System Level Modeling and Optimization of the LTE Downlink

3. Physical Layer Modeling and LTE System Level Simulation3.2.1. Interference-freeIn the first step of verification of the L2S model, a single-cell scenario is considered.This case is equivalent to a throughput evaluation over an SNR range, and aims atreproducing a typical link level simulation. As in Section 3.2.2, results obtained withthe Vienna LTE system level simulator [78], which implements the presented linkabstraction model, are compared with link level results obtained with the ViennaLTE link level simulator [98].In this scenario, we compare the throughput performance of different LTE transmissionmodes and antenna configurations over a range of SNRs, such as in [67, 99],performed both by means of link level simulations and system level simulations.In LTE, the data subcarriers are recovered after a Fast Fourier Transform (FFT),discarding the adjacent guard band subcarriers, including any noise there. As theproportion of guard band subcarriers is not constant over the range of possible LTEbandwidths (see Table 2.3), employing a pre-FFT SNR would make the allocatedbandwidth a parameter to take into account when comparing SNR results. Thus,the choice of employing a post-FFR SNR, denoted as γ post-FFT , which directly refersto the SNR level of the data subcarriers (here denoted for a single subcarrier):{NFFTγ post-FFT = EN toty H yN RX σ 2 n}= N FFT 1N tot σn2 , (3.24)where N FFT are the total number of LTE subcarriers (including bandguard), N totthe number of data subcarriers, and σ 2 n the mean noise power per receive antenna,and y the received signal, as in Equation (3.2).In order to reproduce an SNR range with system level simulations, a single cell isplaced, and a decreasing SNR value is accomplished by positioning the UE fartheraway from the cell center. Note that in the pathloss model, transmit power or noisespectral density values are actually irrelevant. Rather, it is the relation between theUE distance, which scales the received signal power, and the resulting SNR what isimportant, so as to be able to compare link and system level results with a commonSNR definition.Table 3.3 lists the employed configuration parameters, which result in the SNR distributionsurrounding a single cell shown in Figure 3.15 (left). This setup consists ofa single eNodeB with an omnidirectional antenna and depicts the SNR distribution,as defined in Equation (3.24), around the cell center. Being circularly-symmetric, itis thus possible to map the distance from the cell center to an SNR value, which isshown on Figure 3.15 (right).The aforementioned scenario has been simulated at both link and system level bymeans of the Vienna LTE link level simulator [68, 98] and the Vienna LTE system43