Advanced Topics in Wireless Communications COGNITIVE RADIO NETWORKS
Fixed Spectrum Utilization
Maximum Amplitudes
Amplit
ude
(dBm)
Heavy Use Heavy Use
Medium Use Sparse Use
Problems of Fixed Spectrum Utilization
Spectrum usage is concentrated on certain portions of the
spectrum
A significant amount of the spectrum remains unutilized.
According to FCC (Federal Communication Commission):
COGNITIVE RADIO NETWORKS;
DYNAMIC SPECTRUM ALLOCATION NETWORKS (DSANs);
xG INITIATIVE
Dynamic Spectrum Allocation
A “Cognitive Radio” is the key enabling technology
for Dynamic Spectrum Access!!
Capability to use or share the spectrum in an
opportunistic manner. “BANDWIDTH HARVESTING”
1) Determine which portions of the spectrum is available and detect the presence of licensed users when a user operates in a licensed band (Spectrum Sensing)
2) Select the best available channel (Spectrum Decision)
3) Coordinate access to this channel with other users (Spectrum Sharing)
4) Vacate the channel when a licensed user is detected
A
“Cognitive Radio”
is a radio that can
change its transmitter parameters based on
interaction with the environment in which it
operates.
(
F
ederal
C
om
C
ommission’05)
FCC (Non-Federal Use of the Spectrum)
A radio or system that senses its operational EM environment and can dynamically and autonomously adjust its radio
operating parameters to modify system operation, such as maximize throughput, mitigate interference, facilitate
interoperability and access secondary markets..
NTIA (National Telecom and Info Administration)’05
A radio or system that senses and is aware of its
operational environment and can dynamically and
autonomously adjust its radio operating parameters accordingly.
ITU (Wp8A working document)’05
A type of radio that can sense and autonomously reason about its environment and adapt accordingly.
This radio could employ knowledge representation,
automated reasoning, and machine learning mechanisms in establishing conducting or terminating communication or networking functions with other radios.
A RADIO THAT IS COGNITIVE !!!!
Senses RF Environment and modifies
frequency, power or modulation
Allows for Real Time Spectrum Management
Dynamic Frequency Selection (DFS)
Adaptive modulation Transmit Power Control (TPC) Adjust transmit parameters based on location
spectrum sharing between a licensee and a third party
Other functionalities are being developed
Analogy between a Cognitive Radio and a Car Driver
Cognitive Radio’s Capabilities:
Senses, and is aware of, its operational
environment and its capabilities
Can dynamically and autonomously adjust its radio
operating parameters accordingly
Learns from previous experiences
Analogy between a Cognitive Radio and a Car Driver
Car Driver’s Capabilities:
Senses, and is aware of, its operational environment and
its capabilities
Can dynamically and autonomously adjust the driving
operation accordingly
Learns from previous experiences
Deals with situations not planned at the initial time of
Spectrum Hole Concept
Frequency
Spectrum Hole
Ultimate Objective of Cognitive Radio
CR enables the usage of temporally unused spectrum
Spectrum Hole
orWhite Space
.If this band is further used by a licensed user,
CR moves to another spectrum hole or stays in the same band
MAIN CHARACTERISTICS OF CR
A. Cognitive Capability
Cognitive Capability
SPECTRUM AWARENESS!!
– Capture or sense the information (e.g., licensed user’s
activity) from radio environment
– Capture the temporal and spatial variations in radio environment
– Avoid interference to other users
– Identification of unused spectrum portions at a specific time or location
Reconfigurability
(SDR functionality)
Enabling the radio* to be dynamically programmed to transmit and receive on a variety of frequencies according to the radio environment and
Physical Architecture of the Cognitive Radio
(Wideband RF/Analog Front-End Architecture)
Frequency Power Spectrum Density (PSD) Band of interest Available Channel
...
...
PSD of the received licensed signalChallenges for Development of CR RF Front-End
Wideband RF antenna receives signals from various
transmitters operating at different power levels, bandwidths, and locations.
the RF front-end must be able to detect a weak
Alternative Approach:
Directional Antennas
Use multiple antennas
such that signal
filtering is performed in the spatial domain
rather than in the frequency domain.
