an enhanced diversityscheme, and as such is not by many considered"true" MIMO, since it does not actuallyincrease transmission data rate. InLTE, space-frequency block coding baseddiversity scheme is used; this is an openloopscheme where UE precoder feedback(PMI) is not required at the base station.• Beamforming: This is very similar to the diversitytransmission, the main nomenclaturaldifference being that in beamformingone typically considers a physical antennabeam being constructed towards the UE.2 With precoding using UE-specific reference signals ("beamforming")arbitrary number of antennas can be used. However,this transmission scheme is not expected to be supportedin first vendor releases, due to hesitance from operatorsto deploy antenna arrays. The cell edge throughput gainfrom such configuration in rural/road environments wouldbe substantial however, e.g., 9 dB from an 8-antenna panelarray.