Cognitive Radio PPT

Advanced Topics in Wireless Communications

COGNITIVE RADIO NETWORKS

2

INTRODUCTION

3

FIXED SPECTRUM ASSIGNMENT

4

Fixed Spectrum Utilization

Maximum Amplitudes Amplitud

e (dBm)

Heavy Use Heavy Use

Medium Use Sparse Use

Frequency (MHz)

5

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): Utilization of the fixed spectrum assignment is approx. 15-85% based on temporal and geographical variations Limited Available Spectrum and Inefficient Spectrum Usage!

6

COGNITIVE RADIO NETWORKS; DYNAMIC SPECTRUM ALLOCATION NETWORKS (DSANs);

xG INITIATIVE

Dynamic Spectrum Allocation

7

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 Dynamic spectrum access techniques allow the CR to operate in the best available channel.

COGNITIVE RADIO

8

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 (Spectrum Mobility).

SPECTRUM MANAGEMENT FRAMEWORK

9

2. COGNITIVE RADIO

10

A Cognitive Radio is a radio that can change its transmitter parameters based on interaction with the environment in which it operates. (Federal Com Commission05) FCC (Non-Federal Use of the Spectrum)

WHAT IS A COGNITIVE RADIO?

11

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 US Department of Commerce: (NTIA) (FEDERAL USE OF THE SPECTRUM)


12

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


13

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. CRs can be trained to dynamically and autonomously adjust its operating parameters.

IEEE 1900.1 Group


14

A RADIO THAT IS COGNITIVE !!!!

HOW ABOUT???

15

Senses RF Environment and modifies frequency, power or modulation Allows for Real Time Spectrum Management Significantly Increases Spectrum Efficiency

ADVANTAGES OF COGNITIVE RADIO

16

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 as technology progresses

Possible CR Functionalities

17

Analogy between a Cognitive Radio and a Car Driver

Cognitive Radios 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 Deals with situations not planned at the initial time of design

18

Analogy between a Cognitive Radio and a Car Driver

Car Drivers 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 learning to drive

They behave almost exactly the same!!!

19

Spectrum Hole Concept

Time

Frequency

Spectrum Hole Power

Spectrum occupied by Licensed users

20

Ultimate Objective of Cognitive Radio

CR enables the usage of temporally unused spectrum Spectrum Hole or White Space.

If this band is further used by a licensed user, CR moves to another spectrum hole or stays in the same band Alters its transmission power level or modulation

scheme to avoid interference.

21

MAIN CHARACTERISTICS OF CR

A. Cognitive Capability

B. Reconfigurability (SDR)

22

Cognitive Capability SPECTRUM AWARENESS!!

Capture or sense the information (e.g., licensed users

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

Selection of best spectrum and appropriate operating

parameters

23

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 * to use different transmission access technologies supported by its hardware design

24

Physical Architecture of the Cognitive Radio (Wideband RF/Analog Front-End Architecture)

Frequency

Power Spectrum Density (PSD)

Band of interest Available Channel

... ...

Baseband (low freq.) (A/D Conversion/Signal

Processing)

Pass-band (high freq.) (for communications)

PSD of the received

licensed signal

25

Challenges 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 signal in a large dynamic range. Requires a multi GHz speed A/D converter with high resolution infeasible!!

26

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 selectively using beam- forming techniques.

Licensed User f1

Licensed User f2

f1 f2

f1

f2

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3. ARCHITECTURE

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Cognitive Radio Network Architecture

Primary Base-station

Primary User

Primary Network

Licensed Band I

Unlicensed Band

Licensed Band II CR Network Access

Primary Network Access

CR User

Spectrum Band

CR Base-station

Cognitive Radio Network (With Infrastructure)

Other Cognitive Radio

Networks

Spectrum Broker

29



Primary User

Primary Network

Licensed Band I

Unlicensed Band

Licensed Band II


Cognitive Radio Network (Without Infrastructure)

CR Ad Hoc Access

CR User

Spectrum Band

30



Primary User

Primary Network

Licensed Band I

Unlicensed Band

Licensed Band II

CR Network Access


Cognitive Radio Network (Without Infrastructure)

CR Ad Hoc Access

CR User

Spectrum Band

CR Base-station

Cognitive Radio Network (With Infrastructure)

Other Cognitive Radio

Networks

Spectrum Broker

31

Architecture

Primary Network

(Primary User, Primary Base Station)

Cognitive Radio Network

(CR User, CR Base Station)

Spectrum Broker

32

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.