Multiple antennas can
receive signals Licensed User f1 Licensed User f2 f1 f2 f1
Cognitive Radio Network Architecture
Primary Base-station Primary User Licensed Band I Unlicensed BandLicensed Band II CR Network
Access Primary Network Access CR User Spectrum Band CR Base-station Other Cognitive Radio Networks Spectrum Broker
Cognitive Radio Network Architecture
Primary Base-station Primary User Licensed Band I Unlicensed Band Licensed Band II Primary Network Access CR Ad Hoc Access CR User Spectrum BandCognitive Radio Network Architecture
Primary Base-station Primary User Licensed Band I Unlicensed Band Licensed Band II CR Network Access Primary Network Access CR Ad Hoc Access CR User Spectrum Band CR Base-station Other Cognitive Radio Networks Spectrum BrokerArchitecture
Primary Network
(Primary User, Primary Base Station)
Cognitive Radio Network
(CR User, CR Base Station)
Primary Network
* An existing network infrastructure (or ad hoc network) which has an access right to a certain spectrum band.
* Examples include the common cellular and TV broadcast networks.
Primary User
(or Licensed User)
* Has a license to operate in a certain spectrum band.
* This access can only be controlled by the primary base- station and should not be affected by the operations of any other unlicensed users.
REMARK:
Primary Base-Station
(or Licensed Base-Station)
A fixed infrastructure network component which has a
spectrum license such as BTS in a cellular system.
Does not have any CR capability for sharing spectrum with
CR users.
Cognitive Radio Network
(or Dynamic Spectrum Access Network,
or Secondary Network or Unlicensed Network)
* Does not have license to operate in a desired band.
* Hence, the spectrum access is allowed only in an opportunistic manner.
Cognitive Radio User
(or Unlicensed User, Secondary User)
has no spectrum license
Hence, additional functionalities are required
to share the licensed spectrum band.
Cognitive Radio
Base-Station
(or Unlicensed Base-Station or Secondary Base-Station)
– A fixed infrastructure component with CR capabilities.
– CR base-station provides single hop connection to CR users without spectrum access license.
Spectrum Broker
(or Scheduling Server)
– A central network entity that plays a role in
sharing the spectrum resources among different CR networks.
– It can be connected to each network and can serve as a spectrum information manager to enable
Architecture
CR Network Access:
CR users can access their own CR base-station both on licensed and unlicensed spectrum bands.
CR Ad hoc Access:
CR users can communicate with other CR users through ad hoc connection on both licensed and unlicensed spectrum
Classifications
CR Network on Licensed Band
CR user is capable of using bands assigned to
licensed users, apart from unlicensed bands, such as ISM band.
CR Network on Unlicensed Band
CR can only utilize unlicensed parts of radio frequency spectrum.
Cognitive Radio Network on Licensed Band
Primary User Primary Base-Station Primary Network CR Base-station Dynamic Spectrum AccessCR Network on Licensed Band
Temporally unused spectrum holes exist in the
licensed spectrum band.
CR networks can exploit these spectrum holes
through cognitive communication techniques.
In Figure, CR network coexists with the primary
CR Network on Licensed Band
Main purpose of the CR network is to determinethe best available spectrum
Here in the licensed band, CR functions are
aimed at the detection of the presence of primary users.
CR Network on Licensed Band
Interference avoidance with primary users is
the most important issue here
Also if primary users appear in the spectrum
band occupied by CR users, they should
vacate the current spectrum band and move to the new available spectrum immediately
Cognitive Radio Network on Unlicensed Band
Spectrum Broker
Cognitive Radio Network A
CR Base-Station
Cognitive Radio Network B
CR Network on Unlicensed Band
Since there are no license holders, all network entities have
the same right to access the spectrum bands.
Multiple CR networks co-exist in the same area and
communicate using the same portion of the spectrum.
Intelligent spectrum sharing algorithms can improve the
CR Network on Unlicensed Band
CR users focus on detecting the transmissions of
other CR users.
Since all CR users have the same right to access the
spectrum, CR users should compete with each other for the same unlicensed band.
CR Network on Unlicensed Band
REQUIREMENTS:
1. Sophisticated spectrum sharing methods among CR users.
2. Fair spectrum sharing among networks if multiple CR network operators reside in the same unlicensed band.
Cognitive Cycle
A CR determines appropriate communication
parameters and adapts to the dynamic radio
environment
Tasks required for adaptive operation in open
spectrum referred as
COGNITIVE CYCLE
.
Cognitive Cycle
Spectrum Sharing Spectrum Sensing Transmitted Signal Licensed User Detection RF Stimuli Spectrum Hole Radio Environment Spectrum Mobility Decision RequestSpectrum Sensing
A CR monitors the available spectrum bands,
captures their information, and then detects
the spectrum holes.
Spectrum Decision
– Based on the spectrum availability, CR users can determine a channel.
– This operation not only depends on spectrum availability, but it is also determined based on internal
Spectrum Sharing
Multiple CR users try to access the spectrum
CR network access should be coordinated in
order to prevent multiple users colliding in overlapping portions of the spectrum.
