Cognitive Radio Chip-Hardware Challenges - Hardware Conference

Cognitive Radio Chip-Hardware 

Challenges 

Eric Klumperink 

Mark Oude Alink, Amir Ghaffari, Dlovan Mahrov, Saqib Subhan, 

Zhiyu Ru, Niels Moseley, Michiel Soer, Rameswor Shresta, 

Andre Kokkeler, Bram Nauta 

IC Design group, CTIT, University of Twente, Enschede 

http://icd.ewi.utwente.nl (all papers available on-line) 

Cognitive Radio Challenges, Eric Klumperink 

1

Overview 

• Introduction Cognitive Radio (CR) 

• Radio Hardware Challenges & Ideas 

• Cognitive Radio 

• Flexible Receiver 

• Flexible Transmitter 

• Spectrum sensing 

• Conclusions 


2

Static Frequency Allocation 

Almost all spectrum allocated - how find spectrum for mobile internet 


3

Spectrum Allocated, but not in use! 

[Swisscom] 

Cognitive Radio: find & use “white spaces” 


4

Applications 

more capacity more users, reliability, speed, .... 

Examples: 

• E.g. Broadband wireless access (IEEE 802.22) 

– 50-860 MHz (old TV-bands), Range: 17-30km 

• Reliable communication for Emergency Services 

other future 

applications .. 


5

Overview 



• Cognitive Radio – Software Defined Radio 

• Flexible Receiver – Clock Controlled Filter 

• Flexible Transmitter – Multi-path Architecture 

• Spectrum sensing – Mixed Signal X-correlator 



6

Cognitive Radio Hardware Challenge 

Needed: Spectrum Sensor + Transmitter + Receiver 

Desired: flexibility 

– Frequency 

– Bandwidth 

– Modulation 

– ... 

“Software Radio” 

ADC 

DSP 

Analog- 

Digital 

Converter 

Digital 

Signal 

Processing 

How fast can we go 

DAC 

Digital-Analog 

Converter 


7

ADC Bandwidth versus SNDR 

Frequency BW [Hz] [Hz] 

1,E+11 

1,E+10 

1,E+09 

1,E+08 

1,E+07 

1,E+06 

1,E+05 

Radio frequencies 

ISSCC 1997-2008 

VLSI 1997-2008 

ISSCC 2009 

VLSI 2009 

Jitter=1psrms 

Jitter=100fsrms 

1,E+04 

1,E+03 

10 20 30 40 50 60 70 80 90 100 110 120 

SNDR [dB] 

[Murmann, 2009] 

Is 40-50dB Signal to Noise-and-Distortion Range enough 


8

How much Path Loss: “inverse square law” 

Antenna aperture 

~ (lambda/4) 2 

• Total power through sphere is constant 

• Received power in fixed area A reduces 1/R 2 

• Also less due to ground-reflection/multi-path: 1/R n with n~3 

10x more distances 1000x more power loss 

(10x power “10dB”, so 30dB per factor 10x distance) 


9

Reduce path loss Parabolic antenna! 

+ Sensitivity 

- Big dish needed 

- Alignment needed, unpractical for mobile 


10

Long distance large loss High SNDR 

SNDR 

• Assumptions for CR device: no antenna gain (non-directional) 

• Desired range for communication: at least e.g.1m..100m 

Result: Path loss ~ 10 … 70 dB + 10..30dB = 20..100dB 

(frequency-dependent 100MHz ..1GHz) 

6dB per ADC bit => >16 bits ADC needed 


11

ADC Bandwidth versus SNDR 

BW [Hz] 

1,E+11 

1,E+10 

1,E+09 

1,E+08 

1,E+07 

1,E+06 

1,E+05 

1,E+04 

1,E+03 

10 20 30 40 50 60 70 80 90 100 110 120 

SNDR [dB] 

ISSCC 1997-2008 

VLSI 1997-2008 

ISSCC 2009 

VLSI 2009 

Jitter=1psrms 

Jitter=100fsrms 

Cognitive Radio 

(6GHz and 100dB) 