33

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 users do not need any modification or additional functions for co-existence with CR base-stations and CR users.

34

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.

It may be requested to have both legacy and CR protocols

for the primary network access of CR users.

35

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.

* CR networks can be deployed both as an

infrastructure network and an ad hoc network

36

Cognitive Radio User (or Unlicensed User, Secondary User)

has no spectrum license

Hence, additional functionalities are required

to share the licensed spectrum band.

37

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.

Through this connection, a CR user can access

other networks.

38

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 co-existence of multiple CR networks.

39

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 bands.

Primary Network Access: CR users can also access the primary base-station through the licensed band.

40

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.

41

Cognitive Radio Network on Licensed Band

Primary User

Primary Base-Station

Primary Network

CR User

CR Base-station

Cognitive Radio Network

Dynamic Spectrum Access

CR User

42

CR 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 network at the same location and on the same spectrum band

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Main purpose of the CR network is to determine the best available spectrum Here in the licensed band, CR functions are aimed at the detection of the presence of primary users. Channel capacity of the spectrum holes depends on the interference at the nearby primary users.

44


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 called spectrum handoff.

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Cognitive Radio Network on Unlicensed Band

Spectrum Broker

CR User

Cognitive Radio Network A CR Base-Station

Cognitive Radio Network B

CR Base-Station

CR Ad Hoc Network

46

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 efficiency of spectrum usage and support high QoS.

47


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.

48


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.

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4. COGNITIVE CYCLE

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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.

51

Cognitive Cycle

Spectrum Sharing

Spectrum Sensing

Spectrum Decision

Channel Capacity

Transmitted Signal

Licensed User Detection

RF Stimuli

Spectrum Hole

Radio Environment

Spectrum Mobility

Decision Request

52

Spectrum Sensing

A CR monitors the available spectrum bands,

captures their information, and then detects

the spectrum holes.

53

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

(and possibly external) policies.

54

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.

55

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.

56

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 dynamic radio environment.

57

Reconfigurable Parameters

i) Operating Frequency

ii) Modulation

iii) Transmission Power

iv) Communication Technology

58

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

the communication can be dynamically performed on this appropriate operating frequency.

59

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!!

Example: Loss-Sensitive Applications error rate important !

Modulation scheme with low bit error rate..

60

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 decrease the interference.

61

Communication Technology

A CR can be used to provide interoperability

among different communication systems.

62

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

* Appropriate communication protocol parameters and

modulation schemes are used.

63

5. SPECTRUM SENSING

64

What is Spectrum Sensing ?

How to detect spectrum holes by the COGNITIVE RADIO so that it can adapt itself to its environment !!

65

Spectrum Sensing

Spectrum Sharing

Spectrum Sensing

Spectrum Decision

Channel Capacity

Primary User Detection

RF Stimuli

Spectrum Hole

Radio Environment

Spectrum Mobility

Decision Request

Transmitted Signal

66

CR User 1

Primary Tx

Primary Rx

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 communication between CRs

A general CR based communication scenario CR

User 2

Licensed band 1

Licensed band 2

EFFICIENT WAY TO DETECT SPECTRUM HOLES

67

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 How to detect primary users based on local observation of CR users (from its environment)

68

Classification of Spectrum Sensing Techniques

Interference Temperature Management

Transmitter Detection

Spectrum Sensing

Receiver Detection

Matched Filter Detection

Energy Detection

Cyclostationary Feature Detection

69


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.

Transmitter Detection Approach Detection of the signal (weak signal as the worst case) from a primary user through local observations of CR

users.

70

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 H0 Null hypothesis No licensed user signal in a certain spectrum band. H1 Alternative hypothesis There exists some licensed user signal.

1

0

)()(

)()(

Htnths

Htntx

71


Three schemes are generally used for the transmitter detection according to the hypothesis model.


Energy Detection and

Cyclostationary Feature Detection Techniques

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.

72




Spectrum Sensing

Receiver Detection


Energy Detection


73


Need Transmitted signal information s(t) Synchronization for sampling timing (t=T)

s(t): the transmitted signal of the primary user n(t): AWGN T: Symbol interval : Threshold

0 T

s(t) r(t)

0 T

oH

Y

Sample at t = T

Received Signal r(t) = s(t) + n(t)

t

dtTsr0

)()(

Threshold Device

Y

1H

Decide H0 or H1

Matched Filter

0 T

maximum at T

2T 0 T 2T

74


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

75


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. However, since most wireless network systems have pilot,

preambles, synchronization word or spreading codes, these can be used for the coherent detection.

