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Examining LTE-A Release 12 transmitter (Part 2)

30 Mar 2015  | Damian Anzaldo

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Consequently, for downlink CA applications the important system-level benefits of a wideband RF DAC transmitter include: LTE band coverage up to 2.2GHz, significantly fewer IC devices, no IF or base band analogue filters, and a footprint that occupies the smallest possible PCB area.


An RF DAC AAS design example
The following case study highlights a typical RF DAC transmitter application in an AAS design. As described earlier, AAS with embedded RF dedicates a radio transceiver to each cross-polarized antenna element. A dual-column, side-by-side antenna array with 16 cross-polarized elements is used as an example. There are a total of 16 dual-channel RF transceivers (2T2R) with DPD (2DPD) as shown in figure 7.


Figure 7: Assembly diagram showing dimensions of an active antenna system comprising an array of 16 cross-polarized antenna elements.


The TX signal path in each transceiver has its own RF PA, typically 5W, for a total power output of 80W. A high-band antenna measures 305mm (W) x 1270mm (L) x 178mm (D). The available area for electronics inside the antenna is less than 2062 square centimeters. After allowing for mechanical clearance, each of the 16 transceivers (including its power management, all measurement/control functions and RF duplexers) needs to occupy a PCB area of approximately 130 square centimeters (figure 8).


Figure 8: Each AAS radio transceiver (2T2R+2DPD) must occupy about 130 square centimeters.


Essentially a complete 2T2R+2DPD eNodeB radio transceiver must shrink to half the size of a typical shoebox. To overcome this size constraint, its obvious dense RF integration is a key design parameter in next-generation AAS radios. Furthermore, in AAS applications power consumption and heat dissipation become critical because the RF transceiver channel count is high, the antenna units are passively cooled, and the system is exposed to high-ambient outdoor temperatures. An RF DAC transmitter is approximately 1W less power than conventional architectures. Since the antenna has 32 physical transmitter channels, a power savings of 1W per TX channel sums to 32W power savings per antenna system. This power savings and reduced heat dissipation is important in outdoor tower-mount AAS installations where compact size and high reliability are critical. For AAS applications the RF DAC transmitter yields significant power savings and considerable size reduction, while delivering all the previously described system-level benefits.


Conclusion
Over the next five years the volume of mobile data traffic and number of mobile broadband users is expected to grow exponentially. New bandwidth-hungry mobile services like multimedia broadcast, HD video, and file sharing will escalate demand for higher peak data rates. But available cellular spectrum is both limited and a valuable resource that must be utilised efficiently. Further, to stay competitive, mobile network operators must offer the highest quality-of-experience service at the lowest cost per bit. For long-term profitability, it is clear that LTE-Advanced is the wireless service provider's investment of choice. The radio access technologies outlined in 3GPP Rel-12 address the peak data rates, spectrum utilisation, and network efficiency that service providers need to sustain 4G mobile broadband demand. However, wideband CA, AAS with embedded RF, and high-order downlink MIMO present new integration challenges to eNodeB radio designers.

RF analogue integration is essential for overcoming radio integration challenges in 4G base stations. The new RF DAC direct-conversion transmitter is a technology disruption that diverges from conventional solutions. It gives radio engineers the means to shape alternative eNode transmitter architectures. Compared to conventional RF transmitters, an RF DAC like the MAX5868 reduces system cost and complexity, occupies less PCB area, consumes lower power, and delivers ultra-wideband performance with LTE band coverage up to 2.2GHz. To successfully deploy LTE-Advanced Rel-12 features each parameter is critical for next generation eNode transmitters.


About the author
Damian Anzaldo is Principal Member of Technical Staff in Field Applications at Maxim Integrated. During his time with Maxim, Damian has published numerous articles and circuit ideas on topics ranging from power and battery management to precision data conversion and RF/wireless communications. His past roles at Maxim include high-speed data converter Product Definer within the High-Speed Signal Processing group, and Technical Marketing Manager for the wireless communication segment. Damian holds a BSEET from Temple University and ASET from Penn State University.


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