**3. Overview of wavelength division multiplexing (WDM)**

Various methods can be executed to gain full duplex transmission of RoF system and wave‐ length division multiplexing (WDM) is one of it. WDM has characteristic where the discrete wavelength will form an orthogonal set of carriers that can be separated, routed and switched without interfering with each other [11]. With WDM, many wavelengths are able to be trans‐ mitted over a large distance and the downlink and uplink data can be transmitted at the same time through single‐mode fiber.

The design of WDM involves some requirements such as selecting various optical sources that has narrow spectral emissions bands. The most straightforward and simplest method can be done by choosing a series of individual lasers that emits at its own specific wavelength. The process of selecting optical sources requires small number of wavelength channel but can be cumbersome for links carrying many wavelengths. In order to implement WDM network, a combination of many passive and active component is need. The difference between pas‐ sive and active components is passive components and is limited in their ability since passive components did not require an external control to operate, whereas active component can be controlled and has a wide network ability.

WDM contributes simplification to the network. With this method, different wavelengths are allocated toward individually BSs. Thus, the network will be simple and the service upgrades can be done easier. WDM multiplexes optical signals that came from multiple sources, and the signal will be amplified before it is transmitted through optical fiber to under goes demul‐ tiplexer so that it can be addressed to each BS as shown in **Figure 6**.

There are two types of WDM architecture: coarse wavelength division multiplexing (CWDM) [12] and dense wavelength division multiplexing (DWDM) [13]. Typically, CWDM systems provide eight wavelengths which separated by 20 nm, from 1470 to 1610 nm. To increase the

**Figure 6.** Wavelength division multiplexing.

channel of CWDM to 16, number of wavelength could be increased by using 1310 nm win‐ dow. One of the advantages of CWDM is that the cost of the optics is 1/3 of the cost of same DWDM optic. This makes that the CDWM is preferable than DWDM. CWDM is able to match the basic capabilities of DWDM but with lower cost and capacity. Typically, CWDM is used for short‐range communications. In addition, CWDM equipment is more compact and cost‐ effective if compared to DWDM designs.

DWDM network systems provide up to 96 wavelengths, which normally has less than 0.4 nm spacing. DWDM is used for long‐haul transmission where wavelengths are compacted tightly together. Erbium‐doped fiber amplifiers (EDFA) can help the system of DWDM to work over thousands of kilometers. CWDM is not implemented in long haul transmission, where the distance reaches up to thousands of kilometers, causing simpler overall sytem components requirement. This leads to lesser cost implementation, despite of having some limita‐tions especially in propagation distance.
