Electrical Engineering
IIT Madras
Cognitive Radio - An Introduction
R. David Koilpillai
Department of Electrical Engineering
Indian Institute of Technology Madras
IISc-DRDO Workshop on Cognitive Radio
Bangalore – March 14, 2009
Koilpillai / Mar 2009 / Cognitive Radio
IISc-DRDO Seminar on Cognitive Radio
1
IITM Proprietary Information
Electrical Engineering
IIT Madras
Evolution of Wireless …
Rel. 7
Rel. 6
GSM
GPRS
WCDMA
LTE-Adv
Rel. 5
LTE
(HSDPA)
1xEV-DV
cdmaOne
UMB
cdma2000
1xEV-DO
IEEE
802.16 d/e
MIMOWave2
IEEE
802.16 m
Focus is on spectral efficiency – bits / sec / Hz
Koilpillai / Mar 2009 / Cognitive Radio
IISc-DRDO Seminar on Cognitive Radio
2
IITM Proprietary Information
Radio Functionality Evolution
Electrical Engineering
IIT Madras
Source: Prasad et al. IEEE Comm Magazine, April 2008
Koilpillai / Mar 2009 / Cognitive Radio
IISc-DRDO Seminar on Cognitive Radio
3
IITM Proprietary Information
Software Defined Radio (SDR)

Electrical Engineering
IIT Madras
J. Mitola, “The software radio architecture” IEEE Communications Magazine, May 1995
Koilpillai / Mar 2009 / Cognitive Radio
IISc-DRDO Seminar on Cognitive Radio
4
IITM Proprietary Information
Electrical Engineering
IIT Madras
Vanu SDR Architecture


Commercial product
Multistandard
–
–



GSM / GPRS / EDGE
Cdma / EV-DO
Flexibility
Scaleability
Cost-effectiveness
Ref: www.vanu.com
Koilpillai / Mar 2009 / Cognitive Radio
IISc-DRDO Seminar on Cognitive Radio
5
IITM Proprietary Information
Electrical Engineering
IIT Madras
Vanu SDR Architecture
Ref: www.vanu.com
Koilpillai / Mar 2009 / Cognitive Radio
IISc-DRDO Seminar on Cognitive Radio
6
IITM Proprietary Information
Electrical Engineering
IIT Madras
SDR Summary



Many technical challenges have been solved
SDR – now commercially viable and attractive
Drivers for SDR
–
–
–
–

SDR: A flexible platform
–
–


–

New technology development
Technology migration
Focus on basestations and not user equipment
Numerous national and international initiatives
–

Advances in processors, DSPs, FPGAs, …
High speed, high-resolution A/D, …
Multi-standard support, MIMO capability, …
Efficient software tools and structures
Multiple SDR test beds
Open-source material available
SDR Forum – an active group
The next step in SDR  Migration towards Cognitive Radio …
Koilpillai / Mar 2009 / Cognitive Radio
IISc-DRDO Seminar on Cognitive Radio
7
IITM Proprietary Information
Electrical Engineering
IIT Madras
SDR  Cognitive Radio
Koilpillai / Mar 2009 / Cognitive Radio
IISc-DRDO Seminar on Cognitive Radio
8
IITM Proprietary Information
Electrical Engineering
IIT Madras
Cognitive Radio (CR)
Motivation for CR
 Increasing demand for radio spectrum
–

Current approach to spectrum allocation
–


Fixed allocation to licensed users
Existing scenario
–
Under-utilization of spectrum
–
Spatial and temporal “spectral holes” exist
Innovative approach to improve spectrum utilization
–

Broadband wireless demand is rapidly growing
Cognitive Radio
Initiated by FCC – regarding secondary usage of spectrum
Koilpillai / Mar 2009 / Cognitive Radio
IISc-DRDO Seminar on Cognitive Radio
9
IITM Proprietary Information
Utilization of Spectrum
Electrical Engineering
IIT Madras



Frequency range
– 30 MHz – 2.9 GHz
Based on report by M.A. McHenry
Max. utilization ~ 25%
–



TV channels
Average usage ~ 5.2 %
New York City average ~ 13.1%
Significant # white spaces
–
Even in cellular bands
Ref: M.A.McHenry, “NSF Spectrum Occupancy
Measurements Project Summary,” August 2005
Ghasemi and Sousa,
IEEE Communications Magazine, April 2008
Koilpillai / Mar 2009 / Cognitive Radio
IISc-DRDO Seminar on Cognitive Radio
10
IITM Proprietary Information
Electrical Engineering
IIT Madras
CR Approach

