New-Tech Europe Magazine | February 2018

previous example with a simple change to the NB-IoT RB location. For guard-band operation, the NB- IoT RB is set to be greater than zero or greater than N_RB_UL, the upper limit, to operate in the lower or upper guard band, respectively. In- band operation is obtained by setting the NB-IoT RB at any value between these limits. The spectra for an NB- IoT channel operating in guard-band mode is shown in Figure 8. As previously mentioned, a front- end module, power amplifier and antenna design can be added to or substituted for the AWGN channel model, which serves as a placeholder for a DUT. Figure 9 shows an amplifier inserted between the UL transmitter and receiver. The simulation allows designers to sweep various parameters, such as input power, or toggle different NB-IoT subcarrier modulation schemes (π/2 BPSK or π/4 QPSK) to investigate the impact on performance, such as error vector magnitude (EVM). Conclusion The NB-IoT standard specified in 3GPP release 13 leverages the existing LTE network to support a future ecosystem of low-cost IoT devices. While the use of the existing LTE infrastructure with relaxed performance requirements, due to the lower data rates, will help offset some design challenges, the need for low cost, increased coverage area and longer battery life with sustained reachability introduces some difficult- to-achieve requirements. VSS and other system simulation tools aid NB-IoT system development by simulating designs pre-silicon, saving valuable time and effort bringing these new products to market.

Figure 8. NB-IoT and LTE spectra for the guard-band mode.

Figure 9. Test bench with power amplifier (a) and guard-band mode spectra (b).

IoT waveform, passed through an additive white Gaussian noise (AWGN) channel and terminated in an NB-IoT UL receiver for demodulation and decoding of the physical uplink shared channel (PUSCH) signal. For component and system designers, the AWGN channel model can be replaced with a different channel model or device under test (DUT). The test bench in this in-band simulation has been configured to monitor the Tx signal spectrum at various points in the link (see Figure

5), the NB-IoT link performance in the presence of the LTE UL signal, I/Q constellation of the transmitted and demodulated signals, bit error rate (BER) (see Figure 6), block error rate (BLER), throughput (see Figure 7) and the CRC error for each block. Guard-Band Nb-Iot Simulation A related example demonstrates operation of NB-IoT in the guard band of an LTE signal. The project is essentially the same as in the

64 l New-Tech Magazine Europe