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PHASER Report Summary

Project ID: 607087
Funded under: FP7-SPACE
Country: Spain

Final Report Summary - PHASER (High speed, high frequency electro-PHotonic Adc for Space Enabled Routers)

Executive Summary:
PHASER is a high speed, high frequency Electro-Photonic ADC system capable of down-converting and digitalize a high frequency band pass signal composed by a high speed optical sub-sampling architecture as a key element, capable of working in a very wide range of frequencies (from few kHz up to tens of GHz) and down-converting high bandwidth signals without electronic mixing hardware or intermediate frequency stages. After this signal down-conversion, the system uses a suitable electrical ADC to digitalize the signal and a powerful digital processing module for post processing.

As a part of the payload in the satellite, the block diagram would be as shown in Figure 1. The Ka band frequency signals coming from the different antennas input will feed the different sub-sampling blocks. The optical signal coming from a pulsed laser (LRGU) will be distributed through the FoDS to the several sub-sampling modules (SSU+RU), each of them capable of down-converting the input high frequency signal to base band. After that, the electrical ADC will convert the analog based band signal to digital.

Figure 1. Channel distributed payload. Photonic-ADC architecture

Figure 2 shows the selected architecture of the Electro-Photonic ADC that has been developed during the project. As can be observed, the Fiber Optic Distribution System has been simplified with 8 output ports just to demonstrate the concept. However, in a real payload, this subsystem would be composed by several cascaded connected stages, to cover the whole number of ADCs required in the satellite.

Figure 2. PHASER system: Single-channel photonic sampling ADC.

The functional blocks in which the system is divided are:

- Laser Reference Generation Unit (LRGU): Includes the driver and the femtosecond pulsed laser head. The key elements of this block are the diode laser pump module, the free space cavity with high quality factor and a solid-state gain element where the optical pulsed signal is passively generated and the low noise constant current source. The laser is passively mode-locked in order to emit ultrashort pulses in the sub picosecond regime.
- Fibre-optics Distribution Subsystem (FoDS): For a high number of samplers, the pulses from the LRGU will be amplified and easily distributed with passive splitters. The number of samplers to be feed and also the power consumption of the whole system will depend of the output power of the selected optical amplifier (usually an Erbium Doped Fibre Amplifier (EDFA)).
- SubSampling unit (SSU): Receives the optical pulses from the reference laser and the radiofrequency signal. Its task is to subsample the input radiofrequency signal (Ka band) thanks to the high modulation bandwidth of a Mach-Zehnder Modulator (MZM) and to generate the down-converted signal, result of the modulation of the radiofrequency signal with the reference pulsed laser.
- Receiver unit (RU): To receive the down-converted signal in the electrical domain, a photodetection of the optical signal received from the SSU shall be carried out. To photodectect this signal, a commercial Photodiodes (PD) made of InGaAs are used thanks to the maturiry and contrasted performance, also in space applications, of this kind of components. Moreover this unit includes the proper optical block to adjust the peak power (before the photodetection) and the required electrical amplification (after the photodetection) that will adjust the signal level to the ADC.
- ADC block: Performs the A/D conversion with a clock taken from the LRGU. The device then transmits the digital data to an FPGA for post processing.

- Post Processing Algorithms: There is still one extra stage where PHASER performances are improved, the PPA included in the first processing element after the AD conversion. This first processing stage is optimised to adapt it to the signal quality, noise and linearity provided by the previous elements of the PHASER system. In this way an optimised digital processing front is used to improve the overall performance of the system in terms of linearity, noise and stability. One extra advantage of this PPA is that the first filtering stage is performed using the Poliphase Filtering technique. As shown, this technique takes advantage of the high bit rate available at the input to perform an initial decimation of the samples before feeding it to an array of FIR filters. This Poliphase approach has allowed to optimise the decimation ratios so that maximum performances are obtained while reducing the bitrate and thus easing the processing requirements of the rest of the data chain (typical decimation ratios are around 5 to 1). The inherent power of these PPA is that all the parameters (decimation ratios, gains, FIR length, FFT resolution...) can be changed and, therefore, optimised in order to adapt to the signals being digitised.

Project Context and Objectives:
Along the project, the following general objectives were established and accomplished:

1. PHASER Architecture and Tests Definition: The main target for the first project year was to agree in a complete PHASER architecture that delivers the performances needed to cover all the application scenarios envisaged in the Project FPP. Having a clear definition of the final PHASER target and the associated requirements allowed the partners to define a detailed work plan for the rest of the project.

2. Building Block Design: Once closed the PHASER system requirements and its corresponding particularization for each of the building blocks, the second period was devoted to the design and implementation of each of those building blocks. Independent design teams have been set up for each of the blocks optimising the resources and concentrating each of the partners’ know how in those tasks where they excel, in this way, some sub-blocks have been designed by one partner and then integrated in the overall block by other partner, as for example the antialias filter designed by TASE and integrated by DAS in the RU.

3. PHASER System Integration: Once the different building blocks are ready, the final step has been the integration of the complete system and the assessment of it performances. This activity called for several coordination activities as well as for step by step incremental integration and test. Once the complete PHASER system has been integrated and operative, it has been refined to make it attractive for the market. This refinement has taken two lines. On one hand, some of the building blocks have been re-engineered to reduce its footprint and mass, leading to a Mark 2 version that is also fully integrated and tested. On the other hand, the complete demonstrator has been reviewed to make it transportable and attractive so that it can be road showed in different congresses and events as part of the PHASER marketing effort.

