et on one sub-carrier causes inter-(sub)-carrier interference in the time domain, since one sampling interval overlaps that of another sub-carrier. It can be corrected by generating an error term that is used to drive a sampling rate converter. 2.5.4 High Peak to Average Power Ratio (PAPR) A process OFDM signal can have large peaks resulting in a large dynamic range and a high PAPR. For multi-carrier systems, the PAPR value is often expressed in terms of statistics because the probability that all sub-carriers will simultaneously reach peak amplitude is low, even though the simultaneous peak amplitude value is large. If the received signal level is very high it can saturate receiver amplifiers or D/A converters; the result will be a distorted signal. The distortion will increase the SNR needed to maintain adequate performance. Linearity requirements in both the receiver and transmitter must be adjusted to account for PAPR. One way to combat a high PAR on the downlink is to leave the sub-carriers empty that do not need to send information. Thus, no unnecessary energy is added to the transmitted signal. 2.5.5 Phase Noise The phase noise effects the OFDM signal reception in two ways namely inter-carrier interference (ICI) and common phase error (CPE). The ICI can be modeled as Gaussian noise (additive white noise) and it is difficult to remove due to its noise-like characteristics. CPE rotates all sub-carriers equally and can be easily corrected by estimating such rotation through continuous pilot tones embedded in OFDM symbols. Next Generation Mobile Communications Ecosystem 2.5.6 I/Q Imbalance Commonly, the OFDM signals are combined with high order modulation (64-Quadrature Amplitude Modulation) to maximize spectral efficiency and achieve broadband data rates. Sophisticated signal processing algorithms are needed to avoid imperfections that are typically present in low-cost direct conversion RF receivers. The analog in-phase and quadrature (I/Q) modulators and demodulators are often used in OFDM communications. Th