Main steps in CR approach
–
–
–
–
Identify spectral bands not used by Primary User
 Signal sensing (to detect Primary User’s signal)
– Estimation of “Interference Temperature”
– Localised around user
 Spectral hole
– A spectral band assigned to primary user
– Currently unused at geographical location
– Should be done reliably
 Should be able to detect “low” level Primary User signals
Utilize spectrum as “Secondary User”
Increasing utilisation of radio spectrum
 Without causing interference to Primary User
Primary user always has priority
Koilpillai / Mar 2009 / Cognitive Radio
IISc-DRDO Seminar on Cognitive Radio
11
IITM Proprietary Information
Today’s CR Scenario

Electrical Engineering
IIT Madras
CR: Opportunistic Unlicensed Access
–
To temporarily unused frequency bands (across the entire licensed radio spectrum)

A means to increase efficiency of spectrum usage

Stringent safeguards required
–

Spectrum sensing based access
–


On-going licensed operations should not be compromised
Unlicensed user transmits if licensed band is sensed to be free
Main functionality of Cognitive Radios
–
Ability to identify unused frequency bands
–
Sensing must be reliable and autonomous
Conclusion
–
A perceived spectrum scarcity - due to inefficient, fixed spectrum allocation
–
Consider radically different paradigm

Secondary (unlicensed) users

Opportunistic use of unused primary (licensed) band(s)
Koilpillai / Mar 2009 / Cognitive Radio
IISc-DRDO Seminar on Cognitive Radio
12
IITM Proprietary Information
Electrical Engineering
IIT Madras
IEEE 802.22


Project started by IEEE in Nov 2004
Charter: To develop a CR-based WRAN
–

Transmission in unused TV and guard bands (54 MHz – 862 MHz)
–
–

Very favourable propagation characteristics
Channel BW 6 MHz (may be 7 MHz / 8 MHz in some countries)
Spectrum sensing for identifying white spaces
–
–

PHY and MAC specifications
Distributed sensing
 FCC maintained server – info about unused channels (by geographical location
Localised sensing
 CPE’s perform periodic measurements and send measurements to BTS
 BTS makes decision to use the current channel or any other alternatives
Application scenarios
–
Wireless broadband in rural / remote areas

Performance comparable to today’s DSL technology
–
Unlicensed devices  lower cost and increased affordability
–
Attractive for Wireless Internet Service Providers (WISP)
–
TV migration : moving from broadcast to cable and satellite

 Broadcast TV channels available
Koilpillai / Mar 2009 / Cognitive Radio
IISc-DRDO Seminar on Cognitive Radio
13
IITM Proprietary Information
Electrical Engineering
IIT Madras
Comparison of Networks

WRAN Aspects

Large coverage footprint
–
Up to 100 Km

Larger cells than cellular

Leverage two factors

–
Higher EIRP
–
Attractive propgn characteristics
Ideal for rural /remote services
–

Broadband wireless access
Unlicensed devices
Ref: Cordeiro et al., “IEEE 802.22: The First Worldwide
Wireless Standard based on Cognitive Radio,” IEEE, 2005
Koilpillai / Mar 2009 / Cognitive Radio
IISc-DRDO Seminar on Cognitive Radio
14
IITM Proprietary Information
Electrical Engineering
IIT Madras
IEEE 802.22 Specifications

Target specifications
–
–
–

Spectral efficiency – 0.5 b/s/Hz – 5 b/s/Hz
 Average: 3 b/s/Hz  18 Mbps in 6 MHz
 Assuming 12 simultaneous users – 1.5 Mbps (DL) and 384 Kbps (UL)
Range: 33 Km (extend to 100 Km)
CPE Tx power 4W EIRP @ CPE
Air interface
–
–
–
–
Requirements – Flexibility and quick adaptibility
 Link adaptation based on SINR
 Adapt modulation and Coding option
 Frequency agility
OFDM(A) based UL and DL
Transmit Power Control : 30 dB withsteps of  1 dB
Channel Bonding – Utilizing more than one TV channel
 System can use larger BW to support higher throughput
Koilpillai / Mar 2009 / Cognitive Radio
IISc-DRDO Seminar on Cognitive Radio
15
IITM Proprietary Information
Electrical Engineering
IIT Madras
IEEE 802.22 MAC