4. Space Validation of Building Blocks: In order to guarantee the future space qualification of a complete PHASER system, there are several building blocks that lack heritage in space environment. These blocks such as the Laser and the FoDS have been tested in the different environments, namely, vibration, shock, thermal cycling and radiation. Once available the different results have been reviewed to assure that they are compatible with the levels that the PHASER system will be subjected to during actual space operation. In the same way, recommendations to improve the designs regarding its space borne-ability have been provided.

5. Capture of Flight Opportunities: One key objective for the project is to identify and execute the capture process of those early flight opportunities identified during the project. Flight opportunities on technological IOD/IOV missions have been targeted and efforts devoted to provide the mission leaders to include PHASER as one of the candidate technologies to be flown. In the same way, a complete exploitation plan has been put in place identifying the target markets and all the flight opportunities.

Project Results:
PHASER is a project funded by the European Commission under the FP7 Space frame and has involved:

• 360 Person/Month activity
• More than 25 engineers from three countries
• 41 Tasks in 11 Workpackages
• More than 40 meetings
• 2 Prototipes
• Over 270 Test hours at 5 facilities

All this effort has led to achieve:
• A full functional laser disciplined ADC at TRL5 ready to hit the market
• A system that:
o Digitizes Ka band channels directly to Baseband
o Removes the need for RF downconverters
o Allows full flexible processing of up to 2GHz wide channels
o Allows the digital data processing of typical telecom signals without the need of intermediate RF downconversions.
o Provides full flexibility of data allocation within the band, as the frequency plan is implemented through data processing.
o At a cost comparable with the legacy non flexible RF solutions
• The first radiation performance characterization of a femtopulse laser for space
• A Modular approach that eases integration of the solution into small and medium payloads
• An easily scalable solution for large payloads as Sampling Signal is distributed through Optical Fiber providing light weight and EMI immune solution
• The definition of the future front-end of photonic based On Board Processors for Telecom payloads
• Compatible with most Satellite Platforms worldwide.

Potential Impact:
PHASER enables a completely new set of telecom systems and their associated markets. As a general outcome of the project we may describe the PHASER enabled new generation payloads under the following three categories:

8.1 The space full IP router
Adding an On board Processor to a regenerative satellite allows to perform Space Routing, this means that IP traffic is encapsulated in RF signals but the space router receives the RF signals and demodulates and decodes them to recover quasi-error free IP packets. IP packets recovered on board are then routed to the right spot according to final user. This approach has some very interesting advantages such as:
• It removes the need for a Hub
• Isolates uplink and downlink performances
• Allows the implementation of multispot and mesh architectures
• Allows direct User to User connection
• Reduces dramatically the communication delays

Present On Board Processor architectures are tightly linked to the standard signal waveforms programmed in their ASICs. This means that any evolution of the standard will render the Space Router obsolete. With PHASER, the way forward is to replace the On Board Processor (OBP) by a Microprocessor Based OBP (uPOBP) and to apply the PHASER results to digitise directly the RF signal. Once achieved the transition to a PHASER enabled microprocessor based Space Router, a flexible OBP system will be feasible presenting the following features:
• Standard Independent system
• Waveforms and coding schemes defined by software can be updated during satellite lifetime
• Flexible bandwidth allocation through input demultiplexer reprogramming
• Dynamic routing schemes through suitable programming of the routing function
• ACM capabilities
• Dynamic resources allocation
This implementation is extremely attractive to operators due to its flexibility in the resource allocation and its evolution and lifetime capabilities.

8.2 The flexible active antenna
Other usage of PHASER is an electronically steerable multibeam antenna (active antenna) for future communication satellites. The next generation of broadband Internet satellite is often called as High Throughput Satellites (HTS) or Terabit satellites. To achieve an higher throughput, future satellites shall operate in Extremely High Frequencies (typ. above 20 GHz in Ka-band and Q/V-band) at high speed (typ. hundreds of Mbps per carrier) and over a large multibeam (or cellular) coverage (typ. hundreds of spot beams or cells). The utilization of multibeam active antenna seems an attractive solution for generating a cellular coverage while allowing some flexibility in the management of coverage and traffic. Active antennas, also called electronically steerable antennas require a special piece of equipment: a Beam Forming Network (BFN). Among the available technologies (microwave, optical) for BFN, the digital technology is the most promising. Nevertheless, there is a technological bottleneck: the analog to digital conversion prior to the digital processing. The PHASER technology now breaks the system free of this bottleneck.
Furthermore, various types of digital processing implemented in the digital BFN will allow new services and will increase performances. For examples, the Spatial Division Multiple Access (SDMA) technique allows doubling the data rate and increasing the frequency reuse factor (and consequently the satellite capacity); Anti-jamming processing could reduce the interference levels between users or with other satellite systems, all thanks to the full band digitalization provided by PHASER.

8.3 Your personal space router
The two previous payloads can be mixed in a future high-power, high flexibility broadband payload that may completely revolution the way users access to satellite communication links, meaning, in practice, that each individual user or group of users would perceive the satellite as if its entire power and bandwidth were allocated exclusively to them.
The proposed architecture consists of a receiving multispot active array antenna directly digitised through a PHASER system, and then a very high performance On board Processor with Digital beamforming capabilities both in reception and transmission would feed data to a DDS system connected to an active array transmission antenna.

Taking into account that different users would access the satellite at different moments in a burst time slotted fashion, this architecture would allow associating each IP data burst to a certain antenna beamforming configuration. This means that such a system can configure the Rx and Tx ultra narrow antenna lobes to be centred in the user position during its burst time so that during that time, the entire satellite power and antenna gains are dedicated to the users in a very narrow spot, thus allowing the satellite access to be performed with very small devices using very little RF power to obtain high bandwidth.

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