Spectrum Mobility
CR users are regarded as "visitors" to the spectrum. If primary users need a specific portion of the
spectrum then the CR users must continue in another vacant portion of the spectrum.
Reconfigurability
Capability of adjusting operating parameters for the
transmission on-the-fly without any modifications on the hardware components.
This capability enables CR to adapt easily to the
Reconfigurable Parameters
i) Operating Frequency
ii) Modulation
iii) Transmission Power
Operating Frequency
A CR is capable of changing the operating frequency. Based on the information about the radio environment,
the most suitable operating frequency can be determined and
Modulation
A CR should reconfigure the modulation scheme
adaptive to the user requirements and channel conditions.
Example: Delay Sensitive Applications data rate important
Modulation scheme enabling higher spectral efficiency!!
Transmission Power
Transmission power can be reconfigured within the
power constraints.
If higher power operation is not necessary, CR
reduces the transmitter power to a lower level to allow more users to share the spectrum and to
Communication Technology
A CR can be used to provide interoperability
among different communication systems.
Reconfigurable Parameters
Not only at the beginning of a transmission but also during
the transmission.
Parameters can be reconfigured such that
* CR is switched to a different spectrum band * Tx and Rx parameters are reconfigured
What is Spectrum Sensing ?
How to
detect spectrum holes
by the COGNITIVE RADIO so that
Spectrum Sensing
Spectrum Sharing Spectrum Sensing Primary User Detection RF Stimuli Spectrum Hole Radio Environment Spectrum Mobility Decision Request Transmitted SignalCR User 1
No interaction between CR user and Primary Tx/Rx
CR user must rely on locally sensed signals to infer primary user activity
Channels found occupied by CR user (Licensed bands 1 and 2) are now avoided during
A general CR based communication scenario
CR User 2 Licensed band 1 Licensed band 2
EFFICIENT WAY TO DETECT SPECTRUM HOLES !!
Detect primary users that are receiving data within
the communication range of a CR user.
In reality Difficult for a CR to detect primary user activity in the
absence of interaction between primary users and itself.
RECENT RESEARCH
Classification of Spectrum Sensing Techniques Interference Temperature Management Transmitter Detection
Spectrum Sensing
Receiver Detection Matched FilterTransmitter Detection
CR should distinguish between Used and Unused spectrum
bands.
CR should have the capability to determine if a signal from
primary user (transmitter) is locally present in a certain spectrum.
Basic Hypothesis Model for Transmitter Detection
The signal x(t) received (detected) by the CR (secondary) user is
where n(t) AWGN (Additive White Gaussian Noise)
s(t) Transmitted signal of the primary user
h Amplitude gain of the channel
H Null hypothesis No licensed user signal in a certain spectrum band.
1 0 ) ( ) ( ) ( ) ( H t n t hs H t n t x
Transmitter Detection
Three schemes are generally used for the transmitter detection according to the hypothesis model.
– Matched Filter Detection
– Energy Detection and
D. Cabric, S. M. Mishra, and R. W. Brodersen, “Implementation Issues in Spectrum
Sensing for Cognitive Radios,” in Proc. 38th Asilomar Conference on Signals,
Matched Filter Detection
Interference Temperature Management Transmitter Detection Spectrum Sensing Receiver DetectionMatched Filter Energy Cyclostationary Feature Detection
Matched Filter Detection
0 T s(t) r(t) 0 T o H Y Sample at t = T Received Signal r(t) = s(t) + n(t) t d t T s r 0 () ( ) Threshold Device Y 1 H Decide H 0 or H1 Matched Filter 0 T maximum at T 2T 0 T 2TMatched Filter Detection
When the shape of the primary user signal is known to
the CR user, the optimal detector in an AWGN channel is the matched filter since it maximizes the received SNR.
Advantage of Matched Filter:
Requires less time to achieve high processing gain due to coherency
A. Sahai, N. Hoven and R. Tandra, “Some Fundamental Limits in Cognitive Radio, in Proc. Allerton Conf. on Comm., Control and Computing 2004
Matched Filter Detection
But
it requires a priori knowledge of the primary user signal such
as the modulation type and order, the pulse shape, and the packet format
Hence, if this information is not accurate, then the matched
filter performs poorly.
Energy Detection
Interference Temperature Management Transmitter Detection Spectrum Sensing Receiver Detection Matched FilterEnergy Detection
If the CR user cannot gather sufficient information about
the primary user signal s(t), the matched filter is not suitable.
D. Cabric, S. M. Mishra, and R. W. Brodersen, “Implementation Issues in Spectrum
Sensing for Cognitive Radios,” in Proc. 38th Asilomar Conference on Signals,
Systems and Computers, pp. 772776, Nov. 2004.
