## 2 Principle of CCIS Operation Based on SAC-OCDMA Systems  
CCIS is a technique in which a combination of a CW laser (coherent source) and an LED (incoherent source) is adopted together for one user. The signal originating from the CW laser always passes without a subtraction process, while the sliced LED signals are involved in the subtraction process where the MAI is targeted. In Figure 1a, where the DPS code is used, the yellow block of the DPS code is encoded via a sliced LED, which circulated to the subtraction operation, whereas the green color block is encoded via the laser with no overlapping. Likewise, in Figure 1b, where the FRS code is used, the output is ultimately decoded with the desired shape. The single photodiode (SPD) detection technique is investigated in this work for a trade-off between the LED and the CW laser.  
Figure 1: Block representation of (**a**) DPS and (**b**) FRS encoding using CCIS design.  
The wavelength spectrum in the case of SAC is divided into bins similar to the spectral code allocated to certain subscribers [24]. The data in a CCIS design is not distributed across the LED and laser wavelengths. In fact, the split LED spectrum and laser carry the same data bits and are used as a code sequence. For example, the pseudorandom binary sequence (PRBS) of user#1 is coded with the DPS code sequence \"100010010100\" [11] via the on-off keying (OOK) modulation technique. In this modulation, each \"1\" of information is mapped accordingly to a specific code sequence. Figure 2 depicts the data distribution between the LED and laser wavelengths. For example, if the data sequence is \"PRBS = 1001\", then the coded data for the same sequence using the CCIS design will be \"100010 01010000100010 010100\".  
Figure 2: (a) CCIS design; (b) OSA spectrum of coded data using the CCIS technique.  
The DPS code has a feature of chip arrangements that are separated from one another at a regular distance. This feature, fortunately, matches with the CCIS design in which two blocks are proposed; the first block contains p that represents the number of bits \"1s\" having a length of p2 which is encoded via the LED and the remaining block comprises only bit \"1\" having a length of p that is assigned to the laser, as shown in Figure 1. The ultimate goal of the receiving section is to detect and recover the desired spectrum with a high autocorrelation and a low cross-correlation between the desired and interfering subscribers. Figure 3 shows a block diagram of the CCIS network where the incoming signal from the laser always passes without a subtraction process. In contrast, the signal coming from a sliced LED always needs subtraction since an MAI always resides in the LED portion. The detection arrangement at the receiving end is also configured to recover the entire encoded spectrum (containing both the LED and laser diode spectrums) in an attempt to yield maximum autocorrelation (equal to the weight of the code) and minimum cross-correlation. The output of the detector arrangement is sent to a photodiode for optical/electrical conversion and decision-making circuitry, respectively. The decision-making circuit computes the probability density function (PDF) of the received current with a certain threshold value to determine binary '1' or '0'. If the system encounters nonlinearities, such as four-wave mixing (FWM), dispersion, and cross-phase modulation (XPM), then an optical source that trades off all the advantages of coherent and incoherent sources is utilized. The mechanism could be achieved by merging both the coherent and incoherent sources all at once.