**4.4 All-photonic digital radio over fiber**

An all-photonic DRoF architecture has been proposed (Abdollahi et al. 2011) and is depicted in Fig. 15. This architecture uses an electro-optical modulator, which is simultaneously shared as an optical sampling and modulating device at the CS. A photonic ADC (PADC) by using a mode-locked laser (MLL) and an electro-optical modulator is able to scale the timing jitter of the laser sources to the femtosecond level, (Kim et al. 2007; Bartels et al. 2003), which allows designers to push the resolution bandwidth by many orders of magnitude beyond what electronic sampling systems can currently achieve. The proposed system includes an all-photonic signal processing block for optical quantization and wavelength conversion of the sampled and symmetrically split signal's power. By using the WDM technique to distribute the generated traffic over different wavelengths exceeding the modulation bandwidth of the fiber on a particular wavelength is prevented.

Fig. 15. All-photonic DRoF architecture, (downlink), (Abdollahi et al., 2011).

In Fig. 15, at the CS, the RF signal is sampled and modulated by optical train pulses that are generated using a passive mode-locked laser. The optical power of the sampled pulses is split into n levels using a symmetrical optical splitter, where n denotes the number of quantization bits. Finally, the split signals are fed to a photonic signal processing block for quantization and wavelength conversion operations.

An all-photonic DRoF architecture has been proposed (Abdollahi et al. 2011) and is depicted in Fig. 15. This architecture uses an electro-optical modulator, which is simultaneously shared as an optical sampling and modulating device at the CS. A photonic ADC (PADC) by using a mode-locked laser (MLL) and an electro-optical modulator is able to scale the timing jitter of the laser sources to the femtosecond level, (Kim et al. 2007; Bartels et al. 2003), which allows designers to push the resolution bandwidth by many orders of magnitude beyond what electronic sampling systems can currently achieve. The proposed system includes an all-photonic signal processing block for optical quantization and wavelength conversion of the sampled and symmetrically split signal's power. By using the WDM technique to distribute the generated traffic over different wavelengths exceeding the modulation

Fig. 15. All-photonic DRoF architecture, (downlink), (Abdollahi et al., 2011).

quantization and wavelength conversion operations.

In Fig. 15, at the CS, the RF signal is sampled and modulated by optical train pulses that are generated using a passive mode-locked laser. The optical power of the sampled pulses is split into n levels using a symmetrical optical splitter, where n denotes the number of quantization bits. Finally, the split signals are fed to a photonic signal processing block for

( 1) "\*2 *<sup>n</sup> A*

( <sup>0</sup> ) *A*"\* 2

**4.4 All-photonic digital radio over fiber** 

bandwidth of the fiber on a particular wavelength is prevented.

The quantization procedure is performed by the process of Fig. 16 in which A and A' are constant parameters. At the first stage of this process, the stage number is equal to '1' ( S=1). In this process entire stages are equal to number of quantization bits, i.e., for each output bit there is a corresponding quantization stage. For quantization of the most significant bit (MSB) the received signal from output number 'n' of the symmetrical splitter SP\_out(M) that is defined by the generic number 'M' which is equal to 'n' in this stage. This output optical signal is compared with a reference quantization level equal to '2(M-S) \*A'. If the signal power square is greater than or equal to '2(M- S) \*A', the output quantization bit is '1'. Otherwise, it is '0'. In this scheme, for performing the pipeline architecture, the quantized bits are converted back into analog domain. Therefore, in stage number '(M-S)', the converted back analog signals from stages 'n' to '(M-S+1)' of the process , are subtracted from the input of the split output signal SP\_out(M-S). Then, the given signal is compared with '2(M-S) \*A'. The quantization process is repeated in parallel 'n' times for quantizing each sampled optical signal into 'n' bits.

Fig. 16. All-photonic signal processing technique, (Abdollahi et al., 2011).

Subsequent to the wavelength conversion, the digital photonic signals are multiplexed in the wavelength domain by using a WDM and transmitted over a fiber. At the BS, the received signal is demultiplexed by wavelength division demultiplexer (WDD) and fed to the photonic digital-to-analog converter (PDAC). The PDAC subsystem, receives digital optical signals on different wavelengths, and converts them back to the equivalent analog signal at wavelength λ by using a passive PDAC and all-optical wavelength conversion. In the following of this stage, by using a photo diode (PD), the RF signal is recovered and after some RF signal processing it is passed to a multi-band distributed antenna system.

According to results provided (Abdollahi et al., 2011), it is demonstrated that ARoF is more dependent on fiber network impairments and length than DRoF. However, very low phase noise photonic sampling pulses and high speed signal conversion rates can be achieved in an all-photonic DRoF system compared with high-speed electronic circuits generated sampling pulses, signal conversion and processing. Consequently, an all-photonic DRoF system can support a digitized RF signal transmission system for providing superbroadband access to remote distributed wired and wireless access networks. It follows that, compared to the present digital optical communication infrastructures the number of CS would decrease with the introduction of all-photonic DRoF systems and as a result the service providers and network operators cost overheads per bit would be reduced.