H. Tang, “Some Physical Layer Issues of Wideband Cognitive Radio System,” in
Energy Detection
Input 2
) (
Squaring Device Integrator Threshold Device
Decide H0 or H1 ) (t r T dt t r 0 2 ) ( ) ( 2 t r Y
T dt 0 Filtering o H Y 1 H T: Observation (sensing) Time
A. Ghasemi and E. S. Sousa, “Collaborative Spectrum Sensing for Opportunistic
Energy Detection
In order to measure the energy of the received
signal by the CR user, the output signal of bandpass filter with bandwidth
W
is squared and integratedEnergy Detection
Finally, the output of the integrator, Y, is compared with a
threshold, λ, to decide whether a licensed user is present or not. (AWGN case)
Energy Detection
A low Pd missing the presence of the
primary user with high probability
increases the interference to the primary user
A high Pf low spectrum utilization
(since false alarms increase the number of missed opportunities (white spaces)).
Problems of Energy Detection
Performance is susceptible to uncertainty in noise
power. SNR problem!!!
Energy detector cannot differentiate signal types
but can only determine the presence of the signal.
Energy detector is prone to the false detection triggered by the unintended signals.
Energy detector needs longer sensing time
Cyclostationary Feature Detection
Interference Temperature Management Transmitter Detection Spectrum Sensing Receiver DetectionCyclostationary Feature Detection
Modulated signals are in general coupled with sine wave
carriers, pulse trains, repeating spreading, hopping sequences, or cyclic prefixes, which result in built-in periodicity.
D. Cabric, S. M. Mishra, and R. W. Brodersen, “Implementation Issues in Spectrum
Sensing for Cognitive Radios,” in Proc. 38th Asilomar Conference on Signals, Systems and Computers, pp. 772776, Nov. 2004.
A. Fehske, J. D. Gaeddert, and J. H. Reed, “A New Approach to Signal
Classification Using Spectral Correlation and Neural Networks,” in Proc. IEEE DySPAN, pp. 144150, Nov. 2005.
Cyclostationary Feature Detection
These modulated signals are characterized as
cyclostationary since their mean and autocorrelation exhibit periodicity.
These features are detected by analyzing a spectral
Sine based Cyclostationary Detection
Primary Tx frequency repeats over symbol durations at regular intervals T
Cyclostationary Feature Detection
Correlate R(f+ )R*(f- ) Average over T r(t) Feature detect r(t) : Received signal R(f) : Fourier transform of r(t) : Cyclic frequency R*(f) : Complex conjugate of R(f)If cyclostationary with period T then cycle autocorrelation has component at =1/T.
Cyclostationary Feature Detection
This scheme performs better than the energy
detector in discriminating against noise due to its robustness to the uncertainty in noise power.
Computationally complex and requires significantly
long observation time.
H. Tang, “Some Physical Layer Issues of Wideband Cognitive Radio System,” in
Limitations of the Transmitter Detection
Hidden Terminal Problem due to Shadowing CR Transmitter Range CR User Primary Transmitter Range Primary Base-station Primary Transmitter Range Primary User CR Transmitter Range Interference Interference CR User Cannot detect the transmitter Shadowing Problem Receiver Uncertainty Problem
Receiver Uncertainty Problem
With the transmitter detection, the CR user
cannot avoid the interference due to the lack of the primary receiver’s information (Fig.a).
Moreover, the transmitter detection model cannot
Shadowing Problem
A CR user is located in the transmission range of the
primary transmitter, but may not be able to detect the transmitter due to the shadowing (Fig. b).
Consequently, the sensing information from other
Transmitter Detection
Non-Cooperative vs Cooperative Detection
Transmitter Detection
Matched Filter
Detection Detection Energy
Cyclostationary Feature Detection Transmitter Detection Non-Cooperative
Detection Cooperative Detection Detection Method
Detection Behavior
Non-Cooperative vs Cooperative Detection
Non-Cooperative Detection
– CR users detect the primary transmitter signal independently through their local observations.
Cooperative Detection
- Information from multiple CR users are utilized for primary user detection.
Cooperative Detection
Primary Base-station
Multi-path fading
Weak signals are received due to the multi-path fading
may not detect
the primary user
Shadowing Cannot detect the primary user due to the obstacles Detect the primary user correctly By exchanging their sensing information, CR CR User 3 CR User 1 BUSY IDLE BUSY BUSY
Detection and False Alarm Probability
for Cooperative Detection
A. Ghasemi and E. S. Sousa, “Collaborative Spectrum Sensing for Opportunistic
Access in Fading Environment,“ in Proc. IEEE DySPAN, pp. 131-136, Nov. 2005
Assume n CR users have the same sensing capabilities
(same Pd and Pf )
All CR users assume a channel to be occupied even if at
least one CR user detects a primary user in that channel.
