# Candidate Waveforms for ARoF in Beyond 5G
## 2 Requirements to Waveforms for Beyond 5G
### Requirements for General Wireless Communications  
In terms of general requirements for wireless communications, a prospective waveform format should include the following features. * Peak-to-average power ratio: PAPR indicates the relationship between the maximum peak and the average transmitted power of the signal. The worst impairment metrics associated to high PAPR in terms of communication are high power consumption and severe signal distortions. Note that energy efficiency is one of the most important requirements in 5G [13]. Signal distortions due to high power values are caused by nonlinearities in devices such as Mach-Zehnder modulators (MZMs) and power amplifiers, causing spectral regrowth and higher bit error rates (BER) [14]. Thus, a waveform format that produces high PAPR is not suitable for an energy efficient network. * Spectral efficiency: Due to both licensing requirements and the spectrum scarcity resulting from the increasing transmission bandwidth requirement with demand for any time, anywhere, any situation communication this indicator is considered pivotal. The spectral efficiency is a very  
Figure 2: Relationships among requirements for candidate waveforms and 5G requirements. Three subsets of waveform requirements are defined: the ones for general wireless communications, the specific ones for mmWave wireless communications, and the ones for ARoF transport of high-bandwidth signals. important factor in a system because it is directly related to its bit rate achieved. According to the 5G requirements proposed in ITU-R M.2410-0 [13], the peak spectral efficient target is 30 bit/s/Hz and 15 bit/s/Hz for downlink and uplink respectively. * Block processing delay: This requirement is seen relevant since it affects the final latency. A waveform format with high complexity suffers large block processing delays. Additionally, block processing delay is lower-bounded by symbol duration in many cases. The final latency of any communication can not be less than the block processing time. The latter can be reduced by employing techniques such as pipelining, efficient algorithms or by reducing symbol temporal period. One of the most challenging objectives in 5G is to reach communications with a maximum delay of 1 ms for the user plane [13]. * Robustness to frequency-selective channels: Multipath propagation is a phenomenon present in any wireless communication. It is caused by multiple reflections and refraction processes suffered by the transmitted signal, resulting in a received signal that is dispersed in time. Each path features its own delay and, accordingly, the temporal dispersion can induce to inter-symbol interference (ISI). Delay spread is a measure of the multipath profile of a mobile communications channel. As frequency fading can severely impact transmission, waveforms must be designed to be robust to this impairment. * Robustness to time-selective channels: Most multipath channels are of time-varying nature. That nature arises as, for example, either transmitter the receiver are moving, and thus the location of reflectors in the transmission path, which gives rise to multipath, will change over time. Thus, if we repeatedly transmit pulses from a moving transmitter, we will observe changes in the amplitudes, delays, and the number of multipath components corresponding to each pulse. Regarding the 5G requirements proposed in ITU-R M.2410 [13], the 5G network should support a spectral efficiency of 0.45, 0.8, 1.12, and 1.5 bit/s/Hz for a mobility speed of 500, 120, 30, and 10 Km/h, respectively, and thus robustness to time-selective channels is key for candidate waveforms. * Out of band (OOB) emissions: Linked to the spectral efficiency, this parameter is very significant as the radio spectrum is generally shared by different users, providers, and technologies. In order to efficiently support multiplexing of services, both in-band and out-of-band emissions must be kept to a minimum, so that services being transmitted on adjacent frequency channels do not interfere with one another.