Optical Communication System, Advanced: Resume, Radio Over Fiber

 

Performance Evaluation of Millimeter Wave and UWB Signals over Fiber Radio Networks

 

 

Radio-over-fiber (RoF) networks is the integration of wireless and optical networks. RoF networks is considered as a potential solution for increasing capacity and mobility as well as decreasing costs in the access network. RoF can also be called as Fiber radio networks or Hybrid optical-wireless networks. The concept of RoF means to transport information over optical fiber by modulating the light with the radio signal. This modulation can be done directly with the radio signal or at an intermediate frequency. RoF technique has the potentiality to be the backbone of the wireless access network. Such architecture can give several advantages, such as reduced complexity at the antenna site, radio carriers can be allocated dynamically to the different antenna sites, transparency and scalability. The Fiber-Radio networks transmits data from Central office (CO) to Base station (BS) as optical signal and Base station (BS) to Mobile station (MS) as wireless signal. Due to the signal degradation mechanisms present both in fiber and wireless transmissions, the radio signals transmitted using RoF technology undergoes degradation in their quality, which in turn reduces the overall performance of the RoF system. This paper aims to analyze and evaluate the performance of RoF System, by considering Millimeter wave and UWB signal transmission for WLAN. Special attention is made on chirp effect caused by Mach Zehender (MZ) external optical modulator.

 

There are three main RoF system architectures in use in current c ommercial in building wireless deployments. All use either direct modulation of a laser diode or external modulation . The three types of radio over fiber available are as follows:

 

• RF transmission over single mode fiber directly at the radio carrier frequency is the simplest method.

 

• IF transmission over multimode or single mode fiber.

 

• Digitized IF over multimode or single mode fiber.

 

Among the three methods RF is the simplest design due to the lowest cost to implement, low noise and low distortion at high frequency.

 

The RoF technology holds many advantage such as low attenuation loss large bandwidth immunity to electromagnetic interference, easy installation and maintenance, Reduced power consumption, dynamic resource allocation.

 

Modulator chirp effect: The chirp parameter is important in externally modulated systems, since the frequency of the pulse changes with time. Chirp indicates the phase of the output signals from the modulator. While MZ interferometer-type intensity modulators (MZM) can be designed to operate completely chirp-free, in most devices there is a small residual chirp arising from an asymmetry in the overlap of the electric fields at each electrode.

 

Millimeter wave and UWB Signals

 

            Mm-wave is a common name for an electromagnetic wave of frequency between 30 GHz to 300 GHz and the wavelengths for these frequencies are about one to ten millimeters. Large bandwidth, small size of antennas and RF circuits make the distribution of mm-waves cost effective. The FCC defines UWB as any signal that occupies more than 500MHz bandwidth in the 3. 1 to 1O.6GHz band. This is the largest spectrum allocation for unlicensed use that FCC has granted. UWB is emerging solution for the future wireless network. Hence, significant applications of both the mmwave and UWB signals can be found in the field of short-range communication systems when link distances range from a few meters up to one kilometer.

 

 

 

RESULTS AND DISCUSSION

 

Chirp effect of power degradation and SNR is simulated for 30, 60 and 300 GHZ frequencies. According to analysis, for Frequencies above 650Hz the negative chirp value has higher Power degradation and results in low SNR. Therefore designing of WLAN in UWB based RoF system considering Negative chirp value will provide higher performance than millimeter wave.