Medium Access Control (MAC)
–
–
–
–
–
–

Design tailored for Cognitive Radio Technology
Key aspect – adaptability based on dynamic changes in environment
 Spectrum sensing measurements
Two structures
 Frame and Superframe
 Superframe will have Superframe Control Header (SCH) and preamble
 SCH sent by BS in every channel that is “available”
Two types of spectrum measurements
 In-band measurements – in channel currently being used
 Out-of-band measurements – Other channels
Two types of sensing
 Fast sensing - < 1 msec per channel
– Performed by CPE and BS - For quick information gathering
 Fine sensing – up to 25 msec per channel
– Verification / validation of measurements
Deal with large propagation delay (roundtrip delay up to 300 microsec)
MAC deals with a number of issues not addressed in traditional systems
Koilpillai / Mar 2009 / Cognitive Radio
IISc-DRDO Seminar on Cognitive Radio
16
IITM Proprietary Information
Electrical Engineering
IIT Madras
Cognitive Radio =
Sense + Learn + Adapt + Use
Koilpillai / Mar 2009 / Cognitive Radio
IISc-DRDO Seminar on Cognitive Radio
17
IITM Proprietary Information
Electrical Engineering
IIT Madras
Spectrum Sensing
Koilpillai / Mar 2009 / Cognitive Radio
IISc-DRDO Seminar on Cognitive Radio
18
IITM Proprietary Information
Methods of Spectrum Sensing

Energy Detector

Correlation-based detector

Cyclostationarity-based detector

Hybrid Detector

Performance of spectrum sensing

Sensing Criteria (Regulatory aspects)
–
Sensing Period
–
Detection Sensitivity
Koilpillai / Mar 2009 / Cognitive Radio
IISc-DRDO Seminar on Cognitive Radio
Electrical Engineering
IIT Madras
19
IITM Proprietary Information
Electrical Engineering
IIT Madras
Spectrum Sensing

Optimum receiver
–
–
–
–

Alternative – Energy Detector
–
–
–
–
–
–


Measures energy of signal in primary band
Compare with properly set threshold
Declare presence of “white spaces”  primary user absent
Requires longer sensing time to achieve desired level of performanc e
Low computational complexity
Ease of implementation
ED - An attractive candidate for Cognitive Radio
Drawbacks of ED
–
–

If structure of primary signal known
Optimum (in AWGN): Matched Filter (MF) followed by Threshold
Can be implemented for a few specific primary signals (selected bands)
Not practical for large # of primary users
 Need for coherent detector for each transmitted signal
Cannot discriminate between sources of input energy (signal vs. noise)
Uncertainty of noise floor will degrade performance
 Especially at low SNR
ED can be effectively combined with more robust detectors – “Hybrid Detectors”
Koilpillai / Mar 2009 / Cognitive Radio
IISc-DRDO Seminar on Cognitive Radio
20
IITM Proprietary Information
Electrical Engineering
IIT Madras
Spectral Sensing

Binary hypothesis testing problem
H 0 : y [n]  w[n]  Primary Us er absent
H1 : y [n]  x [n]  w [n]  Primary Us er present
n  0,1,  (N-1 ) ( N  sample observatio n window of received signal)
xn  transmitte d signal
w[n]  noise (zero  mean AWGN with vari ance  w2
y[n]  transmitte d signal

Decision statistic (Energy detector)
1

N
N 1

y [ n]
2
and

n 0

 H1
 H0

When signal absent, Δ is Central Chi-Square Variable with N degrees of freedom

When signal present, non-Central Chi-Square Variable
Koilpillai / Mar 2009 / Cognitive Radio
IISc-DRDO Seminar on Cognitive Radio
21
IITM Proprietary Information
Electrical Engineering
IIT Madras
Energy Detector