Detection and False Alarm Probability
for Cooperative Detection
A. Ghasemi and E. S. Sousa, “Collaborative Spectrum Sensing for Opportunistic
Access in Fading Environment,“ in Proc. IEEE DySPAN, pp. 131-136, Nov. 2005
Note: Cooperative detection also increases the
probability of false-alarm. n f f n d d P n Q P n Q ) 1 ( 1 } correctly hole spectrum detect the users CR all Pr{ 1 ) 1 ( 1 } detection the miss users CR all Pr{ 1
Qd is the cooperative detection probability Q is the cooperative false alarm probability
Increasing Qd
Increasing Qf
Detection and False Alarm
Cooperative Detection
Cooperative Methods
– Provide more accurate sensing performance !
– However, they cause overhead traffic and power consumption for exchanging sensing information.
STILL ADDITIONAL PROBLEM:
Primary receiver uncertainty problem caused by
Primary Receiver Detection
Interference Temperature Management Transmitter Detection Spectrum Sensing Receiver DetectionPrimary Receiver Detection
Primary Base-station
CR User Local Oscillator (LO)
Leakage Power
CR users detect the
LO leakage power for the detection of
primary users instead of the transmitted signals
When primary users
receive the signals from the transmitter, they emit the LO leakage
B. Wild and K. Ramchandran, “Detecting Primary Receivers for Cognitive Radio
Primary Receiver Detection
AGC A/D PLL Antenna RF Filter Mixer VCO Channel Selection Filter LNA Local Oscillator- Generates a sine signal for the baseband conversion RF Front-end of the Primary Receiver
How can the LO Leakage Power be detected?
Same methods as before, i.e.,
(Matched filter detection, Energy
detection or Cyclostationary feature
detection )
How can the LO Leakage Power be detected?
Primary receiver detection can solve the receiver
uncertainty problem in the transmitter detection
However, since the LO leakage signal is typically weak,
Interference Temperature Management
Interference Temperature Management Transmitter Detection Spectrum Sensing Receiver Detection Matched FilterInterference Temperature Model
o
Power at Receiver
Original Noise Floor
Interference Temperature Limit
Licensed Signal
New Opportunities for Spectrum Access
Minimum Service Range with Interference Cap
Service Range at Original Noise Floor
Interference Temperature Model
The model shows the signal of a radio designed to
operate in a range at which the received power approaches the level of the noise floor.
As additional interfering signals appear, the noise
floor increases at various points within the service area, as indicated by the peaks above the original
Interference Temperature Model
Model manages interference at the receiver through
the interference temperature limit, which is
represented by the amount of new interference that the receiver could tolerate.
Interference Temperature Model
I.o.w., the interference temperature
model accounts for the cumulative RF energy from multiple transmissions and sets a
maximum cap on their aggregate level.
As long as CR users do not exceed this limit by
Interference Temperature Measurement Problems
No practical way for a CR to measure or estimate the
interference temperature. (CR users cannot distinguish between actual signals from the primary user and
noise/interferences).
Interference temperature limit should be location dependent
Spectrum Decision
Spectrum Sharing Spectrum Sensing Primary User Detection RF Stimuli Spectrum Hole Radio Environment Spectrum Mobility Decision Request Transmitted SignalSpectrum Decision
– Unused spectrum bands will be spread over wide frequency range including both unlicensed and licensed bands.
– CR networks require capabilities to decide the best spectrum band among the available bands
– This notion is called “spectrum decision” and constitutes a rather important but yet unexplored topic in CR networks.
Spectrum Decision
Usually consists of two steps:
1. Each spectrum band is characterized based on not only local observations of CR users but also
Spectrum Decision
1st Stage Spectrum Characterization RF information •Interference •Path Loss •Wireless Link Error •Link layer delay Primary Network Information • Primary User Activity • Holding Time 2nd Stage Decision Single Spectrum Decision Multi-Spectrum DecisionSpectrum Characterization
To describe the dynamic nature of CR networks,
each spectrum hole should be characterized
by considering the time-varying radio environment &
Definitions
* Interference level * Channel error rate * Path-loss
* Link layer delay * Holding time
Interference
Some spectrum bands are more crowded compared to others. Hence, the spectrum band in use determines the
interference characteristics of the channel.
From the amount of the interference at the primary
Path Loss
The path loss increases as the operating frequency
increases.
Therefore, if the transmission power of a CR user
remains the same, then its transmission range decreases at higher frequencies.