[Murmann, 2009] 


12

Downconversion / filtering required 

Resolution (bit) 

20 

16 

12 

8 

4 

1mW 

1 W 

1 kW 

1 W 

downconversion 

filter 

1kHz 1MHz 1GHz 

Signal Bandwidth (Hz) 

band 

filter 

Amplify 

f-shift 

(mixer) 

channel 

filter 

ADC + 

software 

“Software Defined Radio” if still very flexible through software 


13

CR and SDR definitions by ITU 

• ITU = International Telecommunication Union 

• Cognitive Radio (CR): 

– Obtains knowledge from environment/user 

– Adjusts Radio parameters 

– Learns from results 

• Often a CR uses a Software Defined Radio 

• Software Defined Radio (SDR): 

– Adjusts Radio parameter and protocols by software 

– At least frequency, modulation type, power 

– More flexible than predefined set by standard 


14

Overview 









15

Mixer multiplication frequency shift 

sin 

f 

t 

sinf 

t sinf 

f t sinf 

f 

LO 

in 

LO 

in 

LO 

in 

t 

 

Downconversion 

(receiver) 

Upconversion 

(transmitter) 

f IN 

Difference and Sum 

frequencies 

f LO 

Lowpass 

Filter 

f LO -f IN 

Down-conversion 

f IN 

f LO 

Up-conversion 

f LO +f IN 

Band 

Filter 


16

How implement RF band-filtering 

Q 

1 

R 

L/ C) 

AM-radio Receiver (1929): 

• Frequency selection by coil 

inductance L & capacitor C 

resonator, swing 

• Tuned mechanically by 

variable plate-capacitor C 

1 

center frequency : fc 

 

L C 

Now: fixed passive bandfilter 

(external SAW/BAW bandfilter) 

+ mixer with tunable frequency 

CMOS chip: limited Q=f c /bandwidth

Do we really need Filters Why 

Filter 

harmonics and 

much more .... 

2 3 4 5 

non-linearity! 

BB 

RF 

Filter 

Base Band (BB) 

ω 

(digital LO-generator) 

Time-variant BB 

to RF transfer! 

3 5 

But: good RF filters are costly and inflexible 

 


3 

Can we implement suppression more flexibly 

5 

18

Flexible Integrated Filter Idea 

f S 

RC >> T on 

V in 

C 1 C 1 

average 

t 

V out 

C 1........ 

C 1 ... 

C 4 

f S 

1.5f S 

0 

C 1 C 1 

[Franks, ISSCC 1960] 

“N-path filters” 

• High ohmic @ switching frequency 

• Short circuit @ other frequencies 


19

Filter Properties 

Band-pass around f s 

Tunable (clock f s ) 

High Q for high RCf s 

Good linearity & noise 

Unwanted harmonics 

V out / V in 

f S 

Frequency 

V out 

More paths Less Distortion 


20

Filter Properties: flexibly tunable 

Selectivity (Q) 

Compression (P 1dB 

) 

Linearity (IIP3) 

Noise Figure 

3 to 29 

+2dBm 

+19dBm 

Overview 









22

Cognitive Radio Transmitter 

Current Dream: One multi-band Flexible transmitter Upconverter 

LO 

PA 

Filter 

BB 

Signal 

LO 

PA 

Filter 

LO 

PA 

Filter 


23

Multipath Polyphase Technique 

Divide the nonlinear circuit into ‘N’ equal smaller pieces 

Nonlinear 

circuit 

Nonlinear 

circuit 

Nonlinear 

circuit 

Nonlinear 

circuit 


24

Multipath Polyphase Technique (2) 

