As the number of users and higher data usage requirements increase, higher capacity networks require the deployment of more base transceiver station systems, resulting in more transmit and receive channels per base transceiver station system and in increasing medium to small cell sizes. Cellular providers, municipalities across the globe, and wireless OEMs are pushing for higher system efficiencies. Next-generation base transceiver station systems require and must leverage proven and innovative high performance RF components to optimise cost, size and power efficiency.
Evolution in base transceiver stations
As air interface standards have evolved from AMPS (advanced mobile phone system) in the 1980s to the high-performance 3G of today and the 4G systems of tomorrow, so have the architectures of base transceiver station system transceivers.
The high-speed data converters used in base transceiver stations are evolving rapidly as radio transceiver design engineers drive to add LTE and LTE-Advanced functionality, as well as strive to reduce power and system size. These dynamics lead to data converters with more channels (moving from dual to quad), higher sampling rates (to 250Msample/s and beyond for ADCs, to 1.25Gsample/s and beyond for DACs). ADC input bandwidths are increasing to 100MHz and potentially higher, and ADC linearity demands are getting higher; SFDR requirements reach as high as 95dBc. DAC noise figures
are required to be -163dBm/Hz or lower, with 85dBc SFDR linearity. ADC power consumption is driving to approximately 1mW/Msample/s, while DAC power consumption is driving well below 1mW/Msample/s.
Gone are the days of up-convert mixers driving four or five amplifiers on a transmitter that connects to a power amplifier (i.e. class A or feed-forward power amplifier). These transceiver transmit chains have been replaced by ZIF architectures using high-performance quadrature modulators followed by a fixed gain amplifier then a variable gain amplifier before going into the final power amplifier. Variable gain amplifiers have higher gain, wider attenuation range and finer step control than ever before. Pre-driver amplifier devices that were previously only possible in GaAs (Gallium Arsenide) are being replaced by similar performing but significantly more cost effective SiGe (Silicon Germanium) devices that enable the next step in transmit channel integration.
Evolutionary changes
Receivers have continued to change as technologies have enabled evolution from super-heterodyne receivers to low-IF sampling receivers or zero-IF sub-sampling receivers. Technology developments have enabled tower mount amplifiers to decrease in size and weight due to power efficient devices, which do not require massive heat slug and allow passive cooling and multiple stage low noise amplifiers and passives to be combined into smaller modules. Improved SiGe BiCMOS process technologies with improved minimum noise figure and high break down voltage have enabled the integration of mixers and differential IF amplifiers that can handle wider instantaneous bandwidths and higher order modulation schemes (16 and 64QAM) signals demanded by today's leading air interface standards. IF variable gain amplifiers now offer outstanding linearity performance plus smaller gain control steps that directly drive SAW or ceramic filters into ADCs, enabling higher performing IF receiver chains.
At the end of the wired signal transmit chain there is the power amplifier, either mounted at ground level, next to the base transceiver station or close to the antenna, as in a tower mounted amplifier.
Next-generation, LTE systems will occupy frequency bands from 700 to 3600MHz, with ever increasing requirements for power density, efficiency and cost. Power amplifier architectures will continue to evolve around Doherty derivatives, both symmetrical and asymmetrical, with power transistor designs to suit. Power amplifier performance will also be stretched in terms of linearisation, as digital pre-distortion (DPD) techniques evolve and diverge. Within the next five to 10 years the industry will see the emergence of switched mode power amplifiers in commercial systems, necessary to reach new levels of power density and efficiency performance.
The most efficient signal chain
As wireless OEMs continue to push for higher efficiency, higher throughput and smaller sized systems, they continue to look to suppliers that offer a broader product portfolio from multiple semiconductor technologies. Companies like NXP supply components and enabling technologies to construct the entire signal path from bits to the antenna, and from the antenna back to bits. Companies that offer a portfolio of proven RF and converter capabilities for 3G and 4G will be able to provide high performance RF components that enable an array of OEM integration paths to drive cost, size and power efficiency.
Increased mobile data usage will be a driver for integration and size reduction of base transceiver stations and receive line-ups. This integration will drive the component performance and reliability requirements to higher levels.
Figure 1: Mobile data will increase demand for base transceiver stations.