Similarly, if transmission power is increased to
Wireless Link Errors
Depending on the modulation scheme and the
interference level of the spectrum band, the
error rate of the channel changes.
Link Layer Delay
To address different path loss, wireless link
error, and interference, different types of
link layer protocols are required at different
spectrum bands.
This results in different link layer packet
transmission delay.
Primary User Activity
– Since there is no guarantee that a spectrum band will be available during the entire communication of a CR user, it is important to consider how often the primary user appears on the spectrum band.
Holding Time
– Expected time duration that the CR user can occupy a licensed band before getting interrupted.
– Obviously, the longer the holding time, the better the quality would be.
– Since frequent spectrum handoff can decrease the holding time, previous statistical patterns of handoff should be considered while designing CR networks with large expected holding time.
CHANNEL CAPACITY
Can be derived from the parameters explained above,
is the most important factor for spectrum characterization.
Usually, SNR at the receiver is used for capacity
8 6 4 0 2
CHANNEL CAPACITY
However, in order to avoid the interference at the primary users, the
transmission power of CR users may be limited.
Primary user
Primary user
CR user CR user
In case there is no
primary user, CR user can transmit with the max. power
In case the primary user
is detected, the
transmission power of the CR user is constrained to avoid the interference.
4
0 2
CHANNEL CAPACITY
Thus, the channel capacity of CR users depends on
the interference at the licensed (primary) receivers, i.e., limited by a primary user’s activity.
Spectrum Capacity
Spectrum capacity, C, can be estimated as:
) 1 log( I N S B C
SINR (Signal to Interference
plus Noise Ratio)
The received power is
constrained by primary users, which affect the channel
capacity
where B is the bandwidth
S is the received signal power from the CR user N is the CR receiver’s noise power
Spectrum Characterization
Recent work on spectrum analysis only focuses
on spectrum capacity estimation.
Other factors such as delay, link error rate, and
holding time also have significant influence on the quality of services.
Spectrum Characterization
Capacity is closely related to both
interference+noise level and path loss.
A complete analysis and modeling of spectrum in CR
Decision Procedure
Once all available spectrum bands are characterized, appropriate
operating spectrum band should be selected for the current
transmission considering the QoS requirements and the spectrum characteristics.
Thus, the spectrum decision function must be aware of user QoS
SINGLE SPECTRUM DECISION
CR user B Occupied by primary users
CR user A
Idle spectrum band
Frequency(Hz)
Each CR user selects only one spectrum band according to the
application requirements
Problems of Single Spectrum Decision
– Because of the operation of primary networks, CR
users cannot obtain a reliable communication channel for a long time.
Multi-Spectrum Decision
Sub-channels for CR user B Occupied by primary users
Sub-channels for CR user A
Idle spectrum band
Frequency(Hz)
CR users select multiple non-contiguous spectrum bands and use them
Multi-Spectrum Decision
High throughput can be achieved !
Immune to the interference and the primary user activity.
– Transmission in multiple spectrum bands allows lower power to be used in each spectrum band
less interference with primary users is caused
- Even if spectrum handoff occurs in one of the current spectrum bands, the rest of the spectrum bands will maintain current
Further Challenges:
Decision Model
SNR is not sufficient to characterize the spectrum band!
Besides the SNR, many spectrum characterization parameters
would affect QoS.
Applications may require different QoS requirements. Thus, how to combine these spectrum characterization
Further Challenges:
Cooperation with Reconfiguration
CR technology enables the transmission parameters of a radio
to be reconfigured for optimal operation in a certain spectrum band.
For example, if SNR is fixed, BER can be adjusted to
maintain the channel capacity by exploiting adaptive modulation techniques.
Spectrum Sharing
Spectrum Sensing Primary User Detection RF Stimuli Spectrum Hole Radio Environment Spectrum Mobility Decision Request Spectrum Sharing Transmitted Signal Spectrum (Channel) CharacterizationSpectrum Sharing
Spectrum Sharing similar to MAC Problems
– Multiple CR users try to access the spectrum
– Access must be coordinated (to prevent collisions in overlapping portions of the spectrum)
Uniqueness
SPECTRUM SHARING CLASSIFICATION
oIntra-Network SS
– Centralized (Infrastruct. based)
– Distributed (Ad hoc – based)
Inter-Network SS
* Centralized * Distributed
Intra-Network Spectrum Sharing
Sending local observations Sending spectrum allocations
Spectrum sharing entity Spectrum sharing entity
Intra-Network Spectrum Sharing
Spectrum sharing inside a CR network same as MACs Focuses on “spectrum allocation” between the CR users
Coordinates multiple accesses among CR users in order to
prevent their collision in overlapping portions of the spectrum
Also CR users need to access the available spectrum
Inter-Network Spectrum Sharing
Sending Local Observations Sending Spectrum Allocations
Spectrum Sharing Entity
CR Network A
CR Network B
Spectrum Broker (or Spectrum Server)
Inter-Network Spectrum Sharing
Multiple systems are deployed in overlapping
locations and spectrum bands
Spectrum sharing among these systems is an
Game Theory
Definition
– A collection of mathematical models and techniques for the analysis of interactive decision processes
– Provides strategic interactions among agents using formalized incentive structure
Why Game Theory?