Decision statistic
1

N

N 1

y[n]
2
and

 H1

 H0
n 0
If N large, invoke CLT
 
 2 2 2 
 ~ Normal   w , w  for H 0

N 

2
2 2 
 2



x
w

 ~ Normal  ( x   w2 ),


N



N
2
2
Pmissed -detection  Q 2



x
w 
2
 x w
1


Q
Pfa  
2

Threshold    w 1 

N 



Koilpillai / Mar 2009 / Cognitive Radio
IISc-DRDO Seminar on Cognitive Radio

for H1


22
IITM Proprietary Information
Electrical Engineering
IIT Madras
Spectral Sensing Performance (1)

Performance of Energy Detector is validated against analytical performance

In AWGN, ED achieves good performance at very low SNRs ~ -8 dB

Achieves low probability of false alarm

Evaluated for frequency selective fading channels also
Koilpillai / Mar 2009 / Cognitive Radio
IISc-DRDO Seminar on Cognitive Radio
23
IITM Proprietary Information
Electrical Engineering
IIT Madras
Spectral Sensing Performance (2)
AWGN, Effect of sensing Period
Performance in fading

Robustness of energy detector enhanced if longer sensing period is used

Performance in fading is poorer than in AWGN (as expected)
–

Noise uncertainty causes major degradation in performance
Energy detector not suited as a stand-alone detector
Koilpillai / Mar 2009 / Cognitive Radio
IISc-DRDO Seminar on Cognitive Radio
24
IITM Proprietary Information
Electrical Engineering
IIT Madras
Spectrum Sensing Summary

Many methods available
–
Properties utilised: Energy, Correlation, Cyclostationarity
–
Computational complexity and estimation time are important factors
–
Searching over a vast frequency range

Focus on robustness (at low SNR) and reliability

Minimize probability of missed detection
–

To avoid interference to primary user
Uncertainties regarding measurement
–
Noise and interference environment

Strong motivation for Hybrid Detectors

Sensing Criteria (Regulatory aspects)
–
Sensing Period
–
Detection Sensitivity
Koilpillai / Mar 2009 / Cognitive Radio
IISc-DRDO Seminar on Cognitive Radio
25
IITM Proprietary Information
Electrical Engineering
IIT Madras
Regulatory Constraints

Satisfactory protection of primary user from harmful interference
–
Essential for realization of opportunistic spectrum access
–
Regulatory constraints

Sensing Periodicity (Tp)
–

Detection Sensitivity
–

Period with which UL user must check for presence of primary user
Signal level at which the UL user must detect primary user reliably
Sensing Period (Tp)
–
Max. time (delay) UL user unaware of reappearance of primary user
–
Max. duration of harmful interference
–
Determines QoS degradation of primary user
–
Delay of primary user in accessing channel
–
Depends on type of primary user service – delay sensitivity
–
Must be set by regulator for each licensed band
Koilpillai / Mar 2009 / Cognitive Radio
IISc-DRDO Seminar on Cognitive Radio
26
IITM Proprietary Information
Electrical Engineering
IIT Madras
Detection Sensitivity
Ref: Ghasemi et al., IEEE Communications Mag,
April 2008

Pp LR 
Ps LD   Pb
Pp , Ps , Pb  Power of PU, SU, and background noise  interferen ce
R  Max distance between PU transmitte r and receiver
L(d )  Path loss (Shadowing and Fading) at distance d

Threshold to be satisfied even if PU Rx is at edge of coverage
–
Provided SU maintains distance D

 SU (CR) must be able to detect PU at distance (R+D)

Detection Sensitivity
Koilpillai / Mar 2009 / Cognitive Radio
IISc-DRDO Seminar on Cognitive Radio
 min 
Pp LD  R 
Pn
27
IITM Proprietary Information
Electrical Engineering
IIT Madras
Uncertainties in Sensing
Channel Uncertainty

Due to fading / shadowing of PU signal
Noise Uncertainty
Aggregate Interference Uncertainty

PU may experience harmful
interference
–

If multiple CR networks active
Requires more sensitive detectors
–
Detect PU at distance
D  D  R
Ref: Ghasemi et al., IEEE Communications Mag, April 2008

Alternative – system level coordination among CR devices
–
 Cooperative sensing
Koilpillai / Mar 2009 / Cognitive Radio
IISc-DRDO Seminar on Cognitive Radio
28
IITM Proprietary Information
Cooperative Sensing

Sensing of primary user difficult with multipath fading and shadowing
–


Significant fluctuation of signal level (worst case is very severe)
Need to maintain sensing performance
–
 CR requires higher detection sensitivity (lower
–
Requirement becomes very stringent
 min
)
To alleviate the problem …  Cooperative Sensing
–
Independent measurements at different locations / CRs
–
Exchange of sensing information among CR nodes
–
Diversity gain achieved (with respect to fading and shadowing)
–
Improved probability of detecting PU