Add equal but opposite phase shift before and after 

Use equidistant phases: path i uses i*2π/N 

path i=1 

0×2π/N 

Nonlinear 

circuit 

-0×2π/N 

y 1 

x(t) 

path i=2 

1×2π/N 

Nonlinear 

circuit 

-1×2π/N 

y(t) 

y 2 

y N 

path i=N 

Nonlinear 

circuit 

( N-1)×2π/N -(N-1)×2π/N 


25

Example: Output for 3-Path case 

Phase rotation increases with order of nonlinearity/harmonic: 

y 3 

y 

120 

120 

1 y 2 y 3 y 1 

y 1 

y 1 y 2 y 3 

0 

y 0 

0 

0 

3 

y 2 

Fundamental 

(y x) 

Magnitude: 

A 

240 

y 2 

2 nd Harmonic 

(y x 2 ) 

240 

3 rd Harmonic 

(y x 3 ) 

First non-cancelled 

Harmonic 

4 th Harmonic 

(y x 4 ) 

2 3 4 5 6 


26

N-PATH: First Non-cancelled Component 

A 

2 Path 

A 

3 Path 

A 

4 Path 

A 

‘N’ Path 

2 3 4 5 6 

2 3 4 5 6 

2 3 4 5 6 

2 3 4 5 (N+1) 

(N+1) th 

Harmonic 

Multi-tone input: cancels p· 1 ± q· 2 , except if: p+q=j×N+1 


27

Concept of Flexible Transmitter 

BB 

path i=1 

Digital baseband 

phase shifters: 

0×2π/N 

path i=2 

1×2π/N 

Nonlinear 

circuit 

Nonlinear 

circuit 

Analog up-mixers 

with different phases 

LO 

LO 

0 

-2π/N 

y 1 

y 2 

y N 

-(N-1)×2π/N 

path i=N 

(N-1)×2π/N 

Nonlinear 

circuit 

LO 

[Shrestha et al, JSSC 2006] 


28

Cancelling Harmonics/Sidebands 

1-path Desired signal, LO + BB LLO ±B BB 

Power 

18-path 

Power 

Desired signal 

3 LO +3 BB 

(3=0×18+3) 

Non-cancelled terms 

L=j×N+B 

17 LO - BB 

j=…-2,-1,0,1,2,… 

15 LO -3 BB 

(15=1×18-3) 

1 3 5 7 9 11 13 15 17 

[Shresta et al, JSSC 2008] 

/ LO 

29 

Cognitive Radio Challenges, Eric Klumperink

Overview 









30

Spectrum Sensing: shadowing problem 

C does not detect A due to “shadowing” of building 

need for sensing below noise floor 


31

Linearity vs Noise 

Ideal output = actual input ( ) spectrum 

Difference 80dB 


32


Effect of non-linearity (too strong signals): 

(NF=20dB, IIP3=+10dBm, RBW=100kHz, att.=0dB) 


33


Apply attenuation: distortion OK but too much noise 

(NF=20dB, IIP3=+10dBm, RBW=100kHz, att.=48dB) 

Each dB of attenuation: 

- Noise floor up by 1dB 

- Distortion peaks down by 2dB 


34

f test 

[Oude Alink et al, DySPAN 2010] 

Idea to improve Spectrum Analysis 

Idea to “get rid of the noise”: 

– a) Duplicate receiver 

– b) Cross-correlate outputs 

Result: correlated signal (and noise) 

Atten. ADC FFT 

out 

f test 

X-corr 

out 

Atten. ADC FFT 


35

Cross-correlation measurement 

Normalized Measurement Time=2 0 

23 

45 

67 

89 

10 11 12 13 14 

test tone 

RBW=10kHz 

-100dBm 

DC-offset 


36

XCSA: Measurements III 

RBW=10kHz 

signal not 

visible with 

autocorrelation 

more accurate 

amplitude estimation 

NF=17.1dB 

NF=3.1dB 


37

Conclusions 

CR,SDR hardware less fixed analog filters 

stronger interference, nonlinearity, ... 

Ideas to address challenges: 

Clock controlled switched RC-filter 

Cancel via Multipath Polyphase transmitter 

Exploit mixed-signal cross-correlation 


38

Questions 


39

Cognitive Radio Chip-Hardware Challenges - Hardware Conference

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