19 Aug 2015
Utilizing SDR for greener wireless communication
Wael Abdullah, co-founder of Spectratronix looks at the ways in which software defined radios can be used to reduce power consumption and make communications greener.
Generating, storing and saving energy is the hot research arena in every industry and science branch nowadays in order to reach a more sustainable and greener planet.
Wireless communications have already become the fastest growing section in the electronics industry, where the number of mobile subscribers is now around, if not larger than, the Earth inhabitants 7.3 billion users.
Every subscriber needs cheaper bundles, higher data rates, and to charge the mobile device less times per week. On the other hand, operators need more efficient spectrum utilization, wider bandwidth and more economically cooling requirements to cut down both capital and operational expenditures. In addition, governments enforce use of environment-friendly equipment with lower heat and carbon emission.
Base station power reduction
Among the different units in the mobile network, the base station is the most expensive and power hungry component. Unfortunately base stations transmit the wireless power with less than 40% efficiency nowadays, that’s more than 60% of their consumed power is wasted as heat by their transmitters, more specifically, mainly by their power amplifier (PAs).
Figure 1: Basic software defined radio system
Usually PAs work at peak-power region with maximum efficiency, but suffer inherent nonlinearities at this region. These non-linearities cause in-band distortion, damaging the signal quality, and out-of-band spectral regrowth, that affects neighbour systems over the spectrum.
Wireless standards specify the linearity requirements by two metrics: error vector magnitude (EVM), that addresses the in-band distortion, and adjacent channel leakage ratio (ACLR), that addresses the out-of-band emissions. Traditional power back-off solution makes PAs obey the linearity requirements but at the expense of lower efficiency. The industry is calling all parties for having PAs as efficient as possible yet obeying the linearity requirements.
Power devices foundries contribute at the technology level through material selection and device architectures. Gallium nitride (GaN) exceeds the low-cost silicon in power and frequency performance, while high electron mobility transistors (HEMTs) have shown promising advancement leaving the traditional metal-oxide-semiconductor (MOS) transistors out of this arena. That’s why one can notice that GaN HEMT industrial product lines are emerging the market through different providers such as Cree, Qorvo, NXP, Freescale and others.
Power amplifier design
PA designers play their role at both system and circuit levels. At the system level, they deal with linear PA structures, preserving the linearity, and employ smart techniques to keep the efficiency high, such as load modulation in Doherty PAs and drain voltage modulation in envelope tracking (ET) PAs. At the circuit level, they utilize the saturated operation of the devices while employing waveform and harmonics engineering to obtain 80%-90% practical PA efficiency.
Superior results were reported recently highlighting the Class-F and the inverse Class-F designs. The linearity of those PAs can be improved by signal processing linearization techniques, such as feedback/feedforward distortion compensation and pre-distortion linearization, which uses a standalone module to pre-distort the input signal to the PA.
Recently, digital pre-distortion (DPD) showed excellent performance and it tends to be a fundamental component in current and next-generation wireless communication systems. Regardless the circuitry of the PA, DPD systems utilize behavioural modelling techniques to characterize and pre-invert the PA behaviour at the system level. The challenges in the model extraction and the inversion open the door for digital signal processing (DSP) engineers to enter the competition of moving greener wireless communications forward.
Linearity requirements, EVM and ACLR, differ from standard to another, so PAs behaviours do. This calls for smart adaptive DPD modelling algorithms, that are able to capture the various nonlinear behaviours under different standards. Different DPD platforms are being utilized to evaluate the overall system performance, some universities develop their own, while some commercial digital processor vendors, such as Xilinx and Altera, also provide DPD intellectual property (IP) cores. But due to their high flexibility and multiple standards support capability, commercial measurement instruments are widely used to build DPD linearization solutions.
Figure 2 The improved spectral efficiency gain from using an SDR
Software defined radio (SDR) platforms don’t only shorten the time-to-market, but also ease the integration among different parties in the competition, where power devices vendors can show their customers how a linearizable solution can be built by their devices, PA vendors can evaluate the linearizability level of their designs, while DSP IP core providers can evaluate their own algorithms.
Some cost-effective modular instrumentation equipment are already being utilized in DPD utilizing their customization degree of freedom, Spectratronix’ C700 platform is one example, other commercial PXI platforms, offered mainly by Keysight and National Instruments (NI), do also the job. Some vendors of these modular instrumentation hardware began providing a built-in DPD algorithms, while others leave it open to build an arbitrary DPD algorithm for an arbitrary application.
Greener wireless communication is being moving forward through technology power device providers, PA vendors and DSP designers. Modular instrumentation platforms accelerate this movement. One more forward step is being established through universities and research institutes by equipping their labs with these platforms while directing the research to this hot area.
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About the author
Wael Abdullah is an RF/MW design engineer whose applied and theoretical research interests include reconfigurable RF/MW resonators and frontends, efficient RF PAs, metamaterials and MW cloaking. Currently Wael is RF/MW Head of Department in Consultix and Co-Founder in Spectratronix. He received his M.Sc. and B.Sc. degrees from Ain Shams University in 2012 and 2007 respectively.
Spectratronix designs and manufactures Test-beds, Development Platforms, RF/MW Components and Subsystem Modules, for academic, commercial, industrial, telecommunications, test equipment and military applications
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