Without increasing sensitivity of each individual SU Rx
–
Introduces additional communications overhead
–
Requires functionality of “Band Manager” (Fusion Centre)


Electrical Engineering
IIT Madras
Collects information, makes decisions and shares information with all CR nodes
Shadowing is correlated over short distances
–
 Cooperation to be done over larger distances (few nodes)
–
Different from conventional view of Mesh / Ad Hoc networks (many nodes in close proximity)
Koilpillai / Mar 2009 / Cognitive Radio
IISc-DRDO Seminar on Cognitive Radio
29
IITM Proprietary Information
Electrical Engineering
IIT Madras
Cooperative Sensing

Decision making options
–
Hard decision based
–
Soft decision based
Hard Decision


Each SU makes indep decision
–
Reg presence of PU
–
One-bit decision
Band Manager gathers information
–
Shares decision with all CR nodes

Rule: If one of the SUs senses PU signal  Primary User present

ROC – Receiver Operating Characteristic to evaluate performance

Observation
–
HD based decision making – not beneficial if SU SNRs are vastly different
Koilpillai / Mar 2009 / Cognitive Radio
IISc-DRDO Seminar on Cognitive Radio
30
IITM Proprietary Information
Electrical Engineering
IIT Madras
Multicarrier Techniques in CR
Koilpillai / Mar 2009 / Cognitive Radio
IISc-DRDO Seminar on Cognitive Radio
31
IITM Proprietary Information
Electrical Engineering
IIT Madras
Multicarrier Techniques

Multicarrier techniques widely used in Cognitive Radio (PHY)
–
OFDM, Filterbank-based multicarrier, Multi-resolution filter banks
–
Spectrum sensing – determine spectral holes
–
Spectrum usage – communication
Code

Transmit data w/o interfering with Primary user

In non-overlapping parts of spectrum

Multicarrier techniques – efficient and effective

To maximize efficiency
–
CR transmission can be TDD or FDD

TDD has inherent advantages for CR
Tx and Rx in in same band  knowledge of channel


Time
Sidelobes (frequency response) of the subcarriers must be minimized

–
Frequency
Implicit sensing of channel during Rx period (Tx OFF)
802.22 WRAN standard focus on TDD
–
OFDM based
Koilpillai / Mar 2009 / Cognitive Radio
IISc-DRDO Seminar on Cognitive Radio
32
IITM Proprietary Information
Electrical Engineering
IIT Madras
Multicarrier Techniques

OFDM
–
–
–
–
Widely studied and well-understood (based on IFFT / FFT)
Used for spectral sensing
Underlying filter is the Rectangular window
 Poor side-lobe suppression
 Significant interference between sub-carriers
Not suitable for spectral sensing / transmission (non-contiguous bands)



Acceptable for contiguous bands
Approaches to consider
–
Muti-Taper Method (MTM) for spectral estimation
–
Filterbank Multi-Carrier
Filterbank-based approaches can overcome spectral leakage problems
–
Less used than OFDM
Koilpillai / Mar 2009 / Cognitive Radio
IISc-DRDO Seminar on Cognitive Radio
33
IITM Proprietary Information
OFDM Carriers in Available Spectrum
Electrical Engineering
IIT Madras
Spectral Adaptation Waveforms
T
I
M
E
Frequency
Ref: B. Fette, “SDR Technology Implementation for the Cognitive Radio,” General Dynamics
Koilpillai / Mar 2009 / Cognitive Radio
IISc-DRDO Seminar on Cognitive Radio
34
IITM Proprietary Information
Electrical Engineering
IIT Madras
Performance of FFT

Raised cosine filtering before FFT
–

Improved freq selectivity
–

Reduces side-lobes
At expense of lower time selectivity
Frequency response of “FFT filter”
i ( f )  K sinc 2  f  f i  Ts 
f i  Centre frequency of i th sub - channel
Ts  OFDM symbol period (incl. CP)

Filtering at Rx end also possible
–
Similar tradeoff as at Tx
Ref: Boroujeny et al., IEEE Communications Mag, April 2008
Koilpillai / Mar 2009 / Cognitive Radio
IISc-DRDO Seminar on Cognitive Radio
35
IITM Proprietary Information
Electrical Engineering
IIT Madras
Multicarrier Techniques