76

Energy Detection



Spectrum Sensing

Receiver Detection


Energy Detection


77

Energy Detection

If the CR user cannot gather sufficient information about the primary user signal s(t), the matched filter is not suitable.

However, if the CR user is aware of the power of the random Gaussian noise, then the energy detector is optimal.




H. Tang, Some Physical Layer Issues of Wideband Cognitive Radio System, in

Proc. IEEE DySPAN, pp. 151159, Nov. 2005.

78

Energy Detection

Input 2)(

Squaring Device Integrator Threshold Device

Decide H0 or H1 )(tr

T

dttr0

2 )()(2 tr

Y

T

dt0

Filtering

oH

Y1H

T: Observation (sensing) Time

A. Ghasemi and E. S. Sousa, Collaborative Spectrum Sensing for Opportunistic

Access in Fading Environment, in Proc. IEEE DySPAN, pp. 131-136, Nov. 2005

79

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 integrated over the observation interval T.

80

Energy Detection

Finally, the output of the integrator, Y, is compared with a threshold, , to decide whether a licensed user is present or not. (AWGN case)

81

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)). Implementation is easy!!

82

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 Matched filter: T~1/SNR Energy Detector: T~1/SNR2

when detecting weak signals: SNR < 1 (-10dB to -40 dB)

83




Spectrum Sensing

Receiver Detection


Energy Detection


84


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.




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.

85


These modulated signals are characterized as

cyclostationary since their mean and autocorrelation exhibit periodicity.

These features are detected by analyzing a spectral

correlation function. Advantage of the spectral correlation function: differentiates the noise energy from modulated signal energy

86

Sine based Cyclostationary Detection Primary Tx frequency repeats over symbol durations at regular intervals T

Problem: Can these cyclical regularities be detected at the CR user?


87


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.

If the correlation factor is high (greater than the threshold),

there is a primary user.

88


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

Proc. IEEE DySPAN, pp. 151159, Nov. 2005.

89

Limitations of the Transmitter Detection

Hidden Terminal Problem due to Shadowing

Interference due to uncertainty of receiver location

CR Transmitter

Range


CR User

Primary Transmitter

Range

Primary User


Primary Transmitter

Range

Primary User

CR Transmitter

Range Interference

Interference

CR User

Cannot detect the transmitter

Cannot detect the transmitter

Shadowing Problem Receiver Uncertainty Problem

90

Receiver Uncertainty Problem

With the transmitter detection, the CR user cannot avoid the interference due to the lack of the primary receivers information (Fig.a).

Moreover, the transmitter detection model cannot prevent the hidden terminal problem.

91

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 users is required for more accurate detection

Cooperative Detection

92

Transmitter Detection Non-Cooperative vs Cooperative Detection



Energy Detection



Non-Cooperative Detection


Detection Method Detection Behavior

93

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.

Mitigates multi-path fading and shadowing effects improves the detection probability in heavily faded/shadowed environments.

94


Primary User


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 users can detect the primary user under fading and shadowing environments

CR User 2

CR User 3

CR User 1

BUSY

IDLE

IDLE

BUSY BUSY

95

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.

- Increases the cooperative detection probability Qd - Suitable for a highly faded/shadowed radio environments

96

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

ff

n

dd

PnQ

PnQ

)1(1}correctly hole spectrum detect the users CR allPr{1

)1(1}detection themiss users CR allPr{1

Qd is the cooperative detection probability Qf is the cooperative false alarm probability Pd is the non-cooperative (individual) detection probability Pf is the non-cooperative (individual) false alarm probability

97

Cooperative Detection Probability Cooperative False Alarm Probability

Increasing Qd

Increasing Qf

Detection and False Alarm Probability for Cooperative Detection

98


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 the lack of the primary receiver location knowledge is still unsolved!!

99

Primary Receiver Detection



Spectrum Sensing

Receiver Detection


Energy Detection


100



Primary User

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 power.

B. Wild and K. Ramchandran, Detecting Primary Receivers for Cognitive Radio

Applications in Proc. IEEE DySPAN, pp. 124130, Nov. 2005.

101


AGC A/D

PLL

Antenna

RF Filter Mixer

VCO

Channel Selection Filter

LNA

Local Oscillator - Generates a sine signal for the baseband conversion

- CR users detect this signal

RF Front-end of the Primary Receiver

102

How can the LO Leakage Power be detected?