Excellent match in nature to the spectrum
sharing in CR networks.
[Game Theory]
–
Provides a well-defined model to describe
Why Game Theory?
[Spectrum Sharing in CR networks]
– CR users have a common interest to have the
spectrum resources as much as possible.
– However, CR users have competing interests to
maximize their own share of the spectrum resources.
i.e., the activity of one CR user can impact the activities of the others
Why Game Theory?
Provides an efficient distributed spectrum sharing
scheme.
Provides the well-defined equilibrium criteria for the
spectrum sharing problem to measure the optimality
Game Theory: Basic Components
Game: A model of interactive decision process Player: A decision making entity
Actions (Strategies): The adaptations available to the player. Outcomes (Payoffs) : The outputs determined by the actions
Game Theory: Recap
The output (outcomes) of the process (game) is the function of the inputs (actions) from several
different decision makers (players) who may have potentially conflicting objectives (preferences) with regards to the outcome of the process.
Normal Form Games (Strategic Form Games)
Synchronous Single Shot Play:
All players make their decisions simultaneously and take only a single decision without knowing the actions of the other
Three Components:
– A set of players N – Action Space A,
Normal Form Games (Strategic Form Games)
Example: Paper (P) – Rock (R) - Scissors (S) Game – N = {P1, P2}
– A = {(P,P), (P,R), (P,S), …, (S,S)}
– {uj} = {-1, 0, 1} (-1: loss, 0: tie, 1: win)
P R P (0,0) (1,-1) R (-1,1) (0,0) S (-1,1) (1,-1) P1 P2
Nash Equilibrium (NE)
DEFINITION:
A set of actions (strategies) where no player has
anything to gain by changing only his/her own
strategy unilaterally.
Nash Equilibrium (NE)
If each player has chosen a strategy and no
player can benefit by changing his/her own
strategy while other players keep theirs
unchanged,
then the current set of strategy choices and the
corresponding payoffs constitute a NE.
Nash Equilibrium (NE)
SIMPLY:
You and I are in NE if I make the best decision I can, taking into account your decision, and you make the best decision you can, taking into account my decision.
Nash Equilibrium
Example Games a1 a2 b2 1,1 -5,5 NE ayer 1 Player 2How to model CR networks using Game Theory?
Player → CR Users (and Primary Users) Action (Strategy)
– CR Users:
Which licensed channels will be used by the players?
How to model CR networks using Game Theory?
Action (Strategy)
–
PR Users****:
(???)
Which unused spectrum they will lease?
How much they will charge CR users for using
How to model CR networks using Game Theory?
Outcome (Payoff) → Network State (SNR, BW, etc) Utility Functions → Target QoS parameters
Example Models
Player: Two CR Users Action:
Select either a low-power narrowband waveform N, or a higher power wideband waveform W
Outcome: Network States (SNR, BW) Utility Function: Throughput
Example Models
Narrowband Wideband 1 CR Users 1 CR users 2 CR Users 2 Wideband Narrowband Narrowband Wideband FrequencySPECTRUM SHARING CLASSIFICATION
oIntra-Network SS
– Centralized (Infrastruct. based)
– Distributed (Ad hoc – based)
Cooperative
Inter-Network SS
* Centralized
Centralized Spectrum Sharing
A centralized node (e.g., CR base station)
controls the spectrum allocation and access
procedures.
Each CR user in the CR network forwards
Centralized Spectrum Sharing
Spectrum sharing on the unlicensed bands
Spectrum server allocates an optimal schedule
for a set of links in CR networks using:
– Maximum Sum Rate Scheduling
– Max-Min Scheduling
C. Raman, R. D. Yates, and N. B. Mandayam, “Scheduling Variable Rate Links via a Spectrum Server,” Proc. IEEE DySPAN, pp.110118, Nov.’05.
Centralized Spectrum Sharing
Performance Analysis
– Maximum sum rate scheduling with no minimum rate constraint: the
transmission mode with the highest sum rate is chosen. The links which are not a part of this transmission mode are not operated at all.
