The increasing demand for data services on mobile phones puts continuous pressure on base station designs for more bandwidth and lower cost. Many factors influence the overall cost to install and operate additional base stations to serve the increased demand. Smaller, lower power electronics within a macrocell base station help to lower the initial costs as well as the ongoing cost of real estate rental and electrical power consumption for the tower. New architectures such as remote radio heads (RRH) promise to decrease costs even further. Tiny picocell and femtocell base stations extend the services to areas not covered by the larger macrocells. To realize these gains, base station designers need new components with very high levels of integration and yet they cannot compromise performance.
Integration in the RF portion of the radio is especially challenging because of the performance requirement. Over a decade ago, the typical base station architecture required several stages of low noise amplification, down-conversion to an intermediate frequency (IF), filtering and further amplification. Higher performance mixers, amplifiers and higher dynamic range analog-to-digital converters (ADCs) with higher sampling rates have enabled designers to eliminate down-conversion stages to a single IF stage today. However, component integration remains somewhat limited. Mixers are available with buffered IF outputs, integrated balun transformers, LO switches and dividers. A device with a mixer and a PLL for the LO represents a recent advance of integration. Dual mixers and dual amplifiers are available. As yet, no device is available that integrates any portion of the RF chain with the ADC on the same silicon. This is primarily because each component requires unique semiconductor processes. The performance trade-off associated with choosing a common process has been unacceptable for the application.
In parallel, the handset radio has evolved to highly integrated baseband and transceiver ICs and integrated RF front-end modules (FEM). RF functional blocks between the transceiver and antenna include filtering, amplification and switching (with impedance matching incorporated between components where needed). The transceiver integrates the receiver ADC, the transmit DAC and the associated RF blocks. Here the performance requirement is at a level such that a common process is viable. The FEM utilizes a system-in-package (SiP) technology to integrate various ICs and passives, including multi-mode filters and the RF switches for transmit and receive. Here, a common process was not viable but integration was still required.
The performance requirements for the RF/IF, ADC and DAC components in picocell and femtocell base stations tends to be much lower than for macrocell base stations because their range, power output and number of users per sector are lower than for macrocells. In some cases, modified versions of components for handsets can be used for picocell or femtocell base stations, providing the necessary integration, low power and low cost. Here, a common semiconductor process provides sufficient level of performance for all of the functional blocks in the signal chain. How can this level of integration be achieved for macrocell base stations?