Improving performance of solid state amplifiers in satellite earth stations
The design of a satellite communications system is a complex, multidimensional task and no two systems are the same. Having said that there are a number of limiting factors which are common to all systems and every design process involves trade-offs between them. They include:
- Satellite G/T, EIRP & Transponder bandwidths
- Satellite uplink and downlink coverage areas.
- Uplink and downlink adjacent channel (ACI), co-channel (CCI) and multipath interference.
- The G/T, EIRP, beamwidth and pointing accuracy of the ground terminals at each end of the overall link.
- The nature of the signals using the link.
- The propagation characteristics of the frequency bands used, and the location-specific propagation statistics for the ground terminal locations.
The Problem to be Addressed
The satellite owner/operator normally produces a set of specifications that have to be adhered to by the users. These are formulated to ensure that, amongst other things, multiple users sharing the same satellite transponder don’t upset each other’s’ signals; and that different types and data rates of signals can happily coexist in the transponder. The overall aim is to maximise the usable capacity of the transponder in terms of power and bandwidth.
In a linear world things would be comparatively straightforward, but components in both the satellite transponder and the ground terminals – notably the RF power amplifiers – are far from linear and herein lies the fundamental problem; how to maximise the ‘useful’ RF power without causing signal distortion or creating damaging non-linear interference to other signals?
The challenges exist for power amplifiers used in space and on the ground. We look here specifically at the ground segment challenges.
The Ground Terminal Power Amplifiers
The challenge is to maintain signal quality and to minimise interference, bearing in mind the requirements for flexibility, low power consumption (good efficiency) and minimum terminal costs and maintainability? Let’s look at some of them:
Ground Terminal Bandwidth
While an individual satellite transponder will usually have a bandwidth significantly less than the bandwidth allocated to the service type provided (FSS, MSS etc.), this is not necessarily the case for the ground terminals. To permit a ground terminal to operate with a variety of satellites and their different frequency plans the transmit and receive chains need to cover as wide a band as possible; from say 500MHz or more for C-, X- or Ku-bands to up to 3GHz for Ka-band.
EM Solutions has addressed this issue and has departed from the conventional single conversion BUC designs and has introduced double conversion frequency converters from the L-band IF. This has been done in a manner which minimises the effects of increased phase noise and so has minimal impact on signals using higher order modulation schemes, such as 64 or 256 level QAM.
However, the transmit power amplifier itself still has to operate over the full bandwidth and it is well known that both GaAs and GaN amplifiers exhibit higher gain and gain compression at lower frequencies. These effects need to be compensated for in the design of the ground terminal transmit chain.
The nonlinearities of the ground terminal introduce intermodulation, ISI, spectrum spreading and small signal suppression to the transmitted signals.
Linearization reduces in-band distortion in RF signals as well as spectrum spreading into neighbouring signals channels. The use of a Linearizer allows the power amplifier to be driven harder to increase its power output, while at the same time reducing these damaging nonlinear effects on the transmitted spectrum.
Further, linearization in combination with optimal biasing of the power amplifier can significantly improve efficiency.
The EM Solutions Linearizer
There are three types of linearization techniques available; namely feedback, feedforward and predistortion. Feedback linearizers, while providing improvements in linearity, are inherently narrowband. Feedforward linearizers require a second, high(ish) power amplifier and are thus quite inefficient. That leaves the predistortion linearizer which is the approach adopted by EM Solutions.
Analogue predistortion linearizers can be made to have few components, thus increasing ground terminal reliability. The linearizer can be tuned by altering a number of control voltages of variable attenuators, phase shifters or gain controllable amplifiers.
A recent EM Solutions’ linearizer was produced to linearize a 200 Watt C-band Gallium Nitride solid state power amplifier. It successfully increased the output power of the amplifier by at least 1dB over the bans 5.850 to 6.725 GHz, keeping the IM3 levels at lower than -22dBc. The device also improved the flatness of the gain versus input power transfer function over a wide range of input powers and across the whole frequency band.
The linearizer outperformed other linearizers of similar complexity, cost and size on all fronts and opens up commercial opportunities for solid state amplifier as a realistic alternative to legacy travelling wave tube solutions
It is also worthy of note that all of EM Solutions’ devices, subsystems and terminals are both developed and produced in-house. There is therefore no dependence upon external suppliers of key elements, which among other benefits means that the company can provide rapid turnaround for customers’ specific requirements.
EM Solutions Ka Multiband Diamond Series BUCs also utilise the linearization techniques described above with more details available here:
For those who want to delve into the technology further, full details of the linearizer are contained in Marshall Lewis’s AIAA ICSSC2015 paper which is also available via the EM Solutions website.