– Maximum sum rate scheduling with nonzero minimum rate constraint: More than one transmission mode is operated since there is a minimum rate
requirement for each link.
SPECTRUM SHARING CLASSIFICATION
oIntra-Network SS
– Centralized (Infrastruct. based)
– Distributed (Ad hoc – based)
Cooperative
Inter-Network SS
* Centralized
Intra-Network Spectrum Sharing
- Distributed & Cooperative
If infrastructure is not preferred !!
Each CR user is responsible for the spectrum allocation
and access is based on local policies.
Cooperative (or collaborative) solutions consider the effect of the CR
user’s communication on other users.
I.o.w. the interference measurements of each CR user are shared
among other CR users.
Furthermore, the spectrum sharing algorithms also consider this
information.
While all the centralized solutions can be regarded as cooperative,
Intra-Network Spectrum Sharing
- Distributed & Cooperative
SPECTRUM SHARING CLASSIFICATION
oIntra-Network SS
– Centralized (Infrastruct. based)
– Distributed (Ad hoc – based)
Inter-Network SS
* Centralized
Intra-Network Spectrum Sharing
- Distributed & Non-Cooperative
If infrastructure is not preferred !!
Each CR user is responsible for the spectrum allocation
and access is based on local policies.
CR users depend only on their local observations for
Non-cooperative (or non-collaborative, selfish) solutions
consider only the node itself
Selects the channel with the objective of maximum
throughput without taking other users into consideration!
May result in reduced spectrum utilization
Intra-Network Spectrum Sharing
- Distributed & Non-Cooperative
SPECTRUM SHARING CLASSIFICATION
oIntra-Network SS
– Centralized (Infrastruct. based)
– Distributed (Ad hoc – based)
Cooperative
Inter-Network SS
* Centralized
Inter-Network Spectrum Sharing
- Centralized
O. Ileri, D. Samardzija, and N. B. Mandayam, “Demand Responsive Pricing and
Competitive Spectrum Allocation via Spectrum Server,” in Proc. IEEE DySPAN, pp. 194202, Nov. 2005.
Step 2: Iterative bidding process: winner declared Step 1: User specific information is
communicated to the SPS
Step 3: User evaluates
Operator Bidding Scheme
A central spectrum policy server (SPS) is proposed
to coordinate spectrum demands of multiple CR operators.
The operators dynamically compete for customers as
SPECTRUM SHARING CLASSIFICATION
oIntra-Network SS
– Centralized (Infrastruct. based)
– Distributed (Ad hoc – based)
Inter-Network SS
* CentralizedClassification of Spectrum Sharing
based on Spectrum Access Techniques
oPrimary user CR user
Overlay Spectrum Sharing
A CR user accesses the primary network using a portion of
the spectrum that has not been occupied by licensed users.
As a result, interference to the primary system is
Underlay Spectrum Sharing
Underlay spectrum sharing exploits the spread spectrum
techniques developed for cellular networks
Once a spectrum allocation map has been acquired, a CR
user begins transmission such that its transmit power at a certain portion of the spectrum is regarded as noise by
the primary users. (Interference temperature idea)
Comparison of Underlay and Overlay Approaches
Based on the influence of the CR network on the primary network in terms of outage probability
(probability that the primary network will experience
interference from the CR network)
three spectrum sharing techniques have been considered.
R. Menon, R. M. Buehrer, J. H. Reed, “Based Comparison of Underlay and Overlay
Spectrum Sharing Techniques Outage Probability,” in Proc. IEEE DySPAN, pp. 101-109, Nov. 2005.
Comparison of Underlay and Overlay Approaches
METHOD 1: Spreading Based Underlay
requires CR users to spread their transmit power over the full spectrum such as CDMA or UWB.
Comparison of Underlay and Overlay Approaches
METHOD 2: Interference Avoidance Overlay
requires CR users to choose a frequency band to transmit such that the interference at a primary user is minimized.
Comparison of Underlay and Overlay Approaches
METHOD 3: Hybrid Technique (Spreading based Underlay
with Interference Avoidance)
A CR user spreads its transmission over the entire spectrum and also null or notch frequencies where a primary user is transmitting.
Comparison of Underlay and Overlay Approaches
Perfect system knowledge
– Overlay scheme outperforms the underlay scheme in
terms of outage probability.
– Underlay scheme with interference avoidance
Comparison of Underlay and Overlay Approaches
Limited System Knowledge (more realistic)
– The overlay schemes result in poor performance due imperfections at spectrum sensing.
– Underlay with interference avoidance
the interference caused to the primary user is minimized.
– Another important result is that a higher number of CR users can be accommodated by the hybrid scheme