10 dB -10 dB 20 MHz -20 dB -30 dB -40 dB -50 dB 60 dB -70 dB -80 dB Spectral density of the candidate waveforms under ideal conditions 10 dB 110 BB 20 MHz 20-BB 30 dB 404 BB 50 BB 6-60 BB +70 dB -80 BB Spectral density of the candidate waveforms with nonlinear power amplification Figure 3.28 Comparison of a 20 MHz LTE downlink OFDM signal (yellow) with FBMC (blue), UFMC (green), and GFDM (orange) signals under ideal (top) and non-ideal conditions [45]. For color interpretation of this figure, please refer web version. gain of 20 dB. Maximum achievable output power for the PA is + 20 dBm. At 0 dBm input power, the PA starts to enter the saturation region. Higher input power would mean that the PA would be operating in the compression region. Fig. 3.28 further shows the result of a measure- ment with the same signal configuration for an input power of -2 - dBm to the amplifier. The spectral advantages of the candidate waveforms seem to have almost disappeared compared to 350 Chapter 3 a 20 MHz LTE downlink signal. When using typical input power of 0 dBm, the advantages of non-OFDM waveforms will completely vanish [45]. 3.3 Multiple-Access Schemes A cellular network consists of a number of fixed base stations distributed across a geographi- cal area. The coverage area is divided into cells and a mobile station communicates with one or more base stations in its proximity. There are two main issues in the physical and medium access layers of cellular communication schemes: multiple access and interference manage- ment. The first issue addresses how the overall radio resources of the system are shared by the users in the same cell (intra-cell) and the second issue addresses the interference caused by simultaneous signal transmissions in different cells (inter-cell). At the network layer, an important issue is to provide and maintain seamless connectivity to the users as they move from one cell to another and thus switching communication link from one base station to another through an operation known as handover. There