Multitaper Method (MTM)
–
Advanced, non-parametric spectral estimation method
–
A set of filters (Slepian 1978, Bell Labs)

Discrete Prolate Spheroidal Sequences

Optimal trade-off between time selectivity and frequency selectivity
–
Combine the output of a family of filters
–
Near-optimal performance in spectral sensing (Haykin, 2005)
–
Example: A set of 5 DPSS based filters and their responses
Filterbank Method
–
Similar performance to MTM
–
Can be used for sensing and for transmission
–
Lower computational complexity than MTM
–
A rich area for further investigation for CR
Koilpillai / Mar 2009 / Cognitive Radio
IISc-DRDO Seminar on Cognitive Radio
36
IITM Proprietary Information
Electrical Engineering
IIT Madras
Performance of Filterbank

MTM – five filters of length 2048
–

Ref: Boroujeny et al., IEEE Communications Mag, April 2008
Three filters with attenuation more than -60 dB
Filterbank Multicarrier – Length 6x256=1536, 256-channel filterbank
–
Achieves comparable performance to MTM
Koilpillai / Mar 2009 / Cognitive Radio
IISc-DRDO Seminar on Cognitive Radio
37
IITM Proprietary Information
Electrical Engineering
IIT Madras
UWB-based CR
Koilpillai / Mar 2009 / Cognitive Radio
IISc-DRDO Seminar on Cognitive Radio
38
IITM Proprietary Information
Electrical Engineering
IIT Madras
UWB Overview

Cognitive network – an interconnection set of CR devices
–
Aware of radio channel characteristics
–
Interference temperature, spectrum availability, policies, …
–
Devices sharing of information to facilitate CR functions

Suitable wireless technology  facilitate collaboration between CR nodes

Ultra Wideband (UWB)
–
Bandwidth (BW) > 500 MHz or
–
Fractional BW
fH  fL
 0.2
 fH  fL 


 2 

FCC permits unlicensed use of UWB (2002)

Proposed methods for UWB
–
OFDM-based UWB (UWB) – (OFDM-UWB)
–
Impulse radio based UWB (IR-UWB)
Koilpillai / Mar 2009 / Cognitive Radio
IISc-DRDO Seminar on Cognitive Radio
39
IITM Proprietary Information
Electrical Engineering
IIT Madras
UWB Overview


UWB – an underlay system
–
Co-exist with other licensed (primary) / UL users
–
In same temporal, spatial, and spectral domain
–
Signal embedded in noise floor  secure transmission
UWB has multidimensional flexibility
–
Pulse shape, bandwidth (BW), data rate, power

UWB has inherent potential to meet CR requirements

IR-UWB – multiple attractive features
–
High multipath resolution
–
Ranging and positioning

UWB – unlicensed operation in 3.1-10.6 GHz

Tx power limit < -42 dBm/MHz
–
Ensures that UWB does not affect licensed operations
Koilpillai / Mar 2009 / Cognitive Radio
IISc-DRDO Seminar on Cognitive Radio
40
IITM Proprietary Information
Electrical Engineering
IIT Madras
UWB-based CN

An interesting possibility …
–
–
–

UWB as a complement to other CR technologies
For sharing information via UWB
Locating other users
Information exchange in CN
–
–
–
CR nodes must have common understanding of spectrum to be used
  Sharing of sensing information
Possible options
 Common control channel for CR nodes to share information
 A centralized controller that gathers info and decides spectrum availability
– Allocates distinct bands to each CR user
Alternative: Low-power UWB signaling to share information
 Leverage all the advantages of UWB
 Low-throughput needed
 Low-complexity (OOK, with non-coherent detection)
 Issue: range of UWB
Koilpillai / Mar 2009 / Cognitive Radio
IISc-DRDO Seminar on Cognitive Radio
41
IITM Proprietary Information
Electrical Engineering
IIT Madras
Cognitive Networks

Network of nodes with CR functionality

Cognitive networks is attractive for Dynamic Spectrum Access

Sharing via UWB is attractive
–
Point-to-point model
–
Centralised model
–
Draw from research results in UWB-based sensor networks
Source: Arslan et al., Cognitive Wireless Communication Networks, Springer
Koilpillai / Mar 2009 / Cognitive Radio
IISc-DRDO Seminar on Cognitive Radio
42
IITM Proprietary Information
Security in Distributed Sensing
Electrical Engineering
IIT Madras

