# Differential Service in a Bidirectional Radio-over-Fiber System over a Spectral-Amplitude-Coding OCDMA Network

^{1}

^{2}

^{*}

## Abstract

**:**

## 1. Introduction

_{i}and P

_{i}is the number of users and power in class i, respectively, where i = 1, 2. The power level P

_{2}is higher than P

_{1}, so a higher signal-to-noise ratio (SNR) is detected for Class 2 and better service is achieved. Furthermore, the desired user can be identified through MAI elimination even if all coded signals are multiplexed in the same wavelength band.

## 2. The Proposed Bi-Directional RoF System with Power Control Scheme

^{2}+ p + 1, ω = p + 1, and λ = 1, where M is the code length, ω is the code weight, λ is the cross-correlation, and p is a prime number. Two MSSP codes of p = 2—(1, 0, 1, 0, 1, 0, 0) and (0, 1, 0, 1, 1, 0, 0)—are assigned to user#1 and user#2, respectively. The power of BLS#2 is twice than that of BLS#1, the coded spectrum of these two are (0, λ

_{2}, 0, λ

_{4}, λ

_{5}, 0, 0) and (2λ

_{1}, 0, 2λ

_{3}, 0, 2λ

_{5}, 0, 0). The corresponding power spectral densities (PSDs) of the two users are shown in Figure 3.

**C**and

_{i}**C**is described as follows:

_{j}**C**and $\odot $ is the dot-product symbol. From Equations (1) and (2), we find that the MSSP code has unit correlation and can be used for designing the MAI elimination process:

_{j}## 3. System Performance Analysis

_{k}(t) is the current of BS #k, ${i}_{P}(t)$ and $\overline{{i}_{P}}(t)$ are PIIN terms of the correlated and the complimentary correlated signal, and i

_{TH}(t) is the thermal noise. We firstly denote the transmitted spectra from all users as a time-varying vector:

_{i}(t) is the normalized radio signal with mean $E\left[{r}_{i}(t)\right]=0$ and time-average power $E\left[{{r}_{i}}^{2}(t)\right]=1/2$. P

_{sr}and v are the power and the band-width of BLS, respectively. The signal of the desired BS #k for two classes is derived by correlation subtraction in Equation (2):

_{k}(t) = (1 + r

_{k}(t))/2. One can see that the signal amplitude of Class 2 is twice than that of Class 1. PIIN-induced variances of the correlated signal of the complementary one in BS #k are expressed as:

_{TH}is the power spectral density (PDF) of thermal noise. Therefore, the average SNR of the SAC-based RoF system is formulated as follows:

_{k}(t) is binary phase shift keying (BPSK) signal, the BER expression is:

## 4. Simulation Results and Discussion

^{TM}are both demonstrated. Received user power P is defined as P

_{sr}= P

_{1}= P

_{2}/2, and user numbers are K

_{1}= K

_{2}= 3. Other parameters are set as follows: B = 0.6 GHz, R = 0.9 A/W, and ν = 7 THz. A 20 km single mode fiber (SMF) was used to connect the CO and the BS. A 3-dB-gain erbium-doped fiber amplifier (EDFA) was employed in front of the down- and uplink receiver to compensate the insertion loss from the components of OS‘s and optical combiners.

_{n}denotes the user in class n, n = 1, 2. As the user power increases, lower BER values are reached. The BERs of users in Class 2 are better than those in Class 1, since the photocurrent after correlation subtraction leads to a higher SNR. For Class 1, the power penalty is 3.5 dB at BER = 10

^{−9}. Additionally, the simulation results nearly match the numerical ones. The small difference between these two curves are the results of the vibrations of the light source. The effects mentioned above become severe when BLS power gets large, where the light source vibrates in a larger amplitude and more power leakage occurs. Therefore, the discrepancy between software and numerical simulation is more obvious for Class 2.

## 5. Conclusions

## Acknowledgments

## Author Contributions

## Conflicts of Interest

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**Figure 1.**User classification and power distribution in the proposed differential service system with spectral amplitude coding (SAC) optical code-division multiple-access (OCDMA).

**Figure 2.**Configuration of the wavelength-reused radio-over-fiber (RoF) network with power control technique.

**Figure 4.**Bit-error rate (BER) comparison between numerical and simulation results for Classes 1 and 2.

**Figure 5.**Bit-error rate (BER) comparison between numerical and simulation results for Classes 1 and 2.

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**MDPI and ACS Style**

Yang, C.-C.; Chen, K.-S.; Huang, J.-F.; Kuo, J.-C.
Differential Service in a Bidirectional Radio-over-Fiber System over a Spectral-Amplitude-Coding OCDMA Network. *Photonics* **2016**, *3*, 53.
https://doi.org/10.3390/photonics3040053

**AMA Style**

Yang C-C, Chen K-S, Huang J-F, Kuo J-C.
Differential Service in a Bidirectional Radio-over-Fiber System over a Spectral-Amplitude-Coding OCDMA Network. *Photonics*. 2016; 3(4):53.
https://doi.org/10.3390/photonics3040053

**Chicago/Turabian Style**

Yang, Chao-Chin, Kai-Sheng Chen, Jen-Fa Huang, and Jia-Cyuan Kuo.
2016. "Differential Service in a Bidirectional Radio-over-Fiber System over a Spectral-Amplitude-Coding OCDMA Network" *Photonics* 3, no. 4: 53.
https://doi.org/10.3390/photonics3040053