# Constant Envelope Modulation Techniques for Limited Power Millimeter Wave Links  
[PERSON]  
1 Faculty of Engineering, Ariel University, Ariel 40700, Israel; [EMAIL_ADDRESS] (M.P.); [EMAIL_ADDRESS] (Y.P.)1 Faculty of Engineering, Ariel University, Ariel 40700, Israel; [EMAIL_ADDRESS] (M.P.)2 Faculty of Engineering, Ariel University, Ariel 40700, Israel; [EMAIL_ADDRESS] (M.P.)3 Faculty of Engineering, Ariel University, Ariel 40700, Israel; monikap@  
The short wavelength of the millimeter waves enables the utilization of small equipment and antennas. The MMW cell will be a few hundredths of a meter, having multiple antennas communicating in a massive multiple-input multiple-output (MIMO) arrangement [14; 15]. Beamforming techniques will allow for the operation of multiple antennas to direct the transmission to the operators [16; 17; 18].  
The power produced by solid-state devices is limited to several tens of milli-watts in MMWs. This limitation calls for effective signaling utilizing constant envelope (CE) waveforms with a low peak to average power ratio (PAPR). The utilization of constant envelope techniques enables a non-linear class of operation, resulting in a better efficiency of energy consumption. This is particularly important for battery-powered mobile appliances because of their limited power resources [19; 20; 21].  
Constant envelope (CE) orthogonal signaling is proposed as an efficient modulation technique for wireless communication systems [22; 23]. The peak to average power ratio (PAPR) in a CE waveform is 0 dB. The popular orthogonal frequency division multiplexing (OFDM) signaling, which is commonly used in communications, presents envelope fluctuations, resulting in a high PAPR [24].  
Several CE techniques have been suggested. In the literature [25; 26], the information-bearing message signal is transformed into a constant envelope waveform by the utilization of the phase modulation. In the CE-OFDM discussed in the literature [25], the OFDM waveform is used to phase modulate the carrier, while in our proposed CE-M-ary time orthogonal (CE-MTO) technique described in [26], orthogonal waveforms in the time domain were used. In our previous work [26], we showed that the implementation of the CE-MTO technique is expected to be simpler than that of the CE-OFDM, and that the symbol error rate (SER) performances in the presence of additive white gaussian noise (AWGN) and fading channels are comparable for both techniques.  
Interference is a significant challenge of wireless communications that has a detrimental effect on the overall link performances. Although it cannot be completely mitigated, the interference effect can be reduced to a certain extent. Several types of interferences exist [27], one of which is co-channel interference (CCI) [28]. It occurs when the interfering signal overlaps with the desired signal in the frequency domain. In order to mitigate the CCI, many methods have been proposed [29; 30; 31; 32]. Channel coding is a traditional and effective way to reduce the CCI, and also provides some robustness [33].  
In this paper, we examine the symbol error rate (SER) in the presence of CCI and AWGN, for both CE-OFDM and CE-MTO techniques. An analytical expression is derived, from which one can estimate the degradation caused by the CCI in an MMW channel employing constant envelope signaling.  
In Section 2, we review the link budgets for the required signal and the interfering one, both operating in the millimeter wave regime. Section 3 presents an analytic study of the detection for a constant envelope modulated signal in the presence of co-channel interference. In Section 4, additive white Gaussian noise is added to the analysis for calculating the symbol error rate. The simulation results are given in Section 5 for different CCI scenarios. A comparison is made with the results obtained from the analytical calculations. Section 6 summarizes and concludes the paper.