Software Defined Radio`s next wave: Fully

Software Defined Radio’s next wave: Fully Digital Radios
FWIC 2015
Bryan Donoghue
22 June 2015
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What was Software Defined Radio (SDR) meant to be?
 Software / Maths can directly manipulate signals at radio (GHz) frequencies
Transmitter
..0101101…
DAC
Software /
firmware /
digital logic
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Gigahertz Radio
Frequency
Receiver
..1101001…
ADC
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What is Software Defined Radio today?
 Software / Maths can directly manipulate signals at MHz frequencies
– Real SDR at GHz is hard
– Radios remain at least half analogue
– Today’s radios provide many of the benefits of true SDR
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Today’s Software Defined Radios have driven wireless handset performance
 Data rates have increased 1000-fold from GSM to LTE
 Enabled by 20 years of Moore’s Law gains:
– Massive increase in SDR baseband complexity
– Limited benefits to RF e.g. pre-distortion and linearization
Data Rate Evolution
1000
100
Mbps
10
1
0.1
0.01
0.001
GSM
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GPRS
EDGE
WCDMA
4
HSPA
HSPA+
LTE
LTE-A
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What’s the downside of analogue-heavy radio architectures?
 Analogue radios are costly to design
– Analogue design process is lower productivity than digital or software
– Analogue designs do not port easily between processes or geometries
 Analogue circuits do not scale well with Moore’s Law
– Digitals circuits get smaller, cheaper and lower power with each new geometry
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Where next?
 Old solution is running out of steam
– Difficult to squeeze more bits/Hz
– Already getting close to Shannon limits
 New Trends
– Greater flexibility in use of spectrum
– Greater spatial re-use of spectrum
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Trend 1: Greater flexibility in use of spectrum
 All new wireless initiatives and standards are considering flexible use of spectrum
– Often joining together (‘aggregating’) disparate fragments
– It’s the only way that 100 billion Internet of Things ‘things’ will be viable!
 4G ‘Soft Cell’ may simultaneously use licensed and unlicensed spectrum:
Small cell in
unlicensed spectrum
Macro network using
licensed spectrum
 More flexible SDR is needed to support multiple bands and protocols
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Trend 2: Greater spatial re-use of spectrum
 Achieving higher spectral density than 4G requires more spatial processing
– A common theme in “5G” technologies is therefore Massive MIMO
100s of radios form
1000s of signal paths
Antenna
Base
station
Phone
Reflective object
Phone
 Building multiple analogue radios on a single chip is disproportionally more difficult
– A fully-digital radio would allow on-chip integration more readily
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What is Fully-Digital SDR?
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Delta-Sigma converters are key
 Advantages
– Low Cost
– Low Power
– Digital-silicon friendly
 Typically limited to 10’s of MHz
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GHz Delta-Sigma DAC in a Digital ASIC
 Sample rate must be >> bandwidth
– Low-Pass Converter:
– Needs 30GHz sample rate to create 1GHz signal
– Band-Pass Converter:
– Needs 2GHz sample rate to create 1GHz signal
 How to output samples in digital ASIC?
– Use SerDes at 3, 6, 12 or 28GHz
 How to compute a high-SNR and configurable Delta-Sigma stream at GHz?
– Unsolved problem until ‘Pizzicato’
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Breaking the computational bottleneck for Delta-Sigma DACs




Complex DSP calculations at GHz
Feedback loop is bottleneck
Conventional approaches to parallelism fail
‘Pizzicato’ creates new approach to Delta-Sigma parallelism
– Can achieve 100-fold or more speed-up
– Generate RF signals directly at GHz
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Pizzicato Experimental Test System
 Runs on mature-technology FPGA and generates high-quality in-band signals
 Exploits FPGA Serializer/Deserializer (SerDes) for high rate output
 Hints at what is possible
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Project Pizzicato
Pizzicato  DAC performance
 We achieved ~65 dB between sine wave peak and noise floor
 ..with a passband of around 100 MHz wide
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Project Pizzicato
Within the passband, we can transmit any combination of signals
 Example: 14 carriers spread over 80 MHz
Pizzicato excels in transmitter tests
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What are the advantages of Pizzicato transmitter?
 Very high Nyquist Frequency - 14GHz with today’s SerDes
 Very low cost (e.g. 7 cents of silicon in 28nm)
 Very flexible SDR transmitter
– Programmable Centre Frequency
– Programmable Bandwidth
 Multiple transmitters in a single chip
– Massive MIMO applications
 Easy to port between Silicon Geometries (it’s just Verilog plus a SerDes)
 Digital-only solution scales with Moore’s Law
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Project Pizzicato
There are limitations today
 Analogue filtering of the output signal is required
– Reduces unwanted emissions and wasted power
We only want this bit
 Pizzicato is a transmitter only
– A receiver is even more complex
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Project Pizzicato
Amplifying the output
 Pizzicato outputs ~0dBm peak
– Equivalent to a quiet Wi-Fi access point
 Pizzicato can drive a classic RF power
amplifier via an analogue filter
Pizzicato
 For ultimate flexibility and performance a
purely digital amplifier is desirable
– Class-S amplifiers are potentially more
efficient than conventional power amplifiers
Amplified
bit stream
 This is an area of on-going research…
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Pizzicato
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What are the technology implications of a Fully-Digital Radio?
 The design process changes
– Higher productivity digital IC and software tool-flow
– More design re-use of Digital-RF IP modules
– Availability of flexible, frequency-agnostic modems
– More fluid boundary between RF and software-defined domains
 Analogue doesn’t go away
– Fewer, more elemental components (Filters, LNAs, Power Transistors)
– RF design knowledge migrates into the DSP domain
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What are the market implications of Fully-Digital Radio?
 What if the radio becomes an IP core?
– Will silicon IP companies displace chipset manufacturers?
 What are the effects of a ‘soft radio’?
– Design cost of new radio architectures drops
– More innovation and customisation, particularly in standards-exempt bands
 What happens if “RF is Easy”
– New entrants in the chipset market?
– Commoditisation of chipsets?
– Value transfers to the hard-bits e.g. protocol stacks?
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Further thoughts for the future
 Baseband modems today bear no resemblance to the modems of thirty years ago
– Digital implementation has allowed baseband processing to change dramatically
– Simple modems needed analogue implementation for transmitter and receiver
(tones and filters). These are now replaced by abstract mathematical concepts
such as MIMO, SCFDMA, and OFDMA
 But wireless standards today make assumptions about the characteristics of the
radios to be used that are wholly influenced by our “analogue thinking”
 What if future radios were as mathematically abstract as basebands are today?
– Will techniques like carrier aggregation (effectively multiple radios)
appear crude when we look back in ten years time?
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22 June 2015
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