Same methods as before, i.e.,

(Matched filter detection, Energy

detection or Cyclostationary feature

detection )

103

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, implementation of a reliable detector is not trivial.

Currently this method is only feasible in the detection of the TV receivers.

104




Spectrum Sensing

Receiver Detection


Energy Detection


105

Interference 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

Distance from Licensed Transmitting Antenna

106

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

noise floor.

107


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.

108


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 their transmissions, they can use this spectrum band.

109

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

of the primary users which is not easy to determine. Increasing the interference temperature limit will affect

primary networks capacity and coverage.

110

6. SPECTRUM DECISION

111

Spectrum Decision

Spectrum Sharing

Spectrum Decision

Spectrum Sensing

Channel Capacity


RF Stimuli

Spectrum Hole

Radio Environment

Spectrum Mobility

Decision Request

Transmitted Signal

112

Spectrum 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 is closely related to the channel characteristics and the operations of primary users.

113

Spectrum Decision

Usually consists of two steps:

1. Each spectrum band is characterized based on not

only local observations of CR users but also

statistical information of primary networks.

2. Then, based on this characterization, the most

appropriate spectrum band can be chosen.

114

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 Decision

115

Spectrum Characterization

To describe the dynamic nature of CR networks,

each spectrum hole should be characterized

by considering the time-varying radio environment &

the primary user activity and the spectrum band information (e.g., operating frequency and bandwidth).

116

Definitions

* Interference level

* Channel error rate

* Path-loss

* Link layer delay

* Holding time

117

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

receiver, the permissible power of a CR user can be

derived, which is used for the estimation of the channel

capacity.

118

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

compensate for the increased path loss, then this results

in higher interference for other users.

119

Wireless Link Errors

Depending on the modulation scheme and the interference level of the spectrum band, the error rate of the channel changes.

120

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.

121

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.

Primary User Activity is defined as the probability of the primary user appearance during the CR user transmission.

122

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.

123

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 estimation.

124

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.

The location of the primary users can affect the channel capacity of CR users

4

0

2

Transmission range

Received power

125

CHANNEL CAPACITY

Thus, the channel capacity of CR users depends on the interference at the licensed (primary) receivers,

i.e., limited by a primary users activity.

126

Spectrum Capacity

Spectrum capacity, C, can be estimated as:

)1log(IN

SBC

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 receivers noise power

I is the interference power received at the CR receiver due to

the primary transmitter.

127


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.

128


Capacity is closely related to both interference+noise level and path loss.

A complete analysis and modeling of spectrum in CR networks is yet to be developed.

129

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

requirements. Based on the user requirements, the data rate, acceptable error rate,

delay bound, the transmission mode, and the bandwidth of the transmission can be determined.

130

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

Spectrum Handoff

CR user A

CR user B

131

Problems of Single Spectrum Decision

Because of the operation of primary networks, CR users cannot obtain a reliable communication channel for a long time.

CR users may experience temporary disconnections (latency) during the spectrum handoff.

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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 simultaneously for the transmission.

Spectrum Handoff

CR user A

CR user B

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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 transmissions.

How to determine the number of spectrum bands and how to select the set of appropriate bands are still open research issues.

134

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 parameters for the application-adaptive spectrum decision model is still an open issue.

135

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.

Hence, a cooperative framework that considers both

spectrum decision and reconfiguration is required.

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7. SPECTRUM SHARING

137

Spectrum Sharing

Spectrum Decision

Spectrum Sensing

Channel Capacity


RF Stimuli

Spectrum Hole

Radio Environment

Spectrum Mobility

Decision Request Spectrum

Sharing

Transmitted Signal

Spectrum (Channel)

Characterization

138

Spectrum 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

Coexistence with licensed (primary) users

Wide range of available spectrum

139

SPECTRUM SHARING CLASSIFICATION o

Intra-Network SS Centralized (Infrastruct. based)

Distributed (Ad hoc based)

Cooperative

Non-cooperative

Inter-Network SS * Centralized

* Distributed

140

Intra-Network Spectrum Sharing

Centralized Spectrum Sharing

141


Distributed Spectrum Sharing (Cooperative)

Sending local observations Sending spectrum allocations

Spectrum sharing entity

Distributed Spectrum Sharing (Non-Cooperative)

Spectrum sharing entity

142


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 without causing interference to the primary users.

