# A Reconfigurable Radio-Frequency Converter IC in 0.18 \(\upmu\)m CMOS
## 2 Broadcast Architecture  
GPONs have a tree P2 MP structure with passive optical splitters (typically 1:32 or 1:64) to divide the signals and drive them from the service provider endpoint, called _optical line termination_ (OLT), to the customer premises, where a piece of equipment called _optical network terminal_ (ONT) allows the obtaining of the services (Internet, television and telephone) provided with triple-play. Both the up- and down-links in a GPON use the same optical fiber, which is shared using wavelength division multiplexing (WDM). Figure 1 shows a scheme of the distribution of contents in a GPON network from the OLT to the different ONTs, both in residential and industrial areas. A standard implementation of GPON uses _class-B_ transceivers, which allow a transmitter laser power of over 1 dBm and a receiver sensitivity of \(-27\) dBm [14]. Because one splitting stage introduces  
Figure 1: Scheme of a GPON network with an indication of how the distribution of contents from the OLT to the different ONTs using the carriers at 1310 nm and 1490 nm is carried out. roughly 20 dB of attenuation, and the typical losses of the fiber are 0.4 dB/km, the above figures mandate that the distance between the OLT and the ONT in a GPON cannot be longer than 20 km [14]. In terms of the transmitted signal, as it is shown in Figure 1, the down-link uses only one carrier at a wavelength of 1490 nm, which is modulated directly at the OLT at a maximum speed of 2.5 Gbit/s. This signal is then broadcast to all the users connected to the GPON, and it is every ONT that filters out the traffic that is not directed to it. The up-link, in turn, uses time-division multiplexing access (TDMA) to avoid collisions with the other ONTs connected to the GPON. The traffic in the up-link is limited to 1.25 Gbit/s, and the ONTs operate modulating a single carrier at 1310 nm. One solution to alleviate the saturation of the bandwidth in GPON, which has already been adopted by some of the biggest operators in the world, is the use of a third wavelength in addition to WDM to deliver the content. This wavelength, which is known as _third lambda_, is set at 1550 nm, and it is used exclusively to deliver video and television broadcasts to the clients directly in radio-frequency (RF). The third lambda is used only in the down-link from the OLT to the different ONTs; therefore, not having an up-link, it is not suitable for interactive or on-demand content. In this way, the approach to implement a third lambda configuration is to broadcast all traditional television channels using the additional down-link at 1550 nm, freeing the corresponding bandwidth in the 1310 nm and 1490 nm links. Because the RF signal sent in the third lambda allows only very simple (i.e., spectrum inversion) or even no encryption, it is not suitable to transmit premium contents subject to subscription, which are still sent by IP using the conventional up- and down-links at 1310 nm and 1490 nm [15]. Figure 2 shows a detailed scheme of the structure of a GPON to implement triple-play services using the third lambda. The IP packages containing the services of telephone, Internet access, and interactive and premium television are generated at the ONT and modulated over the down-link carrier at 1490 nm. In turn, all traditional television channels coming from the header are modulated over the carrier at the third lambda of 1550 nm. Both signals are combined and fed to the optical fiber, where they travel until they are split before reaching the ONT at the end user's premises; once there, the wavelengths are separated using an optical triplexer. The IP packages carried by the traditional down-link at 1490 nm are sorted out and those directed to the end user are distributed to the appropriate device. For its part, the RF signal containing the television broadcast in the third lambda of 1550 nm is converted to the electrical domain and demodulated at the ONT, from where it can be fed directly to the television set at the end user's premises.