Reliable spectrum sensing is key in CR networks

Shadowing and multipath fading  challenges in sensing

Shadowing leads to “hidden node” problem

Sensing challenges alleviated by “Cooperative Sensing”
–

Two major security issues
–
–


Using multiple distributed CR nodes
Incumbent emulation
 Caused by a malicious secondary
 Gains priority over channel by emulating PU characteristics
Falsification of spectrum sensing data
 False data to mislead band manager
Both are important issues that need to be addressed
Potential countermeasures
–
–
Authentication of the data and the sender
Robust data fusion methods
Koilpillai / Mar 2009 / Cognitive Radio
IISc-DRDO Seminar on Cognitive Radio
43
IITM Proprietary Information
Electrical Engineering
IIT Madras
Information Theoretic Aspects
- Capacity of CR Channel
Koilpillai / Mar 2009 / Cognitive Radio
IISc-DRDO Seminar on Cognitive Radio
44
IITM Proprietary Information
Electrical Engineering
IIT Madras
Information Theoretic Aspects in CR

Current CR scenario

Device X1 transmits only when
channel is free

Device X2 transmits after X1

Or uses different freq band

X2 need not wait until X1 is done
Ref: Devroye et al., “Limits on Communications in a
Cognitive Radio Channel,” IEEE Communications Mag,
June, 2006

Is simultaneous transmission more efficient than time sharing?

What are the achievable rates at which two users (CR capable) could transmit

What are the achievable rates if two users do not have CR capability?
Koilpillai / Mar 2009 / Cognitive Radio
IISc-DRDO Seminar on Cognitive Radio
45
IITM Proprietary Information
Electrical Engineering
IIT Madras
Information Theoretic Aspects in CR
Ref: Devroye et al., “Limits on Communications in a
Cognitive Radio Channel,” IEEE Communications
Mag, June, 2006

Cognitive Radio Scenario
–

Goal: Define and evaluate channel capacity for CR channel
–
–

Simplified model : Two transmitters (X1 and X2) and two receivers, (Y1 and Y2)
Two links: (X1  Y1 ) and (X2  Y2 )
Evaluate max. rate at which information sent over both links
Capacity will be a two-dimensional graph (R1 , R2 )
–
–
–
Capacity regions – max. set of all reliable rates that can be simultaneously achieved
Obtain inner (achievable region) bounds and outer bounds
Usually based on random coding (w/o explicitly constructing codes
Koilpillai / Mar 2009 / Cognitive Radio
IISc-DRDO Seminar on Cognitive Radio
46
IITM Proprietary Information
Electrical Engineering
IIT Madras
Information Theoretic Aspects in CR




Two links:
– (X1  Y1 ) and (X2  Y2 )
X2 is a CR device
(X1  X2 ) exists
– X2 knows message of X1
– Genie aided
X1does not know message of X2
–




An asymmetric problem
An idealized situation
Will provide an upper bound on
rates achievable in practice
An open problem
Achievable region – combination of
–
–
–
Han-Kobyashi interference region
Dirty paper coding
Relaying
Ref: Devroye et al., “Limits on Communications in a Cognitive Radio Channel,” IEEE Communications Mag, June, 2006
Koilpillai / Mar 2009 / Cognitive Radio
IISc-DRDO Seminar on Cognitive Radio
47
IITM Proprietary Information
Electrical Engineering
IIT Madras
Capacity

Computing capacity regions uses three techniques
–
–
–


Han-Kobyashi interference region
Dirty paper coding
Relaying
Two links: (X1  Y1 ) and (X2  Y2 ) and X2 knows message of X1
Two possible actions of X2
–
Selfish Approach
 Try to mitigate own interference  Dirty Paper coding