143

Inter-Network Spectrum Sharing

Distributed Spectrum Sharing Centralized Spectrum Sharing

Sending Local Observations Sending Spectrum Allocations

Spectrum Sharing Entity

CR Network A

CR Network B

CR Network C

Spectrum Broker (or Spectrum Server)

CR Network A

CR Network B

CR Network C

144

Inter-Network Spectrum Sharing

Multiple systems are deployed in overlapping locations and spectrum bands

Spectrum sharing among these systems is an important research topic in CR networks

145

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

Enables the choice of optimal behavior when costs and benefits of each option depend upon the choices of other individuals.

146

Why Game Theory?

Excellent match in nature to the spectrum sharing in CR networks.

[Game Theory]

Provides a well-defined model to describe conflict and cooperation among intelligent rational decision makers

147

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

Also CR users rational decisions require anticipating rivals responses

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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 in various network scenarios.

149

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 and the particular system in which the players are operating

Preference: A decision maker objective (To capture the preference relation in a more compact way; we employ utility functions (payoff functions) where each player assigns a real number to each outcome)

150

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.

151

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,

A set of utility functions {uj}

such that each player j N has its own utility function, uj :A R

(R is a set of real numbers)

Specified by 3-tuple =

152

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)

S

(-1,1)

(1,-1)

(0,0)

P1 P2

153

Nash Equilibrium (NE)

DEFINITION: A set of actions (strategies) where no player has anything to gain by changing only his/her own strategy unilaterally.

NEs correspond to the steady-states of the game and are then predicted as the most probable outcomes of the game.

154


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.

155


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. Likewise, many players are in NE if each one is making the best decision he can, taking into account the decisions of the others.

156

Nash Equilibrium

Example Games

a1

b1

a2 b2

1,1 -5,5

-1,-1 5,-5

NE

Player

1

Player 2

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How 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? Which transmission parameters (transmission power, time duration) to apply? or The price they agree to play for leasing certain channels from the primary users

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Action (Strategy)

PR Users****: (???) Which unused spectrum they will lease? How much they will charge CR users for using their spectrum resources, etc. ?

159


Outcome (Payoff) Network State (SNR, BW, etc)

Utility Functions Target QoS parameters (Throughput, Delay, BER, Cost, etc.)

160

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

Preference: To maximize throughput

161

Example Models

Narrowband Wideband

Narrowband (9.6,9.6) (3.2, 21)

Wideband (21,3.2) (7,7) CR u

ser

1

CR Users 1

CR users 2

CR Users 2

Wideband

Narrowband Narrowband

Wideband

Frequency

(kbps)

Nash Equilibrium

162




Cooperative

Non-cooperative

Inter-Network SS * Centralized * Distributed

163


A centralized node (e.g., CR base station) controls the spectrum allocation and access procedures.

Each CR user in the CR network forwards their measurements about the spectrum allocation to the central node which then constructs a spectrum allocation map.

164


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 Proportional Fair 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.

165


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.

Max-min fair solution: all the links end up getting the same rate.

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Cooperative

Non-cooperative


167

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.

CR users exchange their information with other neighboring users for spectrum access

168

Cooperative (or collaborative) solutions consider the effect of the CR users 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, there also exist distributed cooperative solutions.

Intra-Network Spectrum Sharing - Distributed & Cooperative

169




Cooperative

Non-cooperative


170

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 spectrum access

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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

Requires minimal communication among other nodes.

Intra-Network Spectrum Sharing - Distributed & Non-Cooperative

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Cooperative

Non-cooperative


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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 the offer of the winner

CR user

Operator1 Operator2

Spectrum Policy Server (SPS)

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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 well as portions of available spectrum

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Cooperative

Non-cooperative


176

Classification of Spectrum Sharing based on Spectrum Access Techniques o

Overlay Spectrum Sharing Underlay Spectrum Sharing

Primary user CR user

Frequency Frequency

177

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 minimized.

178

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)

Requires sophisticated spread spectrum techniques and can utilize increased bandwidth compared to overlay techniques.

179

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.

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METHOD 1: Spreading Based Underlay

requires CR users to spread their transmit power over the full spectrum such as CDMA or UWB.

181


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.

182


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.

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Perfect system knowledge

Overlay scheme outperforms the underlay scheme in terms of outage probability.

Underlay scheme with interference avoidance guarantees smaller outage probability than the pure interference avoidance.

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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 than the pure interference avoidance scheme.

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Cognitive Radio PPT