–
Selfless Approach

X2 acts a relay for X1
X2 does not transmit own information

Region where R1 is higher than R2





Achieves region where R2 > R1
Region 1 – Time sharing by X1 and X2
Region 2 – Interference region – both do not know other’s information
Region 3 – Cognitive region
Region 4 – MIMO region – Both X1 , X2 and Y1 , Y2 cooperate
–
This is the region that gives maximum capacity
Koilpillai / Mar 2009 / Cognitive Radio
IISc-DRDO Seminar on Cognitive Radio
48
IITM Proprietary Information
Electrical Engineering
IIT Madras
CR – A Practical Implementation
Koilpillai / Mar 2009 / Cognitive Radio
IISc-DRDO Seminar on Cognitive Radio
49
IITM Proprietary Information
Electrical Engineering
IIT Madras
CorDECT Rural WLL Deployment
CorDECT Network
Cor DECT
CPE
Fixed Wireless Link
Up to 240 Kbps per village
15 Km range
(up to 25 km with repeater)
PSTN
SS7/ R2MF
V5.2
Village B
Cor DECT
CPE
CorDECT
Base
Station
xDSL/E1
CorDECT
CO
Internet
Access Center
Village A
corDECT is deployed in > 15 countries
Koilpillai / Mar 2009 / Cognitive Radio
IISc-DRDO Seminar on Cognitive Radio
50
IITM Proprietary Information
Electrical Engineering
IIT Madras
GSM - CR Combination
GSMLite
CorDECT
CPE
PSTN
PLMN
VoIP
CorDECT Network
GSM
BTS
Fixed Wireless Link
Up to 240kbps per village
15km range
CorDECT
(more reach with
Base Stn
Repeaters)
GSM Hotspot
2 km radius
Village B
CorDECT
CPE
xDSL/E1
CorDECT
CO
SoftSwitch
GSM
BTS
Media&
Signaling
Gateway
Access Center
GSM Hotspot
2 km radius
Village A
Koilpillai / Mar 2009 / Cognitive Radio
IISc-DRDO Seminar on Cognitive Radio
51
IITM Proprietary Information
Electrical Engineering
IIT Madras
CR Techniques for GSM band
Goal: Adaptive freq selection for GSM BTS

Prototype (under field trial):
Interference avoidance using CR
Description:



Support GSM Lite developed by Midas

Usage: rural areas, in-building, femtocells

Based on ADI Blackfin DSP
Challenges

Weak signal detection and monitoring

Listening to other GSM BTS

Hardware and Software Implementation
Approaches for detecting GSM signal

Cross Correlation Detector – training sequence

Cyclostationarity-based


Sensitive to frequency error
Hybrid Detector (developed)


Performance of Hybrid scheme
Combines different schemes
Implementation – “intelligent hopping”
Koilpillai / Mar 2009 / Cognitive Radio
IISc-DRDO Seminar on Cognitive Radio
52
IITM Proprietary Information
Electrical Engineering
IIT Madras
Summary

A technical overview of Cognitive Radio

CR - A paradigm shift in wireless communications

Potential of significant increase in spectrum availability
–

Opportunistic access
Spectrum sensing
–
Understanding the various challenges
–
Technical and regulatory issues
–
Robust and computationally efficient approaches are needed

Cooperative sensing is attractive

Information theoretic aspects – Capacity region for CR

IEEE 802.22 standard

A practical application – CR-based GSM basestation

Overall, CR is an exciting field
Koilpillai / Mar 2009 / Cognitive Radio
IISc-DRDO Seminar on Cognitive Radio
53
IITM Proprietary Information
Electrical Engineering
IIT Madras
My best wishes to
all participants of
IISc-DRDO Seminar on Cognitive Radio
Thank You !
Koilpillai / Mar 2009 / Cognitive Radio
IISc-DRDO Seminar on Cognitive Radio
54
IITM Proprietary Information
Electrical Engineering
IIT Madras
David Koilpillai Profile
Education
B.Tech, IIT Madras, MS, PhD Caltech, USA
Work Experience
IIT Madras (2002 – present)
Professor, TeNeT Group, EE Department
CEWiT – Chief Scientist (Jan 2007 – July 2007

Co-Chair, IIT Hyderabad Task Force (June 2008 – present)
Ericsson Inc, USA (1990-2002)

Director, Advanced Technologies, Research and Patents
(R&D team of 75 engineers, annual budget US $20 Million)
Professional
–
–
–
–
–
Areas of expertise: Cellular, wireless systems, DSP
32 Issued US patents
Publications: 11 Journal, 45 Conference
Research Interests: DSP applications in Wireless
Ericsson Inventor of Year Award 1999
Fellow, Indian National Academy of Engineering
–
Koilpillai / Mar 2009 / Cognitive Radio
IISc-DRDO Seminar on Cognitive Radio
55
IITM Proprietary Information